addressing cognitive processes are discussed, as well as theoretical issues about the role of language in the development of time concepts. Because children's cognitive abilities and linguistic capacities are interdependent, practical issues about how to measure each component individually are also considered. Finally, directions for future research to resolve theoretical and practical issues are proposed.

### CONFLICTING EVIDENCE FROM PAST RESEARCH

The limited literature investigating the emergence of temporal concepts comes from two research lines focusing on children's temporal language acquisition and their temporal cognitive processes, respectively. Psycholinguistic researchers claimed that the separation of event time from speech time indicates children's emerging concept of time and their usage of tensed verbs is evidence of a grasp of the basic distinctions between past, present, and future by age 3 (Weist, 1989). However, researchers focusing on temporal cognition concluded that 4- and 5-year-olds do not yet understand the distinctions between the past, present, and future properly (Friedman, 2003). What is the evidence for these conclusions and how can they be reconciled?

### Acquisition of Temporal Language

Time is encoded in language in many ways. Language is the primary medium through which notions about past and future events are transmitted (Harner, 1982). In English, many devices, such as aspect, tense, and temporal adverbs, are used to denote time and code time-related characteristics of actions (Klein, 2009). For example, aspect delineates the internal contour of the event itself, whereas tense and temporal adverbs denote the position of an event on a timeline. Developmental psychologists have argued that the emergence of temporal markers in children's language indicates changes in their understanding of time (Weist, 1989; Busby Grant and Suddendorf, 2011).

In tensed languages, three important points in time are encoded in speech (Reichenbach, 1947). Speech Time (ST) is the time point of the act of speech. Event Time (ET) is the time when the event occurred, and Reference Time (RT) indicates the speaker's temporal vantage point. It is particularly clear when RT does not coincide with ST and ET, as in the case of the past perfect tense (e.g., in "Peter had gone," RT is between ET and ST) and the future perfect tense (e.g., in "Peter will have gone," RT is after both ST and ET). Based on Reichenbach's theoretical work and observations of language acquisition, Weist (1989) proposed a four-system model of children's temporal language development, with each system reflecting a different level of competence. The first system is the ST system used by children from 12 to 18 months. Children's speech at this stage focuses on here-and-now. It does not include tense, aspect, or modality. Between 18 and 24 months, children begin to use past tense to mark an event anterior to speech time and to use future tense to mark an event posterior to speech time. This corresponds to the ET system, where ET is expressed separately from ST. Later, between 30 and 36 months, children start to use temporal adverbs to indicate when an event occurs, which corresponds to the restricted reference time (RTr) system. For example, a child might say, "Yesterday I was in Lodz" (Weist, 1989, p. 108). Compared to utterances from ET system, e.g., "I was in Lodz," utterances from the RTr system contain both event time (i.e., past tense) and reference time (i.e., yesterday) and both are referenced in contrast to speech time. The last system is the free reference time (RTf ) system, emerging between 36 and 52 months. Compared to the RTr system, children are now capable of manipulating RT, ST, and ET to freely express more temporal configurations. They can use the temporal prepositions "before" and "after," perfect tenses, and even temporal clauses – for example, "While this one is playing [RT], that one will be playing [ET]" (p. 105). Weist believed that the separation of ET from ST (i.e., the use of tense) indicates an emerging concept of time and that reference to specific non-present time points (i.e., the use of temporal adverbs) indicates a developing temporal framework. The development from RTr system to RTf system relates to more complicated cognitive processes such as temporal decentering and relational reasoning.

In support of Weist's model, studies focusing on children's natural language production reveal a haphazard use of inflected verbs from 21 to 22 months (Nelson, 1989). From 22 to 24 months, children develop a present-past-progressive system, which first reflects a contrast between now and not-now, and later takes on direction by specifically coding pastness (Nelson, 1989). However, production of verbs with visible tense marking is rare in 2- to 3-year-olds' spontaneous speech (Valian, 1991). Even when they are asked to imitate adult's utterance of past tense, only 2% of verbs are past tensed by 2-year-olds with low mean lengths of utterance (MLU, from 1.5 to 2.5 words) and 14% of verbs are past tensed by 2-year-olds with high MLU (2.5 to 4.6 words) (Valian and Aubry, 2005). Instead of verb inflections, the future time of an action is conveyed in English by a set of modal auxiliaries such as will, shall, may, must, and can. Research by Ames (1946) and Harner (1981) found that children first spontaneously produced words such as gonna and in a minute to denote future at age 2; later they used is gonna/is going to predominately when referring an action that was just about to happen.

Children's elicited language production indicates that they are able to use past tense and future verb forms quite accurately by age 3. For example, Harner (1981) demonstrated actions within a short timescale (e.g., a doll went down a slide) to 3- to 7-year-olds and asked them Tell me about this one while pointing to either the toy that had completed the action or to an identical toy that always did the same thing; children were asked to either describe what the first toy had done or what the other toy was going to do. Three-year-olds were able to distinguish past and future actions, and the majority of their responses contained past tense (70%) and future verb forms (87%). Other researchers elicited children's temporal language in describing events over a relatively longer timescale by simply asking questions such as, What are you going to do tomorrow? and What did you do yesterday? Threeyear-olds were able to answer the tomorrow question with the appropriate verb form, gonna; 4-year-olds were able to answer the yesterday question using a past tense verb (Ames, 1946; Busby and Suddendorf, 2005).

Language production data indicates that children also begin to use temporal adverbials between 2 and 3 years of age (Ames, 1946; Weist, 1989; Pawlak et al., 2006). Ames (1946) observed 1.5- to 4-year-olds' spontaneous language production and found that references to the present (today) emerged around 24 months, references to the future (tomorrow) appeared around 30 months, and references to the past (yesterday) appeared around 36 months. Similarly, a longitudinal study (Pawlak et al., 2006) found that children produced today and tomorrow earlier than yesterday. However, although young children are able to produce temporal adverbs in the appropriate sentence position, their actual temporal references may be inaccurate (Bloom, 1970; Busby Grant and Suddendorf, 2011). For example, parents evaluated their 3- to 5-year-olds' use of temporal terms such as yesterday and tomorrow as less appropriate than their use of more general terms such as now, soon, and later (Busby Grant and Suddendorf, 2011).

These findings suggest that although children are able to produce temporal terms at 2 and 3 years, their usage may not always be appropriate. Nelson (1991) proposed that very young children have a basic grasp of time and temporal language, but their understanding is still limited as compared to older children. This interpretation raises the question of what temporal components children understand when they first use temporal language. This question has not been fully addressed, but one approach has been to examine children's comprehension of temporal language independent of their production of temporal language.

Temporal language comprehension studies have shown that 2- and 3-year-olds understand how tense is used to denote the past and future, but not how more precisely temporal adverbs locate events in time. For example, Herriot (1969) found that 3-year-olds used inflections and modal auxiliaries to correctly identify past and future actions, even with novel verbs. He presented completed and not-yet-begun actions to children using movable toys (one at the starting point of an action, and the other at the ending point). A novel verb (e.g., gling) was used to describe the action and children were asked, Which one is going to gling? and Which one has glinged? With even younger children, Valian (2006) demonstrated familiar actions such as tying shoes (e.g., one tied and the other about to be tied) and asked children either, Show me the one I did tie or, Show me the one I will tie. Two-year-olds successfully distinguished the auxiliaries will and did for future and past actions. Adding temporal adverbials to questions, such as before or already for the past and in a second or next for the future improved 3-year-olds' performance, but not 2-year-olds' (Wagner, 2001; Valian, 2006).

Understanding how temporal adverbs are used to represent, localize, and organize events in time is more difficult than understanding how verbs denote completed or future events. Weist et al. (1991) compared children's understanding of sentences referring to past and future ET using only tense (e.g., The girl threw/will throw the snow ball) to their understanding of sentences referring to the RTr framework using both tense and temporal adverbs (e.g., The girl will dance tomorrow/in a while). They found that children could parse the temporal relation coded in the ET system (using tense) at 2.5 years, much earlier than they could parse temporal relations coded in the RTr system (using tense and adverbs), which was not achieved until 5.5 years.

What makes sentences with both tense and temporal adverbs more difficult? One possibility is that younger children simply do not understand temporal adverbs and find sentences containing temporal adverbs to be confusing. To test this possibility, researchers examined children's understanding of common temporal adverbs, such as yesterday, today, and tomorrow. In contrast to results from language production research, results from comprehension studies suggest that children's grasp of yesterday (referring to the past) appears before that of tomorrow (referring to the future). Harner (1975) assigned toys to 2- to 4-year-olds to play with on successive days (i.e., yesterday, the testing day, and tomorrow) and asked children to show a toy from yesterday and a toy for tomorrow. She found that 2-yearolds barely understood either yesterday or tomorrow, 3-yearolds performed better on yesterday questions, and 4-year-olds understood both yesterday and tomorrow. Zhang and Hudson (2018) explicitly tested children's understanding of the relational underpinnings of yesterday and tomorrow. They presented 3- to 5-year-olds pairs of pictures of objects with visible changes of state (e.g., a carved pumpkin and an intact pumpkin) and sentences referring to an action about the target object (e.g., I carved the pumpkin yesterday or I'm gonna carve the pumpkin tomorrow). Children were asked temporal questions such as What does it look like now? Compared to Harner's task, this task not only requires children's understanding of yesterday and tomorrow as distinct categories but also their understanding of the underlying temporal relations between the past and the present and between the future and the present. Similar to other comprehension studies, they found that children answered questions about yesterday more accurately than they did for questions about tomorrow.

Thus, there seems to be a lag between children's production of temporal language and their comprehension of relational temporal language. Language production studies showed that children begin to use tense around 2 years old and begin to use temporal adverbs around 3 years old. Language comprehension studies showed that children were able to parse temporal relations from tense around 2 to 3 years, but they could not understand the temporal relations coded by temporal adverbs even at age 4 or 5. Such production-comprehension asymmetries have been observed in many studies of children's language, with comprehension typically found in advance of production (Clark, 1995). Delays in comprehension compared to production are found in children's mastery of pronouns, scalar implicatures, aspect, deictic references, and other linguistic forms (Hendriks, 2014). For example, research showed that children produced pronouns (him, her) from age 2 or 3, but the adult-like comprehension of pronouns was usually not found before age 6 (Sekerina et al., 2004).

Explanations for discrepancies between language comprehension and production come from four perspectives. First, the grammatical account claims that children's immature use of syntactic direction-sensitive constraints causes delays in comprehension. Production begins from the input of meaning to the output of optimal form, whereas comprehension begins

from the input of form to the output of optimal meaning. Comprehension lags production because young children cannot compute the speaker's alternative perspective. They have to acquire a Theory of Mind or develop greater processing capacity to be able to compute constraints from both the hearer's and the speaker's perspectives (van Hout, 2007). Second, the interface account emphasizes the cognitive resources needed for processing and integrating linguistic knowledge, discourse, and situation information (Hendriks and Koster, 2010; Hendriks, 2014). Working memory and cognitive control are required to keep multiple interpretations in mind during comprehension (Reinhart, 2004). Third, the pragmatic account proposes that children lack pragmatic knowledge (e.g., knowledge of implicature, reference, deixis, discourse structure, etc.); they may not yet be aware of the subtleties involved in using certain words or grammar, which makes comprehension more difficult. Fourth, the delay of comprehension may be due to the testing context (Grimm et al., 2011). Most comprehension tasks take place in non-naturalistic and highly controlled situations and test sentences are often presented with minimal context. Children lack contextual knowledge of the testing situation that would support their comprehension of the presented sentences (Papafragou and Musolino, 2003).

With regard to comprehension and production of temporal language, this asymmetry may occur because the understanding of relations conveyed by temporal language not only requires a basic understanding of the meaning of temporal markers in language, but also the ability to mentally represent temporal relations between events, which may develop later. For example, a basic grasp of temporal markers may only involve a discrimination of them (e.g., yesterday refers to different things from that of tomorrow) and a sense of their sentential distribution, which seems enough for young children to produce utterances with temporal markers. Bloom (1970) case study showed that a 30-month-old was able to produce sentences with temporal terms such as today, next Monday, and last night in appropriate positions, but the actual temporal references of these adverbs were inaccurate except for the term now. A mature understanding of temporal markers involves a differentiation of the past and the future in terms of sequence, causal relations, distance to the present, and so on. These distinctions require more cognitive processes in order to represent events, to mentally manipulate event representations, and to map representations and relations to linguistic expressions.

### Children's Temporal Representation of Events

Conceptualizing time and mapping language onto temporal constructs involve a number of cognitive processes, including event representation, memory, and reasoning. Children's implicit and explicit understanding of time derives from their representation of events, including the expected sequence of components within particular events, the representations of sequences of multiple events, and eventually, the localization of events in time (Nelson, 1991). McCormack and Hoerl (1999, 2001) distinguished two frameworks, perspectival and non-perspectival, that can be used for representing the temporal location of events. Temporal representations within a perspectival framework locate entities/events relative to one's own position/point of view, for example, describing an event as days from present. In contrast, representations within a non-perspectival frameworks locate entities/events independent of one's position/point of view, for example, describing an event on a given calendar date. Concepts of the past, present, and future are included in the perspectival framework because the past and future are defined from the vantage point of the present. With a stable but ever changing present, our temporal perspective is dynamic and the contents of past, present, and future keep shifting (Harner, 1982). In this review, we focus on children's acquisition of the perspective framework of time, specifically their acquisition of tense and temporal adverbials as discussed in previous section, and their ability to differentiate the past, the present, and the future as discussed in this section.

Friedman and colleagues (see Friedman, 2003, 2005 for reviews) conducted many studies investigating children's representation of a temporal framework ordered with distinct categories for past, present, and future events. In most of these studies, children were asked to judge the past–future status and temporal distances of events in verbal and spatial (timeline) tasks. Friedman consistently found that children often confused the past–future status; they judged impending events as being a short time ago and recent past events as belonging to the near future (Friedman et al., 1995; Friedman and Kemp, 1998). For example, most 4-year-olds responded "yes" to the question, "Is Halloween coming soon?" in the weeks after the holiday (Friedman, 2000). Friedman argued that this confusion comes from a distance-based process of temporal differentiation, in which distance to the present is a salient cue for children to locate and differentiate the time of events.

For more familiar daily events (such as waking up in the morning, eating breakfast, lunch, dinner, going to bed), Friedman (2002) found similar past–future confusion. When tested after breakfast and before lunch, about 75% of 4-year-olds judged that lunch would occur in the future, but only 50% of them correctly judged that breakfast had occurred in the past. Similar confusions in 3-year-olds' temporal judgments were reported by Tillman et al. (2017) in a study about representions of familiar events using a timeline. This limited ability in discriminating past– future status does not only appear for events with cyclic patterns. Friedman (2003) found that 4-year-olds failed in judging the temporal locations of autobiographical events provided by their parents.

Busby Grant and Suddendorf (2009) tested children's temporal differentiation by using a past timeline and a future timeline separately. Three silhouettes of a person were placed at the appropriate points along the timeline to indicate the passage of time. Children were told that a larger silhouette indicated further

in the future (e.g., "this is a picture of a bigger person, like when you are going to be a bit bigger than you are now"); a smaller silhouette indicated longer ago in the past (e.g., "this is a picture of a smaller person, like when you were a bit smaller than you are now"). Children were asked to locate daily, annual, or remote events along the timeline. For example, the experimenter showed children a picture of toothbrush and asked "when did you last clean your teeth? Was it a little time ago, a long time ago, or a really long time ago? Point to where you think it should go." Three-year-olds discriminated times of past events but failed to discriminate times of future events. Four-year-olds performed well for past events, and differentiated daily events from more remote future events. Five-year-olds differentiated both past and future events across all temporal distances. Hudson and Mayhew (2011) also found that after age 5, children were equally accurate in locating past and future events on a timeline. They showed children pictures of events, either depicting someone else (e.g., "This girl is going to the dentist tomorrow") or themselves (e.g., "When did you go to Sari's birthday party?"), and asked them to place the picture on a timeline made of rectangles representing days. Similarly, they found regardless of the effects of temporal distance, a differentiated sense of the past seemed to emerge earlier than a differentiated sense of future.

The findings from this line of research suggest that children's ability to distinguish between past and future events is not as firm as would be expected from studies of temporal language comprehension and production. One explanation for this discrepancy between findings from language-based and timeline-based studies of children's temporal understanding is that in temporal judgment studies using spatial representations of time, the distance of events to the present is very salient. Young children may focus on the distance of an event to the current time point without considering whether events have already happened or have yet to happen. Another issue with the timeline methodology is that the direction of past and future and the scale of distance vary considerably across spatial tasks. For example, Friedman and colleagues (see Friedman, 2003, 2005 for reviews) used tasks in which time was represented as a road stretching ahead in front of the viewer; whereas other researchers (e.g., Busby Grant and Suddendorf, 2009; Hudson and Mayhew, 2011; Tillman et al., 2017) used horizontal time lines where time flowed from left to right. The variations in spatial representation of temporal direction and the saliency of temporal proximity to the present that are entailed by timeline-based measures are not an issue in language-based measures. This may contribute to the discrepant results from these two methods.

Judging and locating events on a timeline measures children's sequential representation of events which is the cognitive foundation for other types of temporal reasoning, such as sorting out the relations between events in the past, present, and the future. Research has shown that children understand basic sequential relations by age 3. For example, Carni and French (1984) told children stories about familiar events with pictures of events in the story and asked them what happened before or after a specific action. They found that 3-year-olds reliably distinguished between sequential relations of before and after given this highly supportive context.2 Similarly, Fivush and Mandler (1985) presented children pictures of familiar events such as going to the supermarket, and unfamiliar events such as going to parachute jumping. After a careful view of all the pictures, children were asked to put randomly ordered pictures in sequence. They found that 4-year-olds were able to reconstruct the temporal sequences of many familiar events. In general, forward temporal reasoning is easier than backward temporal reasoning for children (Tillman et al., 2015; Zhang and Hudson, 2018). Familiar events in forward order are the easiest to sequence, followed by unfamiliar events in forward order, familiar events in backward order, and finally, unfamiliar events in backward order (Fivush and Mandler, 1985).

Moreover, the temporal organization of an event is also a function of how well the mental representation of the event is encoded (Mandler, 1986). For events with a clear goal, outcome, and internal relationships, event representations are easier to be established and the temporal sequences of event components are encoded automatically during initial construction. Causation is one internal relationship that connects events or event components. Physical causes precede effects; therefore causation inherently contains temporal sequence. Using an elicited imitation paradigm, Bauer and Shore (1987) and Bauer and Mandler (1989) showed that children as young as two recalled events with causal relations better than those lacking causal relations, and when causal relations were interrupted, children were still able to organize their recall around causal relations.3

A sense of the past and future not only involves judging events as belonging to the past or the future, but also an understanding of the conceptual relations between the past and future. For example, a past event, but not a future event, could physically affect the present state of affairs. The past, but not the future, can be known; the future, but not the past, can be altered. Although children's ability to reason about temporal and causal relations develops with age, 3-year-olds already understand that physical causes precede their effects (Gelman et al., 1980). The inherent sequence within causation contributes to children's understanding of conceptual relations between the past and present. Povinelli et al. (1999) presented children with videos and verbal descriptions of two past events in which they just participated such as hiding a puppet. Children as young as 4 years were able to find the puppet in its current location, indicating

2Although evidence of young children's understanding of before and after comes from this investigation of preschool children's performance in a script-based task and from observations of preschool children's spontaneous production of relational terms, such as before, after, because, so, if, but, or (French and Nelson, 1985), a flexible understanding of before and after, as tested by sentence comprehension tasks using more complex time clause structures (e.g., X before Y vs. before Y, X; Y after X vs. after X, Y), is not evident until age 12 (Pyykkönen and Järvikivi, 2012). More discussion of linguistic factors in the acquisition of the terms before and after can be found in Clark (1971) and Blything et al. (2015).

3Temporal-causal connections are also observed in children's personal narratives and stories, and children use temporal conjunctions (then, next, first, before, and after) to sequence actions within narratives (Hudson and Shapiro, 1991; Berman and Slobin, 2013). Because narrative production and story comprehension also depend on several other types of knowledge such as an understanding of episodic structure (a story schema), content knowledge, and metalinguistic knowledge, this literature is not included in our review.

that they understood that the very recent past events causally determined the present. With age, children's grasp of causal relations between past, present, and future becomes flexible and applicable in different contexts. Busby and Suddendorf (2010) investigated children's temporal reasoning by describing two short vignettes to children: one about a character who acquired an object (e.g., a balloon) or knowledge (e.g., a name) in the past, and the other about another character acquiring that object or knowledge in the future. Children were asked which character currently possessed the object or knew the fact. They found that 5-year-olds were able to distinguish past and future changes in both physical and mental states. Friedman (unpublished, cited in Friedman, 2003) also reported that 6-year-olds could articulate the causal relation between both the past and the present, and between the future and the present. This conceptual understanding of the past and future in 6-year-olds correlated with their judgment of the past–future status for autobiographical events, supporting the idea that causal understanding underlies children's temporal reasoning.

A crucial ingredient of temporal reasoning is the ability to envisage events from multiple temporal points of view, referred as temporal perspective taking (McCormack and Hoerl, 2001). It allows individual to switch back and forth from different vintage points of time, i.e., temporal decentering. In temporal reasoning tasks, children are often presented with events that happened at a given time point and are asked to reason about situations based this information. Temporal decentering is involved because the question and the given information are about different time points. Children must retain the relevant information in memory and mentally travel from Time A that was specified in the provided information and infer its effect or implication for Time B. For example, to determine whether a character, who is about to get a balloon tomorrow, has a balloon now, children need to first decenter from the present and project themselves to tomorrow, when the character is acquiring the balloon, take the perspective of this time point, and recall that the question asked about events that happened before this point, then switch back to the present, and respond to the question. Temporal perspective switching and temporal decentering are the keys to this temporal reasoning process.

Moreover, because temporal reasoning is based on a concept of time as a successive series of causally interdependent states, it plays an important role in many higher order cognitive processes, such as planning and problem solving. McColgan and McCormack (2008) examined 3- to 5-year-olds' temporalcausal reasoning in searching and planning. In their search task, children observed a puppet walking through a miniature zoo, passing different cages and taking a Polaroid picture at the kangaroo's cage. At the end of the visit, the puppet noticed the camera was missing. While viewing the photo of the kangaroo, children were asked where in the zoo the camera might have been lost. In their planning task, the same scenario was used, and children were told that a puppet wanted to visit the zoo and take a picture of the kangaroo. Children were asked to preposition the camera in the zoo so that the puppet could take the desired picture when passing by the kangaroo's cage. To make an appropriate choice, children had to combine knowledge about the temporal order of events with causal evidence (in the search task) or knowledge (in the planning task, the camera is a prerequisite for taking pictures). Four- and 5-year-olds, but not 3-year-olds, succeeded in the search task. Only 5-year-olds performed well on the planning task, whereas 3- and 4-year-olds' performance was at chance. Using a closely matched control task requiring mere updating, Lohse et al. (2015) found younger children succeeded in the control task but not the search task. These findings indicate that temporal-causal reasoning is qualitatively different from simple updating. It seems to emerge at around 4 years of age and continues to develop in children from 5 to 6 years old.

In summary, studies focusing on temporal language indicate that children are able to distinguish past and future at 2–3 years, but studies focusing on temporal cognition show that children at age 4 and 5 years still display past–future confusion; they are not capable of reasoning about the past and future until age 5. This controversy may relate to the different methodologies employed in each line of research, for example, production of tense was taken as an indicator of temporal concepts in psycholinguistic studies whereas differentiation of past and future events and their effects was considered as temporal understanding in cognitive developmental studies. However, more importantly, the controversy draws attention to the mental processes involved in mastering temporal language and making temporal judgments and reasoning, and raises crucial questions such as: Does children's early use of temporal language indicate temporal understanding? How much do temporal judgment and temporal reasoning tasks tell us about children's temporal concepts? With both being closelsy involved in conceptual development, how can we identify the mental processes for temporal language and those for temporal cognition? Furthermore, how can we tease apart linguistic and cognitive processes in temporal reasoning tasks? By addressing these questions, we can begin to disentangle the linguistic and cognitive components in the conceptualization of time. Theoretical issues concerning the role of language in the development of temporal understanding and practical issues concerning how to assess cognitive and linguistic components separately are discussed in turn below.

### IS LANGUAGE NECESSARY FOR THE VERY FORMATION OF TEMPORAL CONCEPTS?

Our concepts of time are abstract; they are primarily communicated via language. The relationship between language and concept formation or cognition in general has been discussed by many theorists, including Chomsky, Piaget, Whorf, and Vygotsky. Piaget and Vygotsky focused on the effect of language development on changes in thought. They both assumed that thought and language are distinct representational systems. Piaget (1968) held a cognitive determinism view. He claimed that children's grasp of word meanings changes with development and reflects underlying changes in thought. Language is necessary but not sufficient for the construction of logical operations. Both language and logical operations depend on non-linguistic intelligence. The intellectual unfolding of

children's mind sets the pace for their language development. Vygotsky (1962) emphasized the interaction between language and thought.  Vygotsky also emphasized the impact of social interaction and cultural symbol systems on language and cognitive development. Taking a Vygotskian perspective, Nelson (1991) argued for mutual influences between language, world knowledge, and the sociocultural context. She considered language and cognition as interactive systems with cognitive development inseparable from language. The interdependency between cognition and language is especially salient in children's acquisition of temporal concepts.

What role, then, does language play in constructing temporal concepts? Is language necessary for the very formation of these concepts and not merely for their expression? Do pre-linguistic children have some basic temporal understanding? Although researchers (O'Connell and Gerard, 1985; Bauer and Mandler, 1992) have found evidence of sequential understanding in 11 month-old infants using an elicited imitation paradigm, nonlinguistic concepts of past and future are very difficult to assess. Nelson (1989) proposed four logical possibilities with respect to the relation between the linguistic expression of time and the mastery of time concepts: (1) Concepts of past, present, and future are innate and will be expressed in language when language development has reached a particular level; (2) Concepts of past, present, and future are an inherent part of the human conceptual system, but this system matures independent of linguistic development; (3) Concepts of past, present, and future are constructed. Temporal language may facilitate the construction of the temporal systems by flagging potential distinctions, but the concepts are not wholly dependent upon linguistic expression; (4) Concepts of past, present, and future are dependent upon language expression for their construction.

Nelson (1989) longitudinal study of a 2-year-old child's (named Emily) pre-sleep monologs provides data to support the view that temporal concepts are constructed in response to linguistic coding (possibility 3). Linguistic coding of temporal concepts and relations emerged relatively late in Emily's speech, but correlated with the development of many related notions such as far and near, past, future, general event knowledge, frequency, contingence, and possibility. Further, many temporal adverbs, prepositions, and conjunctions appeared simultaneously in Emily's speech, which, according to Nelson, helped build a system of mutually defining temporal and causal relations and guide the acquisition of temporal concepts. These findings suggest that temporal language facilitates the construction of the temporal systems. Moreover, compared to relative concepts of time, such as temporal perspectives (past, present, future), temporal sequence, duration, and speed of events, arbitrary concepts tied to conventional time systems, such as seasons, months, days of a week, hours, require direct teaching by the language community (Nelson, 1991). In other words, children need explicit discussion and teaching from adults to acquire meanings of such lexical terms. For example, Tillman and Barner (2015) found that preschoolers had little to no knowledge of the absolute durations encoded by duration words (e.g., second, minute, hour, day, etc.). This knowledge is learned when they acquired the formal definitions for the words.

However, many commonly used temporal terms, such as morning, afternoon, night, yesterday, tomorrow, etc., are not directly taught to children. How do children learn these? Everyday communication between parents and children often contains a variety of temporal terms, for example, Tomorrow we're going on a trip, Remember last week we were at grandma's house, etc. These temporal terms (e.g., tomorrow, last week, etc.) refer to pseudo-objects whose meanings are not clear to children initially. They may serve initially as placeholders, which contain little meaning content, but have strong associations with specific contexts. These contexts are situations in which the terms have been used by parents. Children hold basic representations for the placeholders, for example, a rough idea about the domain referred by temporal terms and the distribution of temporal terms in a sentence. At this point, children acquire the forms of words from the discourse context but with little conceptual underpinnings. Their early use of temporal terms is limited to the associated contexts and oftentimes inaccurate. For example, they may produce sentences with temporal adverbs (e.g., yesterday, tomorrow, etc.) in appropriate sentence positions but refer inaccurately to time points (Bloom, 1970). In other words, the reference of their temporal linguistic expressions does not match the actual event time that they intend to express. Nelson (1991) called this "use before meaning;" it is consistent with Vygotsky's account of language acquisition in which "grammar precedes logic" (Vygotsky, 1962, p. 127). Thus, early use of temporal words is not necessarily evidence of early temporal understanding.

Parents' feedback and children's own experience of events allow them to update and refine the meanings of the linguistic forms. As contexts entailing temporal language accumulate and diversify, children's grasp of temporal terms gradually becomes decontextualized. They can now generalize the terms to novel situations. During this process, temporal language facilitates the construction of the conceptual temporal systems by introducing new ideas and flagging potential distinctions, such as using the term yesterday to refer to any not-now event. At the same time, children's level of cognitive ability also affects how much children benefit from hearing and using temporal language (Sachs, 1983). For example, Nelson (1977) observed a 3-year-old who mistakenly reversed the order of past events by describing the recent event first and the second recent event next, so on. This narrative pattern indicates the cognitive difficulty of decentering oneself to a non-present point and following the temporal sequence from there. Children's cognitive readiness for flexibly switching temporal perspectives, and for coordinating and manipulating mental representations of events, affects their use and understanding of temporal language in narrative discourse.

How much conceptual understanding of time can be inferred from children's natural language production? Nelson (1989) argued that appropriate production of temporal terms might not indicate a genuine understanding. Children may use the terms meaningfully in a subset of contexts where adults use them, or

simply copy adults' usage in a particular context. For example, in Emily's pre-bed monolog, she used the expression "just a minute" to request her father to rock her in the crib ("Daddy came in just a minute and rocked me," Nelson, 1991, p. 303). This expression was only used in this context at that time and it was the same context that her father used (he usually responded to Emily's request to be rocked with "I will rock you for just a minute," Nelson, 1991, p. 303). Because of the strong association between the use of "just a minute" and the crib-rocking context, Emily's production of the phrase was not underpinned by a genuine comprehension of meanings (either a duration of 60 s or "a little while" in general). Contexts that entail children's active involvement or interest (e.g., Emily desired to be rocked in crib), as well as repetitive interactions associating the context with a small set of temporal terms, seem to incubate the production of those terms. Such production is context dependent; it is an important mid-point in the continuum of concept mastery from "not at all" to "full command".

For these reasons, researchers should be cautious in making conceptual inferences from language production data. For example, whether children's initial use of past tense encodes ordered time relations or aspectual features is under debate. The aspect-before-tense hypothesis claimed that children initially used past tense to mark the completedness of an action, not the time of the action (Bronckart and Sinclair, 1973; Antinucci and Miller, 1976). Therefore, children could not be said to understand the notion of pastness until they used past tense for both continuous, non-goal-oriented actions and completed, goal-oriented actions. However, other researchers provided evidence suggesting that English-speaking children were able to use past tense to refer a variety of past events, not just to goal-oriented ones with completive aspects (Kuczaj, 1977; Di Paolo and Smith, 1978; Sachs, 1979). They also argued that despite children's earliest tendency to use past forms in their own speech to signal a "present completedness of a past action," they might understand references to past events in the speech of others (Harner, 1982, p. 153). Children's production and comprehension of tense should be analyzed in conjunction with consideration of action types (goal- vs. non-goal-oriented) in making inferences about their understanding of the concept of past.

Research directly addressing the role of language in forming concepts of time is very limited, but the influence of language has been addressed for many other aspects of conceptual development. For example, count nouns are considered "invitations" to children to form categories (Waxman and Markow, 1995). They serve as labels for concrete objects (or sets of concrete objects) and help children form theoretical kinds in mind (Gelman and Coley, 1991). Researchers (Waxman, 1991, 2004) believe that language facilitates children in establishing conceptual organizations such as categorical hierarchies. For young children, nouns highlight higher-order category relations (e.g., animal, plant) and adjectival phrases mark specific, lower-order distinctions (e.g., edible mushrooms, poisonous mushrooms). A majority of the word-learning literature focuses on the mapping process between a conceptual category and its linguistic label. Several conceptual bases or initial constraints, such as the whole-object, taxonomic, and mutual exclusivity assumptions, have been shown to be useful in solving the inverse problem of mapping (Markman, 1991). Beyond categorization, language is an important instrument for children to acquire relational concepts. The use of common labels for relational roles (e.g., daddy, mommy, baby), the possession of relational verb (e.g., buy and sell, come and go), relational adjectives (e.g., high and low, more and less), and even names for relations (e.g., same and different) provide representational tools, which make the restricted implicit understanding of relations into a more powerful explicit one (Gentner, 2003; Christie and Gentner, 2014).

Similarly, children begin to produce no and not between 15 and 27 months, but their grasp of the full range of meanings as a logical operator that flips the truth-value of a proposition comes later (Feiman et al., 2017). This lag echoes the one between production and comprehension of temporal terms, and is also evident in children's acquisition of mental state words. Researchers (Nelson, 1996a; de Villiers and de Villiers, 2003, 2014) investigating the connection between mental state words and the development of Theory of Mind (ToM) noticed that children started to use language about mental states, such as verbs of desire, belief, and knowledge at age 3, around the same time they showed their ability in monitoring others' mental states (Bartsch and Wellman, 1995). Although children's use of mental state terms may not be interpreted as having the same meanings that adults attached to them, having labels for abstract mental states and being able to talk about minds make their representations of mental states more portable.

We can draw three important parallels between children's acquisition of negation terms, mental state terms, and temporal terms: (1) these terms do not refer to concrete objects; (2) children usually produce these words before they fully understand them; and (3) children's understanding is affected by context and pragmatic factors. For example, negative sentences are only hard for children to process when they are pragmatically infelicitous (Nordmeyer and Frank, 2015; Reuter et al., 2018). de Villiers and de Villiers (2014) suggested that more conversation in rich social context allows the meanings for mental state words to emerge. Nelson (1996a,b) also emphasized the role of context in acquiring meanings for words referring to abstract entities. Children learn to use abstract words in contexts where others use them. Through using and interpreting words for abstract entities within their representation of familiar situations, children form a preliminary understanding of these words. As contexts and experiences accumulate, children's understanding is refined and becomes connected to other representations in the construction of a conceptual network. At that time, their understanding is stable, decontexted, and conceptual. Nelson provided an insightful perspective on the constructive function of language, but also proposed that concepts are not wholly dependent upon linguistic expression. There must be some prelinguistics representations onto which language can be mapped. The role of language in constructing abstract concepts in general, and the role of language in building temporal concepts in particular, needs to be addressed by more theoretical discussions and empirical investigations.

## HOW TO TEASE APART AND MEASURE COGNITIVE AND LINGUISTIC COMPONENTS?

Although interrelations between language and conceptual development exist in many aspects of conceptual development, the connections are especially important and complicated for the concept of time. As a fundamental dimension of the universe, time is very abstract. Unlike number and space, it is difficult to instantiate with concrete entities. This makes language a crucial symbolic system for conceptual representation. At the same time, time itself is a conceptual tool to measure change and organize experience. Children's temporal understanding develops in parallel with cognitive development and language development; it is also constructed through the interaction between cognitive processes and linguistic capacities. For a better understanding of the developmental trajectory of temporal concepts, it is necessary to tease apart and measure cognitive and linguistic components separately. However, practical challenges and difficulties exist in devising paradigms to assess children's temporal cognition and temporal language separately (McCormack and Hoerl, 2008).

First, tasks that test children's temporal language cannot easily avoid representational or reasoning demands. This issue is illustrated in research testing children's understanding of yesterday and tomorrow (Tillman et al., 2015; Zhang and Hudson, 2018). In Zhang and Hudson (2018) now task, children needed to answer the question What does it look like now? based on sentences referring to an event occurring yesterday or tomorrow. To respond correctly, children had to first decode the temporality indicated by the sentence linguistically, and then parse the temporal relation between the referred event and the present. Children's performances not only reflected their understanding of yesterday and tomorrow, but also demonstrated their temporal reasoning ability. Because forward temporal reasoning is easier than backward temporal reasoning, in their now task, answering the now question given an event occurring yesterday (I carved the pumpkin yesterday. What does it look like now?) was easier than answering the same question given an event occurring tomorrow (I'm gonna carve the pumpkin tomorrow. What does it look like now?). Similar effects were evident in the study by Tillman et al. (2015). They showed 3- to 5-year-olds pictures of increasing events (e.g., a flower growing) and decreasing events (e.g., a snowman melting) and asked them to answer questions about yesterday and tomorrow. For example, for the event of a flower growing, they presented children a picture of flower today and asked them to select one picture from two alternatives to answer the questions, What did the flower look like yesterday or What will the flower look like tomorrow? Children performed better on questions requiring forward temporal reasoning (i.e., from today to tomorrow) than questions requiring backward temporal reasoning (i.e., from today to yesterday). Performances on these two tasks were affected by the reasoning processes required.

It would be very difficult to completely eliminate reasoning or memory in tasks aiming to measure language ability, but researchers can be aware of the effects of cognitive demands and try to minimize or test for their effects. For example, familiar settings and props can be used to reduce working memory and representation loads. Tasks can be designed to test for the effects of the cognitive demands required. For instance, in studying children's understanding of yesterday and tomorrow, researchers can test and compare children's comprehensions when the two terms are embedded in forward and backward reasoning settings, respectively.

Second, because temporal systems are abstract, we have to rely on language to express them, which means that it is difficult for researchers to only measure the cognitive components of temporal understanding. Many temporal reasoning and representation tasks rely heavily on children's language comprehension. For example, Busby and Suddendorf (2010) investigated children's ability to infer current physical and mental states based on past and future events. Children were told stories, each describing two characters. In the possession stories, one character had acquired an object in the past, and the other was going to acquire it in the future. In the knowledge stories, one character had already acquired the knowledge and the other was going to acquire it. Children were asked "which character has [the object]/knows [the knowledge] right now?" The stories were language heavy; each contained more than eight sentences, which required good language comprehension to understand, as wells as good memory skills to keep all of the relevant information in mind. More importantly, understanding of temporal expressions in the story (e.g., "Yesterday, Emma went shopping. When she went shopping she bought a new toothbrush" vs. "Tomorrow, Mindy is going shopping. When she is shopping she is going to buy a new toothbrush") is the key for success in this task. If children simply do not know the meaning of temporal adverbs included (yesterday, tomorrow) and fail to parse or make use of information in past tense and future verb form, they would likely perform poorly. Therefore, their poor performance in this task could be due to the incorrect understanding of temporal expressions rather than to their inability to perform temporal reasoning. Results from this study showed that 4-year-olds' performance was close to chance level. In a follow-up study, the authors simplified the stories by removing the temporal adverbs and adding auxiliaries did and will (e.g., "Emma went to the beach. She did take some shells home from the beach" vs. "Mindy is going to the beach. She will take some shells home from the beach") and found that 4-year-olds' performance significantly improved (above chance). This indicates that the way information is presented in language and children's comprehension of linguistic information directly affect their temporal reasoning performance.

When studying temporal reasoning and representation, the use of language oftentimes cannot be avoided. In order to minimize language demands, future research focusing on temporal reasoning or judgments can make better use of pictures, props, and live or video demonstrations. For example, visible changes of objects over time (e.g., agents moving, plants growing) can be illustrated by using pictures or demonstrations together with linguistic descriptions. The contextual and visual accommodation may provide children alternatives to figure out the cognitive components asked by the tasks and reduce the

demands for language as well as for memory. Researchers can also differentiate the events or scenarios used to study temporal concepts and relations in terms of familiarity. Familiar events can be used to detect the emergence of temporal reasoning and judgment abilities. Attention can be paid to whether children solve temporal reasoning or judgment problems based on their temporal cognitive skills or their memory of scripts for familiar events. In this case, memory factors can be measured and partialled out in data analyses. Unfamiliar or novel events can be used to test the proficiency of temporal cognitive skills. If children can apply the skills they use for familiar events to novel events, it shows that they have developed temporal cognitive skills that are generalizable and transferable.

Third, several cognitive processes of different complexity are often required when testing children's temporal cognition due to varying tasks employed. Research on children's temporal judgments has largely investigated three aspects: judgments about past–future status, judgments about distance of past/future events relative to the present, and placement of events along a timeline. The cognitive processes involved in each of these judgments are quite different. Past–future status is categorical judgment, which may only require a basic differentiation of the past and future. Temporal distance judgments are both categorical and continuous and require more cognitive processes, such as retrieving memory for the exact event time, representing conventional timeframes, and comparing the event time to the present in this mental timeframe. In general, past/future distance judgments are difficult for children; depending on tasks, they may also require cognitive flexibility or inhibitory control. For example, in Friedman (2003, 2005) studies, children were asked which of two cyclical events occurred longer before in the past, Christmas or the child's birthday. The fact that both events happened in the past and will happen in the future makes the task ambiguous. Children might not fully understand what the task is asking for and simply respond based on the distances of events from present. Further, the question itself is not straightforward; in real life, when an annual event is upcoming, it is rare to be asked how long ago the previous one occurred. It is more cognitively adaptive to represent the upcoming occurrence as closer, rather than the previous one as farther away. To come up with the correct response, children had to closely attend to the question, inhibit the more salient representation, and switch to thinking about distances of past events. Given the complexity of the task, Friedman's conclusion that children at age 4 or 5 still do not have a proper understanding of the distinction between the past and future calls for a careful re-examination.

To provide children with a visual representation of time, timeline-based tasks have employed a variety of forms of spatial representation. Researchers have used horizontal lines from left to right (e.g., Busby Grant and Suddendorf, 2009; Tillman et al., 2017), sagittal lines stretching away from the viewer (see Friedman, 2003 for a review), a line made of rectangles indicating time units (Hudson and Mayhew, 2011), and lines with markers indicating direction and scale (Busby Grant and Suddendorf, 2009; Tillman et al., 2017). These variations make it hard to compare children's performances across studies and also raise interesting questions about the spatial representation of time. The limited research (Tillman et al., 2018) on mapping between time and space shows that children are initially flexible with spatial representations of time and most preschoolers do not represent time as a line spontaneously. Their spatial representation of time becomes increasingly automatic and conventionalized in the early school years.

Similarly, research addressing temporal reasoning has used a variety of stimuli and methods. In research focused on sequencing (Nelson and Gruendel, 1981; Fivush and Mandler, 1985; Bauer and Shore, 1987; Bauer and Mandler, 1989), children are shown pictures of an event and are later asked to arrange randomly ordered pictures in the correct temporal order. The extent of children's sequencing ability has been investigated by varying the types of events (e.g., familiar vs. unfamiliar; causal vs. arbitrary) and the manner of sequencing (e.g., forward vs. backward). Event representations, understanding of sequence, and memory are all required for reconstructing event sequences.

Another line of research has focused on children's reasoning ability about temporal causal changes that requires cognitive abilities beyond event representation and sequencing. In this line of research, investigators were investigating children's understanding of time as series of changes, specifically, their understanding of the causal pathway from the past to the present and the non-causal pathway from the future to the present. Friedman (unpublished, cited in Friedman, 2003) explicitly asked children the effect of a past or future event on the present (e.g., "Michelle had a birthday party yesterday. Can she know all the presents she got? Why or why not?"). Busby and Suddendorf (2010) told children stories about characters who did or will get/know something and asked them who had the thing or knew the information now. Tillman et al. (2015) showed children an event unfolding, such as a flower growing or a snowman melting, and asked them to identify what the item looked like in the past (yesterday) or would look like in the future (tomorrow) based their understanding of the event trajectory. The temporal-causal chain was especially important in McColgan and McCormack's (2008) search and planning tasks. Children faced problems in contexts with many parameters and variables (e.g., the goal, the layout, the sequence, the direction, the time point). Temporal reasoning ability was necessary but not sufficient for them to solve the problems. They also needed to properly represent the goal and structure of the problem, be aware of contributing factors, temporally decenter themselves to envision the steps that needed to be taken forward or backward, and integrate steps and situations, either representational or imaginative, to make decisions. These studies differ in the complexity of the task context, and therefore call on different levels of other cognitive skills, such as working memory, cognitive flexibility, inhibition, and causal reasoning, to work together with temporal reasoning skills. This is perhaps one of the reasons that results vary even within temporal reasoning studies. Future research not only needs to disentangle cognitive and linguistic components in temporal understanding, but also needs to investigate elements of each component more systematically. For example, a series of tasks could be designed with increasing complexity, from processing basic temporal information to coordinating temporal and non-temporal factors in making inferences. Careful controls

and contrasts could then be conducted across the series of tasks.

### IMPLICATIONS FOR FUTURE RESEARCH

Psycholinguistic research has contributed much to our understanding of how children acquire temporal markers in language, but it has not fully explained the conceptual changes driven or brought on by language development. Researchers focusing on temporal representation and reasoning oftentimes utilize tasks that depend heavily on other cognitive abilities and knowledge (e.g., memory, cognitive flexibility, knowledge of annual holidays, etc.). The strengths and limitations of these two lines of research implicate several directions for future research.

In general, to better understand development, it would be helpful for researchers to first delineate a mature state of temporal concepts. The nature of time is perplexing; fundamental debates about the nature of time exist in physics (e.g., whether time exists independently of physical spacetime events or it is just a mere relationship of the causal ordering of events, Lobo, 2008) and philosophy (e.g., whether time is a series of events being either the past, present, or future or it is a series of events that one is "earlier than" another, McTaggart, 1908). Although conceptions of time may vary, psychologists interested in the cognitive understanding of time need to specify the key properties of temporal concepts under investigation. McCormack (2015) proposed three key properties of a mature concept of time. First, time is linear and unidirectional. It does not reoccur and cannot be revisited. Second, time is represented as unified, connected by before/after relations. Every time point is systematically related to every other point. Third, adults can think of time independent of events, that is, they can think about time points independent of events that have occurred or will occur. McCormack (2014) also hypothesized important developmental shifts in concepts of time from those grounded in script-like representations of repeated events, to concepts with distinct categories (happened vs. not yet), to a mature concept of event-independent time. This speculative account provides a way of thinking about development and calls for empirical investigation.

To capture emerging temporal concepts, studies focusing on language or cognitive processes need to adopt tasks that minimize cognitive demands for memory, inference, and inhibition. For example, instead of using verbally described vignettes in temporal language comprehension tasks, straightforward demonstrations of scenarios with child-friendly props could reduce cognitive loads and keep children engaged. Valian (2006) tested children's understanding of temporal language by demonstrating and asking them about the familiar action of tying shoes, which effectively minimized memory and representation demands. Other linguistic factors, such as position of temporal words in a sentence and the telicity of verbs presented in task, should be unambiguous and well controlled. Another way to reduce task complexity would be to design tasks within well-known domains and based on events that are familiar to young children. For example, Friedman (1990) showed that children's temporal reasoning was content-dependent; they were able to arrange familiar daily activities backwardly, but could not do the same for novel events, which demanded greater cognitive resources for memory, leaving fewer for inhibition (children had to inhibit their dominated response of reasoning in forward order). Once the initial starting point for temporal conceptualization is clear, researchers can explore the development of more advanced temporal reasoning by gradually increasing task complexity, for example, by including more temporal factors and inferential reasoning.

Third, multiple perspectives and various methods are needed to construct a full picture of conceptual development with respect to time. Previous research on psycholinguistics and cognitive processes has shed light on how children understand and reason about time, but more studies with well-controlled designs are needed to flesh out these two perspectives, and to facilitate conversations between the two. For example, future investigations could pay more attention to the contexts in which temporal terms emerge or new temporal terms/concepts are introduced to young children. Parents and children talk about events in their daily life and teachers and children talk about schedules and plans for activities in school settings. Adults can facilitate children's language and conceptual learning in many ways. Research on parent-child talk about the past (Nelson and Fivush, 2004; Reese and Newcombe, 2007) showed that mothers' elaborative reminiscing enhanced children's autobiographical memory development. Research on parent– child talk about the future (Hudson, 2002, 2006) suggests that maternal time references contribute to children's understanding and use of temporal terms. Future research could embrace more corpus analyses to find out the contextual factors that help children acquire temporal words and concepts of time. For example, in what context, do children start producing different types of temporal words? What social interactional cues and pragmatic cues are effective for early production? How does the quantity and quality of temporal language exposure affect children's temporal language production and temporal understanding?

More research is also needed to compare and integrate findings from investigations of children's production and comprehension of temporal language. This approach is exemplified in research on children's production and comprehension of no and not in which children's comprehension was measured by experimental tasks and children's production was analyzed with respect to the Macarthur-Bates CDI production norms (Feiman et al., 2017). They found that children's comprehension of the truth-functional no lagged behind their normal production of no by about a year, suggesting that the ability to map the concept of negation to the word no is developmentally challenging. Similarly, Sankaran (2011) investigated the influence of verb semantics on Tamil children's acquisition of aspect markers using both a production task and a comprehension task. She found that children understood the imperfective marker before they actively used it, and although children frequently produced the perfective marker, their understanding of the function of the perfective marker

was limited. This approach to comparing and integrating comprehension and production data can also be used to explicate the construction of temporal concepts. Ideally, future research should consider using within-subjects designs to study children's comprehension and production of temporal language so that stronger claims can be made.

Efforts can also be made to design and employ on-line measures, such as preferential looking or eye tracking. Most previous research on temporal cognition and temporal language has adopted off-line measures, such as sentence-picture matching tasks, truth-value judgments, placement/sequencing task, actout tasks, and question-after-story tasks. Online measures may be more sensitive and informative about the parsing/analyzing process. For example, Sekerina et al. (2004) tested children's comprehension of pronouns using both on-line (eye tracking) and off-line (picture-selection) tasks. They found a dissociative pattern of performance across these two tasks. The eye-tracking task revealed a more adult-like competence than indicated by the picture selection task. Similarly, Brandt-Kobele and Höhle (2010) investigated 3- to 4-year-old German children's comprehension of verb inflection as a cue to subject number using a preferential looking paradigm, where children did not have to perform a specific task, but instead their eye gaze was tracked to measure the comprehension of sentences with verb inflections. Using this paradigm, they found clear evidence that 3- to 4-yearolds were able to infer the number of subjects based on the inflectional information. When a similar task with both eyetracking and pointing was conducted, Brandt-Kobele and Höhle (2010) found weaker evidence from children's eye-movement data, and interestingly, no evidence from their pointing reactions. Children's failure in selecting or pointing to the correct picture may be due to general task demands or to different stages of the interpretation process engaged by the on-line and the off-line measures (Trueswell and Gleitman, 2007). Although researchers are still debating whether preferential looking and picture selection tasks tap the same processes and what these crosstask discrepancies can tell us about comprehension-production

### REFERENCES

asymmetry, for under-researched areas such as the development of temporal concepts, data from both on-line and off-line tasks could advance our understanding of the developmental trajectory.

Useful information and insights can also be obtained from the study of the development of related cognitive abilities and processes requiring temporal understanding. For example, an understanding of time is essential for autobiographical memory and future thinking. Remembering one's own past implies an understanding of the past and a differentiation of past time points. Planning one's own future implies an understanding of the future and a differentiation of future time points. Research has investigated the development of autobiographical memory, planning, and future thinking, but little attention has been paid to the extent that children understand the temporal concepts or temporal language presented in investigations. Considering children's performances on autobiographical memory and future thinking tasks from the perspective of temporal understanding is helpful both for research in these areas themselves, but also for the study of temporal concepts, because an awareness of time is required and used for a pragmatic purpose in these tasks. Children may not fully understand the meaning of a temporal term or reason about temporal relations when asked explicitly, but it is possible that they can make use of their limited grasp of time when asked to recollect their past experiences and to imagine their future selves. Future research would benefit not only from disentangling the linguistic factors and cognitive processes in forming temporal concepts, but also from understanding how temporal concepts contribute to the development of other aspects of cognition and language.

### AUTHOR CONTRIBUTIONS

MZ contributed to the conception and the writing of the manuscript. JH contributed to the writing of the manuscript by providing critical and valuable comments.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2018 Zhang and Hudson. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Positive Effect of Visual Cuing in Episodic Memory and Episodic Future Thinking in Adolescents With Autism Spectrum Disorder

Marine Anger1,2† , Prany Wantzen1† , Justine Le Vaillant1,2, Joëlle Malvy3 , Laetitia Bon1,2 , Fabian Guénolé1,2, Edgar Moussaoui2 , Catherine Barthelemy3 , Frédérique Bonnet-Brilhault3 , Francis Eustache1 , Jean-Marc Baleyte1,4 and Bérengère Guillery-Girard1 \*

#### Edited by:

Danielle DeNigris, Fairleigh Dickinson University, United States

#### Reviewed by:

Alain Morin, Mount Royal University, Canada Catherine A. Best, Kutztown University of Pennsylvania, United States

#### \*Correspondence:

Bérengère Guillery-Girard<EMAIL_ADDRESS><EMAIL_ADDRESS>
†Co-first authors

#### Specialty section:

This article was submitted to Cognition, a section of the journal Frontiers in Psychology

> Received: 19 July 2018 Accepted: 17 June 2019 Published: 09 July 2019

#### Citation:

Anger M, Wantzen P, Le Vaillant J, Malvy J, Bon L, Guénolé F, Moussaoui E, Barthelemy C, Bonnet-Brilhault F, Eustache F, Baleyte J-M and Guillery-Girard B (2019) Positive Effect of Visual Cuing in Episodic Memory and Episodic Future Thinking in Adolescents With Autism Spectrum Disorder. Front. Psychol. 10:1513. doi: 10.3389/fpsyg.2019.01513 1 Normandie Université, UNICAEN, PSL Universités Paris, EPHE, INSERM, U1077, CHU de Caen, Neuropsychologie et Imagerie de la Mémoire Humaine, Caen, France, 2 Service de Psychiatrie de l'Enfant et de l'Adolescent, CHU de Caen, Caen, France, 3 UMR 1253, iBrain, Université de Tours, INSERM, Centre Universitaire de Pédopsychiatrie, CHRU de Tours, Tours, France, 4 Service de Psychiatrie de l'Enfant et de l'Adolescent, CHI de Créteil, Créteil, France

Cognitive studies generally report impaired autobiographical memory in individuals with autism spectrum disorder (ASD), but mostly using verbal paradigms. In the present study, we therefore investigated the properties of both past and future autobiographical productions using visual cues in 16 boys with ASD and 16 typically developing (TD) participants aged between 10 and 18 years. We focused on sensory properties, emotional properties, and recollection, probing past and future productions for both near and distant time periods. Results showed that the ASD group performed more poorly than controls on free recall for recent periods, but performed like them when provided with visual cues. In addition, the ASD group reported fewer sensory details than controls and exhibited difficulties in the experience of recollection for the most remote events. These data suggest a combination of consolidation and binding deficits. Finally, our findings reveal the relevance of using visual cues to probe autobiographical memory, with possible perspectives for memory rehabilitation.

#### Keywords: autobiographical memory, episodic memory, visual cues, sensory details, autism

### INTRODUCTION

Autism spectrum disorder (ASD) is a neurodevelopmental disorder, characterized by deficits in social communication, with restricted and repetitive behaviors. There is growing evidence that people with ASD have atypical memory functioning (Bowler et al., 1997), even if their language skills are intact. Difficulties include, among others, impairment of autobiographical memory (AM). AM is a very long-term memory of personal knowledge and events related to individuals' own lives that are accumulated from a very early age. AM allows individuals to build an identity based on a feeling of continuity (Conway, 2005; Bon et al., 2012).

Current cognitive models of AM distinguish between a semantic component pertaining to general personal knowledge or facts, and an episodic component relating to personal events.

**46**

This episodic component relies on the ability to remember past experiences (i.e., episodic autobiographical memories) and to imagine possible future experiences (episodic future thinking) (Tulving, 1985). Both episodic memories and projections involve autonoetic consciousness, namely the ability to project our states of self into the past, present or future to maintain selfcontinuity. This mental time travel allows individuals to reor pre-experience personal events associated with their original context, giving individuals a feeling of (re)living these events. To evoke episodic events, sufficient phenomenological details (i.e., feelings, emotions, sensory details such as colors, sounds, smells, tactile feelings) must be stored in memory, as they serve as retrieval cues. More specifically, episodic future thinking or projection involves imagining oneself in the future to preexperience a possible scenario (Atance and O'Neill, 2005). This projection is supported in part by episodic memory oriented toward the past (Suddendorf and Corballis, 1997; Wheeler et al., 1997). Moreover, remembered personal events and envisioned future plans have been found to share a common brain network (Viard et al., 2011; D'Argembeau, 2015). This network is thought to support common constructive thought processes that allow for the retrieval and flexible combination of stored information to reconstruct past experiences and construct novel future ones. Besides constructive and executive processes, AM involves a broad range of cognitive processes, ranging from perception (Gottfried et al., 2003) to more integrative processes. Some of these are preferentially related to the self (self-concept: Howe and Courage, 1997; theory of mind: Perner and Ruffman, 1995; Welch-Ross, 1997) and social events (Nelson, 1993), while others refer to narrative abilities (Kleinknecht and Beike, 2004). Hence, the maturation of these cognitive processes during childhood and adolescence supports AM development (Nelson and Fivush, 2004; Bauer et al., 2007; Piolino et al., 2007; Picard et al., 2009).

In ASD, both children and adults produce fewer specific memories and projections, characterized by reduced specificity, elaboration and episodic coherence. The content of these memories is also more semantic (e.g., general or repeated event) than episodic (Bon et al., 2012; Crane et al., 2012, 2013; Terrett et al., 2013; Goddard et al., 2014; McDonnell et al., 2017). Ciaramelli et al. (2018) recently reported that providing a series of standardized questions (e.g., "Where did this event take place") does not seem to increase performance, either for past recollection or for future thinking. Similarly, difficulty retrieving specific memories is observed in children and adolescents with ASD, with poorer access to the remote past (8- to 17-year-olds; Goddard et al., 2014), and impaired episodic future thinking (8 to 12-year-olds; Terrett et al., 2013). Children with ASD also have greater difficulty recalling their own activities than typically developing (TD) children (Millward et al., 2000). However, differences may be observed between children and adults with ASD. For example, discourse analysis has shown that children with ASD aged 6–14 years produce fewer past narrative details, as well as fewer emotional (e.g., happy, scared), cognitive (e.g., thought, believed), and sensory (e.g., seen, heard) terms than TD children (Brown et al., 2012). This difference is more pronounced for remote life events than for recent ones for children aged 5–17 years (Bruck et al., 2007; Brown et al., 2012; Goddard et al., 2014) or future thinking (Terrett et al., 2013). On the contrary, results obtained in adults show that sensory references are more frequent in ASD than in TD for self-defining memories (Crane et al., 2010) and early childhood events (Zamoscik et al., 2016). Hence, some sensory details may be more salient than other features and contribute to the structure of AM in adulthood. This heterogeneity highlights the importance of exploring changes between childhood and adulthood, by focusing on the adolescence period.

The impairment of AM in ASD can be interpreted according to different cognitive theories. First, the theory of mind deficit resulting in difficulty recognizing one's own psychological states and understanding of the self (Williams, 2010) may impact the narration of episodic events (Losh and Capps, 2003; Goldman, 2008; McCabe et al., 2013; Kristen et al., 2014). Second, a detail-focused perceptual style, which refers to perception theory, or the weak central coherence evoked by Happé and Frith (2006), may also have a significant impact on the properties of autobiographical memories. Temple Grandin, a woman with high functioning ASD, reported in her 2006 book Thinking in Pictures (Grandin, 2006) that the visual modality is ubiquitous in her daily life:

"I translate both spoken and written word into full-color movies, complete with sound, which run like a VCR tape in my head. . . [I] see the words in pictures . . . I have a video library. . . When I recall something I have learned, I replay the video in my imagination. The videos in my memory are always specific . . . My imagination works like the computer graphics programs. . . When I do an equipment simulation in my imagination or work on an engineering problem, it is like seeing it on a videotape in my mind. I can view it from any angle, placing myself above or below the equipment and rotating it at the same time. . . I create new images all the time by taking many little parts of images I have in the video library in my imagination and piecing them together. . . Unlike those of most people, my thoughts move from video-like, specific images to generalization and concepts. For example, my concept of dogs is inextricably linked to every dog I've ever known. It's as if I have a card catalog of dogs I have seen, complete with pictures, which continually grows as I add more examples to my video library."

She describes her visual memory as a collection of personal photographs of her own life, which has a direct impact on the formation of visual representations of semantic concepts. Moreover, she is able to take different perspectives but, as suggested by her testimony, these tend to be field perspectives with egocentric navigation. This was experimentally corroborated by Ring et al. (2018). Hence, visual autobiographical memories may be very specific and detailed but more fixed than those of TD people.

Third, the AM deficit in ASD may result from difficulty mentally assembling the details that form the experience (e.g., episodic simulation; Schacter et al., 2012) and elaborating the context of this experience (e.g., scene construction; Hassabis and Maguire, 2007). Scene construction relies on visual imagery which involves the mental generation and maintenance of a single element and the binding of all the properties of the event (e.g., objective and subjective details). Poorer scene construction is

consistent with the impaired binding processes observed in ASD (Bowler et al., 2011; Lind et al., 2014a).

Most studies reporting difficulties with AM were conducted using verbal paradigms that elicit narrative abilities (Goddard et al., 2007; Crane and Goddard, 2008; Crane et al., 2009, 2012). Since these narrative abilities are impaired in ASD, solely using language to investigate AM may bias the assessment of memory properties. Most of the studies that have reported an AM impairment in ASD used questionnaires or a fluency task. However, individuals with ASD performed just as well as controls when other methodologies were used. No differences were observed with the use of a sentence completion test that indexes memory retrieval (Crane et al., 2013), or yes– no questions (Bruck et al., 2007), when the recall test was written rather than oral (Crane et al., 2012) or when the cue words were high in imageability (e.g., letter vs. permission) (Crane et al., 2012). All these tasks provide cues or support at retrieval. These observations are in line with the task support hypothesis that emphasizes the role of retrieval support in improving AM productions (Bowler et al., 2004).

Hence, and as suggested by Temple Grandin's testimony, pictures could be a valuable tool for studying AM, by providing a visual aid to overcome the language constraints associated with the free recall paradigm. Therefore, pictures would constitute a more appropriate mean of testing the properties of episodic memories in ASD. In addition, these visual supports would provide an opportunity to test different kinds of properties, including sensory details, and investigate the possible impact on AM of the impairments in sensory processing observed in ASD (Stevenson et al., 2014).

The main aim of the present study was to investigate the properties of episodic memories and future thinking in high-functioning adolescents with ASD using visual cues. We focused on the sensory and emotional properties and the quality of the experience of recollection associated with autobiographical productions for four time periods: two in the past (i.e., yesterday and last summer vacation) and two in the future (i.e., tomorrow and next summer vacation). First, given the known retrieval deficit in ASD and possible difficulties in scene construction, we predicted that free recall performance would be impaired, but performance would normalize when visual cues were provided. We added a general neuropsychological assessment focusing on cognitive functions involved in AM retrieval, i.e., executive functions, short-term memory, and verbal episodic memory, to discuss our results. Based on the cognitive profile of ASD, we expected to find baseline differences in verbal episodic memory, planning and short-term memory. Second, given the perceptual bias reported in ASD (Mottron et al., 2003) and the frequent references to sensory details reported by adults with ASD (Crane et al., 2010), we predicted that participants would exhibit an atypical pattern of performance concerning sensory properties, with a probable focus on some perceptual modalities to the detriment of others. Third, given the well-known difficulty with emotion processing and reduced recollection capacity in ASD (Gaigg, 2012), we expected participants to perform poorly on emotion and recollection assessment.

### MATERIALS AND METHODS

## Participants

Participants were 16 boys aged 10–18 years (mean = 13.4 years, SD = 2.4) (**Table 1**). They were recruited through autism resource centers in Caen and Tours in France. The recruitment started prior to the 2013 publication of DSM5, hence participants had all been diagnosed with verbally and intellectually highfunctioning autism or Asperger's syndrome according to DSM-IV (American Psychiatric Association [APA], 2000) criteria. The diagnosis was established by experienced professionals using the Autism Diagnostic Interview-Revised (ADI-R; Lord et al., 1994) and/or Autism Diagnostic Observation Schedule (ADOS; Lord et al., 1989). The ADI-R is a detailed semi-structured interview of parents about their child's developmental history and autism symptoms that yields ratings for reciprocal social interaction, language and communication, and restricted repetitive behaviors. The ADOS is also a semi-structured interview and is a standardized assessment of social interaction, communication, play and imaginative use of materials. Participants with ASD were compared with 16 TD controls matched for age, sex, and scores on the Perceptual Reasoning Index (PRI) and Verbal Comprehension Index (VCI) of the fourth version of the Wechsler Intelligence Scale for Children (WISC-IV; Wechsler, 2005). These two indices were calculated according to performances on four WISC-IV subtests: Block Design and Matrices for PRI, and Vocabulary and Similarities for VCI. They allowed us to ensure that participants had no general impairment of language comprehension or perceptual abilities. TD adolescents were recruited from several French schools. Brief interviews ensured that none of the participants met the exclusion criteria: history of previous neurological disorders or psychiatric illness (other than ASD in the ASD group), a firstdegree relative with ASD in the TD group, head trauma, current psychoactive medication, intellectual disability, and learning disabilities. Families were given a comprehensive description of the research. The study was approved by the relevant ethic committees, and written consent was obtained from all the participants (and their parents, in the case of minors), in line with committee guidelines.

### General Cognitive Assessment

Each child also underwent a neuropsychological assessment focusing on the cognitive abilities involved in AM production (Picard et al., 2009). This assessment included tests of five executive and memory functions: inhibition (Stroop test; Albaret and Migliore, 1999), planning (Tower of London; Lussier et al., 1998), verbal short-term memory (forward digit span, WISC), visuospatial short-term memory (Forward Corsi blocks; Pagulayan et al., 2006), and verbal episodic memory (story recall from Children's Memory Scale; Cohen, 2001). Picard et al. (2009) found that these cognitive abilities were

#### TABLE 1 | Mean ages and cognitive data for the ASD and TD groups.

∗Significant differences observed between the ASD and TD groups. nd, not done owing to a ceiling effect; PRI, Perceptual Reasoning Index; VCI, Verbal Comprehension Index; ASD, participants with autism spectrum disorder; TD, typically developing participants.

involved in the production of autobiographical memories in childhood (6–11 years).

Finally, all participants underwent a brief investigation of personal semantic knowledge, in order to exclude a possible major deficit that might interfere with the AM task. This consisted of a questionnaire coupled with visual cues about general personal information on three different topics, adapted from Piolino et al. (2007)'s methodology. Questions concerned acquaintances, school life, and personally relevant famous names (e.g., heroes, stars, etc.). The maximum score was 6 for each of these categories.

### From Past to Future Task

This task explored specific past personal events and future thinking for the day before (recent past), last summer vacation (remote past), next day (near future), and forthcoming summer vacation (distant future). For each period, visual cues were provided to support production (**Figure 1**). All responses were directly manually transcribed by the interviewer. The interviewer had a grid for coding each personal event that was reported (free recall and cued recall of personal event). All other responses were directly coded by the participants themselves.

### Visual Cues

Questions were illustrated with drawings that provided a timeline and visual cues for detailing personal events, contents and perceptions (i.e., colors, smells, tactile feelings, sounds, tastes). Contents could refer to temporal situations, spatial locations (e.g., home, school, beach, etc.), modes of transport (e.g., car, plane, train, etc.), activities (e.g., video games, football, musical instrument, etc.) and people present (e.g., parents, children, etc.). All the pictures were drawn by a professional illustrator who ensured that each type of content was included. For example, for the who content, there was a person of every age (i.e., children, adults, and older adults) and gender. In addition, five types of perceptions were illustrated with drawings. For example, colors were associated with a color chart, while smells were indicated with a trash can or a flower; sounds with a musical note or bell; tastes with a lemon or a sweet; and tactile feelings with a finger placed on a pillow (mushy) or on ice (cold) (**Figure 1E**). Each question included explanations of the properties being tested (e.g., "Did you have tactile feelings?  Participants repeated the property when they selected the drawing that supported their autobiographical production (e.g., "I touched something soft. . ."). This procedure was applied to all visual cues.

### Procedure

Each participant was asked to produce descriptions of memories or projections with as many details as possible, focusing on the past (i.e., one event that happened yesterday and one last summer vacation) and the future (i.e., one event that could happen tomorrow and one next summer vacation). These questions allowed us to manipulate orientation (past vs. future) and temporal distance, either close (yesterday or tomorrow) or remote (last or next summer vacation). For past events, participants were instructed to remember real events that had happened to them (e.g., "Can you remember something that happened to you yesterday? I want you to recall it with plenty of details, as if you were reliving this event, and your description has to allow me to imagine this

(D), perceptions (e.g., color; E), perspective (field or observer; F), mental imagery (G), and reliving (H).

event too"). For future events, participants were instructed to imagine an event that could happen in their lives or else was completely invented (e.g., "Can you imagine what you might do tomorrow, either something planned or completely new, but I want you to imagine what could happen with plenty of details, as if you were living this event, and your description has to allow me to imagine this event"). If 1 min went by without an answer, the interviewer gave the children an open-ended prompt (e.g., "What else can you remember?"). If they were still not able to provide different contents associated with an episodic event, after a further minute, they were helped with visual cues for each of these components. Cues concerned activities (what), temporal situation (when), spatial location (where), course of the event (how), and people present (who) (**Figures 1A,B**). Episodic free recall and cued recall (with visual cues) were each scored out of 5, with 1 point per

TABLE 2 | Episodic memory paradigm, variables, and scoring.

∗Coded by two persons: the interviewer and the psychologist. All other measures were directly coded by participants. #Variables for future periods only. No, number.

type of content: what (theme), when (e.g., beginning, middle or end of the month; morning, afternoon or evening), where (which city and where in that city; e.g., home, garden, beach), how (three different details; e.g., perception, feeling, activity, script), and who (participants). Scoring was performed separately by the interviewer and a psychologist until a consensus was reached (**Table 2**).

Next, we asked participants about the properties of each event. Participants rated their own productions. First, we asked them to rate the emotional feeling associated with the event on a 6 point Likert-like scale featuring smiley faces ranging from very sad to very happy (e.g., "I was happy to do this, so I choose the fifth smiley"; **Figure 1C**). They also rated the level of emotional arousal on a triangular ruler, again with a 6-point Likert-like scale along each side (e.g., "I was happy to do this, but not very excited, so I rate it 2 on the scale"; **Figure 1D**). The Likert-scale was used for all the following questions. Participants were then asked to provide sensory details (i.e., colors, sounds, smells, tactile feelings, tastes; **Figure 1E**), and indicate the importance of each one in their memories or future thinking, using the same 6-point triangular ruler (e.g., "Which colors do you remember being associated with your memories? What was the intensity of each one?"). In the final part of the questionnaire, we collected other information. One question concerned the perspective from which they had relived the event: either their own (field perspective, scored 3/3), that of an observer (observer perspective, scored 1/3), or alternating between the two (scored 2/3) (**Figure 1F**). Another question assessed the mental imagery associated with the personal event, asking participants whether they could visualize the personal event in terms of the number of images (e.g., "When you think about this event? How do you see it? Please rate it on a scale from 0 (No image) to 6 (Lot of distinctive images)"; **Figure 1G**) and accuracy (e.g., "Can you evaluate the accuracy or distinctiveness of these images on a scale from 0 (Completely blurry) to 6 (Very precise)?"; **Figure 1G**). We also asked about the sense of subjective recollection (i.e., feeling of reliving): "When you think about this event do you feel that you are reliving it with all the sensations you had at the time? Are you able to provide many details? And is it so realistic that you feel you are reliving the scene?" We used a film/video metaphor to highlight the nature of recollection: "When you think about this event, imagine that you have rewound the film and are reliving this event as a déjà-vu scene. How do you feel about reliving it with all the sensations you had at the time? Can you rate your feeling of experiencing it on a scale from 0 (No feeling of reliving) to 6 (Very intense feeling)?" (**Figure 1H**). Finally, we asked participants about the memory's personal relevance (e.g., "Was this event important to you? Please indicate your answer on a scale of 0 (Not at all) to 6 (Very important)"), its frequency of evocation (e.g., "How often do you remember or mention this event on a scale of 0 (Not at all) to 6 (Very often)") for past and future events. For future events only, we asked whether they wished them to happen (e.g., "Would you like this event to happen? Please indicate your answer on a scale of 0 (Not at all) to 6 (Very much)"), and the probability of occurrence (e.g., "Please rate the likelihood of this event happening on a scale of 0 (Not at all) to 6 (Certainly)") (**Table 2**). To ensure that the adolescents made appropriate use of the criteria, we asked them to reformulate the instructions. This procedure was adapted to each participant and repeated until the experimenter was confident that the child understood the judgment criteria.

### Statistical Analyses

Statistical analyses were performed using Statistica Version 10 software (StatSoft, Tulsa, OK, United States). The reported values are means and standard deviations.

Due to the limited number of participants and some nonnormally distributed variables (K-S test p < 0.05 in one or both groups), we conducted non-parametric analyses (Friedman ANOVAs and Wilcoxon for within comparisons and Mann-Whitney for between comparisons with Z adjusted).

### RESULTS

### General Cognitive Assessment

As expected, Mann–Whitney U-test revealed that the ASD group performed more poorly than the TD group on verbal episodic memory (Immediate recall z = 2.13; p = 0.03, η 2 = 0.14; Recognition z = 2.46; p = 0.01, η 2 = 0.18), and planning (Tower of London, success at first attempt z = 2.11; p = 0.03, η 2 = 0.14),

but none of the other comparisons including working memory, yielded significant differences (**Table 1**).

Semantic performance plateaued in both groups (**Table 1**) confirming the absence of a major deficit in personal semantic knowledge in ASD.

### Personal Event

Mann–Whitney U-tests on free recall performance revealed significant differences for two periods: recent past (z = 2.93, p = 0.004, η 2 = 0.25), near future (z = 2.41, p = 0.01, η 2 = 0.18) and a marginally significant effect for the distant future (z = 1.95, p = 0.056, η 2 = 0.11). The ASD group produced fewer event memories and projections than the TD group (see **Figure 2A**).

Mann–Whitney U-tests on cued recall performance did not show any differences. However, Friedman ANOVA revealed a significant period effect on performance in the control group (χ 2 = 13.1, p = 0.004, η 2 = 0.84). The control group reported less details for the distant future period compared to the recent past (p = 0.03) and near future periods (p = 0.03) (see **Figure 2B**).

### Emotional Feeling

The analyses of emotion (i.e., valence and arousal) revealed no significant differences between groups (**Table 3**). However, Friedman ANOVA revealed a significant period effect on arousal in the TD group (χ 2 = 13.13, p = 0.004, η 2 = 0.84). The arousal associated to memories for the recent past was lower compared to the remote past (p = 0.02) and distant future periods (p = 0.008). Friedman ANOVA analyses conducted in the ASD group showed a period effect for valence (χ 2 = 7.72, p = 0.05, η 2 = 0.39). Memories associated with the remote past had a more positive valence than the recent past (p = 0.01).

### Sensory Perceptual Details

Analyses on the total number of sensory details showed a significant reduction in the ASD group for the remote past (z = 2.74, p = 0.006, η 2 = 0.23). Analyses of each perceptual modality revealed significant differences between the ASD and control group on color for recent past (number z = 2.48, p = 0.01, η 2 = 0.19 and intensity z = 2.19, p = 0.03, η 2 = 0.15) and for remote past (number z = 2.78, p = 0.005, η 2 = 0.24). We also observed differences on smell for remote past period (number z = 2.61, p = 0.01, η 2 = 0.19 and intensity z = 2.00, p = 0.05, η 2 = 0.12), on sound (intensity for remote past z = 2.21, p = 0.03, η 2 = 0.15 and distant future z = −2.05, p = 0.04, η 2 = 0.13), and tactile feeling for remote past (number z = 2.12, p = 0.04, η 2 = 0.13). Except for sounds for the distant future, the ASD group produced fewer information associated with less intensity than the TD group for all modalities and periods cited above (**Table 3**).

Friedman ANOVA analyses were conducted within each group on each category of sensory perceptual details. First and concerning the TD group, analyses showed a period effect on both the number and intensity of smell (respectively, χ 2 = 12.05, p = 0.007, η 2 = 0.75 and χ 2 = 8.28, p = 0.04, η 2 = 0.44): both scores associated with the near future were reduced compared to the remote past (number p = 0.02, intensity p = 0.01) and distant future (number p = 0.005, intensity p = 0.02). Second and concerning the ASD group, analyses showed a period effect on the intensity of colors (χ 2 = 10.03, p = 0.02, η 2 = 0.58): the intensity of colors associated with the recent past was reduced compared to the remote past (p = 0.01). We also observed in this group a period effect on the intensity of sounds (χ 2 = 10.74, p = 0.01, η 2 = 0.64): sound intensity associated with the remote past was reduced compared to the distant future (p = 0.02).

### Recollection and Other Properties

Mann–Whitney comparisons revealed no significant difference for the measures of perspective, personal relevance, wish for it to happen, or probability of occurrence (**Table 4**). However, the ASD group had lower scores than the TD group on several measures associated to the remote past period: mental imagery (number, z = 2.17; p = 0.03, η 2 = 0.14), subjective recollection (z = 1.98, p = 0.05, η 2 = 0.12), and frequency of evocation (z = 2.3, p = 0.02, η 2 = 0.16). Friedman ANOVA analyses conducted in the TD group showed a period effect on mental imagery (number: χ 2 = 8.01, p = 0.05, η 2 = 0.39 and accuracy: χ 2 = 12.24, p = 0.007, η 2 = 0.39). Number of mental imagery associated with recent past was more important than near (p = 0.05) and distant (p = 0.008) future periods. Accuracy of mental imagery associated with recent past was better than for the remote past (p = 0.008) and distant future periods (p = 0.008) and accuracy of mental

TABLE 3 | Mean (SD) emotional feeling and sensory details for each group and each period. Number of details and importance are reported.

∗Significant differences were observed between the ASD and TD groups (in bold), p < 0.05. # Importance refers to arousal for emotions and intensity for sensory details. ASD, participants with autism spectrum disorder; TD, typically developing participants; ND, mean of number of details (SD); I, mean of importance (SD).

TABLE 4 | Mean (SD) properties of personal events according to group.

∗Significant differences were observed between the ASD and TD groups (in bold), p < 0.05. #For future events only. ASD, participants with autism spectrum disorder; TD, typically developing participants.

imagery associated with near future was better than distant future period (p = 0.05).

### DISCUSSION

The aim of this study was to analyze the properties of past memories and future thinking produced by adolescents with ASD, compared with their TD peers, using a visual cues paradigm. As hypothesized, results revealed difficulty with free recall in the ASD group that contrasted with typical performance on the visually cued task. We found differences between the groups on the total number of sensory details provided only for the remote past period. These differences also appeared when we considered each perceptual modality separately, with the ASD group reporting fewer color, smell, sound, and tactile feeling details and intensity than the TD group. Finally, we did not observe any impairment on the measures of emotion and quality of the experience of recollection, except for number of mental imagery, subjective recollection and frequency of evocation for the remote past.

### Visual Cues in Autobiographical Memory Tasks

Our results showed a significant benefit from visual cues in the production of both past and future episodic autobiographical events. This enhanced performance is in line with the task support hypothesis developed by Bowler et al. (1997), which postulates that performance is better when support is provided at retrieval. Hence, visual cues may be more effective for learning/retrieval, as demonstrated by previous studies that used pictorial prompts for teaching children with ASD (McClannahan and Krantz, 1997; Quill, 1997). AM may be used as a support for social interaction in a social skill program and, for example, ASD participants may use visual cues to share their personal memories.

The impaired performances of participants with ASD on the free recall task were in accordance with their story recall

performances (i.e., on the verbal episodic memory test), and mirror previous findings in individuals with ASD (Lind and Bowler, 2010; Brown et al., 2012; Lind et al., 2014a,b). Our data also corroborate the findings of previous studies on future thinking (Terrett et al., 2013; Ciaramelli et al., 2018). In addition, planning difficulties observed in the ASD participants may have contributed to this result. We went beyond them by considering temporal distance and showing impairments of both near that may extend to distant future projections. These impairments may result from difficulty with scene construction, as suggested by Lind et al. (2014b) and, more recently, by Ciaramelli et al. (2018). These authors reported the production of fewer internal details (i.e., episodic), compared with TD controls, but similar numbers of external details (i.e., semantic). Difficulty describing internal states leads to abnormalities in binding experience directly to the self and establishing bonds between the self and others, and consequently, giving coherent meaning to events (Fivush, 2009). Maister and Plaisted-Grant (2011) also suggested that poorer temporal processing abilities in ASD are related to episodic memory impairments. The difficulty accessing episodic AM seemed less pronounced for memories related to the previous and forthcoming summer vacations. Compared with the recent past (restricted to the previous or next day), the more extended vacation period offered a range of possible autobiographical events, facilitating the retrieval of one specific and especially salient moment. Moreover, in contrast to many other studies (Goddard et al., 2014), our task fixed the time period but not the topic, and consequently allowed participants greater flexibility in choosing their personal events, which may have been more closely related to their concerns.

### Sensory Properties

Contrary to our prediction, the episodic memories provided by the participants with ASD contained just as many sensory details as those produced by controls for three periods. These results are in accordance with Crane and Goddard (2008), who did not observe any difference in sensory or emotional information in adults with ASD. This may result, in part, from the use of visual cues for each perceptual modality. However, a lack of details persists for the remote past that may illustrate consolidation difficulties reported by Goddard et al. (2007) and Bon et al. (2012). This reduction is relatively homogeneous and concerned all modalities except taste. Rather surprisingly, however, the recent episodic memories also lacked color details. The adolescents with ASD did mention colors, but fewer than controls. This finding is in accordance with the accounts of some families, who report particular interest in or aversion to some colors and lights in daily life. Some individuals with ASD may have either an obsession with or phobia of colors, as described by Ludlow et al. (2014) in a case study. Hence, they may have an atypical perception of colors that affects the formation/retrieval of memories, even when support is provided. Very few studies have used colored material to study either working memory (see, for example, Vogan et al., 2014) or long-term memory (Massand and Bowler, 2015) in ASD. When Franklin et al. (2008) investigated color memory per se, they found impaired performance for colors compared with shapes. Two years later, Franklin et al. (2010) also reported a general reduction in chromatic sensitivity. This atypical sensitivity to color may account for the present results.

### Recollection and Emotional Properties

When our participants with ASD were prompted by visual cues, we did not find any difference in the processing of either the valence or intensity of emotions: they produced memories that were just as positive as those of controls. These results further justify the use of visual cues at retrieval to compensate for the difficulty that individuals with ASD have understanding verbally expressed emotions. Moreover, Maccari et al. (2014) demonstrated that individuals with ASD are able to process positive emotional information embedded in pictures just as well as controls. Our results indicate that this ability can be generalized to familiar autobiographical scenes.

Concerning the other properties, we observed differences between the two groups only for the remote past. The ASD group had reduced mental imagery, subjective recollection and frequency of evocation. Participants with ASD produced memories lacking in details and associated with reduced episodic properties, compared to controls. Once more, this result is in accordance with abnormal forgetting previously reported in ASD. These data replicate those of other experimental studies that used anterograde memory paradigms (Bowler et al., 1997; Souchay et al., 2013; Cooper and Simons, 2018). Our participants' recollection difficulties may reflect an additional deficit in relational processes, as demonstrated by Bowler et al. (2014) and Gaigg et al. (2015). Individuals with ASD have difficulty binding together the different features that make up an episodic event (Happé and Frith, 2006). Hence, the ASD group may have been successful in recalling some episodic features separately, with the aid of visual cues, but had difficulty binding them together to generate a feeling of reliving. This may be due to weak central coherence, leading to construction, organization, and retrieval difficulties (Happé and Frith, 2006; Bowler et al., 2011), and possibly impacting other abilities such as theory of mind, as suggested recently by Ciaramelli et al. (2018).

Surprisingly, we did not observe the same pattern of performance for projections into the future. Performance was poorer for future versus past periods in the control group for number and accuracy of mental imagery, as previously demonstrated by Abram et al. (2014), thus reducing differences with the ASD group. Hence, the ASD group had an intact feeling of pre-experiencing the future, supporting the notion that the feeling of reliving previous experiences and the pre-experiencing of future events are subtended by partially distinct mechanisms. The feeling of pre-experiencing may have been the product of reasoning based on vividness, the visual perspective adopted during the questionnaire, and personal relevance, as previously demonstrated by D'Argembeau and Van der Linden (2012). All these features were preserved in our participants. The sense of self may be involved to a more limited extent in the ability to elaborate a mental representation associated with future thinking than in the remembering of past autobiographical events.

### Limitations and Perspectives

fpsyg-10-01513 July 5, 2019 Time: 15:15 # 10

This work presents certain limits. First, the sample size is relatively small, preventing us from generalizing to the ASD population. In addition, since we had the opportunity to include only boys, inclusion of a group of girls would extend our conclusions to ASD as a whole. Second, our groups do not differ in age but have a wide age range. Given the major influence of age on cognitive development, it would be particularly interesting to investigate the relationship between AM development and other cognitive abilities, such as theory of mind which is impaired in ASD. Third, given the interaction between AM development and social interactions, environment and lifestyle (e.g., family, therapies, activities, etc.), largely neglected in previous studies, it is crucial to consider these factors in future research. Fourth, each personal event was manually transcribed and scored according to a grid coding for five components of episodic memory (i.e., what, where, when, how, who). Scoring was obtained separately by the interviewer and a psychologist until a consensus was reached. In future work, recording verbatim productions would refine the analysis in providing a more detailed investigation of each component. Finally, the interviewer was one of the two coders and was thus not blind to groups. It would be relevant to replicate our results with two coders blind to the diagnoses and verify their inter-rater reliability.

### CONCLUSION

Our study suggests that AM impairment may result from a combination of a consolidation deficit for the most remote events associated with a binding deficit and demonstrates the relevance of using visual cues to facilitate AM retrieval. These results are in keeping with other studies and may be relevant to other cognitive abilities, as recently suggested by Ciaramelli et al. (2018). This may offer new methodological opportunities for managing ASD. It also shows that some specific properties associated with episodic memories, possibly colors, may be less important than they are to TD people. This raises the issue of the impact of perception on AM, which requires further

### REFERENCES

investigation. In addition, we observed considerable variability, which we could not analyze because of the small size of our sample. Hence, characterizing the different AM profiles should be the next step in studies of cognition in ASD. This could open up new perspectives for cognitive rehabilitation, such as working on AM as the key to social interactions.

### ETHICS STATEMENT

Families were given a comprehensive description of the research. The study was approved by the relevant ethics committees, and written consent was obtained from the participants (or their parents, in the case of minors) in line with their guidelines.

### AUTHOR CONTRIBUTIONS

MA, JLV, FE, and BG-G contributed to the conception and design of the study. MA, JLV, JM, LB, FG, EM, CB, FB-B, and J-MB organized the database. MA, PW, JLV, and BG-G conducted the statistical analysis. MA, PW, and BG-G wrote the first draft of the manuscript. All authors contributed to the manuscript revision, and read and approved the submitted version.

### FUNDING

This study was supported by the Fondation de France (Grant No.<PHONE_NUMBER>).

### ACKNOWLEDGMENTS

We are grateful to Elizabeth Wiles-Portier, Camille Chapot, and Renaud Coppalle for reviewing the language style. We are indebted to all the children and adolescents and their families who took part in this study, as well as to the teachers and head teachers who kindly accommodated us in their schools.

trajectory of autobiographical memory. Front. Psychol. 3:605. doi: 10.3389/ fpsyg.2012.00605

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Anger, Wantzen, Le Vaillant, Malvy, Bon, Guénolé, Moussaoui, Barthelemy, Bonnet-Brilhault, Eustache, Baleyte and Guillery-Girard. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Temporarily Out of Order: Temporal Perspective Taking in Language in Children With Autism Spectrum Disorder

Jessica Overweg1 , Catharina A. Hartman2 and Petra Hendriks1 \*

1 Center for Language and Cognition Groningen (CLCG), University of Groningen, Groningen, Netherlands, 2 Department of Psychiatry, University Medical Center Groningen, Groningen, Netherlands

Clinical reports suggest that children with autism spectrum disorder (ASD) struggle with time perception, but few studies have investigated this. This is the first study to examine these children's understanding of before and after. These temporal conjunctions have been argued to require additional cognitive effort when conjoining two events in a clause order that is incongruent with their order in time. Given the suggested time perception impairment and well-established cognitive deficits of children with ASD, we expected them to have difficulties interpreting temporal conjunctions, especially in an incongruent order. To investigate this, the interpretation of before and after in congruent and incongruent orders was examined in 48 children with ASD and 43 typically developing (TD) children (age 6–12). Additional tasks were administered to measure Theory of Mind (ToM), working memory (WM), cognitive inhibition, cognitive flexibility, IQ, and verbal ability. We found that children with ASD were less accurate in their interpretation of temporal conjunctions than their TD peers. Contrary to our expectations, they did not have particular difficulties in an incongruent order. Furthermore, older children showed better overall performance than younger children. The difference between children with ASD and TD children was explained by WM, ToM, IQ, and verbal ability, but not by cognitive inhibition and flexibility. These cognitive functions are more likely to be impaired in children with ASD than in TD children, which could account for their poorer performance. Thus, the cognitive factors found to affect the interpretation of temporal language in children with ASD are likely to apply in typical development as well. Sufficient WM capacity and verbal ability may help children to process complex sentences conjoined by a temporal conjunction. Additionally, ToM understanding was found to be related to children's interpretation of temporal conjunctions in an incongruent order, indicating that perspective taking is required when events are presented out of order. We conclude from this that perspective-taking abilities are needed for the interpretation of temporal conjunctions, either to shift one's own perspective as a hearer to another point in time, or to shift to the perspective of the speaker to consider the speaker's linguistic choices.

Keywords: autism spectrum disorder, executive functioning, perspective taking, temporal conjunctions, Theory of Mind

#### Edited by:

Danielle DeNigris, Fairleigh Dickinson University, United States

#### Reviewed by:

Janie Busby Grant, University of Canberra, Australia Wing Chee So, The Chinese University of Hong Kong, Hong Kong

> \*Correspondence: Petra Hendriks<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Cognition, a section of the journal Frontiers in Psychology

Received: 18 April 2018 Accepted: 20 August 2018 Published: 05 September 2018

#### Citation:

Overweg J, Hartman CA and Hendriks P (2018) Temporarily Out of Order: Temporal Perspective Taking in Language in Children With Autism Spectrum Disorder. Front. Psychol. 9:1663. doi: 10.3389/fpsyg.2018.01663

## INTRODUCTION

fpsyg-09-01663 September 4, 2018 Time: 11:45 # 2

Time is an important dimension by which we make sense of the world (Navon, 1978). Time is also deeply rooted in the structural organization of language (Klein, 1994). In language, time is generally conceived as a sequential order of events, where one event follows another from past to present to future. Speakers can use temporal expressions, like before or after, to express the order of events in time either in order of occurrence (i.e., temporally congruent) or out of order (i.e., temporally incongruent). The interpretation of the temporal conjunctions before and after in an incongruent order is found to be difficult for typically developing (TD) children (Clark, 1971; Pyykkönen and Järvikivi, 2012; Blything et al., 2015; de Ruiter et al., 2018). This may hold even more for children with an autism spectrum disorder (ASD). Clinical reports suggest that children with ASD encounter difficulties in time perception (Wing, 1996). Additionally, some studies have suggested that individuals with ASD have difficulty interpreting before and after (Boucher, 2001; Perkins et al., 2006). The present study investigates time perception in language in children with ASD and their TD peers by examining their interpretation of sentences containing temporal conjunctions.

Before and after are viewed as the prototypical linguistic expressions indicating temporal order (Schilder and Tenbrink, 2001). Speakers can use these expressions in several ways to express the order of events. For example, all four sentences below indicate that someone first climbed a tree and next read a book:

The speaker's choice of before in a main-subordinate clause order (1) and after in a reversed clause order (4) result in a congruent presentation of the temporal order of events, whereas before in a subordinate-main clause order (2) and after in a reversed clause order (3) result in an incongruent presentation. Thus, it depends on the speaker's choice of type of conjunction and clause order whether the hearer should interpret the event order as congruent or incongruent.

Developmental studies in TD children report that congruency has an effect on the correct interpretation of before and after (Clark, 1971; Trosborg, 1982; McCormack and Hanley, 2011; Pyykkönen and Järvikivi, 2012; Blything et al., 2015; de Ruiter et al., 2018). Children under the age of 7 have more difficulties interpreting conjunctions in a temporally incongruent order than in a temporally congruent order, and mostly rely on the order of presentation of the events. Pyykkönen and Järvikivi (2012) showed that children between 8 and 12 years old still experience difficulties interpreting temporal conjunctions in an incongruent order, especially when the cue to event order occurs sentencemedially, as in example sentence (3).

Children's difficulties with interpreting temporal conjunctions in an incongruent order have been explained in various ways. For example, these difficulties have been argued to result from a still fragile understanding of the meaning of the temporal conjunctions before and after (Clark, 1971), from difficulty shifting one's perspective to a different point in time (McCormack and Hoerl, 1999; McCormack and Hanley, 2011), from difficulty processing subordinate-main clause orders (Diessel, 2008), and from difficulty holding information active in working memory (WM) during processing to create a chronological mental representation of the events (Blything et al., 2015; Blything and Cain, 2016). In adults, interpreting temporal conjunctions in an incongruent rather than congruent order comes with processing costs and has been shown to tax WM (Münte et al., 1998). So, interpreting temporal conjunctions in an incongruent order may require additional cognitive effort.

According to anecdotal evidence and clinical reports, individuals with ASD encounter difficulties in time perception (Wing, 1996). They often report a need to adhere to rituals and routines and are commonly preoccupied with timetables, clocks, and calendars, which may serve to compensate for their failure to predict future events and their disorientation in time (Allman and DeLeon, 2009). This led Boucher (2001) to suggest that individuals with ASD have an impaired sense of time. So far, few studies have been conducted on time perception in children with ASD. Some studies report intact time perception (Wallace and Happé, 2008; Gil et al., 2012), while other studies suggest that children with ASD experience particular difficulties with understanding temporal ordering and concepts such as duration, succession, past, and future (Gillberg and Peeters, 1995; Boucher et al., 2007; Maister and Plaisted-Grant, 2011). Also, some studies report that children with ASD use fewer temporal expressions in story-telling (Colle et al., 2008) and more often omit tense marking than their TD peers (Roberts et al., 2004). These findings regarding the production of temporal expressions suggest that children with ASD may struggle with their interpretation of temporal conjunctions as well, although a mismatch between their production abilities and their comprehension abilities is also conceivable (see Hendriks, 2014 for an overview and discussion of attested production–comprehension asymmetries in child language).