# PHONOLOGICAL AND PHONETIC COMPETENCE: BETWEEN GRAMMAR, SIGNAL PROCESSING, AND NEURAL ACTIVITY

EDITED BY: Ulrike Domahs, Hubert Truckenbrodt and Richard Wiese PUBLISHED IN: Frontiers in Psychology

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ISSN 1664-8714 ISBN 978-2-88919-809-2 DOI 10.3389/978-2-88919-809-2

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## **PHONOLOGICAL AND PHONETIC COMPETENCE: BETWEEN GRAMMAR, SIGNAL PROCESSING, AND NEURAL ACTIVITY**

Topic Editors: **Ulrike Domahs,** Free University of Bozen-Bolzano, Italy **Hubert Truckenbrodt,** Centre for General Linguistics (ZAS), Germany **Richard Wiese,** Philipps-Universität Marburg, Germany

The present collection of articles brings together experimental work in the field of segmental and prosodic processing and representation in phonology and phonetics. Contributions focus on the exploration of human cognitive, articulatory, and perceptual abilities dealing with all types of phonetic and phonological entities.

Main topics of investigation include: (1) sounds and sound-changing processes—systemic and functional aspects, (2) prosodic units such as syllables and metrical feet—systemic properties, processing, and phonetic consequences, and (3) tones as building blocks of the sentence melody—their relation to the level of linguistic expressions on the one hand, their phonetic realization (e.g., tonal height and contours) and perception on the other hand. In addition, topics (1) and (2) extend to the question how phonological representations are stored in the mental lexicon: specified minimally in terms of categorical phonological information or as variable phonetic imprint of the exemplars in the input.

Diagonally to these thematic domains, the present Research Topic shows a strong focus on up-to-date experimental approaches, going far beyond traditional linguistic analysis, and making use of psycho- and neurolinguistic methodologies.

**Citation:** Domahs, U., Truckenbrodt, H., Wiese, R., eds. (2016). Phonological and Phonetic Competence: Between Grammar, Signal Processing, and Neural Activity. Lausanne: Frontiers Media. doi: 10.3389/978-2-88919-809-2

# Table of Contents

Frank Kügler and Anja Gollrad

## Editorial: Phonological and Phonetic Competence: Between Grammar, Signal Processing, and Neural Activity

#### Ulrike Domahs 1 \*, Hubert Truckenbrodt 2 and Richard Wiese3

1 Faculty of Education, Free University of Bozen-Bolzano, Bolzano, Italy, 2 Zentrum für Allgemeine Sprachwissenschaft, Berlin, Germany, 3 Institut für Germanistische Sprachwissenschaft, University of Marburg, Marburg, Germany

Keywords: prosody, phonetics, phonology, speech perception, speech production, language development, language change

**The Editorial on the Research Topic**

**Phonological and Phonetic Competence: Between Grammar, Signal Processing, and Neural Activity**

## INTRODUCTION

The present collection addresses the place and role of phonology (as an object of study, not as a scientific field) within a wider range of neighboring domains. Generally, the relevance of phonological structure in language may be claimed to derive from the fact that phonology constitutes a domain of its own within language (along with syntax, semantics, morphology), but also interfaces intimately with other domains such as cognition, articulation, and perception in general. From this dual nature, it follows that phonology may be an object of linguistic description and theory (for an overview see Goldsmith, 1995; de Lacy, 2012) as well as an object of cognitive and behavioral studies (for an overview see Cohn et al., 2012). Ideally, however, theoretical and empirical studies keep this dual nature of phonology in mind and pay attention to both sides of the coin.

Articles in the present Research Topic attempt to capture different aspects of this overall discussion. The starting point for this Research Topic was a Priority Programme on experimental research in phonology and phonetics funded by the German Science Foundation (DFG; SPP 1234). Based on this programme, the aim of this Research Topic is to draw together empirical work in the field of segmental and prosodic processing and representation and phonological theory.

Contributions address the interface of the speech sound systems investigated in phonology, the representations of articulated speech, perception, acquisition and processing established in phonetics, psycholinguistics, and neurolinguistics. Main topics of investigation include: (1) sounds and sound-changing processes—systemic and functional aspects, (2) prosodic units such as syllables and metrical feet—systemic properties, processing, and phonetic consequences, and (3) tones as building blocks of the sentence melody—their relation to the level of linguistic expressions on the one hand, their phonetic realization (e.g., tonal height and contours) and perception on the other hand. In addition, topics (1) and (2) extend to the question how phonological representations are stored in the mental lexicon: specified minimally in terms of categorical phonological information or as variable phonetic imprint of the exemplars in the input.

Diagonally to these thematic domains, the present Research Topic shows a strong focus on up-to-date experimental methods. Contributions go far beyond traditional linguistic analysis, and make use of psycho- and neuro-linguistic methods.

#### Edited and reviewed by:

Manuel Carreiras, Basque Center on Cognition, Brain and Language, Spain

#### \*Correspondence: Ulrike Domahs<EMAIL_ADDRESS>
#### Specialty section:

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

Received: 21 November 2015 Accepted: 24 November 2015 Published: 23 December 2015

#### Citation:

Domahs U, Truckenbrodt H and Wiese R (2015) Editorial: Phonological and Phonetic Competence: Between Grammar, Signal Processing, and Neural Activity. Front. Psychol. 6:1899. doi: 10.3389/fpsyg.2015.01899

## THE CONTRIBUTIONS

Sound and sound-changing processes are investigated by Bukmaier and colleagues, Schild and colleagues, Truckenbrodt and colleagues, van der Vijver and Baer, Poellmann and colleagues, and Zimmerer and Reetz. Bukmaier and colleagues present production and perception experiments that provide evidence for a process of sound change in which the neutralization of /s/ and /R / to /R / before stops in Augsburg German is influenced by the Standard German contrast between /s/ and /R /. The continuous function of the exposition to Standard German supports models that adhere to the exemplar theory of speech. Changes in sound perception during early childhood were studied by Schild and colleagues. In an ERP study with pre-schoolers, beginning readers, and adults, the authors investigated how stressed and unstressed syllables prime German word targets when prime and target overlap in phonemes and stress patterns. Age-related differences show that the processing of phonemes, but not the processing of stress is modulated by literacy acquisition.

An MMN-study devoted to investigate whether pre-attentive processing is sensitive to a syllable-related phonological process of German, namely final devoicing, was conducted by Truckenbrodt and colleagues. The authors found MMN effects for deviants violating final devoicing showing that even early pre-attentive auditory processing is modulated by syllablerelated and automatic lexical phenomena. Final devoicing as the cause of voicing alternations in singular–plural pairs is also in the focus of the contribution by van de Vijver and Baer-Henney. In their production study, 5 and 7 years old children and adults produced plural forms out of pseudowords that required either voicing or vowel alternations. Age-related decrease of voicing and increase of vowel alternations show that generalizations are lexicon-based and rely on the frequencies of certain processes that vary between child and adult lexicon.

More indirectly connected to the topic of sound changing processes are two contributions on the production and perception of reduced forms displaying either sound deletions or reductions of phonological features. Poellmann and colleagues performed a series of eye-tracking experiments on the perception of reduced forms in which segments were either reduced or deleted. The experience with inconsistent pronunciations leads to a greater perceptual flexibility in dealing with other forms of reduction than does the experience with consistent pronunciations. The processing of reduced forms is also investigated by Zimmerer and Reetz. More specifically, they were interested in the sensitivity to compensatory acoustic cues left when a final /t/ is deleted, and investigated whether German listeners are able to reconstruct a final /t/ when confronted with reduced forms. They found that /t/ was reconstructed in only 45% of items presented. This finding is discussed in the light of the experimental methodology and stimuli used and the acoustic cues indexing final /t/ deletion in German.

The role of prosodic entities and/or their representation is investigated by Bien and colleagues, Samlowski and colleagues, Domahs and colleagues, Domahs and colleagues, Häuser and Domahs, Heisterueber and colleagues, and also Schild and colleagues. In an ERP study using a word fragment priming paradigm, Bien and colleagues found effects that underpin the relevance of the syllable for language processing and lexical access. Samlowski and colleagues investigated the role of a number of prosodic and grammatical factors for syllable pronunciation in German. Some of these factors (word stress and sentence boundaries, lexical classes) were demonstrated to influence phonetic details (especially duration) of syllables corresponding to prefixes and function words.

How syllables are parsed into feet and whether feet are constructed beginning from the right or left edge of words has been investigated by Domahs and colleagues. The selection of the antepenultimate or final syllable as syllable bearing main stress in trisyllabic pseudowords is found to correlate with the working memory capacity of participants in a pseudoword production task.

A study on foot properties is presented by Domahs and colleagues. Their EEG results support evidence for bimoraic trochaic feet as processing units in the word stress system of Cairene Arabic. In addition, prosodic structure in Cairene Arabic is shown to be generated and constructed actively in online processing. The highly predictable word stress system does not lead to limitations in the sensitivity to word stress, i.e., there is no stress-deafness as predicted, among others, by Peperkamp and Dupoux (2002).

Regarding the question where lexical stress representations are functionally localized, Häuser and Domahs reviewed a series of published patient studies: all patients with a representational deficit in word stress processing had lesions in their languagedominant hemisphere. Word stress processing relies mainly on the functioning of the left hemisphere. However, Heisterueber and colleagues show that stress processing is also subject to interindividual differences, as shown in an fMRI-study performed with German native speakers who participated in a sequence recall task testing the capacity to represent segmental and suprasegmental information on an abstract level. The authors report inter-individual differences in behavioral and neural activation patterns for word stress processing modulated by individual auditory processing and working memory capacities.

Finally, Kügler and Gollrad presented production and perception studies on contrastive meaning components of a risefall contour in German: a pitch accent carrying a particular meaning has a preference to occur with a context that triggers this particular meaning. Their findings suggest that the alignment and scaling of the accentual peak are sufficient to license a contrastive interpretation of the nuclear rise-fall contour.

## AUTHOR CONTRIBUTIONS

UD, HT, and RW conceived the topic of this Research Topic. All authors participated in the editorial work.

## ACKNOWLEDGMENTS

We would like to thank the German Science Foundation (Deutsche Forschungsgemeinschaft) for generously supporting the work presented in this Research Topic.

## REFERENCES

**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 © 2015 Domahs, Truckenbrodt and Wiese. 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) or licensor 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.

## Functional lateralization of lexical stress representation: a systematic review of patient data

#### *Katja Häuser 1,2 and Frank Domahs 3 \**

*1 Department for Communication Sciences and Disorders, McGill University, Montreal, QC, Canada*

*2 Centre for Research on Brain, Language and Music, McGill University, Montreal, QC, Canada*

*3 Institute of Germanic Linguistics, Philipps-University Marburg, Marburg, Germany*

#### *Edited by:*

*Richard Wiese, Philipps-Universität Marburg, Germany*

#### *Reviewed by:*

*Marie Klopfenstein, Southern Illinois University Edwardsville, USA Christina Samuelsson, Linköping University, Sweden*

#### *\*Correspondence:*

*Frank Domahs, Institute of Germanic Linguistics, Philipps-University Marburg, Wilhelm-Röpke-Str., 6a, D-35032 Marburg, Germany e-mail<EMAIL_ADDRESS>According to the functional lateralization hypothesis (FLH) the lateralization of speech prosody depends both on its function (linguistic = left, emotional = right) and on the size of the units it operates on (small = left, large = right). In consequence, according to the FLH, lexical stress should be processed by the left (language-dominant) hemisphere, given its linguistic function and small unit size. We performed an exhaustive search for case studies of patients with acquired dysprosody due to unilateral brain damage. In contrast to previous reviews we only regarded dysprosody at the lexical level (excluding phrasal stress). Moreover, we focused on the representational stage of lexical stress processing, excluding more peripheral perceptual or motor deficits. Applying these criteria, we included nine studies reporting on 11 patients. All of these patients showed representational deficits in word stress processing following a lesion in their language-dominant hemisphere. In 9 out of 11 patients, it was the left hemisphere which was affected. This is a much more consistent pattern as found in previous reviews, in which less rigorous inclusion criteria may have blurred the pattern of results. We conclude that the representation of lexical stress crucially relies on the functioning of the language-dominant (mostly left) hemisphere.

**Keywords: word stress, representational knowledge, left hemisphere, right hemisphere, acquired disorders of language, dysprosody**

## **INTRODUCTION**

According to the functional lateralization hypothesis (FLH; Van Lancker, 1980; Van Lancker Sidtis et al., 2006) the lateralization of speech prosody depends both on its function and the size of the linguistic unit it operates on. Processing of prosody with an emotional function is assumed to be accomplished by the right hemisphere, whereas prosody with a linguistic function should be processed by the left or language-dominant hemisphere. Moreover, the right hemisphere is assumed to operate on larger scale linguistic units such as phrases or sentences, while small units such as syllables should be processed by the left (language dominant) hemisphere. In consequence, according to the FLH, lexical stress should be processed by the left (languagedominant) hemisphere, given its linguistic function and small unit size (Van Lancker, 1980; Wong, 2002).

One relevant source of evidence for the FLH are neuropsychological case studies. If lexical stress processing is found to be impaired in subjects with unilateral brain damage, this would provide insights into the neural substrates that are necessarily involved in the processing of this aspect of prosody. However, so far such studies have yielded mixed results with respect to the FLH, and different reviews have arrived at conflicting results (Baum and Pell, 1999; Wong, 2002). Whereas the authors in one review concluded that there is sufficient evidence in favor of a consistent involvement of left hemisphere substrates in lexical stress processing (Baum and Pell, 1999), another review found the results too inconclusive to fully support the hypothesis of functional lateralization (Wong, 2002). These contradicting conclusions can partly be attributed to diverging methods and interpretations of the results. For example, Wong (2002) stated that since not all reviewed studies consistently include an LHD, RHD, and normal control group, some results are impossible to evaluate against the hypothesis of functional lateralization. Another potential problem is the fact that most previous studies have intermixed tasks involving different stages of lexical stress processing (such as perception, representation, and production), although it seems implausible that these processing stages are accomplished by the same neural regions at all (for a review, see Zatorre and Gandour, 2008). This has possible consequences for lateralization according to the FLH and could also explain why previous reviews did not reach a consistent conclusion in this matter. Finally, existing reviews often included clinical case studies conducted in English, some of which have insufficiently distinguished the size of the linguistic units under consideration. In some studies, compound noun phrases (green 'house vs. 'greenhouse) have been investigated on the same level as noun/verb minimal pairs ('convict vs. con'vict). Such an approach is potentially problematic, since noun phrases have greater semantic and syntactic complexity than compound nouns or simplex nouns and verbs (Wasow, 1997). Consequently, 'green house is not minimally distinct from green 'house in regards to word stress alone. Crucially, they also differ in the size of linguistic units involved which has implications for the lateralization of processing according to the FLH. In sum, various reasons ranging from differing methodologies to contrasting interpretations could explain the rather mixed evidence that has been discussed with respect to the FLH so far.

The goal of the present study was to review existing case reports with respect to the functional lateralization of lexical stress. In contrast to previous reviews (Baum and Pell, 1999; Wong, 2002), we only considered clinical case studies that investigated prosody at a purely lexical level, thus excluding studies on noun phrase or compound noun stress processing. Moreover, we focused on the representational stage of lexical stress processing, excluding perceptional and articulatory deficits. Our aim was to evaluate evidence informative to the claim that lexical stress is represented in the language-dominant (i.e., mostly left) hemisphere.

## **METHODS**

We conducted an exhaustive search on the data bases Google Scholar, PubMed, and Entrez, using the search terms *lexical stress* AND *brain damage*, *lexical stress* AND *hemisphere*, and *lexical stress* AND *aphasia*. We discarded all studies that did not focus on individuals with unilateral brain damage and/or did not investigate stress assignment at a purely lexical level (for example, studies on tone or phrase level stress). In addition, we excluded all studies in which prosodic impairments in speech production could also result from more peripheral perceptual or articulatory difficulties (e.g., cases of dysarthric or apraxic impairment). After the application of these exclusion criteria, 12 articles remained for analysis, reporting on 15 patients with representational impairments in lexical stress processing (meaning that these patients displayed impairments in stress assignment). We reviewed all 12 studies for hemispheric site of lesion and language-dominant hemisphere of the patient. If both types of information were missing and could not be inferred based on the information provided

#### **Table 1 | Table of patients included in the review.**

by the authors, the study and/or subject was excluded from further analysis, resulting in the exclusion of four studies/subjects (excluded studies: Lloyd, 1999; Howard and Smith, 2002; Janssen, 2003. excluded subject: DE in Black and Byng, 1986). An overview on all studies and patients included is provided in **Table 1**.

The languages spoken by the patients included in our analyses were English, German, and Italian—all languages with variable stress. This means that although in all three languages, word stress assignment shows some regularities (for an overview, see Van der Hulst, 1999), the assignment of stress to individual words cannot be inferred by phonemic or orthographic rules alone and thus requires activation of word-specific (i.e., lexical) phonological representations (Miceli and Caramazza, 1993). The word stress errors reported by the studies included in this review particularly affected words with infrequent or "irregular" stress patterns (Coltheart et al., 1983; Chiacchio et al., 1993; Miceli and Caramazza, 1993; Cappa et al., 1997; Rozzini et al., 1997; Galante et al., 2000; Laganaro et al., 2002; Janssen and Domahs, 2008), typically leading to shifts in stress assignment to the most frequent pattern ("over-regularisations", e.g., Marshall and Newcombe, 1973; Black and Byng, 1986; Cappa et al., 1997; Laganaro et al., 2002; Janssen and Domahs, 2008).

#### **RESULTS**

#### **CLASSIFICATION BASED ON HANDEDNESS**

This analysis is based on the fact that handedness is closely related to hemispheric dominance for language (e.g., Knecht et al., 2000). Out of the ten studies with 12 patients that remained in the pool (see **Table 1**), data on both the impaired hemisphere and the patient's handedness were available in eight cases. All of these eight patients presented with systematic errors in stress assignment following a lesion in their language-dominant hemisphere

*Hand, reported handedness; Hemisphere, lesioned hemisphere; Site, site of lesion (if available); T, Temporal; Ven, Ventricle; P, Parietal; F, Frontal. Etiology: DEG, Degenerative; OHI, Open Head Injury; CVA, Cerebral Vascular Accident.*

*Tasks: nam, picture naming; read, reading; repet, repetition; lex. dec., lexical decision task performance is indicated in % errors. x's indicate missing information.*

(which was the LH in seven patients and the RH in one patient), as inferred from handedness.

### **CLASSIFICATION BASED ON LINGUISTIC IMPAIRMENT**

All 12 patients (including the four cases where handedness information was not available) showed major linguistic impairments, suggesting that they suffered from lesions in their languagedominant hemisphere. This yields a total of 12 out of 12 patients who showed representational deficits in word stress processing following a lesion in their language-dominant hemisphere. In 10 out of 12 patients, it was the left hemisphere which was affected.

## **DISCUSSION**

The goal of this study was to review evidence from acquired language impairment regarding the functional lateralization hypothesis (FLH, Van Lancker, 1980; Van Lancker Sidtis et al., 2006), which states that function and size of a prosodic unit determine the cortical hemisphere it is processed in. Specifically, we were interested in the representation of lexical stress, which according to the FLH is a property of the left, language-dominant hemisphere. To this end, we reviewed clinical case studies that focused on brain-damaged patients with representational impairments in lexical stress assignment. Ten studies reporting on 12 cases remained for analysis after the application of all exclusion criteria. The results showed that in all of these patients, impairments in lexical stress assignment followed a lesion in the languagedominant hemisphere. In contrast to earlier reviews, which have arrived at mixed results, our data thus fully support the functional lateralization hypothesis.

The sample of studies that met our inclusion criteria was rather small, given that only few studies addressed the representation (rather than perception or articulation) of lexical stress. However, our rigorous and hypothesis-driven approach yielded a very clear pattern of results, in comparison to previous reviews that investigated speech prosody. In fact, a closer look at patients which were excluded from our analyses because they did not fulfill our criterion of a representational impairment (either because of a speechmotor deficit, e.g., dysarthria, or because of a non-specified deficit affecting lexical stress processing) revealed a much less consistent pattern: 82 of those patients had a left-hemisphere lesion, in comparison to 74 patients with right-hemisphere damage. Furthermore, 65 patients were reported to have lesions at the side of their dominant hand. Clearly, allowing for less precision in the nature of lexical dysprosody (as in previous reviews) would have led to a more impressive number of cases but to a blurred pattern of results. After all, it is highly plausible that perceptual and motor stages of lexical stress processing are subserved by bilateral brain areas whereas the more abstract linguistic representation of word prosody may reside in the language-dominant (left) hemisphere. This could explain the mixed evidence that earlier reviews yielded with respect to the FLH (Baum and Pell, 1999; Wong, 2002).

Our findings are consistent with evidence from dichotic listening showing that stress typicality effects (indicative of the representational stage of stress processing) only appeared in repetition and noun/verb-classification when stimuli were presented to the right ear/left hemisphere (Arciuli and Slowiaczek, 2007). More generally, our findings are also consistent with previous studies (Baum and Pell, 1999) that have rejected a strict division of labor regarding the hemispheric representation of prosody. Even though our results support the notion that lexical stress is a property of the language-dominant hemisphere, it seems that any global "all-left" or "all-right" account with respect to the hemispheric lateralization of *all* prosodic functions is an oversimplification and fails to account for the data. In this context, it seems that to date the FLH is the most promising account put forward to describe the neural substrates of prosody, since it does not set up an all-or-none division for prosodic functions but allows for gradedness of prosodic representation, depending on their function and the size of the processing units involved. This claim is also substantiated by findings in neuro-imaging, which have demonstrated bilateral cortical activations for lexical stress processing (Aleman et al., 2005; Wildgruber et al., 2006; Klein et al., 2011; Domahs et al., 2013). Yet, it is the methodological strength of lesion studies to highlight the functional relevance of brain regions for cognitive functions (Rorden and Karnath, 2004).

In sum, based on the data at hand we conclude that the representation of lexical stress crucially relies on the functioning of the language-dominant (mostly left) hemisphere.

## **ACKNOWLEDGMENTS**

This research was supported by a grant from the LOEWE initiative of excellence of the Hessian Ministry of Research and the Arts (project LingBas).

## **REFERENCES**

framework for the study of the cerebral representation of prosody. *Brain Lang.* 97, 135–153. doi: 10.1016/j.bandl.2005.09.001

**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.

*Received: 28 January 2014; accepted: 26 March 2014; published online: 10 April 2014. Citation: Häuser K and Domahs F (2014) Functional lateralization of lexical stress representation: a systematic review of patient data. Front. Psychol. 5:317. doi: 10.3389/ fpsyg.2014.00317*

*This article was submitted to Language Sciences, a section of the journal Frontiers in Psychology.*

*Copyright © 2014 Häuser and Domahs. 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) or licensor 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.*

## Processing word prosody—behavioral and neuroimaging evidence for heterogeneous performance in a language with variable stress

## *Miriam Heisterueber 1,2\*, Elise Klein2,3,4, Klaus Willmes 1,2,4, Stefan Heim1,5,6 and Frank Domahs 7*

*1 Section Neurological Cognition Research, Department of Neurology, Uniklinik RWTH Aachen, Aachen, Germany*

*2 Faculty of Medicine, Brain Imaging Facility of the Interdisciplinary Centre for Clinical Research, Uniklinik RWTH Aachen, Aachen, Germany*

*3 KMRC – Knowledge Media Research Center, Tuebingen, Germany*

*4 Section Neuropsychology, Department of Neurology, Uniklinik RWTH Aachen, Aachen, Germany*

*5 Department of Psychiatry, Psychotherapy and Psychosomatics, Uniklinik RWTH Aachen, Aachen, Germany*

*6 Research Centre Juelich, Institute of Neuroscience and Medicine (INM-1), Juelich, Germany*

*7 Institute of Germanic Linguistics, Philipps University, Marburg, Germany*

#### *Edited by:*

*Richard Wiese, Philipps-Universität Marburg, Germany*

#### *Reviewed by:*

*Maren Schmidt-Kassow, Goethe University, Germany Anna J. Simmonds, Imperial College London, UK*

#### *\*Correspondence:*

*Miriam Heisterueber, Section Neurological Cognition Research, Department of Neurology, Uniklinik RWTH Aachen, Pauwelsstraße 30, 52074 Aachen, Germany e-mail: miriam.heisterueber@ rwth-aachen.de*

In the present behavioral and fMRI study, we investigated for the first time interindividual variability in word stress processing in a language with variable stress position (German) in order to identify behavioral predictors and neural correlates underlying these differences. It has been argued that speakers of languages with variable stress should perform relatively well in tasks tapping into the representation and processing of word stress, given that this is a relevant feature of their language. Nevertheless, in previous studies on word stress processing large degrees of interindividual variability have been observed but were ignored or left unexplained. Twenty-five native speakers of German performed a sequence recall task using both segmental and suprasegmental stimuli. In general, the suprasegmental condition activated a subcortico-cortico-cerebellar network including, amongst others, bilateral inferior frontal gyrus, insula, precuneus, cerebellum, the basal ganglia, pre-SMA and SMA, which has been suggested to be dedicated to the processing of temporal aspects of speech. However, substantial interindividual differences were observed. In particular, main effects of group were observed in the left middle temporal gyrus (below vs. above average performance in stress processing) and in the left precuneus (above vs. below average). Moreover, condition (segmental vs. suprasegmental) and group (above vs. below average) interacted in the right hippocampus and cerebellum. At the behavioral level, differences in word stress processing could be partly explained by individual performance in basic auditory perception including duration discrimination and by working memory performance (WM). We conclude that even in a language with variable stress, interindividual differences in behavioral performance and in the neuro-cognitive foundations of stress processing can be observed which may partly be traced back to individual basic auditory processing and WM performance.

#### **Keywords: word stress, fMRI, interindividual differences, segmental processing, stress processing**

## **INTRODUCTION**

In some languages (e.g., Czech, Finnish, Polish, Turkish, Persian, or French) main stress always falls on the same position within a word (fixed stress; for a typological overview see Van der Hulst, 1999). In those languages, no minimal pairs of words exist which do only differ in terms of their stress position. Accordingly, in fixed stress languages word stress is not contrastive and does not carry lexical information. In consequence, the processing and representation of word stress is not particularly relevant in the use of such languages. In this vein, it has been repeatedly reported that speakers of languages with fixed stress encounter difficulties when confronted with tasks requiring processing or representation of word prosody (Dupoux et al., 1997; Peperkamp et al., 1999, 2010; Mehler et al., 2004; Domahs et al., 2012, 2013a).

In contrast, other languages (e.g., English, Spanish, Russian, or German) have variable stress positions. Word stress may be contrastive, carrying lexical information. Thus, there may be minimal pairs, which only differ in their suprasegmental makeup, i.e., stress pattern, their segmental sequence being identical (e.g., German verbs *umfáhren* vs. *úmfahren*, to drive around vs. to knock over). Therefore, the processing and representation of word stress is particularly relevant in languages with variable stress and speakers of those languages are typically found to be highly sensitive to suprasegmental manipulations, showing relatively good performance in a variety of tasks tapping on word stress (Domahs et al., 2008; Molczanow et al., 2013; for a direct comparison between speakers of a language with fixed stress (French) and with variable stress (Spanish or German) see Dupoux et al., 2001, 2008; Schmidt-Kassow et al., 2011a).

However, comparing speakers of different languages typically ignores the possibility that there may be substantial interindividual variability in stress processing performance even within a given language. Thus, the present study addresses the questions whether there are interindividual differences in stress processing in a language with variable stress (German) and, if so, which neural correlates may underlie those differences. Before the details of the present study will be outlined, a brief summary of research on stress processing will be given by describing word stress assignment in German and discussing evidence on the neuronal basis of stress processing.

### **WORD STRESS ASSIGNMENT IN GERMAN**

Given that German is a language with variable stress, the stress pattern of individual words is largely unpredictable and has thus to be lexicalized (Eisenberg, 2006; Domahs et al., 2008). This lexical knowledge can be used to distinguish between the elements of minimal pairs and to activate the correct meaning related to each of the members of a minimal pair. Beyond complete lexicalization, there are some rules and regularities in German stress assignment which become apparent, when participants are asked to pronounce pseudowords or have to deal with stress violations:

Phonetically, German word stress is marked by a combination of the following cues: duration, (global) intensity, fundamental frequency (pitch), vowel formants and voice quality (for a comprehensive overview see Lintfert, 2010). Haake et al. (2013) found a significant relationship between auditory perception of duration cues and the representation of word stress both in children with specific language impairment and in typically developing children acquiring German. Heim and Alter (2006, 2007) provided EEG evidence that context stress, e.g., in a sentence, can be used as additional information to identify stress patterns.

## **THE NEURAL BASES OF WORD STRESS PROCESSING**

There are currently only very few functional neuroimaging studies investigating the neural correlates of word stress processing (Aleman et al., 2005; Klein et al., 2011; Domahs et al., 2013b). In the study by Aleman et al. (2005) participants had to identify weak-initial and strong-initial words. The bilateral supplementary motor area (SMA) and the left inferior frontal gyrus (IFG), the superior temporal gyrus (STG) as well as the superior temporal sulcus (STS), and the insula were associated with the processing of word stress compared to a semantic control condition. In the study by Klein et al. (2011) participants were asked to solve an identity matching task with pseudowords. Processing of word stress minimal pairs as compared to segmental minimal pairs was associated with activation in a bilateral fronto-temporal network. Klein et al. (2011) suggested that there is a basic system for word stress processing in the left hemisphere, whereas the right hemisphere supports the left in case of increasing task difficulty. Domahs et al. (2013b) investigated the neural correlates of processing correctly vs. incorrectly stressed words. They observed activations of the left posterior angular and retrosplenial cortex when contrasting the processing of correct vs. incorrect stress. In the inverse contrast, bilateral STG were found to be involved. The analysis of severe vs. mild stress violations revealed activations of the left superior temporal and left anterior angular gyrus. Frontal activations, including Broca's area and its right homolog, were found when contrasting mild with severe stress violations.

With respect to interindividual differences in stress processing, Boecker et al. (1999) performed an ERP study using a word stress discrimination task. Based on the median split of the behavioral outcome, they defined two groups of participants: good and poor performers. The authors found a significant N400-effect for sequence-final words with a weak-strong pattern only in the group of good performers, but not in the group of poor performers, providing first evidence to the possibility of substantial interindividual differences in word stress processing in a language with variable stress (Dutch).

### **THE PRESENT STUDY**

While differences in word stress processing between speakers of languages with fixed vs. variable stress have been described repeatedly (Dupoux et al., 2001, 2008; Peperkamp et al., 2010; Schmidt-Kassow et al., 2011a,b), interindividual differences within one type of language—although observed—remained largely ignored or unexplained (Boecker et al., 1999; Peperkamp et al., 1999; Domahs et al., 2008, 2013b; Dupoux et al., 2010). In general, it has been argued that speakers of a language with variable stress should perform relatively well in word stress processing (Dupoux et al., 1997, 2001, 2008, 2010; Peperkamp et al., 1999; Schmidt-Kassow et al., 2011a). Although interindividual variance in word stress processing in German has not been the focus of previous research, such variability has been observed (albeit ignored) in adult participants in previous studies (Domahs et al., 2008, 2013b). In a recent study, (Haake et al., 2013) reported interindividual variability in word stress processing in both children with specific language impairment and typically developing children. This variance was at least partly predicted by individual perceptual processing of auditory cues related to word stress (e.g., duration).

The aim of the current study was to investigate interindividual performance differences in the processing of word stress. To this end, native speakers of German had to perform a variant of a sequence recall task, adapted from Dupoux et al. (2001; see also Haake et al., 2013). Studies on languages with fixed stress using this task have shown that when demands on working memory increase, performance of speakers of such languages in reproducing pseudoword minimal pairs (e.g., míkuta vs. mikutá) decreases disproportionately (Dupoux et al., 1997, 2001). We used a suprasegmental variant of this task to investigate interindividual heterogeneity in word stress processing in native speakers of German, a language with variable stress, while a segmental variant of this task served as a control condition. Note that speakers of German should be highly familiar with both suprasegmental and segmental features since both are essential in the use of this language.

In sum, the research questions of the present study were the following: (i) Are there substantial interindividual differences in word stress processing within a group of native speakers of German, a language where this feature is functional? (ii) Which neural correlates in functional magnetic resonance imaging (fMRI) are associated with word stress processing in good and poor performers? Following the results of previous neuroimaging studies on word stress processing (Aleman et al., 2005; Klein et al., 2011; Domahs et al., 2013b), we expected to find clusters of activated voxels in the left IFG, the bilateral superior temporal gyrus/sulcus and in the insula as well as bi-hemispheric activation in the SMA. (iii) Can predictors for interindividual variability be identified (e.g., working memory abilities and/or basic auditory processing)?

## **MATERIALS AND METHODS**

#### **PARTICIPANTS**

Twenty-five right-handed native German-speaking healthy volunteers (nine female; mean age = 28.8 years, *SD* = 10*.*1 years) participated in the study after having given their written informed consent. The study was approved by the Institutional Review Board of the Medical Faculty at RWTH Aachen University (EK 182/06).

#### **STIMULI**

Stimulus material consisted of trisyllabic pseudowords obeying German phonotactic constraints. The pseudowords were built from five different consonants (plosives: p, t, k; nasals: n, m) and three different vowels (a, u, i). All items had the same syllable structure (CV.CV.CV). Minimal pairs of pseudowords were created such that they either differed only with respect to word stress (suprasegmental condition, SSEG) or only with respect to one consonant (segmental condition, SEG). There were two suprasegmental contrasts and two segmental contrasts, each consisting of two items, respectively (see **Table 1**). In the suprasegmental condition, penultimate stress (PU) was compared to final stress (U) and antepenultimate stress (APU) was contrasted to final stress (U). In the segmental condition, the consonants differed either in place of articulation (POA) or in a combination of place and manner of articulation (MOA). In the POA condition the consonants /m/ vs. /n/ and /k/ vs. /p/ were contrasted, whereas in the MOA condition /t/ vs. /f/ and /k/ vs. /s/ were contrasted.

For each type of stimulus, different tokens were recorded such that in each minimal pair one token was spoken by a female speaker (native speaker of Polish) and one token was spoken by a male speaker (native speaker of Persian), with the order being counterbalanced across conditions. Each pseudoword was recorded multiple times from each speaker so that different tokens from the same word were presented in the experiment. In this way, phonetic variance of stimuli was increased, disfavoring purely auditory/phonetic strategies and encouraging a more abstract, phonological type of target comparison. The duration of the pseudowords was approximately 1000 ms. Stimuli were recorded using Amadeus Pro sound editing software (HairerSoft, Kenilworth, UK).

### **PRETEST PROCEDURE**

Each participant completed pretests to evaluate his/her basal auditory processing performance. The following three auditory cues were examined, because they are critical for word stress perception: pitch, duration and skewness. The tasks testing for pitch and length discrimination were taken from the Seashore-Test (Stanton, 1928). Skewness discrimination was determined using the procedure developed by (Haake et al., 2013). The procedure was similar to the one used in the Seashore Test. Basically, skewness discrimination required the ability to distinguish the intensity of sounds (stronger vs. weaker). All items were presented via headphones employing Adobe Audition 1.5 (Adobe Systems, San Jose, CA, USA).

*Contrasts are highlighted in bold face. APU, antepenultimate stress; PU, penultimate stress; U, final stress; POA, place of articulation; MOA, combination of place and manner of articulation.*

Moreover, given that working memory was crucial for the sequence recall task used in the present study, measures of working memory span were determined for each participant (letter word span forward and backward, following the German version of the Wechsler Memory Scale for number word span forward and backward; Tewes, 1991). Participants were asked to repeat sequences of letters which were given by the examiner. For letter span forward, participants had first to repeat two sequences of three different letters, respectively (for example: f-b-i and c-g-e). At the second level of complexity two sequences of four different letters had to be repeated, respectively, and so forth. On the heighest (sixth) level participants had to repeat two sequences of eight letters. For the letter span backward task participants were asked to repeat two sequences of two up to eight letters, respectively, in inverted order. The test procedure was stopped when a participant repeated both sequences on a given level incorrectly.

#### **fMRI PROCEDURE**

The experiment was a combined behavioral and fMRI study. Participants were lying in the scanner, listening to the pseudowords presented via headphones. They had response boxes in both hands and were instructed to press the correct response buttons with the index finger of the respective hand. Head movements were prevented by using soft foam pads. To familiarize participants with the task and to reduce potential training effects during fMRI data acquisition, all participants were given the opportunity to practice two blocks (one per type of contrast) in a separate room before entering the scanner. The same pseudowords as employed in the scanner served as practice items, but spoken by different speakers (a female native speaker of Dutch and a male native speaker of German).

The experiment had a block design and comprised 8 blocks, each one of which lasted about 73.8 s. Each block consisted of two phases: a learning phase and an experimental phase. There were two types of blocks: Block A contained the segmental condition, and Block B the suprasegmental condition. Blocks were separated by pauses of 30 s. The blocks were presented in an alternating fashion, either starting with Block A (A-B-A-B etc.) or starting with Block B (B-A-B-A etc.), counterbalanced over participants (see **Figure 1**).

In each learning phase the two pseudowords needed for the following experimental task were presented, such that participants could familiarize with both words and their association with the respective response button (see **Figure 1**). Participants were instructed to respond to the first pseudoword encountered by pressing the right button. In this way the right button was always correct for the first pseudoword, such that no further explanation of the correct association between pseudowords and response buttons was needed. When hearing the second pseudoword of the learning phase, participants had to decide whether it matched with the first one (pressing the right button) or not (pressing the left button). Here matching refers to a phonological (type-based) rather than a phonetic (token-based) match. The participants had to make this decision in a sequence of 12 pseudowords per learning phase in pseudorandomized order such that no more than two identical items were presented in a row. The items were spoken either by the male or the female speaker, but no more than three times in a row by the same speaker. Participants were instructed to respond as fast and as accurately as possible by pressing the corresponding button after stimulus presentation. Maximum duration of response time was set to 2000 ms. Only in the learning phase Feedback was presented immediately after each trial only in the learning phase: a "Smiley" for a correct response and a "Frowney" for an incorrect or missing response. The learning phase lasted for about 44.3 s per block. At the end of the learning phase, participants had learned the correct correspondence between both pseudowords and their associated response buttons, which was also valid for the following experimental task.

In the experimental phase participants were presented with pairs of pseudowords from the set of items learned in the preceding learning phase. The task was to press the respective response buttons (as learned in the preceding learning phase) in the order the pseudowords had just been presented. No feedback was provided during the experimental phase. Eight item pairs were presented in random order per block. There were 12 different randomized orders of items for each block, such that only three to four participants had the same order of items. In each item pair, one item was spoken by the male und one by the female speaker. The duration of the experimental phase was 29.5 s per block (see **Figure 1**). Between pairs in the experimental phase, the background color was slightly modified (a different shade of gray for each sequence) to visually indicate the start of a new pair. Overall, the experiment took 13:34 min. The

experiment was presented with Presentation software (version 14.5, Neurobehavioral Systems, Albany, USA).

## **IMAGING ACQUISITION**

For each participant, a high-resolution T1-weighted anatomical scan was acquired with a 3T Philips Magnetom MRI system using the standard head coil (*TR* = 9*.*89 s, matrix 256 × 256 mm, 176 slices, voxel size = 1 × 1 × 1 mm3; *FOV* = 256 mm, *TE* = 4*.*59 ms; flip angle = 8◦). Moreover, one functional imaging block sensitive to blood oxygenation level-dependent (BOLD) contrast was recorded for each participant (T2∗-weighted echo-planar sequence, *TR* = 2*.*89 s; *TE* = 30 ms; flip angle = 79◦; *FOV* = 240 mm; 80 × 80 matrix; 42 slices, voxel size = 3 × 3 × 3 mm3, gap = 0.5 mm).

## **ANALYSIS OF BEHAVIORAL DATA**

Behavioral data analysis was based responses in the experimental phase only. Furthermore, items with response latency faster than 200 ms were not considered. Analyses focused on accuracy data since reaction times in the suprasegmental condition were confounded with different "points of uniqueness" when participants were able to detect the stress difference in a pair of pseudowords (e.g., earlier point of uniqueness in "míkuta" vs. "mikúta" compared to "míkuta" vs. "mikutá").

Participant's individual performance in word stress processing was evaluated employing accuracy data of the suprasegmental condition. Based on a median split of the number of correct trials in the suprasegmental condition (see **Figure 2**), each participant was assigned either to a group of poor performers (below average) or to a group of good performers (above average).

In an initial step, a 2 × 2 repeated measures Analysis of Variance (ANOVA) on accuracy was performed with the withinparticipant factor condition (segmental vs. suprasegmental) and

above average group, gray dots = participants of the below average group.

the between-participant factor group (above vs. below average word stress processing).

To pursue the potential association between performance in basal auditory processing, working memory, and suprasegmental processing, a stepwise multiple regression analysis with mean accuracy in the suprasegmental condition as criterion variable was conducted, which was stopped when the inclusion of another predictor would not increase R2 significantly (at *p <* 0*.*05). The predictors incorporated were performance measures from the pretest tasks, i.e., pitch discrimination, duration discrimination, skewness discrimination, a combined measure of these three auditory processing tasks (mean auditory processing accuracy), and working memory span.

## **ANALYSIS OF IMAGING DATA**

The anatomical scans were normalized and averaged in SPM8 (http://www*.*fil*.*ion*.*ucl*.*ac*.*uk/spm/software/spm8/). The fMRI time series were corrected for movement in SPM8. Images were motion corrected and realigned to each participant's first image. Data was normalized into standard MNI space. Images were resampled every 2.5 mm using 4th degree spline interpolation and smoothed with a 6 mm FWHM Gaussian kernel to accommodate inter-subject variation in brain anatomy and to increase signal tonoise ratio in the images. The data were high-pass filtered (128 s) to remove low-frequency signal drifts and corrected for autocorrelation assuming an AR(1) process. Brain activity was convolved over all experimental trials with the canonical haemodynamic response function (HRF) and its derivative.

On the first level, the intraindividual beta contrast weights for segmental and suprasegmental processing were evaluated. On the second level, both main effects and their interaction were evaluated in a 2 × 2 (flexible factorial) ANOVA with the between-subject factor group (above vs. below average) and the within-participant factor condition (segmental vs. suprasegmental). For the anatomical localization of effects, the anatomical automatic labeling tool (AAL) in SPM8 (http://www.cyceron.fr/ index.php/en/plateforme-en/freeware) was used to identify Brodmann Areas (BA). If possible, the SPM Anatomy Toolbox (Eickhoff et al., 2005), available for all published cytoarchitectonic maps from www.fz-juelich.de/ime/spm\_anatomy\_toolbox, was additionally used and in the results will be indicated by an "Area" specification.

## **RESULTS**

## **BEHAVIORAL DATA**

Accuracy in the segmental task ranged from 56.3 to 96.9% and in the suprasegmental task from 56.3 to 100%. The group classification was based on a median split for the accuracy results in the suprasegmental condition (see **Figure 2**). The ratio of male and female participants was comparable between both groups (good: 8m/5f, poor: 8m/4f). A descriptive overview of the results is provided in **Figure 3**.

A repeated measures ANOVA over arcsine-transformed error rates revealed a significant main effect of group [*F*(1*,* 23) = 12*.*16, *p <* 0*.*01], indicating that good performers made significantly less errors (in total) than poor performers (16.0 vs. 29.0%, see **Figure 3**). There was no main effect of condition [*F*(1*,* 23) *<* 1]. However, there was a significant two-way interaction of condition and group [*F*(1*,* 23) = 9*.*3, *p <* 0*.*01]. The effect of condition was only significant for poor performers [*t*(11) = 3*.*24; *p <* 0*.*01], meaning that in this group the error rate in the suprasegmental condition was higher than in the segmental condition (36.2 vs. 21.9%). In contrast, for good performers the effect of condition did not reach significance [*t*(12) = 1*.*78, *p* = 0*.*10]. However, it should be noted that, in contrast to the poor performers, error rate was numerically higher in the segmental than in the suprasegmental condition (19.7 vs. 12.3%).

Crucially, both groups differed significantly only in the suprasegmental condition [*t*(24) = 6*.*21, *p <* 0*.*001] indicating that the good performers performed reliably better (87.7%) than the poor performers (63.8%).There was no significant difference between groups for the segmental condition [*t*(24) = −0*.*4, *p* = 0*.*70], see **Figure 3**. Furthermore, no correlation was observed between stress processing (suprasegmental) and consonant processing (segmental) (Spearman rho = 0.072, *p* = 0*.*733).

In order to examine whether performance in the suprasegmental condition was influenced by basic auditory processing abilities and/or working memory skills, a stepwise multiple linear regression analysis was performed over arcsine-transformed error rate of the suprasegmental condition. The final model comprised the predictors auditory processing and working memory span forward [*R*2 = 0*.*400, adjusted *R*2 = 0*.*345, *F*(2*,* 24) = 7*.*3, *p <* 0*.*01].

#### **fMRI DATA**

Analysis of fMRI data was based on all trials in the experimental phase. In a first step, a conjunction analysis was conducted to identify common overall activation in the paradigm irrespective of group and condition.

#### **OVERVIEW: CONJUNCTION ANALYSIS**

A conjunction over all conditions and groups was calculated (SEG in poor performers, SSEG in poor performers, SEG in good performers, SSEG in good performers) to show joined activation at an uncorrected voxelwise *p <* 0*.*0001. Please note that

this more rigorous *p*-value had to be used in the conjunction (compared to the level of *p <* 0*.*001 for the complex contrasts reported below) to visualize the different maxima of activation (cf. Wood et al., 2009; Klein et al., 2010). However, all activations reported here remain significant following family-wise error correction (FWE) at a cluster-level of *p <* 0*.*05. Significant activations in the entire primary auditory cortex were present (see **Table 2** and **Figure 4**). Bilateral activation was found in the superior temporal gyrus/sulcus (STG; STS) and the middle temporal gyrus (MTG). Furthermore, left-hemispheric clusters of activated voxels were observed in the inferior frontal gyrus (IFG; Area 44, Area 6 (BA 44); SPM Anatomy Toolbox, Amunts et al., 1999; cf. Eickhoff et al., 2005), the insula, the inferior parietal sulcus (IPS; hIP2, IPC (PF, PFm), hIP1 (BA 7); SPM Anatomy Toolbox, Choi et al., 2006; cf. Eickhoff et al., 2005) the SMA, and the middle frontal gyrus (MFG). In the right hemisphere voxels in the IFG, inferior parietal lobule (hIP2, SPL (7PC), hIP1, hIP3; SPM Anatomy Toolbox, Choi et al., 2006; Scheperjans et al., 2008a,b; cf. Eickhoff et al., 2005) and the cerebellum were activated, while the precentral gyrus was found active bilaterally (see **Table 2**, **Figure 4**).

**Table 2 | Maxima of the conjunction analysis over both conditions (segmental and suprasegmental) as well as both groups (above and below average) at an uncorrected voxelwise** *p <* **0***.***0001 (cluster-corrected FWE of** *p <* **0***.***05).**

*IPS, inferior parietal sulcus; LH, left hemisphere; RH, right hemisphere; SMA, supplementary motor area.*

*aMinor maximum.*

**uncorrected voxelwise** *p <* **0***.***0001 (cluster-corrected FWE of** *p <* **0***.***05).**

**CONDITION-BASED COMPARISONS**

#### *Suprasegmental vs. segmental processing*

Suprasegmental was contrasted to segmental processing at an uncorrected voxelwise threshold of *p <* 0*.*001 and a cluster size of *k* = 10 voxels (see **Figure 5A**, **Table 3**). Larger activation for suprasegmental processing was found bilaterally in the IFG (Area 44 and Area 45 (BA44 and BA 45); SPM Anatomy Toolbox, cf. Eickhoff et al., 2005) as well as in the insula. Furthermore, in the left hemisphere the thalamus, the IPS (hIP1, hIP3 (BA 7); SPM Anatomy Toolbox, cf. Eickhoff et al., 2005) and the pre-SMA (BA 6) were activated, while in the right hemisphere the pallidum as well as the right SMA (BA 6) revealed stronger activation in stress processing compared to consonant processing. Further clusters of activated voxels were found in the bilateral precentral gyrus, in the left MFG (BA 10) and in the cerebellum, bilaterally.

#### *Segmental vs. suprasegmental processing*

Inspection of the inverse contrast (uncorrected *p <* 0*.*001, *k* = 10 voxel) revealed activation in the bilateral SMA (BA 6), the right middle orbital gyrus and the left precuneus (see **Figure 5B**, **Table 3**).

#### **GROUP-BASED COMPARISONS**

#### *Poor performers vs. good performers*

Poor performers revealed significantly stronger activation than good performers in the left MTG at an uncorrected voxelwise *p <* 0*.*001 and a cluster size of 10 voxels (see **Figure 6A**, **Table 3**).

#### *Good performers vs. poor performers*

When comparing good performers vs. poor performers (uncorrected *p <* 0*.*001, *k* = 10 voxel), significantly more activation was found in the left precuneus (see **Figure 6B**, **Table 3**).

#### **INTERACTION BETWEEN GROUP AND CONDITION**

We conducted an ANCOVA over participants on the fMRI data with working memory and auditory performance from the pretest as covariates, to correct the segmental and suprasegmental activations for working memory and auditory abilities. In this context, we also examined whether there is additional fMRI variance, which is exclusively explained by the covariates. However, at the threshold given (FWE-cluster threshold corrected) there was no such additional activity to be found.

Group and condition interacted significantly in the right hippocampus (CA (BA 27), SPM Anatomy Toolbox, Amunts et al., 2005; cf. Eickhoff et al., 2005) and cerebellum at an uncorrected voxelwise *p <* 0*.*001 and a cluster size of 10 voxels (see **Figure 7**, **Table 3**). However, especially in the cerebellum the interactions in signal change seem to be mostly due to different degrees of deactivation. However, it can be seen that good performers showed relatively more activation (or less deactivation, respectively) in the segmental condition in the right hippocampus and cerebellum compared to poor performers, whereas poor performers revealed relatively stronger activation compared to good performers in these areas in the suprasegmental condition.

## **DISCUSSION**

The current study set off to examine whether there are interindividual differences in word stress processing performance in native speakers of German and, if so, which neural correlates underlie these differences. So far, most studies focused on typologically motivated processing differences between speakers of languages with fixed vs. variable stress. In particular, Dupoux, Peperkamp and colleagues compared speakers of Spanish (variable stress pattern) to speakers of French (fixed stress pattern; see Dupoux et al., 1997, 2001; Peperkamp et al., 1999; Peperkamp and Dupoux, 2002) and found superior performance of the former compared to the latter (for similar results in a comparison between French and German see Schmidt-Kassow et al., 2011a). Interindividual differences within one language although repeatedly observed—were treated as noise (Peperkamp et al., 1999; Domahs et al., 2008, 2013b; Dupoux et al., 2010) or were left unexplained (Boecker et al., 1999).

In the present study, participants were examined in both suprasegmental as well as segmental variants of the sequence recall task both at a behavioral and at a neuro-functional level. Indeed, based on behavioral results we were able to identify considerable interindividual differences within native speakers of German (accuracy in the suprasegmental task ranging from floor to ceiling performance).

To explore more thoroughly, which factors modulate suprasegmental processing differences, working memory span as well as auditory processing abilities were analyzed. In fact, we demonstrated that suprasegmental performance was predicted by both basic auditory processing abilities (i.e., duration, time, skewness discrimination) and working memory span. The influence of working memory on performance in the suprasegmental task seems highly plausible since working memory was clearly task-relevant. Crucially, the fact that a combined measure of duration, time, and skewness discrimination predicted individual performance in word stress processing, provides a first hint toward an explanation for the interindividual variability observed. This result fits nicely with findings recently reported by Haake et al. (2013), who observed that word stress processing in children with specific language impairment as well as in typically developing children is predicted by auditory processing of duration cues. Obviously, basic auditory processing performance may exert its influence not only in children, but also in healthy adults for whom the recognition and interpretation of word stress is relevant in their native language.

In sum, there was substantial interindividual variability in word stress processing. Hence, two groups were defined based

SSEG, suprasegmental.

condition (uncorrected *p <* 0*.*001, *k* = 10 voxels). IFG, inferior frontal gyrus;

*IPS, inferior parietal sulcus; LH, left hemisphere; RH, right hemisphere; SMA, supplementary motor area.*

*aMinor maximum.*

on a median split of individual accuracy results in the suprasegmental task. Neural correlates of segmental and suprasegmental processing and their interaction with group membership were investigated and will be discussed in the following.

## **NEURAL CORRELATES OF SEGMENTAL AND SUPRASEGMENTAL PROCESSING**

The conjunction analysis revealed a large cluster of activation in auditory cortex across performance levels and conditions

**FIGURE 7 | Interaction between group and condition (uncorrected** *p <* **0***.***001,** *k* **= 10 voxels).** The bar charts next to the activation figure depict the corresponding beta estimates for the respective brain region.

(cf. **Figure 4**, **Table 2**), extending from the superior temporal gyrus to the middle temporal gyrus and to the insula. This finding is highly plausible, because participants had to process auditory linguistic stimuli. More specifically, previous studies reported activation in the STG or STS for processing of prosodic information in general (e.g., Dogil, 2003; Ischebeck et al., 2008), and for processing of word stress in particular (Aleman et al., 2005; Klein et al., 2011; Domahs et al., 2013b).

In addition, activation in the bilateral supplementary motor area (with left-hemispheric peak activation within a large cluster extending into the right hemisphere) and in the bilateral inferior parietal sulcus was found. This may be related to the fact that participants had to determine either stress localization or consonant differences by button presses since the SMA has been suggested to subserve decision making (Kong et al., 2005). Additionally, a combination of working memory related BA 44 and intraparietal BA 7 activation indicated that participants had to hold the sequences of pseudowords in working memory. Moreover, bilateral activation in the precentral gyrus was observed, probably indicating motor processing associated with finger movements and button presses (Zilles and Rehkämper, 1998).

Beyond these task-related effects, cerebellum, temporal cortex, premotor cortex, preSMA/SMA and inferior frontal cortex have been described as part of a network involved in speech perception, especially engaged in the temporal processing of speech (Grahn and Brett, 2007; Kotz et al., 2009; Kotz and Schwartze, 2010).

### **SUPRASEGMENTAL vs. SEGMENTAL PROCESSING**

In the behavioral data, no correlation was observed between stress processing (suprasegmental) and consonant processing (segmental). This suggests that the linguistic abilities underlying these two conditions may be to a certain degree independent, although they were tested with a comparable paradigm in the present study.

When the suprasegmental task was contrasted to the segmental task, a subcortico-cortico-cerebellar network of brain regions was revealed, including bilateral IFG (BA44 and BA 45), bilateral insula, bilateral precentral gyrus, bilateral cerebellum, left thalamus, left pre-SMA (BA 6), right globus pallidus, and right SMA (BA 6). There is accumulating evidence, that this network is involved in processing spectro-temporal aspects of speech (Lutz et al., 2000; Lewis et al., 2004; Bengtsson et al., 2005; Riecker et al., 2006; Grahn and Brett, 2007; Coull et al., 2008; Geiser et al., 2008; Kotz et al., 2009; Kotz and Schwartze, 2011; Schwartze et al., 2012a,b, see Kotz and Schwartze, 2010, for a review). This finding seems very plausible, given that duration is the most relevant acoustic cue to word stress in German (Jessen and Marasek, 1997; Classen et al., 1998; Schneider, 2007; Schneider and Möbius, 2007; Lintfert, 2010) and performance in auditory discrimination in general and duration discrimination in particular predicts performance in the more complex task related to word stress (behavioral results of the present study, see Haake et al., 2013, for evidence from German speaking children).

More specifically, bilateral activation in the inferior frontal gyri related to the suprasegmental condition is in line with previous studies, which reported these areas to be activated in processing linguistic aspects of prosody (e.g., Wildgruber et al., 2004; Li et al., 2010; Klein et al., 2011; Domahs et al., 2013b).

Furthermore, activation in the left insula related to suprasegmental processing is consistent with previous studies, which found this area activated for auditory temporal processing (Lewis et al., 2000; Ackermann et al., 2001; Lewis and Miall, 2003), for pitch-related stimuli (Zarate and Zatorre, 2005) as well as for auditory timing perception (Geiser et al., 2008) and word stress processing proper (Aleman et al., 2005; Klein et al., 2011).

Activation in the bilateral inferior parietal sulcus may reflect the fact that participants had to store information in working memory and to respond by button presses. Possibly, they employed a spatial representation of the pseudowords (e.g., first syllable = left, last syllable = right) and of response buttons to come to the correct decision. Amongst others, the intraparietal cortex has been suggested to subserve mental imagery (Just et al., 2004). Moreover, the IPS has been frequently reported to be involved in the processing of proximity relations (see Dehaene et al., 2003 for a review). Recall that stress is an inherently relational property and requires the comparison of acoustic cues (e.g., duration, pitch, and skewness) between stressed and unstressed syllables. In the present study, the inferior parietal sulcus may be associated with mental imagery and with the evaluation of gradual differences in acoustic cues related to word stress. This might comprise positional information, which has to be encoded in the IPS and held in working memory as well as the actual comparison process of the positional information within the sequences of CV-syllables—a process also most probably associated with the intraparietal cortices (cf. Klein et al., 2011). In particular, bilateral inferior parietal cortex has been found activated in tasks tapping on suprasegmental compared to segmental aspects of words (Li et al., 2010; Klein et al., 2011).

Beyond temporal processing of speech input, activation in the supplementary motor area may be related to the fact that in general the suprasegmental task in this study was somewhat more difficult than the segmental task. The SMA has been found to support operation procedures (Kong et al., 2005). Interestingly, Domahs et al. (2013b) observed increased activation in bilateral SMA in a difficult compared to an easy condition in a word stress violation task. Moreover, SMA activation in the suprasegmental condition together with a significantly increased activation in the precentral gyrus could point to an involvement of the central motor system. Given that both the SMA and the precentral gyrus were activated bilaterally, these findings may reflect control of finger movements in participants (e.g., Shibasaki et al., 1993; Catalan et al., 1998). Possibly, participants may have needed higher control of their finger movements in the more difficult suprasegmental condition. An alternative explanation could be that in more difficult conditions participants may establish a correspondence between their fingers and the positional information of stress, for instance, by using finger counting. This would be also in line with the activation pattern observed in SMA, precentral and intraparietal areas. However, this account remains speculative so far and needs further evaluation in future studies.

## **INTERINDIVIDUAL DIFFERENCES**

The middle temporal gyrus was found activated in both conditions (segmental, suprasegmental) for both groups (cf. **Table 3**). This fits well with the fact that the MTG has been associated with phonology (Graves et al., 2010) and, more generally, with complex sound and speech processing (Scott et al., 2000). Nevertheless, poor performers showed stronger activation in this region.

Further significant changes in the BOLD signal were found in the precuneus. These findings are rather difficult to interpret since for good performers the BOLD signal in the precuneus seemed to be close to zero in both the segmental and the suprasegmental conditions (see **Figure 6B**), whereas in poor performers the precuneus was strongly deactivated in both conditions. Considering that the amplitude of the BOLD signal indicated by SPM is subject to arbitrary factors (such as the definition of the baseline), the present findings can only be interpreted in relative terms, not in terms of "activation" or "deactivation." Generally, the precuneus has been suggested not only to subserve learning of motor-sequences (Sadato et al., 1996; Sakai et al., 1998) but also to be involved in mental imagery (Dehaene et al., 1996; Huijbers et al., 2011). Possibly, good performers may have relied more on mental imagery or motor-sequence learning to solve the task correctly, compared to poor performers. Nevertheless, we are well aware of the fact that currently this explanation remains speculative.

One may conclude that both groups activated the MTG for phonological processing of stimuli in both conditions, but that poor performers required more resources. It may be speculated that good performers have used a combination of visual and auditory representations to solve the tasks, whereas poor performers only relied on auditory information (but to a higher degree). Possibly, a combination of visual and auditory processing may be advantageous.

Although native speakers of German are highly familiar with the use of suprasegmental features in their mother tongue, the present study shows that their performance in an experimental task tapping on this aspect of language may nevertheless be very heterogeneous. Until now, it was assumed that native German speakers should be "naturally" competent in word stress processing, since this is a relevant feature of their language, which is acquired early. Preverbal infants learn the typical stress pattern of their mother tongue and can use it in speech segmentation (Hoehle et al., 2009). Importantly, even those participants, who showed poor performance in the specific suprasegmental task in the present study, were competent speakers of German. Note that the stress pattern of real words is stored in the lexicon. However, in the present study, participants had to process pseudowords which by definition cannot be stored in the mental lexicon. Thus, processing word stress in everyday language requires lexical retrieval, whereas the suprasegmental task in our experiment may have required other types of prosodic knowledge (e.g., rule-based knowledge). Furthermore, every-day language is typically embedded in a redundant context, which helps in resolving ambiguities related to word stress, e.g., in the interpretation of minimal pairs. Therefore, the specific difficulties in suprasegmental processing of pseudowords observed in the present study are subclinical with no obvious impact on language use.

## **INTERACTION BETWEEN GROUP AND CONDITION**

Behaviorally, a two-way interaction of condition (segmental vs. suprasegmental) and group (below vs. above average) indicated that the good performers were numerically better in suprasegmental than in segmental processing, whereas the poor performers were significantly better in segmental than in suprasegmental processing (see **Figure 3**). Importantly, a two-way interaction of condition and group was also revealed in the neuro-functional data (see **Figure 7**, **Table 3**). Good performers showed relatively more activation (or less deactivation, respectively) in the segmental condition in the right hippocampus and cerebellum compared to poor performers, whereas poor performers revealed relatively stronger activation in these areas in the suprasegmental condition compared to good performers.

Hippocampal cells have been shown to be involved in auditory working memory in rats (Sakurai, 1990, 1994). More recently, the hippocampus has been argued to contribute to performance in a variety of cognitive tasks including working memory and perception, when these tasks require high-resolution binding of features and relational information (Yonelinas, 2013). Clearly, the sequence recall task used in the present experiment does require such a complex and demanding type of binding. Interestingly, activation in the right hippocampus was related to relative task difficulty: Poor performers seemed to need relatively more cognitive resources in the suprasegmental task (which they performed worse than the segmental task), but good performers seemed to put relatively more effort into the segmental task (which they performed worse than the suprasegmental task).

Furthermore, a similar pattern of (de-)activation was observed for the interaction in the right cerebellum. The cerebellum has been considered to be part of a network related to the processing of spectro-temporal aspects of speech (Kotz and Schwartze, 2010). The interaction in the cerebellum suggests that poor performers may have needed the cerebellum relatively more for the suprasegmental task (although achieving inferior results) than good performers. The opposite pattern was observed in the segmental condition. Again, these interpretations have to be considered very cautiously and remain speculative, because the interaction pattern consists only of different degrees of deactivation.

## **CONCLUSION AND PERSPECTIVES**

The present study is a first step toward a more comprehensive understanding of the processing of word stress. In particular, it highlights the need to examine brain activation data not only at the second level in group analyses, but also to analyze individual data at the first level. Taken together, our results provide behavioral and neuro-functional evidence for substantial interindividual differences within a group of native speakers of German, a language with variable stress, in word stress processing. They suggest that part of the behavioral variance is explained by basic auditory processing and working memory performance. It would be interesting to explore, whether speakers of a language with fixed stress (e.g., Czech, Finnish, Polish, Turkish, Persian, or French) show similar interindividual heterogeneity.

## **SUPPLEMENTARY MATERIAL**

The Supplementary Material for this article can be found online at: http://www.frontiersin.org/journal/10.3389/fpsyg. 2014.00365/abstract

## **REFERENCES**

**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.

*Received: 29 January 2014; accepted: 07 April 2014; published online: 29 April 2014.*

*Citation: Heisterueber M, Klein E, Willmes K, Heim S and Domahs F (2014) Processing word prosody—behavioral and neuroimaging evidence for heterogeneous performance in a language with variable stress. Front. Psychol. 5:365. doi: 10.3389/ fpsyg.2014.00365*

*This article was submitted to Language Sciences, a section of the journal Frontiers in Psychology.*

*Copyright © 2014 Heisterueber, Klein, Willmes, Heim and Domahs. This is an openaccess 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) or licensor 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.*

## Phonetic detail in German syllable pronunciation: influences of prosody and grammar

#### *Barbara Samlowski 1 \*, Bernd Möbius 2 and Petra Wagner 1*

*1 Work Group Phonetics/Phonology, Faculty of Linguistics and Literary Studies, Bielefeld University, Bielefeld, Germany 2 Department of Computational Linguistics and Phonetics, Saarland University, Saarbrücken, Germany*

#### *Edited by:*

*Richard Wiese, Philipps-Universität Marburg, Germany*

#### *Reviewed by:*

*Christiane Ulbrich, University of Marburg, Germany Marzena Zygis, Centre for General Research & Humboldt University, Germany*

#### *\*Correspondence:*

*Barbara Samlowski, Work Group Phonetics/Phonology, Faculty of Linguistics and Literary Studies, Bielefeld University, Universitätsstraße 25, Bielefeld 33615, Germany e-mail: barbara.samlowski@ uni-bielefeld.de*

This study presents two experiments designed to disentangle various influences on syllable pronunciation. Target syllables were embedded in carrier sentences, read aloud by native German participants, and analyzed in terms of syllable and vowel duration, acoustic prominence, and spectral similarity. Both experiments revealed a complex interaction of different factors, as participants attempted to disambiguate semantically and syntactically ambiguous structures while at the same time distinguishing between important and unimportant information. The first experiment examined German verb prefixes that formed prosodic minimal pairs. Carrier sentences were formulated so as to systematically vary word stress, sentence focus, and the type of syntactic boundary following the prefix. We found clear effects of word stress on duration, prominence, and spectral similarity as well as a small influence of sentence focus on prominence levels of lexically stressed prefixes. While sentence boundaries were marked by particularly high prominence and duration values, hardly any effect was shown for word boundaries. The second experiment compared German function words which were segmentally identical but appeared in different grammatical roles. Here, definite articles were found to be shorter than relative pronouns and still shorter than demonstrative pronouns. As definite articles are also much more common than the other two lexical classes, effects of lemma frequency might also have played a role.

**Keywords: prominence, duration, stress, syntactic boundaries, lexical class, lemma frequency**

## **INTRODUCTION**

Syllables can vary strongly in the way they are pronounced, even when in canonical pronunciation they are segmentally identical. One important source of variation is prominence, i.e., the degree of emphasis which is placed on syllables and with which they are perceived. Such emphasis may be realized by means of higher duration and intensity values, overall larger articulatory effort, as well as the presence and shape of pitch accents (Wagner, 2002). Among other things, prominence differences are used to distinguish between lexically stressed and unstressed syllables. Duration seems to be a main correlate of word stress in German, but differences were also found for formant values, fundamental frequency, and various voice quality parameters (e.g., Kohler, 1987; Claßen et al., 1998; Kleber and Klipphahn, 2006; Schneider and Möbius, 2007; Lintfert, 2010). Studies specifically investigating word stress in focused and unfocused sentence positions have confirmed duration as a strong signal of word stress which operates independently of sentence accent (Dogil and Williams, 1999 for German; Okobi, 2006 and Cho and Keating, 2009 for English; Sluijter and van Heuven, 1996 for Dutch). However, for English, Plag et al. (2011) found no effect at all of word stress on duration, while Campbell and Beckman (1997) discovered stress-related duration differences only in one of the two unaccented contexts examined. For English and Dutch, spectral tilt, i.e., the intensity in higher compared to lower frequency bands, appeared to be another robust correlate of word stress in accented as well as unaccented contexts (Sluijter and van Heuven, 1996; Okobi, 2006; Plag et al., 2011). Although Dogil and Williams (1999) found no significant differences between accented and unaccented words in German in terms of fundamental frequency, intensity, or duration, studies for other languages showed stress-related differences in fundamental frequency and intensity to be strongly reduced when target words were not accented (Sluijter and van Heuven, 1996; Plag et al., 2011). Apart from signaling sentence focus, prominence differences are also used to distinguish important from unimportant information on the level of lexical class. In German speech synthesis, lexical class has been used as an important indicator for predicting prominence levels (Widera et al., 1997; Windmann et al., 2011). Frequency and predictability effects have an influence on word pronunciation as well. There is evidence for English that words tend to be spoken at a faster rate if they are frequent or easily predictable from their context (Bell et al., 2003; Aylett and Turk, 2004; Baker and Bradlow, 2009). Although effects of word frequency and lexical class are often confounded, both factors were found to play an important role (Jurafsky et al., 2000; Pluymaekers et al., 2005; Bell et al., 2009). The present study consists of two controlled production experiments. The first experiment aims to disentangle influences of lexical stress, sentence accent, and syntactic boundaries, while the second experiment analyzes effects of lexical class and word frequency.

## **METHODS**

### **PARTICIPANTS**

Thirty participants took part in the two experiments (15 men, 15 women, ages ranging between 19 and 47). All were native speakers of German. They were paid for their participation in the study.

## **MATERIAL**

### *Experiment 1: Stress, accent, and syntactic boundaries*

Certain German verbs can differ in meaning depending on whether their lexical stress falls on the prefix or the verb stem. For example, the word [PUn.t5.Stε.l@n] ([unter]prefix−[[stell]stem−[en]ending]verb), literally "to underput") means "to store / take shelter" when stressed on the prefix, but "to insinuate" when stressed on the stem. This ambiguity is not visible in all inflections, however. In most finite forms, lexically stressed prefixes are separated from the verb and placed at the end of the clause. As the verb prefixes used for this experiment are segmentally identical to prepositions or conjunctions, we were able to use them to analyze effects of syntactic boundaries as well. We examined the effects of word and sentence stress as well as word and sentence boundaries on the production of the four German verb prefixes "um" ([PUm] – "around"), "unter" (['PUn.t5] – "under"), "über" (['Py:.b5] – "over"), and "durch" ([dUKç] – "through") in a reading task (see also Samlowski et al., 2012). The phonetic transcriptions given here are canonical. The glottal stop preceding onset vowels may be omitted or realized through vowel glottalization, and the [K] in "durch" is commonly rendered as [5].

Target items consisted of the prefixes combined with two different verb stems each. Each of the eight resulting verbs was placed in seven different carrier sentences. In sentences 1–4, word and sentence stress were varied, while sentences 5–7 compared different types of syntactic boundaries (see **Table 1**). As participants needed to be able to infer the correct stress pattern from the sentence context, a different set of carrier sentences was created for each verb. Sentence stress differences were not elicited in a uniform manner, either. While sentences belonging to the categories "w+s+" and "w−s+" were formulated so as to imply a broad focus, sentences in categories "w+s−" and "w−s−" contained elements designed to attract a contrasting focus and thereby move the sentence stress away from the main verb. Among the strategies used for this were the inclusion of two contrasting objects, topic fronting, and the addition of an emphasized modifier. For the sake of brevity in this paper we refer to the first four sentence categories in terms of stressed and unstressed prefixes ("w+" vs. "w−") in accented and unaccented conditions ("s+" vs. "s−"). Nonetheless, it is important to note that the categories do not reflect the actual stress patterns used by the participants. Instead they describe potential differences in word and sentence stress due to different word meanings and the presence or absence of an additional motivation for deaccentuating the verb. Our aim is to discover the extent to which these conceptual differences are realized in the acoustic production of the target syllables.

We deliberately decided against using underline, font style, or a question-answer structure to indicate lexical stress and sentence focus, since we wanted to avoid potentially evoking exaggerated responses by attracting the participants' attention to the intended

*Word and sentence stress status and succeeding syntactic boundary for the target items in each of the 7 sentences (manipulated parameters in bold).*

reading. This meant that the context was not controlled across verbs and only up to a limited degree within each set of sentences. For the first four sentences within one set, the half syllable preceding and following the prefix were kept constant. Sentence 5 ("sb") used the same preceding half-syllable as the first four. While the prefix in sentence 6 ("mb") fulfilled the same conditions as in sentence 3 ("w−s+"), its target sentence was formulated so that the preceding and following half-syllables matched those of the identical prepositions or conjunctions in sentence 7 ("wb").

### *Experiment 2: Lexical class and word frequency*

While different words are used for German demonstrative pronouns, relative pronouns, and definite articles, depending on gender, number, and case, these words are often segmentally identical across the three lexical classes. Definite articles are much more common than the segmentally identical demonstrative or relative pronouns. According to the DeWaC corpus (Baroni and Kilgarriff, 2006), a 1.5 billion word database of German internet articles which was automatically tagged for lexical classes, the words "der" ([de:5]), "die" ([di:]), "das" ([das]), "dem" ([de:m]), and "den" ([de:n]) were used as definite articles 89.8% of the time, while 7.2% of their appearances were classified as relative pronouns, and only 3% were demonstrative pronouns.

To examine whether these differences in frequency of occurrence have an influence on pronunciation, we compared their realizations in different grammatical roles (see also Samlowski et al., 2013). Sentences containing relative and demonstrative pronouns were formulated so as to match definite articles already appearing in one of the other carrier sentences from the two experiments. As each of the lexical classes required different types of surrounding grammatical structure, only the half-syllables preceding and following the target word were held constant across each group of 3 sentences. For each of the investigated words, 3 sentence groups were assembled (see **Table 2**), resulting in a total of 48 new sentences containing relative and demonstrative pronouns.

### **PROCEDURE**

Sentences from both experiments were placed in a quasi-random order, which was not varied across participants. Care was taken to avoid repetitions of the same verb and provide a good mixture of sentences from both experiments, allowing them to function as mutual distractors. Acoustic recordings took place in a

#### **Table 2 | Target items.**

Samlowski et al. Phonetic detail in syllable pronunciation

**FIGURE 1 | Example illustration for Experiment 1—"unterstellen" (category "w+s+").** Corresponding sentence: "Wir wollten uns *unterstellen*, weil es so stark regnet." (English: "We wanted to *take shelter* because it is raining so heavily.")

*Orthographic form, phonetic transcription, grammatical description and number of stimuli used for each word.*

sound-treated chamber at Bielefeld University. One sentence at a time was presented on a computer screen to the participants, who proceeded through the experiment in a self-paced manner. To further clarify the intended word meaning and improve understanding of the reading content, sentences were illustrated using the text-to-scene conversion program WordsEye (Coyne and Sproat, 2001, see **Figures 1** and **2**). Participants looked at each sentence and the accompanying picture and then read the sentence out loud. Beforehand, they were instructed to repeat any sentences in which they made a mistake or slip of the tongue. These sentences as well as sentences where participants hesitated noticeably while reading were omitted from analysis. Target items were also discarded if they or their immediate context was impaired through speech errors, noise, or unexpected vowel elision.

The remaining recordings were analyzed in terms of syllable and vowel duration, acoustic prominence, and spectral similarity. For the duration and prominence analysis, syllable and vowel boundaries of the target items as well as the preceding and following syllables were manually annotated with Praat (Boersma, 2001). Acoustic prominence was investigated by means of an automatic prominence tagger which analyzed annotated syllable nuclei in terms of pitch movement, duration, intensity, and spectral emphasis. Values for the last three parameters were normalized across all investigated syllables in the utterance using *z*-scores and the individual factors were weighted so as to model perceptual ratings of German prominence (Tamburini and Wagner, 2007). In the present study, only the syllables immediately preceding or following the target items were used as context for the tagger. If the vowel of a context syllable tended to be elided, the preceding/following syllable nucleus was taken as context syllable instead. We also compared pairs of segmentally identical syllables produced by the same speaker in terms of spectral similarity, using a method developed by Wade and Möbius (2007) and Lewandowski (2011). Amplitude envelopes were computed for 4 frequency bands (equally spaced on a logarithmic scale ranging

**FIGURE 2 | Example illustration for Experiment 2—"den" (rp).** Corresponding sentence: "Es war deutlich, dass der Fuchs den See beobachtete, *den* Enten als ihre Heimat gewählt hatten." (English: "It was clear that the fox was watching the lake *which* ducks had chosen as their home.")

from 80 to 7800 Hz), using a sampling rate of 500 Hz. The spectral similarity of two syllables was calculated by cross-correlating pairs of envelopes for each frequency band, taking the maximum of the cross-correlation as an indicator for the degree of similarity. Although spectral similarity is not a direct measure of vowel quality and degree of coarticulation, it can serve as an indication of how strongly the target items varied in their pronunciation across contexts and categories. Statistical analysis and visualization was performed with R (R Development Core Team, 2010). As residuals from analyses of variances only followed a normal distribution in the case of the duration results of the second experiment, the other investigations were analyzed with Wilcoxon rank sum tests. Significance values were Bonferroni-corrected for multiple comparisons.

## **RESULTS**

## **EXPERIMENT 1: STRESS, ACCENT, AND SYNTACTIC BOUNDARIES**

Of the 1680 sentences collected (8 verbs × 7 sentences × 30 participants), 113 were discarded. As two of the prefixes used are bisyllabic, the following analyses are based on a total of 2278 syllables. The results were analyzed in terms of sentence category ("w+s+," "w+s−," "w−s+," "w−s−," "sb," "mb," "wb") and syllable identity ([PUm], [PUn], [t5], [?y:], [b5], [dUKç]).

## *Duration*

**Figure 3** gives an overview of vowel duration results for the seven sentence categories examined. Wilcoxon rank sum tests

#### **Table 3 | Mean duration.**

comparing sentence categories across syllables (corrected for 21 comparisons) showed phrase-final prefixes ("sb") to be significantly longer than those in the other categories (*W >* 87*,*000, *p <* 0*.*0001). A small influence of word stress was also observed, with syllables and vowels being longer when appearing in lexically stressed compared to unstressed prefixes ("w+s+" vs. "w−s+," "w+s−" vs. "w−s−," *W >* 62*,*000, *p <* 0*.*0001). Vowel duration of lexically stressed prefixes was slightly reduced if the verb was not in the focus of the sentence ("w+s+" vs. "w+s−," *W* = 59,074, *p <* 0*.*05). Finally, there was a small tendency for prepositions or conjunctions to have slightly longer syllables and vowels than segmentally identical bound prefixes ("wb" vs. "mb," *W >* 59*,*000, *p <* 0*.*05).

Syllable and vowel durations were also analyzed for combinations of syllable identity and sentence category (corrected for 861 comparisons, see **Table 3** for mean values). All investigated syllables were significantly longer when they occurred in separated sentence-final prefixes than in other contexts ("sb" vs. others, *W >* 2400, *p <* 0*.*0001). Differences in vowel duration were significant for all syllables except [PUn]. Here, differences between separated prefixes and bound prefixes in lexically stressed and potentially accented positions ("sb" vs. "w+s+") failed to reach significance, and comparisons between separated prefixes and segmentally identical function words ("sb" vs. "wb") were significant on a lower level (*W* = 2386, *p <* 0*.*01) than the other comparisons (*W >* 2200, *p <* 0*.*0001). No significant influences were shown for word boundary ("mb" vs. "wb") or sentence stress ("w+s+" vs. "w+s−," "w−s+" vs. "w−s−"). Effects of word stress on syllable and vowel duration are summarized in **Table 4**.

### *Prominence*

Prominence estimates for the individual syllables in the seven sentence categories are shown in **Figure 4**. Wilcoxon rank sum tests for sentence categories across syllables (corrected for 21 comparisons) showed that lexically stressed prefixes tended to receive significantly higher prominence values than unstressed ones in accented as well as unaccented conditions ("w+s+" vs.

*Mean duration values in milliseconds for syllables (above) and vowels (below) in the seven sentence categories.*

#### **Table 4 | Duration statistics for lexical stress.**

*W values with significance levels for syllables (above) and vowels (below), n.s.: p* ≥ *0.05, \*p < 0.05, \*\*p < 0.01, \*\*\*\*p < 0.0001.*

"w−s+," *W* = 72*,*651, *p <* 0*.*0001; "w+s−" vs. "w−s−," *W* = 59,677, *p <* 0*.*01). Sentence stress differences were significant for lexically stressed prefixes ("w+s+" vs. "w+s−," *W* = 63,503, *p <* 0*.*0001). Separated, phrase-final prefixes were particularly high in prominence ("sb" vs. others, *W >* 76*,*000, *p <* 0*.*0001), while no effect of word boundary was observed ("mb" vs. "wb").

In tests for combinations of sentence categories and syllables (corrected for 821 comparisons) differences related to word and sentence stress mostly failed to reach significance. Word stress effects were found for [Py:] and [dUKç] in accented conditions as well as for [PUm] and [PUn] in unaccented conditions (see **Table 5**). Effects of sentence stress were only shown in the case of lexically stressed [dUKç] ("w+s+" vs. "w+s−," *W* = 2189, *p <* 0*.*0001). In separated, phrase-final prefixes, syllables often received significantly higher prominence values than in the other categories ("sb" vs. others, *W >* 1900, *p <* 0*.*05). Exceptions for this last tendency were found for [t5] ("sb" vs. "w+s+," "sb" vs. "w+s−"), [b5] ("sb" vs. "w+s+," "sb" vs. "w−s+"), and [dUKç] ("sb" vs. "w+s+," "sb" vs. "w−s−"). No significant differences appeared between bound prefixes and corresponding prepositions or conjunctions ("mb" vs. "wb") or between unstressed prefixes in accented and unaccented conditions ("w−s+" vs. "w−s−").

## *Spectral similarity*

For each target syllable in each sentence category, we calculated the level of similarity between prefixes produced by the same speaker in the two verb contexts. Wilcoxon rank sum tests comparing sentence categories across syllables (corrected for 21 comparisons) showed significant differences in syllable similarity for stressed versus unstressed prefixes in accented conditions ("w+s+" vs. "w−s+," *W* = 16*,*183*.*5, *p <* 0*.*01, mean values: 0.889 vs. 0.848). Sentence stress differences in stressed prefixes had only a marginally significant effect ("w+s+" vs. "w+s−," *W* = 14*,*189*.*5, *p* = 0*.*052, mean values: 0.889 vs. 0.864). Separated, phrase-final prefixes ("sb," mean: 0.897) received significantly higher similarity values (*W >* 16*,*000, *p <* 0*.*001) compared to all examined categories except for stressed and potentially accented prefixes ("w+s+"). Effects were most pronounced for the syllables [Py:], [d*υ*Kç], and, to a lesser extent, [PUn], although results failed to reach significance when combinations of syllables and sentence categories were investigated (corrected for 821 comparisons).

In an analysis of spectral similarity between sentence categories for syllables produced by the same speaker in the same verb context, comparisons with separated, phrase-final prefixes tended to result in lower values than comparisons between the other

*W values with significant levels for comparisons between lexically stressed and unstressed prefixes, n.s.: p* ≥ *0.05, \*p < 0.05, \*\*p < 0.01, \*\*\*\*p < 0.0001.*

sentence categories ("sb" vs. others). This effect was shown to be significant (*W >* 57*,*000, *p <* 0*.*0001) in tests for combinations of sentence categories (corrected for 210 comparisons). Lexically stressed prefixes were significantly closer to those in sentence-final prefixes than syllables in unstressed prefixes ("w+s+" and "sb" vs. "w−s+" and "sb," "w+s−" and "sb" vs. "w−s−" and "sb," *W >* 64*,*000, *p <* 0*.*001, mean values: 0.794 vs. 0.748 and 0.799 vs. 0.765). Here as well as for the comparisons within sentence categories, effects were most clearly visible for [Py:] and [dUKç]. An analysis of similarity between sentences in the "sb" category and those in the other categories combined with syllable identity (corrected for 630 comparisons) showed significant differences between stressed and unstressed [Py:] in accented conditions ("w+s+" and "sb" vs. "w−s+" and "sb," mean values: 0.816 vs. 0.709, *W* = 2552, *p <* 0*.*0001).

#### *Discussion*

Apart from Dogil and Williams (1999), there have been hardly any studies examining the interaction of word and sentence stress in German. In our paper, we examine the extent to which canonical word stress differences and additional semantic contrasts triggered differences in the word and sentence stress patterns which in turn were visible in the acoustic realization of the target syllables. Based on German language corpus studies as well as evidence from other Germanic languages, we expected lexically stressed syllables to be longer than unstressed syllables in accented as well as unaccented conditions. We also predicted an effect of word and sentence stress on acoustic prominence levels compared to the immediate surroundings. Although spectral parameters have been shown to be affected by stress, we had no clear hypotheses as to how word and sentence stress might influence similarity across and within sentence categories. Our study indeed showed a significant influence of lexical stress on duration values for all investigated prefixes apart from [PUm]. When sentences were given a broad focus, even the lexically unstressed second syllables of the prefixes ['PUn.t5] and ['Py:.b5] were affected. This result may be explained by accentual lengthening of the word carrying sentence stress, as there is evidence that in English and Dutch this effect is stronger to the right of the lexically stressed syllable than to the left (Cambier-Langeveld and Turk, 1999). There was also a tendency for stressed syllables to be higher in prominence and more similar to syllables in sentence-final prefixes than unstressed ones. When no deaccentuation cues were given, lexically stressed syllables were more similar across verb contexts than unstressed syllables. Results for prominence and spectral similarity mostly failed to reach significance in a syllableby-syllable analysis. One reason for the small size of the word stress effects might be that all investigated syllables except [Py:] had lax vowels, since these have been found to have a considerably reduced effect of lexical stress on duration (Mooshammer et al., 1999; Kleber and Klipphahn, 2006). Although there was a slight effect of sentence stress on duration and prominence values of lexically stressed syllables, it almost never reached significance in a syllable-by-syllable analysis. Although the data was not analyzed perceptually, auditory impressions suggest that participants often placed a secondary accent on the target verb in unaccented conditions—perhaps because they wanted to better clarify the intended word meaning or because the given cues were not strong enough. Particularly in the case of the verbs ['dUKç.SaU.@n] ("to look through") and ['Um.fa:.K@n] ("to run over"), effects of final lengthening might also have played a role, as these were sentence-final in the unaccented, but not in the accented conditions. The unusually strong effect of sentence stress on prominence levels for [dUKç] may have been due to the fact that ["dUKç.SaU.@n] was one of the few verbs where the potentially contrasting sentence stress in the unaccented condition would actually fall on the syllable used as preceding context by the tagger.

As was to be expected, a large effect of sentence boundary on syllable and vowel duration was observed. All examined syllables, including the first syllables of the prefixes ['PUn.t5], and ['Py:.b5], were considerably lengthened when appearing in sentence-final, separated prefixes. The results confirm findings by Kohler (1983) and Silverman (1990), according to which sentence-final lengthening extends beyond the final syllable. Effects of sentence boundary were also found for prominence and spectral similarity, although not all syllables were affected equally. The interpretation of possible word boundary effects is not straightforward. A longer duration of free words might be expected due to effects of word-final lengthening (e.g., Beckman and Edwards, 1990) or polysyllabic shortening (e.g., Turk and Shattuck-Hufnagel, 2000; White, 2002), as bound prefixes were not followed by a word boundary and therefore appeared in longer words than the corresponding prepositions or conjunctions. Also, bisyllabic items had lexical stress on the first syllable as free words, but not as bound prefixes. On the other hand, there might have been counteracting influences of word frequency and accentual lengthening, as the verbs used were generally less frequent than the matching function words and tended to attract sentence focus. In our study, syllables in bound prefixes tended to be slightly shorter than when they occurred in segmentally identical prepositions or conjunctions, with the first syllable of the bisyllabic ['PUn.t5] and ['Py:.b5] being affected more strongly than the second syllable. No influence was found for prominence and similarity values, and the word boundary effect was not significant in a separate investigation of the individual target syllables.

## **EXPERIMENT 2: LEXICAL CLASS AND WORD FREQUENCY**

Of the 2160 items recorded (8 words × 3 contexts × 3 lexical classes × 30 participants), 310 had to be omitted from the analysis. Results are based on the remaining 1850 items, which were investigated with regards to the factors lexical class ("dp," "rp," "da") and word identity ("der masc.," "der fem.," "die sg.," "die pl.," "das," "dem masc.," "dem neut.," "den").

## *Duration*

In terms of syllable and vowel duration, demonstrative pronouns tended to be slightly longer than segmentally identical definite articles, with relative pronouns usually falling somewhere in between. This trend was especially noticeable for feminine "der" as well as masculine and neuter "dem." Differences for "den," masculine "der," and singular "die" were less pronounced, while hardly any changes were observed for "das" and plural "die" (see **Table 6** for mean values). Two-Way ANOVAs were computed to examine the influence of word identity and lexical class on logtransformed syllable and vowel duration values. Significant effects (*p <* 0*.*0001) were found for word identity [syllable duration: *F*(7*,* 1824) = 147*.*8, vowel duration: *F*(7*,* 1824) = 61*.*2], lexical class [syllable duration: *F*(2*,* 1824) = 123*.*2, vowel duration: *F*(2*,* 1824) = 109*.*3], and their interaction [syllable duration: *F*(14*,* 1824) = 8*.*6, vowel duration: *F*(14*,* 1824) = 11*.*0]. Tukey's HSD tests were used to further investigate the data. In terms of syllable as well as vowel duration, significant differences (*p <* 0*.*001) were found between masculine and neuter "dem" and between masculine and feminine "der," but not between singular and plural "die." Significance

levels for the interaction between lexical class and word identity are given in **Table 7**.

## *Prominence*

Across items, prominences were higher for demonstrative pronouns than for relative pronouns and definite articles. Definite articles were minimally less prominent than relative pronouns. **Figure 5** shows results by lexical class for the individual words. Combinations of word identity and lexical class were analyzed using Wilcoxon rank sum tests (corrected for 276 comparisons, see **Table 8**). No significant differences between lexical classes were found for neuter "dem" or "den." For all other items except masculine "dem," demonstrative pronouns tended to receive higher prominence values than relative pronouns. Demonstrative pronouns were more prominent than definite articles for masculine and feminine "der" and masculine "dem." While definite articles tended to be more prominent than relative pronouns for masculine "der," singular and plural "die," and "das," an opposite trend was visible for masculine "dem."

## *Spectral similarity*

Similarity levels were computed for segmentally identical items belonging to the same lexical class and produced by the same speaker in different contexts. Across words, definite articles (mean value: 0.814) appeared to be minimally less consistent in their pronunciation than demonstrative or relative pronouns (mean values: 0.823, 0.823). The difference, however, was only significant in Wilcoxon rank sum tests (*W >* 739*,*000, *p <* 0*.*05, corrected

*Mean duration values of demonstrative pronouns (dp), relative pronouns (rp) and definite articles (da) in milliseconds for syllables (above) and vowels (below).*

*Adjusted p-values (Tukey HSD) for comparisons of syllable duration (above) and vowel duration (below) between demonstrative pronouns (dp), relative pronouns (rp) and definite articles (da).*

**Table 8 | Prominence statistics for differences in lexical class.**

*W values with significance levels for comparisons between definite pronouns (dp), relative pronouns (rp) and definite articles (da), n.s.: p* ≥ *0.05, \*\*p < 0.01, \*\*\*p < 0.001, \*\*\*\*p < 0.0001.*

for 3 comparisons) when similarities were calculated regardless of gender or class. No effects were found when word identity as well as segmental identity was controlled (corrected for 3 comparisons), or when lexical classes were compared separately for individual word identities (corrected for 276 comparisons). We also examined similarity levels between words belonging to different lexical classes (paired for speaker, word identity, and context). Here, we found a significant difference between similarity measures of relative and demonstrative pronouns on the one hand and relative pronouns and definite articles on the other (mean values: 0.861 vs. 0.850, *W* = 155,459.5, *p <* 0*.*05, corrected for 3 comparisons). In separate comparisons for individual word identities (corrected for 276 comparisons), this tendency was only confirmed for masculine "dem" (mean values: 0.893 vs. 0.831, *W* = 2651, *p <* 0*.*001).

#### *Discussion*

Definite articles were expected to have smaller duration values than segmentally identical relative or demonstrative pronouns due to effects of frequency and predictability. Not only are they much more common than the other lexical classes, the carrier sentences for the pronouns were specifically constructed to mirror the phonetic context of definite articles found in other sentences, probably increasing their artificiality and reducing the predictability of the target words. According to exemplartheoretic approaches, definite articles might also be more strongly adapted to their surroundings, which would lead to lowered spectral similarity values across contexts. However, differences in pronunciation cannot always be explained by lemma frequency, and lexical classes may vary in the degree to which they can be emphasized. For instance, Jurafsky et al. (2000) found that although the English word "that" was most commonly used as a demonstrative pronoun, it tended to be longer in this function than when it was produced as a segmentally identical relative pronoun, complement, or determiner. In order to monitor for differences in emphasis, we also analyzed the target words' level of acoustic prominence in relation to their immediate context. In our investigation, we discovered significant differences between all three lexical classes in terms of syllable and vowel duration. Although these differences were not contradictory to lemma frequency effects, they did not mirror the fact that in German, frequency differences between the two types of pronouns are minimal compared to their difference to definite articles. The comparatively high duration of demonstrative pronouns was probably due to their semantic role, as it is their function to point out and emphasize the entity to which they refer. Results for acoustic prominence confirm that participants tended to emphasize demonstrative pronouns more strongly than relative pronouns or definite articles. Contrary to our expectations, we found only minimal effects and no consistent patterns in terms of spectral similarity within and between lexical classes.

A closer examination of the data revealed that the individual target words varied in the ways and extent to which they were affected by changes in lexical class. Duration differences were most stable in comparisons between demonstrative pronouns and definite articles. Relative pronouns often tended to be closer in duration to demonstrative pronouns than to definite articles. Plural "die" showed no duration effects whatsoever, and the only significant duration effect found for "das" was a slight difference in vowel duration between relative and demonstrative pronouns. Concerning acoustic prominence, it was striking that while any significant differences between demonstrative pronouns and definite articles was accompanied by significant effects of syllable and vowel duration, several words showed prominence differences between relative pronouns and the other two categories without any corresponding duration effects. Although relative pronouns were generally longer than definite articles, prominence levels tended to be lower, with only masculine "dem" showing a significant effect in the opposite direction. Only singular "die" showed contradictory duration and prominence results which were both significant. The conflicting prominence findings may have resulted from the difficulty in controlling the context of the target items. As relative pronouns are generally used to introduce relative clauses, the syllables preceding them tended to be clausefinal and therefore subject to final lengthening. It is very likely that relative pronouns received particularly low prominence ratings by the tagger due to their relatively prominent preceding context. In the case of feminine "der," masculine and neuter "dem," and one sentence used for "den," possible context lengthening was avoided by placing the relative pronouns in prepositional phrases. For these words, there was indeed no tendency for relative pronouns to be less prominent than definite articles, and prominence differences were supported by differences in syllable and vowel duration.

### **SUMMARY**

This paper describes results from two experiments designed to disentangle various influences on syllable pronunciation in German. In the first experiment, we found clear differences due to word stress and sentence boundaries, while effects of sentence stress and word boundaries were smaller in size and less consistent across stimuli. In the second experiment, differences between segmentally identical demonstrative pronouns, relative pronouns, and definite articles were found that could be related to lemma frequency, semantic function, and sentence structure. In both experiments, duration was shown to be the most robust of the investigated cues for disambiguating word meanings. Measures of acoustic prominence added valuable information on how strongly syllables were emphasized, but also proved to be highly sensitive to differences in context. Finally, an examination of spectral similarity revealed that syllables in lexically stressed prefixes were less variable across contexts and closer in pronunciation to sentencefinal realizations than unstressed prefixes. Separate investigations of individual target syllables often failed to reach significance in terms of acoustic prominence and spectral similarity, suggesting that other influences may also have been of importance. Especially prominence and similarity measures often failed to reach significance in these detailed analyses. A larger study covering a greater number of contexts and using a separate quasi-random order of sentences for each speaker, possibly followed by a perception study to confirm the results, might lead to more robust findings.

## **ACKNOWLEDGMENTS**

This study was funded by the German Research Foundation (DFG), Priority Program 1234, grant MO 597/4. We would like to thank Natalie Lewandowski for providing the Matlab scripts used for calculating spectral similarity measures. We also thank the two anonymous reviewers for their insightful and constructive comments.

#### **SUPPLEMENTARY MATERIAL**

The Supplementary Material for this article can be found online at: http://www*.*frontiersin*.*org/journal/10*.*3389/fpsyg*.*2014*.* 00500/abstract

## **REFERENCES**

**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.

*Received: 28 January 2014; accepted: 07 May 2014; published online: 27 May 2014. Citation: Samlowski B, Möbius B and Wagner P (2014) Phonetic detail in German syllable pronunciation: influences of prosody and grammar. Front. Psychol. 5:500. doi: 10.3389/fpsyg.2014.00500*

*This article was submitted to Language Sciences, a section of the journal Frontiers in Psychology.*

*Copyright © 2014 Samlowski, Möbius and Wagner. 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) or licensor 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.*

## Use what you can: storage, abstraction processes, and perceptual adjustments help listeners recognize reduced forms

## *Katja Poellmann1,2\*†, Holger Mitterer 1† and James M. McQueen1,3*

*1 Language Comprehension Department, Max Planck Institute for Psycholinguistics, Nijmegen, Netherlands*

*2 International Max Planck Research School for Language Sciences, Nijmegen, Netherlands*

*3 Behavioural Science Institute and Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen, Nijmegen, Netherlands*

#### *Edited by:*

*Ulrike Domahs, University of Marburg, Germany*

#### *Reviewed by:*

*LouAnn Gerken, University of Arizona, USA Judith Koehne, Saarland University, Germany*

#### *\*Correspondence:*

*Katja Poellmann, Department of Speech-Language Pathology and Audiology, Northeastern University, 360 Huntington Avenue, 226 FR, Boston, MA 02115, USA e-mail<EMAIL_ADDRESS>
#### *†Present address:*

*Katja Poellmann, Department of Speech-Language Pathology and Audiology, Northeastern University, Boston, USA; Holger Mitterer, Department of Cognitive Science, University of Malta, Msida, Malta*