 much as a four-fold increase in reported duration. Specifically, longer INMI episodes were reported in a diary study requiring more detailed reports of the experience in comparison with a second study using a shorter report form, indicating a possible tendency to prioritize reporting longer INMI episodes over more fleeting experiences in the first study.
Methodological progress in INMI research has largely been achieved by adapting existing methods that have parallels in the involuntary thought and memory research traditions (see Hyman et al., 2015). These methods have proven that self-reporting can produce systematic, retest reliable results regarding INMI (Cotter, Christensen, & Silvia, 2016; Floridou et al., 2015). This was elegantly illustrated in a study that showed how results based on self-reports can be used to identify individual differences in cortical structure (Farrugia, Jakubowski, Cusack, & Stewart, 2015). Empirical studies have also produced novel methods for inducing and capturing INMI. For instance, recording INMI by tapping to its beat (Jakubowski, Bashir, Farrugia, & Stewart, 2018; Jakubowski et al., 2015) has provided strong evidence on the similarities between INMI, voluntary imagery, and perception.
The biggest remaining methodological gap concerns the sole reliance on self-report data. Unfortunately, the direct detection of the INMI experience—for instance, from brain or motor system activation—has not yet been demonstrated as feasible, even though certain aspects of musical imagery have been reliably observed in brain imaging (Kraemer, Macrae, Green, & Kelley, 2005). Given the findings of voluntary musical imagery research (Halpern & Zatorre, 1999; Halpern, Zatorre, Bouffard, & Johnson, 2004), it is predicted that INMI may reveal similar activations in auditory and frontal cortices to voluntary musical imagery (see Hemming & Merrill, 2015), as well as patterns previously witnessed in involuntary memory studies (Hall, Gjedde, & Kupers, 2008). In addition, recent work has shown that activity in laryngeal muscles recorded using surface electromyography may be an objective indicator of voluntary musical imagery (Pruitt, Halpern, & Pfordresher, 2019), suggesting that such methods may be promising for INMI research as well.
An additional point of methodological concern is the nonreplication of certain findings within the literature. For instance, one study (Beaman & Williams, 2013) found that several detailed results deviated from their prior work—for instance, a failure to identify any individual differences factors to explain the frequency of INMI reports (Beaman & Williams, 2010). Such anomalies may result from the fact that few studies have used standardized measures to quantify INMI and its phenomenology. It appears that small changes in a survey’s wording may quickly render single-item responses incomparable across studies. This is made more complicated by the fact that not all studies have fully disclosed the instructions, survey instruments, and experience sampling forms they have used to manipulate and measure INMI. Fortunately, a standardized survey instrument for INMI has recently been proposed (Floridou et al., 2015) and already adopted by other researchers (see Cotter et al., 2016; Farrugia et al., 2015), which may help to address this source of methodological variation. This survey utilizes scales rather than individual items, which further reduces variation in the measured dimension. It is also important that self-report instruments do not presume an either–or relationship between even closely related activities of music listening and imaging, as both may occur simultaneously (Bailes, 2007).
The studies considering phenomenology examined questions on INMI’s existence: What are the experiences like in quantitative and qualitative terms? The dynamics of INMI theme refers to research that investigates how INMI experiences emerge, are maintained, or terminate; such investigations are typically made possible through experimental manipulations. A further step is taken in studies of individual differences, which attempt to reveal systematic differences in the phenomenology or dynamics of INMI between people. The fourth included theme is a distinct question on the importance of the features of the music in evoking INMI.
A quantitative assessment of the research themes shows that phenomenology is by far the most studied theme, followed by dynamics (see Fig. 4). The remaining two themes were covered in around one quarter of the studies. The following sections synthesize the findings under each theme.
Studies of INMI phenomenology describe the characteristics and subjective experience of INMI. These descriptions have been predominantly quantitative and aggregated, with only a few exceptions describing individual experiences (Brown, 2006; Hemming & Merrill, 2015; Williamson & Jilka, 2014).
Four measures have been widely adopted to describe INMI: frequency, duration, veridicality, and subjective pleasantness. Frequency is the measure that appears in most publications. The simplest frequency measure is a binary indication of whether an INMI episode has been experienced or not.
Several studies indicate that people across the English-speaking world recognize INMI and spontaneously and publicly discuss their INMI experiences (Liikkanen et al., 2015; Williamson et al., 2012). Similar evidence can be found in French, German, and Finnish language-speaking countries (Hemming, 2008; Liikkanen, 2012c; McNally-Gagnon, 2016).
In a study of Twitter users’ INMI experiences, Liikkanen et al. (2015) collated 80,620 short social media messages known as tweets (see http://twitter.com/about) using a keyword search to isolate possible INMI-related content. Twenty-eight percent of the data were classified as reports of ongoing or past INMI experiences. These tweets came from 173 countries, indicating that English-speaking people across the world spontaneously report INMI experiences.
The most comprehensive accounts of INMI frequency are based on retrospective reporting. Liikkanen (2012c) reported that INMI was experienced at least every week by 92% of the Finnish respondents in a large, Internet-based sample (N = 11,910), as shown in Fig. 5. No large-scale studies have since tried to replicate this finding (see, e.g., Floridou et al., 2015; Müllensiefen et al., 2014, for smaller scale replications).
ESM studies also support the idea of frequent INMI experiences. The prevalence of episodes occurring at the time of probe ranges from 17% in a study of general musical imagery experiences (Beaty et al., 2013) to 47% in a study of INMI in particular (Floridou & Müllensiefen, 2015). This gives further credibility to the claims about the commonality of the phenomenon, at least among Western people.
Estimating INMI duration via retrospective self-report methods can be problematic, and thus confident reports on this topic are few. In a novel instance of INMI duration measurement in an experimental setting (Moeck, Hyman, & Takarangi, 2018), participants were asked to hold down the computer keyboard space bar throughout the duration of any INMI episodes that they experienced, releasing it at the end of each episode. Individual INMI episodes lasted less than 10 seconds on average, and multiple episodes accumulated during a 5-minute thought-monitoring period lasted a mean duration of 44 seconds altogether.
Diary studies have reported INMI episodes lasting from under a minute to a median or average duration of 8, 27, or 36 minutes (Beaman & Williams, 2010; Halpern & Bartlett, 2011). Retrospective studies provide even longer duration estimates, from hours to days (Beaman & Williams, 2010). For instance, Hyman et al. (2015) have reported “a few hours” as the modal response for INMI duration. This indicates that experimental, diary, and survey methods result in different estimates of INMI duration.
INMI has several features in common with voluntary musical imagery. One of these shared features is the high faithfulness of musical images. This has been a repeated finding in both in-depth qualitative studies (Brown, 2006; Williamson & Jilka, 2014) and quantitative research (Jakubowski et al., 2018; Jakubowski et al., 2015). Williamson and Jilka (2014) state that musical imagery is frequently “comparable to an actual music listening experience” (p. 667), and INMI experiences can be described as simplified versions of the canonical recording. In a study of the general musical imagery experiences of music students, Bailes (2007) reported higher vividness ratings for melody and lyrics over other musical features. She mentions the limitations of the human voice in reproducing timbre or texture, in comparison with pitch or words, as a possible explanation for this difference in the representation of musical features.
Going beyond self-report measures, the veridicality of INMI has also been behaviorally verified. The tempo of INMI songs has been investigated using an innovative technique of recording self-caught INMI episodes (Jakubowski et al., 2018; Jakubowski et al., 2015). In both studies, participants wore wristband accelerometers. They were instructed to use the associated hand to tap to the beat when they experienced INMI and record the details of the INMI incident in a diary. The tempi of INMI episodes were calculated from the tapping data, and it was found that INMI tempo deviated only 15% on average from canonical, recorded versions of the music (Jakubowski et al., 2015; see Fig. 6). These results were replicated in a subsequent study, which revealed that INMI did not significantly differ from voluntary musical imagery for the same songs in terms of veridicality of tempo recall (Jakubowski et al., 2018).
Research shows that the subjective assessment of INMI pleasantness depends on at least two factors: the features of the experience and the mode of inquiry.