Eye movement research has attracted increasing interest in recent decades as a fruitful approach to studying cognitive factors underlying domain expertise, including eye movements during music reading. Indeed, this approach to visual expertise research is well suited to the act of music reading, for a number of reasons. First, musical symbols can roughly be said to have motor counterparts. This enables the researcher to verify that each to-be-read symbol is being processed throughout the course of eye-movement recording, at least to the extent that it is correctly performed, as in studies of typing or reading aloud. This detailed, ongoing verification of the reading process is not achievable in many other natural visual tasks, such as silent reading of text or viewing of complex images. Second, as fluent music-reading and instrumental skills are not typically taught in general schooling but music is a profession for some, it is possible to identify performers with different levels of domain expertise, from novice to expert. Third, as a universally used system, Western music notation is not restricted by language borders. In addition, the impact of this work extends beyond academia: music-reading skill is relevant for both professionals and amateurs (and, indeed, for their teachers). As such, the research is of wide potential interest and has clear practical implications.

When reading music, our eyes do not move linearly across the musical score. Instead, and as with all visual processing, the reading consists of short moments when our eyes are somewhat still, called fixations, and rapid shifts between these ‘stops’, called saccades. In practice, during one fixation, we only see a few note symbols accurately and everything else in our visual array remains blurred. With a saccade we then move our area of accurate vision to fixate on the next note symbols, and to see them clearly. (For more information on eye movements, see [1, 2].)

It is now generally acknowledged that we only gain visual information during fixations and suppress it during the very fast saccades [2]. Thus, research of cognitive factors involved in visual tasks often studies the duration, location, order, and timing of fixations. In their review in 2008, Madell and Hébert set the average fixation duration during music reading at 200-400 ms [3]. More recently, however, Penttinen, Huovinen and Ylitalo [4] and Arthur, Khuu and Blom [5] reported slightly higher average durations (500-700 ms), and, overall, it seems likely that fixation durations are greatly affected by task- and performer-related factors. There is also large variability in the case of a single performer and a performance: Goolsby [6], for instance, noted that the fixation durations of one singer varied from 99 to 1640 milliseconds during one single performance.

To roughly summarize recent published work on music reading, eye movements are affected both by ‘top-down’ and ‘bottom-up’ factors (e.g., [4, 5, 7, 8, 9, 10, 11]). Not surprisingly, then, reading depends in practice both on ‘who is reading’ and ‘what is being read’. Regarding the former issue, increased expertise seems to have the overall effect of reducing average fixation durations and increasing fixation frequency during music reading [3, 4] (but see also Performance Conditions section below). This finding aligns with the reported actions of experts in some other domains [12]. Expertise may also result in an increase in how much the eyes are ahead of the ongoing performance [3, 11]. This distance, called the eye-hand span, has been suggested to average roughly around 1 s [4, 13].

However, less can be said about the issue of ‘what is being read’. One obvious drawback of previous work on music reading is that a focus on the effects of general expertise has directed attention away from the effects of the musical stimuli; indeed, in their review, Madell and Hébert [3] called for more work on the eye-movement effects of stimulus features. We can say that groups of note symbols seem to be processed, at least on occasion, as visual chunks (e.g. [7, 14]) and that violating melodic or harmonic expectations (by asking a musician to perform something that feels ‘wrong’ according to musical convention) causes performers to adjust their reading at the level of eye movement [4, 8, 15].

Overall, and even despite the early start by Jacobsen [16] and Weaver [17], this line of research is still at an early stage. It is therefore understandable that the field lacks methodological coherence and has yet to establish any standard approach. This absence of systematic research settings in this narrow field of study unfortunately hampers comparison and generalization of the scattered findings at any level of detail. In a growing area of research, with much to do and little to build on, we argue that a more detailed review of methodological choices in previous studies would be of benefit to researchers in formulating research questions and positioning new work, all in the interest of establishing a more systematic research tradition.

The aim of this review is to support the crafting of more well-founded research hypotheses and the more systematic design of experiments in future work on music reading. To this end, we will review, in some detail, methodological choices in eye-tracking studies of music reading from 1994 to the present day, focusing on (a) choice of performed music, (b) performance conditions, (c) performers’ musical expertise, and (d) handling of performance and eye-movement data. In each section, we discuss how these choices may have affected interpretation of the studies’ findings and alignment, and we offer recommendations for increasing the field’s coherence. We focus on studies of ‘sight-reading’ during a musical performance (i.e. reading at first sight) (see also Performance Conditions section below) or reading with varying amounts of prior exposure to the performed material. We ignore non-performance music-reading tasks (sometimes called ‘silent reading’; see, [18]). In addition to lacking motor components, silent-reading tasks make quite different cognitive demands on the reader (e.g. note or chord identification, error detection), compared to reading music while performing.

The papers selected for this review had to fulfill a number of criteria. First, they had to be published in 1994 or after, and available in 2017. Year 1994 marked the slow but evident growth of interest in this topic, following publication of Goolsby’s two seminal papers in Music Perception: An Interdisciplinary Journal. Second, the papers had to be published in peer-reviewed journals and written in English. Third, papers had to include a task involving music reading and simultaneous musical performance, (i.e. singing, tapping rhythms, or playing an instrument). Through search engines and author contact, 15 publications were identified that met these criteria (Table 1).

To begin, we focus on the first issue mentioned above:‘what is being read’. Music notation can provide the performer with a wealth of information on the music in question; typically, the central elements are rhythm, melody, and harmony, along with other additional information (see 8-bar excerpt in Figure 1). Rhythms—that is, the lengths of individual notes and the patterns of their durational relationships—are implied by the stems, flags and heads of individual note symbols, which are then positioned between the vertical bar lines according to the given meter (see marking “3/4” in Figure 1). Rhythm relates to motor planning; in Figure 1, for instance, a pianist needs to make six successive key presses in bar one, whereas in bar three, only one chord— three simultaneously played notes—is performed. The melody—the succession of pitch heights—is reflected in the horizontal locations of concurrent note heads; in Figure 1, the melody first ascends slightly and then starts to descend after measures 3 and 4. Harmony is presented by groups of simultaneously performed notes or by chord symbols placed above the staff lines (see Figure 1), and additional information is given in textual form (e.g. instructions to perform the piece “vividly” or “slowly”), or by symbols. In Figure 1, the “8va” and dotted line signal that the whole sequence is actually performed one octave higher than where it is written, and the symbol below measures 5 and 6 indicate that the music should be played with decreasing loudness toward the end of the melody. Phrasing, signalled by note-binding arches as in Figure 1, has several meanings. A pianist regards the phrasing in measure 1 in Figure 1 as a guide for binding the notes as much as possible (more of an expressive guideline), whereas for a violinist the arch is also a signal for choosing the bowing for the measure, and for a clarinettist to use one single blow to perform it. In the final measure in Figure 1, the note-binding arch means that the last of the notes is not played, but the duration of the previous note is lengthened by the latter note’s duration.

In general, a performer tends to focus his or her gaze on note symbols or expressive markings that are relevant for motor execution, avoiding, for instance, vertical bar lines [6, 19, 20]. Fixations do not always land exactly on the note symbols, however; it seems to suffice to fixate close enough to a symbol to have it within in the area of accurate vision (see, e.g [19]). For the same reason, groups of notes may be inspected with single fixations [4, 6, 14]. In Figure 1, for example, it is likely that each of the three pairs of eighth-notes (joined with vertical beams) in bar one would be fixated on only once, as would the three-note chord in bar three. Importantly, written music only on occasion gives information about how to actually execute the note symbols. Instead, the motor protocol (which finger to use on a keyboard next, or which string and finger to use on the violin) needs to be either practiced beforehand or decided on the fly while performing.

Researchers have opted for one of the following two main approaches in terms of selecting performed music for their studies: the Natural Approach, where musicians are invited to perform authentic pieces, or the Experimental Approach, with specifically designed musical tasks. When applying the first of these (Table 2a), the focus of the studies has been in addressing global differences in eye movements during music reading with respect to the amount of visual information in the notated pieces [7, 21], performers’ skill levels and their perceptual and/or eye-hand spans [13, 20, 1013,] or the presence or absence of auditory models and/or fingerings [9]. To be sure, when studying expert-like music reading, the Natural Approach creates a more ecologically valid performing situation in which experts can use their domain knowledge and plan their motor responses to the stimuli exactly as they would ‘ordinarily’ do. This approach is very fitting for descriptive purposes—that is, when pointing out general pattern-like differences between reading by experts and novices, or when piloting and experimenting for future studies.

In the Experimental Approach (Table 2b), focus has been on the eye-movement effects of violating melodic and harmonic expectations [4, 8, 15], unusual visual layout [5, 8], or on the very basic reading mechanisms explored with extremely simple musical tasks [11, 14, 19, 22]. With simple tasks, the leading idea has been to keep some factors of the stimuli constant and only vary one: for instance, Kinsler and Carpenter [14] only asked their performers to tap rhythms, whereas Penttinen and Huovinen [22] and Huovinen et al. [11] created melodies where all notes were of the same duration (see also [19]).

In reviewing the findings of all these studies in parallel, the great variability in the stimuli and lack of consistency in creating them presents an obvious challenge; but this is especially so in the case of studies involving authentic music. As Figure 1 demonstrates, Western music notation is a complex symbolic system, where each note provides information about rhythm, melody, and harmony. These ‘chunks’ of information then form more or less conventional sequences and, in turn, still larger ‘chunks’ or patterns (at least for experts in this domain) (cf. [26]). For this reason, the lack of control over the visual information in the musical scores makes it impossible, in practice, to say what characteristics of the score may have caused the observed effects and why the experts or the novices read it as they did. How would we know whether those differences were an effect of musical expertise alone, or of the melodic or rhythmic elements of the music, or of a slightly less typical harmonic progression, or of difficulty in motor execution, or of a combination of some or all of these elements? We can only note differences; without a baseline understanding of the effects of various stimulus features in guiding eye movements, we cannot fully explain them.

Thus, the Natural Approach makes comparison across pieces challenging. In Goolsby’s [6, 21] studies, the one-staff stimuli contained not only note symbols but textual information and other types of markings referring to temporal and expressive features of the music. Similarly, in Wurtz et al.’s [7] study, violinists were given detailed information on bowing in one piece (signaled by note-binding arches) but not in the other (which, for the violin, means that each note is performed with its own bow movement). Comparing, for instance, average fixation durations across pieces with such differing amounts and types of information guides us at only a very general level. Another issue (as discussed later) is whether all performers actually focus on and/or execute all the instructions provided in the score.

The amount of information provided in the studies varies to the extent that, in some cases, pianists were required to read from two-staff systems, meaning that the music is written separately for the right and left hand (Tables 2a and 2b). As Weaver [17] noted in his early study, a two-staff system prompts vertical eye movements in skipping from one staff to the other (for illustrations, see [13]). Naturally, adding a staff often also adds to the visual information the performer must process and execute. Other studies employing one staff of music (as in Figure 1) eliminated the need to coordinate reading and performing from two parallel staves. These experiments studied either singers or violinists (who typically read only one-staff systems), or asked pianists to perform with only their right or left hand (Tables 2a and 2b).

As an example of the range of all this variation with respect to performed music and its visual layout, studies have investigated eye-hand span when performing a professional-level sonata accompaniment [10] or modified Bach chorales written for piano on two [20], one-staff tasks such as playing complex violin pieces [7], simple Bartok piano melodies performed with only the right or left hand [19], or a one-hand piano performance of a children’s song [4]. In addition, the length of music material varied in these studies from three to 30 bars, presenting the performers with very different conditions for the study of ‘looking ahead’. With short stimuli, the longest advance inspections (although very long ones seem somewhat rare) simply cannot occur. All this permits only broad overall comparisons between results, rather than a full meta-analysis of eye-hand span and the factors affecting it.

At the other extreme, analyses of eye-movement data based on the Experimental Approach (Table 2b) are of course, affected by their simplicity. Here, musicians do not need to perform at their maximum capacity. This relaxation of visual-motor challenges seems equally likely to affect the reading — especially for highly skilled performers (about selection of musical material for performers of different skill levels, see Performers’ Expertise Levels section below). However, following Madell and Hébert [3], we argue that to formulate hypotheses on expert-like behavior that go beyond the most general and advance this field of research, it will be necessary to devote greater attention to the systematic selection of stimuli when building research settings. Understanding the effects of the most basic features of music notation on the targeting and timing of eye movements seems essential before combining these observations with the effects of expertise, added visual elements, violation of musical expectations in complex settings, or even the distribution of attention between two staves.

To be sure, tasks designed according to the Experimental Approach can be quite far away from every-day music-making. It is therefore important to keep in mind that these simplified tasks are not the ‘actual’ targets of study: the findings we are after are not about the size of the eye-hand span in a certain task and for particular groups of performers, even though these may be the results of single experiments. Instead, we wish to move, one step at a time, toward understanding the process of transforming read note symbols into motor activity—and with musical meaning. One way to proceed is to systematically revisit previously studied stimuli under different conditions, or to modify or contrast them. This systematic commentary of tasks applied in prior research would aid in gradually moving toward the use of more complex musical stimuli. The work of Kinsler and Carpenter [14] on rhythm reading or of Penttinen and Huovinen [18] and Huovinen et al. [11] on reading of large melodic intervals (i.e. large “skips” between two consecutive pitch heights) may serve as useful points of departure for building an understanding of the effects of these music-structural features on eye movement. Their stimuli could quite easily be re-tested as well as complemented: melody could be added to the rhythms of Kinsler and Carpenter, and different rhythm patterns to the two other studies.

Having decided on the appropriate stimuli in accordance with a set of research questions, two key issues to be considered with regard to performance conditions are: the time allowed for completing the task and whether performers should be allowed to familiarize themselves with the music before the performance.

With regard to performers’ skill levels, our review indicates that three approaches have dominated earlier work; either one group of performers has been selected as representing (presumably skilled) performers or participants have been divided into groups, based on their musical background or, more specifically, on their sight-reading skill. In the first category, studies applying what we refer to as the Skilled-Only Approach (see Table 3a) have examined one group’s reading of authentic material [7, 10] or of more experimental performance tasks [4]. In practice, the focus has often been on the effects of certain stimulus characteristics, although the interpretation of the findings has been hindered by a lack of control of stimuli and study conditions.

Papers reporting the use of what we call the Sight-Reading Skill Approach (see Table 3b) focus on performers whose overall performance ability and musical background is assumed to match but who differ in terms of their sight-reading ability. In other words, these studies specifically study between-group differences but among trained musicians. (Again, however, the reader is reminded of the different definitions of ‘sight-reading’ in these studies; see previous section.) In the studies by Goolsby [6, 21] and Gilman and Underwood [20], participants were selected according to background criteria, and their sight-reading skills were pre-tested. The internal coherence of these groups supported the creation of hypotheses, ensuring that observed differences were due to effects of sight-reading skill rather than, for instance, performance (motor) abilities. In Gilman and Underwood [20], the highest grade level was used as a general reference point (see also [5]). Along with Goolsby [6], these studies illustrate the importance of separately assessing sight-reading and performance skills. Clearly, even among these high-level performers, there are still great differences in sight-reading skills. Unfortunately the failure to fully control tempo in these studies meant that better sight-readers were quicker in performing tasks. Having established this, the same sampling approach could be used in modified research settings.

The ‘Musical Background Approach’ represents the most typical way of addressing skill differences in empirical studies of expertise. Here, performers with differing levels of musical expertise were invited to participate (Table 3c). Musical background was typically established by means of background questionnaires, and some studies reported post hoc checks on performance duration or accuracy in experimental tasks [21, 22]. However, these studies varied considerably in approach, especially in their definition of ‘less-skilled’ performers, who ranged from complete musical novices to ‘novices’ with little training, and from ‘non-experts’ with some prior training to students minoring in music education (see Table 3c). As in the Skilled-Only Approach, it is therefore somewhat challenging to assess performance levels across participants in the different studies. For instance, the ‘non-experts’ in Drai-Zerbib et al. [9] and Arthur et al. [5] may share more similar backgrounds than the ‘active pianists’ who were sole representatives of music readers in Hadley et al. [15] (Table 3a). More standardized pre-performance and sight-reading tests would aid comparison of these findings, as would more systematic vocabulary for describing participants.

For all studies involving participants with differing performance or sight-reading abilities, the selection of musical stimuli is undoubtedly a significant issue. Furneaux and Land [13], for instance, resolved this issue by presenting participants with pieces that matched their skill level, but this meant that stimuli were completely different across the three skill-based groups. In other studies, less skilled performers and/or sight readers have been made to struggle through tasks that were too challenging for them. For example, in Goolsby’s [6] illustrative case study, it was apparent that the poorer sight singer (who could barely perform the tasks at all) was unable to process all the information while the skilled sight singer performed the melody and the expressive and temporal markings with greater accuracy. It seems likely, then, that with such differences in sight-singing skills and outputs, the material was not even used in the same manner by the two readers. Gilman and Underwood [20] also report significant data loss in terms of performance accuracy, especially in their study 2 (see Table 3b). It remains unclear whether the skill-based groups of prior studies that produced very different performance outcomes were performing the ‘same’ tasks; while some excelled in expressive interpretation, others struggled to get through.

To overcome these difficulties, some Musical Background Approach studies used stimuli that were simple enough to be performed correctly even by less-skilled performers (see [4, 22]), as did some studies applying the Skilled-Only Approach [11, 14, 15]. Here, the idea is to examine performances that are as similar as possible, minimizing performance errors. Naturally, again, the task is easier for some than for others, but at least the outputs are similar in terms of the performed music.

In reviewing earlier studies and planning for future work, two further issues seem important: the quality and handling of performance data and eye movement data.

In this review, we have discussed the methodological aspects of recent eye-tracking research in the domain of music and noted potentially fruitful next steps to increase the field’s coherence and systematicity. In particular, the review focuses on choices of performed music, the conditions under which it is performed (e.g. controlled tempo and music-reading protocol), performers’ levels of musical expertise and, finally, the handling of performance errors and eye-movement data for analysis.

While important progress has undoubtedly been made in many respects, there remains a clear need to ask and answer research questions concerning the basic elements of a music-reading task before embarking on more complex research designs where potential effects are blurred by other as yet unidentified factors. In particular, the effects of performance tempo have only rarely been addressed in a controlled way [10, 11, 13], and information generally remains scarce on the effects of most of the basic elements of music notation, including rhythm, melody, harmony and the placement of music on two staves. The differing definitions of ‘sight-reading’ suggest a need for separate study of initial encounters, where music is performed without prior exposure, and rehearsed readings (see for example [6, 21]). Importantly, we should also distinguish these acts by name (for instance, ‘sight-reading’ and ‘rehearsed reading’) [26]. In relation to eye movements, musical expertise (the defining of which should be more consistent) and performance tempo may well be more intertwined with the musical stimuli than has been thought and research settings and analytical choices should be created so that such complexities can be addressed (see [11]). Finally, the role of motor planning, which seems likely in particular to affect the need to ‘look ahead’ while reading, is only hinted at in studies asking participants to perform something ‘odd’ or ‘surprising’ and has not yet been systematically investigated. The fact that symbols must be executed at a given tempo is what makes music reading so interesting as a visual-motor task.

With a slightly more complete sense of the role of such characteristics, we could begin to explore in more detail the relation of sight reading and rehearsed reading to silent reading of music notation and other types of visual ‘reading’ (such as text or code reading), and to bridge studies about visual expertise in music with work done elsewhere on the performer-related characteristics affecting the music-reading skill (e.g. [30, 31, 32]). With respect to eye movements and the learning of music-reading skill, there is almost nothing but open questions; some studies do address the repeated reading and thus the learning of particular musical material [6, 10, 13, 14, 21], but the variability between the studies and lack of control in their designs hinder the making of strong conclusions. In addition, there is almost a complete lack of studies about beginners, as only Penttinen and Huovinen [22] have reported a data set that focused on ‘true’ novices in training. We should also keep in mind that there is still plenty of scope for more lenient, descriptive takes on this topic, creating research settings accordingly. Qualitative information gained in this way (as for instance in Goolsby’s [6] case studies) would help in formulating research hypotheses that could later be tested by a stricter statistical approach. No one researcher can tackle all these issues; thus, the benefits of a systematic, collaborative, and multidisciplinary study seem numerous.

Given the recent increase in research interest, we now have the box for the music-reading puzzle, but as yet, it contains only a few pieces. At this early stage, we have a wonderful opportunity to work towards a more coherent paradigm, in which research teams employ similar eye-movement measures and methods of analysis to build systematically on stimuli tested by others. Ideally, technical choices (related, for instance, to eye trackers and algorithms for defining fixations) would also converge. In pursuing those goals, the minimum requirement for now is to carefully report the detail of applied research designs; although this review has focused on the most basic elements of experimental studies (stimuli, task, participants, and data analysis), such details were not always provided in the reviewed papers. Precise descriptions of method and openness about successes and failures of choices made seem essential if other research teams are to learn from and build on each other’s work.