The "ticktock" of our internal clock: direct brain evidence of subjective accents in isochronous sequences

Implicit memory in music and language

Research on music and language in recent decades has focused on their overlapping neurophysiological, perceptual, and cognitive underpinnings, ranging from the mechanism for encoding basic auditory cues to the mechanism for detecting violations in phrase structure. These overlaps have most often been identified in musicians with musical knowledge that was acquired explicitly, through formal training. In this paper, we review independent bodies of work in music and language that suggest an important role for implicitly acquired knowledge, implicit memory, and their associated neural structures in the acquisition of linguistic or musical grammar. These findings motivate potential new work that examines music and language comparatively in the context of the implicit memory system.

The integration of stimulus dimensions in the perception of music

A central aim of cognitive psychology is to explain how we integrate stimulus dimensions into a unified percept, but how the dimensions of pitch and time combine in the perception of music remains a largely unresolved issue. The goal of this study was to test the effect of varying the degree of conformity to dimensional structure in pitch and time (specifically, tonality and metre) on goodness ratings and classifications of melodies. The pitches and durations of melodies were either presented in their original order, as a reordered sequence, or replaced with random elements. Musically trained and untrained participants (24 each) rated melodic goodness, attending selectively to the dimensions of pitch, time, or both. Also, 24 trained participants classified whether or not the melodies were tonal, metric, or both. Pitch and temporal manipulations always influenced responses, but participants successfully emphasized either dimension in accordance with instructions. Effects of pitch and time were mostly independent for selective attention conditions, but more interactive when evaluating both dimensions. When interactions occurred, the effect of either dimension increased as the other dimension conformed more to its original structure. Relative main effect sizes (| pitch η(2) - time η(2) |) predicted the strength of pitch-time interactions (pitch × time η(2)); interactions were stronger when main effect sizes were more evenly matched. These results have implications for dimensional integration in several domains. Relative main effect size could serve as an indicator of dimensional salience, such that interactions are more likely when dimensions are equally salient.

Musicians and the Metric Structure of Words

The present study aimed to examine the influence of musical expertise on the metric and semantic aspects of speech processing. In two attentional conditions (metric and semantic tasks), musicians listened to short sentences ending in trisyllabic words that were semantically and/or metrically congruous or incongruous. Both ERPs and behavioral data were analyzed and the results were compared to previous nonmusicians' data. Regarding the processing of meter, results showed that musical expertise influenced the automatic detection of the syllable temporal structure (P200 effect), the integration of metric structure and its influence on word comprehension (N400 effect), as well as the reanalysis of metric violations (P600 and late positivities effects). By contrast, results showed that musical expertise did not influence the semantic level of processing. These results are discussed in terms of transfer of training effects from music to speech processing.

Perception-production relationships and phase correction in synchronization with two-interval rhythms

Two experiments investigated the effects of interval duration ratio on perception of local timing perturbations, accuracy of rhythm production, and phase correction in musicians listening to or tapping in synchrony with cyclically repeated auditory two-interval rhythms. Ratios ranged from simple (1:2) to complex (7:11, 5:13), and from small (5:13 = 0.38) to large (6:7 = 0.86). Rhythm production and perception exhibited similar ratio-dependent biases: rhythms with small ratios were produced with increased ratios, and timing perturbations in these rhythms tended to be harder to detect when they locally increased the ratio than when they reduced it. The opposite held for rhythms with large ratios. This demonstrates a close relation between rhythm perception and production. Unexpectedly, however, the neutral "attractor" was not the simplest ratio (1:2 = 0.50) but a complex ratio near 4:7 (= 0.57). Phase correction in response to perturbations was generally rapid and did not show the ratio-dependent biases observed in rhythm perception and production. Thus, phase correction operates efficiently and autonomously even in synchronization with rhythms exhibiting complex interval ratios.

Decomposing rhythm processing: electroencephalography of perceived and self-imposed rhythmic patterns

Perceiving musical rhythms can be considered a process of attentional chunking over time, driven by accent patterns. A rhythmic structure can also be generated internally, by placing a subjective accent pattern on an isochronous stimulus train. Here, we investigate the event-related potential (ERP) signature of actual and subjective accents, thus disentangling low-level perceptual processes from the cognitive aspects of rhythm processing. The results show differences between accented and unaccented events, but also show that different types of unaccented events can be distinguished, revealing additional structure within the rhythmic pattern. This structure is further investigated by decomposing the ERP into subcomponents, using principal component analysis. In this way, the processes that are common for perceiving a pattern and self-generating it are isolated, and can be visualized for the tasks separately. The results suggest that top-down processes have a substantial role in the cerebral mechanisms of rhythm processing, independent of an externally presented stimulus.

Listening strategy for auditory rhythms modulates neural correlates of expectancy and cognitive processing

A recently described auditory tempo perception paradigm revealed individual differences in perceived stimulus timing for identical stimulus sequences. The current study takes advantage of this paradigm by recording event-related potentials (ERPs) concurrent with task performance in order to reveal brain responses that reflect individual differences in timing strategy. No strategy-related differences were observed in sensory encoding of tones, as measured by the P1-N1-P2 complex. However, the contingent negative variation (CNV) leading up to the final tone of the sequence varied as a function of strategy, as did a parietal-maximum late positive component (P3b) that occurred following the final tone. These data suggest that temporal expectancy for and cognitive processing of the final tone of rhythmic sequences underlies differences in strategy during rhythm perception.

Brain lateralization of metrical accenting in musicians

The perception of meter, or the alternation of strong and weak beats, was assessed in musically trained listeners through magnetoencephalography. Metrical accents were examined with no temporal disruption of the serial grouping of tones. Results showed an effect of metrical processing among identical standard tones in the left hemisphere, with larger responses on strong than on weak beats. Moreover, processing of occasional increases in intensity (phenomenal accents) varied as a function of metrical position in the left hemisphere, but not in the right. Our findings support the view of a relatively early, left-hemispheric effect of metrical processing in musicians.

Event-related Brain Potentials Suggest a Late Interaction of Meter and Syntax in the P600

Many studies refer to the relevance of metric cues in speech segmentation during language acquisition and adult language processing. However, the on-line use (i.e., time-locking the unfolding of a sentence to EEG) of metric stress patterns that are manifested by the succession of stressed and unstressed syllables during auditory syntactic processing has not been investigated. This is surprising as both processes rely on abstract rules that allow the building up of expectancies of which element will occur next and at which point in time. Participants listened to metrically regular sentences that could either be correct, syntactically incorrect, metrically incorrect, or doubly incorrect. They either judged syntactic correctness or metric homogeneity in two different sessions. We provide first event-related potential evidence that the metric structure of a given language is processed in two stages as evidenced in a biphasic pattern of an early frontal negativity and a late posterior positivity. This pattern is comparable to the biphasic pattern reported in syntactic processing. However, metric cues are processed earlier than syntactic cues during the first stage (LAN), whereas both processes seem to interact at a later integrational stage (P600). The present results substantiate the important impact of metric cues during auditory syntactic language processing.

Structural decomposition of EEG signatures of melodic processing

In the current study we investigate the EEG response to listening and imagining melodies and explore the possibility of decomposing this response according to musical features, such as rhythm and pitch patterns. A structural model was created based on musical aspects and multiple regression was used to calculate profiles of the contribution of each aspect, in contrast to traditional ERP components. By decomposing the response, we aimed to uncover pronounced ERP contributions for aspects of the encoding of musical structure, assuming a simple additive combination of these. When using a model built up of metric levels and contour direction, 81% of the variance is explained for perceived, and 57% for imagined melodies. The maximum correlation between the parameters found for the same melodic aspect in perception vs. imagery was 0.88, indicating similar processing between tasks. The decomposition method is shown to be a novel analysis method of complex ERP patterns, which allows subcomponents to be investigated within a continuous context.

Feeling the beat: premotor and striatal interactions in musicians and nonmusicians during beat perception

Little is known about the underlying neurobiology of rhythm and beat perception, despite its universal cultural importance. Here we used functional magnetic resonance imaging to study rhythm perception in musicians and nonmusicians. Three conditions varied in the degree to which external reinforcement versus internal generation of the beat was required. The "volume" condition strongly externally marked the beat with volume changes, the "duration" condition marked the beat with weaker accents arising from duration changes, and the "unaccented" condition required the beat to be entirely internally generated. In all conditions, beat rhythms compared with nonbeat control rhythms revealed putamen activity. The presence of a beat was also associated with greater connectivity between the putamen and the supplementary motor area (SMA), the premotor cortex (PMC), and auditory cortex. In contrast, the type of accent within the beat conditions modulated the coupling between premotor and auditory cortex, with greater modulation for musicians than nonmusicians. Importantly, the response of the putamen to beat conditions was not attributable to differences in temporal complexity between the three rhythm conditions. We propose that a cortico-subcortical network including the putamen, SMA, and PMC is engaged for the analysis of temporal sequences and prediction or generation of putative beats, especially under conditions that may require internal generation of the beat. The importance of this system for auditory-motor interaction and development of precisely timed movement is suggested here by its facilitation in musicians.

Perceiving rhythm where none exists: event-related potential (ERP) correlates of subjective accenting

Previous research suggests that our past experience of rhythmic structure in music results in a tendency for Western listeners to subjectively accent equitonal isochronous sequences. We have shown in an earlier study that the occurrence of a slightly softer tone in the 8th to 11th position of such a sequence evokes a P300 event-related potential (ERP) response of different amplitudes depending on whether the tone occurs in putatively subjectively accented or unaccented sequence positions (Brochard et al., 2003). One current theory of rhythm processing postulates that subjective accenting is the result of predictive modulations of perceptual processes by the attention system. If this is the case then ERP modulations should be observed at an earlier latency than the P300 and these should be observed in ERPs to both standard and softer tones. Such effects were not observed in our previous study. This was possibly due to the use of a linked-mastoid reference which may have obscured lateralized differences. The aim of the present study was to replicate the previous auditory P300 subjective accenting findings and to investigate the possibility that these effects are preceded by ERP changes that are indicative of rhythmic modulation of perceptual processing. Previous auditory P300 findings were replicated. In addition and consistent with current theories of rhythm processing, early brain ERP differences were observed both in standard and deviant tones from the onset of the stimulus. These left lateralized differences are consistent with a rhythmic, endogenously driven, modulation of perception that influences the conscious experience of equitonal isochronous sequences.

Anticipation is the key to understanding music and the effects of music on emotion

There is certainly a need for a framework to guide the study of the physiological mechanisms underlying the experience of music and the emotions that music evokes. However, this framework should be organised hierarchically, with musical anticipation as its fundamental mechanism.

Evidence of beat perception via purely tactile stimulation

Humans can easily tap in synchrony with an auditory beat but not with an equivalent visual rhythmic sequence, suggesting that the sensation of meter (i.e. of an underlying regular pulse) may be inherently auditory. We assessed whether the perception of meter could also be felt with tactile sensory inputs. We found that, when participants were presented with identical rhythmic sequences filled with either short tones or hand stimulations, they could more efficiently tap in synchrony with strongly rather than weakly metric sequences. These observations suggest that non-musician adults can extract the metric structure of purely tactile rhythms and use it to tap regularly with the beat induced by such sequences. This finding represents a challenge for present models of rhythm processing.

Rhythm and Beat Perception in Motor Areas of the Brain

When we listen to rhythm, we often move spontaneously to the beat. This movement may result from processing of the beat by motor areas. Previous studies have shown that several motor areas respond when attending to rhythms. Here we investigate whether specific motor regions respond to beat in rhythm. We predicted that the basal ganglia and supplementary motor area (SMA) would respond in the presence of a regular beat. To establish what rhythm properties induce a beat, we asked subjects to reproduce different types of rhythmic sequences. Improved reproduction was observed for one rhythm type, which had integer ratio relationships between its intervals and regular perceptual accents. A subsequent functional magnetic resonance imaging study found that these rhythms also elicited higher activity in the basal ganglia and SMA. This finding was consistent across different levels of musical training, although musicians showed activation increases unrelated to rhythm type in the premotor cortex, cerebellum, and SMAs (pre-SMA and SMA). We conclude that, in addition to their role in movement production, the basal ganglia and SMAs may mediate beat perception.

Hearing a melody in different ways: multistability of metrical interpretation, reflected in rate limits of sensorimotor synchronization

Music commonly induces the feeling of a regular beat (i.e., a metrical structure) in listeners. However, musicians can also intentionally impose a beat (i.e., a metrical interpretation) on a metrically ambiguous passage. The present study aimed to provide objective evidence for this little-studied mental ability. Participants were prompted with musical notation to adopt different metrical interpretations of a cyclically repeated isochronous 12-note melody while tapping in synchrony with specified target tones in the melody. The target tones either coincided with the imposed beat (on-beat tapping) or did not (off-beat tapping). An adaptive staircase method was employed to determine the fastest tempo at which each synchronization task could be performed. For each metrical interpretation, a significant advantage for on-beat over off-beat tapping was obtained - except in a condition in which participants, instead of synchronizing, were in control of the target tones. By showing that a self-imposed beat can affect sensorimotor synchronization, the present results provide objective evidence for endogenous perceptual organization of metrical sequences. It is hypothesized that metrical interpretation rests upon covert rhythmic action.

Infant music perception: domain-general or domain-specific mechanisms?

We review the literature on infants' perception of pitch and temporal patterns, relating it to comparable research with human adult and non-human listeners. Although there are parallels in relative pitch processing across age and species, there are notable differences. Infants accomplish such tasks with ease, but non-human listeners require extensive training to achieve very modest levels of performance. In general, human listeners process auditory sequences in a holistic manner, and non-human listeners focus on absolute aspects of individual tones. Temporal grouping processes and categorization on the basis of rhythm are evident in non-human listeners and in human infants and adults. Although synchronization to sound patterns is thought to be uniquely human, tapping to music, synchronous firefly flashing, and other cyclic behaviors can be described by similar mathematical principles. We conclude that infants' music perception skills are a product of general perceptual mechanisms that are neither music- nor species-specific. Along with general-purpose mechanisms for the perceptual foundations of music, we suggest unique motivational mechanisms that can account for the perpetuation of musical behavior in all human societies.

Gamma-band activity reflects the metric structure of rhythmic tone sequences

Relatively little is known about the dynamics of auditory cortical rhythm processing using non-invasive methods, partly because resolving responses to events in patterns is difficult using long-latency auditory neuroelectric responses. We studied the relationship between short-latency gamma-band (20-60 Hz) activity (GBA) and the structure of rhythmic tone sequences. We show that induced (non-phase-locked) GBA predicts tone onsets and persists when expected tones are omitted. Evoked (phase-locked) GBA occurs in response to tone onsets with approximately 50 ms latency, and is strongly diminished during tone omissions. These properties of auditory GBA correspond with perception of meter in acoustic sequences and provide evidence for the dynamic allocation of attention to temporally structured auditory sequences.

The role of melodic and temporal cues in perceiving musical meter

A number of different cues allow listeners to perceive musical meter. Three experiments examined effects of melodic and temporal accents on perceived meter in excerpts from folk songs scored in 6/8 or 3/4 meter. Participants matched excerpts with 1 of 2 metrical drum accompaniments. Melodic accents included contour change, melodic leaps, registral extreme, melodic repetition, and harmonic rhythm. Two experiments with isochronous melodies showed that contour change and melodic repetition predicted judgments. For longer melodies in the 2nd experiment, variables predicted judgments best at the beginning of excerpts. The final experiment, with rhythmically varied melodies, showed that temporal accents, tempo, and contour change were the strongest predictors of meter. The authors' findings suggest that listeners combine multiple melodic and temporal features to perceive musical meter.

The influence of metricality and modality on synchronization with a beat

The great majority of the world's music is metrical, i.e., has periodic structure at multiple time scales. Does the metrical structure of a non-isochronous rhythm improve synchronization with a beat compared to synchronization with an isochronous sequence at the beat period? Beat synchronization is usually associated with auditory stimuli, but are people able to extract a beat from rhythmic visual sequences with metrical structure? We addressed these questions by presenting listeners with rhythmic patterns which were either isochronous or non-isochronous in either the auditory or visual modality, and by asking them to tap to the beat, which was prescribed to occur at 800-ms intervals. For auditory patterns, we found that a strongly metrical structure did not improve overall accuracy of synchronization compared with isochronous patterns of the same beat period, though it did influence the higher-level patterning of taps. Synchronization was impaired in weakly metrical patterns in which some beats were silent. For the visual patterns, we found that participants were generally unable to synchronize to metrical non-isochronous rhythms, or to rapid isochronous rhythms. This suggests that beat perception and synchronization have a special affinity with the auditory system.