Musical Expertise and rhythm processing

For decades, researchers have been trying to understand how the human mind/brain processes rhythm and time in general. Within this framework, many studies have explored the influence of long-term musical training on the neural and behavioral correlates of rhythm processing. Some pieces of evidence point to enhanced rhythm processing in musicians as one of the consequences of the structural and functional changes in many brain areas involved in auditory processing, motor synchronization and cognitive control. Yet there is still more controversy than consensus on this field. Indeed, several behavioral and neural studies report opposite results and describe contrasting effects associated to rhythm perception in musicians and non-musicians. The aim of the project described in this thesis was to shed new light on the effects of long-term musical training on the behavioral and neural correlates of rhythm processing. First, I addressed whether musical expertise influences rhythm processing when this is not task-relevant. Next, I expanded the investigation to the ability of musicians to orient efficiently attention in time. I explored these questions by looking at behavioral and electroencephalographic (EEG) correlates associated to the detection of auditory deviant stimuli. Chapter 1 is a general introduction to the current knowledge on rhythm processing. After a description of the most influential theories of temporal elaboration, it introduces some of the electrophysiological correlates associated to regularity violation. Then, it provides a detailed description of the neural and behavioral changes triggered by a long-term musical training, focusing on rhythm processing. At the end of this introduction, the aims and hypotheses of each experiment are presented in detail. In Chapter 2, I describe two behavioral experiments that explored how the processing of different temporal structures (rhythmical, non-rhythmical) influences the detection of deviant stimuli (Experiment 1), and addressed the role of a refined metrical representation in musicians and non-musicians (Experiment 2). The results revealed an overall superior performance of musicians in all experimental conditions, thus pointing to an enhanced auditory perception as consequence of their musical training. The results also highlight a large-scale processing of rhythm, independent of musical expertise. Furthermore, the results on response speed indicate a refined metrical processing only in musicians. Thus, the first part of the thesis demonstrates that long-term musical training boosts meter processing, whereas (some form of) rhythm processing appears to be overall present in all individuals. In Chapter 3, I report an EEG experiment which was aimed at exploring the effects of long-term musical training on the neural correlates of auditory deviance violation (Mismatch Negativity – MMN and Middle Latency Responses - MLRs), by inserting deviant stimuli in rhythmical (at strong and weak metrical positions) and non-rhythmical structures. Deviant stimuli within rhythmical structures elicited larger MMN compared to non-rhythmical ones in all participants. Moreover, the MMN was also modulated by meter as showed by the smaller amplitude for deviants at strong than weak positions. Interestingly, a deeper investigation of the neural modulations associated to the strong positions revealed a stronger response in musicians than non-musicians. This demonstrates the prevailing effect of stimulus salience (strong metrical positions and frequency deviant at these positions) over the effect of prediction for musicians. Finally, effect of long-term musical training modulated the MLRs for deviants within rhythmical structures. Taken together, these results indicate an effect of musical expertise at early and late stages of deviance perception, as evidenced by modulations of the MLR and MMN responses. Chapter 4 reports the results of a cross-modal cueing experiment aimed at investigating the influence of long-term musical training on the ability to orient attention in time using external cues. Besides behavioral and ERP responses, here I examined a particular neural response associated to entrainment: the steady state evoked potential (SS-EP). The results showed that auditory cues greatly facilitate attention orienting in time. Furthermore, targets preceded by short intervals were highly expected and this was visible both at the behavioral (high efficiency and more anticipations) and neural (larger CNV and reduced P300 amplitude) levels. Effects of musical expertise were present only in behavioral data and only when considering the mostly trained sensory modality (audition). Finally, musicians were less synchronized to the rhythm than non-musicians (reduced SS-EPs). In sum, these results indicate that the auditory modality better guide temporal orienting than the visual one, and that this effect is magnified for musicians. Finally, weaker synchronization to rhythm in musicians may mirror the ease with which they process rhythm. Chapter 5 is a summary of the main results and of their interpretation.