Synchronisation rythmique déficiente chez l'humain : bases comportementales

Humans across cultures show the propensity to move in time with the beat of music. Despite the spontaneous nature of this behavior, it is one that requires the abstraction of periodicities from acoustically complex signals and the temporally precise coupling between auditory perception and motor action. It is therefore not surprising that some individuals exhibit difficulties to synchronize movements with the beat of music; a few such cases have been reported in recent years. Importantly, their lack of synchronization seems to be attributable to a disorder specific to beat processing, as these individuals did not present deficient intelligence, motor or hearing disabilities, or deficient pitch-related musical processing in previous studies.   The aim of this thesis was to clarify the nature of this beat synchronization impairment, as a way to shed light on theories of beat processing in humans. As a first step, we used ecological conditions - natural movements and commercially available music - to screen for synchronization deficits in a large pool of young adults. This approach revealed a gradient of synchronization difficulties across movement types and stimulus complexity. While some individuals displayed an inability to bounce their body and clap their hands with all stimuli, others failed to bounce to a few songs only.    Next, we studied a group of nine individuals ("beat-impaired") with deficient synchronization across tapping and bouncing movement forms. We tested their capacity to detect a misalignment of a superimposed metronome soundtrack to the beat of music. We found lower performance for this detection task in the beat-impaired group as compared to typical adults. This association of perception and production deficits is in line with a motor theory of beat finding, according to which beat perception depends on neural oscillations in motor regions of the brain. Nevertheless, we observed deficient synchronization in six beat-impaired cases who were able to perceive the beat normally, albeit in the low range. The opposite pattern, i.e. low but normal synchronization performance in spite of deficient detection, was observed in two other participants initially from the typical group. These dissociations question the motor theory of beat finding, albeit not strongly so, because the eight cases performed poorly in general. Our results thus point towards a central origin of beat impairments, which affects both perception and synchronization.   In line with this view, we also showed that deficient synchronization to a musical beat is linked to faulty timekeeping core mechanisms. In this last study, we used two tapping conditions: spontaneous production of a regular sequence and synchronization-continuation with metronome stimuli covering the range of predictive timing in humans (225-1709 milliseconds). By testing eight of the nine beat-impaired cases from the previous study, we found higher temporal variability across conditions and poorer rate flexibility in paced tapping, as compared to a group of closely matched control participants.   Altogether, our findings highlight an association between beat perception and synchronization impairments, which may emerge from disrupted timekeeping functions.

Keyphrases: Beat, Music, Beat, Synchronization, Movement, Timekeeping, Sensorimotor Impairments