Simulation de l'amusie dans le cerveau normal

It is estimated that approximately 1.5% of the population has a perceptual deficit that interferes with the development of normal musical abilities. This disorder is commonly referred to as congenital amusia, and is characterized by important difficulties in perceiving music and discriminating between pitches, despite the absence of any other perceptual (e.g. hearing loss) or cognitive (e.g. intellectual disability, speech delay) impairments. The core deficit in amusia consists in a lack of conscious access to processed pitch deviances, as reflected by the absence of typical electrical brain responses to deviants. Anatomical and functional anomalies found in the right fronto-temporal network of amusic individuals point to a breakdown in communication between the inferior frontal gyrus and the auditory cortex as being responsible for the manifestation of amusia. However, no study has systematically investigated the potential causal role played by the fronto-temporal network in the behavioral and electrophysiological expression of the disorder. The first goal of the thesis was to investigate whether it was possible to induce transient characteristics of amusia in the normal brain, by deactivating frontal and temporal regions during the detection of pitch deviations. To do so, we applied cathodal (inhibitory) transcranial direct-current stimulation (tDCS) over right frontal and temporal regions during separate sessions. We also recorded participants’ electrophysiological (EEG) activity before and after tDCS stimulation, while they performed a pitch change detection task. The present findings indicate that it is possible to induce EEG changes when processing small pitch deviants, which mimic electrophysiological markers of amusia. They also support the involvement of the right fronto-temporal network in the processing of small pitch deviants and its role in the manifestation of amusia. A recent meta-analysis highlighted the presence of a double dissociation between acoustic and musical processing in the amusic population, which suggests that amusia is characterized by heterogeneous phenotypes. The processing of incongruous notes in a melody requires the representation of the musical structure in addition to pitch discrimination abilities, and it is currently unclear whether any brain areas preferentially process pitch violations in musical contexts in comparison to non-musical contexts. The second goal of the thesis was to address this issue. Using functional magnetic resonance imaging (fMRI), we measured the brain activity of normal participants while they completed a pitch-change detection task in a series of non-melodic repeated tones and in tonal melodies. A contrast between brain activation patterns elicited by deviants in the melodic task and deviants in the non-melodic task revealed greater activity in the right inferior parietal lobule, which likely represents working memory processes related to the maintenance of tonal pitch structure during pitch discrimination. Taken together these findings add to our understanding of the brain anomalies underlying congenital amusia, and motivate future research that could lead to the development of neuromodulation-based interventions that could help individuals with neurodevelopmental disorders such as amusia to improve their deficits.

Keyphrases: Congenital amusia, Transcranial direct-current stimulation, Electroencephalography, Functional magnetic resonance imaging, Right fronto-temporal network, Inferior parietal lobule, Event-related potentials