Losing the beat: brain and behavior basis of human rhythm

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A defining characteristic of our interactions with music is the ability to identify and move to the “beat”. This complex ability is universal and spontaneously acquired early in life. Yet, as we have recently shown, some individuals fail to find the beat in music and to synchronize with it, despite having normal hearing, normal motor and intellectual abilities. This disorder, to which we refer as beat deafness (a new form of congenital amusia), provides a natural experiment, a rare chance to examine how a selective cognitive deficit emerges and how it is expressed in the brain. The goal of this proposal is to characterize the functional and neural anomalies of beat deafness. To this aim, we will recruit and test groups of abnormal (beat deaf) and control adults. Beat-deaf adults will be selected according to the same criteria as those used to diagnose our first case. Its essential behavioral manifestation is poor synchronization of body movements to selections of dance music, as measured by motion capture. The disorder appears specific to rhythm because it spares melodic pitch processing. Hence beat deafness is distinct from the most common form of congenital amusia, pitch deafness that affects pitch processing. The comparison of beat-deaf and pitch-deaf performance will serve to fractionate beat-based entrainment while controlling for impoverished musical experience. In PART I of the proposal, we will examine how deficits in beat-tracking can shed light on characteristics of human beat-based entrainment. Typically, people tap in advance of the beat to eliminate the time lag introduced by sensorimotor reaction. Such anticipatory tendencies are considered to be unique to humans. We will test here whether beat-deaf individuals are more reactive or exhibit stronger influences of preferred endogenous rates in anticipatory tapping to metronome-like stimuli as compared to control and pitch-deaf performance. In PART II, we will test the music-specificity of entrainment mechanisms. We predict that normal participants should tap less variably and with fewer asynchronies to song than to regular speech. In contrast, beat deaf individuals should be equally poor whether they tap on natural, regular speech or songs, and pitch deaf performance will show the opposite pattern with best performance on regular speech since songs contain interfering pitch variations. In PART III, neural entrainment to music will be measured with a novel electrophysiological (EEG) technique that captures beat-related steady-state evoked potentials, following the idea that natural brain oscillations can entrain to the periodicities of the auditory stimulus. In the brains of beat deaf listeners, the evidence for the presence of an internal representation of the beat should be weak. An interesting alternative is that the EEG reveals normal neural entrainment in listening but not in tapping. Such a finding would suggest that the deficit is not perceptual but arises from poor auditory-motor coupling. Finally, in PART IV, we will search for brain anomalies with several Magnetic Resonance Imaging (MRI) techniques that have proven to be informative in pitch deafness. Beat deafness represents a rare chance to identify which parts of the neural network (from the basal ganglia to the dorsal premotor cortex) are essential for entrainment. By combining these diverse neuropsychological methods, this project should provide theoretical insight on the neurobiological basis of human rhythm.