On the neurobiology of beat induction: sensorimotor and drive mechanisms

Neil Todd

University of Manchester, UK

In this paper I present a theoretical interpretation of recent electrophysiological experiments concerned with the brain mechanisms underlying beat induction (Todd and Siess, 2004; Todd et al. 2002). The interpretation includes the operation of (1) an auditory-motor timing mechanism, accounting for the basic pulse sensation, and (2) a vestibular-mesolimbic drive mechanism which accounts for the compulsion for beat inducing rhythms to be bassy and loud. The timing mechanism involves two distinct sensorimotor circuits: (a) an automatic, internally driven circuit involving supplementary motor area (SMA) and (b) an attention dependent, externally driven circuit involving posterior parietal cortex (PPC). Both circuits are co-activated during the presentation of a rhythmic stimulus but the relative role of the two depends on the predictability of the rhythm. When a rhythm is isochronous or highly regular little attention is required to maintain an internal pulse or active synchronization and automated circuits likely involving basal ganglia and cerebellum can deal with the required processing. For more complex rhythms involving syncopated or unexpected events an attention dependent externally guided mechanism comes into play. The vestibular drive mechanism involves a primitive projection from the otolith organs, which are sensitive to loud low-frequency sound and vibration, to the basal ganglia and prefrontal cortex via the parabrachial nucleus and medial forebrain bundle.