Cerebellar Involvement in Cognitive Sequencing

Although there is increasing recognition that the cerebellum, which contains half of the brain's neurons, is involved in cognition as well as motor function, the manner in which the cerebellum contributes to cognition is uncertain. The uniform circuitry of the cerebellum and the extensive connectivity of the cerebellum with numerous neocortical regions has suggested to some researchers that there is a common computation that the cerebellum performs for both motor and cognitive functions. The cerebellar sequencing hypothesis posits that the cerebellum acquires sequence information, makes sequence predictions, and detects sequence violations. These functions, executed via a forward model, could underlie cerebellar involvement in both motor and cognitive behavior. In motor control, such predictions can be used to guide limb trajectory without reliance on movement-generated sensory feedback. In cognition, sequencing requirements are prominent in both verbal working memory (VWM) and language acquisition; eg, in VWM, keeping a phone number in mind requires encoding and rehearsing a sequence of digits. In language, words consist of sequences of syllables, and the learning of syllable transition probabilities is an important component of recognizing legal words in a language. Importantly, prominent cerebellar activation has been observed in many functional MRI (fMRI) VWM and language studies. However, the brainstem/cerebellar neural correlates of sequencing in cognition, and the influence of cerebellar sequence predictions on neocortical targets, are poorly understood. In this experiment, studying healthy individuals the investigators will Investigate the cerebellum as a source of sequence prediction and its influence on forebrain areas. The cerebellum is hypothesized to provide its forward model sequence prediction to forebrain targets, but to date no study has attempted to visualize with concurrent TMS/fMRI the consequences of disrupting this cerebellar input on forebrain activation. In the investigators previous work, the investigators show that transcranial magnetic stimulation (TMS) during the rehearsal of a sequence of letters results in errors in determining if a probe letter matches the next letter in the sequence, suggesting that TMS disrupted this predictive input. In accordance with this finding, the investigators hypothesize that, using concurrent TMS/fMRI, TMS disruption during a sequencing task will produce greater changes in neocortical activation relative to an analogous control task that does not have the predictive component brought out by sequencing demands. The investigators further hypothesize that different patterns of neocortical activations in response to cerebellar TMS will be observed depending on whether a probe letter matches the expected next letter in a sequence.