The supplementary motor area in motor and perceptual time processing: fMRI studies

Supplementary motor area and presupplementary motor area: targets of basal ganglia and cerebellar output

We used retrograde transneuronal transport of neurotropic viruses in Cebus monkeys to examine the organization of basal ganglia and cerebellar projections to two cortical areas on the medial wall of the hemisphere, the supplementary motor area (SMA) and the pre-SMA. We found that both of these cortical areas are the targets of disynaptic projections from the dentate nucleus of the cerebellum and from the internal segment of the globus pallidus (GPi). On average, the number of pallidal neurons that project to the SMA and pre-SMA is approximately three to four times greater than the number of dentate neurons that project to these cortical areas. GPi neurons that project to the pre-SMA are located in a rostral, "associative" territory of the nucleus, whereas GPi neurons that project to the SMA are located in a more caudal and ventral "sensorimotor" territory. Similarly, dentate neurons that project to the pre-SMA are located in a ventral, "nonmotor" domain of the nucleus, whereas dentate neurons that project to the SMA are located in a more dorsal, "motor" domain. The differential origin of subcortical projections to the SMA and pre-SMA suggests that these cortical areas are nodes in distinct neural systems. Although both systems are the target of outputs from the basal ganglia and the cerebellum, these two cortical areas seem to be dominated by basal ganglia input.

Repetitive TMS of cerebellum interferes with millisecond time processing

Time processing is important in several cognitive and motor functions, but it is still unclear how the human brain perceives time intervals of different durations. Processing of time in millisecond and second intervals may depend on different neural networks and there is now considerable evidence to suggest that these intervals are possibly measured by independent brain mechanisms. Using repetitive transcranial magnetic stimulation (rTMS), we determined that the cerebellum is essential in explicit temporal processing of millisecond time intervals. In the first experiment, subjects' performance in a time reproduction task of short (400-600 ms) and long (1,600-2,400 ms) intervals, were evaluated immediately after application of inhibitory rTMS trains over the left and right lateral cerebellum (Cb) and the right dorsolateral prefrontal cortex (DLPFC). We found that rTMS over the lateral cerebellum impaired time perception in the short interval (millisecond range) only; for the second range intervals, impaired timing was found selectively for stimulation of the right DLPFC. In the second experiment, we observed that cerebellar involvement in millisecond time processing was evident when the time intervals were encoded but not when they were retrieved from memory. Our results are consistent with the hypothesis that the cerebellum can be considered as an internal timing system, deputed to assess millisecond time intervals.