SOMATOSENSORY-EVOKED MAGNETIC-FIELDS ARISING FROM SOURCES IN THE HUMAN CEREBELLUM

Somatosensory evoked neuromagnetic activity of human cerebellum was re corded noninvasively with a 122-channel whole-scalp magnetometer. Cere bellar source areas activated 13-19 ms after unilateral electric stimu lation of the median nerve. The first signals preceded those occurring in the primary sensorimotor cortex at around 20 ms and overlapped in time with the activation of thalamic sources. The orientation and loca tion of most prominent cerebellar activation suggest that the detected signals represent synchronized postsynaptic activity of spinocerebell ar cortex. These signals are probably elicited by the first afferent s ensory volley from peripheral nerve endings and mediated by spinocereb ellar (cuneocerebellar) tracts. The results imply strong coherent acti vation of cerebellar neuronal populations after purely sensory stimula tion. Moreover, with presented methods the millisecond-scale temporal resolution of neurophysiological measurements can be more generally ap plied to the study of neuronal population activity in intact human cer ebellum.