Epicortical electrical mapping of motor areas in primates

The neocortical motor areas of monkeys were discovered by epicortical electrical stimulation. Ferrier in 1875 drew non-overlapping circles for face, limbs and tail. His 'centres' were postcentral as well as precentral. In the 1880s Horsley added an arm and face area on the medical surface (where Penfield's supplementary motor area is now ensconced). At the turn of the century the newborn science of cortical architectonics discovered striking differences between precentral and postcentral areas. Sherrington got no responses from the postcentral gyrus of apes. At mid-century, however, Woolsey reinstated the postcentral motor map (his Sm 1). The discrepancies between these classical maps could probably be explained by sensitivity to levels of anaesthesia and to arbitrarily chosen configurations of stimuli whose intracortical actions remained obscure. Some of the modes of action of epicortical stimulation have since been worked out. These are relevant to the results of electrical and magnetic stimulation of the human brain through scalp and skull. Today's research is differentiating the functions of the precentral and postcentral areas (4, 6PM, 6SMA, 3a, 3b, 1): at the microscopic level, by studies of sampled neurons whose discharges can be related to specific components of learnt motor performances, and whose connectivities can be traced by electroanatomical and microscopical labelling of their perikarya, axons and synapses; at the macroscopic level, by studies of Bereitschaftspotential and regional cerebral metabolism in motor performances in intact humans.