Intracortical Sources and Surface Topography of the Motor Potential and Somatosensory Evoked Potential in the Monkey

This chapter discusses the intracortical sources and surface topography of the motor potential and somatosensory evoked potential in the monkey. The neural sources and functional significance of the components of the human motor potential (MP) have been inferred principally from their timing and scalp topography. In the absence of direct intracerebral recordings, however, the scalp data may be misleading because of the complex spatial relations of the MP generators that include the cortex forming the banks of the central sulcus. MP components reflecting efferent and afferent processes have positive or negative polarities at the cortical surface, depending on the specific cellular population involved. They also overlap in time, and are volume conducted to the scalp in a direction determined by the orientation of their sources. As a consequence, some potential summate and others cancel, yielding a “resultant” morphology and scalp distribution. Since it is unlikely that the extensive intracranial recordings necessary to elucidate these interactions can be performed in man, one examines the sources and surface topography of the MP in monkeys trained to perform self-initiated wrist extension movements.