Topographical localization in the motor cortex of the cat for somatic afferent responses and evoked movements

Microwires chronically implanted in the pericruciate cortex of free-to-move cats were used to record extracellularly from cortical neurones and to deliver intracortical stimulation. Natural stimulation of cutaneous and/or deep mechanoreceptors in limbs and trunk evoked discharges in 89% of 165 neurones, 57% of which were pyramidal tract neurones. Out of 112 cells with receptive fields on the contralateral forelimb, 41% had cutaneous fields, 29% had fields involving deep tissues and 30% were driven from both sources. Cutaneous receptive fields were much commoner than deep ones among cells with fields including the forefoot; this relationship was reversed for cells with more proximal fields. Many more cells had distal than proximal fields. The 'zones' of the forelimb (i.e. foot, wrist, elbow, shoulder) provided input to widespread and overlapping cell populations within the coronal gyrus and the lateral parts of the anterior and posterior sigmoid gyri. Despite the overlap a somatotopy existed with successively more distal limb zones represented successively further laterally in the pericruciate area. Intracortical stimulation (eleven cathodal pulses, duration 0.2 ms, frequency 330 Hz, intensity 35 microA or less) evoked flick movements of the contralateral limbs which were abolished by pyramidectomy. In the forelimb, shoulder movements were commonest and elbow, wrist and digits were represented with decreasing frequency. Both for 35 microA and for threshold stimulation the distributions of the effective electrodes revealed an overlapping somatotopy such that the wrist movements were almost restricted to the coronal gyrus and shoulder movements were most often evoked from the lateral part of the anterior sigmoid gyrus. The movement and receptive field somatotopies overlapped heavily but the former showed a distinct lateral shift relative to the latter. As a result shoulder movements were not uncommonly evoked from the coronal gyrus although the shoulder provided almost no input to cells in that area.