CORTICAL UNIT ACTIVITY FOLLOWING TRANSCALLOSAL VOLLEYS

Abstract

The response of 216 cortical units evoked by contralateral homologous cortical stimulation has been examined in cats anesthetized with chloralose; 191 of the units discharged during the surface transcallosal response (TCR). Ninety per cent of the units were in the suprasylvian cortex. The mean latencies of initial discharge ranged from 1 msec. to more than 100 msec. The units were found at all cortical depths and no significant correlation between depth and latency was observed. A small number of units believed to be antidromically discharged were scattered throughout the depth of the cortex; units thought likely to be monosynaptically discharged were also found at various depths. These results suggest that the cells of origin of transcallosal fibers are not confined to any layer or depth, and that the terminals of these fibers are distributed in all cortical layers as Nauta''s histological studies have demonstrated. When the unit discharges were correlated with the concomitantly recorded surface TCR, it was noted that three fourths of the units discharged initially during the positive wave of the surface TCR. The units designated as antidromically activated discharged very early, before the TCR or during the brief spike deflections super-imposed on the early part of the positive wave. Antidromic firing is therefore believed to contribute little to the surface TCR. Units firing later displayed larger variability in latency of discharge to successive shocks when stimulus parameters remained constant and when the stimulus intensity was systematically varied. These units would follow only lower rates of stimulation. Thus, unit activity appears to be relayed over multisynaptic routes and the discharge of these later firing units might also be expected to contribute to the surface-evoked response.