ELECTRICAL RESPONSES TO ACOUSTIC CLICKS IN MONKEY: EXTENT OF NEOCORTEX ACTIVATED

Abstract

The extent of neocortex from which an electrical response could be evoked by acoustic clicks was mapped in 7 monkeys. Although it was customary to resect the inferior portion of the frontal and parietal lobes to gain access to the supratemporal plane, in the present experiments the banks of the sylvian fissure were gently separated. A larger extent of neocortex of monkey was activated by acoustic clicks than has hitherto been described. The responsive area comprises the posterior supratemporal plane, the posterior superior temporal gyrus, the posterior insula, the parietal operculum, the anterior half of the inferior parietal operculum, and both banks of the anterior half of the intraparietal fissure and the inferior half of the central fissure. The extent of cortex activated was essentially the same whether intense or threshold stimuli were used. Stimuli delivered to ipsilateral and con-tralateral ears evoked responses at approximately the same cortical locations. The latency of onset of the initial positive deflection delimits a subdivision of "auditory" cortex. This subdivision is a band reaching along the anterior margin of the responsive area on the supratemporal plane, insula, and operculum. The initial latencies are considerably longer in this area (from 12 to 18 msec.) than in the rest of the responsive cortex (7.5-9.5 msec). This long latency area can be inferred to be homologous with a similar area in the cat (called "secondary" by Ades and Bremer). It is well established that responses to acoustic signals are relayed through the medial geniculate nucleus of the thalamus to the cortex of the posterior supratemporal plane. Resection of the supratemporal cortex did not change the parietal response to acoustic clicks in acute preparations. When such resection antedated the electrophysiological experiment sufficiently to allow retrograde degeneration of the microcellular portion of the medial geniculate body, all neocortical responses on the side of the involved nucleus disappeared. These results suggest that the electrical response to acoustic clicks recorded from parietal cortex is mediated by collaterals which branch from the main projection and connect the medial geniculate nucleus with the cortex of the supratemporal plane. This inference, if supported and extended to other systems, would provide an anatomical base for electro-physiological findings; at present the area of cortex afferently connected to the periphery is wider than can be demonstrated by the technique of retrograde thalamic degeneration.