Electrical responses to acoustic clicks in monkey: extent of neocortex activated.

THE POSTERIOR HALF of the cortex of the supratemporal plane in the monkey has been related to the auditory system by anatomical and electrophysiological techniques (2, 11, 15, 20). One investigation has indicated that the posterior insula also is activated by stimulation of the auditory sysLem (25, 27). Because of the inaccessibility of the supratemporal plane, experimenters usually resect the inferior portions of the frontal and parietal lobe in order to expose the cortex of the inferior bank of the sylvian fissure. I n recent experiments on the cortex of the operculum, insula, and supratemporal plane, the customary resection was abandoned and the sylvian fissure was opened by gentle dissection and retracted by packing (16). The present study was undertaken in order to utilize this procedure in a reinvestigation of the extent of the cortex of monkey activated by acoustic stimulation. The unexplained difference in size between the relatively large area of "auditory cortex" in rodent and carnivore and the comparable area in primate suggested that this new research might prove fruitful. Maps may vary with (i) the dimension of the stimulus or (ii) the criteria of response. For the stimulus, clicks were chosen because they evoke the most nearly consistent electroneural response and appear to activate the cortex extensively. With electrical responses the experimenters must determine which aspects of the responses are to be employed in mapping the cortex. (Since the techniques chosen were used to measure the activity of large populations of responding elements, all-or-none measurements could be made only when "all" and "none" were delimited.) In this study the effect of ::coustic clicks of various intensities on the electrical activity of the neocortex of the monkey was mapped in terms of various criteria. The resulting congruence and incongruence of maps provides topics for discussion.