Modification of unit discharges in the medial geniculate nucleus by click-shock pairing.

The present experiment was concerned with some discharge properties of single neurons in the medial geniculate body of the locally anesthetized paralyzed cat. The effect of pairing clicks with paw shock upon discharge rate and pattern was of particular concern. Twelve neurons obtained from 11 cats were studied exhaustively for periods up to 4 hr. Under control conditions, rate stationarity for both spontaneous and click-evoked activity was found in only 3/12 of the units. Click-shock pairing produced rate changes in lo/12 of the cells; an increase in rate predominated. The pattern of discharges was altered in 9/E of the cells as a consequence of click-shock pairing. Specifically, the initial short-latency discharge was modified; there was a reduction in the proportion of spikes in the first peak to the total number of spikes in the poststimulus time histogram. This reduction was not merely a consequence of increases in over-all rate of discharge. In the case of one cell which was inhibited rather than excited by click stimulation, the click-shock pairing resulted in a reduction in the duration of inhibition. Control findings indicated that the pattern modifications were not due to a change in stimulus intensity, the unconditioned effects of the shock itself, or to an increase in arousal level.