Interaction between auditory and somatosensory systems: a study of evoked potentials in the inferior colliculus.

It is known that some neurons in the external nucleus of the inferior colliculus (ICX) receive both auditory and somatosensory input. In recordings of evoked potentials from the ICX in the rat we show evidence that somatosensory activation can modify the auditory response to click stimulation. Electrical stimulation of the median nerve affects the click-evoked response that is recorded from the surface of the ICX and different components of the response were affected differently. The magnitude of the changes was dependent on the time interval between electrical stimulation of the median nerve and presentation of the click stimulation. The amplitude of the two earliest peaks in the click-evoked ICX response increased slightly (16% and 20%, respectively; average of recordings in 10 animals) as a result of stimulation of the median nerve, while the amplitudes of two later peaks decreased as a result of stimulation of the median nerve (the average decrease in amplitude of these two peaks in recordings in 10 animals was 18 and 26%, respectively). The latencies of the 3 peaks we studied were only slightly affected, and the maximal change was 0.07 and 0.25 msec, respectively, for the 2 earliest peaks. The decrease in the amplitude of specific peaks of the ICX response confirms that some neurons of the extralemniscal system receive both auditory and somatosensory input, and that such somatosensory input is mainly inhibitory on these neurons. We interpret the increase in the amplitude of the early peak to be a result of median nerve stimulation caused by a decreased centrifugal inhibitory influence on superior olivary neurons.