Differentiated short latency response increases after conditioning in inferior colliculus neurons of alert rat

The responses of 35 inferior colliculus multiple units (MUs) to tone onset were measured in 10 freely moving rats before and after differential behavioral conditioning. MU response changes were found in the 16.8 msec after tone onset (includes 2 msec air travel time) after learning. Responses to CS+ onset increased in 12 of the 35 individual MUs, and in the group of MUs as a whole, after conditioning. The CS+--CS- difference increased in 17 of the 35 individual MUs, and in the group of MUs, after conditioning. The response differentiation was significant in the 3.6-6.4 msec interval after the tone reached the animal's ears, the time at which neuronal responses were first evident. Since the inferior colliculus increases were differentiated between CS+ and CS-, they could not be explained by sensitization caused by changed middle ear contractions or by alterations in orientation to the speakers.