The emergence of changed unit responses in the auditory system of the alert rat during classical conditioning

Alert albino rats were trained that one of two tones signalled food presentation after a 300 msec CB US interval. Unit responses to tone onset were recorded via microelectrodes chronically implanted in inferior colliculus, medial geniculate, posterior nucleus, pretectal region and cortex before and after tone food pairing. Changes in the size of these unit responses during the early (1-150) msec and late (151-300 msec) portions of the CS US interval were examined in the trial sequence. In the first 150 paired trials, late response changes in most units occurred before early ones. However, there was a subgroup of units within each brain region in which the early responses changed first in trials. These responses would have been masked had population measures been studied. Units which changed within the first 60 trials were found in thalamus and cortex but not in colliculus, which changed in the 61-150 trial period. Because of their late appearance, these units seemed more involved in processing related to response consolidation. Units in all brain regions showed marked response increases after an 8 hr rest interval. These increases indicated that considerable consolidation and reorganization had occurred during this period.