Cortical and Midbrain Mediation of a Conditioned Discrimination of Acoustic Intensities

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Abstract
Cats were conditioned to changes (increments) in intensity of 3 tonal stimuli. Following conditioning, differential intensity thresholds were detd. by decreasing the intensity increments until the animals failed to respond. Cortical and midbrain auditory centers were extirpated systematically, and the animals were tested post-operatively to determine the effect of ablations on (a) conditioned discrimination and (b) differential intensity limens (DLs.). After total and bilateral removal of the auditory cortex, the animals required varying amts. of retraining to meet the 90% criterion of conditioning. The post-operative amnesia is interpreted as indicating that the conditioned reflex (CR) is cortically mediated in the intact animal. All 4 animals were able to relearn the habit, however, following ablation. The postoperative DLs did not differ significantly from the pre-operative. After bilateral destruction of the inferior and superior colliculi, the CR was retained perfectly. The DLs did not change significantly. In one animal which had been retrained following auditory cortex destruction, the inferior colliculi was bilaterally destroyed. This animal had to be retrained. It was concluded that relearning after cortical destruction may have been mediated by these midbrain auditory centers. In the absence of both the cortex and colliculi the DLs are markedly increased. Removal of the superior colliculi after prior ablation of the auditory cortex again resulted in a post-operative amnesia. The animals could be retrained. In the absence of the inferior or superior colliculi, partial destruction of the auditory cortex brought about no habit loss and no change in the DLs. These results are interpreted as indicating within the auditory nervous system 3 functional levels, cortical, midbrain and bulbar.