Neural structures mediating differential sound intensity discrimination in the cat

Abstract

Nine cats were trained to discriminate changes in intensity of a 1000 c/sec tone. Differential intensity limens were measured. Limens were redetermined following (1) unilateral and bilateral 2-stage ablation of auditory cortex, (2) bilateral section of the brachium of the inferior colliculus, (3) bilateral ablation of the inferior colliculus, and (4) bilateral cortical ablation in combination with one or the other subcortical lesion. Four cats needed retraining after bilateral cortical ablation, but had little or no change in limen. No significant loss was seen after removal of the inferior colliculus. In 3 animals with both cortical and inferior colliculus lesions, the differential intensity limen was elevated by 5.0–6.8 dB. Bilateral section of the brachium of the inferior colliculus caused elevation of the differential limen by about 10 dB. One animal subsequently given a cortical showed no further impairment. Another cat given a bilateral grachium lesion following prior cortical ablations had a 20 dB threshold elevation. This cat sustained extensive damage in the midbrain tegmentum in addition to destruction of the acoustic brachium. Findings that lesions of the brachium of the inferior colliculus caused greater losses than either cortical or inferior colliculus lesions are attributed to interruption of extralemniscal fiber systems which bypass the inferior colliculus as well as the medial geniculate body and its cortical projection field.