The effect of superior temporal lesions on the recognition of species-specific calls in the squirrel monkey.

Eleven squirrel monkeys (Saimiri sciureus) were trained to discriminate species-specific calls from non-species-specific complex sounds in a go, no-go procedure with social contact as positive reinforcement. The task required that the animals not only responded to a particular call but that this response should be generalized to any squirrel monkey call, whether or not it had been presented previously in training. After having reached a performance level of 75% correct responses in three consecutive sessions, seven animals received bilateral lesions of the auditory cortex; the other four animals served as controls. It was found that small lesions within the superior temporal gyrus did not interfere with the discrimination task. Lesions destroying about three quarters of the auditory cortex led to loss of retention; during retraining the animals did not reach criterion, but performed significantly above chance. These animals were able, however, to master a simplified version of the task where one species-specific call had to be discriminated from one non-species-specific sound. Animals with almost total ablation of the auditory cortex were capable of mastering neither the generalized task nor the simplified version. From these results, together with those of the literature, it is concluded 1) that recognition of complex sounds is not possible after complete auditory cortex ablation, probably because of interference with gestalt-formation processing, and 2) that species-specific calls are processed in the auditory system in the same way as other complex sounds.