Preservation of cross-modal transfer of a rate discrimination in the bushbaby (Galago senegalensis) with lesions of posterior neocortex.

The effect of lesions of posterior neocortex was assessed, using a test method that permits the demonstration of cross-modal transfer in intact bushbabies. Eight bushbabies were trained to discriminate light flashes of 18/sec and 3/sec in a go-no-go shock-avoidance task. On completion of training, four bushbabies received lesions of posterior neocortex by aspiration. After 6 wk both lesion and intact animals were returned to training in the visual discrimination. On the day following criterion performance on the visual tests, auditory clicks off the same rate and contingencies were substituted and maintained to criterion. All eight bushbabies demonstrated rapid transfer and the lesion animals were not retarded as compared with intact subjects. The cross-modal transfer of a specific rate discrimination was thus preserved in the absence of posterior intersensory neocortex. The results are discussed in terms of a hypothetical subcortical system capable of the amodal coding of simple stimulus dimensions.