Performance of decorticated rats in a two-choice visual discrimination apparatus

Normal rats and rats with 98.8% (S.D. +/- 1.4) of neocortex surgically removed were trained in a two-choice discrimination apparatus for food reward. The animals were first trained to run the apparatus with identical non-patterned goal box doors followed by discrimination and reversal training using horizontal and vertical striped panels attached to the doors. These patterned stimuli were matched for total luminous flux and contour length and control procedures were included for unintended discriminative cues from other sensory modalities. The two groups did not differ in the number of trials taken to complete preliminary training but the decorticates were significantly faster at running the apparatus than the normals. All animals solved the visual discrimination and were able to reverse it. There were no significant differences between the groups in trials taken to acquire the visual discrimination except at the very highest criterion levels employed, though reversal of the discrimination was more difficult for the lesioned group. The preliminary training data are taken as further evidence that total decortication does not necessarily impair instrumental performance for food reward and that this lesion does not introduce any limiting sensory or motor deficit in the alleyway situation. It would also appear that neocortex is not essential for the solution of a horizontal/vertical stripe problem. It is not clear, however, that such a solution necessarily depends on a true pattern discrimination, and both lesioned and normal animals may have been employing strategies based on local cues within the two patterned stimuli. The appropriateness of the total decorticate preparation for a resolution of the problem of cortical vs subcortical substrates of residual vision following visual cortex lesions is discussed. The possibility that undamaged neocortical tissue remaining after visual cortex lesions may inhibit the full expression of residual subcortical function is raised by these data.