Principal sulcus and posterior parieto-occipital cortex lesions in the monkey

In order to assess the roles of the posterior parieto-occipital and principal sulcus cortices in processing spatial information, both with and without delays, monkeys were given lesions of one or the other area or no lesion and tested on a series of conditional discrimination tasks. There were six basic tasks, and the cue locations differed from the response locations within each task. The first two had mixed dimensions in that the location of the cue indicated the color of the correct response alternative or the color of the cue indicated the correct response location. The animals were trained preoperatively on these tasks and then tested for postoperative retention. For the next two tasks color was the only relevant dimension, but different colors were used for the cues and response alternatives. For the final two tasks location was the only relevant dimension. When an animal learned a given task it was subsequently tested on it with a 5-sec. delay. The deficits observed suggest that the roles of these areas can not be expressed simply in terms of spatial or delay functions. No group appeared to be impaired on any delay task on which it was tested. The animals with the posterior cortex removed had a great deal of difficulty in using information from one dimension to identify the correct response alternative by the other dimension, but not when the relevant cue and response dimensions were the same. These results suggest that the posterior parieto-occipital area is involved in making associations across dimensions with in the visual modality. The animals with the principal sulcus removed appeared to have difficulty only when they were required to respond with reference to the colors of the response alternatives. On such tasks the relevant dimension (color) was not available until the presentation of the response array. On the other tasks the correct response location could be determined from the cue presentation alone, that is, before the presentation of the response array. This suggests that these animals had a tendency to respond without reference to the color dimension, but rather just to the presentation of the response array, and therefore a principal sulcus function of inhibiting a response until the appropriate information has been processed.