FUNCTIONAL DOUBLE DISSOCIATION BETWEEN 2 INFERIOR TEMPORAL CORTICAL AREAS - PERIRHINAL CORTEX VERSUS MIDDLE TEMPORAL GYRUS

There is both anatomic and cytoarchitectural evidence for dorsal-ventr al subdivisions of the inferior temporal cortex. Despite this, there h as been only limited evidence of corresponding functional subdivisions and no evidence that two adjacent cortical areas within the inferior temporal cortex, namely area TE and the perirhinal cortex, have distin ctly different roles in vision and memory. We assessed the color discr imination abilities of cynomolgus monkeys with either bilateral ablati on of the perirhinal cortex or bilateral ablation of the middle tempor al gyrus. The stimuli were isoluminant colored squares presented on a touch screen. Ln each trial the subject had to learn to discriminate a nd select the correct choice (green) from among a maximum of eight oth er foils, each varying in either hue or saturation. Relative to unoper ated controls, monkeys with middle temporal gyrus lesions were severel y impaired in the color discrimination task, whereas monkeys with peri rhinal lesions were unimpaired on this task. We also assessed the visu al recognition abilities, as measured by a basic delayed nonmatching-t o-sample task with trial-unique objects presented in a Wisconsin Gener al Test Apparatus, of rhesus monkeys with bilateral middle temporal gy rus lesions. We then tested the monkeys' postoperative performance on a delayed nonmatching-to-sample task with delays and extended list len gths. The results from this experiment were compared with those from t wo other groups of rhesus monkeys, an unoperated control group and a g roup with bilateral perirhinal cortex lesions, both of which had perfo rmed the identical tasks in a previous experiment. Relative to unopera ted controls, monkeys with perirhinal cortex lesions were severely imp aired both in relearning the basic delayed nonmatching-to-sample task and on the postoperative performance test. In contrast, monkeys with m iddle temporal gyrus lesions were only mildly affected in relearning t he basic nonmatching task and were unimpaired on the postoperative per formance test. Thus our data demonstrate a clear functional double dis sociation between the perirhinal cortex and the middle temporal gyrus. This result gives strong support to the hypothesis that the perirhina l cortex and the adjacent area TE have distinctly different roles in v isual learning and memory.