The role of the inferior prefrontal convexity in performance of delayed nonmatching-to-sample.

Findings in an earlier study (Bachevalier, J. and Mishkin, M. Behav. Brain Res. 20, 249-261, 1986) indicated that ventromedial prefrontal cortex, which receives thalamic projections from the limbo-diencephalic system, is critical for visual recognition; whereas the dorsolateral prefrontal cortex, which receives no such thalamic projections, makes little or no contribution to this type of memory. In the present study, we examined the role in recognition of another prefrontal area outside the thalamic projection zone of the limbo-diencephalic system, namely, the inferior prefrontal convexity. In the first experiment, monkeys with lesions of this cortex (Group IC) were found to be impaired in relearning delayed nonmatching-to-sample (DNMS), but not on the subsequent DNMS performance test in which memory demands were greatly increased. In a second experiment, monkeys with combined lesions of the inferior and dorsolateral prefrontal cortex (Group LAT) were also found to be impaired in relearning DNMS, but in this case they were impaired, in addition, on the subsequent performance test. Neither group (IC or LAT) showed retardation in acquiring visual discrimination habits. Analysis of the DNMS behavior of both groups suggested that their deficits on this task were due not to a loss in recognition memory, but to various forms of perseverative interference. The results support the view that the inferior prefrontal convexity, like the dorsolateral prefrontal cortex, lies outside the limbo-diencephalic memory system not only anatomically but also functionally.