Memory after frontal/temporal disconnection in monkeys: conditional and non-conditional tasks, unilateral and bilateral frontal lesions

Seven Cynomolgus monkeys (Macaca fascicularis) learned a series of reward-visual conditional discrimination problems, in which the arrival or non-arrival of a food pellet at the beginning of each trial acted as an instruction cue, signalling which of two visually distinct stimulus objects the animal should choose on that trial in order to obtain a further food pellet reward. Following surgical removal of the ventrolateral prefrontal cortex in one hemisphere and the inferior temporal cortex in the contralateral hemisphere, combined with forebrain commissurotomy, the four operated animals were severely impaired at relearning this task. They were not impaired, however, in non-conditional visual discrimination learning. Extending the unilateral frontal lesion to include the ventromedial prefrontal cortex had no detrimental effect, nor did complete unilateral removal of the frontal cortex. In a third experiment, the operated animals underwent a further surgery to remove either ventrolateral, ventral or complete frontal cortex similar to that in the opposite hemisphere. Compared to their previous level of performance, the animals with bilateral ventrolateral prefrontal lesions were now mildly impaired and the animals with the bilateral lesion extended to the ventromedial cortex more severely impaired on the non-conditional visual discrimination task. The bilaterally lobectomized animals were unable to relearn the task. We suggest that behaviour in visual learning tasks is controlled by cortical convergence upon subcortical structures, possibly by striatal efferents from both the visual cortex and frontal cortex, and that intrahemispheric convergence of these two efferents within the corpus striatum of one hemisphere could allow detailed control of visual choices by non-visual information, while subcortical interhemispheric transfer allows only less detailed, more general control.