Object recognition and location memory in monkeys with excitotoxic lesions of the amygdala and hippocampus

Earlier work indicated that combined but not separate removal of the amygdala and hippocampus, together with the cortex underlying these structures, leads to a severe impairment in visual recognition. More recent work, however, has shown that removal of the rhinal cortex, a region subjacent to the amygdala and rostral hippocampus, yields nearly the same impairment as the original removal. This raises the possibility that the earlier results were attributable to combined damage to the rostral and caudal portions of the rhinal cortex rather than to the combined amygdala and hippocampal removal. To test this possibility, we trained rhesus monkeys on delayed nonmatching-to-sample, a measure of visual recognition, gave them selective lesions of the amygdala and hippocampus made with the excitotoxin ibotenic acid, and then assessed their recognition abilities by using increasingly longer delays and list lengths, including delays as long as 40 min. Postoperatively, monkeys with the combined amygdala and hippocampal lesions performed as well as intact controls at every stage of testing. The same monkeys also were unimpaired relative to controls on an analogous test of spatial memory, delayed nonmatching-to-location. It is unlikely that unintended sparing of target structures can account for the lack of impairment; there was a significant positive correlation between the percentage of damage to the hippocampus and scores on portions of the recognition performance test, suggesting that, paradoxically, the greater the hippocampal damage, the better the recognition. The results show that, within the medial temporal lobe, the rhinal cortex is both necessary and sufficient for visual recognition.