Effects of selective neonatal temporal lobe lesions on visual recognition memory in rhesus monkeys.

Ten-month-old infant monkeys that had received neonatal ablations of either inferior temporal cortex (area TE) or the medial temporal region were compared with age-matched normal infant monkeys in visual delayed nonmatching-to-sample with trial-unique objects. Both types of early damage caused impairment in visual recognition, but the degree of deficit after early area TE lesions differed sharply from that after early medial temporal removals. Thus, whereas early medial temporal damage yielded a marked decline in visual recognition when the delays and lists were gradually increased, early area TE damage yielded normal recognition up to a delay of 60 sec and only mild impairment at longer delays and lists. The data indicate that, unlike adult monkeys, which suffer severe and nearly equivalent losses in visual object recognition after both types of ablation, the infant monkeys' recognition ability is largely spared after early damage to area TE but not after early damage to the medial temporal lobe. Together with recent clinical reports of profound memory loss in children with early dysfunction of the medial temporal region, the present findings demonstrate that medial temporal lobe structures operate early to sustain visual recognition memory, and recovery from early damage is limited at best. Early damage to higher-order visual cortex, however, can be largely compensated, presumably by one or more of the visual cortical areas that were left intact.