Earlier we had assessed visual recognition in monkeys after neonatal damage either to the entire medial temporal region or to neocortical visual area TE, both of which are known to be essential for this mnemonic ability in adult monkeys. The results indicated substantial functional sparing after the neonatal neocortical but not after the neonatal medial temporal removals. These findings, together with those of follow-up studies with larger lesions of extrastriate visual neocortex, suggest that, during infancy, visual recognition functions are widely distributed throughout many visual neocortical areas and that they become critically dependent on (i.e. localized to) area TE only after cortical maturation. In a new follow-up study, we have examined the effects of neonatal medial temporal removals limited to the perirhinal/entorhinal (i.e. rhinal) periallocortex, which is known from lesion studies in adult monkeys to be the critical medial temporal substrate for visual recognition memory. Our findings indicate that rhinal lesions in neonates, just like rhinal lesions in adults, produce a long-lasting impairment of visual recognition that is nearly as severe as that produced by removal of the entire medial temporal region, including amygdala, hippocampus, and rhinal plus parahippocampal cortices. Thus, unlike the mnemonic functions of neocortical visual area TE, those of the rhinal periallocortex cannot be assumed by any other region even when the damage occurs neonatally. Apparently, the functions of periallocortex are more firmly fixed at birth than those of neocortex.
