Neonatal aspiration lesions of the hippocampal formation impair visual recognition memory when assessed by paired-comparison task but not by delayed nonmatching-to-sample task.

Previous experiments showed that neonatal aspiration lesions of the hippocampal formation in monkeys yield no visual recognition loss at delays up to 10 min, when recognition memory was assessed by a trial-unique delayed nonmatching-to-sample (DNMS) task. The present study examined whether neonatal hippocampal lesions also have no effect on visual recognition when assessed by a visual paired-comparison (VPC) task. In the VPC task, animals are looking at visual stimuli and their preference for viewing new stimuli is measured. Normal adult monkeys showed strong preference for looking at the novel stimuli at all delays tested. By contrast, adult monkeys with neonatal hippocampal lesions, which included the dentate gyrus, cornus ammon (CA) fields, subicular complex, and portions of parahippocampal areas TH/TF, showed preference for novelty at short delays of 10 s but not at longer delays of 30 s to 24 h. This visual recognition loss contrasts with the normal performance of the same operated animals when tested in the DNMS task. The discrepancy between the results obtained in the two recognition tasks suggests that, to perform normally on the DNMS task, the operated monkeys may have used behavioral strategies that do not depend on the integrity of the hippocampal formation. In this respect, VPC appears to be a more sensitive task than DNMS to detect damage to the hippocampal region in primates.