Stimulus similarity and encoding time influence incidental recognition memory in adult monkeys with selective hippocampal lesions

Recognition memory impairment after selective hippocampal lesions in monkeys is more profound when measured with visual paired-comparison (VPC) than with delayed nonmatching-to-sample (DNMS). To clarify this issue, we assessed the impact of stimuli similarity and encoding duration on the VPC performance in monkeys with hippocampal lesions and sham-operated controls. The novelty preference was compared for pictures of dissimilar vs. similar objects and for encoding duration of 30, 10, 5, and 1 sec. The novelty preference was spared after hippocampal lesions with dissimilar (colored or black and white [BW]) stimuli and an encoding time ≥10 sec, but declined with similar stimuli or a short encoding time of 1 or 5 sec. Therefore, the severe VPC impairment reported earlier after hippocampal damage cannot be attributed to the long encoding time used (30 sec) relative to DNMS (1-5 sec). However, it may result, at least in part, from the poorer distinctiveness of the stimuli typically used for VPC (BW slides of pictures of equal size and brightness of objects differing in shape) relative to the actual objects used for DNMS, differing in shape, color, size, brightness, and texture. This conclusion fits well with current models that view the hippocampus as a comparator capable of individualizing the representations of highly overlapping inputs.