Selective neurotoxic damage to the hippocampal formation impairs performance of the transverse patterning and location memory tasks in rhesus macaques

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Abstract
Monkeys with neurotoxic (ibotenic acid) damage to the hippocampal formation and unoperated controls were trained on two sets of transverse patterning problems (A+/B−, B+/C−, C+/A−, and D+/E−, E+/F−, F+/D−) and a delayed nonmatching‐to‐location paradigm (DNML) with delays of 10s, 30s, 120s, and 600s. Hippocampal lesions produced a size‐ and area‐dependent impairment on transverse patterning. Damage largely limited to the right hippocampus in one subject had no effect on performance on the task. Of the remaining four subjects, two with hippocampal damage greater than 40% bilaterally were unable to solve the two transverse patterning sets, but could solve the linear set of discriminations (A+/B−, B+/C−, C+/X−). The two remaining operated animals were impaired in acquisition of both sets, but were eventually able to solve one of the two transverse patterning discrimination sets. All five operated monkeys were impaired relative to normal controls on DNML, but not on the standard delayed nonmatching‐to‐sample (DNMS) version with trial‐unique objects. The results confirm our previous findings (Alvarado et al., Hippocampus 12:421–433, 2002) using aspiration lesions of the hippocampal formation and strengthen the view that the hippocampal formation is critical for object and spatial relational memory.