Hippocampal formation lesions produce memory impairment in the rhesus monkey

There is much debate over the role of temporal lobe structures In the abili ty to learn and retain new information. To further assess the contributions of the hippocampal formation (HF), five rhesus monkeys received stereotact ically placed ibotenic acid lesions of this region without involvement of s urrounding ventromedial temporal cortices. After surgery, the animals were trained on two recognition memory tasks: the Delayed Non-Match to Sample (D NMS) task, which tests the ability to remember specific trial unique stimul i, and the Delayed Recognition Span Task (DRST), which tests the ability to remember an increasing array of stimuli. Relative to normal control monkey s, those with HF lesions demonstrated significant impairments in both learn ing and memory stages of the DNMS task. Additionally, the HF group was sign ificantly impaired on spatial, color, and object versions of the DRST. Cont rary to suggestions that damage to the entorhinal and parahippocampal corti ces is required to produce significant behavioral deficits in the monkey, t hese results demonstrate that selective damage to the HF is sufficient to p roduce impairments on tasks involving delayed recognition and memory load. This finding illustrates the importance of the HF in the acquisition and re tention of new information. (C) 1999 Wiley-Liss, Inc.