Neurotoxic hippocampal lesions have no effect on odor span and little effect on odor recognition memory but produce significant impairments on spatial span, recognition, and alternation

Recent work has shown that lesions of the hippocampus in monkeys cause defi cits in the capacity to remember increasing numbers of objects, colors, and spatial locations (Beason-Held et al., 1999). However, others have observe d that hippocampectomized monkeys can show intact memory for a list of obje cts or locations (Murray and Mishkin, 1998). We wished to explore the effec ts of hippocampal damage on the capacity of memory in the rodent and, to do so, developed novel "span" tasks in which a variable number of odors or lo cations had to be remembered. In the odor span task (experiment 1), rats we re trained on a nonmatching to sample task in which increasing numbers of o dors had to be remembered. Half of the trained rats received ibotenic acid lesions of the hippocampus. Postoperatively, hippocampectomized animals did not differ from control animals even when required to remember up to 24 od ors. However, when tested on delayed retention of a list of 12 odors, rats with hippocampal lesions were impaired at a long delay. Also, these rats we re impaired on a subsequent test of delayed spatial alternation. In a spati al span task (experiment 2), naive rats were trained on a nonmatching to sa mple task in which a variable number of locations had to be remembered. Aft er this, half of the animals received ibotenic acid lesions. Postoperativel y, hippocampectomized animals performed above chance levels when required t o remember a single cup location, but were unable to remember more. Subsequ ent testing on another spatial delayed alternation task suggested that hipp ocampectomized rats could recognize, but could not inhibit their approach t o previously visited locations.