Conditional learning and memory impairments following neurotoxic lesion of the CA1 field of the hippocampus.

Monkeys with bilateral lesions of the CA1 field of the hippocampus produced by the injection of neurotoxin diagonally along the length of the hippocampus were found to have a severe impairment on the retention of a conditional task learnt prior to surgery and on the new acquisition of several types of this task. They were equally impaired on conditional tasks that required a spatial response or an object choice in response to either visual or spatial cues. They were not impaired on simple visual discrimination tasks, simple spatial discrimination tasks or reversal learning of these tasks. This patterns of impairment resembles that seen in the same species with neurotoxic lesions within the vertical limb of the diagonal band of Broca or transection of the fornix. Monkeys with subtotal lesions of the adjacent medial temporal area were not consistently impaired on any of these tasks. The results suggest that hippocampal lesions produce anterograde and retrograde amnesia for information other than reward association.