Non-spatial acquisition and retention deficits following small excitotoxiclesions within the hippocampus in monkeys

Marmoset monkeys with excitotoxic lesions confined to cornu ammonis subfiel ds 1-3, subiculum and pre-subiculum, but sparing the entorhinal cortex, wer e impaired on retention and learning of conditional object-choice discrimin ations. For each of these discriminations, the monkeys were required to cho ose one of two objects depending on which of two patterned backgrounds was used on each trial. Two styles of order of trial presentation were used.. ' random' presentation which maximised the degree of interference between tri als, and 'runs' presentation which was intended to encourage the monkeys to learn each component of the discrimination separately. Before surgery monk eys found the discriminations more difficult to learn when the trials were presented in the 'runs' style than when presented in the 'random' style sug gesting that the task is best learnt by applying a conditional rule, After surgery a significant 'group x style' interaction indicated that the 'runs' style was especially difficult for the lesioned monkeys. From these results we suggest that the hippocampus is involved in learning about and remembering non-spatial. conditional relations between objects. ( C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.