Basal forebrain-lesioned monkeys are severely impaired in tasks of association and recognition memory

Six squirrel monkeys received ibotenic acid-induced lesions targeted to the basal nucleus of Meynert (BNM). Postoperatively, the monkeys were trained in a broad spectrum of learning tasks and compared to normal and sham-operated controls. The lesioned monkeys showed severe and enduring learning and memory deficits in a visual reversal task, several concurrent object discriminations, a delayed nonmatch-to-sample task, and, most outstanding, and angle threshold discrimination. Although in some of the monkeys the BNM was lesioned only partly with some surrounding tissue affected as well, the lesion of the BNM appears to be crucial in the large and apparently irreversible learning deficits of the monkeys. However, final proof for this suggestion is still needed.