Impairments in visual discrimination learning and recognition memory produced by neurotoxic lesions of rhinal cortex in rhesus monkeys

Much work on the cognitive functions of the primate rhinal (i.e. entorhinal plus perirhinal) cortex has been based on aspiration lesions of this struc ture, which might disrupt fibres passing nearby and through the rhinal cort ex in addition to removing the cell bodies of the rhinal cortex itself. To determine whether damage limited to the cell bodies of the rhinal cortex is sufficient to impair visual learning and memory, four rhesus monkeys (Maca ca mulatta) were preoperatively trained on a battery of visual learning and memory tasks, including single-pair discrimination learning for primary re inforcement, single-pair discrimination reversals, concurrent discriminatio n learning and reversal, and delayed matching-to-sample. Following acquisit ion of these tasks and a preoperative performance test, ibotenic acid was i njected bilaterally into the rhinal cortex, and the monkeys were retested. Consistent with the results of studies using aspiration lesions, the monkey s were impaired on single-pair discrimination learning as well as recogniti on memory performance postoperatively, although reliable reversal learning impairments were not observed. The magnitude of postoperative impairment in discrimination learning was not correlated with the magnitude of postopera tive impairment in recognition memory, suggesting a possible dissociation b etween these functions within the rhinal cortex. The correspondence of beha vioural deficits following aspiration and neurotoxic lesions of the rhinal cortex validates the attribution of various cognitive functions to this str ucture, based on the results of studies with aspiration lesions.