Visual discrimination learning impairments produced by combined transections of the anterior temporal stem, amygdala and fornix in marmoset monkeys

Marmoset monkeys (Callithrix jacchus) with bilateral transections of the an terior temporal stem, amygdala and fornix were unable to relearn a 2-choice object discrimination first learnt prior to surgery, and were very severel y impaired at relearning a concurrent object discrimination task which they had learnt and relearnt prior to surgery, indicating that they had a dense retrograde amnesia. They also had difficulty learning new visual object di scriminations but were only mildly impaired on spatial learning. When teste d on new learning of concurrent discriminations 8 to 10 weeks after surgery , three operated monkeys were unable: to reach criterion in 300 trials whil e the remaining two operated monkeys performed within the normal range. The operated monkeys were subsequently shown to be impaired on acquisition of shape discriminations using black objects. These anterograde effects sugges t that the impair ment runs mainly in the domain of visual analysis. The mo nkeys also exhibited many of the features of the Kluver-Bucy syndrome. Hist ological analysis indicated that in addition to cutting some of the subcort ical temporal lobe efferent pathways, the surgical procedures had cut the c holinergic afferents to the temporal neocortex, entorhinal cortex, and hipp ocampus. In a second experiment we found that treatment with the cholinergi c agonist pilocarpine, which is effective in monkeys with specific choliner gic lesions, was unable to remediate the lesion-induced impairments. This s uggests that transection of the non-cholinergic afferents, or the temporal lobe subcortical efferents, contributed to the behavioural syndrome and the learning and retention deficits seen in these monkeys. (C) 2001 Elsevier S cience B.V. All rights reserved.