Further analysis of the effects of immunotoxic lesions of the basal nucleus of Meynert reveals substantial impairment on visual discrimination learning in monkeys

In this paper we undertake a combined analysis of several studies in which marmoset monkeys received immunotoxic lesions of the cortical cholinergic projections from the basal nucleus of Meynert (NBM) bilaterally and/or in combination with immunotoxic lesions of other parts of the cholinergic system or ablations of the target inferotemporal neocortical area. Analysis of the mean learning scores across all visual discriminations learning tasks for each lesion combination revealed highly significant impairments where the NBM was lesioned bilaterally or where an NBM lesion in one hemisphere was crossed with an inferotemporal cortical ablation in the other hemisphere. This demonstrates that the cholinergic projection from the NBM to the major target area of neocortex involved in visual discrimination learning, i.e. the inferotemporal cortex, makes an important contribution to the perceptuo-mnemonic processes necessary for this type of learning. A new study demonstrates a significant effect of a subtotal bilateral cholinergic lesion confined to the NBM on a concurrent object-reward association task using black objects which is perceptually and mnemonically demanding. These results do not preclude the possibility that cholinergic projections from the NBM to other parts of the neocortex make a contribution to other cortical functions which are not mnemonic. It is well established that lesions of the cholinergic projection from the diagonal band of Broca disrupts the mnemonic functions of the hippocampus. The results described here suggest that degeneration of the cholinergic projections in Alzheimer's disease and other dementias will contribute to the loss of those mnemonic functions which are dependent on the neocortex.