A specific form of cognitive rigidity following excitotoxic lesions of the basal forebrain in marmosets

The effects of N-methyl-D-aspartate-induced lesions of the basal forebrain were studied on performance of a series of visual discrimination tests that examined a range of cognitive functions in the marmoset. These included the ability to attend to the various dimensional properties of stimuli and to use just one of these properties in order to solve a discrimination (intra-dimensional shift); to switch attention from one dimension to another (extra-dimensional shift); to learn the reinforcement value of specific exemplars within a dimension (new learning); and to relearn their reinforcement value following reversal of the reward contingencies (serial reversals). Lesions of the basal forebrain did not impair the ability either to attend selectively to the dimensional properties of the stimuli or to switch attention from one dimension to the other. However, the lesion did affect various aspects of associative learning including a transient impairment of new learning and a marked disruption of serial reversal learning. The reversal deficit could be characterised as a tendency to perseverate on the previously correct stimulus and as a failure to to show the formation of a reversal learning set. In addition, the lesion prevented disruption of performance of a well-learned discrimination when novel exemplars from the irrelevant dimension were introduced (probe test). It is suggested that the functional effects of the basal forebrain lesion reflect impaired learning of stimulus-reward associations and behavioural rigidity. The finding, however, that there was no effect of the lesion on attentional set-shifting suggests that any loss of inhibitory control was specific to the level of stimulus-response or stimulus-reward associations, inhibitory control at the level of attentional selection remaining intact. The similarity of the effects of damage to the basal forebrain to those seen following damage to the orbitofrontal cortex and the amygdala are discussed in the context of the close anatomical and functional relationships that exist among these three structures.