Cognitive inflexibility after archicortical and paleocortical prefrontal lesions in marmosets

The behaviour and cognitive abilities of marmosets with bilateral ablations of either the archicortex on the mediodorsal surface or the paleocortex on the ventrolateral surface of the frontal lobes were assessed in terms of the types of information being handled, the mode of action (inhibitory versus productive mechanisms), and the level of functioning of these frontal areas. Although there was a tendency for the ventrolateral ablation to produce a greater impairment on object than on spatial discrimination in comparison to the mediodorsal ablation, the differences between the two groups was not marked in this respect. The effect of ablations of either of these areas was more marked on tasks which required the inhibition of prepotent responses than on tasks where novel or arbitrary responses were appropriate. The major effect of both lesions was the production of perseverative behaviour which occurred at the level of motor output in the first few days after surgery but was then observed at the level of choice behaviour and attentional set in formal cognitive tests. Impairments were also seen on postoperative retention of tasks learnt prior to surgery. It is suggested that all these impairments arise from an inability to use or modify stored information.