Crossed unilateral lesions of the medial forebrain bundle and either inferior temporal or frontal cortex impair object-reward association learning in Rhesus monkeys.

In an accompanying paper we showed that combined transection of the fornix, amygdala and temporal stem in monkeys produced dense amnesia, including an impairment in visual object-reward association learning. We proposed that this combined surgical section had its effect by isolating temporal cortex from the ascending projections of the basal forebrain and midbrain structures. To test this hypothesis, in the present experiment we disconnected the inferior temporal cortex from these basal forebrain and midbrain structures, while sparing cortical white matter, by crossed unilateral lesions of the medial forebrain bundle in one hemisphere and inferior temporal cortex in the opposite hemisphere. The aim of the medial forebrain bundle lesion was to section axons of cells, both those that project to the cortex via the medial forebrain bundle, and those which control the activity of these same structures. A single unilateral lesion alone had no effect on the ability to learn and remember visual object-reward associations, but the crossed unilateral lesions produced an impairment in this task which was equal in severity to the impairment seen earlier after bilateral section of the fornix, amygdala and temporal stem. The impairment was not an effect of interrupting fibres to the cortex from the ventromedial hypothalamus, or of unilateral sensory neglect. This supports the hypothesis that these midbrain and basal forebrain afferents to the inferior temporal cortex are important for new visual learning. Furthermore, an impairment of equal severity was demonstrated in a separate group of animals that received crossed unilateral lesions of the medial forebrain bundle in one hemisphere and of the frontal cortex in the opposite hemisphere. We propose that the frontal cortex acts to modulate basal forebrain activity which in turn reinforces object representations in the inferior temporal cortex during learning.