Interaction of Temporal Lobe and Frontal Lobe in Memory
Memory impairments which follow temporal lobe lesions can also be produced by lesions in the prefrontal cortex. One approach to understanding the memory function of the frontal lobe, therefore, is to ask how it interacts with the temporal lobe. Clinical and experimental evidence indicates that the memory function of the hippocampus in primates is critically dependent on its diencephalic connections via the fornix and anterior thalamus, through which it can interact with the cingulate and prefrontal cortex. Is there a similar arrangement for other specialized memory systems of the temporal lobe?
In macaques, lesions of the ventromedial frontal cortex (VF) or of the mediodorsal thalamus (MD) produce memory impairments similar to those which are produced by lesions of the amygdala (A). However, these structures do not interact only via a single serial pathway (A → MD → VF); disconnection of that pathway, by crossed unilateral lesions of A in one hemisphere and of either MD or VF in the other hemisphere, produced only a partial impairment. The full impairment was produced, however, by unilateral lesions of A in one hemisphere and of both MD and VF in the other hemisphere. These results show that, in addition to the A → MD → VF pathway, the amygdala also interacts with the frontal lobe via other independent pathways, which could include both the direct projection from A to VF and the neostriatal loop beginning with the projection from A to the tail of caudate and posterior putamen; the pathway from A to the ventral striatum is not functionally critical in the memory tasks we have examined, since crossed unilateral lesions of A and the ventral striatum were without effect.
The visual association cortex of the temporal lobe (area TE) can also interact with the prefrontal cortex via multiple routes, some of which are independent of the amygdala and hippocampus. Two such routes are the direct cortico-cortical projection via the uncinate fascicle, and the neostriatal loop. Simple visual discrimination learning for an immediate local visible reinforcer was not impaired by combined amygdalectomy, fornix transection and uncinate fascicle section; this kind of visual learning must, therefore, proceed via the neostriatal outputs of TE. However, uncinate fascicle section alone does produce an impairment in conditional learning tasks, in which a visual stimulus acts as an instruction or cue for a separate choice.
Thus, both the amygdala and TE have multiple routes of interaction with the the prefrontal cortex, and these routes are themselves in some cases known to be specialized in function. The critical dependence of the hippocampus on a single pathway, via the fornix, appears to be unique among the memory mechanisms of the temporal lobe.
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