We studied the perception of simple computer-generated scenes by normal and fornix-transected Dark Agouti rats. In Experiment 1, the rats were rewarded for approaching trial-unique variable scenes differing from a constant scene that was the same across trials (constant-negative paradigm). The groups performed equivalently when scenes differed only in their objects or only in the occupied positions; however, when two scenes shared an object-place combination, the normal rats were more likely to see them as similar than were the fornix-transected rats. In Experiment 2, the rats learned to discriminate pairs of scenes. Again, there was no lesion effect when scenes differed by a single cue, object or position, but when the two scenes comprised the same objects interchanged in position, fornixtransected rats learned relatively easily. Fornix transection reduces rats' sensitivity to object-place combinations within scenes, consistent with D. Gaffan's account of scene memory as an animal analogue of episodic memory deficits in amnesia.
The 5-hydroxytryptamine1a (5-HT1a) receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT; 0.15 mg/kg) impaired rats' rapid visual learning on a computerized maze. This treatment also increased decision time (DT) but the learning impairment was not necessarily a side-effect of slower responding because, in this task, responses made at long DT are more accurate than those at short DT. The selective 5-HT1a receptor antagonist WAY-100635 (0.3 mg/kg) was itself without effect on accuracy, but was effective in reversing effects of 8-OH-DPAT (on both accuracy and DT). Within problems (i.e., over the 40-60 trials of a single discrimination), performance was reduced by treatment with 8-OH-DPAT at all stages of learning. We conclude that this effect is mediated through the 5-HT1a receptor site (rather than through some other serotonergic receptor site or non-specific mechanism) as it was reversible by treatment with WAY-100635. Although it could still arise from behaviourally non-specific effects, the performance deficit finds its best account in terms of the psychological processes necessary to visual learning. Its reversal with WAY-100635 offers support to the hypothesis that 5-HT1a receptor antagonists could improve cognitive function, under conditions of pre-existing impairment due to overactive serotonergic inhibition, as is thought to occur in Alzheimer's disease.
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