Excitotoxic lesions of the perirhinal cortex leave intact rats' gustatory sensory preconditioning.

We report findings from two sensory preconditioning experiments in which rats consumed two flavoured solutions, each with two gustatory components (AX and BY), composed of sweet, bitter, salt, and acid elements. After this pre-exposure, rats were conditioned to X by pairing with lithium chloride. Standard sensory preconditioning was observed: Consumption of flavour A was less than that of B. We found that sensory preconditioning was maintained when X was added to A and B. Both experiments included one group of rats with lesions of the perirhinal cortex, which did not influence sensory preconditioning. We discuss our findings in the light of other sensory preconditioning procedures that involve the perirhinal cortex and conclude that differences in experimental variables invoke different mechanisms of sensory preconditioning, which vary in their requirement of the perirhinal cortex.

Resilient allocentric reorientation following transfer from the inside to the outside of an arena.

In two virtual spatial-navigation experiments, participants were trained to find a hidden goal that was located adjacent to one of the right-angled corners of a cross-shaped virtual environment. The location of the goal was defined solely with respect to the geometry of the environmental structure. Training trials started from a single central start location (Experiment 1) or from multiple start locations over 2, 4, or 16 training trials (Experiment 2). Following training, participants were placed onto the outside of the same environment and asked to again find the hidden goal (which, unbeknown to participants, was removed) during a single test trial. The results from both experiments revealed that participants spent more time searching in regions on the outside of the environment that were closest to the location where the hidden goal was positioned during the previous training stage. In contrast, participants spent very little time searching in regions whose visual appearance matched those regions that contained the hidden goal during training. These results reproduce the findings from previous research which supports the idea of an allocentric encoding of the shape of the environment during navigation and further implies that this encoding is relatively resilient to manipulations that might be expected to undermine it.

Familiarity-based stimulus generalization of conditioned suppression.

We report that stimulus novelty/familiarity is able to modulate stimulus generalization and discuss the theoretical implications of novelty/familiarity coding. Rats in Skinner boxes received clicker → shock pairings before generalization testing to a tone. Before clicker training, different groups of rats received preexposure treatments designed to systematically modulate the clicker and the tone's novelty and familiarity. Rats whose preexposure matched novelty/familiarity (i.e., either both or neither clicker and tone were preexposed) showed enhanced suppression to the tone relative to rats whose preexposure mixed novelty/familiarity (i.e., only clicker or tone was preexposed). This was not the result of sensory preconditioning to clicker and tone. (PsycINFO Database Record

Excitotoxic perirhinal cortex lesions leave stimulus-specific habituation of suppression to lights intact.

Previous experiments demonstrate a normal decline in unconditioned responding in rats with perirhinal cortex lesions but attenuated performance on spontaneous object recognition (SOR), a finding supporting the assertion that distinct systems support these phenomena. This finding informs on the nature of these two fundamental forms of learning and may be taken as support for certain contemporary theories of memory. However, we cannot quantify the relative contributions of genuine habituation and alternative, trivial sources in response decline from effector fatigue and sensory adaptation in these demonstrations. An important implication of this problem is that previous reports may have missed perirhinal-dependent habituation. We report perirhinal cortex lesions to be without effect in rats' habituation of suppression to lights when any influence of effector fatigue and sensory adaptation is eliminated. Theoretical implications of this finding are discussed.

Familiarity-based stimulus generalization of conditioned suppression in rats is dependent on the perirhinal cortex.

We report that bilateral, excitoxic lesions of the perirhinal cortex attenuate rats' familiarity-based stimulus generalization. After surgery, rats were preexposed either to 2 auditory stimuli (A and B) or to only 1 auditory stimulus (B). Following preexposure, all rats received pairings of A and a footshock before assessment of generalized responding (conditioned suppression) to B. Sham rats' generalization was greater when preexposure was to both A and B than when preexposure was to B only. That pattern was abolished in lesioned rats, though no general deficiency was found in other measures of auditory processing. Our findings suggest that the perirhinal cortex is required for rats to encode familiarity as part of stimulus representations.