Abstract
Two experiments using rats assessed the effect of dorsal hippocampal lesions on the orienting response (OR) elicited by a discrete light and on the subsequent associability of that light. In both experiments subjects received extensive nonreinforced exposure to the light, and it was found that the decline in the OR evident in sham-operated animals was severely attenuated by hippocampal lesions. Subsequently the light was paired with condensed milk in Experiment 1, and it was found that conditioning was faster in the lesioned than in the sham-operated group. Experiment 2 used a conditioned inhibition procedure whereby the light was presented in a nonreinforced compound with a tone which, when presented alone, acted as a signal for condensed milk. Again this discrimination was mastered more rapidly by lesioned animals. Relevant control groups indicated no differences between lesioned and sham-operated animals in learning about a novel light. Thus, hippocampal lesions influence both the loss of associability of a stimulus and the decline in the OR it elicits, which, in turn, suggests that the OR acts as an index of stimulus associability.
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