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Abstract:
Pavlovian delay conditioning differs from Pavlovian trace
conditioning in at least two important respects. Procedurally, the
predictive conditional stimulus (CS) and the reinforcing
unconditional stimulus (US) are temporally contiguous in delay
conditioning. In trace conditioning, a brief empty period separates
the two stimuli. Additionally, trace but not delay conditioning is
reduced by hippocampal lesions. We sought to determine if
experience with cognitive demands following hippocampal damage
reduced susceptibility to the amnestic effects of such damage. Mice
received excitotoxic lesions of the hippocampus and then were group
housed in either a complex environment that was changed (including
the location of food) every 3-4 days or in standard cages. After
one month of living in these housing conditions the mice were
trained in a trace fear conditioning task, where a 20 sec tone CS
was followed by a shock US after a 20 sec empty interval. One day
after training, fear of the tone was tested in a novel environment.
In intact mice, the trace conditioned tone produced fear as
measured by freezing. Hippocampal lesions reduced fear only in the
normally housed mice. Thus exposure to a spatially complex
"enriched" environment enhanced the ability of hippocampal animals
to acquire trace conditioned fear.
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