<p class="first" id="P1">Prediction error (PE) plays a critical role in most modern
theories of associative
learning, by determining the effectiveness of conditioned or unconditioned stimuli
(CS or USs). Here, we examined the effects of lesions of central (CeA) or basolateral
(BLA) amygdala on performance in overexpectation tasks. In two experiments, after
two CSs were separately paired with the US, they were combined and followed by the
same US. In a subsequent test, we observed losses in strength of both CSs, as expected
if the negative PE generated on reinforced compound trials encouraged inhibitory learning.
CeA lesions, known to interfere with PE-induced enhancements in CS effectiveness,
reduced those losses, suggesting that normally the negative PE also enhances cue associability
in this task. BLA lesions had no effect. When a novel cue accompanied the reinforced
compound, it acquired net conditioned inhibition, despite its consistent pairings
with the US, consonant with US effectiveness models. That acquisition was unaffected
by either CeA or BLA lesions, suggesting different rules for assignment of credit
of changes in cue strength and cue associability. Finally, we examined a puzzling
autoshaping phenomenon previously attributed to overexpectation effects. When a previously-food-paired
auditory cue was combined with the insertion of a lever and paired with the same food
US, the auditory cue not only failed to block conditioning to the lever, but also
lost strength, as in an overexpectation experiment. This effect was abolished by BLA
lesions but unaffected by CeA lesions, suggesting it was unrelated to other overexpectation
effects.
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