Overexpectation: Response Loss During Sustained Stimulus Compounding in the Rabbit Nictitating Membrane Preparation
Abstract
Rabbits were given reinforced training of the nictitating membrane (NM) response using separate conditioned stimuli (CSs), which were a tone, light, and/or tactile vibration. Then, two CSs were compounded and given further pairings with the unconditioned stimulus (US). Evidence of both overexpectation and summation effects appeared. That is, responding to the individual CSs declined despite their continued pairing with the US on compound trials (overexpectation), and responding on the compound trials was greater than responding to the individual CSs (summation). The response loss appeared regardless of the testing regime, that is, whether the test presentations of the individual CSs were themselves reinforced (Experiment 2), not reinforced (Experiment 1), or deferred until the end of compound training (Experiment 2). The results are discussed with respect to the roles of excitatory versus inhibitory processes, elemental versus configural processes, and the possible roles of cerebellar and hippocampal pathways.
Footnotes
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↵2 Reducing the total number of trials and increasing the ITI for the compounding groups necessarily increased the spacing of the trials. However, on the basis of previous findings in the rabbit NM preparation (Kehoe and Macrae 2002), this increase, at worst, introduced a bias against seeing an overexpectation effect by perhaps slightly elevating the overall level of responding to the compound and elements. In contrast to this bias, the alternative tactic of using 40 TL trials per session at a 60-sec ITI could have artifactually produced an overexpectation effect. According to the Rescorla-Wagner model, doubling the number of tone and light presentations in the compounding groups would exaggerate the overexpectation effect by roughly doubling any decline in the associative strength of the elements relative to the control groups.
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Article and publication are at http://www.learnmem.org/cgi/doi/10.1101/lm.77604.
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- Accepted June 10, 2004.
- Received March 29, 2004.
- Cold Spring Harbor Laboratory Press
