Abstract
Two seemingly contradictory principles describe conditioned response (CR) acquisition when a sequence of two conditioned stimuli (CSA-CSX) signal the unconditioned stimulus (US) for the rabbit′s nictitating membrane (NM) response. For long CSA-CSX intervals, responding to CSA shows facilitation similar to second-order conditioning. For shorter CSA-CSX intervals, responding to CSX shows impairments similar to blocking and overshadowing. Whether and how the two processes interact has been unclear. The present study interpolated extra CSs between CSA and CSX, thus introducing the full range of CS-CS intervals into a single sequence. The interpolated CSs maintained the facilitation of CRs to CSA and progressively impaired CRs to CSX. The interpolated CSs themselves tended to show facilitated CR acquisition. Thus, the two processes can act concurrently. In theoretical terms, the higher-order CSs appear to have a parasitic relationship to the first-order CS, effectively stealing its associative strength.
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