Each of four groups of rabbits received nictitating membrane (NM), heart rate (HR) and respiration rate (RR) classical discrimination conditioning to tones and shocks at interstimulus intervals of.25,.75, 2.25 and 6.75 sec. Percentages of NM CRs were inversely related to ISI; whereas magnitude of RR responding was directly related to ISI, and maximum HR conditioning occurred at 2.25 sec. Discrimination conditioning failed to occur in acquisition in the RR response system, and occurred maximally at. 75 sec in the NM system and 2.25 sec in the HR system.
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