Unconditional-stimulus locus and interstimulus-interval shift in rabbit (Oryctolagus cuniculus) nictitating membrane conditioning.

The nictitating membrane response of rabbits was conditioned at a 200 msec interstimulus interval (ISI) with either circumorbital (C) or paraorbital (P) shock as the unconditional-stimulus locus. After 3 acquisition days half of each group was shifted to a 700 msec interstimuls interval. Results indicated: (1) more rapid acquisition for Group C, (2) postshift response decrements for both groups, (3) more rapid and stable, as well as complete return to preshift performance levels for Group C. Results were discussed in terms of the response-shaping hypothesis and the contiguity-substitution hypothesis in explaining both conditional response emergence and subsequent modifications of CR topography.