Double responding in classical nictitating membrane conditioning with single-CS dual-ISI training.

Rabbits were classically conditioned under a single-CS dual-ISI paradigm. Contrary to a previous experiment by Frey (1970), a separate CR was conditioned at each interstimulus interval (ISI), producing a double response (DR) on long-ISI trials. Further experiments indicated that DR development is facilitated by (a) circumorbital US-location and (b) relatively high US-intensity. However, subsequent shifts to nonoptimal conditions did not produce substantial decreases in DR frequency. The presence of DRs appears to be readily amenable to interpretation in terms of a Pavlovian stimulus-trace mechanism in which the US becomes associated with a particular temporal portion of the CS. An instrumental-response-shaping hypothesis of classical conditioning can explain these data only if the hypothetical reinforcement mechanism is assumed to be considerably more complex than had previously been supposed.