UNBLOCKING WITH QUALITATIVE CHANGE OF UNCONDITIONED STIMULUS

Conditioned barpress suppression was used to examine the effects of qu alitative changes in the unconditioned stimulus (US) between Phases 1 and 2 of a blocking paradigm. In Phase 1, rats received pairings of a conditioned stimulus (CS) with footshock. In Phase 2, experimental sub jects received a single trial of the same CS or a different CS compoun ded with a second stimulus and followed either by a footshock or an ic e water dunking. These two USs were equated in their potential to elic it conditioned suppression of barpressing. Less blocking of the second stimulus (i.e., unblocking) was observed in subjects that received a qualitative change in US between phases than in subjects for which the US was consistent between phases. This unblocking effect is discussed with respect to the differences between various models of conditionin g and several prior successes and failures to demonstrate unblocking. (C) 1997 Academic Press.