Ontogenetic differences in the effects of unpaired stimulus preexposure oneyeblink conditioning in the rat

Learned irrelevance (LIr) is a Pavlovian conditioning phenomenon in which r andom or unpaired preexposure to a conditional stimulus (CS) and to an unco nditional stimulus (US) retards subsequent paired conditioning involving th ese stimuli. A previous development study of eyeblink conditioning in the r at suggested that LIr is not present on postnatal Day 20. Stanton, Fox, and Carter (1998) showed that unpaired preexposure to a CS and a US on postnat al Day 17 failed to retard (and, in fact, facilitated) subsequent paired co nditioning involving these stimuli on postnatal Day 20. The present experim ent 1, LIr was observed when rat pups were tested for eyeblink conditioning as described in Stanton et al. (1998), except that preexposure occurred on postnatal Day 27, and acquisition testing occurred on postnatal Day 30, an d four types of preexposure were compared: chamber only, CS alone, US alone , or unpaired presentation of CS and US. Unpaired preexposure impaired acqu isition relative to that of the remaining three groups, which did not diffe r. Experiment 3, showed that under the conditions of Experiment 2, LIr fail ed to appear on postnatal Day 20, but was observed on postnatal Days 25 and 30. These findings suggest that learning that events are unrelated emerges between postnatal Days 20 and 25 in the rat. Possible behavioral and neura l mechanisms underlying this effects are discussed.