Associative Basis of UCS Preexposure in Excitatory and Inhibitory Rabbit Eyelid Conditioning
Exposure to the unconditional stimulus (UCS) prior to its pairing with the conditional stimulus (CS) retards subsequent excitatory conditional response (CR) acquisition in a variety of Pavlovian conditioning preparations. Traditionally, this UCS preexposure effect has been attributed to nonassociative, adaptational-like processes. Recently, however, it has been suggested that associative processes, involving the formation of an association between the UCS and stimuli of the context in which the UCS is preexposed, play a crucial role in the effects of UCS exposure. The experiments reported in this thesis were designed to determine whether conditioning to contextual stimuli may, in fact, mediate the effects of UCS preexposure in rabbit eyelid conditioning.
The results of Experiment 1 demonstrate that the UCS preexposure effect in excitatory eyelid conditioning is evident only if CS-UCS pairings are administered in the same context as UCS preexposure. Thus, mere repeated exposure to the UCS is not sufficient for production of the UCS preexposure effect as would be expected on the basis of any entirely nonassociative account. Experiments 2 and 3 demonstrate that the detrimental effect of UCS preexposure on subsequent excitatory CR acquisition may be attenuated by associative manipulations of contextual stimuli of the preexposure environment. In Experiment 2, contextual stimuli were "latently inhibited" (a procedure known to reduce conditioning to stimuli) prior to the start of UCS preexposure. This procedure, which should have had the effect of reducing conditioning to contextual stimuli, attenuated the UCS preexposure effect. In Experiment 3, contextual stimuli were extinguished (a procedure known to weaken established conditioning). This procedure also attenuated the UCS preexposure effect. The results of Experiment 1,2, and 3 establish that associative processes involving contextual stimuli are crucially involved in the UCS preexposure effect in excitatory eyelid conditioning.
In Experiment 4, the effect of UCS preexposure on inhibitory eyelid conditioning was examined. UCS preexposure faciltated inhibitory learning, an effect that would be expected if conditioning to contextual stimuli occurs during UCS preexposure, but not if UCS preexposure involves only nonassociative processes.
The results were discussed in relation to various theoretical accounts of UCS preexposure. Although other factors may be involved, the results of the present thesis demonstrate an associative basis for the effects of UCS preexposure in rabbit eyelid conditioning. The role of conditioning to contextual stimuli in other preconditioning stimulus exposure procedures (e.g., latent inhibition, truly random CS/UCS presentations), and in effects involving postconditioning UCS exposure (reinstatement of fear) was also discussed. In conclusion, it is argued that conditioning to contextual stimuli occurs in a variety of stimulus exposure procedures, and that such conditioning may be importantly involved in mediating a variety of learning phenomena.