Análisis del efecto de preexposición al EI con un procedimiento de condicionamiento apetitivo

Resumen

This thesis made use of the conditioned flavour preference (CFP) paradigm, with rats subjects, in order to investigate the effect of prior exposure to an appetitive unconditioned stimulus (US) on a conditioning treatment (the US-preexposure effect). Previous work, mainly using aversive paradigms, has focused on two main explanations for this effect - associative blocking and in terms of a non-associative habituation process. In this thesis, the nature of the mechanisms operating in the appetitive version of the US-preexposure effect was explored. The experiments reported in Chapter 2 first established the basic US-preexposure effect using the CFP procedure by demonstrating that preexposing rats to sucrose (US), will produce a reduced conditioned response (CR) to the conditioned stimulus (CS) when it is presented alone in a test. Chapter 3 then explored the role blocking by contextual cues might play in this demonstration of the US-preexposure effect. The experiments reported in Chapter 3 produced results that did not support a blocking-by-context explanation. Accordingly an alternative explanation was suggested that explains these results in terms of a modified version of the blocking-by-context hypothesis, in which not the context, but the taste of the US serves to block the acquisition to the CS during conditioning. Chapter 4 explored this alternative account for the US-preexposure effect by using a different substance as the US. Results from the experiments reported in Chapter 4 showed that using maltodextrin as the US (a substance with similar nutritive consequences to sucrose but a less salient taste), produces only a weak US-preexposure effect; this does not always occur and it depends on various procedural parameters. In Chapter 5 a comparison was made between animals trained in different motivational states. Varying the motivational state of animals, allowed the experiments in Chapter 5 to test the blocking-by-taste hypothesis of the US-preexposure effect. If the mechanism underlying the effect relies on blocking by taste, then the US-preexposure effect should be demonstrated more readily in hungry animals than in those animals that are sated. This result is found in Experiment 9. The experiments in Chapter 6 demonstrated, however, that the effect could be found both when the US is sucrose (providing both a sweet taste and motivational post-oral consequences) and when it is saccharin (a substance that lacks any caloric properties). This latter result does not support the blocking-by-taste account previously offered. A different mechanism is offered in terms other than blocking-by-taste for the saccharin case. The implications of these results for interpretations of the US-preexposure effect are considered in Chapter 7.