An analysis of late-developing learning and memory systems in rats: fear-potentiated startle and context-specific latent inhibition and extinction

摘要

Eleven experiments examined two late-developing learning and memory systems in rats: fear-potentiated startle (FPS) and the contextual regulation of latent inhibition and extinction. The first study was based on three previous developmental findings on FPS: (1) FPS to an odour CS emerges at postnatal day (PN) 23; Rats conditioned at PN16 to an odour CS express freezing but not FPS when tested at PN23, and (3) FPS to an odour CS trained at PN16 is activated if rats are also trained to a difference odour at PN23 (Yap, Stapinski, & Richardson, 2005). Yap et al. (2005) hypothesised that the activation effect only occurs if rats are given training to the second odour at an age when FPS has emerged. Study 1 assessed this hypothesis and trained the second odour CS at either PN23 or PN20. Contrary to expectations, the results of this study showed the activation effect for both groups of rats. Surprisingly, the results also revealed a significant FPS effect to the odour CS trained at PN20. Subsequent experiments examined this unexpected result, and found that learning to odour 1 at PN16 facilitated the age of onset for FPS at PN20. The results of Study 1 are discussed in relation to past findings on enrichment, cumulative learning, and neurobiological models of conditioned fear. The second section of this thesis (Studies 2 and 3) examined the context-specificity of two memory interference paradigms, latent inhibition and extinction, in developing rats. The studies found that both phenomena were context-specific at PN23-25 but not at PN16-18. Moreover, the results suggest that the context-specificity of both latent inhibition depended on the age of the rat during the second phase of training, but not their age during the first phase of training or their age at test. The implications of these findings for theoretical and neural models of learning, as well as the occurrence of latent inhibition and extinction during development are discussed.