Abstract
Higher-order conditioning phenomena, including context conditioning and blocking, occur when conditioning to one set of stimuli interacts with conditioning to a second set of stimuli to modulate the strength of the resultant memories. Here we analyze higher-order conditioning in the nematode worm Caenorhabditis elegans, demonstrating for the first time the presence of blocking in this animal, and dissociating it from context conditioning. We present an initial genetic dissection of these phenomena in a model benzaldehyde/NH4Cl aversive learning system, and suggest that blocking may involve an alteration of memory retrieval rather than storage. These findings offer a fundamentally different explanation for blocking than traditional explanations, and position C. elegans as a powerful model organism for the study of higher order conditioning.
Highlights
In context conditioning, context cues present during training become associated with a memory such that retrieval of the memory is enhanced in the presence of the context cues, and hindered in alternate contexts
We first attempted to determine conditions which would allow for NH4Cl/food-deprivation learning equal in extent to benzaldehyde/food-deprivation learning, since previous research has shown that equivalent conditioning strengths are a facilitating condition for blocking[15]
We found that training to 100 mM NH4Cl, followed by testing on a gradient produced by a 5 μL point of 2.5 M NH4Cl opposite a 5 μL point of 20 mM NaAc, produced the greatest alteration of post-training chemotaxis relative to untrained controls, and that which was most similar to benzaldehyde/food-deprivation conditioning (Fig. 1)
Summary
Context cues present during training become associated with a memory such that retrieval of the memory is enhanced in the presence of the context cues, and hindered in alternate contexts. Blocking is often explained as a failure to attend to the non-associated component of the conditioned stimulus, because the prior association between the associated component and the unconditioned stimulus is sufficient to fully predict the US. In this view, it is necessary that the US be “surprising” to the subject to form new associative memories[7,8]. The insight that a cue must be “surprising” to be learned about was later expanded and formalized in the influential Rescorla-Wagner model of classical conditioning[9], which proposed that conditioned stimuli become associated with an unconditioned stimulus based on the scale of the mismatch between the strength of pre-existing associations with the US and the strength of the present association. Correspondence and requests for materials should be addressed to Chemotaxis Index 1% Bnz 2 mM NaAc 20 mM NaAc 200 mM NaAc 2 M NaAc www.nature.com/scientificreports/
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