Context-US learning in Aplysia californica.

Two experiments explored learning about the relation between an unconditioned stimulus (US) and the context in which that US was delivered in Aplysia californica. Subjects were exposed to 2 different contexts. They received a series of moderate electric shock USs in one context but not in the other. Experiment 1 revealed that the context-shock pairings were sufficient to establish an association between that context and the shock. That association was demonstrated by the ability of the reinforced context to enhance a defensive reflex and to reduce learning about a conditioned stimulus (CS) that was subsequently paired with the shock US. Experiment 2 provided evidence that the context-US association could be extinguished by simple nonreinforced exposure to the reinforced context. These results show that context conditioning is not unique to vertebrate learning. We argue that this learning is difficult to accommodate within current contiguity-based neuronal models of learning.