Classical conditioning, differential conditioning, and second-order conditioning of the Aplysia gill-withdrawal reflex in a simplified mantle organ preparation.

Nonassociative learning was previously examined in a simplified preparation consisting of the isolated mantle organs and abdominal ganglion of Aplysia californica that is advantageous for relating cellular events to behavior (T. E. Cohen, S. W. Kaplan, E. R. Kandel, & R. D. Hawkins, 1997). Results of the current study show that the gill-withdrawal reflex in that preparation also underwent 2 associative forms of learning: classical conditioning and differential conditioning. In addition, the reflex underwent second-order conditioning with either forward or simultaneous pairing of a novel conditioned stimulus (CS2) and a previously conditioned stimulus (CS1). Moreover, extinction of CS1 after simultaneous second-order conditioning was accompanied by a decrease in responding to CS2, suggesting that the conditioning might have involved formation of an association between the CSs. In each of these paradigms, learning in the Aplysia mantle organ preparation resembled learning in vertebrates.