Transformation of siphon responses during conditioning of Aplysia suggests a model of primitive stimulus-response association.

A semi-intact preparation was used to study the effects of classical conditioning on the type of siphon response elicited by a conditioned stimulus to the mantle of Aplysia. Five pairings of the conditioned stimulus with an unconditioned stimulus to nerves from the tail transformed the constricting alpha response of the siphon into a conditioned flaring response resembling the unconditioned response to stimulation of the tail nerves. Although some pseudoconditioning occurred, an associative component was indicated by the significantly greater incidence of flaring responses after paired training than after unpaired presentations of the conditioned and unconditioned stimulus or the unconditioned stimulus alone. Previously described cellular plasticity in the underlying neural circuits suggests a testable model based on cell-wide rather than synapse-specific mechanisms, which can account for specific conditioned responses. In this model, effective stimulus-response associations are produced by a concatenation of stimulus-specific facilitation of sensory neurons (a mechanism for alpha conditioning) and response-specific facilitation of motor neurons (a mechanism for pseudoconditioning).