Extinction of associative learning in Hermissenda: behavior and neural correlates.

The nudibranch mollusk Hermissenda crassicornis' normal attraction to light can be suppressed by repeated pairings of light with rotation. The present study examines the effects of extinction procedures on this simple form of associative learning. Presentation of non-reinforced light steps following associative conditioning resulted in an attenuation of phototaxic suppression, evident at both short- and long-term retention intervals. The absence of habituation of phototaxic behavior, coupled with the failure to demonstrate spontaneous recovery of extinguished conditioned suppression, indicates that extinction of associative conditioning in Hermissenda depends little upon non-associative learning processes. Electrophysiological evidence indicates that the Type B photoreceptors, which have been causally implicated in the acquisition and retention of associative learning, play an important role in mediating extinction as well. Enhanced input resistances and light responses of B cells, which are produced by associative training, are absent for animals subsequently exposed to light unaccompanied by rotation. In terms of both behavioral and electrophysiological measures, extinction appears to result primarily from a reversal of the original acquisition process.