Associatively reduced withdrawal from shadows in Hermissenda: a direct behavioral analog of photoreceptor responses to brief light steps.

When the nudibranch Hermissenda crassicornis encounters a shadow in an otherwise uniformly illuminated field, it stops and turns back into the light within seconds. Associative conditioning, with paired light and rotation stimuli, produces learned modifications of phototaxis in illumination gradients. This same training procedure significantly reduced the ability of paired, but not random or naive control animals, to withdraw from shadows. In naive animals, after 13 min of dark adaptation, withdrawal from shadows was less apparent when animals encountered this stimulus the first time than after the second encounter. This difference in responsiveness to the first and second edge stimulus paralleled differences in type B photoreceptor impulse frequencies recorded during and after first and second steps of light. Earlier studies have shown that associative training of Hermissenda increases a long-lasting depolarization (LLD) which follows a light step. Our present findings suggest a functional relationship between the LLD of the type B photoreceptor and the behavioral response to light-dark differences. This supports the view that membrane changes which cause modifications of LLD magnitude store the learned association for later recall.