Implicating causal relations between cellular function and learning behavior.

Learning in the nudibranch mollusc Hermissenda shows many features of vertebrate associative conditioning. Pairings of light and rotation produce conditioned suppression of phototaxis, which is retained for days, shows savings, extinction, contingency sensitivity, and, recently, temporal specificity. In addition, specific features of the behavior have been shown to undergo classical Pavlovian conditioning. Extensive analysis of the neural networks mediating the flow of visual and graviceptive information have demonstrated convergent pathways at specific cellular loci. These cells are critically implicated for a primary role in the conditioned modifications of behavior. A variety of experimental approaches consistently support the proposal that reductions of specific K+ currents in the Type B photoreceptor soma play a causal role for several different behavioral expressions of the conditioning. In this article, we review several of these behaviors to show how the demonstrated close temporal correspondence of cellular and behavioral functions further implicates certain causal relations. For example, studies of the shadow withdrawal behavior of Hermissenda suggest a causal relation between the long-lasting depolarization of the Type B photoreceptor and the animal's reduced ability to turn towards the light at light/dark boundaries. Whereas the shadow response corresponded to cellular events at the end of a light step, responses to the onset of light or rotation were largely unexplored. By using a different approach, we identified behavioral responses during the first few seconds of stimulation with light and rotation. These responses, for which Pavlovian conditioning was demonstrated, correspond closely in time to known cellular correlates.(ABSTRACT TRUNCATED AT 250 WORDS)