ABSTRACT
Comparative psychology has attracted the interest of behavioral and brain scientists who hope that fundamental, phylogenetically general principles of behavioral organization can be identified and applied to the study of human behavior and brain function. T. C. Schneirla, one of the seminal comparative psychologists, discerned a pattern common in adults of simpler species and in the early ontogenetic stages of more complex species, namely that “low intensities of stimulation tend to evoke approach reactions, high intensities withdrawal reactions” (Schneirla, 1959). He elaborated this general observation into a biphasic approach/withdrawal theory which was applied primarily to problems in behavioral development, such as the nature-nurture controversy. Schneirla’s observations of biphasic responses to environmental stimulation raise three interesting neurobiological questions, aside from the question of the developmental roles of such response patterns. Are intensity-dependent, biphasic response patterns common in the animal kingdom, as Schneirla claimed? How can an animal generate opposite responses to qualitatively similar stimuli? How does an organism change its choice of responses as a consequence of behavioral experience? As described below, recent investigations of the gastropod mollusc, Aplysia californica, address these questions, and pose a fourth question. How can we balance the promises and pitfalls of reductionist analysis in our efforts to understand neuronal substrates of behavior?
