Operant conditioning of head-waving in Aplysia. I. Identified muscles involved in the operant response

A basic goal in the neurosciences is to understand the cellular mechanisms underlying associative learning. The 2 major forms of associative learning are classical conditioning and operant conditioning. In recent years considerable progress has been made towards a cellular analysis of classical conditioning in a number of different preparations. In contrast, the cellular mechanisms underlying operant conditioning are poorly understood. Since the marine mollusc Aplysia has proved to be a powerful preparation for studying cellular and molecular mechanisms of a variety of forms of learning, including classical conditioning, we asked whether Aplysia might also be capable of exhibiting operant conditioning. In previous experiments we found that a naturally occurring behavior, the head-waving response of Aplysia, could be operantly conditioned (Cook and Carew, 1986). In the present paper we have carried out a quantitative analysis of the horizontal and vertical components of the head-waving response at the level of individual muscle groups. We have identified a discrete, bilateral band of neck muscles, the lateral columellar muscles (LCMs), whose electromyographic (EMG) activity is significantly correlated with the horizontal component of head-waving, the component modified by operant conditioning. Since head-waving is a complex behavioral response involving a variety of different muscle groups, using a restricted system such as the LCMs as an analog of the head-waving response will greatly facilitate a cellular analysis of operant conditioning in Aplysia.