Developmental Analysis in Aplysia Reveals Inhibitory as Well as Facilitatory Effects of Tail Shock

Recent studies that were performed to examine the development of learning in the siphon withdrawal reflex in Aplysia have shown that habituation, dishabituation, and sensitization emerge according to different developmental timetables (Nolen & Carew, 1988; Rankin & Carew, 1987b). In the course of those studies, an inhibitory process triggered by tail shock was also described. In this study we examined some of the features of this inhibitory process in young juvenile animals (stage 11 and early stage 12) which developmentally lack sensitization. In stage 11 animals, the magnitude of inhibition significantly increased as a function of tailshock intensity. A second process triggered by tail shock, the facilitatory process of dishabituation, exhibited a surprising monotonic decrease in magnitude as a function of increased tailshock intensity. The same pattern of results was observed in early stage 12 animals, although the inhibition appeared somewhat weaker. These results, in conjunction with previous findings examining an intermediate level of tail shock (Rankin & Carew, 1988), revealed a monotonic increase in inhibition, and a parallel monotonic decrease in dishabituation, with increased levels of tail shock. Collectively, these findings suggest the hypothesis that the inhibitory process, which is progressively recruited with increased levels of tail shock, becomes progressively more effective in competing with the expression of the facilitatory process of dishabituation. Our developmental studies have allowed us to dissociate four behavioral processes: two decrementing (habituation and inhibition) and two facilitatory (dishabituation and sensitization). These results, together with those from earlier studies by Whitlow (1975), Wagner (1976), and Hochner, Klein, Schacher, and Kandel (1986a, 1986b), suggest that a dual-process view of nonassociative learning (Carew & Kandel, 1974; Groves & Thompson, 1970) requires revision and that a multiprocess view is required to adequately account for the mechanisms underlying nonassociative learning.