Feeding behavior in Aplysia fasciata and A. oculifera is modified by pairing the behavior with reinforcing consequences. Successful and unsuccessful attempts to transfer food from the buccal cavity to the crop act as positive and negative reinforcers, respectively. A number of changes in feeding behavior occur as a result of pairing of feeding with the negative reinforcer: feeding responses become less effective in leading to the entry of food into the buccal cavity; when food does enter the buccal cavity, it exits sooner; swallowing responses after food entry are less likely to occur; Aplysia eventually cease responding to food. Pairing successful transfer of food into the crop with feeding behavior produces opposite effects. Behavioral change is specific to pairing, as shown by lack of change when reinforcement is explicitly unpaired with feeding behavior. Behavioral change is specific to foods with a particular taste and texture; generalization to alternate foods was not observed. In spite of cessation of feeding, animals remain aroused, as shown by low response latency to alternate foods. Memory of response change persists for at least 48 hr.

Bilateral sectioning of the esophageal nerves that innervate the gut of Aplysia was found to have profound effect on response decrement to inedible food: Time to criterion for cessation of feeding was elevated, no memory of the decrement was present 24 hr after training, and motor patterning during training was altered. The parametric features of response decrement to sustained lip stimulation were examined to determine their resemblance to parameters of response decrement to inedible food after esophageal nerve sectioning. Parameters of response decrement were similar, indicating that after esophageal nerve sectioning response decrement is likely to be the result of sustained lip stimulation. Bilateral nerve sectioning had no effect on decrement due to sustained lip stimulation. Unilateral lesions and lesions of either of the two major divisions of the esophageal nerves had no effect on learning that food was inedible. The data indicate that bilateral nerve sectioning eliminates all stimuli causing negative reinforcment of feeding due to failure to consume food. Based on the data in this and the previous paper, a model is presented suggesting sites of action and mechanisms for learning that foods are edible or inedible in Aplysiu.An important problem in neuroscience is determining the relationship between mechanisms underlying different forms of plasticity affecting the same efferent pathway. The present paper examines the relationship between two types of plasticity causing inhibition of feeding in Aplysia: decrement due to learning that a food is inedible and decrement caused by sustained stimulation of the lips with food (presumably due to sensory adaptation or habituation).In the preceding paper (Susswein et al., 1986), data were presented indicating that the reinforcing consequences of feeding behavior affect subsequent responsiveness to food. Successful consumption of food acts as a positive reinforcer of feeding, while failed attempts to consume food act as negative reinforcers. Potentially, any of a number of stimuli could signal Aplysia as to whether they have succeeded in consuming food. These stimuli could be carried into the CNS via afferents traveling in a number of peripheral nerves. In a preliminary report (Schwarz and Susswein, 1984), we presented data suggesting that the esophageal nerves innervating the gut carry afferents that inform the CNS about whether food has been consumed; these afferents are necessary for reinforcement of feeding. When gut inputs that reinforce feeding are not activated, decreased feeding occurs because of other processes. The present paper examines this hypothesis in greater detail. The parametric features of inhibi-

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