Fig. 2.

Operant reward learning in Aplysia’s feeding behavior. a Protocol for appetitive operant conditioning. Three groups of animals (Control, Contingent reward, Non-contingent reward) were subjected to a sustained food stimulus to the lips (top of frames) to incite the maintenance of feeding posture (left) and the generation of repeating cycles of radula biting movements (blue circles at right). Control: no additional food reward stimulus was provided. Contingent reward: a small volume of seaweed juice was additionally injected into the buccal cavity (at green rectangles) in association with each spontaneous radula bite during a 40-min training period. Non-contingent: the food reward was delivered at regular intervals and independently of bite occurrences. b Behavioral changes induced by operant reward training. In control animals (left), radula bites occurred erratically (blue circles) and in a random temporal succession as indicated by the corresponding autocorrelation histograms (below). In contrast, contingently rewarded animals (middle) generated radula movement cycles at a higher frequency and in stereotyped rhythmic succession (as indicated by the autocorrelation histogram fit with a Gabor sinusoidal function). This intensified rhythmic behavior was a specific consequence of the contingent operant reward association, because it was not expressed by non-contingently rewarded animals (right) which continued to express erratic, irregular biting behavior similar to control animals