The learning abilities of planarian worms (Dugesia tigrina) were assessed by using a number of Pavlovian conditioning paradigms. Experiment 1 showed that planaria were susceptible to basic conditioning in that they readily developed a conditioned response to a change in ambient luminance when it was consistently paired with an electric shock over a number of trials. In Experiment 2, the change in luminance was presented in a compound with a vibration stimulus during conditioning. Subsequent tests revealed poor conditioning of the elements compared with control groups in which the animals were conditioned in the presence of the elements alone, an instance of overshadowing. In Experiment 3, pre-training of one of the elements before compound conditioning resulted in blocking of learning about the other element. These results add to other studies that have reported cue competition effects in animal species belonging to different phyla (chordate, mollusk, arthropod), suggesting that learning in these phyla could be ruled by similar principles. The results are discussed adopting an evolutionary-comparative approach.

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Elucidation of the conditions in which associative learning occurs is a critical issue in neuroscience and comparative psychology. In Pavlovian conditioning in mammals, it is thought that the discrepancy, or error, between the actual reward and the predicted reward determines whether learning occurs. This theory stems from the finding of Kamin’s blocking effect, in which after pairing of a stimulus with an unconditioned stimulus (US), conditioning of a second stimulus is blocked when the two stimuli are presented in compound and paired with the same US. Whether this theory is applicable to any species of invertebrates, however, has remained unknown. We first showed blocking and one-trial blocking of Pavlovian conditioning in the cricket Gryllus bimaculatus, which supported the Rescorla–Wagner model but not attentional theories, the major competitive error-correction learning theories to account for blocking. To match the prediction error theory, a neural circuit model was proposed, and prediction from the model was tested: the results were consistent with the Rescorla–Wagner model but not with the retrieval theory, another competitive theory to account for blocking. The findings suggest that the Rescorla–Wagner model best accounts for Pavlovian conditioning in crickets and that the basic computation rule underlying Pavlovian conditioning in crickets is the same to those suggested in mammals. Moreover, results of pharmacological studies in crickets suggested that octopamine and dopamine mediate prediction error signals in appetitive and aversive conditioning, respectively. This was in contrast to the notion that dopamine mediates appetitive prediction error signals in mammals. The functional significance and evolutionary implications of these findings are discussed.

Schedule-induced drinking has been a theoretical question of concern ever since it was first described more than 50 years ago. It has been classified as adjunctive behavior; that is, behavior that is induced by an incentive but not reinforced by it. Nevertheless, some authors have argued against this view, claiming that adjunctive drinking is actually a type of operant behavior. If this were true, schedule-induced drinking should be controlled by its consequences, which is the major definition of an operant. The present study tested this hypothesis. In a first experimental phase, a single pellet of food was delivered at regular 90-s intervals, but the interfood interval could be shortened depending on the rat's licking. The degree of contingency between licking the bottle spout and hastening the delivery of the food pellet was 100 %, 50 %, and 0 % for 3 separate groups of animals. Rats that could shorten the interval (100 % and 50 % contingency) drank at a higher rate than those that could not (0 %), and the level of acquisition was positively related to the degree of contingency. In a second phase of the experiment, all groups were exposed to a 100 % contingency, which resulted in all rats developing high levels of schedule-induced drinking. Licking is enhanced if it hastens reinforcement, and can do so at delay characteristics of those present in studies of schedule-induced drinking, thus supporting the view that adjunctive behavior is an operant.Keywords Schedule-induced drinking . Adjunctive vs. operant behavior . Lick-food contingency . Rats Schedule-induced drinking was first described by Falk when he was running an experiment with food-deprived rats on bar pressing under a variable-interval 1-minute food reinforcement schedule in which the animals had continuous access to a bottle of water (Falk, 1961). He observed that rats, despite not being water deprived, consumed large quantities of water (polydipsia) by following the regular pattern of accumulating small quantities after the delivery of each food pellet. As deprivation of food normally leads to a decrease in water intake (e.g., Bolles, 1961), and drinking was not explicitly reinforced, this excessive consumption of water was described by Falk as Babsurd^ (Falk, 1971, p. 577). The main characteristics of schedule-induced drinking are therefore that (a) it takes place after the food pellet has been delivered and (b) in an exaggerated manner. This large consumption of water occurs under an intermittent food schedule and with no apparent reason, as it does not appear to be reinforced during training. Characterization of schedule-induced drinking as an adjunctive behaviorOne of the approaches to explain schedule-induced drinking is to consider this as a phenomenon belonging to a separate category of behavior: adjunctive (Falk, 1971; see a critical analysis by Wetherington, 1982). According to this point of view, adjunctive drinking is induced by the delivery of periodic food but is not reinforced by it (see Baum, 2012, for a recent treatment of ...