The role of quinine chlorohydrate in conditioned inhibition of the tarsal reflex in Drosophila melanogaster

The proboscis extension reflex (PER) can be elicited by applying a sucrose stimulation to the tarsus of a walking fly. This reflex decreases in frequency with repetition, presumably habituation, a nonassociative learning. If each sucrose stimulation is followed by a bitter stimulation, quinine chloride, the PER declines more rapidly, probably the result of conditioning, an associative learning. The present work shows that quinine chloride does not always inhibit PER suppressions but depends on the moment of delivery, being most effective when presented immediately after a sucrose stimulation. A bitter stimulus presented before, or simultaneously with a sucrose stimulation is less effective than habituation to sucrose alone. This experiment provides evidence for an interpretation in terms of cognitive association. The model of learning is not a Pavlovian conditioning as advanced by Medioni and Vaysse (1975), but corresponds to the punishment paradigm of Dyal and Corning (1973).