Postponed Conditioning: Testing a Hypothesis About Synaptic Strengthening

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Abstract
A connectionist neural circuit model of motivated behavior that uses a variant on Hebbian synaptic plasticity (Halperin, 1990, 1991) predicts that transient excitatory conditioning should occur to a stimulus presented after the US has been removed, provided that this CS presentation is postponed until just before the end of a behavioral afterdischarge. We report here some experimental confirmations of this prediction, using the social display system of male Siamese fighting fish, in which afterdischarge is visually observable. A mirror was presented and removed, to elicit social display followed by display afterdischarge. A CS object was presented just as the afterdischarge faded. After 10 to 12 such pairings, the CS shown alone elicited display. The model, however, also predicts that no excitatory conditioning should occur to a CS presented near the beginning of a long afterdischarge. This arrangement and an unpaired CS condition were used as controls, and the prediction was confirmed. However, the model further suggests that a CS presented at the beginning of an afterdischarge should show excitatory conditioning if the display afterdischarge could be artificially shortened so that the CS presentation would come just before the end of the afterdischarge. In a second experiment, the CS was presented immediately after mirror removal, and then afterdischarge was ended by feeding the fighting fish a guppy. Since the CS now predicted the arrival of food as well as the end of the afterdischarge, we depressed conditioned feeding responses by food-satiating the fish before presenting the CS alone. Display conditioning was confirmed. Controls were included for the effects of feeding.