Motivational control of instrumental performance following a shift from thirst to hunger.

Thirsty rats were trained to press a lever for either a sucrose solution or saline before performance was tested in extinction while the animals were either hungry alone or experiencing both hunger and a sodium appetite. Reinforcer-specific motivational control was observed in that the animals trained with the sucrose solution pressed more than those trained with the saline when they were tested hungry, but not when they were tested under combined hunger and sodium appetite. In order to assess the role of a Pavlovian incentive process in this effect, thirsty animals received non-contingent pairings of one stimulus with the sucrose solution and another with saline in the second experiment. In an extinction test the sucrose stimulus augmented lever pressing relative to the saline stimulus when the animals were hungry, but not when they were thirsty. In the subsequent experiments the contribution of the Pavlovian process was equated by giving concurrent training with both incentives. Lever pressing and chain pulling were reinforced concurrently, one with the sucrose solution and the other with saline, while the animals were thirsty. Once again, the animals pressed more in extinction if this action had been trained with the sucrose solution rather than the saline, but only if they were hungry rather than thirsty. Thus, instrumental performance across a thirst-to-hunger shift can also be controlled by an instrumental incentive process. The direct engagement of the instrumental process by this motivational shift contrasts to the absence of such control following a hunger-to-thirst transition (Dickinson & Dawson, 1987a), a fact attributed to the asymmetrical motivational interactions produced by water and food deprivation.