Ontogeny of successive negative contrast and its dissociation from other paradoxical reward effects in preweanling rats.

Abstract

The effects of abrupt reductions in reward magnitude (the successive negative contrast or SNC effect) were studied in rats at a number of ages and under conditions of dry pellet and milk rewards. In experiment (exp) 1, SNC was shown in 61-68 day old rats, but not in 17-24 day olds with food pellet reward. Exp 2 showed strong SNC at 34-35 days and marginal contrast at 25-26 days, again wit food pellet reward. With milk reward (exp 3) there was a clear SNC effect at 25-26 days, a slight effect, at 20-21 days but no effect at all in 16-17 day olds. Apparently SNC occurs earlier with milk than with dry food reward. Exp 4 repeated the 16-17 day milk-reward condition of exp 3 with more highly massed trials. Again, contrast did not appear at this age, although, as in all other experiments, reduction of reward size reduced performance to the level of an appropriate small reward control. Exp 5 demonstrated a dissociation of SNC and other paradoxical reward effects at 16-17 days of age. Despite the absence of SNC at 16-17 days, the extinction effects of partial reinforcement (PREE), partial delay of reinforcement and varied magnitude of reward were large and clear. The magnitude-of-reward extinction effect, an extreme case of contrast, was not found. These results are related to earlier work on the ontogeny of SNC in even younger animals, in which suckling from a dam was the reward system, and they are discussed in relation to an apparent transitional period of the PREE in ontogeny.