Effects of Preshift Small Magnitude of Reward Received during Goalbox Placements on the Negative Contrast Effect

During preshift, one experimental group of rats was given a large magnitude of food reward following a traversal of a straight alley and during a goalbox placement, while the other experimental group was given a small reward during goalbox placement and a large reward following a run. During postshift, all experimental groups were given a small reward of food following a traversal down the runway and during a goalbox placement. A control group was maintained on small reward during placements and following a traversal throughout the study. Only the group who received preshift large reward during placement and following a runway response ran slower to small reward during postshift than the control group maintained on small reward (negative contrast effect).

Multiple Shifts in Magnitude of Reward

Three groups of 5 rats were administered either large reward (10 pellets), small reward (2 pellets), or multiple shifts (Iarge-small-large-etc.) in an alleyway. The multiple-shift group received a total of 7 large and 6 small phases of reinforcement. Early in training the shifted group exhibited positive contrast effect to a shift to large reward and negative contrast effect to a shift to small reward. Later in training, the same group showed neither effect perhaps because experience with the shift provided a smaller discrepancy between the upshifts and downshifts in magnitude of reward.

Effects of Shifts in Delayed Food Reward: Repeated Shifts

In a repeated-shifts study, a negative contrast effect was obtained when a group of rats was shifted from 0- to 20-sec. delayed food reward only in the first, but not the second, shift. No positive contrast effect was obtained in either shift when a group of rats was shifted from 20- to 0-sec. delayed food reward. Shifts in delay are not repeatedly effective in inducing negative contrast and not effective at all in inducing positive contrast.