Successive Positive Contrast in One-Way Avoidance Learning

The main finding of these experiments was a positive contrast effect in one-way avoidance learning. Experiment 1 showed that increasing safety time during one-way avoidance training led to improved performance, surpassing that of a control group that had received the high reward (safe time) from the beginning of training. Experiment 2 showed that a similar positive contrast effect occurred when the time spent in the danger compartment before the onset of the warning signal was shortened. These results suggest that time spent in a safe context acts as a reinforcer of the avoidance response; however, its incentive value depends not only on its duration, but also on the length of the time spent in the danger compartment before the onset of the signal. Overall, results also suggest that the avoidance response is a mixture of flight (motivated by fear) and approach (to a safe place) behaviour. The specific weight of the flight or approach component may be a function of the time and the amount of activation of each emotional state (fear or relief) due to opponent homeostatic compensatory processes that occur in the danger and safe compartments during one-way avoidance learning.

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.

Behavioral Contrast in Pigeons with Qualitatively Different Reinforcers

Three white Carneaux pigeons were trained to respond on a mult VI 1-min. (milo reinforcement), VI 1-min. (pea reinforcement) schedule when each component was associated with a different key, feeder, and reinforcer. The experiment was divided into four phases. In Phases 1 and 3, baseline rates of responding were established. In experimental Phases 2 and 4, one component of the multiple schedule was changed to extinction. During the experimental phases, response rates decreased in the extinction component and increased in the unchanged component (positive behavioral contrast). The increase in responding in the unchanged component was greater when the more valued reinforcer was extinguished. These findings are very similar to those reported by Beninger and Kendall (1975) and extend the positive contrast effect to another species, pigeons.

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.

Runway Performance as a Function of Magnitude of Runway Reward and Intertrial Reinforcement

Rats were trained to traverse a straight alley for either 1 or 10 food pellets as reward. Each runway trial was preceded by an intertrial reinforcement of 1 or 10 pellets. A negative contrast effect obtained and persisted throughout the 72 trials conducted. Although there was a strong suggestion of a positive contrast effect throughout training, the effect did not prove reliable. The contrast effect results were interpreted as challenging the view that with extended training ITRs become “irrelevant” to runway reward.