Dan M. Wrather

Preshift reward magnitude and positive contrast in the rat

Four groups of rats were given 24 trials of training in a straight runway with 1, 2, 4, or 8 food pellets, and then shifted to 8 pellets. There was a 20-sec delay of reinforcement on each trial, to prevent a ceiling effect on running speeds. The three shifted groups showed stable positive contrast relative to the unshifted control group. Postshift running speed was inversely related to magnitude of preshift reward.

Magnitude of negative reinforcement and resistance to extinction

Abstract In the first experiment rats experienced large or small magnitude of negative reinforcement (shock reduction) in a straight alley. Half of the subjects in each magnitude group received continuous reinforcement, and the other half received a 50% partial reinforcement schedule (nonreinforcement consisting of no shock reduction in the goal box). In extinction the groups were ordered: large partial > small-partial > small-continuous > large-continuous. In the second experiment rats received large, small and nonreinforcement in various sequences using the runway-negative reinforcement procedure and were ordered: SNL>LNL>SNL>LNS in resistance to extinction (letters represent the magnitudes in the sequence experienced in acquisition). The results of these experiments indicate a commality between positive and negative reinforcement with respect to behavioral phenomena and theoretical accounts of those phenomena.

Transfer of the partial reinforcement extinction effect between escape (shock) and appetitive (food) conditioning

Abstract In the first experiment rats were given partial reinforcement or continuous reinforcement in either an escape or an appetitive paradigm. Subsequently, the rats received continuous reinforcement training under motivational conditions opposite those experienced earlier. Finally, responses were extinguished according to the motivational conditions experienced in the second phase. The results indicated that partial reinforcement in the initial phase operated to increase resistance to extinction in the last. In a second experiment this intermotivational partial reinforcement extinction effect was shown to survive interpolated experiences with extinction, a 1-week rest period, and continuous reinforcement reacquisition. A third experiment examined the influence of intramodal versus intermodal nonreinforcement-reinforcement sequences on the intermotivational partial reinforcement extinction effect. Interactive effects between similarity of aversive outcome (escape nonreinforcement, appetitive nonreinforcement) and reinforcement type (negative, positive) were found. The theoretical implications of the data from all three experiments are discussed.

Partial reinforcement effect: The expectancy of reward on nonreward trials*

In order to determine the importance of the development of expectancy of reward prior to partial reward trials; rats were given 20 continuously reinforced trials prior to 20 partially reinforced trials (CRF-PRF) and compared to Ss given only 20 partially reinforced trials (PRF). Control groups received 20 or 40 continuously reinforced trials (CRF-20, CRF-40) to determine the effect of differing numbers of acquisition trials. Results showed that terminal acquisition differences were minimal in the run segment of the alley and that Group CRF-PRF was more resistant to extinction than Group PRF, and both were more resistant to extinction than the CRF-20 and CRF-40 groups, which did not differ from each other. These results were interpreted as supporting the notion that the expectancy of reward on nonreward trials during partial reinforcement acquisition is a determiner of the magnitude of the partial reinforcement extinction effect.

Transfer of Persistence of Responding across Motivational and Reward Conditions

Rats were trained with a continuous or partial reinforcement schedule under water deprivation-reward conditions or food deprivation-reward conditions; half were then shifted to the opposite motivational-reward conditions and half remained as in the first phase for a period of continuous reinforcement. Extinction under the second-phase motivational conditions showed that a partial reinforcement effect occurred for all groups (i.e., partial groups were more resistant than the appropriate comparison continuous groups), although a shift in motivational-reward condition tended to reduce the size of the partial reinforcement effect. These results support the notion that the partial reinforcement effect is a robust phenomenon in the rat, not subject to narrow limits as has sometimes been suggested.

Contrast effects with shifts in punishment level

The present experiment was designed to investigate the effects of shifts in punishment level using a successive shift procedure. Rats were given a constant reward (two pellets) throughout training but received varying intensities of brief electric shock (punishment) in the goalbox. During preshift, subjects ran for 40 trials to either.1,.4, or.8-mA shocks in the goalbox. All subjects were then shifted to.4 mA in the goalbox for 40 trials. The results showed that subjects shifted to a higher intensity shock ran slower than subjects originally trained on that higher intensity shock (negative contrast). There was no evidence of a corresponding positive contrast effect. The data were discussed with respect to their implications for theories that attempt to treat reward and punishment in comparable theoretical fashion.