If every food reinforcer delivered on a VI schedule is preceded by a brief flash of light, rats leverpress for food more slowly than if light flashes are given independent of the food and their responding. This retardation in rate has been attributed to the overshadowing of the response–food association by the light–food association. Such an explanation assumes that response rate is positively correlated to the response association strength. Exps I and II investigated this account by using prefeeding and extinction procedures to test the strength of responding in 44 Long-Evans rats that had received food-correlated or random-light signals. Acquisition response rate was lower in the signaled condition (consistent with previous studies), but responding was more resistant to both extinction and satiation, regardless of whether the light cues were presented during the extinction/prefeeding phase. Exp III, with 27 Ss, revealed no difference between the signaled and random conditions either in terms of acquisition response rates or resistance to satiation when a VR schedule of reinforcement was used. Results are inconsistent with an overshadowing account of the acquisition rate difference. Instead, the signal seems to enhance the rat's sensitivity to the contingencies present on VI schedules. (44 ref)

A review of data and theories on delay of primary reinforcement in discrete trial instrumental learning is provided. The major areas of investigation that are reviewed include delay of negative reinforcement; measurements of delay effects; cue utilization in acquisition and extinction; resistance to extinction as a function of constant delay, partial delay, patterns of partial delay; contrast; and discrimination learning.

Rats pressed levers for food reward which was delivered, when appropriate, 0·4 s after the response. For one group, the delay interval was filled by a light cue; for the other group, the same number of lights was given but they were not correlated with food delivery. In Experiment I, all lever presses were reinforced and there were no differences in response rate between groups. In Experiments II and III, lever pressing was rewarded according to a VI and VR schedule respectively. Group differences were observed in Experiment II but they disappeared in Experiment III. The results of Experiments I and II show that a reward-related stimulus does not overshadow a lever response unless the stimulus is a better predictor of reward. Differences in salience or competition from sign-tracking behaviors were ruled out as causes of this phenomenon. Experiment III demonstrated, however, that a weak response-reward correlation is not a sufficient condition for the overshadowing effect. A fourth experiment replicated the results of Experiment III using naive animals. The results of these last two experiments are not consistent with an information theory approach unless (a) a response-units concept is adopted or (b) the cue involved in overshadowing is not the pre-food light but the end of a temporal interval, whose salience is enhanced by the light.

Previous research has shown that response rates on a variable interval (VI) schedule of reinforcement decrease if a brief response-produced signal is given prior to reward. One explanation is that the signal overshadows the response because it is a better predictor of reinforcement. The S-R overshadowing effect does not occur with variable ratio (VR) schedules, however. Tarpy, Lea, and Midgley (1983) explained this fact by suggesting that the signal functions to enhance the salience of the temporal interval offset on the VI schedule (a characteristic not possessed by VR schedules), which then overshadows the response. In this experiment, the salience of the temporal interval was enhanced in another way: signaled or unsignaled reward was provided to rats responding on either a VI or fixed interval (FI) reward schedule. As predicted, rates were lowest for animals receiving signaled reinforcement on an FI schedule and highest for those receiving unsignaled reinforcement on a VI schedule.

Rats were given 6 days of operant bar training, I session of avoidance training, and differential rest intervals of 0, I, 4, 24, and 168 hr. 5s were then tested in the operant training situation where rate suppression during avoidance C5 presentation was used as an index of fear. Fear increased up to 4 hr. and diminished only slightly after I week.