A note on behavioural contrast and frustration

The suggestion has been made that positive contrast, the increase in response rate in one successively-presented stimulus following a change in conditions that decreases responding in the other, may not depend on differential stimulus control. Evidence is reviewed suggesting that contrast effects on most multiple interval schedules do depend upon discrimination, but that effects similar to contrast that result from the omission of reinforcement on fixed-interval schedules (the frustration/omission effect) reflect other factors. The link between contrast and the omission effect is the discrimination mechanism which allows animals to respond only at times or places correlated with food delivery. Contrast is a direct result of this mechanism; but the omission effect depends on the difference between the inhibitory discriminative after-effects acquired by food on fixed-interval schedules, and those acquired by a “neutral” stimulus presented in its place.