If multiple discriminative stimuli are differentially correlated with reinforcement, then stimulus control may be stronger to those stimuli that more reliably signal the occurrence and nonoccurrence of reinforcement. Such stimuli are often said to be more valid or diagnostic cues.
For example, suppose that in Case 1, compound discriminative stimuli AX and BX are explicitly associated with reinforcement and extinction, respectively. We would expect behavioral control by the A and B elements to exceed that by the X element.
Now, suppose that in Case 2, the compound discriminative stimuli AX and BX are equally often associated with reinforcement and extinction. Here, we would expect that control by the A and B elements would be similar to that by the X element.
These expectations have been borne out by behavioral evidence in both human beings and laboratory animals. Critically, behavioral control by the X element in Case 1 is much lower than in Case 2, despite the fact that the X element is equally often associated with reinforcement and extinction in each case.
This relative validity effect shows that the behavioral control that is exerted by one element of a compound discriminative stimuli (here X) depends on the relative discriminative validity of other stimuli (here A and B) with which it occurs in compound.
Casually speaking, organisms learn to attend to the stimuli that are the most diagnostic of events of importance to them.
A discriminative stimulus when it is presented alone may exert strong stimulus control over operant behavior. However, if that discriminative stimulus is accompanied by another, then stimulus control by the first (or overshadowed) stimulus may be reduced or eliminated by the second (or overshadowing) stimulus.
For example, I might easily recognize my son by the cowlick in his hair. But, I would be more likely to recognize him from his distinctive gait when he is walking in the playground. We would say that control by his cowlick was overshadowed by control by his gait.
If a discriminative stimulus (A) is first presented alone and is followed by reinforcement, then that stimulus may gain strong stimulus control over operant behavior. If that discriminative stimulus is later combined with a novel stimulus (X) and the stimulus compound (AX) is followed by the same reinforcement, then little or no control may be exerted by the second stimulus (X) when tests with with it alone are conducted.
An additional comparison is critical to show that stimulus control by X has been blocked by prior training with A. Here, only AX training is given. Stimulus control by X alone in this comparison condition greatly exceeds that in the first condition.
For example, the printed word "stop" on a black sign might easily be able to control a motorist's braking response. But, after a long history of braking at red stop signs, a motorist might very well speed past a black sign with "stop" printed on it. We would then say that the color of the stop sign had blocked the lettering on it in controlling the motorist's braking behavior.