The effect of prior experience on a current learning task is of the utmost relevance to theories of learning. In special cases, prior experience may even prevent subsequent learning. These cases can generally be thought of as instances of the blocking phenomenon (e.g., Kamin, 1969; Wagner, Logan, Haberlandt, and Price, 1968). Specifically, if a stimulus A is paired with a reinforcer so that a response comes under the control of that stimulus, then when a novel stimulus B is simultaneously compounded with A and also paired with the reinforcer, stimulus B does not acquire control over the response, as measured by presentation of B alone in extinction. Stimulus A is said to "block" the establishment of stimulus control by B.