Transfer of stimulus control: measuring the moment of transfer.

Three severely retarded boys acquired simple form discriminations errorlessly. Each was first taught to press a red key versus a simultaneously present white key. After this discrimination had been established, black figures were superimposed on the red and white keys. Each correct response affected the next trial by delaying the onset of the red stimulus an additional 0.5 sec. Transfer of stimulus control to the figures was indicated when the subjects responded correctly before the onset of the red stimulus. A series of errorless discrimination reversals was accomplished with this technique, during which the number of trials to transfer systematically decreased with successive reversals.