Fig. 1.

Conditioned responding was assessed in probe trials in which the stimuli—light or tones—were presented alone and not followed by food reward. Responses in the 1 sec before stimulus onset were subtracted from responses during the 1 sec stimulus. Theleft panel shows responding to the light; theright panel shows responding to the tone. The rats were preconditioned, either with the light (white bars), the tone (black bars), or the compound (gray bars) stimulus. The number of hopper entries was highest when the preconditioned stimulus was presented and lowest when the blocked stimulus was presented. Intermediate values were observed for rats trained with the compound stimulus.