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Experiment 1 showed that the three stimuli associated with three chained fixed-interval links could be used to maintain observing behavior. Experiment 2 showed that three stimuli correlated with the passage of time since the last reinforcement in a fixed-interval schedule could be used to maintain observing behavior. In both experiments most observing responses occurred midway between reinforcements. Few occurred just before or just after reinforcement. Experiment 3 showed that the decline in the rate of observing behavior just before reinforcement was reduced when more stimuli could be observed. The relatively high terminal rate of observing behavior that resulted was maintained even when at least 4 sec intervened between the reinforcement and the last observed stimulus.

The form of classically conditioned suppression of instrumental performance associated with a fixed duration pre-aversive stimulus (CS) was quantitatively analysed over 300 acquisition trials. The temporal discriminations characteristic of this CER procedure were found to vary in form according to the stage of training. When the pre-aversive stimulus was subsequently made variable from trial to trial and its termination unpredictable, no change in average suppression during the CS occurred although the temporal discrimination was lost. Increasing the intensity of the aversive stimulus (US) produced an increase in average conditioned suppression of about the same degree whether the aversive stimulus was preceded by a fixed or variable duration CS.

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