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Pigeons were exposed to three successive matching‐to‐sample procedures. On a given trial, the sample (red, green or blue light) appeared on a center key; observing responses to this key produced the comparison stimuli on two side keys. Seven different experimental conditions could govern the temporal relations between the sample and comparison stimuli. In the “simultaneous” condition, the center key response was followed immediately by illumination of the side key comparison stimuli, with the center key remaining on. In “zero delay” the center key response simultaneously turned the side keys on and the center key off, while in the “variable delay” conditions, intervals of 1, 2, 4, 10, and 24 sec were interposed between the offset of the sample and the appearance of the comparison stimuli on the side keys. In all conditions, a response to the side key of matching hue produced reinforcement, while a response to the non‐matching side key was followed by a blackout. In procedure I all seven experimental conditions were presented in randomly permutated order. After nine sessions of exposure (at 191 trials per session, for a total of 1719 trials) the birds gave no evidence of acquisition in any of the conditions. They were therefore transferred to Procedure II, which required them to match only in the “simultaneous” condition, with both the sample and comparison stimuli present at the same time. With the exception of one bird, all subjects acquired this performance to near 100% levels. Next, in Procedure III, they were once more exposed to presentation of all seven experimental conditions in random order. In contrast to Procedure I, they now acquired the delay performance, and were able to match effectively at delays of about 4 sec.

An invisibly small thumb‐contraction was conditioned under secondary positive reinforcement (money) in four adult human subjects without their observation of the response. Electromyographic detection enabled the experimenter to reinforce the response by advancing on the subject's illuminated scoreboard the count of nickels earned. A light‐beam galvanometer recorded on photosensitive paper not only those instances of the response which were of the size pre‐selected for reinforcement but also those too small or too large to qualify. From the developed record cumulative response curves were constructed for each of the variously sized subclasses of the operant. Histograms, too, were plotted showing response‐frequency by subclass for each 10‐min interval of the experimental session. Before conditioning, response frequency was radically skewed toward the large‐amplitude end of the distribution. The effect of conditioning was to normalize the distribution, with the middle‐sized subclass (the one reinforced) becoming modal. This entailed reduced frequency of responses in subclasses smaller than the one reinforced. In extinction the original skew was strikingly restored in three of the four cases.

Male albino rats were trained on an adjusting avoidance schedule in which each lever press accumulated a given amount of shock‐free time. Multiple auditory and visual stimuli were programmed for each discrete temporal distance from the shock in an effort to place the avoidance behavior under the control of the shock proximity. The effects of the stimuli were further examined by presenting part of them and then by removing them altogether. With the combined auditory and visual stimuli, the rat spent most of the time relatively close to the shock and usually started to respond only when the shock was near. With the visual stimuli only, the rat kept the shock at intermediate temporal distances and responded more variably. The behavior with the auditory stimuli alone was quite similar to that produced by the combined stimuli, thus indicating that the auditory stimuli exercised the greater control. When all stimuli were removed, the animal usually kept the shock as far away as the procedure permitted. When only a single pre‐shock stimulus was presented, the rat remained quite close to the shock and started to respond predominantly in the pre‐shock step.

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