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Two pigeons were trained to perform discrimination tasks along two dimensions, wavelength of a circular spot of light and orientation of a white line. Discriminability among stimuli along these dimensions was established for both subjects by means of a steady state testing procedure. The two dimensions were then combined by superimposing the white line upon the colored background. Subjects were given a series of tests in which a correct response could be made on the basis of either of the two components of the stimulus compounds. Discriminability among these redundant compound stimuli was found to be better than that among wavelength and tilt stimuli alone. A second series of tests was administered using both redundant and conflicting compound stimuli. The results of this test series are consistent with a response strategy in which subjects first examine both elements of a compound and then emit a choice response on the basis of the element that best predicts reinforcement.

The learning efficiency of an Atlantic bottlenose dolphin was evaluated using auditory discrimination learning‐set tasks. Efficiency, as measured by the probability of a correct response on Trial 2 of a new discrete‐trial, two‐choice auditory discrimination problem, reached levels comparable to those attained by advanced species of nonhuman primates. Runs of errorless problems in some cases rivaled those reported for individual rhesus monkeys in visual discrimination learning‐set tasks. This level of stimulus control of responses to new auditory discriminanda was attained through (a) the development of a sequential within‐trial method for presentation of a pair of auditory discriminanda; (b) the extensive use of fading methods to train initial discriminations, followed by the fade‐out of the use of fading; (c) the development of listening behavior through control of the animal's responses during projection of the auditory discriminanda; and (d) the use of highly discriminable auditory stimuli, by applying results of a parametric evaluation of discriminability of selected acoustic variables. Learning efficiency was tested using a cueing method on Trial 1 of each new discrimination, to allow the animal to identify the positive stimulus before its response. Efficiency was also tested with the more common blind baiting method, in which the Trial 1 response was reinforced on only a random half of the problems. Efficiency was high for both methods. The overall results were generally in keeping with exceptations of learning capacity based on the large size and high degree of cortical complexity of the brain of the bottlenose dolphin.

The relation between performance during discriminative training and subsequently obtained measures of stimulus control was investigated. Pigeons served as experimental subjects. In the discriminative training phase, a single peck on the center key, transilluminated by a bright or dim white light, resulted in the onset of the side keys, one red and one green. If the center key was brightly lighted, a response on the red side key was correct. A response on the green side key was correct if the center key was dimly lighted. Correct responses were reinforced on independently arranged variable‐interval schedules. Following discriminative training, tests of stimulus control were administered during which white light of 11 intensities was projected on the center key and responses on the red and green side keys recorded. The proportion of correct responses in the presence of a bright or dim center‐key stimulus decreased with decreases in the frequency of reinforcement of correct red or correct green responses, respectively. The slopes of the stimulus control gradients were related to the extent of response bias during training. The greater the bias to respond on the green key, the flatter the gradient showing the proportion of green‐key responses to each stimulus and the steeper the corresponding gradient of red‐key responses.

The key pecking of six pigeons was reinforced according to a variable‐interval 1‐min schedule during each of two successively presented stimuli. When the key was illuminated by a black line on a white background, reinforcement was delayed for 10 sec. When the key was illuminated by a plain white light, reinforcement was not delayed. All subjects responded at a lower rate during the presentation of the black line. A subsequent generalization test along the line‐orientation dimension produced a U‐shaped gradient, with the nadir located at or near the training stimulus, for each subject. These gradients suggested that the lower rate of response during the stimulus associated with delayed reinforcement may have been due to an inhibition of responding.

The first experiment studied the effects of punishment on rats' lever pressing maintained by a fixed‐interval schedule of food reinforcement and on the associated schedule‐induced licking. When licking was followed by shock, licking was suppressed but lever pressing was largely unaffected. When lever pressing was followed by shock, lever pressing was suppressed but licking was unaffected. In both cases, the punished behavior recovered its previous unpunished level when the shocks were discontinued. In a second experiment, the rats' lever pressing was maintained by a variable‐interval schedule of food reinforcement under which polydipsic licking also developed. Both lever pressing and licking were partially suppressed during a stimulus correlated with occasional unavoidable electric shocks. With a higher shock intensity, both behaviors were suppressed further. Both lever pressing and licking recovered their previous levels when shocks were discontinued. These results show that schedule‐induced licking, which has been described as adjunctive behavior, can be suppressed by procedures that suppress reinforced lever pressing, an operant behavior.

A 1‐min tone and light signal that preceded two free pellets of food suppressed the random‐ratio responding of four rhesus monkeys, but accelerated the same subjects' responding on a differential‐reinforcement‐of‐low‐rate schedule in separate sessions. Both schedule‐specific interactions occurred during the first presentations of the signal that previously had been paired with food outside the operant sessions. Thus, neither effect was adventitiously produced. In two subjects, both the direction and magnitude of the prereward change in differential‐reinforcement‐of‐low‐rate responding appeared related to baseline response rates: the more rapid the baseline responding, the less was the acceleration during the signal. Suppression and acceleration did not appear as dichotomous effects with separate parameters, but as related effects at least partly determined by the characteristics of the baseline operant performance.

Three pigeons pecked for food on a synthetic variable‐interval schedule of reinforcement that had two independent parts: a variable‐interval schedule that arranged a distribution of interreinforcement intervals, and a device that randomly assigned each reinforcement to one of 10 classes of interresponse times. The frequencies of reinforcement for the 10 classes of interresponse times were systematically varied, while the overall frequency of reinforcement was held within a comparatively narrow range. The 10 classes extended either from 0.1 to 0.6 sec in 0.05‐sec intervals, or from 1.0 to 6.0 sec in 0.5‐sec intervals. In the former case, some control by reinforcement was obtained, but it was weak and no simple relationships were discernible. In the latter case, the relative frequency of an interresponse time was a generally increasing function of its relative frequency of reinforcement, and two simple controlling relationships were found. First, the function relating interresponse times per opportunity to reinforcements per opportunity was, over a restricted range, approximately linear with a slope of unity. Second, when all 10 classes of interresponse times were reinforced equally often, the relative frequency of an interresponse time approximately equalled the relative reciprocal of its length.

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