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Two experiments were performed to investigate the effects on pigeons' keypecking behavior of stimuli that signal different kinds of aversive events: time‐out from positive reinforcement, electric shock, loud noise, and loud tone. Behavior maintained by a variable‐interval schedule of reinforcement was suppressed by a stimulus before shock, was accelerated by a stimulus before time‐out from positive reinforcement, and was unchanged by a stimulus before loud noise or a stimulus before loud tone. Conditioned acceleration with time‐out from positive reinforcement and conditioned suppression with shock were obtained regardless of whether a response contingent or response‐independent procedure was employed.

Pigeons responded under a schedule in which food was presented only after a fixed number of fixed‐interval components were completed. Two such second‐order schedules were studied: under one, 30 consecutive 2‐min fixed‐interval components were required; under the other, 15 consecutive 4‐min fixed‐interval components were required. Under both schedules, when a 0.7‐sec stimulus light was presented at completion of each fixed interval, positively accelerated responding developed in each component. When no stimulus change occurred at completion of each fixed interval, relatively low and constant rates of responding prevailed in each component; a similar result was obtained when a 0.7‐sec stimulus change occurred at completion of each fixed interval except the one which terminated with primary reinforcement. The 0.7‐sec stimulus correlated with food delivery was an effective conditioned reinforcer in maintaining patterns of responding in fixed‐interval components despite low average frequencies of food reinforcement.

Conditioned suppression is a decrease in response rate during a relatively short duration stimulus that terminates independently of the animal's behavior and coincidentally with a brief unavoidable shock. The degree of conditioned suppression was measured for each of three birds on three variable ratio schedules; that is, the number of responses required for food reinforcement was varied around a mean of 50, 100, or 200. The results indicated a slight and possibly negligible decrease in the degree of suppression as the mean number of responses required on the schedule was increased from 50, to 100, and 200. In general, it was found that all of the variable ratio schedules tested were quite insensitive to the conditioned suppression procedure, although almost complete suppression was obtained on a few occasions. Since the reinforcement was contingent upon the emission of responses, the birds typically displayed a high rate of response during the pre‐shock stimulus on all schedules. In addition, the rate during the pre‐shock stimulus often changed abruptly independent of the presentation of a reinforcement. As a result of the high rate of response and the abrupt changes in rate, the degree of suppression from trial to trial was quite variable. A clear analysis of an experimental variable on this baseline is thus difficult.

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Two albino rats were trained to terminate an aversive light for 1 min by pressing a bar. After 19 hr of conditioning they were exposed to successive delays of 1, 2, 5, and 10 sec imposed between occurrence of the escape response and light termination. No stimulus change accompanied the delay interval, and any additional responses made at this time reset the delay timer. For both rats the relative frequency of escape responses with very long latencies increased as the delay interval increased. The modal escape latency, however, remained essentially unchanged for all delay values of greater than 1 sec. “Superstitious” responding was observed during the delay interval.

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Because the frequency distribution of IRTs showed little or no control by a DRL schedule, the schedule was modified so that the pigeon's behavior after each IRT would indicate whether or not it had discriminated the duration of the IRT. After every two pecks on a red key, the key changed to blue for 30 sec. Then it automatically became red again. Pecks on the blue key were reinforced with food on a VI schedule only when the preceding IRT on the red key had been longer than 18 sec. The birds did not selectively emit longer IRTs on the red key: the value of IRTs/op did not increase with IRT duration. However, they did discriminate the duration of the IRT emitted on the red key: the rate of pecking on the blue key was an increasing function of the duration of the preceding IRT on the red key.

Eight groups of rats were trained on an auditory intensity discrimination in which the discriminative stimuli were separated by 10 decibels (db). Four pairs of stimuli were selected from different regions along a 60 — 100 db (SPL) intensity continuum. Counterpart groups were trained on each stimulus pair, with the relative intensity positions of the reinforced stimulus (SD) and the non‐reinforced stimulus (SΔ) reversed for the two groups. Discrimination acquisition curves were compared to determine whether stimuli separated by equal logarithmic units were of comparable “difficulty”, and to determine the relative effectiveness of an SD serving as the more versus less intense member of a stimulus pair. It was concluded that: (1) When SD is the more intense, auditory intensities of constant logarithmic separation are graded in “difficulty” along the intensity continuum; high intensity discriminative stimuli are most readily discriminated. When SΔ is the more intense, this graded effect is not evident. (2) For a given continuum location, discrimination is inferior when SΔ is the more intense. This effect is most pronounced at the high intensity end of the continuum and is chiefly attributable to differences in the rate of SΔ responding.

In Experiment I some discriminative functions of food pellets were studied by developing a multiple schedule of reinforcement (mult FR 30 FI 3) in which the delivery of a standard laboratory food pellet as a reinforcer set the occasion for reinforcement on every 30th response (FR 30), and the delivery of a sucrose food pellet as a reinforcer set the occasion for reinforcement after a 3‐min interval (FI 3). Discriminative stimulus control by the type of pellet was also demonstrated by reversing the operant discrimination and having the standard pellet control the FI 3 and the sucrose pellet control the FR 30. In Experiment II a mult FR 30 FR 30 with two bars was developed; a standard food pellet was followed by an FR 30 on Bar 1 and extinction (ext) on Bar 2, while a sucrose pellet was followed by an FR 30 on Bar 2 and ext on Bar 1. A control rat was placed, for comparison, on a mixed (mix) FR 30 FR 30 schedule with two bars, but neither bar correlated with the type of food pellet. In Experiments I and II the similarity between pellet controlled multiple schedules and multiple primed schedules was discussed, as was the comparability of transitions and effectiveness of control between pellet controlled multiple schedules and multiple schedules providing continuous exteroceptive stimuli.

Operant conditioning techniques were used to establish a discriminative echolocation performance in a porpoise. Pairs of spheres of disparate diameters were presented in an underwater display, and the positions of the spheres were switched according to a scrambled sequence while the blindfolded porpoise responded on a pair of submerged response levers. Responses which identified the momentary state of the display were food‐reinforced, while those which did not (errors) produced time out. Errors were then studied in relation to decreased disparity between the spheres. As disparity was decreased, errors which terminated runs of correct responses occurred more frequently and were followed by longer strings of consecutive errors. Increased errors and disruption of a stable pattern of collateral behavior were associated. Since some sources of error other than decreased disparity were present, the porpoise's final performance did not fully reflect the acuity of its echolocation channel.