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Pigeons learned to respond to the middle‐sized member (SD) of a set of three simultaneously presented stimuli with responses to: the SD reinforced on a VI 1 schedule. They were then tested for several days with other sets of three stimuli. One procedure presented reinforcements on a VI 1 schedule during the test independent of the stimulus chosen when a reinforcement was programmed. The tests were also given under extinction conditions. With the testing carried out with extinction, preference consistently was for the test stimulus most similar in physical size to the SD. However, when the tests were with reinforcement, random responding resulted. Another effect of testing with reinforcement was an increase in incorrect responding with the training set. Such a test procedure was unsatisfactory for determining the effective aspect of the SD. The conclusion, based on the data of the extinction series, was that pigeons learned the intermediate size problem on the basis of the discrimination of absolute stimulus properties.

Pigeons exposed to a schedule which reinforces interresponse times (IRTs) longer than a given value (DRL schedule) eventually reach a stable pattern of responding which is shown to be a function both of the DRL value and of previous experience with other DRL values. On any given DRL schedule, the stable performance of most pigeons which have been previously exposed to a variety of such schedules, shows an IRT distribution with median equal to the DRL value. For DRL values longer than about 30 sec, however, the median IRT falls short of the DRL value; this failure of adjustment to longer values appears to be a species characteristic of pigeons. The function relating reinforcement rate to 1/DRL value is also shown to be approximately linear over the same range, with variable slope (less than 45°) and a downturn in the vicinity of DRL 30.

This study investigated some of the serial patterns among interresponse times during Sidman avoidance. It was found that the momentary probability of a response was determined by the length of exposure to the avoidance procedure during a session, by the time since the last shock, and by the duration of the preceding IRT.

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A tone ending with electrical shock was periodically presented to pigeons while they pecked a key for food. Pairs of birds were run simultaneously under a yoked program which insured that both birds received the same number and temporal distribution of shocks. For one of the birds, shock was always initiated by a peck; for the other, shock was unavoidable. Both procedures led to reduced rates of pecking in the presence of the tone, and gradients of stimulus generalization were obtained. But the effects of response contingent shock extinguished more rapidly than the effects of unavoidable shock. In general, birds exposed to unavoidable shock tended to respond at intermediate rates throughout tone, whereas those exposed to response contingent shock ceased to peck for part or all of the tone period.

Rats trained to discriminate between SD and SΔ for food reinforcement showed marked impairments in this discrimination when strong, unavoidable shocks occurred at the termination of a third stimulus. The predominant feature of this impairment was a supernormal rate of unreinforced (SΔ) behavior. Shocks delivered without exteroceptive warning also led to a discriminative breakdown. The effect was a direct function of shock intensity. When behavior was strongly suppressed in the third stimulus by response‐correlated shock (“punishment”), instead of unavoidable shock, breakdowns were only temporary; as soon as responding recovered from its overall suppression, discriminative performance returned to normal. The discriminative deterioration may be interpreted as an emotional by‐product of frequent aversive stimulation, but accidental contingencies could also have played a role.

Children learned to respond differentially to four tones when correct responses were indicated by re‐sounding the tone and flashing a signal lamp (simple knowledge of results). Correct responses were then made to advance a display counter. With this procedure, rate of response increased, but accuracy decreased. Mild punishment—points subtracted from the count—was then arranged for incorrect responses. Accuracy returned to or above its previous level. Response rate tended to remain above its initial level. These results indicate that when supplementary reinforcers are employed, precise contingencies must be arranged to ensure desired behavior.

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