In three experiments, human subjects were trained on a five-component multiple schedule with different fixed intervals of monetary reinforcement scheduled in the different components. Subjects uninstructed about the fixed-interval schedules manifested high and generally equivalent rates regardless of the particular component. By comparison, subjects given instructions about the schedules showed orderly progressions of rates and temporal patterning as a function of the interreinforcement intervals, particularly when feedback about reinforcement was delivered but also when reinforcement-feedback was withheld. Administration of the instructions-reinforcement combination to subjects who had already developed poorly differentiated behavior, however, did not make their behavior substantially better differentiated. When cost was imposed for responding, both instructed and uninstructed subjects showed low and differentiated rates regardless of their prior histories. It was concluded that instructions can have major influences on the establishment and maintenance of human operant behavior.

Michael (1975) reviewed efforts to classify reinforcing events in terms of whether stimuli are added (positive reinforcement) or removed (negative reinforcement). He concluded that distinctions in these terms are confusing and ambiguous. Of necessity, adding a stimulus requires its previous absence and removing a stimulus its previous presence. Moreover, there is no good basis, either behavioral or physiological, that indicates the involvement of distinctly different processes, and on these grounds he proposed that the distinction be abandoned. Despite the cogency of Michael's analysis, the distinction between positive and negative reinforcement is still being taught. In this paper, we reconsider the issue from the perspective of 30 years. However, we could not find new evidence in contemporary research and theory that allows reliable classification of an event as a positive rather than a negative reinforcer. We conclude by reiterating Michael's admonitions about the conceptual confusion created by such a distinction.

Young men pulled a plunger on mixed and multiple schedules in which periods of variable-interval monetary reinforcement alternated irregularly with periods of extinction (Experiment 1), or in which reinforcement was contingent on different degrees of effort in the two alternating components (Experiment 2). In the baseline conditions, the pair of stimuli correlated with the schedule components could be obtained intermittently by pressing either of two observing keys. In the main conditions, pressing one of the keys continued to produce both discriminative stimuli as appropriate. Pressing the other key produced only the stimulus correlated with variable-interval reinforcement or reduced effort; presses on this key were ineffective during periods of extinction or increased effort. In both experiments, key presses producing both stimuli occurred at higher rates than key presses producing only one, demonstrating enhancement of observing behavior by a stimulus correlated with the less favorable of two contingencies. A control experiment showed that stimulus change alone was not an important factor in the maintenance of the behavior. These findings suggest that negative as well as positive stimuli may play a role in the conditioned reinforcement of human behavior.

Rats responded under progressive-ratio schedules for sweetened milk reinforcers; each session ended when responding ceased for 10 min. Experiment 1 varied the concentration of milk and the duration of postreinforcement timeouts. Postreinforcement pausing increased as a positively accelerated function of the size of the ratio, and the rate of increase was reduced as a function of concentration and by timeouts of 10 s or longer. Experiment 2 varied reinforcement magnitude within sessions (number of dipper operations per reinforcer) in conjunction with stimuli correlated with the upcoming magnitude. In the absence of discriminative stimuli, pausing was longer following a large reinforcer than following a small one. Pauses were reduced by a stimulus signaling a large upcoming reinforcer, particularly at the highest ratios, and the animals tended to quit responding when the past reinforcer was large and the stimulus signaled that the next one would be small. Results of both experiments revealed parallels between responding under progressive-ratio schedules and other schedules containing ratio contingencies. Relationships between pausing and magnitude suggest that ratio pausing is under the joint control of inhibitory properties of the past reinforcer and excitatory properties of stimuli correlated with the upcoming reinforcer, rather than under the exclusive control of either factor alone.

We investigated the possibility that human-like fixed-interval performances would appear in rats given a variable-ratio history (Wanchisen, Tatham, & Mooney, 1989). Nine rats were trained under single or compound variable-ratio schedules and then under a fixed-interval 30-s schedule. The histories produced high fixed-interval rates that declined slowly over 90 sessions; differences as a function of the particular history were absent. Nine control animals given only fixed-interval training responded at lower levels initially, but rates increased with training. Despite differences in absolute rates, rates within the intervals and postreinforcement pauses indicated equivalent development of the accelerated response patterns suggestive of sensitivity to fixed-interval contingencies. The finding that the histories elevated rates without retarding development of differentiated patterns suggests that the effective response unit was a burst of several lever presses and that the fixed-interval contingencies acted on these units in the same way as for single responses. Regardless of history, the rats did not manifest the persistent, undifferentiated responding reported for humans under comparable schedules. We concluded that the shortcomings of animal models of human fixed-interval performances cannot be easily remedied by including a variable-ratio conditioning history within the model.

Younger and older men (18-25 vs. 65-76 years of age) were given extended practice (44 hr) with a memory scanning procedure in which lists of visual and auditory items were presented singly (single-task condition) or together with a second visual or auditory list (dual-task condition). For both age groups, identification of test items was slower with the dual-task procedure, and experience with the tasks produced only small changes in response latencies. When time limits were placed on responding, latencies decreased substantially, and the difference between single and divided tasks was reduced. Although the older men were slower throughout the experiment, the time limit contingency reduced age differences in responses to both tasks. The task effect was larger for the older men, but this age difference also was reduced by the time limit procedures.

An apparatus and data analysis methodology is described which allows determination of response time to oxygen concentration changes of luminescent oxygen sensor coatings. Utilizing a solenoid valve, a sample chamber creates a pressure jump from 0.1 to 700 Torr in 600 μs that is followed by 15 ms of ringing. An optical detection system measures the response of porphyrin-based luminescent oxygen sensors to the pressure jump. The pressure in the chamber is measured simultaneously and independently with a piezoresistive pressure transducer. Data analysis techniques using nonlinear least-squares and numerical convolution of the luminescent response to the pressure rise allow determination of response times of the oxygen sensor. The response to pressure jumps of several luminescent oxygen sensitive coatings designed for video luminescent barometry are measured with this computer-controlled instrument. Several coatings were studied with response times of ∼2.5 s, ∼400 ms, 11 ms, 1.5 ms, and <25 μs. Studies of the system suggest that we can determine response times down to about 10 μs.

Young adult 5s were trained to press a key in response to a 1,300-Hz. pure tone. Members of 4 groups were given discrimination training with S-either 1,000, 1,200, 1,400, or 1,600 Hz. For a fifth group S-was white noise, and a control group was given single-stimulus training. Generalization tests with pure tones in the range 1,000-1,600 Hz. yielded symmetrical gradients centered about S+ for the white noise and single-stimulus groups. For the remaining groups the postdiscrimination gradient was displaced in the direction away from S -. Further, the extent of the displacement was inversely related to the size of the SH-S-difference, i.e., the shift was greater when the difference was 100 Hz. than when it was 300 Hz. The similarities of the present findings with human 5s to the results of animal studies were attributed to the inability of 5s to label the pure tones that served as the stimuli.