The effects of varying the distribution of generalization stimuli within a constant range upon the bisection of a sound-intensity interval by rats

Two male, albino rats were trained on a two-valued, self-paced, discrete-trials auditory discrimination. In the presence of a high-intensity stimulus (90 decibels SPL, 4 kiloHertz), response A was reinforced; in the presence of a low-intensity stimulus (50 decibels SPL, 4 kiloHertz), response B was reinforced. When discrimination performance was asymptotic, stimuli intermediate in intensity were presented with the training stimuli in a maintained generalization paradigm. Generalization gradients were derived from the relative frequencies of response A in the presence of each stimulus. A relative frequency of 0.50 was then determined and used as the bisection point of the intensity interval defined by the 90- and 50-decibel stimuli. The bisection point varied with the distribution of the stimuli presented in generalization. This effect was similar to context effects seen in human psychophysics.     

Fixed versus variable sequences of food and stimulus presentation in second-order schedules

Three pigeons were exposed to a second-order schedule in which the behavior specified by a fixed-interval component schedule was reinforced according to a ratio overall schedule. The completion of components not followed by food was signalled by a brief stimulus never paired with food. Food and the stimulus occurred in a random sequence or in fixed alternation, but the overall schedules (variable ratio 2 or fixed ratio 2) ensured that an equal number of food and brief-stimulus presentations occurred in each session. The control exerted by the food and by the brief stimulus was measured by overall response rates, mean pauses, frequency distributions of pauses, and response patterning across components. In general, the stimulus controlled patterns of behavior more similar to those controlled by food when food and the stimulus occurred in a random sequence than when they occurred in fixed alternation.     

The role of verbal behavior in human learning: infant performance on fixed-interval schedules

The performances of two infants less than one year old were investigated on fixed-interval schedules. When the infants touched a cylinder either music or food was presented according to fixed-interval schedules ranging in value from 10 to 50 seconds. With respect to two principal criteria, namely, pattern of responding and sensitivity to the schedule parameter, the subjects' behavior closely resembled that of animals but differed markedly from that of older children and adults. Negatively accelerated responding in the course of the fixed interval in the early sessions gave way to a scalloped pattern, consisting of a pause after reinforcement followed by an accelerated response rate. This scalloped pattern was the final form of responding on all schedule values. Analysis of data after performance had stabilized showed that postreinforcement pause was a negatively accelerated increasing function, and running rate (calculated after excluding the postreinforcement pause) was a declining function, of schedule value. On each schedule, the durations of mean successive interresponse times declined in the course of the fixed interval and were directly related to schedule value. The results supported Lowe's (1979) suggestion that verbal behavior may be responsible for major differences in the schedule performance of older humans and animals.     

Choice and foraging

In Experiment 1, six naive pigeons were trained on a foraging schedule characterized by different states beginning with a search state in which completion of a fixed-interval on a white key led to a choice state. In the choice state the subject could, by appropriate responding on a fixed ratio of three, either accept or reject the schedule of reinforcement that was offered (either a variable-interval five-second or a variable-interval 20-second). If the subject accepted the schedule, it entered a “handling state” in which the appropriate variable-interval schedule was presented. Completion of the variable-interval schedule produced food. The independent variable was the fixed-interval value in the search state, and the dependent variable was the rate of acceptance of the long variable-interval in the choice state. Experiment 2 was identical except that the search state required completion of a variable-interval, instead of a fixed-interval, schedule. The rate of acceptance of the long variable-interval schedule in both experiments was a direct function of the length of the search state, in accordance with both optimality theory and the delay-reduction hypothesis.     

Behavioral and dimensional contrast in rats.

Rats pressed a nose key for brain stimulation reinforcement presented on a fixed-interval schedule. Stimuli were drawn at random from a continuum of 12 white noise intensities in the range 62-95 decibels, spaced in 3 decibel steps. Experiment 1 varied the number of stimuli and the reinforcement contingencies associated with them. In Condition I (baseline) all stimuli signaled reinforcement; in Conditions II and III stimuli from one half of the continuum signaled reinforcement and those from the other half, extinction. However, in Condition II the 6 stimuli from the middle of the continuum were omitted. Experiment 2 held constant the number of stimuli and varied their spacing. In Condition I, each of 6 sounds signaled reinforcement. In Conditions II and II, three stimuli from one half of the continuum signaled reinforcement and three from the other half, extinction. However, in Condition II the stimuli were near the extremes of the continuum (Stimuli 1, 3, 4, 9, 10, 12). Condition III replaced Stimulus 3 with Stimulus 6 and Stimulus 10 with Stimulus 7. Behavioral contrast was seen in an increase over baseline in response rate to the stimuli associated with the constant schedule component when the variable component was changed to extinction. Dimensional contrast was seen in a further elevation of rate to intermediate positive stimulus values when stimuli were added to the border region between positive and negative values.     

Effects of ethanol on multiple fixed-interval fixed-ratio schedule performances: dynamic interactions at different fixed-ratio values.

Key pecking by three pigeons was maintained under a multiple fixed-interval fixed-ratio schedule of food presentation. The fixed-interval value remained at 3 minutes, while the fixed-ratio size was increased systematically in 30-response increments from 30 to either 120 (two pigeons) or 150 (one pigeon). At least two lower fixed-ratio values were also redetermined. The effects of ethanol (5 to 2.5 g/kg) were assessed at each of the different schedule parameters. Both overall and running response rates under the fixed-ratio schedule decreased with increases in the size of the fixed-ratio schedule; pause duration under the fixed-ratio schedule was directly related to increases in fixed-ratio size. Overall and running rates of responding under the fixed-interval schedule changed little with increases in the size of the fixed-ratio schedule. Despite the relative invariance of fixed-interval responding across the different fixed-ratio values, the effects of ethanol on responding under the fixed-interval schedule differed depending on the size of the fixed-ratio schedule. Greater increases occurred in both overall and in lower local rates of responding under the fixed-interval schedule when the fixed-ratio value was 120 or 150. The effects of ethanol on responding under the fixed-ratio schedule also depended on the size of the fixed ratio. Increases in responding under the fixed-ratio schedule were typically greater at the higher fixed-ratio values where response rates were lower. When the effects of ethanol were redetermined at the lower fixed-ratio parameter values, rates and patterns of responding were comparable to those obtained initially. However, the dose-effect curves for responding under both fixed-ratio and fixed-interval schedules were shifted up and to the right of those determined during the ascending series. The effects of ethanol can depend on rate or responding, behavioral history, and the context in which behavior occurs.     

Contingency and stimulus change in chained schedules of reinforcement

Higher rates of pecking were maintained by pigeons in the middle component of three-component chained fixed-interval schedules than in that component of corresponding multiple schedules (two extinction components followed by a fixed-interval component). This rate difference did not occur in equivalent tandem and mixed schedules, in which a single stimulus was correlated with the three components. The higher rates in components of chained schedules demonstrate a reinforcing effect of the stimulus correlated with the next component; the acquired functions of this stimulus make the vocabulary of conditioned reinforcement appropriate. Problems in defining conditioned reinforcement arise not from difficulties in demonstrating reinforcing effects but from disagreements about which experimental operations allow such reinforcing effects to be called conditioned.     

The puzzle of responding maintained by response-contingent shock.

Four squirrel monkeys were first exposed to a sequence of procedures that reliably generate responding maintained by brief response-contingent electric shocks arranged according to a fixed-interval schedule. After responding had become stable on the fixed-interval schedule, additional contingencies were added in tandem, whereby after completion of the interval, the spacing of responses affected shock delivery. In one procedure, responses had to be spaced more widely than their previous median value if shock were to be delivered. In the other procedure, responses had to be spaced more closely to produce shock. On the first of these procedures, decreased but stable responses rates would indicate that shock functioned as a positive reinforcer; on the second, increased response rates would indicate the positively reinforcing function. Instead, response rates accelerated on the procedure that targeted more widely spaced responses for shock delivery, and decelerated or ceased on the procedure that arranged for shocks to be produced by more closely spaced responses. Consistent with other recent findings, these results question the interpretation of performances maintained by response-contingent shock as engendered by positive reinforcement and are consistent with aversive-control interpretations. The details of that aversive control are not entirely clear, however, and these same procedures would be informative if applied to shock-maintained behavior that is generated in other ways.