Inhibitory control by one stimulus after an increase in the frequency of reinforcement associated with another stimulus

Pigeons were trained on a multiple variable-interval 5-min variable-interval 5-min schedule and then shifted to either a multiple variable-interval 1-min variable-interval 5-min or a multiple variable-interval 30-sec variable-interval 5-min schedule. A generalization test was subsequently administered along the dimension containing the stimulus associated with the variable-interval 5-min component. The generalization gradients for subjects that received multiple variable-interval 1-min variable-interval 5-min training were not consistent in shape. However, an incremental gradient was obtained from each subject that received multiple variable-interval 30-sec variable-interval 5-min training. Thus, a sufficiently large reduction in merely the relative frequency of reinforcement during a stimulus resulted in that stimulus’ acquiring inhibitory control over responding.     

Rates of responding in the pigeon generated by simple and complex schedules which provide the same rates of reinforcement

Four pigeons pecked for food reinforcement on variable interval 1-min schedules and on the variable-interval 1-min components of multiple, concurrent, and pseudoconcurrent schedules. The pseudoconcurrent schedule provided only one schedule of reinforcement; but, any reinforcer could be collected by responding on either of two keys. The rate of responding generated by the variable interval schedule was not greater than the rates of responding generated by the components of the complex schedules. But, the rate of reinforcement obtained from the variable interval schedule was greater than the rates of reinforcement obtained from the components of the multiple schedule. These results may contradict the equation proposed by Herrnstein (1970). The equation predicts that the rate of responding generated by a schedule of reinforcement will be greater when the schedule appears alone, than when it appears as one component of a complex schedule.     

Autocontingencies: Rats count to three to predict safety from shock

Following training on a variable-interval food-reinforcement schedule, rats were exposed to three unsignaled shocks during each 30-min session. Although leverpressing was initially suppressed, responding was significantly accelerated following offset of the third shock, regardless of when in the session it occurred. Control sessions in which only two shocks were programmed, one early and one late, did not yield baseline acceleration. Evidence of “counting to three” was less obvious in subjects simultaneously exposed to a temporal autocontingency, that is, for which each shock also predicted a minimum 3-min safety period. The addition of a signal prior to each shock eliminated evidence of counting behavior altogether. We conclude that rats may be taught to count, but such behavior is highly unnatural and may be blocked or overshadowed by more salient sources of information.     

Schedule-induced behavior in rats: Pellets versus powder

Food-deprived rats develop polydipsia on an intermittent schedule (fixed time 60 sec) of food pellet delivery, but not on an identical schedule of food powder delivery. This result was demonstrated with separate groups receiving each type of food and was replicated using rats as their own controls. Powdered food not only prevented the development of polydipsia, but it abruptly terminated ongoing polydipsia in rats that were switched from the scheduled delivery of pellets to powder. Ethological analysis of the behavior showed that the rats receiving powder were not engaging excessively in some behavior other than drinking. After discounting several factors, we concluded that the amount of oral activity associated with feeding, which occurred immediately after food delivery, was reciprocally related to the level of drinking.     

Schedule-induced drinking facilitates schedule-controlled feeding

Four food-deprived rats barpressed with food reinforcement on a 1-min fixed-interval schedule in 7-h sessions with water either present or absent. They all became polydipsic, drinking after many pellets early in sessions but after fewer pellets later in sessions. Pellets were secured at near maximum rates for about 3 h. Thereafter, the three heaviest drinkers obtained more food with water than without it. The results indicate that schedule-induced drinking does not continue indefinitely and that it can be adaptive with regard to feeding.     

Preference and resistance to change in concurrent variable-interval schedules

Pigeons were trained on a multiple schedule in which separate concurrent schedules were presented in the two components of the schedule. During one component, concurrent variable-interval 40-sec variableinterval 80-sec schedules operated. In the second component, concurrent variable-interval 40-sec variableinterval 20-sec schedules operated. After stable baseline performance was obtained in both components, extinction probe choice tests were presented to assess preference between the variable-interval 40-sec schedules from the two components. The variable-interval 40-sec schedule paired with the variableinterval 80-sec schedule was preferred over the variable-interval 40-sec schedule paired with the variableinterval 20-sec schedule. The subjects were also exposed to several resistance-to-change manipulations: (1) prefeeding prior to the experimental session, (2) a free-food schedule added to timeout periods separating components, and (3) extinction. The results indicated that preference and resistance to change do not necessarily covary.     

Transfer of conditioned suppression and conditioned acceleration from instrumental to consummatory baselines

Following training on a variable-interval food reinforcement schedule, rats were exposed to Pavlovian procedures which produced reliable conditioned suppression and conditioned acceleration of the leverpressing (instrumental) baseline. When free food was simultaneously made available in the test cage, all subjects spent the majority of each session “freeloading,” that is, eating food from a dish rather than leverpressing for it. When superimposed upon the freeloading baseline, the conditioned suppression and conditioned acceleration procedures affected the rate of pellet consumption identically in magnitude and direction to their previous effects on leverpressing. These results suggest a motivational mechanism for conditioned suppression and acceleration, rather than one which depends upon spurious punishment of specific response sequences.     

Choice responding following multiple schedule training

Pigeons’ choice responding on 10-sec interpolated probes was studied after baseline training on multiple variable-interval variable-interval schedules of food reinforcement. Unreinforced choice following training with three different relative reinforcement rates (Experiment 1), with a 3-ply multiple schedule (Experiment 2), and with three different relative reinforcement durations (Experiment 3) was examined. Least squares lines were fit to choice relative response rate and schedule relative response rate as functions of training relative reinforcement rate; choice slope was significantly greater than schedule slope in all three experiments. This result is counter to the prediction of Herrnstein’s (1970) theory that these slopes should not differ. Luce’s (1959) theory also failed to account for the data. It was concluded that choice responding was controlled by both approach to the stimulus associated with the smaller mean interreinforcer interval or the longer duration, and avoidance of the other stimulus.     

Wheel running in the rat induced by a fixed-time presentation of water

Four female water-deprived albino rats were given free access to a running wheel and food in baseline sessions with water present for the first 375 sec of each daily 100-min session. In experimental sessions, water was presented intermittently according to a fixed-time (FT) 2-min schedule, which delivered water on a periodic basis independently of the rat’s behavior, resulting in 50 7.5sec presentations of the water tube. Food consumption did not vary as a function of the experimental condition, whereas running increased for all Ss during the schedule conditions. Increases in running appeared to be the result of increases in both initiations of running and run-burst lengths.     

Rats' performance on an interval time-place task: Increasing sequence complexity

Rats were trained on an interval time-place learning (TPL) task in which the location of food availability depended on the time since the start of the session. Each of four levers (numbered 1, 2, 3, 4) provided food on an intermittent schedule for two nonconsecutive 3-min periods. The order in which the levers provided food was 1, 2, 4, 3, 2, 3, 1, 4. This order was consistent across sessions. Previous research conducted in our lab has shown that when only four “places” are used, rather than the eight in the present study, rats use a timing strategy to track the location of food. Pizzo and Crystal (2004) recently trained rats on an interval TPL in which each of eight arms of a radial arm maze provided food. They found evidence suggesting that rats used both spatial and temporal information. In the present study, in which a revisiting strategy was used (i.e., each lever provided food on more than one occasion), the rats tracked both the spatial and the temporal availability of food for the first half of the session. Interestingly, in the second half of the sessions, the rats appeared to be timing the availability of food even though they did not know where it would occur. That is, the rats knew the temporal, but not the spatial, contingencies for the second half of the session. It appears that the requirement of revisiting a previously reinforced lever resulted in rats' no longer being able to solve the spatial aspect of the task.     

A concurrent assessment of the positive and negative properties of a signaled shock schedule

A choice and a conditioned suppression procedure were used to assess concurrently the positive and negative properties of stimuli within a signaled shock schedule, Occasional shocks were presented to Ss responding on a variable-interval food schedule. Ss could choose whether shocks occurred alone or whether they were preceded by a 1-min signal. All Ss chose the signaled shock condition over the unsignaled one, even though food reinforced responding in the presence of the signal was suppressed. Rate of responding for food varied across stimulus conditions, with the lowest rate in the presence of the signal and the highest rate in its absence. An intermediate rate occurred under the unsignaled shock schedule. A safety analysis was applied to the data.     

Rate of reinforcement and session duration as determinants of within-session patterns of responding

Rats pressed levers for Noyes pellets or keys for sweetened condensed milk reinforcers delivered by multiple schedules. Session length and baseline rates of reinforcement were varied in two experiments. Rates of responding increased during the early part of the session and then decreased for both responses and reinforcers, as well as for all subjects and values of the independent variables. Changes in response rates across the session sometimes exceeded 500%. Respoiise rates peaked approximately 20 min after the beginning of the session, regardless of session duration, when subjects responded on a multiple variable interval 1-min variable interval 1-min schedule. The function was flatter for longer sessions than it was for shorter sessions. The function was flatter, more symmetrical, and peaked later for lower rates of reinforcement than for higher rates of reinforcement. The function appeared early in training, and further experience moved and reduced its peak. Variables related to reinforcement exerted more control over some aspects of this function than did variables related to responding. These within-session patterns of responding may have fundamental implications for experimental design and theorizing.     

Development of preference and spontaneous recovery in choice behavior with concurrent variable-interval schedules

Pigeons pecked on two response keys that delivered reinforcers on a variable-interval schedule. The proportion of reinforcers delivered by one key was constant for a few sessions and then changed, and subjects’ choice responses were recorded during these periods of transition. In Experiment 1, response proportions approached a new asymptote slightly more slowly when the switch in reinforcement proportions was more extreme. In Experiment 2, slightly faster transitions were found with higher overall rates of reinforcement. The results from the first session, after a switch in the reinforcement proportions, were generally consistent with a mathematical model that assumes that the strength of each response is increased by reinforcement and decreased by nonreinforcement. However, neither this model nor other similar models predicted the “spontaneous recovery” observed in later sessions: At the start of these sessions, response proportions reverted toward their preswitch levels. Computer simulations could mimic the spontaneous recovery by assuming that subjects store separate representations of response strength for each session, which are averaged at the start of each new session.     

Omission training and positive conditioned suppression in the rat

Suppression of operant responding during a conditioned stimulus (CS) was studied in two procedures. In both procedures, operant leverpressing was maintained by a variable-interval 1-min food-delivery schedule, and insertion of a second lever served as the CS. In the first procedure, autoshaping, food followed each CS presentation irrespective of a subject’s behavior during the CS. In the second procedure, omission training, contact with the CS canceled the delivery of food scheduled for the end of that CS. In the first experiment, subjects were exposed to omission training followed by autoshaping; these procedures were reversed in the second experiment. In each experiment, the omission contingency resulted in fewer CS contacts and less suppression of operant responding during the CS than did autoshaping. These differences were more notable in subjects receiving the sequence autoshaping→omission training (Experiment 2). Direct observations in Experiment 2 revealed that, for subjects that were contacting the CS frequently when the omission contingency was introduced, reductions in signal contacts were accompanied by redistributions of behavior. The form of these redistributions depended upon behavior allocation at the time the omission contingency was imposed.     

Differences between rates of responding emitted during simple and multiple schedules

Pigeons pecked keys for food reinforcers delivered by several variable-interval and multiple variable-interval schedules. The rates of responding emitted during the simple schedules were not systematically different from the rates emitted during the multiple schedules when the components of the multiple schedule were identical. The rates of responding emitted during the components were usually greater than the rates emitted during comparable simple schedules when the components were more favorable than the added components of the multiple schedules. Response rates during the components were not significantly lower than those during comparable simple schedules when the components were less favorable. The observation of higher rates of responding during the more favorable components conforms to a prediction of several additive theories (e.g., Rachlin, 1973) but violates a prediction of Herrnstein’s (1970) theory. However, the additive theories are brought into question by the fact that changing the location of the discriminative stimuli did not change the pattern of results.     

Instrumental responding by rats on free-operant schedules with components that schedule response-dependent reinforcer omission: Implications for optimization theories

In two experiments, we assessed whether rats optimize the number of reinforcers per response. In Experiment 1, one group of rats was trained to leverpress for food reinforcement on a simple variable-interval (VI) 60-sec schedule. For another group, a negative fixed-ratio component was imposed on the VI schedule to produce a conjoint contingency in which reinforcement became available on the VI schedule but was omitted when the ratio criterion was satisfied. In Experiment 2, one group of rats responded on a VI schedule and also received response-independent food. For another group, responding above a certain rate canceled the response-independent food. Despite the negative contingency experienced by the groups in Experiments 1 and 2, responding was maintained at a level at which the number of obtained reinforcers was reduced substantially below the maximum number possible. In addition, in both experiments, the groups that experienced the negative contingency responded at a lower rate than did a yoked control group that experienced the same frequency of reinforcement but lacked the negative component. These results demonstrate that although rats do not optimize their behavior with respect to reinforcement, they are nevertheless sensitive to some aspect of the instrumental contingency in operation.     

Reductions in resistance to extinction and spontaneous recovery as a function of changes in transportational and contextual stimuli

Thirty rats received 10 sessions of baseline training in which leverpressing was reinforced according to a variable-interval (VI) 60-sec schedule. Twenty-four of the subjects were then assigned to one of four groups that received five sessions of extinction, with groups being differentiated in a 2 by 2 factorial design on the basis of: (1) changes in stimuli accompanying transportation of subjects from home cages to the laboratory and placement in the apparatus, and/or (2) changes in contextual stimuli within the apparatus. During the sixth session of extinction, the transportational and contextual stimuli previously associated with baseline training were reinstated. The remaining six rats experienced changes in both transportational and contextual stimuli but were maintained on the VI 60-sec schedule of reinforcement. Changes in either transportational or contextual stimuli reduced resistance to extinction and spontaneous recovery, and substantial increments in responding occurred upon reinstatement of the transportational and contextual stimuli associated with baseline training. Evidence for summation of the two sources of stimulus change was obtained. Changes in transportational and contextual stimuli produced only a brief disruption in responding when reinforcement of leverpressing was continued.     

Behavioral contrast and responding during multiple food-food,food-water,and water-water schedules

Five pigeons pecked lighted keys for food reinforcers delivered by several multiple variable interval 2-min variable interval 2-min schedules. At different times, the components of the multiple schedule both supplied food reinforcers, both supplied water, or one supplied food and the other supplied water. Rates of responding during the water component of the food-water schedule were lower than the rates during comparable components of the water-water schedules (negative contrast). But, the rates of responding during the food component of the food-water schedule were not greater than the rates of responding during comparable components of the food-food schedules (absence of positive contrast) at two different levels of water deprivation. These results raise questions about several theories of behavioral contrast, and they may restrict the scope of any theory that attributes positive and negative contrast to symmetrical factors.     

Adjunctive drinking during variable and random-interval food reinforcement schedules

Rats were trained to leverpress for food and subsequently exposed to either arithmetic series or random variable-interval reinforcement schedules. Adjunctive drinking developed in all subjects exposed to arithmetic variable-interval reinforcement, but did not develop in six of the eight animals trained on the random schedule. The results suggest that adjunctive drinking is the result of an interaction between the tendency of rats to drink after eating and the ability of locally low probabilities of reinforcement within schedules to induce conditioned behavioral states.     

Behavioral contrast in pigeons and rats: A comparative analysis

The effects of reinforcement rate on behavioral contrast were examined in pigeons and rats. Each species was exposed to a series of 12 multiple variable-interval schedules, divided into four 3-schedule series. Each series consisted of a standard contrast manipulation, and baseline schedules provided a different rate of reinforcement in each of the series. The functions relating reinforcement rate to the magnitude of contrast were different across species. Rats showed a U-shaped function, with reliable contrast occurring only at high reinforcement rates. Pigeons showed an inverted U-shaped function, with contrast occurring on all schedules except the schedule providing the lowest rate of reinforcement. Pigeons discriminated between schedule components better than rats did, although differences in discrimination were probably not responsible for the differences in contrast. The results suggest that behavioral contrast in rats may be a different phenomenon from behavioral contrast in pigeons. The results cannot be explained by current theories, which view contrast as the product of a single general process.