Positive behavioral contrast as a function of time-out duration when pigeons peck keys on a within-session procedure

Three pigeons pecked keys for food reinforcers delivered by multiple variable interval variable interval schedules in the first part of each session (baseline) and by multiple variable interval extinction schedules in the second part of each session (contrast). The variable interval schedules delivered reinforcers after an average of 4 min or 30 sec in different conditions. The duration of a time-out between the components varied in five steps from 5 to 120 sec. Positive contrast occurred for all time-out durations in both experiments. That is, the rate of responding emitted during the constant, variable interval component was greater during the contrast than during the baseline schedules. The size of contrast did not change systematically with changes in timeout duration. These results violate most theories of contrast. They are compatible with the idea that animals integrate reinforcers over intervals longer than 2 min.     

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.     

Contrast during multiple schedules with different component response requirements

Positive and negative behavioral contrast were examined when pigeons were required to keypeck in one component and treadle press in the other component of a series of multiple schedules. The experimental conditions were constructed so that the positive and negative contrast groups were exposed to complementary conditions. Positive keypeck and negative treadle-press contrast occurred in all subjects. Positive treadle-press contrast seemed to depend on the order of schedule presentation. Only one subject exhibited strong negative keypeck contrast. The results indicate that symmetrical conditions are not sufficient to produce positive and negative contrast for a given response when topographically different responses are used in the components of a multiple schedule. The results are globally consistent with the competition theory of behavioral contrast.     

Behavioral contrast as a function of component duration for leverpressing using a within-session procedure

Ten rats pressed levers for food reinforcers delivered by multiple schedules. Behavioral contrast was measured using a within-session procedure that presented the baseline and contrast schedules within single sessions. The absolute sizes of both positive and negative contrast increased and then decreased as components lengthened. Negative induction occurred when components were very short. These results question theories that predict that the size of contrast will vary inversely with component duration. They support theories that attribute positive and negative contrast to similar theoretical mechanisms. A comparison of the present results with those of past studies indicates that keypecking by pigeons and leverpressing by rats change as different functions of component duration. Treadlepressing by pigeons and leverpressing by rats change as similar functions. These findings challenge general process theories that argue that all responses obey the same behavioral laws.     

Behavioral contrast and type of reward: Role of elicited response topography

Groups of pigeons were exposed to multiple variable-interval variable-interval and multiple variable-interval extinction schedules of either food or water reinforcement for keypecking. Discriminative stimuli associated with component schedules were located either on the operant key or on a second “signal” key. When the stimuli were projected on the operant key, positive contrast appeared during discrimination conditions with either food or water as the reinforcer. When the stimuli were projected on the signal key, overall responding to the operant and signal keys showed contrast with food, but negative induction with water as the reinforcer. In the latter condition, the signal for the variable-interval shcedule of water reinforcement elicited a variety of water-related behavior, only some of which was directed at the signal. Thus, the type of reward and location of discriminative stimuli interacted to determine the presence or absence of behavioral contrast effects. In large part, these results support and extend the autoshaping view of contrast.     

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.     

Habituation may contribute to within-session decreases in responding under high-rate schedules of reinforcement

Two experiments tested the hypothesis that habituation contributes to within-session decreases in responding. In Experiment 1, rats’ leverpressing was reinforced under a fixed ratio (FR) 4 schedule throughout the baseline sessions. During the dishabituation sessions, the first 21 min and the last 21 min were FR 4; dishabituating events occurred during the middle 3 min. The dishabituating events altered the manner of reinforcer delivery in four different ways. Response rates increased after all dishabituating events. In Experiment 2, rats responded on several FR and variable ratio (VR) schedules. The ratio requirement varied from 3 to 15. Within-session decreases in responding were steeper during FR schedules than during VR schedules. In addition, response rates were well described as linear functions of cumulative number of food pellets eaten within sessions. These results support the habituation hypothesis but do not rule out the possibility that other satiety variables might contribute simultaneously.     

Schedule-induced polydipsia contrast in the rat

Schedule-induced polydipsia was studied using a behavioral contrast paradigm. Food pellets were delivered to food-deprived rats on a response-independent FT 1-min schedule. Licking on a tube produced water on a MULT FR 10 FR 10, MULT FR 10 EXT, or MIXED FR 10 EXT for three rats (Experiment 1) and on a MULT VI VI, MULT VI EXT, or MIXED VI EXT schedule for three other rats (Experiment 2). On the FR schedules, rats could drink more water by increasing lick rates, but on the VI schedules the amount of drinking was fixed by the schedule parameters and was relatively unaffected by lick rates. Relative to MULT FR FR, positive polydipsia contrast was clearly demonstrated on MULT and MIXED FR EXT; but relative to MULT VI VI, contrast was not demonstrated on MULT and MIXED VI EXT. These data suggest that polydipsia contrast occurs only if increased licking permits increased drinking.     

Behavioral contrast and inhibitory stimulus control

Two experiments examined the presumed relationship between behavioral contrast and inhibitory stimulus control. In Experiment I, pigeons were exposed to mult VI 1-min VI 1-min or mult VI 5-min VI 5-min during baseline training prior to mult VI 1-min VI 5-min discrimination training. Half of the subjects received a timeout (TO) component during baseline in order to reduce the degree of contrast during discrimination training. Only 3 of 8 subjects receiving the TO showed contrast while all other subjects showed various degrees of contrast. Postdiscrimination generalization gradients indicated excitatory rather than inhibitory control by the stimulus associated with the VI 5-min schedule. During baseline training in Experiment II, responding to all the generalization stimuli was reinforced. In addition, some subjects received the TO stimulus. The subjects were next exposed to mult VI 1-min EXT, mult VI 1-min VI 5-min, or just the VI 5-min component. Generalization gradients indicated inhibitory control by the stimulus associated with EXT or VI 5-min for 19 of 20 subjects even though some subjects did not show contrast. These results question the presumed relationship between behavioral contrast and inhibitory stimulus control.     

Extinction of operant behavior: An analysis based on foraging considerations

In two experiments, the frequency of food reinforcement provided by variable interval (VI) schedules prior to extinction was varied. In the first experiment, two-component multiple schedules resulted in a greater number of responses in extinction in the presence of the stimulus previously associated with the richer of the two component schedules than that previously associated with the leaner schedule. In the second experiment, different groups of animals were trained on different VI schedules. Responding in extinction was analyzed into bouts of responding showing that the number of response bouts increased and the number of responses per bout decreased with decreasing frequency of reinforcement during training. These data are compatible with an analysis of operant behavior based on an analogy to processes that presumably-occur-in naturalistic foraging situations. According to this analogy, behavior associated with search for a food source (i.e., number of response bouts) and that of procurement of food from a source (i.e., responses per bout) represent aspects of behavior that are differentially strengthened by different VI schedules. Extinction serves to reveal this differential strengthening.     

Value transfer in concurrent-schedule discriminations by pigeons

When pigeons are trained on a discrete-trial simultaneous discrimination, some of the value associated with the positive stimulus appears to transfer to the negative stimulus (Zentall & Sherburne, 1994). Pigeons preferred a negative stimulus that had been discriminated from an always-positive stimulus (S+) over a negative stimulus that had been discriminated from a sometimes-positive stimulus (S±). A very different finding (suggestive of transitivity of preference or contrast) was reported by Belke (1992). On concurrent probe tests of stimuli associated with equal variable interval (VI) schedules but originally trained in alternative concurrent pairs (one with a richer schedule, the other with a poorer schedule—VI 20 sec vs. VI 40 sec and VI 40 sec vs. VI 80 sec), the stimulus originally paired with the poorer schedule was preferred. But Belke’s results may have been obtained because the pigeons had been trained to peck the VI 40 sec paired with the poorer schedule and they had been trained not to peck the VI 40 sec paired with the richer schedule. In the present experiment, we avoided this bias by training pigeons on two concurrent schedules in which the tested stimuli both had been associated with the poorer schedule of the pair [A(VI 20 sec) vs. B(VI 80 sec) and C(VI 40 sec) vs. D(VI 80 sec)]. Evidence for value transfer was demonstrated when on probe trials pigeons preferred B over D.     

Additive summation by stimulus compounding irrespective of behavioral contrast during discrimination training: An investigation with positive reinforcement and avoidance schedules

The similarity in the discrimination training leading to behavioral contrast and that preceding tests producing response enhancement to combined discriminative stimuli suggested that the two phenomena might be related. This was investigated by determining if contrast indiscrimination training was necessary for this outcome of stimulus compounding. Responding to tone, light, and to the simultaneous absence of tone and light (T + L) was maintained during baseline training by food reinforcement in Experiment I and by shock avoidance in Experiment II. During subsequent discrimination training, responding was reduced in T + L by programming nonreinforcement in Experiment I and safety or response-punishment in Experiment II. In the first experiment, one rat exhibited positive behavioral contrast, i.e., tone and light rates increased while his T + L rate decreased. In Experiment II, rats punished in T + L showed contrast in tone and light, this being the first demonstration of punishment contrast on an avoidance baseline with rats. The discrimination acquisition data are discussed in the light of current explanations of contrast by Gamzu and Schwartz (1973) and Terrace (1972). During stimulus compounding tests, all subjects in both experiments emitted more responses to tone-plus-light than to tone or light (additive summation). An analysis of the terminal training baselines suggests that the factors producing these test results seem unrelated to whether or not contrast occurred during discrimination training. It was concluded that the stimulus compounding test reveals the operation of the terminal baseline response associations and reinforcement associations conditioned on these multicomponent free-operant schedules of reinforcement.

     

Multiple-schedule interactions and discrimination

Pigeons received variable-interval (VI) reinforcement for keypecking during randomized presentations of seven line-orientation stimuli forming a continuum ranging from horizontal (0 deg) to vertical (90 deg). Each line presentation lasted for 30 sec and was preceded and followed by 30-sec time-outs. After responding stabilized, only responding in the two extreme stimuli (0 and 90 deg) was reinforced. As discrimination training proceeded, strong behavioral contrast and dimensional contrast effects appeared. However, only marginal local effects (local contrast and local dimensional effects), exerted by one line-orientation component upon a second, appeared, indicating that behavioral and dimensional contrast may be independent of parallel local effects. An attempt was made to apply Blough’s (1975) quantitative model of operant generalization and discrimination to the present discrimination procedure. However, this model did not predict the generalization gradient shape that was experimentally obtained. This experiment also yielded two serendipitous findings: (1) Positive behavioral contrast appeared in an extinction-related stimulus (time-out) when other stimuli were switched from reinforcement to extinction (hitherto, positive behavioral contrast had been observed only in responding to a reinforcementrelated stimulus when other stimuli were switched from reinforcement to extinction), and (2) final responding was higher in the presence of an extinction stimulus that had always been an extinction stimulus than it was in the presence of other extinction stimuli that had previously been paired with VI reinforcement.     

Positive behavioral contrast,autoshaping, and omission responding in the goldfish (Carassius auratus)

Two experiments with goldfish were performed to investigate the role of stimulus-reinforcer vs. response-reinforcer relationships in omission training and the role of stimulus localizability in a positive behavioral contrast paradigm. The directed behavior of fish, like that of pigeons and rats in other studies, was greatly influenced by positive stimulus-reinforcer correlations, as evidenced by maintained contacts of a signal for food, even though such responses terminated the signal and cancelled reinforcement delivery. Goldfish exhibited positive behavioral contrast when the signals for reinforcement and nonreinforcement were displayed directly on the response key, but no contrast was observed when variations in a diffuse houselight stimulus were used as signals for reinforcement or nonreinforcement. Analysis of sequential-trial data yielded effects analogous to Pavlovian positive and negative induction. Theoretical and methodological problems were briefly considered.     

A matching law analysis of the reinforcing efficacy of wheel running in rats

Previous research has demonstrated that running in a rotating wheel functions as a reinforcer for leverpressing in rats. In these studies, the pattern of responding was similar to the pattern of responding maintained by consummatory reinforcers, such as food and water. The present study investigated quantitative features of responding maintained by running. In previous experiments in which responses were reinforced according to variable-interval (VI) schedules and food and water served as the reinforcer, the equation for a rectangular hyperbola described the relationship between response rate and reinforcement rate. This experiment tested whether this quantitative regularity also applies to leverpressing maintained by the opportunity to run in a wheel. Fourteen male Wistar rats responded on levers for the opportunity to run. In each session, subjects were exposed to a series of VI schedules. An opportunity to run for 60 sec was the reinforcing consequence. Results showed that response rate was a negatively accelerated function of reinforcement rate, and the relationship between these two variables was described well by the equation for a rectangular hyperbola. To further test the similarity between running and consummatory reinforcers, the response requirement and access were manipulated. In previous experiments with food and water, these types of manipulations differentially changed the two parameters of the hyperbola. A similar pattern of results was obtained with wheel running. Thus, the equation appears to apply to running about as well as it does to consummatory reinforcers.     

The effect of discrimination training on the response rate and response duration of the rat

Response rate (responses per minute) and response duration (time in seconds per press) were recorded in rats shifted from multiple variable-interval variable-interval (mult VI VI) to multiple extinction variable-interval (mult EXT VI). This procedure produced positive contrast (i.e., a decrease in response rate in EXT and an increase in VI) in 11 of 12 rats and, in addition, produced an increase in response duration (i.e., bar holding), which, for the most part, was confined to the EXT component of the mult EXT VI condition. One of the 12 rats showed negative induction (i.e., a decrease in response rate in both the EXT and VI components), and also an increase in response duration in both the EXT and VI components of the mult EXT VI condition. The results indicate that generalization of behavior from the EXT to the VI component can produce negative induction, and that such generalization must be prevented if positive contrast is to occur in the rat.     

Stimulus preference and the transitivity of preference

Four pigeons were exposed to multiple schedules with concurrent variable interval (VI) components and then tested for preference transfer. Half of the pigeons were trained on a multiple concurrent VI 20-sec, VI 40-sec/cuncurrent VI 4G-sec5 VI 80-sec schedule. The remaining pigeons were trained on a multiple concurrent VI 80-sec, VI 40-sec/concurrent VI 40-sec, VI 20-sec schedule-After stability criteria for time and response proportions were simultaneously met, four preference transfer tests were conducted with the stimuli associated with the VI 40-sec schedules. During the transfer tests, each pigeon allocated a greater proportion of responses (M=0,79) and time (M=0.82) to the stimulus associated with the VI 40-sec schedule that was paired with the VI 80-sec schedule than lo the VI 40-sec schedule stimulus paired with the VI 20-sec schedule. Absolute reinforcement rates on the two VI 40 sec schedules were approximately equal and unlikely to account for the observed preference. Nor was the preference consistent with the differences in local reinforcement rates associated with the two stimuli. Instead, the results were interpreted in terms of the differential value that stimuli acquire as a function of previous pairings with alternative schedules of reinforcement.     

Pigeon and human performance in a multi-armed bandit task in response to changes in variable interval schedules

The tension between exploitation of the best options and exploration of alternatives is a ubiquitous problem that all organisms face. To examine this trade-off across species, pigeons and people were trained on an eight-armed bandit task in which the options were rewarded on a variable interval (VI) schedule. At regular intervals, each option’s VI changed, thus encouraging dynamic increases in exploration in response to these anticipated changes. Both species showed sensitivity to the payoffs that was often well modeled by Luce’s (1963) decision rule. For pigeons, exploration of alternative options was driven by experienced changes in the payoff schedules, not the beginning of a new session, even though each session signaled a new schedule. In contrast, people quickly learned to explore in response to signaled changes in the payoffs.     

The maintenance of matching behavior when contrast occurs in multiple concurrent concurrent schedules of reinforcement

Pigeons were trained on a multiple schedule of reinforcement in which each component was a concurrent schedule. The concurrent schedules were programmed by the changeover-key procedure. The primary purpose was to determine if the relative behavior allocated to two response alternatives is affected when absolute changes in these behaviors occur; i.e., to determine if matching is affected when positive behavioral contrast occurs. Results showed that (1) relative behavior in the unaltered component of the multiple schedule is not disrupted when positive contrast occurs in that component, (2) positive contrast occurred when the overall frequency of reinforcement in the reinforcement-correlated component(s) was high, but not when it was low, (3) changeover behavior was susceptible to positive contrast effects, and (4) changeover contrast and food-key contrast are independent phenomena.     

Stimulus control in multiple variable-ratio schedules of reinforcement

One component of a multiple variable-ratio 150 variable-ratio 150 schedule remained unchanged while the reinforcement schedule of the other component was periodically changed to extinction and then returned to variable ratio 150. When the reinforcement schedule of the changed component was an unmodified variable ratio 150 schedule, the magnitude of negative contrast during baseline recovery was equal to the magnitude of positive contrast observed during the previous change from multiple variable-ratio 150 variable-ratio 150 to multiple variable-ratio 150 extinction. When the schedule of the changed component was modified during baseline recovery so that responses terminating interresponse times greater than the average baseline interresponse time in the unchanged component were not reinforced, the magnitude of the unchanged-component response rate decrease was reduced. The magnitude of negative contrast was attenuated even though response and reinforcement rates in the variable component increased to levels at or above their prior multiple variable-ratio 150 variable-ratio 150 baseline levels. The results support a general theory that negative contrast results from both (1) the removal of certain positive-contrast-producing operations and (2) changes in the interresponse time-reinforcer relations that occur as a byproduct of schedule manipulations.