Compounding discriminative stimuli controlling free-operant avoidance

The performances of three rats were stabilized on a multiple schedule that maintained responding by a free-operant avoidance schedule during independent presentations of tone and light. The simultaneous absence of these stimuli signalled shock-free periods and controlled response cessation. Subsequently, test sessions were administered consisting of independent presentations of each stimulus and these stimuli compounded (tone-plus-light). During an extinction test, additive summation was observed to the compounded stimuli, i.e., more responses were emitted to the compound than to either tone or light. During a series of 28 maintenance-test sessions in which the shock schedule remained operative, the compounded stimuli produced a generally enhanced response rate and fewer pauses terminating with shock than either single stimulus condition. These results extend the generality of free-operant additive summation to responding maintained by aversive control. In addition, a comparison of the present study with previous experiments reporting additive summation of positively reinforced responding indicates that similar variables—rate and aversive differences between training stimulus conditions—should be considered in accounting for response distributions during stimulus compounding when responding is controlled by either positive or negative contingencies.     

Compounding of discriminative stimuli correlated with chained and multiple schedules

In studies of stimulus compounding (1) the stimuli are presented randomly, (2) primary reinforcement is correlated with each stimulus, (3) a specific response is emitted during each stimulus, and (4) the response is necessary to produce the reinforcer. The present experiments assessed the importance of these procedures by (1) presenting light and tone stimuli in fixed order, (2) removing reinforcement (food) during one stimulus, (3) preventing the response (lever pressing) from being emitted, and (4) eliminating the contingency between lever pressing and food. These variables were presented in various combinations within the context of chained and multiple schedules. When the stimuli were combined in the schedule component correlated with each stimulus, the frequency of lever pressing increased in most instances (additive summation). This suggests that the effect of combining stimuli was not closely tied to the specific procedures used in previous experiments. However, presenting the stimuli in a fixed order did have an effect: the level of responding to the compound was generally greatest when the stimuli were combined in the component correlated with the higher frequency of lever pressing to the single stimulus. Additive summation failed to occur consistently when response-independent food was correlated with each stimulus, and when both lever pressing and food were eliminated during one stimulus.     

Compounding of discriminative stimuli from the same and different sensory modalities

Rats' responding was maintained by fixed-interval schedules of reinforcement in the presence of a tone or two separate lights. The lights were either of low, moderate, or high intensity. Compounds of these single discriminative stimuli each maintained a greater frequency of response than did the single stimuli, and the compound composed of stimuli from different sensory modalities (light+tone) maintained a greater level of responding than did the compound composed of stimuli from the same sensory modality (light+light). Combining lights of different intensity had no differential effect on responding. However, in the second experiment, a compound composed of a light and a tone, each of greater intensity than the light and tone of another compound, initially maintained a higher frequency of response, demonstrating intensity effects during stimulus compounding when the increase in intensity occurs through the component stimuli. This intensity effect, however, was only transitory.     

Effects of chlorpromazine on fixed-ratio responding: modification by fixed-interval discriminative stimuli.

Effects of chlorpromazine (1 to 100 mg/kg) were assessed on two pigeons' responding under various modifications of a multiple schedule of food delivery. During a fixed-interval component, the first response after 5 min produced food; during the subsequent, fixed-ratio component, the 30th response produced food. Modifications of the schedule entailed changes in stimulus conditions imposed during the fixed-ratio component that did not systematically alter characteristics of performance under non-drug conditions. In the first phase of the experiment, distinctive visual stimuli were correlated with each schedule component (conventional multiple schedule); chlorpromazine produced small decreases in fixed-ratio responding (20% at 30 mg/kg). When each response during the fixed-ratio component produced the stimulus correlated with the fixed-interval schedule (fixed-interval discriminative stimulus) for 1.2 s, effects of chlorpromazine were not different from those under the conventional multiple schedule. Chlorpromazine produced greater decreases in fixed-ratio responding (55% at 30 mg/kg) when either the first response of each fixed ratio changed the stimulus correlated with the fixed-ratio schedule to the fixed-interval discriminative stimulus for the remainder of the fixed-ratio component, or when the fixed-interval discriminative stimulus was presented independently of responding according to a matched temporal sequence. When the fixed-interval discriminative stimulus was present continuously during the fixed-ratio component (mixed schedule), chlorpromazine produced even more substantial decreases in fixed-ratio responding (greater than 80% at 30 mg/kg). Effects of chlorpromazine on fixed-interval responding were also modified by the schedules of fixed-interval discriminative stimulus presentation. The effects of chlorpromazine were a joint function of the stimuli prevailing during the multiple schedule and the degree to which responding influenced these stimuli.     

Control of responding by the elements of a compound discriminative stimulus and by the elements as individual discriminative stimuli

In the first of two studies, the responding of four albino rats was differentially reinforced in the presence of noise and light together and then tested in the presence of the noise and the light separately during extinction. The light exercised substantially more control of responding than did the noise. In the second study the responding of a similar group of four rats was differentially reinforced in the presence of the noise and the light separately. Control of responding by the light developed more rapidly than control by the noise. Results suggest that levels of control by stimuli after differential reinforcement with respect to the stimuli together can be predicted by the rates of development of control during differential reinforcement with respect to the stimuli separately.     

Compounding of pre-aversive stimuli

When two discriminative stimuli, each capable of maintaining a response, are combined, their compound will maintain a frequency of response greater than the frequencies maintained by the individual stimuli. This has been called additive summation. The present experiments extended the investigation of this phenomenon to a converse situation in which two pre-aversive stimuli were combined. Each pre-aversive stimulus was capable of reducing the frequency of an ongoing response. The combination of these stimuli reduced the relative frequency of response below that resulting from either stimulus. Furthermore, the compounding of two highly suppressive stimuli produced more suppression than the compounding of two less-suppressive stimuli. Evidence was also presented to suggest that the compound continued to reduce responding even when the single stimuli were no longer effective. A fourth experiment demonstrated that summation of response tendencies could not be accounted for in terms of stimulus intensity or sensory interaction.     

Differential responding in the presence and absence of discriminative stimuli during multielement functional analyses

We evaluated the extent to which discriminative stimuli (S(D)s) facilitate differential responding during multielement functional analyses. Eight individuals, all diagnosed with mental retardation and referred for assessment and treatment of self-injurious behavior (SIB) or aggression, participated. Functional analyses consisted of four or five assessment conditions alternated in multielement designs. Each condition was initially correlated with a specific therapist and a specific room color (S(D)s), and sessions continued until higher rates of target behaviors were consistently observed under a specific test condition. In a subsequent analysis, the programmed S(D)s were removed (i.e., all conditions were now conducted by the same therapist in the same room), and sessions continued until differential responding was observed or until twice as many sessions were conducted with the S(D)s absent (as opposed to present), whichever came first. Results indicated that the inclusion of programmed S(D)s facilitated discrimination among functional analysis conditions for half of the participants. These results suggest that the inclusion of salient cues may increase either the efficiency of functional analyses or the likelihood of obtaining clear assessment outcomes.     

Temporal properties of responding during stimuli that precede response-independent food

Pigeons' keypecks were reinforced with grain on the average of once per minute by schedules that maintained low response rates and by schedules that maintained high response rates. During these schedules, a fixed-duration conditioned stimulus (CS) ranging from 7.5 to 120 sec in duration across conditions terminated with response-independent food. Response rates during the CS were inversely related to CS duration. The rates and the temporal patterns of responding during the shortest CS were similar whether the ongoing schedule maintained high response rates or low response rates. As CS duration increased, the rate and pattern of responding during the CS converged on the rate and pattern of responding maintained by the baseline schedule. These data indicate that changes in responding during stimuli that signal response-independent reinforcement are not homogeneous throughout the CS; that response measures, such as “suppression ratios”, which presume homogeneity may mislead us; and that conditioned suppression and conditioned enhancement may be better talked about in terms of species-specific approach and avoidance than in terms of emotional states.     

Control of responding during stimuli that precede transitions in reinforcement frequency

Pigeons' responses were reinforced on a variant of a mixed variable-interval extinction schedule of reinforcement in which the transition to the higher reinforcement rate was signaled by a trace stimulus projected on the response key prior to the onset of the component correlated with food delivery. In the first of two experiments, the duration of the trace stimulus preceding the component correlated with food delivery was varied from 1.5 to 50.0 s and in the second experiment, the reinforcement frequency in the same component was varied from 10 to 60 reinforcers per hour. Pigeons pecked at the trace stimulus preceding the onset of the component correlated with food delivery even though responding was not reinforced in its presence and only one of the changes in reinforcement rate (i.e., from extinction to reinforcement) was signaled. The rate of pecking during the trace stimulus was a function of its duration but not of the reinforcement frequency in the following component. Higher rates generally occurred at the shorter trace-stimulus durations. Component responding following the offset of the trace stimulus was under discriminative control of the trace stimulus whether or not responding occurred in the presence of the trace stimulus.     

Summation of response rates to discriminative stimuli associated with qualitatively different reinforcers

In Experiment I, a four-ply multiple schedule was used to study the effects on rate of responding in rats of food, water, and food and/or water reinforcement under different deprivation conditions. Food and water were associated separately with different stimuli, the combination of which was associated with food and water together, or with food or water randomly. Rates in the presence of the combined stimuli were consistently intermediate to the rates generated by the separate stimuli, a result seemingly incompatible with a "summation" hypothesis. Experiment II was a simplified systematic replication of Experiment I, verifying the major findings.     

Compound stimulus control by discriminative stimuli associated with high and moderate response rates

Four rats received water on a fixed-ratio schedule for lever pressing in the presence of a tone (or light) stimulus and on a variable-interval schedule in the presence of a light (or tone) stimulus. Following stabilization of a high response rate during the fixed-ratio component and a moderate response rate during the variable-interval component, brief periods with the light and tone presented simultaneously but with no responses reinforced were inserted into the regular training schedule. Response rates during the compound stimuli were intermediate between the response rates controlled by the individual fixed-ratio and variable-interval associated stimuli.     

The use of rats as discriminative stimuli

A FREE OPERANT PROCEDURE WAS USED TO DETERMINE WHETHER OR NOT ONE RAT COULD DISCRIMINATE: (1) between the presence and absence of a second rat, and (2) between two other rats of the same species and sex. The subjects were four male Wistar rats. The discriminatory response was a bar press and food was used as reinforcement during training. Although there were wide individual differences in rate of learning, all subjects learned to make both discriminations.     

Effects of stimulus control and deprivation upon discriminative responding

Discriminative responding in pigeons was studied under multiple variable-interval extinction schedules in which extinction was correlated with either a tone or a white keylight. The two procedures resulted in weak and strong stimulus control, respectively. In the first experiment, there was no interaction between schedule components when stimulus control was strong and reinforcement was omitted under the previously reinforced component. However, there was marked induction between components when stimulus control was weak and responding was extinguished under the previously reinforced component. In the second experiment, hours of food deprivation was varied under two levels of stimulus control. Deprivation mainly influenced response rates under the extinction stimulus, with greater absolute rate increases occurring the lower the existing level of stimulus control. Increases in responding during the extinction stimulus were four times as great from 24 to 72 hours of deprivation as from 24 to 48 hours under conditions of both high and low stimulus control.     

Some effects of punishment shock intensity upon discriminative responding

Three pigeons received visual discrimination training under both multiple variable-ratio extinction and variable-interval extinction schedules. All birds developed nearly perfect discrimination. When punishment for every tenth response during food reinforcement was presented, responding decreased as shock intensity increased. At the same time, responding during extinction, which was not punished, increased at intermediate punishment intensities, but returned to low levels under severe punishment. A second procedure, in which punishment and no-punishment sessions alternated unsystematically, was employed with two of the birds. The results under this procedure essentially replicated the data obtained as punishment shock intensity increased gradually.     

Analysis of discriminative control by social behavioral stimuli

Visual discriminative control of the behavior of one rat by the behavior of another was studied in a two-compartment chamber. Each rat's compartment had a food cup and two response keys arranged vertically next to the clear partition that separated the two rats. Illumination of the leader's key lights signaled a “search” period when a response by the leader on the unsignaled and randomly selected correct key for that trial illuminated the follower's keys. Then, a response by the follower on the corresponding key was reinforced, or a response on the incorrect key terminated the trial without reinforcement. Accuracy of following the leader increased to 85% within 15 sessions. Blocking the view of the leader reduced accuracy but not to chance levels. Apparent control by visual behavioral stimuli was also affected by auditory stimuli and a correction procedure. When white noise eliminated auditory cues, social learning was not acquired as fast nor as completely. A reductionistic position holds that behavioral stimuli are the same as nonsocial stimuli; however, that does not mean that they do not require any separate treatment. Behavioral stimuli are usually more variable than nonsocial stimuli, and further study is required to disentangle behavioral and nonsocial contributions to the stimulus control of social interactions.     

Free-operant compounding of variable-interval and low-rate discriminative stimuli

Four rats were trained on a schedule containing stimuli associated with variable-interval 30-sec and differential-reinforcement-of-low-rate 20-sec schedules of reinforcement. Subsequently, a stimulus compounding test was administered that included individual presentations of two intensities of each stimulus plus compounds of these stimuli. In training, extremely high rates were emitted to the variable-interval stimulus, and very low rates to the differential-reinforcement-of-low-rate stimulus. Compounding the two training stimuli always produced an overall response rate intermediate between the rates controlled by the two stimuli separately presented. Essentially the same relationship held with different stimulus intensities. These results resolve the confounding of response and reinforcement variables present in previous conditioning studies reporting response averaging. They are discussed in terms of the incompatibility of the response chains associated with the individual stimuli compounded.     

Separating discriminative and function-altering effects of verbal stimuli

Ever since Skinner's first discussion of rule-governed behavior, behavior analysts have continued to define rules, either explicitly or implicitly, as verbal discriminative stimuli. Consequently, it is not difficult to find, in the literature on rule-governed behavior, references to stimulus control, antecedent control, or to rules occasioning behavior. However, some verbal stimuli have effects on behavior that are not easily described as discriminative. Such stimuli don't evoke behavior as discriminative stimuli, but rather alter the functions of other stimuli in a manner analogous to operant and respondent conditioning. Hence, this type of control has been called function altering. Any known stimulus function (e.g., evocative, or [conditioned] reinforcing or punishing functions) can apparently be altered by such function-altering stimuli. Describing these stimuli as discriminative stimuli obscures their possible function-altering effects and consequently may retard inquiry into them. This paper encourages behavior analysts to begin separating the discriminative and function-altering effects of verbal stimuli and suggests that by doing so, behavior analysts may better understand what may be most unique about these stimuli. Results from several experiments, especially those in which children served as subjects, are analyzed. Finally, some speculations are offered concerning the genesis of function-altering stimuli.     

Short-term remembering of discriminative stimuli in pigeons.

Pigeons learned to peck the left or right of two white keys depending on whether a red or a green stimulus was displayed on a third key. The opportunity to peck the white keys was then dealyed for zero to six seconds after the red or green (to-be-remembered) stimulus. On half the trials, the feeder operated during the delay to interrupt behavior that might mediate discriminated responding. No events were scheduled on the remaining trials. In a later condition, the pigeons had the opportunity to peck the white keys during the delay. In general, accuracy decreased as delay increased in all conditions, but performance was least accurate following feeder operations and most accurate when pecking was allowed during the delay. The procedures may be analogous to varying the opportunity for rehearsal in studies of human short-term memory.