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.     

Summation of punishment suppression

In two experiments, eight rats were trained to lever press with food on a variable-interval schedule. Bar pressing produced shock on a variable-interval schedule in the presence of two independently presented stimuli, a light and a tone. Two rats in each experiment received alternative presentations of the light and the tone and were consequently always in the presence of a stimulus that signalled variable-interval punishment. The other two rats in each experiment were treated similarly except that they received periods in which neither light nor tone was present. During these periods, bar pressing was not punished. The two stimuli that signalled punishment were then presented simultaneously to evaluate the effect of stimulus compounding on response suppression. The subjects trained without punishment-free periods did not show summation to the compound stimulus; the subjects trained with punishment-free periods showed summation of suppression. The major difference between the two experiments was the longer mean interval of variable-interval punishment used in the second experiment. This manipulation made the summation effect more resistant to extinction and thus increased its magnitude.     

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.     

Stimulus control and compounding with ambient odor as a discriminative stimulus on a free-operant baseline

Previous experiments have demonstrated that the simultaneous presentation of independently established discriminative stimuli can control rates of operant responding substantially higher than the rates occasioned by the individual stimuli. This "additive summation" phenomenon has been shown with a variety of different reinforcers (e.g., food, water, shock avoidance, cocaine, and heroin). Discriminative stimuli previously used in such studies have been limited to the visual and auditory sensory modalities. The present experiment sought to (1) establish stimulus control on a free-operant baseline with an ambient olfactory discriminative stimulus, (2) compare olfactory control to that produced with an auditory discriminative stimulus, and (3) determine whether compounding independently established olfactory and auditory discriminative stimuli produces additive summation. Rats lever pressed for food on a variable-interval schedule in the presence of either a tone or an odor, with comparable control developed to each stimulus. In the absence of these stimuli responding was not reinforced. During stimulus compounding tests, the tone-plus-odor compound occasioned more than double the responses occasioned by either the tone or odor presented individually. Thus, the current study (1) established stimulus control with an ambient olfactory discriminative stimulus in a traditional free-operant setting and (2) extended the generality of stimulus-compounding effects by demonstrating additive summation when olfactory and auditory discriminative stimuli were presented simultaneously.     

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.     

Summation of conditioned suppression

Four rats were trained to suppress responses in the presence of two separately presented stimuli that signalled shock in a conditioned-suppression paradigm. The two stimuli that signalled shock were then presented simultaneously to evaluate the effect of stimulus compounding on conditioned suppression. Two rats were tested by presenting the compound conditioned stimulus while conditioned suppression was being maintained to the individual conditioned stimuli. The other two rats were tested by giving them random presentations of the compound conditioned stimulus and the single conditioned stimuli during extinction of conditioned suppression. All four rats showed greater suppression to the compound stimulus than to either stimulus presented alone.     

Summation of responding maintained by fixed-interval schedules

A light and tone were separately correlated with responding maintained by fixed-interval schedules, in which the level of responding varied continuously throughout the duration of the interval. Responding during the presence of the single stimuli and their compound was compared during the successive segments of the interval. The following results were obtained: (1) more responses were emitted during compounding than were emitted during either stimulus alone in all segments of the interval; (2) increases in the number of responses across the interval during compounding paralleled increases during single-stimulus presentations. The sum of the responses emitted during the single stimuli was similar to the number of responses emitted during compounding, suggesting that the response tendencies correlated with the single stimuli combined in a summative or additive fashion.     

Pavlovian phenomena in conditioned acceleration—Stimulus summation

Four naive male rats were separately fear-conditioned to two stimuli (light and sound) by pairing these stimuli with shocks. During Sidman avoidance these unreinforced stimuli elicited increases in response rates (conditioned acceleration). In a multiple schedule with light-shock and sound-shock reinforcements in the first component, the rats were tested for summation effects in the second (avoidance) component by presenting the light twice, the sound twice and the compound twice. They were then extinguished by eliminating the shocks in the first component. The effects of compounding were evidenced by: (1) a sizeable and reliable amplitude increase to the compound stimulus during conditioning; (2) a smaller and less consistent decrease in latencies during conditioning in all four subjects; and (3) a greater resistance to extinction for the compound stimulus as measured by mean amplitudes for all four subjects and on six of seven extinction sessions. Latency data during extinction were inconclusive. An additional unexpected finding was what appeared to be a two-factor secondary extinction of the avoidance itself as a result of Pavlovian extinction. The responding in the first four minutes of avoidance was inhibited to such an extent that the number of shocks received during this period increased 700 per cent from the last three conditioning sessions to the first three extinction sessions.     

Enhancement and summation in the perception of two successive vibrotactile stimuli

Enhancement and summation were found to be fundamentally different perceptual processes affecting the sensation magnitude of two successive vibrotactile stimuli. Enhancement, defined operationally as an increment in the subjective magnitude of one stimulus due to the presentation of a prior stimulus, and summation, defined as an increment in overall subjective magnitude of the two stimuli, were measured for sinusoidal vibration of the thenar eminence of the hand. The effect of summation was maximum when the two stimuli greatly differed in frequency, whereas maximum enhancement effects were found when both stimuli were close in frequency. The summation effect showed little decay as the interstimulus interval was increased to as much as 500 msec, whereas enhancement effects decayed to zero at approximately 500 msec. Results were similar to those obtained in comparable studies of audition and support the hypothesis that there are at least two distinct information-processing channels for the perception of cutaneous vibration.     

Humans’ responses to novel stimulus compounds and the effects of training

Twenty-four college students participated in an experiment with stimulus compounds. Subjects learned to discriminate color stimuli that correlated with varying probabilities of reinforcement. Reinforcement consisted of points. For all subjects, two colors signaled a .80 reinforcement probability, and two others signaled a .20 probability. For compound-trained subjects, a fifth compound stimulus (composed of a high-probability color and a low-probability color) was correlated with a .10 reinforcement probability. During testing, interspersed probe trials required subjects to choose between two alternatives: a compound stimulus and either one of its constituent stimuli. Compound-untrained subjects preferred the compound over either individual stimulus, thus showing response summation. However, compound-trained subjects, having had experience with an exemplar compound, showed significantly lower choice proportions for a test compound, indicating that subjects’ responding to novel stimulus compounds is modifiable by experience with a single similar compound.     

The startle response and auditory temporal summation in neonates

The present study assessed temporal summation of transient and sustained stimuli in the startle eyeblink response system in neonates during quiet sleep. Subjects received 100-dB(A), fast-rising broadband noise bursts of two types: (a) single stimuli varying in duration from 20 to 100 ms and (b) pairs of 3-ms bursts presented at interpulse intervals corresponding to the single-stimulus durations. In addition, a single 3-ms pulse was used as an anchor point for both stimulus types. For startle amplitude, single stimuli were more effective than were paired stimuli, but the temporal summation functions were similar for the two types of stimuli. Response amplitude increased as stimulus duration/interval increased to 50 ms, but not beyond. For startle probability, temporal summation was similar for single and paired stimuli at 20 ms. Pairs of pulses were equally effective at 20, 35, and 50 ms, beyond which the second pulse was not effective. Increasing the duration of single stimuli from 20 to 35 ms resulted in increased probability, illustrating a contribution of sustained summation beyond that of transient summation. Response latency was generally greater for paired than for single stimuli. The results suggest that temporal summation of brief stimuli is deficient in the neonate. These data were compared to adult data from an analogous study, and suggest that the transient system is immature in infants, and that this immaturity is expressed differently by startle amplitude, probability, and latency.     

The Relation of the Galvanic Skin Reflex Recovery Curve to Reactivity,Resistance Level,and Perspiration

The effect of stimulus compounding in classical conditioning was investigated by conditioning one group of rats to a compound CS consisting of a buzzer and light and then conditioning separate groups of rats to the individual elements of the compound CS. On hurdle-jump test trials, the group of Ss conditioned to the compound CS performed better than Ss conditioned to the elements of the compound. Strength of conditioning to each of the elements of the compound CS was about equal. There was some evidence of a summation effect resulting from conditioning to the compound CS. Strength of conditioning to the compound CS was somewhat greater than the sum of the response strengths conditioned to the elements of the compound CS.     

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.     

Overexpectation: response loss during sustained stimulus compounding in the rabbit nictitating membrane preparation

Rabbits were given reinforced training of the nictitating membrane (NM) response using separate conditioned stimuli (CSs), which were a tone, light, and/or tactile vibration. Then, two CSs were compounded and given further pairings with the unconditioned stimulus (US). Evidence of both overexpectation and summation effects appeared. That is, responding to the individual CSs declined despite their continued pairing with the US on compound trials (overexpectation), and responding on the compound trials was greater than responding to the individual CSs (summation). The response loss appeared regardless of the testing regime, that is, whether the test presentations of the individual CSs were themselves reinforced (Experiment 2), not reinforced (Experiment 1), or deferred until the end of compound training (Experiment 2). The results are discussed with respect to the roles of excitatory versus inhibitory processes, elemental versus configural processes, and the possible roles of cerebellar and hippocampal pathways.     

Steady state responding based upon simple and compound stimuli

Two pigeons were trained to perform discrimination tasks along two dimensions, wavelength of a circular spot of light and orientation of a white line. Discriminability among stimuli along these dimensions was established for both subjects by means of a steady state testing procedure. The two dimensions were then combined by superimposing the white line upon the colored background. Subjects were given a series of tests in which a correct response could be made on the basis of either of the two components of the stimulus compounds. Discriminability among these redundant compound stimuli was found to be better than that among wavelength and tilt stimuli alone. A second series of tests was administered using both redundant and conflicting compound stimuli. The results of this test series are consistent with a response strategy in which subjects first examine both elements of a compound and then emit a choice response on the basis of the element that best predicts reinforcement.     

Combined stimulus control over time allocation in response-independent schedules

Rats were exposed to concurrent schedules in which reinforcers occurred independently of behavior. In Experiment 1, rats could control time spent in the following conditions: (1) a light, (2) white noise, and (3) the absence of both light and noise. Response-independent reinforcers occurred at the same rate during the light and the noise and at either a higher rate or not at all in the absence of both stimuli. In subsequent tests, the rats spent more time in a light and noise compound than in either light or noise alone after the absence of both stimuli had signaled no reinforcers. When the absence of both stimuli had signaled a higher rate of reinforcement, however, the rats typically spent less time in the compound than in light or noise alone. In Experiment 2, rats could control time spent in the presence of a light and of a buzzer. The reinforcement rate in the light was twice that in the buzzer. In a later test, the rats spent more time in a light and buzzer compound than in the buzzer, but less time in the compound than in the light. The results show that additive summation, suppressive summation, and stimulus averaging of time allocation occur and that response rate differences between training stimuli are not necessary for these phenomena.     

Ipsi-and bilateral interactions in taste

In two experiments, the integration over spatial extent in taste was investigated for threshold sensitivity to NaCl and for suprathreshold intensity perception of saltiness. The area of stimulation was doubled by adding either an ipsilateral or a bilateral stimulus. The two stimuli could be of equal or unequal intensity. The data showed that at threshold level a probability summation model applied to all bilateral and most of the ipsilateral stimulus combinations. Probability summation failed to predict detection probability when two stimuli with different intensities were presented at the same tongue side. For suprathreshold stimuli, the magnitude of the saltiness sensation as estimated by a line-length method depended on the level of stimulation. The possible peripheral interaction mechanisms and central factors contributing to the taste response were discussed.     

Compounding of discriminative stimuli that maintain responding on separate response levers

In Experiment 1, rats' responses were reinforced on a fixed-interval 30-sec schedule in the presence of either a light or a tone and were not reinforced in their absence. Each stimulus was correlated with its own response lever, with only one lever present during a session. When light and tone were compounded in the presence of the tone-correlated lever, no change in responding occurred. However, when tone was compounded with light in the presence of the light-correlated lever, level of responding was greater than to light alone (response summation). Summation was also found when each stimulus was correlated with the same lever. Next, light and tone were again correlated with separate levers, but both levers were always simultaneously present. Compounding produced both summation and emission of most responses on the light-correlated lever. This prepotency of light was reduced (1) by leaving a houselight on throughout the session; and (2) by correlating each stimulus with a different schedule (either fixed-interval 4.7-sec or fixed-interval 30-sec). With a medium- and high-intensity houselight and with the different reinforcement schedules, similar results were obtained during compounding, regardless of whether compounding occurred in the presence of the light- or tone-correlated lever.     

Temporal integration of acoustic and cutaneous stimuli shown in the blink reflex

Temporal integration of pairs of brief blink-eliciting acoustic and cutaneous stimuli was investigated to determine if there was integration of stimuli from different modalities. Reflexes elicited by a tone burst or by a brief electrical shock to the supraorbital nerve followed by a second tone burst or shock at short stimulus onset asynchronies (SOAs) were larger and faster than control reflexes elicited by a single stimulus identical to the lead stimulus of the stimulus pairs. Reflex amplitude was augmented at longer SOAs where there was no effect on latency. Temporal integration was evident for all stimulus pairs, showing that it is due, at least in part, to processes that occur outside specific sensory pathways. Heterogeneous stimulus pairs produced greater reflex enhancement than did homogeneous stimulus pairs. This finding was examined further in Experiment 2, which showed that reflex enhancement with pairs of acoustic pulses was unaffected by the frequency of the second stimulus, suggesting that sensory masking was not acting to suppress reflex expression with acoustic pulse pairs. Integration of reflexogenic acoustic stimuli shown in the blink reflex is restricted to shorter intervals than is integration of acoustic stimuli shown by psychophysical procedures, suggesting that the two methods reflect different aspects of stimulus processing. Integration of reflexogenic stimuli may result from summation of activity associated more directly with reflex expression than with perceptual awareness.     

Do chickens (Gallus gallus f. domestica) decompose visual figures?

To investigate whether learning to discriminate between visual compound stimuli depends on decomposing them into constituting features, hens were first trained to discriminate four features (red, green, horizontal, vertical) from two dimensions (colour, line orientation). After acquisition, hens were trained with compound stimuli made up from these dimensions in two ways: a separable (line on a coloured background) stimulus and an integral one (coloured line). This compound training included a reversal of reinforcement of only one of the two dimensions (half-reversal). After having achieved the compound stimulus discrimination, a second dimensional training identical to the first was performed. Finally, in the second compound training the other dimension was reversed. Two major results were found: (1) an interaction between the dimension reversed and the type of compound stimulus: in compound training with colour reversal, separable compound stimuli were discriminated worse than integral compounds and vice versa in compound training with line orientation reversed. (2) Performance in the second compound training was worse than in the first one. The first result points to a similar mode of processing for separable and integral compounds, whereas the second result shows that the whole stimulus is psychologically superior to its constituting features. Experiment 2 repeated experiment 1 using line orientation stimuli of reversed line and background brightness. Nevertheless, the results were similar to experiment 1. Results are discussed in the framework of a configural exemplar theory of discrimination that assumes the representation of the whole stimulus situation combined with transfer based on a measure of overall similarity.