Effects of S+ and S- separation on gradient shifts in humans

Following single stimulus training, responding during a generalization test tends to be distributed around the positive stimulus (S+). However, if participants are trained instead to discriminate the S+ from a negative stimulus (S-), the response gradient often shifts away from the S- and toward the opposite end of the stimulus continuum. In this experiment, the author examined the basis of gradient shifts with 72 college undergraduates. The research especially examined how gradient shifts are affected by the physical similarity of the S+ and the S- and by the ease with which the two stimuli can be compared. For the former manipulation, the author randomly assigned participants to either a control condition in which only the S+ was shown or a discrimination condition in which the S- was either near to or far from the S+ on the continuum. For the latter manipulation, the author randomly assigned participants to a condition in which S+ and S- presentations were separated by intervals of 1, 15, or 30 s. The results showed that marked shifts occurred when the S+ and S- were relatively similar, but temporal separations did not affect responding.     

Discrimination learning, the peak shift, and behavioral contrast

A discrimination between two successively alternating stimuli was trained under conditions that maintained equal frequencies of reinforcement in the presence of each of the discriminative stimuli (S1 and S2) but that also reduced the rate of responding to S2. These conditions included a multiple variable-interval differential-reinforcement-of-low-rate schedule and a multiple variable-interval variable-interval schedule in which responses to S2 were punished. Whenever the rate of responding to S2 was reduced, rate of responding to S1 (behavioral contrast) increased, and the peak of a subsequently obtained generalization gradient did not occur at the expected location (between S1 and S2) but was displaced away from S2, below S1. Discrimination training in which the frequencies of reinforcement earned in S1 and S2 were not equal (variable-interval 1-min variable-interval 5-min training) produced contrast and the peak shift only if the rate of responding to S2 had been reduced, as after non-differential reinforcement in which variable-interval 1-min schedules were correlated with S1 and with S2. It was concluded that a sufficient condition for the occurrence of behavioral contrast and the peak shift was reduction of the rate of responding to one of two alternating discriminative stimuli and that a peak shift will occur only if contrast had occurred during discrimination training.     

Generalization and discrimination of shape orientation in the pigeon

Pigeons learned to peck a green key on which parallelogram-shapes were projected; they then received generalization tests in which the orientation of the parallelogram was varied. Nondifferential training produced very little eventual stimulus control along the orientation dimension, but when training included S- trials (absence of the parallelogram) subjects responded consistently more to certain orientations than to others. Unlike typical results for visual generalization (e.g., line-tilt), the tilt gradients obtained for this complex stimulus were bimodal, supporting predictions on the basis of human perceptual data. However, unimodal gradients could be produced by specific discrimination training along the orientation dimension. Other forms of intradimensional training also produced relatively steep gradients, often characterized by unexpected but consistent secondary peaks. An attempt to obtain inhibitory gradients (S+: green key; S-: parallelogram on a green background) resulted in virtually zero responding all along the shape-orientation dimension; therefore, specific inhibitory control could not be evaluated. All these experiments suggest that definition of this complex stimulus dimension in terms of mere "angular orientation" is inappropriate, and alternative interpretations are discussed.     

Attention in the pigeon: testing for excitatory and inhibitory control by the weak elements.

In two experiments, pigeons were trained on a successive discrimination between a color and either a compound S+ or a compound S- consisting of a form superimposed on a second color. Two stimulus control tests followed discrimination training: an attention test in which the form and colors used in training were presented singly and in combination, and then a resistance-to-reinforcement test using the form element of S+ or S- and a novel form. In the attention test, the birds trained with a compound S+ responded most to the S+ compound, less to the S+ color alone, and still less to the S+ form on a dark key. Few responses were made to the negative stimulus, either alone or with the S+ form added. The birds trained with a compound S- pecked most at the S+ color and to a compound of the S+ color with the S- form added. The resistance-to-reinforcement test showed that the birds trained with a compound S+ responded more to the S+ form than to a novel form. However, the birds trained with a compound S- did not reliably respond more to a novel form than to the S- form. These findings suggested that the form element of a compound S+ gains some excitatory control, but the form element of a compound S- does not acquire inhibitory control. The possibility existed that low levels of responding to the S+ form on a dark background in the first experiment were due to use of a darkened key to separate S+ and S- periods during discrimination training. However, the essential findings were the same in a second experiment in which darkening of the chamber separated S+ and S- periods.     

Stimulus generalization: the ordering and spacing of test stimuli

Twenty-four pigeons learned a successive discrimination between 500 mmu (S+) and 574 mmu (S-). When tested in extinction, some birds received stimuli around S+, with no S- presentations. These birds showed a positive peak shift, with maximum responding not at 550 mmu, but displaced to 538 mmu and 544 mmu. Other birds were tested with stimuli around S-, with no S+ presentations. These birds showed a negative shift, with least responding not at 574 mmu, but at 586 mmu. Though the first group was tested around S+ and the second around S-, total responding between groups did not differ. When retested on the other half of the continuum, however, birds that had gone from the S+ half to the S- half responded fewer times than those that had gone from the S- half to the S+ half. In a second experiment, reducing stimulus spacing from 6 mmu to 2 mmu produced flatter gradients and decreased the amount of positive shift. In a third experiment, birds were tested across the whole continuum with stimuli presented in serial order. A sequence from 538 mmu to 586 mmu produced no responding after the first part of the session; a sequence from 586 mmu to 538 mmu produced responding throughout the session.     

An investigation of peak shift and behavioral contrast for autoshaped and operant behavior.

Instrumental treadle press and nonreinforced key peck responses were monitored during discrimination training and generalization testing in pigeons on positive and negative reinforcement schedules. In Experiment 1, six pigeons pressed a treadle for food on a multiple variable-interval extinction schedule. In Experiment 2, three pigeons pressed a treadle to avoid shock on a multiple free-operant avoidance extinction schedule. Different color keylights signaled S+ and S- components. Some positive behavioral contrast occurred during discrimination training, but the effect was small. Pecking occurred to the S+ keylight in Experiment 1 but not in Experiment 2. On stimulus generalization tests, all subjects displayed a positive peak shift when pressing the treadle for food or to avoid shock. However, peak shift was not found for nonreinforced "autopecks" on the stimulus key, although an area shift was observed in Experiment 1. This is the first demonstration of peak shift for pigeons pressing treadles and the only reliable demonstration of peak shift when negative reinforcement maintained responding. These results, in combination with previous demonstrations of peak shift for rats pressing levers and pigeons pecking keys, indicate that peak shift is a general by-product of operant discrimination learning, since it occurs across a variety of the organisms, responses, and reinforcers.     

An auto-adjusting shaping procedure

A variable interval (VI) schedule is described that automatically adjusts the programmed rates of reinforcement in accordance with the rates of responding of subjects during the two immediately preceding 30-sec time intervals. The schedule prescribes that as rate of responding decreases, programmed reinforcement rate increases, and that when rate of responding increases, reinforcement rate decreases. Thus, programmed reinforcement rate is adjusted continuously until some target value is reached. Ten rats were exposed to this procedure five times a day at 1-h intervals. The target, set at VI 120 sec, was reached by most subjects within 4 days of training. Subsequently, all subjects responded consistently during five daily 1-h sessions with VI 120 sec. This procedure speeds up the training of subjects on long VI schedules and substantially reduces the time and effort spent observing the subjects and adjusting the schedule parameter value during the early development of responding.     

Simultaneous Discrimination Learning in Pigeons: Value of S - Affects the Relative Value of its Associated S+

In a simple simultaneous discrimination involving a positive stimulus (S+) and a negative stimulus (S-), it has been hypothesized that positive value can transfer from the S+ to the S- (thus increasing the relative value of the S-) and also that negative value can transfer from the S- to the S+ (thus diminishing the relative value of the S+; Fersen, Wynne, Delius, & Staddon, 1991). Evidence for positive value transfer has been reported in pigeons (e.g. Zentall & Sherburne, 1994). The purpose of the present experiments was to determine, in a simultaneous discrimination, whether the S- diminishes the value of the S+ or the Sis contrasted with the S+ (thus enhancing the value of the S+). In twoexperiments, we found evidence for contrast, rather than value transfer, attributable to simultaneous discrimination training. Thus, not only does the S+ appear to enhance the value of the S-, but the S appears to enhance rather than reduce the value of the S+.     

A search for contrast effects in discrete-trial discrimination learning by pigeons

Behavioral contrast has often been observed in free-operant experiments with pigeons, rarely in discrete-trial experiments with rats. Although Jenkins (1961) and Terrace (1963) have reported a discrete-trial contrast effect in pigeons, a series of experiments reported here found no evidence that latency of responding to S+ in a discrete-trial situation was reliably decreased by alternating reinforced trials to S+ with nonreinforced trials to S?. Latency of responding to S+ was affected neither by the length of the preceding intertrial interval (within the range of 10–60 sec), nor by whether the preceding trial had been to S+ or to S?. The results of two of these experiments suggested that the appearance of positive contrast in Jenkins's experiment was a consequence of differences in the variability of the intertrial interval experienced by control and discrimination groups. In two final experiments, employing standard free-operant procedures, contrast was observed as an increase in rate of responding to S+, but not as a decrease in latency of the first response on each S+ trial. The implication is that contrast effects are more readily observed with the rate measures of free-operant experiments than with the latency measures of discrete-trial experiments.     

Stimulus generalization along a light flicker rate continuum after discrimination training with several S−'s

Four pigeons were trained with VI reinforcement to peck a key which was briefly illuminated by a flickering light. Generalization gradients were then obtained with nine different rates of flicker, four faster than S+ and four slower. Two birds were then trained to discriminate between S+ and the fastest stimulus (S−). These birds were then trained to discriminate between S+ and the two fastest stimuli, alternated as S−'s. This procedure was continued, adding one new S− at a time, until all four stimuli faster than S+ were S−'s. The remaining two birds were trained on this latter discrimination without intervening training. In a final stage, using the first two birds, the slowest stimulus was added as a fifth S−. Generalization gradients in extinction were obtained from each bird after each stage of training. As more stimuli from one end of the continuum served as S−'s, responding increased in the presence of stimuli from the other end of the continuum, and the gradient tended to become flattened at this end.     

Stimulus generalization following extra-dimensional training in educationally subnormal (severely) children.

The use of discrimination learning paradigms in the study of attentional transfer is discussed. The technique of go/no-go discrimination learning followed by stimulus generalization testing is contrasted with the more familiar simultaneous learning paradigm followed by a shift in the relevant cues. In the former paradigm the effect of training a discrimination on one dimension on the slope of the stimulus generalization gradient on an independent gradient dimension (extra-dimensional training) is assessed. A steepening of the gradient relative to appropriate control procedures is taken as evidence of positive attentional transfer. The relevance of the technique to the detailed study of attentional transfer in educationally subnormal (severely) (ESN(S)) children is considered. In Expt. I nine ESN(S) children were trained in a go/no-go discrimination involving stimuli differing in orientation, and were generalization tested on a dimension that was orthogonal, namely hue. Of the six subjects who learnt the discrimination five showed clear decremental gradients on the hue dimension. In contrast a Pseudo-Discrimination group (PD) of eight subjects matched to those in the TD group showed no gradients. These subjects were not trained in the orientation discrimination, but were reinforced for responding on 50 per cent of each of the S+ and S- stimulus presentations. They thus received equal exposure to, but no differential training on, the orientation dimension. An S+ only group of four subjects who received no exposure to the orientation stimuli showed no gradients when stimulus generalization testing on the hue continuum was carried out. The result is discussed in terms of transfer deriving from stimulus control by relational aspects of the stimuli; in terms of control by constant irrelevant stimuli; and in terms of the study of stimulus control in ESN(S) children. In Expt. II the influence of the codability of the colours on the location of the peak of the stimulus generalization gradients in the TD group is investigated.     

The generality of selective observing

Four rats obtained food pellets by poking a key and 5-s presentations of the discriminative stimuli by pressing a lever. Every 1 or 2 min, the prevailing schedule of reinforcement for key poking alternated between rich (either variable-interval [VI] 30 s or VI 60 s) and lean (either VI 240 s, VI 480 s, or extinction) components. While the key was dark (mixed-schedule stimulus), no exteroceptive stimulus indicated the prevailing schedule. A lever press (i.e., an observing response), however, illuminated the key for 5 s with either a steady light (S+), signaling the rich reinforcement schedule, or a blinking light (S-), signaling the lean reinforcement schedule. One goal was to determine whether rats would engage in selective observing (i.e., a pattern of responding that maintains contact with S+ and decreases contact with S-). Such a pattern was found, in that a 5-s presentation of S+ was followed relatively quickly by another observing response (which likely produced another 5-s period of S+), whereas exposure to S- resulted in extended breaks from observing. Additional conditions demonstrated that the rate of observing remained high when lever presses were effective only when the rich reinforcement schedule was in effect (S+ only condition), but decreased to a low level when lever presses were effective only during the lean reinforcement component (S- only condition) or when lever presses had no effect (in removing the mixed stimulus or presenting the multiple-schedule stimuli). These findings are consistent with relativistic conceptualizations of conditioned reinforcement and extend the generality of selective observing to procedures in which the experimenter controls the duration of stimulus presentations, the schedule components both offer intermittent food reinforcement, and rats serve as subjects.     

Responding to a positive stimulus by “satiated” pigeons

Skinner (1938) found that rats given discrimination training (Phase I) and then reinforced to “satiation” for responses in the presence of the negative stimulus (S?) (Phase II), began to respond again when the positive stimulus (S+) was reintroduced (Phase III). Experiment I replicated Skinner's finding with pigeons, alternating S+ and S? presentations during Phase III. In Experiment II, Phase II was extended, and Phase III results were similar to those of Experiment I, demonstrating that the recovery of S+ responding could not be attributed to a lax Phase II satiation criterion. In Experiment III, a uniform schedule of reinforcement was maintained throughout the three phases, and results similar to those of Experiment I were found, indicating that renewed S+ responding was not due to the shift in schedule between phases. In Experiment IV, Phase I consisted of discrimination training with two positive stimuli (S+_s and S+_n), and Phase II consisted of reinforcement for responses in the presence of S? and S+_s. During Phase III, significantly more responding was found to S+_s and S+_n than to S?, but no difference in responding was found between the two positive stimuli. In Experiment V, Phase I consisted of simple discrimination training, and during Phase II, responses in the presence of both S? and a novel stimulus (So) were reinforced. During Phase III, significantly more responding was found to S+ than to either S? or So, with no difference found between S? and So responding. Renewed responding to S+ during Phase III in the present experiments is best explained by behavioral contrast developed during Phase I.     

Peak shift revisited: a test of alternative interpretations

In Experiment 1, 2 groups of human subjects were trained to respond to 1 of 2 light intensity stimuli, S2 or S4, and then were tested for generalization with a randomized series of increasing values from S1 to S11. Both groups, including the group trained to respond to dimmer value, showed peak shifts to a brighter more centrally located test stimulus. In Experiment 2, which used line angle stimuli, both the size of the difference between S+ and S- and the range of test stimuli that extended beyond S+ were varied. The larger the S(+)-S- separation and the larger the range, the greater was the peak shift obtained. In Experiment 3, training involved an S- (line angle) surrounded by 2 S+ values with testing symmetrical about the training values and covering either a narrow or a wide range. The wide range produced greater peak shifts in both directions from S-. All 3 experiments support an adaptation-level interpretation of intradimensional discrimination learning and generalization test performance in human subjects. Related work with animals suggests the presence of similar processes.     

The peak shift in stimulus generalization: equivalent effects of errors and noncontingent shock

Terrace suggested that the peak shift in stimulus generalization occurs because the training stimulus not correlated with reinforcement has become aversive. This hypothesis is plausible in the light of instances where the peak shift is obtained compared with those where it fails to appear. The present experiment attempted to test implications of this hypothesis. Two groups of pigeons learned the same two-stimulus discrimination between colors by different training methods in a free-operant situation. When the discrimination was trained with many errors, a large peak shift was obtained in a subsequent generalization test of wavelength; after discrimination training with few errors, a negligible shift was observed. Half of each group then received noncontingent aversive shock during presentations of the stimulus not correlated with reinforcement in continued discrimination training. After this treatment, the errorless-shock subgroup showed a large peak shift and the error-shock subgroup tended to show a larger shift than before. Nonshocked control groups showed little change in the peak shift. It was concluded that pairing aversive shock with a stimulus not correlated with reinforcement is sufficient to produce or enhance a peak shift. In their effect on the peak shift, aversive shock and large amounts of nonreinforced responding appear to be equivalent.     

Latent timing in human conditioned avoidance

Four experiments explored signal timing in human conditioned avoidance. Participants received discrimination training with different duration signals that announced the outcome (S+) or not (S-). Temporal discrimination and superposition of performance to S+ signals of different length (3, 6, or 9 s) was found both in within-subjects (Experiment 1a) and between-subjects (Experiment 1b) designs. S- signals also produced a temporal discrimination and superposition effect during a single test trial conducted after the meaning of the signals was reversed through instructions. Experiments 2a and 2b replicated these results in a situation in which (a) the durations of the S+ and S- signals were different (4.5 or 9 s) to prevent any temporal generalization between them (Experiment 2a), and (b) only S- signals were presented during training, precluding developing of inhibition to S- (Experiment 2b). These results show that participants time both S+ and S- signals in human conditioned avoidance, and they further suggest that the timing of a cue is independent of reinforcement.     

Stimulus function in simultaneous discrimination

In discrimination learning, the negativity of the stimulus correlated with nonreinforcement (S-) declines after 100 training trials while the stimulus correlated with reinforcement (S+) is paradoxically more positive with lesser amounts of discrimination training. Training subjects on two simultaneous discrimination tasks revealed a within-subjects overlearning reversal effect, where a more-frequently presented discrimination problem was better learned in reversal than was a discrimination problem presented less frequently during training.     

Effective use of the blank comparison procedure in simple discrimination by infant capuchin monkeys: A methodological note

In studies of simple and conditional discrimination, procedures are needed to measure those aspects of stimuli that control behavior. The blank comparison procedure is one such procedure. It was designed explicitly for assessing S+ and S- functions when discriminative stimuli are presented simultaneously. In this procedure, a neutral stimulus serves sometimes as S+ and sometimes as S-. Its discriminative function is defined in relation to other stimuli in the display. The present study aimed to prepare 2 infant female capuchin monkeys for the effective use of the blank comparison procedure in a simple discrimination task. First, simple discrimination training was applied up to a stable accuracy criterion of ≥90%. This training was followed by the replacement of S+ and then of S- stimuli with new stimuli. Ultimately, trials with the blank comparison were introduced. Following this sequence, both monkeys immediately displayed highly accurate blank-comparison performances without the need for stimulus control shaping or other preparatory discrimination training. Thus, this procedure sequence may be an efficient, effective method for establishing blank-comparison baselines for experimental analyses of S+/S- discriminative functions and perhaps for other applications in teaching simple and conditional discrimination performances to this species and others.     

Emergent simple discrimination in children: Preference for non-preferred stimuli

Previous research (Smeets, 1991) suggested that when given a new discrimination, children respond on the basis of physical similarity with previously discriminated stimuli. They respond to a stimulus similar to another preferred stimulus (S+ transfer) and respond away from a stimulus similar to another nonpreferred stimulus (S- transfer). When both types of transfer apply to the same stimulus, S+ transfer prevails, S+ Priority Transfer (S+PT). The present study demonstrated that S+PT also occurs when the criterion task consists of two nonpreferred stimuli. When given a choice between two previously nonpreferred stimuli, one similar and one dissimilar to other preferred stimuli, children select the first one. They do not so, however, when a nonpreferred stimulus resembling another preferred stimulus is presented with a new nonpreferred stimulus. These findings suggest that the children's preferences were not based on the physical resemblance with other (non)preferred stimuli but on the functions (S+, S-, S0) of individual stimulus components. A theoretical model is presented that accounts for all experimental data reported in the previous and present study. The model implies that discriminative responding not only results from but also determines the functional properties of individual stimulus elements.     

Conditional Discriminations by Preverbal Children in an Identity Matching-to-Sample Task

This study sought to develop methodology for assessing whether children aged 16-21 months could learn to match stimuli on the basis of physical identity in conditional discrimination procedures of the type routinely used in stimulus equivalence research with older participants. The study was conducted in a private room at a daycare center for children and toddlers. The child and the research sat together on the floor facing an apparatus with two windows. Stimuli to be discriminated were toys especially designed to attract the child's attention and maintain continued interest. On simple discrimination and discrimination reversal trials that were programmed in initial training, S+ and S- toys were displayed within the two windows. When the child touched the window containing the toy defined as S+ on a given trial, s/he was allowed to manipulate/play with that toy. Selections of the S- toy ended the trial without a play opportunity. On subsequent identity matching-to-sample trials, the child was first allowed to manipulate a sample toy. Then, S+ (matching) and S- (nonmatching) comparison toys were displayed within the windows, and the selection consequences were the same as on simple discrimination trials. The study provides evidence that preverbal children can master simple and conditional discrimination performances via such procedures, perhaps setting the stage for subsequent studies aimed at establishing procedural control of the discrimination baselines needed to assess the stimulus equivalence potential of children in this age range.