Inhibitory stimulus control following errorless discrimination learning

Three generalization procedures were used to investigate inhibitory stimulus control following discrimination learning with few errors. Three groups of pigeons acquired a discrimination between a green stimulus (the positive stimulus) and a vertical or horizontal line (the negative stimulus) through differential autoshaping followed by multiple schedule presentation of the two stimuli with gradually increasing stimulus durations. Genereralization testing was along a line-tilt continuum. For one group, the test involved a resistance-to-reinforcement procedure in which responses to all line tilts were reinforced on a variable-interval schedule. For a second group, also tested with the resistance-to-reinforcement procedure, the lines were superimposed on the green field that formerly served as the positive stimulus. A third group was tested in extinction with the combined stimuli. Control groups had no discrimination training but responding to green was nondifferentially reinforced. The control subjects responded more to all line tilts during testing than did the comparable experimental subjects, indicating that the negative stimulus had become an inhibitory stimulus. Both resistance-to-reinforcement groups revealed inhibitory gradients around the negative stimulus, but the gradient for the extinction group was relatively flat. These data are consistent with others that modify Terrace's early conclusion concerning the failure of inhibition to develop during errorless training.     

Contrast and stimulus generalization following prolonged discrimination training

Different groups of pigeons received discrimination training in which the reinforcement-associated and extinction-associated stimuli were respectively either (a) a line tilt vs a blank key, (b) a blank key vs a line tilt, or (c) two different line tilts. The high response rates that developed to the positive stimulus in all groups during discrimination learning were maintained over 64 sessions of training. After these sessions, all subjects were tested for stimulus generalization along the line-tilt dimension. Gradients of relative (per cent) generalization around the stimulus associated with reinforcement (so-called excitatory gradients) and around the stimulus associated with extinction (so-called inhibitory gradients) were as steep as they typically are after much briefer training periods. These results do not support several of Terrace's predictions on the basis of the hypothesis that emotional responses develop to the stimulus associated with extinction during discrimination learning with errors, but eventually dissipate after extended training.     

The acquisition of observing.

Pigeons were exposed to stimuli correlated with the presence or absence of a variable-interval 60-second schedule of reinforcement only while they depressed a crossbar or "perch." In the first experiment, the stimuli were different tilts of a line displayed on the key. When the difference in brightness between the line and the background (salience) was maximal, seven of eight birds acquired the discrimination, but when the difference was reduced by 50%, only one succeeded. In the second experiment, wavelength of chamber illumination served as the relevant dimension. Neither experiment showed a large effect attributable to the magnitude of the difference (disparity) between the positive and the negative stimulus. Individual differences in time spent observing were positively correlated with level of discrimination in the presence of the stimuli. All birds produced the positive stimulus for a greater proportion of the available time than they did the negative stimulus. This may be the mechanism that provides selective reinforcement of observing. Finally, the formation of a discrimination was analyzed in terms of changes in the proportion of time spent in contact with the discriminative stimuli.     

Behavioral contrast and inhibitory stimulus control as related to extended training

Pigeons received discrimination training in which the presence of a white field was correlated with variable-interval reinforcement and the presence of a monochromatic field was correlated with extinction. Responses during the negative stimulus prolonged its duration. Five experimental groups each received a different number of discrimination sessions up to 70 sessions. The last session was followed by a wavelength generalization test. The control group was tested both before and after four discrimination sessions. Responding to the positive training stimulus was enhanced in all groups by the discrimination procedures. This enhancement tended to decrease over sessions in some animals but it never disappeared in others. Responding to the test stimuli preceding discrimination training was minimal around the negative stimulus and increased in either direction away from that wavelength. Responding to the test stimuli was not systematically related to the amount of discrimination training.     

Stimulus generalization, discrimination learning, and peak shift in horses.

Using horses, we investigated three aspects of the stimulus control of lever-pressing behavior: stimulus generalization, discrimination learning, and peak shift. Nine solid black circles, ranging in size from 0.5 in. to 4.5 in. (1.3 cm to 11.4 cm) served as stimuli. Each horse was shaped, using successive approximations, to press a rat lever with its lip in the presence of a positive stimulus, the 2.5-in. (6.4-cm) circle. Shaping proceeded quickly and was comparable to that of other laboratory organisms. After responding was maintained on a variable-interval 30-s schedule, stimulus generalization gradients were collected from 2 horses prior to discrimination training. During discrimination training, grain followed lever presses in the presence of a positive stimulus (a 2.5-in circle) and never followed lever presses in the presence of a negative stimulus (a 1.5-in. [3.8-cm] circle). Three horses met a criterion of zero responses to the negative stimulus in fewer than 15 sessions. Horses given stimulus generalization testing prior to discrimination training produced symmetrical gradients; horses given discrimination training prior to generalization testing produced asymmetrical gradients. The peak of these gradients shifted away from the negative stimulus. These results are consistent with discrimination, stimulus generalization, and peak-shift phenomena observed in other organisms.     

Effects of criterion-level probing on demonstrating newly acquired discriminative behavior.

The purpose of the study was to determine whether probes of the final criterion-level discrimination administered during and after training provided an accurate measure of acquisition. Training and probe stimuli were designed to make training and probe trials initially very discriminable and then progressively less discriminable as training progressed. Initially, the discrimination required on probe trials was more difficult than the discrimination required on training trials. However, this difference in difficulty was gradually eliminated as training stimuli were topographically altered and made identical to probe stimuli by the end of training. Results showed that while correct responding was maintained throughout training, error patterns occurred on all probe trials administered during training. Error patterns developed regardless of whether probe trials occurred only at the beginning of training sessions (temporally discriminable probes) or were randomly interspersed in the training sessions (temporally indiscriminable probes). Probe error patterns seemed to be controlled by the stimulus properties of training and probe trials. Thus, probes did not measure acquisition as it occurred during training. Probe error patterns were maintained when probes were administered after completion of training. This final measure of acquisition did not agree with the demonstration of acquisition provided by the final training trial. The results suggest that probe trials can measure a different stimulus-response relationship from that trained when training starts with an easier or known discrimination and probes involve a final or criterion test of a more difficult or unknown discrimination. Stimulus control of correct responses versus error patterns is discussed.     

Discrimination of word-like compound visual stimuli by monkeys

In Experiment I, two monkeys solved a successive visual discrimination in which the four positive stimuli were the visual arrays RIM, LID, RAD and LAM while the four negative stimuli were RID, LIM, RAM and LAD. In Experiment II the same monkeys first learned a discrimination where the positive stimuli were pairs of letters (e.g. OB and AK) while the negative stimulus was the letter I; in a subsequent generalization test with all four possible pairings of the stimulus elements that had been positive during training (i.e. with OB, AK, OK and AB) the monkeys responded more strongly to the pairs that had been present in initial training. These results were discussed in relation to the theoretical analysis of configurational cues in animal discrimination learning and to the mechanism underlying visual discrimination of words by people.     

A new approach to the formation of equivalence classes in pigeons

Four pigeons were given simultaneous discrimination training with visual patterns arbitrarily divided into two sets, with the stimuli in one set designated A1, B1, C1, and D1 and those in the other set designated A2, B2, C2, and D2. In sequentially introduced training phases, the pigeons were exposed to a series of reversals to establish AB and then CD equivalences. In subsequent testing sessions, a subset of stimuli from one set served as positive stimuli and those from the other set as negative stimuli on training trials, and transfer of the reinforced relation to other members of the sets was tested with nonreinforced probe trials. The pigeons were trained further on AC and BD equivalences and then were tested for the emergence of untrained AD and BC equivalences. Two of the 4 pigeons exhibited the emergence of one of these untrained equivalences, evidence for the emergence of transitive relations. This finding suggests that the pigeons established three-member functional equivalence classes by incorporating separately trained multiple equivalence relations. Repeated reversal training and probe testing enabled us to explore the formation and expansion of functional equivalence classes in pigeons.     

Optional-shift behaviour in rats: a novel procedure for assessing attentional processes in discrimination learning

A novel, optional-shift procedure was used to assess changes in the attention paid to stimuli that occur over the course of discrimination learning. In Phase 1, rats were trained on a conditional instrumental discrimination using audiovisual stimulus compounds; one stimulus dimension (auditory or visual) was relevant to the solution of the discrimination, the other was irrelevant. In Phase 2, all animals received three sessions of training with novel audiovisual compounds where both dimensions were equally diagnostic. Results from probe test trials indicated that, during Phase 2, animals learnt most about cues belonging to the stimulus dimension that was relevant during Phase 1. These results suggest that over the course of discrimination learning there was an increase in the amount of attention paid to relevant stimuli and/or a decrease in the attention paid to irrelevant stimuli.     

Optional-shift behaviour in rats: A novel procedure for assessing attentional processes in discrimination learning

A novel, optional-shift procedure was used to assess changes in the attention paid to stimuli that occur over the course of discrimination learning. In Phase 1, rats were trained on a conditional instrumental discrimination using audiovisual stimulus compounds; one stimulus dimension (auditory or visual) was relevant to the solution of the discrimination, the other was irrelevant. In Phase 2, all animals received three sessions of training with novel audiovisual compounds where both dimensions were equally diagnostic. Results from probe test trials indicated that, during Phase 2, animals learnt most about cues belonging to the stimulus dimension that was relevant during Phase 1. These results suggest that over the course of discrimination learning there was an increase in the amount of attention paid to relevant stimuli and/or a decrease in the attention paid to irrelevant stimuli.     

Human peak shift: Analysis of the effects of three-stimulus discrimination training

In a series of studies, the effects of different types of intradimensional discrimination training on human auditory frequency generalization were examined. When subjects were trained with a single S− located on one or the other side of S+, postdiscrimination gradients were displaced away from S−. Subjects trained with two negative stimuli both on one side of S+ showed a greater extent of displacement with true peak shift. In a second experiment the procedures were repeated with two fixed amounts of training: either 12 or 42 training trials. Again the subjects trained with two negative stimuli showed more shift than those trained with one S−, and this effect was independent of amount of training. Experiment 3 showed increased peak shift when two positive stimuli surrounded a central S− as compared to groups with a single S+ and S−. The general conclusion is that training with more difficult, three-stimulus discrimination problems results in enhanced peak shift.     

Generalization during acquisition, extinction, and transfer of matching with an adjustable comparison

Three groups of pigeons were given conditional discrimination training in which the number of standard stimuli was varied across groups. In the presence of each standard, a pigeon adjusted the comparison stimulus on a second key until the two keys matched. A report of this match (response on the first key) was reinforced. Transfer of the matching performance was investigated by adding new standards to the ones already available. All pigeons were exposed to two extinction sessions after 155 sessions of training. Rapidity of acquisition was inversely related to the number of standards presented. Generalization gradients derived from the several comparison stimuli showed that all pigeons reached a high level of accuracy in the presence of at least one standard, and some pigeons did so in the presence of as many as four of the six standards. There was no evidence of a systematic effect of extinction upon overall accuracy, or the individual generalization gradients. When a new standard was added, a given pigeon's performance (in terms of responding to the comparisons) was similar to performance in the presence of one of the old standards. However, the pigeons did not show evidence of confusion among the comparisons.     

The effects of amount of training per reversal on successive reversals of a color discrimination

Three groups of pigeons were trained on a red-green discrimination in which the stimuli were alternately presented in a multiple schedule of reinforcement. The discrimination was reversed 24 times. Groups were given 1, 2, or 4 hr of training on each discrimination. Increasing the length of training had two principal effects on reversal performance: it increased the rate of extinction of responding to one of the stimuli and increased the rate of reacquisition of responding to the other. The latter effect involved both an increase in reacquisition of responding to a positive stimulus within reversals and an increase in recovery of responding to the previous negative stimulus between reversals. Improvements in performance of each group over the series of reversals were qualitatively similar to the two effects of length of training on each discrimination, and were analogous to effects obtained in other studies involving overtraining and successive reversals of simultaneous discriminations.     

Local contrast and Pavlovian induction

Two experiments examined the effects of number and similarity of stimuli on local contrast. In the first experiment, local contrast effects differed in magnitude as a function of the similarity among stimuli; greater positive local contrast appeared when stimuli were less similar, though this effect sometimes reversed for very dissimilar stimuli. In the second experiment, both positive and negative local contrast appeared transiently during the course of training a discrimination including two quite dissimilar stimuli. When two new stimuli were added, both effects reappeared in several cases. The effects remained when the discrimination was rendered more difficult by substituting a new stimulus very similar to one of the original pair. These and other data suggest that local contrast depends on the same factors that produce Pavlovian induction; in the absence of an alternative account, Pavlov's interpretation may be useful in suggesting further research that will help identify the mechanisms involved in both classical and operant discrimination learning.     

Absolute and relative measures of dimensional contrast.

Two sets of experiments examined the discriminative performance of pigeons on a visual flicker-rate continuum using a maintained generalization procedure. In the first experiment, responses during the intermediate stimulus value were not reinforced, whereas responses during all other stimuli were reinforced periodically. In the second experiment, training was similar to the first, with the exception of one condition in which stimuli adjacent to the negative stimulus border were eliminated from the discrimination set. Results from both experiments show that positive dimensional contrast seems to represent a relative enhancement of discrimination gradient form, rather than an absolute increase in responding with respect to prior baseline. Further, the form and magnitude of positive dimensional contrast are not predictable from the form and magnitude of baseline response rates. Results from the second experiment indicate that eliminating border stimuli increased response-rate differences between positive and negative stimuli, but did not necessarily diminish the magnitude of positive dimensional contrast.     

Postdiscrimination Gradients With Familiar and Unfamiliar Faces

Discrimination training is often used to improve accuracy in stimulus recognition. However, intradimensional discrimination training with a single positive exemplar and a single negative exemplar may reduce accuracy during a generalization test. More specifically, discrimination training can cause a shift in responding from the positive exemplar to stimuli unlike the negative exemplar. Previous studies have shown that the length of discrimination training affects the degree to which a shift occurs. We sought to determine whether pre-experimental exposure (“familiarity”) to the stimuli would have a similar effect. Twenty participants were trained and tested with two different sets of facial images. Each set of images consisted of stimuli that ranged from the naturally asymmetrical version of the face to a symmetrical version of the face. In one condition, the images were based on the participant’s own face; in the second condition, the images were based on the face of a stranger. In both cases, the median face served as the positive exemplar. In half of the conditions the naturally asymmetrical face served as the negative exemplar, and in the other half, the symmetrical face served as the negative exemplar. Images based on a stranger’s face produced greater generalization than images based on the participant’s own face. A significant gradient shift occurred only with familiar faces.

     

Discrimination of polymorphous stimulus sets by pigeons

Pigeons were trained to perform a visual discrimination between stimulus sets in which the presence of any two of three positive features made a stimulus positive, while any two of three negative features made it negative (there were thus three different positive and three different negative stimuli). After training, the birds were exposed to test stimuli that contained either all three positive or all three negative features. In Experiment I three pigeons were successfully trained by a successive method, and subsequently responded to the test stimuli as though they were positive or negative respectively. In Experiment II four pigeons were trained by a simultaneous method. Three learned the discrimination and generalized appropriately to the test stimuli, but they showed no preference between positive test and positive training stimuli, nor any consistent difference in speed of response to them; and similar results were found for negative stimuli. It is argued from this that the pigeons learned to respond to the stimuli as patterns (configurations of features) rather than to the constituent features, but that they generalized to the test stimuli by using the common features. The experiments show that pigeons could in principle learn to discriminate natural polymorphous classes (such as “pigeon” or “person”) without using any single feature, but neither the present experiments nor earlier ones demonstrating discriminations of such natural classes establish that pigeons make use of polymorphous concepts in the same way as people.     

Development of neuronal responsiveness in the mediorostral neostriatum/hyperstriatum ventrale during auditory filial imprinting in domestic chicks

Chronic electrophysiological recordings of slow field potentials from tone-imprinted chicks show significantly enhanced fast Fourier transform (FFT) power during playback of rhythmic 400-Hz imprinting tone stimuli in the presence of a surrogate mother. The FFT power was already significantly higher during the very first imprinting session, when the chick was exposed to the imprinting tone stimuli in the presence of the surrogate mother compared to spontaneous activity (EEG recordings). During discrimination tests, where individual chicks were exposed to the imprinting tone stimuli in alternation to rhythmic 700-Hz tone stimuli (discrimination tone stimuli), the FFT power was significantly higher during playback of the imprinting tone stimuli than the FFT power during playback of the discrimination tone stimuli. Chicks which were imprinted in the absence of the surrogate mother also show enhanced FFT power in the course of the imprinting sessions; however, in contrast to the first group, they did not show significant differences in the FFT power during playback of either the imprinting or discrimination tone stimuli in the discrimination tests. Our results suggest that the high FFT power of a potential imprinting stimulus or situation, which is expressed in newborn (still naive) chicks, is maintained only when the chicks form an association between the tone stimuli and a positive emotional situation (represented by the surrogate mother).     

Generalization of auditory intensity as a function of small amounts of discrimination training

Rats were trained on a two-valued auditory-intensity discrimination for 5, 2, 1 or 0 days. Then, the bar-press response rate was sampled in the presence of seven added discrimination stimuli (generalization) for seven days. Daily training sessions were 4 hr in length throughout the experiment. Only the five-day group exhibited a marked superiority on the multi-valued discrimination when contrasted with the group having had no prior simple discrimination training.     

Transfer of matching-to-sample in pigeons

In Experiment I, pigeons were first trained on simultaneous matching-to-sample with either color stimuli or form stimuli, and then shifted to stimuli on the other dimension. Matching performance in the first session with stimuli on a given dimension was not affected by prior matching training with stimuli on the other dimension. However, in the first six color-matching sessions pooled, birds with prior form-matching training performed significantly better than birds without any prior matching training. In Experiment II, birds with experience matching both colors and forms in separate sessions were tested with novel stimulus configurations involving either novel stimuli or novel combinations of familiar colors and forms. Matching performance was not affected by novel stimulus configurations, except that performance dropped to a chance level or below when the standard stimulus was novel. In Experiments II, III, and IV, three of four tests did not show any effect of prior reinforcement of pecks at a novel stimulus, presented alone, on subsequent matching of that stimulus. The results were interpreted as indicating that matching performance in pigeons depends on the learning of stimulus-response chains involving the specific stimuli employed during training. An incidental observation in Experiments I and II was that there were typically more excess pecks at the standard stimulus during form-matching sessions than during color-matching sessions, which may be related to the fact that form matching is more difficult than color matching.