Discrimination of relative numerosity and stimulus mixture by pigeons with comparable tasks.

Arrays of small squares of 2 colors were presented in various proportions to pigeons on a video screen. Birds pecked differentially at the left or right side of the screen to obtain grain. In Experiment 1, pecking at 1 side was correct when more blue than red elements were Proportions of responses to the 2 locations reflected the proportions of elements in an orderly manner and were little affected by alterations in spacing or size of elements. When red Experiment 2, 1 side of the screen was correct when uniform red or blue arrays were presented; the other was correct for mixed arrays. Orderly gradients of response location reflected degree of stimulus mixture. Good transfer was obtained with green and blue elements. These results support the robust nature of discriminations of emergent properties of complex arrays when stimuli are equally associated with reinforcement and when response location, and not response rate, indicates stimulus control.

Cued and uncued terminal links in concurrent-chains schedules.

Pigeons were trained on a concurrent-chains schedule. The initial links were concurrent variable-interval schedules arranged on two side keys. Each terminal link was a fixed-interval schedule arranged on the center key. In cued conditions, different center-key colors signaled the two terminal-link schedules. In uncued conditions, the same center-key color appeared for both terminal links. Experiment 1 arranged unequal initial links and equal terminal links. Preference for the shorter initial-link schedule was greater when the terminal links were uncued. Experiment 2 arranged equal initial links and unequal terminal links. Preference for the shorter terminal-link schedule was greater when the terminal links were cued. Although the results of Experiment 2 successfully replicated previous research, the results of Experiment 1 are not easily reconciled with conditioned-reinforcement or discriminative-stimulus accounts of the role of terminal-link cues. Rather, terminal-link cues appear to decrease sensitivity to initial-link contingencies.

Transfer of a discrimination by pigeons (Columba livia) between pictured locations and the represented environments.

In Experiment 1, pigeons (Columba livia) were trained in a successive slide presentation procedure to discriminate between pictures of 2 ends of a room and then trained to find food in the actual room. A congruent-transfer (CT) group learned the spatial discrimination more quickly than an incongruent-transfer (IT) group. In Experiment 2's replication we used a simultaneous slide presentation procedure and added a control group. The IT group required significantly more trials than the CT or control groups. In Experiment 3, order of the training conditions was reversed. CT and IT conditions had no effect on the speed of acquisition of the discrimination. This indicates that pigeons acquire a representation of spatial locations from pictures, which can then direct behavior, but the direction of transfer observed was unidirectional. This suggests that a discrimination between spatial locations may not be accurately represented in pictorial form.

Discrimination by pigeons of mixture and uniformity in arrays of stimulus elements.

In 5 experiments, pigeons learned to discriminate uniform arrays of colored elements from mixed arrays containing 2 colors in various proportions. When only the uniform arrays were positive, mixture-negative gradients were obtained. When only the arrays containing equal numbers of red and blue elements were positive, mixture-positive gradients were obtained. When elements of different colors from 2 independent mixture-negative discriminations were recombined, mixture-negative gradients were obtained with the transfer patterns. When elements from a mixture-negative discrimination were combined with a new set of positive elements, responding was not suppressed. Appropriate combinations of the elements in mixture discriminations reduce responding, but the elements themselves do not acquire inhibitory properties.

Pigeons can discriminate locations presented in pictures.

The present experiments were designed to teach pigeons to discriminate two locations represented by color photographs. Two sets of photographs were taken at two distinctive locations on a university campus. These sets represented several standpoints at each location. For the true-discrimination group, pictures from the two locations were differentially associated with reward; for the pseudodiscrimination group, half of the views from each location were arbitrarily but consistently associated with reward. The former group acquired the discrimination much more rapidly. These birds also showed good transfer to new views from the standpoints used in training and to a new standpoint at each location not used in training. In a second experiment, another group of pigeons could terminate any training trial by pecking an "advance" key. Three of 4 subjects used this option to reduce the duration of trials in which pictures from the negative location were presented. These data suggest that pigeons can integrate views shown in pictures into a "concept" of a location. The method used here may be the experimental analogue of a common, natural process by which animals learn to identify locations.

The legacy of Guttman and Kalish (1956): Twenty-five years of research on stimulus generalization.

This paper is a selective review of the methods, problems, and findings in the area of operant stimulus generalization over the 25 years since the publication of the original paper by Guttman and Kalish (1956) on discriminability and spectral generalization in the pigeon. The paper falls into five main sections, which encompass the main themes and problems stemming from the Guttman and Kalish work and its immediate successors. The first section addresses the relationship between stimulus generalization and stimulus control, as well as the variety of testing procedures and dependent variables used to measure generalization. The next section reviews the limited literature on the effects of early rearing on the generalization gradient. The relationship between discriminability among test stimuli and the slope of the spectral gradient is discussed in the third section, with emphasis upon recent reassessments of the pigeon's hue discriminability function. The fourth section reviews the topic of inhibitory stimulus control, one which developed with the discovery of the peak shift following intradimensional discrimination training. Problems of definition and measurement are discussed in conjunction with the gradient forms used to index inhibitory control. The last section is devoted to attentional effects and the two principal theories postulated to account for them. A survey of different attentional paradigms is provided and the possible role of constant irrelevant stimuli as a source of control is examined. A brief conclusion summarizes the contribution of the generalization technique toward an understanding of the nature and acquisition of stimulus control.

Control of choice in conditional discriminations by sample-specific behaviors.

When sample-specific responding is occasioned by sample stimuli in matching and oddity tasks with pigeons, such responding controls the choices between comparison stimuli. This control was investigated in four experiments. In Experiments 1 and 2, differential responding to the samples in line-line matching and line-line oddity was reversed following acquisition of these problems. The reversal interfered with reacquisition of the same conditional discrimination but facilitated acquisition of the opposite discrimination. In Experiment 3, pigeons initially trained on line-line matching were shifted to hue-line matching. Positive transfer occurred when correct choices in both tasks were paired with the same sample-response patterns. Conversely, negative transfer occurred when correct choices were paired with opposite patterns. In Experiment 4, two concurrent conditional discriminations were designed so that sample-response patterns were paired with specific sample stimuli, but not with correct choices. Faster acquisition occurred when response patterns differed within rather than between sample dimensions. Furthermore, sample-specific responding controlled choices when the dimensional stimuli were difficult to discriminate but not when they were easy to discriminate. The combined results are interpreted in terms of overshadowing. Visual stimuli that control choice can be overshadowed by sample-specific responding if the latter facilitates conditional discrimination acquisition.

Stimulus duration as a measure of stimulus generalization.

Four pigeons in the line-positive group were trained with a vertical line on a green background that signalled intermittent reinforcement while a plain green field signalled extinction. Four pigeons in the line-negative group were trained with the opposite discrimination. Response to a control key terminated any trial and initiated the next trial. The birds also used the control key during generalization tests to control the durations of trials in which various line orientations were presented. These durations were summed to provide generalization gradients of stimulus duration that were positive or negative in accordance with the trained discriminations. In Experiment 2, birds from the line-positive group were tested with a procedure in which the control key was not available on some trials. This provided an independent assessment of response rates to the test stimuli. These rates were used to predict the stimulus durations obtained when the control key was available. The findings supported a general model for the prediction of response distributions among concurrent stimuli from rates observed with single stimuli.

Maintenance of behavior controlling the duration of discriminative stimuli.

THE MAINTENANCE OF A RESPONSE CONTROLLING THE DURATION OF POSITIVE AND NEGATIVE DISCRIMINATIVE STIMULI IN A MULTIPLE SCHEDULE WAS EXAMINED WITH RESPECT TO THE POSSIBLE CONSEQUENCES OF THE RESPONSE: none, escape from the negative stimulus, production of the positive stimulus, and initiation of the reinforcement schedule associated with the latter. The last two seemed to be the major factors in producing and maintaining the response. Escape from the negative stimulus maintained it in most subjects, but only at a much lower level.

Effects of monochromatic rearing on spectral discrimination learning and the peak shift in chicks.

Chicks were hatched and raised in white or monochromatic sodium (589 nm) light. They were trained on a 590 (+) vs. 580 (-) nm successive discrimination. The combined results of two experiments indicated that rearing illumination did not affect discrimination acquisition. All subjects given generalization tests after discrimination training exhibited peak shifts that were equivalent for the two rearing conditions. The peak shifts exhibited by the monochromatically reared subjects represent maximum responding to stimuli they had not previously seen. This result further confirms the notion that behavioral control by the spectral dimension in birds is organized independently of differential early experience on that dimension.

Pigeon concept formation: successive and simultaneous acquisition.

Pigeons were trained to discriminate the presence of one or more human forms in displays projected on a panel above the response key. This task was mastered, although imperfectly, with successive and with simultaneous presentations of positive and negative instances. The course of acquisition of the discrimination was similar for the two training procedures. Animals were able to transfer the discrimination from the successive to the simultaneous situation. Various tests were carried out to control for artifactual cues on which the discrimination might have been based. The discrimination was maintained when new displays were presented, when reinforcement was omitted, and when displays were inverted 180 degrees . Animals were also able to discriminate between pairs of displays that were identical, except that one member of the pair contained a human form. The research extends the techniques used by Herrnstein and Loveland (1964), and confirms their finding that pigeons can master the concept of "person-present" in a visual display.

The role of discrimination training in the generalization of punishment.

Pigeons were trained to respond equally to various orientations of three parallel lines projected on a response key. One group was then punished for responding to the vertical lines, but not punished in a line-absent condition. Two other groups were also punished but had no opportunity to make such a discrimination. Orderly generalization gradients were obtained from the discrimination group during recovery from punishment, with least responding to the vertical lines and higher rates to other orientations. Gradients obtained from the non-discrimination groups were flat. A discrimination of punishment contingencies appears to be necessary for a stimulus correlated with punishment to acquire control over its reductive effects.