Domestic pigeons (Columba livia) discriminate between photographs of male and female pigeons

We investigated whether pigeons are able to discriminate color photographs of male and female pigeons, using a categorical discrimination procedure. In Experiments 1 and 2B, 10 out of 14 pigeons learned the discrimination. Of these, 5 pigeons showed transfer to novel stimuli, demonstrating the categorical nature of the trained discrimination. Experiment 3 showed that the discriminative behavior was based primarily on the body, as opposed to the head and the neck region. In 1 out of 3 pigeons, the discriminative behavior was maintained by the black-and-white photographs. The results suggest that some pigeons have the ability to discriminate the sex of conspecifics without behavioral cues.     

Visual discrimination of real objects and pictures in pigeons

Pigeons were trained on two visual discrimination tasks. One task was discrimination between real objects and their photographs. Training stimuli consisted of real food (grains), photographs of food, and nonedible junk objects and their photographs. The other task was discrimination between food and nonfood using the same stimuli as those in the first task. The pigeons learned either task and showed generalization of these classifications to novel stimuli. These results suggest that pigeons can either discriminate or integrate real stimuli and their photographs. Bilateral ectostriatal lesions caused deficits in food versus nonfood discrimination, but not in object versus picture discrimination. This dissociation suggests different brain mechanisms between the two visual discriminations using the same stimuli.     

Elemental versus configural perception in a people-present/people-absent discrimination task by pigeons

In the present experiment, we investigated whether pigeons rely exclusively on elemental information or whether they are also able to exploit configural information in apeople-present/people-absent discrimination task. Six pigeons were trained in a go/no-go procedure to discriminate between 800 color photographs characterized by the presence or absence of people. Thepeople-present stimuli were designated as positive, and thepeople-absent stimuli were designated as negative. After training and a subsequent generalization test, the pigeons were presented with both familiar and novel people-present stimuli containing human figures that were distorted in one of seven different ways. All the pigeons learned the initial discrimination and also showed generalization to novel stimuli. In the subsequent test, performance on all types of distorted stimuli was diminished in comparison with that on the intact original pictures from which they had been derived. At the same time, however, peck rates clearly exceeded the level of responding found for regular people-absent stimuli. This result strongly suggests that responding was controlled by both the constituting target components and their spatial relations and, therefore, points to the dual importance of elemental and configural information.     

Recognition of moving video images of conspecifics by pigeons: Effects of individuals, static and dynamic motion cues, and movement

Two groups of pigeons were trained with a go/no-go procedure to discriminate video images of conspecifics based on the individuals or else on their actions. Both groups showed rapid acquisition, and the discrimination transferred to new scenes in Experiment 1 and to static scenes in Experiment 2. In Experiment 3, experimentally naive pigeons were trained to discriminate video images of particular birds showing different actions. Transfer to novel scenes, including a new bird and a new motion, revealed the dominance of motion as a cue to discriminate video images. In Experiment 4, the pigeons trained to discriminate video scenes of 2 pigeons showing a variety of activities successfully recognized these stimuli regardless of whether the video was played forward or backward, and transferred the discrimination to still scenes. The findings suggest that pigeons’ discrimination of video images is primarily based on information that is invariant across static and dynamic conditions.     

Factors affecting conditional discrimination learning by pigeons: II. Physical and temporal characteristics of stimuli

Four experiments were performed to determine the stimulus characteristics that favor the development of conditional stimulus control in the single reversal paradigm with pigeon subjects. In Experiment 1, pigeons were trained on a successive discrimination between tone frequencies ranging from 350 to 3500 Hz in a particular houselight context condition (houselight-on or -off). The subjects then were trained on the reversal of the tone discrimination in the alternative context. Subsequent tone-frequency generalization testing in the two contexts indicated that they had failed to gain conditional control over the pigeons’ discriminative performance. Such control was obtained in Experiment 2, in which the two problems were alternated daily for 32 sessions of training. The gradients then peaked at the appropriate S+ value in each context. In Experiment 3, the key colors (blue vs. red) served as contexts while pigeons learned a successive discrimination in which the discriminative cues were houselight-on versus houselight-off conditions. This was followed by a reversal of the discrimination in the alternative key-color context condition. The key colors were effective conditional cues in this situation. In a previous experiment (Thomas, McKelvie, & Mah, 1985), key color had been ineffective as a conditional cue when the discriminative cues were lines superimposed on the colored background. In Experiment 4, key color was effective when the color and lines were presented on a single key as in the earlier experiment, but were sequenced such that the onset of the key color preceded and then overlapped the presentation of the lines. We concluded that conditional discriminations are easiest for pigeons when visual cues are used, but the conditional and discriminative cues must be presented in such a way that they do not combine to form a psychological compound.     

Attempts to demonstrate magnetic discrimination by homing pigeons in flight

Eight homing pigeons, trained to fly between two elevated feeders within a flight tunnel, were tested for their ability to discriminate between two magnetic field stimuli and two acoustic stimuli, using a unitary discrete-trials procedure with successive presentation of stimuli. Magnetic stimuli consisted of the ambient magnetic field and a reduced magnetic field in which the vertical component of the field was reduced to 50% of its ambient value. Acoustic stimuli consisted of an ambient white noise and the white noise plus a tone. Stimuli were paired with food reward and either a time penalty (Experiment 1) or electric shock (Experiment 2). Although subjects could discriminate sounds with our procedures, none of the subjects demonstrated discrimination of magnetic fields. The failure of pigeons to discriminate magnetic stimuli is discussed as a consequence of either the failure to provide conditions sufficient for such discrimination or the absence of a magnetic sense in these animals.     

Response rate is not an effective mediator of learned stimulus equivalence in pigeons

We explored response rate as a possible mediator of learned stimulus equivalence. Five pigeons were trained to discriminate four clip art pictures presented during a 10-sec discrete-trial fixed interval (FI) schedule: two paired with a one-pellet reinforcer, which supported a low rate of responding, and two paired with a nine-pellet reinforcer, which supported a high rate of responding. After subjects associated one stimulus from each of these pairs with a discriminative choice response, researchers presented two new clip art stimuli during a 10-sec FI: one trained with a differential reinforcement of low rate schedule (DRL) after the FI and the other trained with a differential reinforcement of high rate schedule (DRH) after the FI. Each of the stimuli that were withheld during choice training was later shown to see if the choice responses would transfer to these stimuli. The results suggest that response rate alone does not mediate learned stimulus equivalence.     

Recognition of static and dynamic images of depth-rotated human faces by pigeons

In three experiments, we examined pigeons’ recognition of video images of human faces. In Experiment 1, pigeons were trained to discriminate between frontal views of human faces in a go/no-go discrimination procedure. They then showed substantial generalization to novel views, even though human faces change radically as viewpoint changes. In Experiment 2, the pigeons tested in Experiment 1 failed to transfer to the faces dynamically rotating in depth. In Experiment 3, the pigeons trained to discriminate the dynamic stimuli showed excellent transfer to the corresponding static views, but responses to the positive faces decreased at novel viewpoints outside the range spanned by the dynamic stimuli. These results suggest that pigeons are insensitive to the three-dimensional properties of video images. Consideration is given to the nature of the task, relating to the identification of three-dimensional objects and to perceptual classifications based on similarity judgments.     

Discrimination of relative numerosity by pigeons

In three experiments, pigeons were trained to discriminate between uniform arrays of two elements that differed in color, form, or size. They were then tested with arrays that contained different proportions of the two elements on these dimensions. In all cases, orderly discrimination gradients reflected these proportions. The discrimination readily transferred to new arrays with similar stimuli, but with different total numbers of elements. In Experiment 4, the pigeons were taught to discriminate between two groups of categorical stimuli: pictures of birds and pictures of flowers. A test with different proportions of each again produced a gradient based on relative numerosity. Experiment 5 demonstrated transfer of stimulus control on the numerosity dimension when pigeons were trained with one set of instances from two categories, and then were tested with new instances from the same categories.     

Pigeons discriminate pictures of a geographic location

Four pigeons previously trained to home to the roof of the University of British Columbia psychology building and 4 nonhoming pigeons were trained to discriminate between two sets of color slides projected onto a pecking panel of a Skinner box. One slide set consisted of photographs taken in the vicinity of the psychology building; the other set consisted of similar views taken at locations not previously visited by the homing subjects. All subjects were rewarded for pecking during slides from the first but not the second set. Every few sessions, new “Home” and “Away” slides were introduced during transfer tests. In a final transfer test, a completely new tray of Home and Away slides was introduced. The homing pigeons were slightly (but not statistically significantly) better at discriminating Home from Away slides. The implications of these results for understanding pigeons’ homing behavior, concept attainment, and spatial memory are discussed.     

Oddity of visual patterns conceptualized by pigeons

Pigeons were trained to learn an instrumental oddity-from-sample discrimination involving visual forms. One group, the “few examples” group, dealt with 5 patterns in 40 different combinations. Another group, the “many examples” group, dealt with 20 patterns in 160 different combinations. After both groups had reached asymptotic performance and had learned to operate under partial reinforcement conditions, they were tested for transfer under extinction conditions with two different groups of 5 novel patterns, each in 40 combinations. All animals showed significant above chance transfer to both of these novel stimulus sets. Transfer performance with test stimuli of similar geometric design to training stimuli was better than performance with stimuli of markedly different design. The transfer performance of the “many examples” group was marginally better than that of the “few examples” group, even though the latter’s performance on the training stimuli was better throughout. It is concluded that pigeons can learn to employ an oddity concept and that this may be promoted by the use of many training exemplars. Furthermore, it is inferred that pigeons may normally use a mixture of strategies to solve oddity and identity problems.     

The role of item- and category-specific information in the discrimination of people versus nonpeople images by pigeons

Herrnstein and Loveland (1964, pp. 549–551) successfully trained pigeons to discriminate pictures showing humans from pictures that did not. In the present study, a go/no-go procedure was employed to replicate and extend their findings, the primary focus of concern being to reevaluate the role of item- and category-specific information. The pigeons readily acquired the discrimination and were also able to generalize to novel instances of the two classes (Experiment 1). Classification of scrambled versions of the stimuli was based on small and local features, rather than on configural and global features (Experiment 2). The presentation of gray-scale stimuli indicated that color was important for classifying novel stimuli and recognizing familiar ones (Experiments 1 and 2). Finally, the control that could possibly be exerted by irrelevant background features was investigated by presenting the pigeons with images of persons contained in former person-absent pictures (Experiment 3). Classification was found to be controlled by both item- and category- specific features, but only in pigeons that were reinforced on person-present pictures was the latter type of information given precedence over the former.     

Recall of three-item sequences by pigeons

Pigeons were trained to recall an arbitrary sequence on a delayed matching-to-successive-samples (DMTSS) task. Sample items were presented successively and then displayed simultaneously. Subjects were required to respond to them in the order in which they appeared. In Experiment 1, pigeons responded correctly on 75% of the trials on a two-item DMTSS task but at a chance level of accuracy on a three-item task. In Experiment 2, pigeons who learned to produce a three-item sequence prior to DMTSS training mastered a three-item DMTSS task at a 75% level of accuracy. Control groups, trained initially with the same items on nonserial tasks, performed as poorly on a three-item DMTSS task as the naive subjects of Experiment 1. It was hypothesized that pigeons that first learned to produce a three-item list were able to recall three-item samples in DMTSS because they had learned to represent three-item sequences.     

Dynamic object recognition in pigeons and humans

We investigated the role of dynamic information in human and pigeon object recognition. Both species were trained to discriminate between two objects that each had a characteristic motion, so that either cue could be used to perform the task successfully. The objects were either easy or difficult to decompose into parts. At test, the learned objects could appear in their learned motions, the reverse of the learned motions, or an entirely new motion, or a new object could appear in one of the learned motions. For humans, any change in the learned motion produced a decrement in performance for both the decomposable and the nondecomposable objects, but participants did not respond differentially to new objects that appeared in the learned motions. Pigeons showed the same pattern of responding as did humans for the decomposable objects, except that pigeons responded differentially to new objects in the learned motions. For the nondecomposable objects, pigeons used motion cues exclusively. We suggest that for some types of objects, dynamic information may be weighted differently by pigeons and humans.     

Time-course of control by specific stimulus features and relational cues during same-different discrimination training

We trained 7 pigeons to discriminate visual displays of 16same items from displays of 16different items. The specific stimulus features of the items and the relations among the items could serve as discriminative stimuli. Unlike in most studies of same-different discrimination behavior, we gave a small number of probe tests during each session of acquisition to measure the time-course of control by the learning of specific stimulus features and relational cues. Both the specific stimulus features and relational cues exerted reliable stimulus control, with the specific stimulus features exerting more control during the final three fourths of same-different learning. These findings replicate research suggesting that pigeons encode both the specific stimulus features and relational cues, and for the first time document the time-course of control by each kind of cue.     

Categorical discrimination of objects and pictures by pigeons

With a three-choice instrumental discrimination procedure, pigeons were taught to distinguish small spherical objects from nonspherical objects. Spherical objects were defined as positive, nonspherical objects as negative. A device allowing an automatic presentation of the stimuli was employed. The subjects actually pecked the objects, and grain rewards were presented directly beside the correct objects. Acquisition was rapid, with the birds reaching a criterion of 80% correct choices within less than 150 trials. There was evidence that more than 200 objects were remembered individually over 3 months. Pigeons transferred the discrimination of spherical/nonspherical objects to novel objects. The criteria by which the birds judged the sphericity of objects seemed to be similar to those applied by humans. They could apply the categorization in a relational manner and generalize it to apply to photographs and drawings of objects. The categorization competence was retained for at least 3 months.     

Generalization along the dimension of sound intensity in pigeons

Two groups of four pigeons each were trained on a discrimination between two intensities of white noise. The low-intensity group had a 60-dB intensity as the negative discriminative stimulus (S?) and a 70-dB intensity as the positive discriminative stimulus (S+): the high-intensity group had a 95-dB intensity as S? and an 85-dB intensity as S+. Generalization stimuli were all of higher intensity than S+ for the former group and all of lower intensity than S+ for the latter group. The rate of acquisition of the discrimination was faster for the Ss in the high-intensity group. In both groups, the maximum of the generalization function was shifted toward the middle values of the set of test stimuli, away from the training stimuli. Responding showed a decline at the far end of the range of test stimuli. Responding to the positive training stimulus was initially as great as it had been on the preceding training sessions, but became markedly depressed relative to responding to the other stimuli as the test progressed.     

The recency effect in pigeons’ long-term memory

Four experiments are reported in which pigeons first learned one wavelength discrimination (green S+, yellow S?) and then the reversal; finally, after various delays, they were tested for wavelength generalization in extinction. In Experiment 1, the two problems were learned in different contexts; testing in Context 1 produced maximal responding to green in only half of the subjects, even when testing was delayed 30 days. In Experiment 2, testing of the subjects repeatedly in both contexts showed good control by each context after a 30-day delay. In Experiment 3, both problems were learned in the same context, and all gradients showed recency, peaking at yellow, even after 30 days. In Experiment 4, the subjects learned a series of reversals in the same context, terminating in yellow, S+, green, S?, and their gradients peaked at yellow, even after a 30-day delay. In Experiments 3 and 4, the gradients became flatter with increasing delays, and they were flatter in Experiment 4 (after three reversals) than in Experiment 3 (after one reversal). The location of the peak was not affected by delay, but only by testing in a context that had been uniquely associated with Problem 1 (Experiments 1 and 2). It is proposed that the location of gradient peaks indicates what is being remembered, whereas the slope of the obtained gradients indicates how well the target memory has been retrieved.     

Limits of dynamic object perception in pigeons: Dynamic stimulus presentation does not enhance perception and discrimination of complex shape

A go/no-go procedure was used to train pigeons to discriminate pictures of human faces differing only in shape, with either static images or movies of human faces dynamically rotating in depth. On the basis of experimental findings in humans and some earlier studies on three-dimensional object perception in pigeons, we expected dynamic stimulus presentation to support the pigeon’s perception of the complex morphology of a human face. However, the performance of the subjects presented with movies was either worse than (AVI format movies) or did not differ from (uncompressed dynamic presentation) that of the subjects trained with a single or with multiple static images of the faces. Furthermore, generalization tests to other presentation conditions and to novel static views revealed no promoting effect of dynamic training. Except for the subjects trained on multiple static views, performance dropped to chance level with views outside the training range. These results are in contrast to some prior reports from the literature, since they suggest that pigeons, unlike humans, have difficulty using the additional structural information provided by the dynamic presentation and integrating the multiple views into a three-dimensional object.     

The effect of the controllability of auditory discriminative stimuli in the performance of go/no-go discriminations by pigeons

Compared with their performance with localized (on-key) visual stimuli, pigeons are notoriously poor at performing go/no-go discriminations when keypecking for food in the presence of auditory discriminative stimuli. The difference might reflect the fact that an aversive visual onkey stimulus signaling nonreward can be escaped by looking away and not pecking, which contributes to the measure of good discriminative performance, while an auditory stimulus cannot be escaped. In Experiment 1, discriminative performance was significantly improved by providing pigeons with a response incompatible with keypecking by which they could escape a tone S+ and a tone S?. However, the pattern, frequency, and duration of escape responses were found to be insufficient to explain the improvement. In Experiment 2, it was found that the capacity to escape only S+ or only S? enhanced discriminative performance as much as the capacity to escape both. It is theorized that the Pavlovian relationship between the absence of the discriminative stimuli and the nonoccurrence of food might transfer to the instrumental relationships learned in a go/no-go discrimination. The possibility that intermittent stimuli command more attention than continuous stimuli is also considered.