Reflexivity in pigeons

A recent theory of pigeons' equivalence-class formation (Urcuioli, 2008) predicts that reflexivity, an untrained ability to match a stimulus to itself, should be observed after training on two "mirror-image" symbolic successive matching tasks plus identity successive matching using some of the symbolic matching stimuli. One group of pigeons was trained in this fashion; a second group was trained similarly but with successive oddity (rather than identity). Subsequently, comparison-response rates on novel matching versus mismatching sequences with the remaining symbolic matching stimuli were measured on nonreinforced probe trials. Higher rates were observed on matching than on mismatching probes in the former group. The opposite effect--higher rates on mismatching than matching probes--was mostly absent in the latter group, despite being predicted by the theory. Nevertheless, the ostensible reflexivity effect observed in former group may be the first time this phenomenon has been demonstrated in any animal.     

Self-reinforcement in pigeons

Pigeons were trained to reward their own performances by eating from a freely available food source only after pecking a disc. The self-reinforcement pattern was established by fading in the work requirement and punishing noncontingent self-feeding by food withdrawal. The animals maintained faultless self-reinforcement for hundreds of trials after the punishment contingency was removed so that the birds could safely feed themselves without performing any pecking responses. In successive induction and extinction of the phenomenon, the number of responses per self-reward was observed to covary with self-reinforcement rate. By progressively raising the work requirements an animal was trained to adopt increasingly higher performance standards of self-reward. After maintaining a high response output for each self-reward the animal promptly discarded all self-imposed work contingencies and quickly resumed them again, though less durably, following additional training.     

Uncertainty in pigeons

Pigeons classified a display of illuminated pixels on a touchscreen as sparse or dense. Correct responses were reinforced with six food pellets; incorrect responses were unreinforced. On some trials an uncertain response option was available. Pecking it was always reinforced with an intermediate number of pellets. Like monkeys and people in related experiments, the birds chose the uncertain response most often when the stimulus presented was difficult to classify correctly, but in other respects their behavior was not functionally similar to human behavior based on conscious uncertainty or to the behavior of monkeys in comparable experiments. Our data were well described by a signal detection model that assumed that the birds were maximizing perceived reward in a consistent way across all the experimental conditions.     

Avoidance responding in pigeons

Four pigeons were trained in a one-way shuttle box avoidance situation. Three of the birds met the criterion of 90% avoidances; the fourth, although frequently avoiding successfully, was too erratic to meet the criterion. Avoidance responding in two of the birds was subsequently extinguished, showing that the response was true avoidance, and not escape from the buzzer warning stimulus. In Experiment 2, the three birds that had met criterion in Experiment 1 were trained in a two-way avoidance task, and all three met the criterion of 90% avoidances. The shuttle box therefore provides a rapid and reliable method of obtaining avoidance performance in pigeons.     

What-where-when memory in pigeons

The authors report a novel approach to testing episodic-like memory for single events. Pigeons were trained in separate sessions to match the identity of a sample on a touch screen, to match its location, and to report on the length of the retention interval. When these 3 tasks were mixed randomly within sessions, birds were more than 80% correct on each task. However, performance on 2 different tests in succession after each sample was not consistent with an integrated memory for sample location, time, and identity. Experiment 2 tested binding of location and identity memories in 2 different ways. The results were again consistent with independent feature memories. Implications for tests of episodic-like memory are discussed.     

Impulse control in pigeons

Pigeons were given a small, immediate food reinforcement for pecking a key, and a larger, delayed reinforcement for not pecking this key. Most subjects pecked the key on more than 95% of trials. However, when pecking a differently colored key at an earlier time prevented this option from becoming available, three of 10 subjects consistently pecked it, thereby forcing themselves to wait for the larger reward. They did not peck the earlier key when it did not prevent this option. This is an experimental example of psychological impulse and a learnable device to control it. Although only a minority of the subjects learned it, the fact that such learning is possible at all argues for a theory of delayed reward that can predict change of preference as a function of elapsing time.     

Observing responses in pigeons

Pigeons were trained on an observing-response procedure in which periods of VR 100 and EXT alternated unpredictably during a white light (mixed stimulus). During VR 100, responses on a food-producing key (the first key) were intermittently reinforced. Responses on the observing key (the second key) produced a green light (positive stimulus) when VR 100 was in effect, and a red light (negative stimulus) for EXT. The birds did not respond on either key during the negative stimulus, but they responded on the food-producing key when the positive stimulus appeared. When observing responses produced the positive or negative stimulus on FR, observing responses were maintained until the FR reached a maximum; beyond this, only food-producing responses occurred. When observing responses did not produce either stimulus, the observing-response rates fell to zero. With prolonged exposure to an FR 20 schedule of observing, observing-response rates during EXT were higher than during VR 100. Chlorpromazine hydrochloride decreased the total response output but markedly increased observing-response rates except when it was administered before sessions of observing response extinction.     

Shared attention in pigeons

Two pigeons performed a three-key matching-to-sample task. The comparison (side key) stimuli were either solid colors or white lines. The sample (center key) stimuli were either compounds (white lines on colored grounds) or elements (white lines on black grounds on some trials, and solid colors on other trials). Sample stimuli were presented for nine sample stimulus durations ranging between 0.04 and 5.00 sec. Within each daily session, both compound and element samples were presented at each sample duration in a random sequence. Compound samples controlled matching responses less effectively than did element samples at all sample stimulus durations.     

Perceptual grouping in pigeons

Animal studies reveal that many species perceive partially occluded objects in the same way as do humans. Pigeons have been a notable exception. We re-investigated this anomaly of pigeon perception using a different approach from previous studies. With our method, we show that pigeons perceive occluded objects in the same manner as do other species. In addition, we report that pigeons can recognize perceptually transparent surfaces when the effect is induced by the same perceptual mechanisms as occlusion. These results give behavioral evidence that the perception of both occlusion and transparency is a common visual function shared by pigeons and humans, despite the structural differences between their visual systems.     

Episodic-like memory in pigeons

It has been proposed that memory for personal experiences (episodic memory, rather than semantic memory) relies on the conscious review of past experience and thus is unique to humans. In an attempt to demonstrate episodic-like memory in animals, we first trained pigeons to respond to the (nonverbal) question "Did you just peck or did you just refrain from pecking?" by training them on a symbolic matching task with differential responding required to the two line-orientation samples and reinforcing the choice of a red comparison if they had pecked and the choice of a green comparison if they had not pecked. Then, in Experiment 1, after providing the conditions for (but not requiring) the pigeons to peck at one new stimulus (a yellow hue) but not at another (a blue hue), we tested them with the new hue stimuli and the red and green comparisons. In Experiment 2, we tested the pigeons with novel stimuli (a circle, which they spontaneously pecked, and a dark response key, which they did not peck) and the red and green comparisons. In both experiments, pigeons chose the comparison appropriate to the response made to the test stimulus. Thus, the pigeons demonstrated that they could remember specific details about their past experiences, a result consistent with the notion that they have the capacity for forming episodic-like memories.     

A Simon effect in pigeons

Pigeons pecked left versus right keys contingent upon the color presented at 1 of those locations. Spatial-response latencies were shorter when the color appeared at the same location as the required response than at the opposite location. This Simon effect occurred when the stimulus on the alternative key was constant, varied from trial to trial, or changed when the color cue appeared and when the reinforcement probability for correct responses was the same on corresponding as on noncorresponding trials. Humans performing the same task by touching the keys also showed the Simon effect. These findings demonstrate that for pigeons, too, a relevant symbolic cue activates a spatial code that produces faster responses at the location corresponding with the activated code.     

Dynamics of waiting in pigeons

Two experiments used response-initiated delay schedules to test the idea that when food reinforcement is available at regular intervals, the time an animal waits before its first operant response (waiting time) is proportional to the immediately preceding interfood interval (linear waiting; Wynne & Staddon, 1988). In Experiment 1 the interfood intervals varied from cycle to cycle according to one of four sinusoidal sequences with different amounts of added noise. Waiting times tracked the input cycle in a way which showed that they were affected by interfood intervals earlier than the immediately preceding one. In Experiment 2 different patterns of long and short interfood intervals were presented, and the results implied that waiting times are disproportionately influenced by the shortest of recent interfood intervals. A model based on this idea is shown to account for a wide range of results on the dynamics of timing behavior.     

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.     

Van Gogh, Chagall and pigeons: picture discrimination in pigeons and humans

The author has previously reported that pigeons can discriminate paintings by different artists. Here, I replicated the previous findings, carried out additional tests and compared discrimination by pigeons with that of humans. In Experiment 1, pigeons were trained to discriminate between paintings by Van Gogh and Chagall. After training, the subjects were tested with different paintings by the same artists. The subjects showed generalization to these paintings. The subjects maintained their discriminative ability for black-and-white paintings and partially occluded paintings. When they were tested with mosaic paintings, the number of correct responses decreased, depending on the level of processing needed. In Experiment 2, human subjects were tested with the same paintings. The subjects showed generalization and decrement of correct responses depending on the degree of mosaic processing. These observations suggest that the visual cognitive function of pigeons is comparable to that of humans. The convergence of higher visual cognition between the two species may stem from similar selection pressures on avian ancestors and our ancestors through their evolution. Accepted after revision: 19 August 2001 Electronic Publication     

Nonstable concurrent choice in pigeons

Six pigeons were trained on concurrent variable-interval schedules in which the arranged reinforcer ratios changed from session to session according to a 31-step pseudorandom binary sequence. This procedure allows a quantitative analysis of the degree to which performance in an experimental session is affected by conditions in previous sessions. Two experiments were carried out. In each, the size of the reinforcer ratios arranged between the two concurrent schedules was varied between 31-step conditions. In Experiment 1, the concurrent schedules were arranged independently, and in Experiment 2 they were arranged nonindependently. An extended form of the generalized matching law described the relative contribution of past and present events to present-session behavior. Total performance in sessions was mostly determined by the reinforcer ratio in that session and partially by reinforcers that had been obtained in previous sessions. However, the initial exposure to the random sequence produced a lower sensitivity to current-session reinforcers but no difference in overall sensitivity to reinforcement. There was no evidence that the size of the reinforcer ratios available on the concurrent schedules affected either overall sensitivity to reinforcement or the sensitivity to reinforcement in the current session. There was also no evidence of any different performance between independent and nonindependent scheduling. Because of these invariances, this experiment validates the use of the pseudorandom sequence for the fast determination of sensitivity to reinforcement.     

Shape from shading in pigeons

Light is the origin of vision. The pattern of shading reflected from object surfaces is one of several optical features that provide fundamental information about shape and surface orientation. To understand how surface and object shading is processed by birds, six pigeons were tested with differentially illuminated convex and concave curved surfaces in five experiments using a go/no-go procedure. We found that pigeons rapidly learned this type of visual discrimination independent of lighting direction, surface coloration and camera perspective. Subsequent experiments varying the pattern of the lighting on these surfaces through changes in camera perspective, surface height, contrast, material specularity, surface shape, light motion, and perspective movement were consistent with the hypothesis that the pigeons were perceiving these illuminated surfaces as three-dimensional surfaces containing curved shapes. The results suggest that the use of relative shading for objects in a visual scene creates highly salient features for shape processing in birds.     

Discriminated time-out avoidance in pigeons

Performances of two pigeons were studied on a concurrent discriminated TO avoidance-VR schedule. Each avoidance response postponed a TO from a VR 140 for a specified RS interval. The warning stimulus on the TO avoidance schedule was a discontinuous clock which consisted of a series of discrete color changes that varied systematically with the RS interval. Experimental manipulations established that the avoidance behavior was under the control of the avoidance schedule and the discontinuous clock. Five-min TOs maintained higher avoidance rates than shorter TO durations; a 15-min TO maintained less avoidance responding than the 5-min TO. Chlorpromazine hydrochloride increased TO avoidance behavior and decreased concurrent VR behavior.     

Visual search asymmetry in pigeons

Pigeons received an odd-item search task that involved an array of 12 patterns containing 11 similar distractors and a single target. Pecks to the target resulted in the delivery of food. Accuracy was greater on trials when a distinctive feature was located in the target but not in the distractors, rather than when the feature was in the distractors but not in the target. This search asymmetry was influenced by the similarity of the target to the distractors. The results are similar to those obtained with humans.