The pigeon's discrimination of movement patterns (Lissajous figures) and contour-dependent rotational invariance

The ability of pigeons to discriminate complex motion patterns was investigated with the aid of moving Lissajous figures. The pigeons successfully learned to differentiate two successively presented cyclic trajectories of a single moving dot. This suggests that they can recognize a movement Gestalt when information about shape is minimal. They also quickly learned a new discrimination between moving-outline stimuli with repetitively changing contour patterns. Contrasting results were obtained when the dot or outline stimuli were axis-rotated through 90 degrees. Rotational invariance of pattern discrimination was clearly demonstrated only when moving contours were visible. Nevertheless, pigeons could discriminate the axis-orientation of a moving-dot or moving-outline pattern when trained to do so. Discrimination did not seem to depend on single parameters of motion but rather on the recognition of a temporally integrated movement Gestalt. The visual system of pigeons, as well as that of humans, may be well adapted to recognize the types of oscillatory movements that represent components of the motor behaviour shown by many living organisms.     

Dynamic object perception by pigeons: discrimination of action in video presentations

Two experiments examined the discrimination by pigeons of relative motion using computer-generated video stimuli. Using a go/no-go procedure, pigeons were tested with video stimuli in which the camera's perspective went either "around" or "through" an approaching object in a semi-realistic context. Experiment 1 found that pigeons could learn this discrimination and transfer it to videos composed from novel objects. Experiment 2 found that the order of the video's frames was critical to the discrimination of the videos. We hypothesize that the pigeons perceived a three-dimensional representation of the objects and the camera's relative motion and used this as the primary basis for discrimination. It is proposed that the pigeons might be able to form generalized natural categories for the different kinds of motions portrayed in the videos. Accepted after revision: 23 March 2001 Electronic Publication     

Pigeons can discriminate “good” and “bad” paintings by children

Humans have the unique ability to create art, but non-human animals may be able to discriminate “good” art from “bad” art. In this study, I investigated whether pigeons could be trained to discriminate between paintings that had been judged by humans as either “bad” or “good”. To do this, adult human observers first classified several children’s paintings as either “good” (beautiful) or “bad” (ugly). Using operant conditioning procedures, pigeons were then reinforced for pecking at “good” paintings. After the pigeons learned the discrimination task, they were presented with novel pictures of both “good” and “bad” children’s paintings to test whether they had successfully learned to discriminate between these two stimulus categories. The results showed that pigeons could discriminate novel “good” and “bad” paintings. Then, to determine which cues the subjects used for the discrimination, I conducted tests of the stimuli when the paintings were of reduced size or grayscale. In addition, I tested their ability to discriminate when the painting stimuli were mosaic and partial occluded. The pigeons maintained discrimination performance when the paintings were reduced in size. However, discrimination performance decreased when stimuli were presented as grayscale images or when a mosaic effect was applied to the original stimuli in order to disrupt spatial frequency. Thus, the pigeons used both color and pattern cues for their discrimination. The partial occlusion did not disrupt the discriminative behavior suggesting that the pigeons did not attend to particular parts, namely upper, lower, left or right half, of the paintings. These results suggest that the pigeons are capable of learning the concept of a stimulus class that humans name “good” pictures. The second experiment showed that pigeons learned to discriminate watercolor paintings from pastel paintings. The subjects showed generalization to novel paintings. Then, as the first experiment, size reduction test, grayscale test, mosaic processing test and partial occlusion test were carried out. The results suggest that the pigeons used both color and pattern cues for the discrimination and show that non-human animals, such as pigeons, can be trained to discriminate abstract visual stimuli, such as pictures and may also have the ability to learn the concept of “beauty” as defined by humans.     

Perception of motion transparency in 5-month-old infants

We investigated the perceptual development of motion transparency in 3- to 5-month-old infants. In two experiments we tested a total of 55 infants and examined their preferential looking behaviour. In experiment 1, we presented transparent motion as a target, and uniform motion as a non-target consisting of random-dot motions. We measured the time during which infants looked at the target and non-target stimuli. In experiment 2, we used paired-dot motions (Qian et al, 1994 Journal of Neuroscience 14 7357-7366) as non-targets and also measured target looking time. We calculated the ratio of the target looking time to the total target and no-target looking time. In both experiments we controlled the dot size, speed, the horizontal travel distance of the dots, and the motion pattern of the dots. The results demonstrated that 5-month-old infants showed a statistically significant preference for motion transparency in almost all stimulus conditions, whereas the preference in 3- and 4-month-old infants depended on stimulus conditions. These results suggest that the sensitivity to motion transparency was robust in 5-month-olds, but not in 3- and 4-month-olds.     

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.     

Categorical Discrimination of Human Facial Expressions by Pigeons: A Test of the Linear Feature Model

Pigeons were trained to discriminate human facial expressions, happiness and anger, in a go/ no-go discrimination procedure. Five pigeons learned to discriminate photographs of the happy and angry faces of 25 different people and showed high levels of transfer to novel faces expressing the training emotions. The pigeons directed their pecks predominantly to the mouth, eyes, or the area between these features. The pigeons were then tested with familiar stimuli in which the upper and lower parts of the face were manipulated separately by substitution or removal of facial features ('eyes-and-eyebrows' and 'mouth'). It was shown that the salience of particular features differed considerably among the birds, but that a linear feature model adequately accounted for discriminative performance of the birds with these stimuli. Furthermore, the discrimination was maintained when these features were inverted. Thus, the so-called Thatcher illusion did not occur. It is suggested that the discrimination is based not on a feature configuration or perceptual gestalt but on an additive integration of individual features.     

Visually guided capture of a moving stimulus by the pigeon (Columba livia)

Although the pigeon is a popular model for studying visual perception, relatively little is known about its perception of motion. Three experiments examined the pigeons’ ability to capture a moving stimulus. In Experiment 1, the effect of manipulating stimulus speed and the length of the stimulus was examined using a simple rightward linear motion. This revealed a clear effect of length on capture and speed on errors. Errors were mostly anticipatory and there appeared to be two processes contributing to response locations: anticipatory peck bias and lag time. Using the same birds as Experiment 1, Experiment 2 assessed transfer of tracking and capture to novel linear motions. The birds were able to capture other motion directions, but they displayed a strong rightward peck bias, indicating that they had learned to peck to the right of the stimulus in Experiment 1. Experiment 3 used the same task as Experiment 2 but with naïve birds. These birds did not show the rightward bias in pecking and instead pecked more evenly around the stimulus. The combined results indicate that the pigeon can engage in anticipatory tracking and capture of a moving stimulus, and that motion properties and training experience influence capture.     

Peck tracking: a method for localizing critical features within complex pictures for pigeons

The pigeon is a standard animal in comparative psychology and is frequently used to investigate visuocognitive functions. Nonetheless, the strategies that pigeons use to discriminate complex visual stimuli remain a difficult area of study. In search of a reliable method to identify features that control the discrimination behaviour, pecking location was tracked using touch screen technology in a people-absent/people-present discrimination task. The correct stimuli contained human figures anywhere on the picture, but the birds were not required to peck on that part. However, the stimuli were designed in a way that only the human figures contained distinguishing information. All pigeons focused their pecks on a subarea of the distinctive human figures, namely the heads. Removal of the heads significantly impaired performance, while removal of other distinctive parts did not. Thus, peck tracking reveals the location within a complex visual stimulus that controls discrimination behaviour, and might be a valuable tool to reveal the strategies pigeons apply in visual discrimination tasks.     

Discrimination of five-dimensional stimuli by pigeons: Limitations of feature analysis

Pigeons were trained to discriminate between stimuli constructed using five orthogonal two-valued features. The stimuli consisted of stylized monochrome drawings of seeds. Two different training procedures (conditional and simultaneous discrimination) were used. In the first two experiments, the discrimination required was between polymorphous categories, in which a positive stimulus was defined as one in which three or more of the five features took their positive values. Discrimination in both experiments was imperfect; the pigeons' behaviour only came under the control of a subset of the available features (one to three in Experiment 1, three or four in Experiment 2). In Experiment 3, single features had to be discriminated, while the remaining features varied. It was found that all five features of the “seed” stimuli could be discriminated, but one of them was exceptionally difficult. The results show that pigeons do not reliably use all the features available to them when making category discriminations. This casts doubts on feature analysis as a basis for the excellent performance pigeons show when required to discriminate between categories of natural objects.     

Temporal parameters of the feature positive effect

Trial duration and intertrial interval duration were parametrically varied between groups of pigeons exposed to a discrimination involving the presence vs. the absence of a dot. Half the groups received the dot as the positive stimulus (feature positive groups) and half the groups received the dot as the negative stimulus (feature negative groups). Faster learning by the feature positive birds (feature positive effect) was found when the trial duration was short (5 sec) regardless of whether the intertrial interval was short (5 sec) or long (30 sec). No evidence for a feature positive effect was found when the trial duration was long (30 sec) regardless of the length of the intertrial interval (30 sec or 180 sec). The results suggest that short trial duration is a necessary condition for the occurrence of the feature positive effect, and neither intertrial interval nor trial duration/intertrial interval ratio are important for its occurrence. The suggestion that mechanisms underlying the feature positive effect and autoshaping might be similar was not supported by the present experiment since the trial duration/intertrial interval ration parameter appears to play an important role in autoshaping but not the feature positive effect.     

Motion as a natural category for pigeons: Generalization and a feature-positive effect

Three groups of pigeons were trained with a modified discriminative autoshaping procedure to discriminate video images of other pigeons on the basis of movement. Birds of all groups were shown the same video images of other pigeons, which were either moving or still. The group to whom food was presented only after moving images learned the discrimination very quickly. A second group, to whom food was given only after still images, and a pseudocategory group, to whom food was presented after arbitrarily chosen stimuli, showed no evidence of discrimination during acquisition training. Extinction conditions led to clear differences in peck rates to moving and still images in the second group but not in the pseudocategory group. The result is related to the feature-positive effect. Generalization tests showed that the discrimination performance was based on visual features of the stimuli but was invariant against changes of size, perspective, brightness, and color. Furthermore, discrimination was maintained when novel images of pigeons under different viewing angles and seven other types of motion categories were presented. It is argued that the discrimination is based not on a common motion feature but on motion concepts or high-order generalization across motion categories.     

Does the use of natural stimuli facilitate amodal completion in pigeons?

Three experiments were carried out to investigate whether amodal completion in pigeons can be facilitated by the use of colour photographs instead of highly artificial stimuli such as geometrical shapes. Ten pigeons were trained in a go/no-go procedure to discriminate between photographs of complete and of incomplete pigeon figures. In the subsequent test, the birds classified pictures of partly occluded pigeons as though they were complete (experiment 1). However, we found evidence that classification was based on spurious stimulus features that paralleled the intended class rule of figural completeness versus incompleteness. In particular, classification was shown to be guided by white background gaps that separated the parts of the fragmented pigeon figures (experiment 2), as well as by cues related to overall Gestalt (experiment 3). In summary, the present results indicate that the use of more natural stimuli such as photographs instead of geometrical shapes is insufficient for providing amodal completion in pigeons. It is suggested that a combination of various cues, including, eg, 3-D information and common motion in addition to surface and contour properties, may be required to induce a perceptual bias favouring visual completion of occluded portions.     

Amodal completion of moving objects by pigeons

In a series of four experiments, we explored whether pigeons complete partially occluded moving shapes. Four pigeons were trained to discriminate between a complete moving shape and an incomplete moving shape in a two-alternative forced-choice task. In testing, the birds were presented with a partially occluded moving shape. In experiment 1, none of the pigeons appeared to complete the testing stimulus; instead, they appeared to perceive the testing stimulus as incomplete fragments. However, in experiments 2, 3, and 4, three of the birds appeared to complete the partially occluded moving shapes. These rare positive results suggest that motion may facilitate amodal completion by pigeons, perhaps by enhancing the figure - ground segregation process.     

Category discrimination by pigeons using five polymorphous features

Eight pigeons were trained to discriminate between sets of color photographs of natural scenes. The scenes differed along five two-valued dimensions (site, weather, camera distance, camera orientation, and camera height), and all combinations of the feature values were used. One value of each dimension was designated as positive, and slides containing three or more positive feature values were members of the positive stimulus set. Thus, each feature had an equal, low, correlation with reinforcement, and all features had zero correlations with each other. Seven of the 8 pigeons learned this discrimination, and their responding came under the control of all five features. Within the positive and negative stimulus sets, response rates were higher to stimuli that contained more positive feature values. Once discrimination had been achieved, reversal training was given using a subset of the slides. In this subset, only a single feature was correlated with reinforcement. All pigeons learned this reversal successfully and generalized it to additional photographs with the same feature content. After reversal, the original reinforcement contingencies were reinstated, and training was continued using all the slides except those that had been used in reversal. Reversal generalized to these slides to some extent. Analysis of the response rates to individual slides showed that, compared with prereversal training, only the feature that had been subjected to reversal contingencies showed a reversed correlation with response rate. The remaining features showed the same correlation with response rate as they had before reversal training. Thus, reversal on some members of a category following category discrimination training led to generalization to stimuli within the category that were not involved in the reversal, but not to features that were not reversed. It is therefore inappropriate to refer to the pigeons as learning a concept.     

Discrimination of artificial polymorphous categories by rhesus monkeys (Macaca mulatta)

Monkeys (Macaca mulatta) were trained to discriminate between sets of artificial stimuli such as those used by Jitsumori (1993) for pigeons and humans. The stimuli were arrays of symbols differing along three two-valued (positive or negative) dimensions. The discrimination required was between polymorphous categories in which a positive stimulus was defined by possession of any 2 out of 3 positive features. Of the 5 monkeys, 3 learned the discrimination much faster than did pigeons, but transfer to novel stimuli was less impressive than had been shown in pigeons. The 3 monkeys showed high levels of transfer to the stimuli that contained either all 3 positive or all 3 negative features, but 2 of the 3 monkeys failed to show transfer to stimuli that had 1 of the 3 features replaced with a novel one. Analysis of the monkeys' performance raised doubts on the additive integration of features but supported learning of feature combinations as a basis for the discrimination of polymorphous categories by this species.     

Learning of an oddity rule by pigeons in a four-choice touch-screen procedure

Six pigeons were trained to peck at a target (odd stimulus) that was presented on a touch-screen together with three identical distractors (non-odd stimuli). The target could be either a square or a circle that was either blue or green, and the distractors in each trial were always of the opposite form and color to the target. Thus, the birds could solve the task by attending to color, form, or both. Transfer tests showed that performance was not disrupted by novel forms, stimulus sizes, distractor numbers, and display configurations, but broke down with novel stimulus types (textured stimuli, clip art images, and photographs). Transfer to novel colors was, for the most part, restricted to trials in which only one component—target or distractors, but not both—had a novel color. This suggested that the pigeons used a couple of if–then rules rather than an oddity concept to solve the task, and that color differences between target and distractors were the only cue upon which responding was based. A control experiment with the order of color and form tests being reversed excluded the possibility of the prevalence of color being an artifact of task order and reinforcement contingencies.     

Some determiners of attention

Three experiments, using a total of 13 pigeons, examined the stimulus control acquired by the separate components of a compound visual stimulus transilluminating the pecking key. Experiment I measured the control acquired by components of compound discriminative stimuli used in discrimination training. Experiment II sought to demonstrate the effect of pretraining a single stimulus discrimination on control acquired by each component in a compound stimulus discrimination. It also investigated the effect of training the compound stimulus discrimination before the single stimulus discrimination. Experiment III sought a continuous stimulus control function when pretraining stimulus intensities were varied. The results suggest that the extent to which a bird "pays attention" to a stimulus, defined in terms of the degree of stimulus control acquired by that stimulus, is determined by how well it previously learned to discriminate that stimulus from other stimuli.     

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.     

Anisotropic motion coherence sensitivities to expansion/contraction motion in early infancy

We investigated 2- and 3-month-olds' motion coherence sensitivities to radial expansion/contraction by using the preferential looking method. The infants were tested with a stimulus composed of two dynamic random dot patterns placed side by side: an expansion (or a contraction) pattern and a random directional pattern. The results showed that the 3-month-old infants tested with both a contraction and random directional pattern could discriminate between those two motions significantly, even when the contraction motion coherence was relatively low (50%). On the other hand, the 3-month-old infants who were tested with both expansion and random directional pattern could not discriminate between those two motions. None of the 2-month-old infants showed significant discrimination between the expansion/contraction and random motion patterns. Results of the present study suggest that anisotropic motion coherence sensitivities to radial expansion/contraction emerge at around 3 months of age.     

Errorless discrimination established by differential autoshaping

In Experiment I, pigeons exposed to a differential autoshaping procedure pecked a key in the presence of the stimulus associated with reinforcement but did not peck, or pecked infrequently, in the presence of the stimulus associated with nonreinforcement. In Experiment II, pigeons were exposed to a differential autoshaping procedure in which one stimulus was associated with reinforcement and two stimuli were associated with nonreinforcement. The birds initially responded in the presence of one stimulus associated with nonreinforcement but never responded in the presence of the second stimulus associated with nonreinforcement. They were subsequently exposed to an autoshaping procedure in which reinforcement followed both these stimuli. The number of stimulus-reinforcement pairings required to establish pecking in the presence of the stimulus during which responses had not previously occurred suggested that such stimuli are inhibitory. These findings have implications for autoshaping, errorless discrimination, inhibition, and theories of discrimination byproducts.