The ties that keep us bound: Top-down influences on the persistence of shape-from-motion

The phenomenon of perceptual persistence after the motion stops in shape-from-motion displays (SFM) was used to study the influence of prior knowledge on the maintenance of a percept in awareness. In SFM displays an object composed of discontinuous line segments are embedded in a background of randomly oriented lines. The object only becomes perceptible when the line segments that compose the object and the lines that compose the background move in counterphase. Critically, once the movement of the line segments stops, the percept of the object persists for a short period of time. In the present study, perceptual persistence for digits exceeded that reported for nonsense shapes composed of the same line segments. This result is taken as evidence that the processes involved in the persistence of SFM, and therefore sustained perception, are sensitive to top-down influences.     

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.     

Pigeons (<Emphasis Type="Italic">Columba livia</Emphasis>) know when they will need hints: prospective metacognition for reference memory?

Despite their impressive cognitive abilities, avian species have shown less evidence for metacognition than mammals. We suspect that commonly used tasks such as matching to sample might be too demanding to allow metacognitive processing within birds’ working memory. Here, we examined whether pigeons could control their behavior as a function of knowledge levels on a three-item sequence learning task, a reference memory task supposedly requiring fewer working memory resources. The experiment used two types of lists differing in familiarity. One was familiar to the pigeons through repeated exposure, whereas the other was novel in every new session. In test sessions, pigeons could choose between a trial with a hint specifying the next item to peck and one with no hint. However, successful responses in trials with a hint resulted in lowered rates of primary reinforcement: .60 in the first test and .75 in the second. Results showed that two of four pigeons chose the trial with a hint significantly more often before receiving a novel list than the familiar list in the four sessions of the first test, and three did so in the second test. Impressively, one bird showed robust evidence in the very first sessions in both tests. These results suggest that pigeons may monitor their long-term knowledge states and thereby control their environment before starting to solve a task.     

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.     

Lateralized cognition: asymmetrical and complementary strategies of pigeons during discrimination of the "human concept"

This study was aimed at revealing which cognitive processes are lateralized in visual categorizations of "humans" by pigeons. To this end, pigeons were trained to categorize pictures of humans and then tested binocularly or monocularly (left or right eye) on the learned categorization and for transfer to novel exemplars (Experiment 1). Subsequent tests examined whether they relied on memorized features or on a conceptual strategy, using stimuli composed of new combinations of familiar and novel humans and backgrounds (Experiment 2), whether the hemispheres processed global or local information, using pictures with different levels of scrambling (Experiment 3), and whether they attended to configuration, using distorted human figures (Experiment 4). The results suggest that the left hemisphere employs a category strategy and concentrates on local features, while the right hemisphere uses an exemplar strategy and relies on configuration. These cognitive dichotomies of the cerebral hemispheres are largely shared by humans, suggesting that lateralized cognitive systems already defined the neural architecture of the common ancestor of birds and mammals.     

Hippocampus lesions impair landmark array spatial learning in homing pigeons: a laboratory study

Hippocampal (HF)-lesioned pigeons display impaired homing ability when flying over familiar terrain, where they are presumably relying on a map-like representation of familiar landmarks to navigate. However, research carried out in the field precludes a direct test of whether hippocampal lesions compromise the ability of homing pigeons to navigate by familiar landmarks. To examine more thoroughly the relationship between hippocampus and landmark spatial learning, control, neostriatum-lesioned, and HF-lesioned homing pigeons were trained on two open field, laboratory, conditional discrimination tasks. One was a visual landmark array task, and the other was a room color discrimination task. For the tasks, the correct of three differently colored food bowls was determined by the spatial relationship among a group of five landmarks and room color, respectively. Intact control birds successfully learned both tasks, while neostriatum-lesioned birds successfully learned the landmark array task-the only task on which they were trained. By contrast, HF-lesioned birds successfully learned the room color task but were unable to learn the landmark array task. The data support the hypothesis that homing performance deficits observed in the field following hippocampal lesions are in part a consequence of an impairment in the ability of lesioned pigeons to use familiar visual landmarks for navigation.     

Plucks and bows are not categorically perceived

Letter-like targets (a circle and a square) were presented in one of two fixed and cued visual field locations and were shown alone, flanked by a noise stimulus on the peripheral side (side of target farthest from fixation), on the central side, or on both sides simultaneously. The adjacent target and noise stimulus borders had similar featural properties (both curved or both straight lines) or dissimilar properties (one being a curved line and one a straight line). Each of 10 subjects made a go, no-go response only when his or her designated target appeared in a display. The results showed: (1) single targets were discriminated more accurately and more rapidly than were targets shown simultaneously with noise stimuli, (2) targets having dissimilar border relationships with noise items were discriminated more accurately than were targets having similar border relationships, (3) targets in central-noise displays were discriminated more accurately and rapidly than were targets in peripheral-noise displays, and (4) there was no interaction between border relationships and noise position. The principal result relating to target-noise border featural relationship was consistent with predictions derived from models which place the locus of noise effects at the stage of stimulus feature extraction. Aspects of the results were, however, seen to be consistent with both feature extraction and response competition conceptualizations.     

Long-term memory of pigeons for stimulus-outcome associations involving multiple stimuli, each seen in a single brief trial

Experiment 1 explored performance of pigeons in two versions of a shortterm recognition memory procedure. In one version responding to entirely novel slides was rewarded, and responding to familiar slides (slides seen once, for 10 sec) was not rewarded; in the other version, responding to familiar slides was rewarded. Performance was initially below chance in both versions of the procedure. This result indicated that in this procedure associations were formed between the slides and the outcome (reward or non-reward) that followed their presentation. The result also suggested that the true capacity of pigeon recognition memory cannot be assessed using these procedures, as performance is inevitably disrupted by the bird's associative memory. The tendency of pigeons to form one-trial associations was exploited in Experiment 2. Phase 1 consisted of 16 two-session cycles: in Session 1 of each cycle, birds were shown 20 novel slides and were rewarded for responding to 10 of those slides; in Session 2, the same slides were shown again, with the same reinforcement contingencies. The birds showed significant overnight retention of the one-trial associations formed in Session 1 of each cycle. Phase 2 showed significant retention over periods of more than 20 days of associations involving 320 slides seen twice only. Phase 3 re-exposed for nine daily sessions one of the sets of 20 slides used in Phases 1 and 2; a high level of discrimination emerged rapidly and 4 (of 8) birds showed, by the end of training, no overlap in response rates to positive and negative slides. Comparative implications of the results are discussed.     

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.     

Extended conditioning and 24-hour retention in infants.

The successive contribution of brief daily conditioning sessions to the relatively long-term retention of stimulus-response interaction was assessed in 30 3-month-old infants. On each of 3 days, infants received a characteristic acquisition and extinction period, preceded by a 24-hr retention test. In addition to kick rate, duration of attention to a visual conjugate reinforcer (mobile) was measured. On the fourth day, the specificity of original learning was explored by exposing half the infants to a novel reinforcing agent while the remaining infants continued to view the original training mobile. Both kicks and attention significantly increased across successive retention tests for the first 3 days; on Day 4, infants viewing the novel mobile kicked significantly less and attended significantly longer than infants viewing the familiar mobile. Subsequent reattainment of high kick levels by the experimental group on Day 4 appeared to be highly individualistic and, in many instances, discontinuous.     

Categorization of natural movements by pigeons: visual concept discrimination and biological motion

In three experiments, pigeons were exposed to a discriminated autoshaping procedure in which categories of moving stimuli, presented on videotape, were differentially associated with reinforcement. All stimuli depicted pigeons making defined responses. In Experiment 1, one category consisted of several different scenes of pecking and the other consisted of scenes of walking, flying, head movements, or standing still. Four of the 4 birds for which pecking scenes were positive stimuli discriminated successfully, whereas only 1 of the 4 for which pecking was the negative category did so. In the pecking-positive group, there were differences between the pecking rates in the presence of the four negative actions, and these differences were consistent across subjects. In Experiment 2, only the categories of walking and pecking were used; some but not all birds learned this discrimination, whichever category was positive, and these birds showed some transfer to new stimuli in which the same movements were represented only by a small number of point lights (Johansson's “biological motion” displays). In Experiment 3, discriminations between pecking and walking movement categories using point-light displays were trained. Four of the 8 birds discriminated successfully, but transfer to fully detailed displays could not be demonstrated. Pseudoconcept control groups, in which scenes from the same categories of motion were used in both the positive and negative stimulus sets, were used in Experiments 1 and 3. None of the 8 pigeons trained under these conditions showed discriminative responding. The results suggest that pigeons can respond differentially to moving stimuli on the basis of movement cues alone.     

Not all same-different discriminations are created equal: Evidence contrary to a unidimensional account of same-different learning

In Experiment 1, we trained four pigeons to concurrently discriminate displays of 16 same icons (16S) from displays of 16 different icons (16D) as well as between displays of same icons (16S) from displays that contained 15 same icons and one different icon (15S:1D). The birds rapidly learned to discriminate 16S vs. 16D displays, but they failed to learn to discriminate 16S vs. 15S:1D displays. In Experiment 2, the same pigeons acquired the 16S vs. 15S:1D task after being required to locate and peck at the odd-item in the 15S:1D displays. Acquisition of the 16S vs. 15S:1D task had little effect on discriminative performance in the concurrent 16S:16D task, suggesting that a unidimensional entropy explanation for mastery of these two same-different tasks is not viable. During testing, the birds transferred discriminative performance in both tasks to displays composed of different visual stimuli. Such concurrent discrimination learning, performance, and transfer suggest that pigeons are flexible in the way they process the displays seen in these two same-different tasks.     

The role of edges in object recognition by pigeons

In three experiments, we explored how pigeons use edges, corresponding to orientation and depth discontinuities, in visual recognition tasks. In experiment 1, we compared the pigeon's ability to recognize line drawings of four different geons when trained with shaded images. The birds were trained with either a single view or five different views of each object. Because the five training views had markedly different appearances and locations of shaded surfaces, reflectance edges, etc, the pigeons might have been expected to rely more on the orientation and depth discontinuities that were preserved over rotation and in the line drawings. In neither condition, however, was there any transfer from the rendered images to the outline drawings. In experiment 2, some pigeons were trained with line drawings and shaded images of the same objects associated with the same response (consistent condition), whereas other pigeons were trained with a line drawing and a shaded image of two different objects associated with the same response (inconsistent condition). If the pigeons perceived any correspondence between the stimulus types, then birds in the consistent condition should have learned the discrimination more quickly than birds in the inconsistent condition. But, there was no difference in performance between birds in the consistent and inconsistent conditions. In experiment 3, we explored pigeons' processing of edges by comparing their discrimination of shaded images or line drawings of four objects. Once trained, the pigeons were tested with planar rotations of those objects. The pigeons exhibited different patterns of generalization depending on whether they were trained with line drawings or shaded images. The results of these three experiments suggest that pigeons may place greater importance on surface features indicating materials, such as food or water. Such substances do not have definite boundaries cued by edges which are thought to be central to human recognition.     

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.     

Visual gap and offset discrimination and its relation to categorical identification in brief line-element displays

Visual processing was investigated in judgments of relative line position. Stimulus continua were generated by bisecting a straight line and displacing the segments. Experiment 1 measured discrimination of pairs of longitudinally displaced segments at equal steps along the continuum. At long (2 s) durations discrimination fell smoothly, but at short (100 ms) durations it was sharp-peaked. In Experiment 2 the short-duration stimuli were labeled with subsets of the labels no gap, just a gap, and more than just a gap. Theoretical discrimination performances were computed and the one based on no gap and just a gap closely fitted observed performance. Experiments 3 and 4 were similar to 1 and 2, with lateral replacing longitudinal displacement. Similar "categorical" performance was obtained. It was concluded that there are discrete mechanisms for early detection of relative line position and that 2 labels can be used to characterize performance in each direction.     

Retrospective coding in pigeons' delayed matching-to-sample

In this study we examined how coding processes in pigeons' delayed matching-to-sample were affected by the stimuli to be remembered. In Experiment 1, two groups of pigeons initially learned 0-delay matching-to-sample with identical comparison stimuli (vertical and horizontal lines) but with different sample stimuli (red and green hues or vertical and horizontal lines). Longer delays were then introduced between sample offset and comparison onset to assess whether pigeons were prospectively coding the same events (viz., the correct line comparisons) or retrospectively coding different events (viz., their respective sample stimuli). The hue-sample group matched more accurately and showed a slower rate of forgetting than the line-sample group. In Experiment 2, pigeons were trained with either hues or lines as both sample and comparison stimuli, or with hue samples and line comparisons or vice versa. Subsequent delay tests revealed that the hue-sample groups remembered more accurately and generally showed slower rates of forgetting than the line-sample groups. Comparison dimension had little or no effect on performance. Together, these data suggest that pigeons retrospectively code the samples in delayed matching-to-sample.     

Degree of representation of the matching concept in pigeons (Columba livia)

In Experiment 1, 12 pigeons (Columba livia) were trained on a simultaneous matching-to-sample task with 2 stimuli and then tested with 2 novel stimuli. Half of the birds were trained with a fixed ratio schedule requirement of 1 (FR1) or 20 (FR20) pecks on the sample stimulus. None of the birds showed any evidence of concept-mediated transfer. In Experiment 2, 12 pigeons were trained with 3 stimuli and then tested with the same novel stimuli used in Experiment 1. Half of the birds in each group were trained with either an FR1 or FR20 requirement on the sample stimulus. Two of the FR20 birds showed high levels of transfer to the novel stimuli similar to that of monkeys in a previous study.     

Pigeons (Columba livia) learn to link numerosities with symbols

After responding to each element in varying, successive numerosity displays, pigeons (Columba livia) had to choose, out of an array of symbols, the symbol designated to correspond to the preceding number of elements. After extensive training, 5 pigeons responded with significant accuracy to the numerosities 1 to 4, and 2 pigeons to the numerosities 1 to 5. Several tests showed that feedback tones accompanying element pecks, the familiarity of element configurations, and the shape of the elements were not crucial to this performance. One test, however, indicated that the number of pecks issued to the elements was important for numerosities above 2. An additional test confirmed that the birds chose the symbol that corresponded to a particular numerosity rather than the positions that the symbols had held during training.     

Transfer of oddity-from-sample performance in pigeons

Four pigeons were trained on a modified three-key oddity-from-sample task in which an observing response to the sample (center-key) stimulus lighted a single comparison (side-key) stimulus. If the comparison stimulus was different from the sample stimulus, a single peck to the lighted comparison was reinforced. If the comparison and sample stimuli were identical, the pigeons had to refrain from pecking the comparison for 4.6 seconds to terminate the matching comparison and to produce immediately a nonmatching comparison on the remaining side key. Each peck to the matching comparison reset the 4.6-second delay interval. Three hues were used during acquisition. During tests for transfer of the oddity performance, two novel hues were substituted either individually or together for one or two of the original training hues. For three birds, latencies to novel nonmatching hues were identical to baseline nonmatching latencies. Latencies to novel matching hues were shorter than baseline matching latencies but were consistently longer than novel nonmatching latencies. These transfer data demonstrate that the pigeons learned the oddity concept.     

Transfer to intermediate forms following concept discrimination by pigeons: chimeras and morphs

Two experiments examined pigeons' generalization to intermediate forms following training of concept discriminations. In Experiment 1, the training stimuli were sets of images of dogs and cats, and the transfer stimuli were head/body chimeras, which humans tend to categorize more readily in terms of the head part rather than the body part. In Experiment 2, the training stimuli were sets of images of heads of dogs and cats, and the intermediate stimuli were computer-generated morphs. In both experiments, pigeons learned the concept discrimination quickly and generalized with some decrement to novel instances of the categories. In both experiments, transfer tests were carried out with intermediate forms generated from both familiar and novel exemplars of the training sets. In Experiment 1, the pigeons' transfer performance, unlike that of human infants exposed to similar stimuli, was best predicted by the body part of the stimulus when the chimeras were formed from familiar exemplars. Spatial frequency analysis of the stimuli showed that the body parts were richer in high spatial frequencies than the head parts, so these data are consistent with the hypothesis that categorization is more dependent on local stimulus features in pigeons than in humans. There was no corresponding trend when the chimeras were formed from novel exemplars. In Experiment 2, when morphs of training stimuli were used, response rates declined smoothly as the proportion of the morph contributed by the positive stimulus fell, although results with morphs of novel stimuli were again less orderly.