Donald S. Blough

Spectral Sensitivity in the Pigeon

Measurements of light- and dark-adapted absolute thresholds were obtained from three pigeons at 15 wavelengths ranging from 380 mμ to 700 mμ. Pecking responses caused a stimulus patch to fluctuate in intensity up and down across the pigeon’s threshold, and a record of the intensity provided the sensitivity data. The sensitivity of the birds was followed throughout a period of 80 min following a standard pre-exposure to white light. Four complete dark adaptation curves were obtained from each bird at each wavelength. Spectral sensitivity functions derived from these curves place the photopic maximum at 560–580 mμ, and the scotopic maximum at about 500 mμ. The scotopic function is fitted closely by aphakic human data. The photopic function shows inflections that may be related to similar inflections in corresponding human curves. The functions are quite similar to those found in electrophysiological studies of the pigeon eye. They also correspond rather well to the absorption spectra of chicken rhodopsin and iodopsin.

Pigeons see the Ponzo illusion

Three experiments were performed to address factors contributing to the Ponzo illusion. In Experiment 1, pigeons learned to peck at the longer of two bars in varying line contexts. When these lines converged, the birds had difficulty in learning several patterns in which a Ponzo illusion would reduce the perceived difference. In Experiment 2, the subjects chose one response if a stimulus bar was longer than a predetermined length and a second response for shorter bars. The subjects were more likely to choose “long” as the stimulus bar approached the apex of converging lines. These results suggested that pigeons experience the Ponzo illusion. In Experiment 3, the effects of contexts that did and did not form a texture gradient were compared. The magnitude of the illusion did not differ according to context lines. This result failed to support a perspective theory of the Ponzo illusion.

Discrimination of letters and random dot patterns by pigeons and humans.

In a three-choice discrimination task, pigeons learned to distinguish each letter of the alphabet from all the other letters and each of 16 random dot patterns from all the others. Discrimination errors were used to generate a matrix of interletter and interpattern similarities. Human estimates of letter similarity were obtained from the literature, and human subjects rated the similarity of the dot patterns. Pigeon and human performances were described and compared through correlation, multidimensional scaling, and cluster analysis. Fits of the data by simple-feature and template models were computed and compared. The correlation between pigeon and human similarity matrices was .68 for letters and .72 for dot patterns. The other analyses revealed broadly similar patterns of results from the two species but suggested also that, relative to human data, the best fits to the pigeon data required fewer dimensions, fewer features, and fuzzier templates. There was some indication that pigeon discriminations depended on relatively simple features, and several of these were tentatively identified. The different methods employed might have influenced these apparent differences between pigeons and humans, but, overall, the results suggest considerable cross-task and cross species generality in the processing of these simple forms.

SOME EFFECTS OF DRUGS ON VISUAL DISCRIMINATION IN THE PIGEON

Joseph V. Brady, elsewhere in these pages, has mentioned some of the operant conditioning techniques that are being developed to assess the behavioral effects of drugs. In this paper I shall describe some preliminary results obtained with still another method of this sort. Both psychotic behavior and drug-induced behavior are often characterized by a lack of appropriateness to environmental stimuli. To put it in another way, stimuli appear to have less control over behavior than usual. The present method is specifically concerned with the stimulus control of behavior and druginduced changes in this control. The procedure derives from B. F. Skinner’s operant-conditioning methodology. It is rather complex and will be analyzed in detail in a future paper. ’

Form perception and attention in pigeons

This report selectively reviews the authors’ research on stimulus control by visual forms. Most studies employed visual search of computer-generated displays, and the main behavioral measure was search reaction time. Pigeons classify simple line forms much as do humans, and there was some evidence for feature extraction, though a search asymmetry probe failed to identify several putative features. Dimensional analysis revealed a new quantitative relation between similarity and the probability of detecting a target. It also identified candidates for integral and separable stimulus dimensions. Similarity and set-size variables influenced the extent to which attention was engaged. Further studies addressed the manner in which attention is focused, revealing relationships between priming and search images. In these cases, attention to a selected form is probably driven by a top-down assessment of predictability.

Feature-based search asymmetries in pigeons and humans

Pigeon and human subjects searched for one target item amidst a number of identical distractors. Simple line forms were used. The target differed from the distractors only in terms of the presence or absence of a feature (a line or a gap); in some experimental series, the feature was present in the target; in others, the feature was in the distractors. The pigeons pecked at the target; the human subjects either reported the presence of the target or pointed to it with a light pen. The time between display onset and this response was recorded. Varied across experimental conditions were the number of distractors in the display, the nature of the stimulus forms, and certain procedural parameters; five conditions were run with pigeons and three with humans. Under all test conditions, the results from the human subjects replicated the previously reported search-asymmetry effect. That is, search speed was greater and decreased less with display size when the target bore the feature (line or gap) than when the distractors bore the feature; both yes/no and localization-response conditions yielded this effect. However, pigeons failed to show search asymmetry; neither line nor gap in a target facilitated search. The results suggest that early visual processing differs for pigeons and humans, that pigeon features differ from human features, or that search asymmetry was eliminated by the long practice given the pigeons.

Effects of the inclination of context lines on perception of the Ponzo illusion by pigeons

Pigeons discriminated the length of a bar located between two context lines. Responses to one key were reinforced when the bar was longer than a predetermined length, and those to the other key were reinforced when the bar was shorter. The inclination of the context lines was systematically varied from 54.6° (converging upward) to 125.4° (converging downward). Five out of 6 subjects tended to report “long” when the bars were located near the apex of the context lines, regardless of whether the context lines were oriented upward or downward. The magnitude of the illusion varied almost linearly with the ratio of the length of the stimulus bar to the gap between the bar and the context lines. This relationship held equally for upward- and downward-converging context lines.

Quantitative relations between visual search speed and target-distractor similarity.

Three pigeons worked on visual search tasks in which arrays of 32 small forms appeared in computer-driven displays. Their task was to peck at a unique target among identical distractors; the main dependent variable was the speed with which this target was found. The target and distractor forms for each trial were chosen from a set of forms (usually 16); in most experiments, all possible pairs of forms appeared in each experimental session. Six experiments were run; each yielded a matrix of mean reaction times (RTs), one for each of the form pairs. Such matrices were subject to multidimensional scaling, from which best-fitting interform distances were determined. In four experiments with disparate form sets, Euclidean distance between pairs of forms in multidimensional space was found to approximate an exponential decay function of RT-K, whereK was a constant set for each subject. Two additional experiments showed that RT distributions for single-form pairs were well fit by the convolution of a fixed distribution, approximately Gaussian, and an exponential distribution whose decay parameter varied with interform similarity. The exponential decay parameter may be taken to reflect momentary detection probability, and interform distance in multidimensional space may be taken to measure similarity. Thus, the data as a whole suggest an exponential relation between the probability of detecting a target and the similarity of that target to its surrounding distractors. This relationship is analogous to Shepard’s law of generalization, which states an exponential relation between response probability and similarity.

Odd-item search by pigeons: Method, instrumentation, and uses

In odd-item visual search, subjects confront a display on which a number of stimulus items appear. All but one of these items are identical; the subject must respond to the one item (the target) that in some way differs from all the others (the distractors). The time required to find the target reflects the similarity between the target form and the distractor form. A matrix of search times for all possible pairs of a set of 20 or more items can be obtained in a single session. Such similarity matrices may reflect stimulus features, dimensions, and categories, among other things. A method is described through which pigeons learn odd-item search rapidly and perform with high accuracy despite the appearance of each form as a target on some trials and as a distractor on others. The paper also describes the essential apparatus and exemplifies displays and data.

The discrimination of mirror-image forms by pigeons

Four experiments investigated the discriminability of mirror-image forms with pigeons. Experiments 1–3 used U-shaped forms, oriented up, down, right, and left. The first experiment found non-mirror images better discriminated than mirror images on a matching-to-sample task. Experiment 2 found up-down reversals better discriminated than left-right mirror images on a three-key simultaneous discrimination task. Experiment 3 replicated this result with a single stimulus presentation method on a go/no-go discrimination. The final experiment repeated Experiment 2 with F-shaped forms whose mirror images could not be produced by a 180-deg rotation, but only by a reflection about the horizontal or vertical axis. These results are consistent with most findings for human and nonhuman subjects, but contrary to a recent suggestion by Hollard and Delius (1982) that mirror-image forms are distinct for pigeons.