Real and subjective contour Poggendorff illusions: no differences based on eye color

Measures of illusion magnitude for a real contour and two subjective contour Poggendorff figures were obtained from a large number of dark-eyed and light-eyed undergraduates. Illusion magnitude varied as a function of figure type. However, eye color neither had a significant overall effect on illusion magnitude, nor did it interact with the figure-type variable. These data indicate that the presence of intersecting lines is not the basis of the iris-pigmentation effect reported by Coren and Porac.     

The effects of display and observer strategy variables on Bourdon assimilation illusions

When a solid (filled-in) triangle is mirror-reflected about its vertex such that one angle arm and its reflection form a straight edge, this straight edge appears as a chevron bent in the direction of the other sides of the triangles, an effect directionally opposite to the well-known tilt illusion that occurs with outline angles. It has been proposed that these negative solid-angle illusions (assimilation effects) which occur in dual-angle (Bourdon-type) displays result from a failure to discriminate between the test (judged) edges and the bisectors of the solid angles. In dual-angle outline displays, near-zero effects have been attributed to the availability of collinearity cues. These hypotheses were tested in two experiments in which cues to collinearity were reduced by inserting gaps between the angles and in which collinearity information was increased by adding thick 'necks' to the displays. The results are consistent with predictions and implicate not only the nature of the angle display, but also the way in which observers perform the matching task in the production of assimilation effects.     

In search of closure: subjective contour illusions, Gestalt completion tests, and implications

This study investigates the construct validity of perceptual closure tests (CTs), and isolates a common processing demand from the right-hemisphere. Sixty-seven patients with focal unilateral lesions (34 right side, 33 left side), and 80 control subjects participated. Multivariate analyses indicated that there was substantial age-related variance in all CTs, while sex was variably significant; CTs are not uniform in their discriminating ability, and hence factorially complex; and their specifically right hemisphere-sensitive dimension was subjective contour illusions, and interestingly unrelated to facial discrimination ability. The methodological and theoretical implications are discussed.     

Determinants of the rod-and-frame effect: Role of organization and subjective contour

In Experiment 1, performance on a conventional (full) rod and frame was compared with that on three other displays. These included a closure condition in which only the corners of a luminous frame were present, a subjective contour condition in which the rod was seen against ablack square subjective surface, and a pattern condition in which four luminous disks were arranged to form the corners of a square pattern. In all cases, a square-like form was perceived. The rod-and-frame effect (RFE) was greatest on the full frame, which differed significantly from all the rest. Closure produced the next greatest influence, while the responses to subjective and pattern conditions did not differ from each other. Experiment 2 showed that a subjective frame was no more effective than the subjective surface of Experiment 1, and verified the importance of 90-deg corner elements in the rod-and-frame display. The conclusion drawn was that equivalent form organizations are not sufficient to produce equivalent levels of the RFE, but certain luminance-difference contours appear to be essential.     

Determinants of stability in the perception of subjective contour

The present study constituted an initial experimental effort to examine the fragmentation characteristics of subjective contours within the photopic and upper 5cotopic ranges of illumination. Four stimulus factors known to influence the visibility of subjective contours—target luminance, inducing area size and contrast, and contour orientation—were examined. Results indicated that subjective contours are indeed unstable perceptual phenomena. On the average, fragmentation or fading occurred after only 15 sec of observation, and some farm of stimulus outage was present for 28% of the viewing time of each stimulus. Fragmentation latency was significantly shorter and total time in fragmentation longer for diamond than for square contours, and total time in fragmentation varied inversely with inducing-area size. Fragmentation tendedto occur in whole units rather than in isolated elements, a result reminiscent of the fading of real Contours under impoverished viewing conditions.     

Determinants of the Bourdon effect

The Bourdon illusion is the apparent inward bending of straight, collinear edges in a solid figure consisting of two elongated triangles meeting at their apexes. This effect was investigated in five experiments. In the first and third experiments, it was shown that the apparent bending is greatest when the apical angles are about 12 deg and the axis of the figure is oriented at about 22 deg from the vertical. The second experiment was a control involving visual acuity for angular departures of two lines from collinearity and served as a basis of selection for subjects in Experiments 3, 4, and 5. Experiments 4 and 5 showed that the illusion occurs strongly in a solid (“filled in”) figure but is notably smaller in outline figures of the same size and shape. It tends to be negative in outline figures with boundaries formed by continuous and broken lines. The relationship between the Bourdon illusion and the “negative” Zöllner illusion is considered.     

Determinants of subjective novelty

Subjects were exposed to sequences of colored shapes and required torate each stimulus on a 7-point scale for “novelty.” The sequences were such that 12 hypotheses regarding determinants of subjective novelty could be tested by orthogonal comparisons. The results of two experiments confirmed eight of the hypotheses.     

Attentional processing and the subjective contour illusion

Leading explanations of the subjective contour illusion can be classified as being either "bottom-up" or "top-down." Bottom-up explanations assert that peripheral, physiological mechanisms often associated with the perception of real contours also account for subjective contour (SC) perception. In contrast, top-down explanations posit a more central locus of SC perception and are formulated on a molar, psychological level. A major aspect of bottom-up perceptual processing is that it is largely automatic. On the other hand, top-down processing implies a greater role for selective attention. In an effort to distinguish between bottom-up and top-down accounts of SC perception, the present investigation used a dual-task paradigm to test the relative attentional demands of real contour perception versus SC perception. In the primary task, subjects made speeded same-different discriminations of either paired SC forms or their real contour analogues. Half the subjects performed this primary task in conjunction with a six-digit short-term memory load secondary task. If subjective forms indeed impose a greater limited-capacity processing load than real forms, then the need to share processing capacity with a secondary task was expected to produce a greater increment in reaction time (RT) for subjective relative to real forms. The results indicated that the expected enhanced RT increment for subjective relative to real forms with the addition of a concurrent memory load was limited to same trials. This result implies that the nature of response indicators must be considered in assessing capacity requirements with the sort of dual-task paradigm used in the present investigation. Nevertheless, the fact that the increment in same RT with the addition of a concurrent memory load was greater for subjective relative to real forms accords with expectations derived from the notion that the perception of SCs is more attention demanding than that of real contours. If the interpretation of the present study is correct, then a comprehensive theory of SC perception will most likely be formulated within the top-down perspective of conceptually driven visual information processing.     

Yang's iris illusion: External contour causes length‐assimilation illusions1

The Delboeuf illusion and the Ebbinghaus illusion (also known as the Titchener illusion) demonstrate that an external contour can lead to size‐assimilation and size‐contrast perception. This paper explores a novel illusion, revealing that neighboring external contours can also lead to a distortion in length perception. The illusion was originally discovered from a face stimulus (Experiment 1) in which a face was depicted alongside its mirror image so as to make the four irises absolutely equidistant. The distance between the middle two irises was underestimated in Asian faces, but overestimated in Caucasian faces. The illusion was also maintained when the facial stimuli were replaced by line drawings of eyes (Experiment 2). However, the illusion vanished when the irises were presented alone. Further scrutiny of the differences in facial characteristics between Asian and Caucasian faces reveals that the illusion might be elicited by the relative position of the eye shapes. This hypothesis was confirmed in Experiment 3, in which the distances between the eye shapes and the irises were manipulated.     

Contour: A hypermedia environment for teaching about subjective contours and other visual illusions

Contour is an interactive HyperCard stack for the Apple Macintosh computer, which presents users with a wealth of information (text, graphics, sound, and animation) concerning the history, theory, and methodology used in the research of a class of visual illusions calledsubjective contour illusions. The stack is divided into six sections (Introduction, Findings, Index, References, Map, and File Creator), which allow users to access differing types of information in several ways. As users navigate through the stack, they are presented with multimedia information that they can trace through author, theory, or type of illusion. The stack allows users to make notes to themselves on the information presented, to execute a literature search of the most relevant articles in the subjective contour literature, and to save them in text file for printing in a word processing program. The study of subjective contour illusions can be viewed as a microcosm of issues in the field of visual perception.     

The effects of illumination level and retinal size on the depth stratification of subjective contour figures

The apparent stratification in depth of subjective contour figures over their backgrounds was investigated as a function of illumination level, figure size, and viewing distance. Magnitude estimation, with a real contour figure serving as the modulus, was used to measure the stratification in depth of a subjective contour figure over its background. Illumination level and retinal size both had significant effects on the depth stratification of the subjective contour figures. The greatest apparent depth differences were obtained for figures of small retinal size under low levels of illumination. These results paralleled previous findings for judgments of subjective contour strength. Consequently, both contour clarity and depth stratification of subjective contour figures are affected in similar ways by illumination level, figure size, and viewing distance. The implications of this response coupling are discussed in terms of current theories of subjective contours.     

Everything is subjective under water surface,too: visual illusions in fish

Animal Cognition - The study of visual illusions has captured the attention of comparative psychologists since the last century, given the unquestionable advantage of investigating complex...     

Effects of contour proximity and lightness on Delboeuf illusions created by circumscribed letters

32 observers judged the size of a letter, either an "A" or an "S," which was surrounded by a circle. Both letters were overestimated, but larger surrounding circles reduced the illusion. Decreasing the lightness contrast of the surrounding circle relative to the central letter diminished the illusion. The results suggest that, like the Delboeuf illusion, these circumscribed letters illusions are produced by interactions among size-coding neurons.     

Response times to Ehrenstein illusions of varying subjective magnitude: complementarity of psychophysical measures

The following experiments investigate the effects of contrast polarity, inducer spacing, and inducer type on three dependent variables measuring the perception of an illusory surface in Ehrenstein figures: subjective magnitude, response time, and frequency of perception. It was found that response time generally decreased when the other two behavioral indicators increased. However, it was also shown that subjective magnitude provided more discriminating measures of relatively strong illusory percepts, whereas frequency of perception and response time provided more discriminating measures of relatively weak illusory percepts. The findings generally confirm earlier work on the effect of inducer spacing and contrast polarity on the perceived strength of brightness illusions, and in particular reveal the complementarity of subjective magnitude, response time, and frequency of perception as critical measures of configurational effects in the perceptual processing of these phenomena. Thanks are due to J. E. Hoffman, T. Parks, and an anonymous referee for their insightful comments and suggestions on an earlier version of this paper.     

Poggendorff and Müller-Lyer illusions: common effects

Previous investigations have shown that the magnitude of the Müller-Lyer illusion is a function of the linear and angular dimensions of the figure. If the Müller-Lyer and Poggendorff illusions share a common basis, then the magnitude of the Poggendorff illusion should similarly be a function of the analogous configural dimensions. A study is reported in which changes were made in the dimensions of the Poggendorff figure that are analogous to the dimensions of the Müller-Lyer figure: the length of the parallel components (analogous to the wings of the Müller-Lyer figure); the length of the intertransversal extent (analogous to Müller-Lyer shaft length); and the angle formed between the parallel components and the intertransversal extent (analogous to the angle of wing attachment in the Müller-Lyer figure). The relationship between the magnitude of the illusion and the dimensions of the Poggendorff figure was found to be generally in line with previous findings relating to the Müller-Lyer illusion. Adaptation-level theory and the positive-context model accommodate the major findings of the present study.     

Perceptual organization and apparent brightness in subjective contour figures

According to a number of theories subjective contours arise from brightness contrast and/or assimilation. The apparent brightness gradients generated by these effects are assumed to give rise to the perception of contours delineating the gradients. A study is reported in which naive observers were shown a subjective contour display and asked to report what they saw. They were then asked to judge whether the center or the surround of the display appeared brighter. Subjects whose reports indicated that they had seen the subjective contour figure showed an overwhelming preference for the center of the display being brighter than the surround. However, subjects who did not see the subjective contour figure did not differ significantly in their selection of the center over the surround. This finding presents difficulties for any theory which derives subjective contours from the apparent brightness difference.