Figure-ground perception and random geometry

Constructs of random geometry were applied to the problem of figure-ground perception. Random-dot images of black and white dots with various area fractions and tesselations (square and triangular lattices) were used as stimuli. The constructs of random geometry are correlation functions of n-th order and some functionals defined on them. The only parameter which is independent of the tesselation used is the first-order correlation which is the area fraction. It was first conjectured and then experimentally verified that figureground perception is not affected by the various tesselations used. Thus, figure-ground phenomena depend only on the area fraction of the white and black dots in the stimulus. There is a perceptual bias for white, that is, figure-ground reversal is easiest at 40 percent white-black area fraction. It was also experimentally shown that size-constancy prevails in figure-ground perception, but brightnes s-constancy does not.     

Judgments of proportions

This study investigated the processes that underlie estimates of relative frequency. Ss performed 4 tasks using the same stimuli (squares containing black and white dots); they judged "percentages" of white dots, "percentages" of black dots, "ratios" of black dots to white dots, and "differences" between the number of black and white dots. Results were consistent with the theory that Ss used the instructed operations with the same scale values in all tasks. Despite the use of the correct operation, Ss consistently overestimated small proportions and underestimated large proportions. Variations in the distributions of actual proportions affected the extent to which Ss overestimated small proportions and underestimated large proportions in the direction predicted by range-frequency theory. Results suggest that proportion judgments, and by analogy probability judgments, should not be taken at face value.     

The concave cusp as a determiner of figure-ground

The tendency to interpret as figure, relative to background, those regions that are lighter, smaller, and, especially, more convex is well known. Wherever convex opaque objects abut or partially occlude one another in an image, the points of contact between the silhouettes form concave cusps, each indicating the local assignment of figure versus ground across the contour segments. It is proposed that this local geometric feature is a preattentive determiner of figure-ground perception and that it contributes to the previously observed tendency for convexity preference. Evidence is presented that figure-ground assignment can be determined solely on the basis of the concave cusp feature, and that the salience of the cusp derives from local geometry and not from adjacent contour convexity.     

Semantic context and figure-ground organization

Summary The semantic-priming paradigm was used to investigate the effect of semantic context on the latency to identify visually ambiguous and unambiguous targets. Ambiguous targets had reversible figure-ground organizations analogous to Rubin's vase-profiles picture in which the figural vase is shaped by the same contour that, if reassigned, shapes two figural profiles. The two organizations of the ambiguous targets were either a series of black irregular shapes or a familiar word in white letters; subjects were required to achieve the latter organization. Unambiguous target words were not reversible. Related primes, unlike unrelated primes, facilitated the identification of both types of target; but the magnitude of facilitation was greater for ambiguous targets. The results demonstrate that semantic context influences the speed of figure-ground organization and add perceptual ambiguity to the target manipulations that interact with context to affect encoding.     

The role of numerosity in processing nonsymbolic proportions

The difficulty in processing fractions seems to be related to the interference between the whole-number value of the numerator and the denominator and the real value of the fraction. Here we assess whether the reported problems with symbolic fractions extend to the nonsymbolic domain, by presenting fractions as arrays of black and white dots representing the two operands. Participants were asked to compare a target array with a reference array in two separate tasks using the same stimuli: a numerosity task comparing just the number of white dots in the two arrays; and a proportion task comparing the proportion of black and white dots. The proportion task yielded lower accuracy and slower response, confirming that even with nonsymbolic stimuli accessing proportional information is relatively difficult. However, using a congruity manipulation in which the greater numerosity of white dots could co-occur with a lower proportion of them, and vice versa, it was found that both task-irrelevant dimensions would interfere with the task-relevant dimension suggesting that both numerosity and proportion information was automatically accessed. The results indicate that the magnitude of fractions can be automatically and holistically processed in the nonsymbolic domain.     

When figure-ground segmentation modulates brightness: the case of phantom illumination

In the phantom illumination illusion, luminance ramps ranging from black to white induce a brightness enhancement on an otherwise homogeneous dark background. The strength of the illusion was tested with regard to the extension of the brightness inducing perimeter, surrounding the target area by manipulating the number of inducers (exp. 1) and the size of the inducers (exp. 2). Participants' task was to rate the difference in brightness between the target area and the background. Results show that the illusion occurs only when the target area is not completely segregated from the background by luminance ramps; vice versa, when the target area is delimited by a continuous gradient, it appears darker than the background. These findings suggest a major role of figure-ground organization in the appearance of the illusion. This hypothesis was tested in a rating task experiment with three types of target area shapes circumscribed by four types of edges: luminance contours, illusory contours, no contours, and ambiguous contours. Illusory contours, just as luminance contours, hinder the illusion and produce a darkening of the target area. A control experiment measured the brightness of the previous stimuli without luminance ramps: all configurations resulted in a darkening of the target area. Results from all experiments suggest that figure-ground segmentation plays a major role in the determination of both illumination and lightness in stimuli with luminance gradients.     

Determinants of perceived surface transparency in two-dimensional achromatic textured patterns

Achromatic transparency in 2-D surfaces composed of three adjacent areas, one created from the others, occurs when in the created area it is possible to see the two colours of the adjacent areas. Displays with two white and black intersecting bars were produced to verify the possibility of perceiving transparency in the intersection area when this was subdivided into a different number of white and black diamonds. By increasing the spatial frequency of the elements in the intersection area we investigated the impression of transparency, and found that (i) the impression of transparency increases up to a spatial frequency of 10 cycles deg(-1) and then levels off, (ii) there is a transition from a perception of transparency in which white and black are well discriminable to a perception in which a perceptually uniform grey is seen, (iii) the perception of transparency occurring when colours are discriminated seems consistent with Helson's area-luminance hypothesis [1964, in Adaptation Level Theory (New York: Harper & Row) pp 282-292], according to which the increase of spatial frequency determines the transition from the phenomenon of contrast to that of assimilation.     

Perception of motion in equiluminous kinematograms

Two fields of random dots that were identical except for a slight shift in a central square region were presented in rapid alternation. This produced a vivid impression of a square moving back and forth above the background. When the kinematogram is presented in equiluminous red/green, the motion of the central region can still be seen, although over a narrower range of alternation rates, interstimulus intervals, and displacements than for black/white presentation. The perception of motion for equiluminous stimuli indicates that colour and motion can be analyzed conjointly by the visual system. However, as originally reported by Ramachandran and Gregory, the segregation of the oscillating central square from the background is lost at equiluminance. This segregation process therefore appears to be colour-blind.     

Perceptual grouping between successively presented stimuli and its relation to visual simultaneity and masking

Summary A 4×4 matrix of dots was presented by a tachistoscope. The first and the third columns of the stimulus pattern were presented first for 20 ms and then the second and the fourth columns were presented for 20 ms with SOAs of 0 to 170 ms. Luminous dots on a dark background, black dots on a white background, and black dots and small outline circles on a white background were used in Experiments I, II, and III, respectively. In each experiment, 5 Ss were asked to report whether the stimulus dots were seen as 4 horizontal rows or 4 vertical columns, and whether they were seen simultaneously or successively. The experimental results showed that the time range of successive grouping was nearly the same as that of perceptual simultaneity in Experiments I and II, but was much greater than the latter in Experiment III where the similarity factor favored successive grouping. Successive grouping generally occurred in the same time range with visual masking of dot patterns by random dots, temporal organization of patterns from successive stimulus elements. But the time range was generally wider than those of contrast reduction caused by temporal luminance summation, metacontrast, and apparent movement.A part of this study was conducted during the junior author's stay at Chiba University and presented by him at the 20th International Congress of Psychology at Tokyo, August 13–19, 1972 (Yamada and Oyama, 1972)     

The perception of assimilation and brightness contrast as derived from code theory

It is shown that assimilation and brightness contrast effects are evoked by structural aspects of patterns. In a pilot experiment, variously shaped gray patterns were used as stimuli. The backgrounds used with each of these shapes were identical: half black and half white. If the gray area against the black part was judged to be more black than the gray area against the white part, an assimilation effect will have occurred; when the reverse occurred, this was called a contrast effect. The task was to rank-order the stimuli on the assimilation-contrast scale. It is argued that the two effects are due to two different interpretations, each derivable from a different code of a pattern. The simpler the contrast code is with respect to the assimilation code, the more it will be perceptually preferred. In the specification of pattern complexity, structural information theory was used. A significant correlation was discovered between the theoretical preference for the contrast interpretation and the contrast preference of subjects.     

The reversed Müller-Lyer illusion and figure-ground organization theory.

When the shaft is shortened and reaches neither of the vertices of the two pairs of wings, a reversed Müller-Lyer illusion is observed: a shaft between inward-pointing wings appears to be longer than a shaft between the outward-pointing wings. In this paper it is examined whether this illusion can be explained in terms of figure-ground organization. A circle was used as the focal area, instead of a shaft or a pair of dots, so that the figure-ground character could be seen more definitely in this focal area. The apparent size of the focal circle was measured under different conditions with three variables (enclosure, wings direction, and depth). The focal circle appeared to be largest in the condition where the circle should appear most readily as a hole, ie in the single, wings-in, space condition. The circle appeared to be smallest in the condition where the circle should appear most readily as a disc, ie in the separate, wings-out, object condition. This is consistent with an explanation of the usual, as well as the reversed, Müller-Lyer illusion in terms of figure-ground organization theory.     

Visual perception of touchdown point during simulated landing

Experiments examined the accuracy of visual touchdown point perception during oblique descents (1.5 degrees -15 degrees ) toward a ground plane consisting of (a) randomly positioned dots, (b) a runway outline, or (c) a grid. Participants judged whether the perceived touchdown point was above or below a probe that appeared at a random position following each display. Although judgments were unacceptably imprecise and biased for moving dot and runway displays, accurate and unbiased judgments were found for grid displays. It is concluded that optic flow per se does not appear to be sufficient for a pilot to land an airplane and that the systematic errors associated with optic flow under sparse conditions may be responsible for the common occurrence of landing incidents in so-called "black hole" situations.     

Lower region: a new cue for figure-ground assignment

Figure-ground assignment is an important visual process; humans recognize, attend to, and act on figures, not backgrounds. There are many visual cues for figure-ground assignment. A new cue to figure-ground assignment, called lower region, is presented: Regions in the lower portion of a stimulus array appear more figurelike than regions in the upper portion of the display. This phenomenon was explored, and it was demonstrated that the lower-region preference is not influenced by contrast, eye movements, or voluntary spatial attention. It was found that the lower region is defined relative to the stimulus display, linking the lower-region preference to pictorial depth perception cues. The results are discussed in terms of the environmental regularities that this new figure-ground cue may reflect.     

Perceptions of depth elicited by occluded and shearing motions of random dots

A computer-controlled display of random dots was used to study perceptions of depth. In this display, a field of stationary random dots surrounded a rectangular area in which random dots moved with uniform velocity in a single direction. The boundaries of this rectangle did not move. When dot motion was perpendicular to the longer boundary of the rectangle (occluded motion), the rectangle seemed to be behind the stationary background surround. Motion parallel to the longer boundary of the rectangle (shearing motion) made it appear in front of the surround. The relative lengths of the sides of the rectangle determined which effect predominated. Thus, for motion perpendicular to the long axis of the rectangle the occlusion predominated and naive subjects reported that the central area seemed farther away than the surround. For shearing motion parallel to the long axis, the subjects reported that the rectangle was closer than the surround and the strength of both effects also depended on the length-to-width ratio of the rectangle. If there was occluded motion along the long axis, as the length-to-width ratio increased so did the likelihood that subjects would report seeing the rectangle behind the surround. Conversely, with shearing motion along the long axis, increasing the length-to-width ratio increased the likelihood that the rectangle would appear unambiguously in front of the surround. Some subjects integrated the two cues with the resulting perception being a rotating cylinder. The occlusion effect was stronger than the shearing effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

The reference frame of figure-ground assignment

Figure-ground assignment involves determining which visual regions are foreground figures and which are backgrounds. Although figure-ground processes provide important inputs to high-level vision, little is known about the reference frame in which the figure's features and parts are defined. Computational approaches have suggested a retinally based, viewer-centered reference frame for figure-ground assignment, but figural assignment could also be computed on the basis of environmental regularities in an environmental reference frame. The present research used a newly discovered cue, lower region, to examine the reference frame of figure-ground assignment. Possible reference frames were misaligned by changing the orientation of viewers by having them tilt their heads (Experiments 1 and 2) or turn them upside down (Experiment 3). The results of these experiments indicated that figure-ground perception followed the orientation of the viewer, suggesting a viewer-centered reference frame for figure-ground assignment.     

Inhibitory competition between shape properties in figure-ground perception

Theories of figure-ground perception entail inhibitory competition between either low-level units (edge or feature units) or high-level shape properties. Extant computational models instantiate the 1st type of theory. The authors investigated a prediction of the 2nd type of theory: that shape properties suggested on the ground side of an edge are suppressed when they lose the figure-ground competition. In Experiment 1, the authors present behavioral evidence of the predicted suppression: Object decisions were slower for line drawings that followed silhouettes suggesting portions of objects from the same rather than a different category on their ground sides. In Experiment 2, the authors reversed the silhouette's figure-ground relationships and obtained speeding rather than slowing in the same category condition, thereby demonstrating that the Experiment 1 results reflect suppression of those shape properties that lose the figure-ground competition. These experiments provide the first clear empirical evidence that figure-ground perception entails inhibitory competition between high-level shape properties and demonstrate the need for amendments to existing computational models. Furthermore, these results suggest that figure-ground perception may itself be an instance of biased competition in shape perception.     

Children with spina bifida perceive visual illusions but not multistable figures

We compared 32 children with spina bifida and 32 age-matched controls on two classes of illusory perception, one involving visual illusions and the other, multistable figures. Children with spina bifida were as adept as age peers in the perception of visual illusions concerned with size, length, and area, but were impaired in the perception of multistable figures that involved figure-ground reversals, illusory contours, perspective reversing, and paradoxical figures. That children with spina bifida reliably perceive illusions that rely on inappropriate constancy scaling of size, length, and area suggests that their brain dysmorphologies do not prevent the acquisition of basic perceptual operations that enhance the local coherence of object perception. That they do not perceive multistable figures suggests that their visual perception impairments may involve not object processing so much as poor top-down control from higher association areas to representations in the visual cortex.     

Color charts, esthetics, and subjective randomness

Color charts, or grids of evenly spaced multicolored dots or squares, appear in the work of modern artists and designers. Often the artist/designer distributes the many colors in a way that could be described as "random," that is, without an obvious pattern. We conduct a statistical analysis of 125 "random-looking" art and design color charts and show that they differ significantly from truly random color charts in the average distance between adjacent colors. We argue that this attribute generalizes results in subjective randomness in a black/white setting and gives further evidence supporting a connection between subjective randomness and what is esthetically pleasing.     

In sight but out of mind: do competing views test the limits of perception without awareness?

Over a century's worth of research suggests that "perception" without awareness is a genuine phenomenon. However, relatively little research has explored the question of whether all visually presented information activates representations in long term memory without awareness. Two experiments explored the use of a figure-ground display consisting of competing views in which one view dominates the other such that subjects are (initially) unaware of the non-dominant view. Neither experiment provided evidence that the non-dominant view (an embedded word) activated its representation in long term memory when the subject failed to report being aware of the embedded word in that priming was not seen on a subsequent stem completion task. In contrast, priming was seen when subjects reported being aware of the embedded word. It is suggested that two competing figure-ground relations are not concurrently computed unconsciously.