Subjective contours in stereoscopic space

Stereoscopic depth and subjective contour clarity were manipulated by varying the type of monocular configuration as well as magnitude and direction of disparity. The clarity of the subjective contours was influenced significantly by both magnitude and direction of disparity and by the type of monocular configuration. Subjective contours were always less clear when the objective monocular contour was discontinuous regardless of disparity. Stereoscopic depth estimates varied directly with magnitude and direction of disparity; however, depth magnitude reports were truncated when the disparity was carried by discontinuously defined patterns.     

Visual inter-attribute contour completion

A new visual phenomenon, inter-attribute illusory (completed) contours, is demonstrated. Contour completions are perceived between any combination of spatially separate pairs of inducing elements (Kanizsa-like 'pacman' figures) defined either by pictorial cues (luminance contrast or offset gratings), temporal contrast (motion, second-order-motion or 'phantom' contours), or binocular-disparity contrast. In a first experiment, observers reported the perceived occurrence of contour completion for all pair combinations of inducing elements. In a second experiment they rated the perceived clarity of the completed contours. Both methods generated similar results contour completions were perceived even though the inducing elements were defined by different attributes. Ratings of inter-attribute clarity were no lower than in either of the two corresponding intra-attribute conditions and seem to be the average of these two ratings. The results provide evidence for the existence of attribute-invariant Gestalt processes, and on a mechanistic level indicate that the completion process operates on attribute-invariant contour detectors.     

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.     

Evidence relating subjective contours and interpretations involving interposition

Subjective contours, according to one theory, outline surfaces that are apparently interposed between the viewer and background (because of the disruption of background figures, sudden termination of lines, and other interposition 'cues') but are not explicitly outlined by intensity discontinuities. This theory predicts that if the cues are not interpreted as evidence of interposition, no intervening surface need be postulated, hence no subjective contours would be seen. This prediction, however, is difficult to test because observers normally interpret the cues as interposition evidence and normally see the subjective contours. Tests are reported on a patient with visual agnosia who is unable to make the usual interposition interpretations and unable to see subjective contours, but has normal ability to interpret standard visual illusions, stereograms, and in particular, stereogram versions of the standard subjective contour figures, which elicit to him strong subjective edges in depth (corresponding to the subjective contours viewed in the monocular versions of the figures.     

Retinal mechanisms in the perception of subjective contours: the contribution of lateral inhibition.

One mechanism frequently proposed for the creation of subjective contours and their related brightness effects involves lateral neural interactions on the retina, such as the lateral inhibitory effects that underlie brightness contrast. Subjective contour stimuli were displayed under an intermittent light source, with rapid onset and slow offset as has been shown to increase lateral inhibitory interactions by allowing summation of neural onset transients. A sample of forty subjects, using magnitude estimates, reported increased subjective contour clarity and brightness effects under these exposure conditions. The effects were larger for relative brightness differences than for contour visibility. It appears that this technique may have applications in exploring retinal contributions to other aspects of the perception of subjective contours.     

Occlusion cues contribute to orientation judgments of occlusion-defined contours

Occlusion cues defining a contour in a 2-D stimulus pattern were shown to contribute to the accuracy of orientation judgments of that contour. The stimulus pattern was altered so that the occlusion cues became ambiguous, by introducing a textured background suggesting transparency of the stimulus pattern. Orientation judgments then became significantly less accurate. This finding shows that occlusion cues in 2-D patterns can be behaviorally relevant, in addition to generating the subjective percept commonly known as an illusory contour. The disruptive effect of the textured background on orientation judgments remained when no texture elements were present in the vicinity of the contour. This suggests that the generation of occlusion-defined contours relies as much on an evaluation of the surfaces at either side of the contour as being opaque as it does on local encoding of occlusion cues close to the contour. Finally, orientation sensitivity measured with contours defined by other than occlusion cues was not altered after the introduction of a textured background.     

The effects of illumination level and retinal size on the apparent strength of subjective contours

The apparent strength of subjective contours was investigated as a function of illumination level, figure size, and viewing distance. Magnitude estimation, with a real contour standard as the modulus, was used to measure the perceived strength of the illusory contours. It was found that illumination level and retinal size are both powerful determinants of the apparent strength of subjective contours, generating magnitude estimates varying from 20% to 96% of the strength of the real contour modulus. Particularly strong subjective contours were reported for figures of small retinal size (1.2 to 4.8 deg) under very dim illumination (.10 log lx).     

The role of junctions in surface completion and contour matching

It has been suggested that contour junctions may be used as cues for occlusion. Ecologically, T-junctions and L-junctions are concurrent with situations of occlusion: they arise when the bounding contour of the occluding surface intersects with that of the occluded surface. However, there are other image properties that can be used as cues for occlusion. Here the role of junctions is directly compared with other occlusion cues--specifically, relatability and surface-similarity--in the emergence of amodal completion and illusory contour perception. Stimuli have been constructed that differ only in the junction structure, with the other occlusion cues kept unchanged. L-junctions and T-junctions were eliminated from the image or manipulated so as to be locally inconsistent with the (still valid) global occlusion interpretation. Although the other occlusion cues of relatability and surface similarity still existed in the image, subjects reported not perceiving illusory contours or amodal completion in junction-manipulated images. Junction manipulation also affected the perceived stereoscopic depth and motion of image regions, depending on whether they were perceived to amodally complete with a disjoint region in the image. These results are interpreted in terms of the role of junctions in the processes of surface completion and contour matching. It is proposed that junctions, being a local cue for occlusion, are used to launch completion processes. Other, more global occlusion cues, such as relatability, play a part at a later stage, once completion processes have been launched.     

Amodal completion, depth stratification, and illusory figures: a test of Kanizsa's explanation.

Subjective contours have been explained by Kanizsa as being a consequence of amodal completion of incomplete figures. According to the theory of amodal completion, figural incompleteness triggers the emergence of an illusory object superimposed on the gaps in the inducers, which in turn hide parts of the pattern, thus suggesting that the plane of the illusory object must always be seen to be above the plane of the inducers. A figure was created in which subjective contours are seen despite the fact that the perceived depth relationships run counter to that required by the theory of amodal completion. In four experiments, this depth relationship is confirmed by using direct and indirect measures which assess both registered and apprehended depth. By emphasizing a logical inconsistency in the explanation based on amodal completion, the results show that amodal completion, at least in Kanizsa-like patterns, cannot be considered as a causal factor for subjective contour figures.     

Rapid changes in visual-spatial attention distort object shape

Shifts of attention due to rapid cue onsets have been shown to distort the perceived location of objects, but are there also systematic distortions in the perceived shapes of the objects themselves from such shifts? The present study demonstrates that there are. In three experiments, oval contours were presented that varied in width and height. Two brief, bright white dots were presented as cues and were positioned horizontally or vertically either inside or outside the oval contour. Observers had to judge whether the oval was taller than wide. The results show that the perceived shape of an oval was changed by visual cues such that the oval contours were repelled by the cues (Exp. 1). This effect only occurred when the cues preceded the ovals, providing sufficient time between the presentations to attract involuntary attention (Exp. 2). Moreover, an explanation based on figural aftereffects was ruled out (Exp. 3).     

Properties of long-range illusory contours produced by offset-arcs

Researchers have used several different types of illusory contours to investigate properties of human perception. One rarely used illusory contour is a combination of the abutting grating and Kanizsa illusions. We call this the offset-arcs illusion and provide an empirical investigation of the illusion. Through a series of four experiments, using different methods of measurement, we show that changes to the phase of the abutting-grating part of the inducing stimulus can dramatically change the perceived strength and clarity of the long-range illusory contour. The easy manipulation of illusion strength should make the offset-arcs illusion applicable to a wide range of studies that utilize long-range illusory contours. The lack of a brightness component to the illusion should allow the offset-arcs illusion to help separate perceptual grouping from surface brightness effects that are often confounded in other illusory contours.     

How configurations of binocular disparity determine whether stereoscopic slant or stereoscopic occlusion is seen

A partially occluded contour and a slanted contour may generate identical binocular horizontal disparities. We investigated conditions promoting an occlusion resolution indicated by an illusory contour in depth along the aligned ends of horizontally disparate line sets. For a set of identical oblique lines with a constant width added to one eye's view, strength, depth, and stability of the illusory contour were poor, whereas for oblique lines of alternating orientations the illusory contours were strong, indicating a reliance on vertical size disparities rather than vertical positional disparities in generating perceived occlusion. For horizontal lines, occlusion was seen when the lines were of different lengths and absolute width disparity was invariant across the set. In all line configurations, when the additional length was on the wrong eye to be attributed to differential occlusion, lines appeared slanted consistent with their individual horizontal disparities. This rules out monocular illusory contours as the determining factor.     

The illusion of transparency and chromatic subjective contours

Subjective contours can be produced that include an illusion of edge and an extension of color throughout the area of the illusion. The phenomenological appearance is of a transparent colored shape in front of the background. Two explanations of this illusion are proposed. The first is that there is an assimilation of color analogous to brightness assimilation. The second is a variant of the stratification of depth theory of subjective contours. In it, the pattern elements lead to the illusion of a surface in front of the pattern elements. We thus predicted that an illusion of transparency would enhance the subjective contour, Metelli’s model of transparency was used to quantify our prediction, and it was found that the possibility of transparency was a powerful predictor of the chromatic subjective contour.     

Transparency: relation to depth, subjective contours, luminance, and neon color spreading

The perception of transparency is highly dependent on luminance and perceived depth. An image region is seen as transparent if it is of intermediate luminance relative to adjacent image regions, and if it is perceived in front of another region and has a boundary which provides information that an object is visible through this region. Yet, transparency is not just the passive end-product of these required conditions. If perceived transparency is triggered, a number of seemingly more elemental perceptual primitives such as color, contour, and depth can be radically altered. Thus, with the perception of transparency, neon color spreading becomes apparent, depth changes, stereoscopic depth capture can be eliminated, and otherwise robust subjective contours can be abolished. In addition, we show that transparency is not coupled strongly to real-world chromatic constraints since combinations of luminance and color which would be unlikely to arise in real-world scenes still give rise to the perception of transparency. Rather than seeing transparency as a perceptual end-point, determined by seemingly more primitive processes, we interpret perceived transparency as much a 'cause', as an 'effect'. We speculate that the anatomical substrate for such mutual interaction may lie in cortical feed-forward connections which maintain modular segregation and cortical feedback connections which do not.     

Depth separation and lateral interference

In four experiments, the perceptual interaction between an annulus and a Landolt C enclosed within it was investigated as a function of their perceived relative depth positions and of the perceived lateral distance between the inner edge of the annulus and the outer edge of the C. To permit facile and unconfounded manipulation of perceived depth, the stimuli were stereoscopic contours formed from dynamic random-lement stereograms. Either one or both stimuli were visible continuously. The effect of the annulus on the Landolt C was assessed by forced-choice recognition thresholds of the C and by judgments of its apparent clarity. The main results were: (1) For both threshold recognition and apparent clarity, perceived depth separation has a strong effect on the strength of perceptual interaction; (2) the effect is asymmetrical in that the stimulus perceived as in front of its partner and closer to the observer has greater perceptual potency; and (3) as spacing between the elements increases, perceptual interaction declines independently of depth position. The implications of these data for general theories of stimulus interaction in three-dimensional space are discussed.     

Displacement of the path of perceived movement by intersection with static contours

Observation of a moving dot gives rise to a perceived movement path, which has properties similar to those of real contours. If the dot crosses a line inclined to a horizontal direction of movement, it appears to undergo a vertical displacement. This displacement was found to be greatest for a line orientation of around 15° with respect to the movement. At other relative orientations, the size of the perceived displacement varied in the same manner as the perceived expansion of angles formed by intersection between static contours. Movement path distortions were measured with background fields like those that produce the Hering and Zöllner illusions with continuous lines. Illusory displacements of perceived movement were found to be equivalent to the static forms. The subjective contour formed by observation of movement can therefore give rise to illusions similar to those obtained with real lines.     

The perceived strength of illusory contours.

Illusory contours are not well understood, partially because a lack of physical substance complicates their specification via physical standards. One solution is to gauge illusory contours with respect to luminance-defined contours, which are easily quantified physically. Accordingly, we chose a metric (perceived contrast) that expresses illusory contour strength in terms of the physical contrast of luminance-defined contours. Using this metric, adult observers adjusted the contrast of a luminance-defined contour until it matched the perceived contrast of an illusory contour. Illusory contour length, inducer size, and inducer contrast all influenced illusory contour strength. The results are adequately explained via low-level visual processes. It appears that matching paradigms can be beneficial in quantitative studies of illusory contours.     

The perceived strength of illusory contours

Illusory contours are not well understood, partially because a lack of physical substance complicates their specification via physical standards. One solution is to gauge illusory contours with respect to luminance-defined contours, which are easily quantified physically. Accordingly, we chose a metric (perceived contrast) that expresses illusory contour strength in terms of the physical contrast of luminance-defined contours. Using this metric, adult observers adjusted the contrast of a luminance-defined contour until it matched the perceived contrast of an illusory contour. Illusory contour length, inducer size, and inducer contrast all influenced illusory contour strength.. The results are adequately explained via low-level visual processes. It appears that matching paradigms can be beneficial in quantitative studies of illusory contours.     

Subjective contours independent of subjective brightness

Two theories of subjective contours are distinguished according to the interrelationship of subjective contours and subjective brightness effects. In one view, subjective contours are illusory brightness gradients generated from grouped local brightness effects. In another view, subjective contours are the edges of subjective forms created on the basis of gestalt factors; subjective brightness is a secondary consequence of form perception. Two experiments which use rating scales to separate judgments of subjective contour and subjective brightness are presented. The first shows that subjects may judge contour to be strong when there is no subjective brightness gradient. In the second, gestalt grouping factors are shown to be more important than factors which should influence brightness according to local effects theories. Both experiments support the view that subjective brightness occurs through interactions at the level of form perception.     

Kanizsa-type subjective contours do not guide attentional deployment in visual search but line termination contours do

We used visual search to explore whether attention could be guided by Kanizsa-type subjective contours and by subjective contours induced by line ends. Unlike in previous experiments, we compared search performance with subjective contours against performance with real, luminance contours, and we had observers search for orientations or shapes produced by subjective contours, rather than searching for the presence of the contours themselves. Visual search for one orientation or shape among distractors of another orientation or shape was efficient when the items were defined by luminance contours. Search was much less efficient among items defined by Kanizsa-type subjective contours. Search remained efficient when the items were defined by subjective contours induced by line ends. The difference between Kanizsa-type subjective contour and subjective contours induced by line ends is consistent with physiological evidence suggesting that the brain mechanisms underlying the perception of these two kinds of subjective contours may be different.