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.     

The perception of subjective contours and neon color spreading figures in young infants

The goal of the present habituation—dishabituation study was to explore sensitivity to subjective contours and neon color spreading patterns in infants. The first experiment was a replication of earlier investigations that showed evidence that even young infants are capable of perceiving subjective contours. Participants 4 months of age were habituated to a subjective Kanizsa square and were tested afterward for their ability to differentiate between the subjective square and a nonsubjective pattern that was constructed by rotating some of the inducing elements. Data analysis indicated a significant preference for the nonsubjective pattern. A control condition ensured that this result was not generated by the difference in figural symmetry or by the local differences between the test displays. In the second experiment, infant perception of a neon color spreading display was analyzed. Again, 4-month-old infants could discriminate between the illusory figure and a nonillusory pattern. Furthermore, infants in a control group did not respond to the difference in symmetry and the local differences between two nonillusory targets. Overall, the results show that young infants respond to illusory figures that are generated by either implicit T-junctions (Experiment 1) or implicit X-junctions (Experiment 2). The findings are interpreted against the background of the neurophysiological model proposed by Grossberg and Mingolla (1985).     

Modal and amodal completion generate different shapes

Mechanisms of contour completion are critical for computing visual surface structure in the face of occlusion. Theories of visual completion posit that mechanisms of contour interpolation operate independently of whether the completion is modal or amodal--thereby generating identical shapes in the two cases. This identity hypothesis was tested in two experiments using a configuration of two overlapping objects and a modified Kanizsa configuration. Participants adjusted the shape of a comparison display in order to match the shape of perceived interpolated contours in a standard completion display. Results revealed large and systematic shape differences between modal and amodal contours in both configurations. Participants perceived amodal (i.e., partly occluded) contours to be systematically more angular--that is, closer to a corner--than corresponding modal (i.e., illusory) contours. The results falsify the identity hypothesis in its current form: Corresponding modal and amodal contours can have different shapes, and, therefore, mechanisms of contour interpolation cannot be independent of completion type.     

T-junctions,apparent depth,and perceived lightness contrast

Observers performed lightness matches for physically equivalent gray targets of a simultaneous lightness contrast display and displays in which both targets were on the same background. Targets either shared a common line-texture pattern with their respective backgrounds or did not. Results indicate that when targets share a line-texture pattern with their respective backgrounds, a contrast effect is obtained. However, when the target's pattern is different than the background's pattern, perceived contrast is significantly reduced and the target appears as a separate 3-D entity. This result applies to both vertically and horizontally oriented displays, to targets that are increments or decrements, and to line-texture patterns that are black or white. Line patterns that are shared by targets and backgrounds result in T-junctions that provide occlusion information. We conclude that targets and backgrounds perceived to be on separate planes because of T-junctions are less likely to be perceptually grouped together and that their luminance values are less likely to be compared with one another.     

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.     

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 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).     

Detection and discrimination of subjective contours defined by offset gratings

Three experiments were conducted to refine our understanding of the mechanisms that encode subjective contours. In Experiment 1, discrimination thresholds (stimulus onset asynchronies [SOAs] yielding 81% correct) were measured in a backward masking paradigm for subjective contours defined by offset gratings. For large apertures, thresholds increased as carrier frequency increased. For the smallest aperture, thresholds were a U-shaped function of carrier frequency. Experiment 2 showed that these threshold results were generally consistent with the rated strength of the subjective contours. Experiment 3 showed that detection thresholds (SOAs yielding 81% correct) again increased with carrier spatial frequency, increased for obliquely oriented carriers, and, for a particular frequency and orientation of the carrier, were lower when the subjective contour was orthogonal to the carrier. All of these results are well explained by a two-stage process in which a second-layer filter integrates the responses of end-stopped mechanisms to the terminators defining the subjective contour. In the model, the end-stopped mechanisms have low-pass sensitivity to carrier spatial frequency, and the sizes of the second-layer filters are proportional to the scale of the end-stopped mechanisms from which they draw their input.     

Stereopsis and subjective contours

Ten Ss rated perceived depth and contour clarity of figures containing binocularly disparate subjective contours. There was no tendency for stereoscopic depth cues to enhance the perceived clarity of subjective contours. Disparity cues that were incompatible with monocular depth cues reduced the depth sensation but did not affect contour clarity. Although subjective contours can be perceived stereoscopically, they are seen in less depth than real contours with the same degree of horizontal disparity.     

Perception of curves.

In 4 experiments, in which 66 subjects participated, the perception of 6 types of curved contours in two-dimensional shapes was studied. Random polygons and their curvilinear transformations were presented for detection under low-luminance contrast conditions, oddity-type discrimination problem solving, tachistoscopic identification, and identification involving visual acuity in distance vision. In all experiments curvature affected perception at statistically significant levels, but the extent of this effect was a function of (1) the locus and direction of curvature, (2) the level of compactness-jaggedness of the figure, and (3) the nature of perceptual task. Shapes with acute corners were more easily perceived than shapes with curved corners. Within these two classes of shapes, those with convex sides were perceived as having greater curvedness than those with concave contours. However, the degree to which curvature affected response was determined primarily by the nature of the perceptual task.     

Uniform connectedness and classical Gestalt principles of perceptual grouping.

We assessed whether uniform connectedness (UC; Palmer & Rock, 1994) operates prior to effects reflecting classical principles of grouping: proximity and similarity. In Experiments 1 and 2, reaction times to discriminate global letters (H vs. E), made up of small circles, were recorded. The small circles were respectively grouped by proximity, similarity of shapes, and by UC. The discrimination of stimuli grouped by similarity was slower than those grouped by proximity, and it was speeded up by the addition of UC. However, the discrimination of stimuli grouped by proximity was unaffected by connecting the local elements. In Experiment 3, similar results occurred in a task requiring discrimination of the orientation of grouped elements, except that the discrimination of stimuli grouped by UC was faster than that of those grouped by weak proximity. Experiment 4 further showed that subjects could respond to letters composed of discriminably separate local elements as fast as to those without separated local elements. The results suggest that grouping by similarity of shapes is perceived slower than grouping by UC, but grouping by proximity can be as fast and efficient as that by UC.     

Uniform connectedness and classical gestalt principles of perceptual grouping

We assessed whether uniform connectedness (UC; Palmer & Rock, 1994) operates prior to effects reflecting classical principles of grouping: proximity and similarity. In Experiments 1 and 2, reaction times to discriminate global letters (H vs. E), made up of small circles, were recorded. The small circles were respectively grouped by proximity, similarity of shapes, and by UC. The discrimination of stimuli grouped by similarity was slower than those grouped by proximity, and it was speeded up by the addition of UC. However, the discrimination of stimuli grouped by proxhnity was unaffected by connecting the local elements. In Experiment 3, similar results occurred in a task requiring discrimination of the orientation of grouped elements, except that the discrimination of stimuli grouped by UC was faster than that of those grouped by weak proximity. Experiment 4 further showed that subjects could respond to letters composed of discriminably separate local elements as fast as to those without separated local elements. The results suggest that grouping by similarity of shapes is perceived slower than grouping by UC, but grouping by proximity can be as fast and efficient as that by UC.     

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.     

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.     

Parallel discrimination of subjective contours defined by offset gratings

Recent physiological studies (von der Heydt & Peterhans, 1989) suggest that the orientation of subjective contours is encoded very early in the visual system (V2 in monkey). This result is seemingly at odds with existing psychophysical data which suggest that the detection of subjective contours involves selective attention. It is argued that certain subjective contours are registered in a reflexive (bottom-up) manner by the visual system but that selective attention may be needed to gain access to this representation. To assess this suggestion, a visual-search task was used in which subjects were to detect the presence of a horizontal (vertical) subjective contour (defined by offset gratings) in a variable number of vertical (horizontal) subjective contours (also defined by offset gratings). When there were no competing organizations within the display, detection was indeed independent of the number of nontarget distractors, that is, selective attention was unnecessary. In a second experiment, we found that a curved form (a crescent defined by subjective contours) was easier to detect in a background of vertical bars (also defined by subjective contours) than vice versa, namely, a search asymmetry paralleling those found by Treisman and Gormican (1988). A final experiment showed that when the horizontal and vertical bars of the first experiment formed textured regions, they could be discriminated at very brief display durations (30–120 msec), However, when the line terminations aligned along the subjective contour were tapered rather than abrupt, discrimination dropped off with the degree of tapering. The latter result is consistent with the assumption that the registration of subjective contours in V2 involves the integration of responses from aligned, end-stopped cells found in VI (von der Heydt & Peterhans, 1989).     

Parallel discrimination of subjective contours defined by offset gratings.

Recent physiological studies (von der Heydt & Peterhans, 1989) suggest that the orientation of subjective contours is encoded very early in the visual system (V2 in monkey). This result is seemingly at odds with existing psychophysical data which suggest that the detection of subjective contours involves selective attention. It is argued that certain subjective contours are registered in a reflexive (bottom-up) manner by the visual system but that selective attention may be needed to gain access to this representation. To assess this suggestion, a visual-search task was used in which subjects were to detect the presence of a horizontal (vertical) subjective contour (defined by offset gratings) in a variable number of vertical (horizontal) subjective contours (also defined by offset gratings). When there were no competing organizations within the display, detection was indeed independent of the number of nontarget distractors, that is, selective attention was unnecessary. In a second experiment, we found that a curved form (a crescent defined by subjective contours) was easier to detect in a background of vertical bars (also defined by subjective contours) than vice versa, namely, a search asymmetry paralleling those found by Treisman and Gormican (1988). A final experiment showed that when the horizontal and vertical bars of the first experiment formed textured regions, they could be discriminated at very brief display durations (30-120 msec). However, when the line terminations aligned along the subjective contour were tapered rather than abrupt, discrimination dropped off with the degree of tapering. The latter result is consistent with the assumption that the registration of subjective contours in V2 involves the integration of responses from aligned, end-stopped cells found in V1 (von der Heydt & Peterhans, 1989).     

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.     

Stereoscopic depth and the occlusion illusion

In the occlusion illusion, the visible portion of a partly occluded object appears larger than a physically identical nonoccluded region. Stereoscopic displays allowed for a direct test of the apparent-distance hypothesis. In Experiments 1A and 1B, we measured both the perceived size and the perceived depth of partly occluded targets when the binocular disparity of both targets and occluders was varied. Stereoscopic occlusion greatly increased perceived target size but not perceived target distance. A reduced illusion was still present when the target was stereoscopically in front of the abutting rectangle, however. Experiments 2A and 2B showed similar results, even when the occluding figures were illusory rectangles that formed no explicit T-junctions. Experiment 3 showed that an unexpected negative size illusion on control trials was primarily due to adaptation to the occlusion illusion on other trials. The present findings eliminate apparent-distance explanations of the occlusion illusion but are consistent with other hypotheses, such as partial modal completion and selective dimensional expansion.     

Associative grouping: Perceptual grouping of shapes by association

Perceptual grouping is usually defined by principles that associate distinct elements by virtue of image properties, such as proximity, similarity, and occurrence within common regions. What role does learning play in forming a perceptual group? This study provides evidence that learning of shape associations leads to perceptual grouping. Subjects were repeatedly exposed to pairs of unique shapes that co-occurred within a common region. The common region cue was later removed in displays composed of these shapes, and the subjects searched the display for two adjacent shapes of the same color. The subjects were faster at locating the color repetition when the adjacent shapes with the same color came from the same trained groups than when they were composed of two shapes from different trained groups. The effects were perceptual in nature: Learned pairings produced spatial distortions similar to those observed for groups defined by perceptual similarity. A residual grouping effect was observed even when the shapes in the trained group switched their relative positions but was eliminated when each shape was inverted. These results indicate that statistical co-occurrence with explicit grouping cues may form an important component of perceptual organization, determining perceived scene structure solely on the basis of past experience.     

Implicit/explicit motive discrepancies and volitional depletion among managers

The compensatory model of motivation and volition is based on the assumption that discrepancies between implicit and explicit motives lead to psychological conflict, and that resolution of this conflict requires volitional regulation and consumes volitional strength. This suggests that implicit/explicit motive discrepancies (IED) are responsible for decreases in volitional strength. A longitudinal field study with 82 managers was conducted to test this proposition. Results show that IED longitudinally predicted decreases in volitional strength. Furthermore, structural equation modeling revealed that volitional strength mediated the relation between IED and impaired subjective well-being. The theoretical and practical implications of these results are discussed, particularly with respect to volitional depletion.