The gender-based facing bias in 3-D biological motion perception.

In biological motion perception, movements of several point lights can evoke a vivid impression of living animals, including humans. Recent studies have reported that male point-light walkers tend to be perceived as facing toward the viewer more than female walkers, and have hypothesized that the gender-based facing bias arises from motion signals. The purpose of this study was to test this hypothesis under experimental conditions where binocular disparity was added to biological motion stimuli. In the two experiments reported here, participants were presented with disparity-defined female and male point-light figures facing toward or away from the viewer. In Experiment 1, we measured "facing-the-viewer" responses in upright and inverted walker configurations. It was found that the facing bias was greater for the male walker than for the female walker in most disparity magnitudes, regardless of walker inversion. In Experiment 2, the walker stimuli were replaced by static snapshots of the walkers. The results showed that the facing bias did not differ between the female and male static figures. These results suggest that motion signals play an important role in producing the gender-based facing bias, even when binocular disparity is added to biological motion stimuli.

Stereoscopic Slant Contrast and the Perception of Inducer Slant at Brief Stimulus Presentations.

Slant contrast refers to a stereoscopic phenomenon in which the perceived slant of a test object is affected by the disparity of a surrounding inducer object. Slant contrast has been proposed to involve cue conflict, but it is unclear whether this idea is useful in explaining slant contrast at short stimulus presentations (<1 s). We measured both slant contrast and perceived inducer slant while varying the presentation duration (100-800 ms) of stereograms with several spatial configurations. In three psychophysical experiments, we found that (a) both slant contrast and perceived inducer slant increased as a function of stimulus duration, and (b) slant contrast was relatively stable across different test and inducer shapes at each short stimulus duration, whereas perceived inducer slant increased when cue conflict was reduced. These results suggest that at brief, not long stimulus presentations, the cue conflict between disparity and perspective plays a smaller role in slant contrast than other depth cues.

Stereoscopic Depth Contrast in a 3D Müller-Lyer Configuration: Evidence for Local Normalization.

Depth contrast is a stereoscopic visual phenomenon in which the slant of an element is affected by that of adjacent elements. Normalization has been proposed to be a possible cause of depth contrast, but it is still unclear how depth contrast involves normalization. To address this issue, we devised stereograms consisting of a vertical test line accompanied by several inducer lines, like a three-dimensional variation of the well-known Müller-Lyer configuration. The inducer lines had horizontal binocular disparities that defined a stereoscopic slant about a horizontal axis with respect to the endpoints of the test line. The observer's task was to adjust the slant of the test line about a horizontal axis until it appeared subjectively vertical. The results of two psychophysical experiments found that slant settings were affected by the slant of local inducers, but not by the overall slant of the whole stimulus. These results suggest that, at least for line patterns, the stereo system normalizes depth locally.

Eggs illusion: Local shape deformation generated by a grid pattern.

In this study, we report a new visual shape illusion, the eggs illusion, in which circular disks located at the midpoints between adjacent grid intersections are perceived as being deformed to ellipses. In Experiment 1, we examined the eggs illusion by using a matching method and found that grid luminance and patch size play a critical role in producing the illusory deformation. In Experiment 2, we employed several types of elliptic or circular patches to examine the conditions in which the illusory deformation was cancelled or weakened. We observed that the illusory deformation was dependent on local grid orientation. Based on these results, we found several common features between the eggs illusion and the scintillating grid illusion. This resemblance suggests a possibility that similar mechanisms underlie the two phenomena. In addition to the scintillating grid illusion, we also considered several known perceptual phenomena that might be related to the eggs illusion, i.e., the apparent size illusion, the shape-contrast effect, and the Orbison illusion. Finally, we discuss the role of orientation processing in generating the eggs illusion.

Wing-shaped walls: A directional effect of obstacles on manual avoidance.

Visual information can be used to plan, start, and coordinate manual movements in obstacle avoidance. An intriguing example of visuomotor coordination is the effect of wing-shaped walls, in which walls are oriented away from or toward a moving agent. A historical story from medieval Japan recounts that wing-shaped walls disrupted the agent's movement more when oriented toward the agent than when oriented away from the agent. This study aimed at examining whether the disruptive effect of wing-shaped walls occurs in a schematic situation represented on a 2D plane. In this study, we conducted psychophysical experiments in which participants were asked to move a stylus from a start point to a goal while avoiding multiple line obstacles that were arranged alternately at a course. In the two experiments, we manipulated the orientation and the size of the visible parts of the obstacles systematically. We found that the obstacles oriented toward the agent produced frequent contacts with the agent and attracted manual movements to the endpoints of obstacles. We discussed possible interpretations of the results in the context of attentional guidance.

Large-scale cortico-cerebellar computations for horizontal and vertical vergence in humans.

Horizontal and vertical vergence eye movements play a central role in binocular coordination. Neurophysiological studies suggest that cortical and subcortical regions in animals and humans are involved in horizontal vergence. However, little is known about the extent to which the neural mechanism underlying vertical vergence overlaps with that of horizontal vergence. In this study, to explore neural computation for horizontal and vertical vergence, we simultaneously recorded electrooculography (EOG) and whole-head magnetoencephalography (MEG) while presenting large-field stereograms for 29 healthy human adults. The stereograms were designed to produce vergence responses by manipulating horizontal and vertical binocular disparities. A model-based approach was used to assess neural sensitivity to horizontal and vertical disparities via MEG source estimation and the theta-band (4 Hz) coherence between brain activity and EOG vergence velocity. We found similar time-locked neural responses to horizontal and vertical disparity in cortical and cerebellar areas at around 100-250 ms after stimulus onset. In contrast, the low-frequency oscillatory neural activity associated with the execution of vertical vergence differed from that of horizontal vergence. These findings indicate that horizontal and vertical vergence involve partially shared but distinct computations in large-scale cortico-cerebellar networks.

[Neural Mechanism Underlying Stereoscopic Depth Perception: Why are the Two Eyes Aligned?]

This paper explains the basics of depth perception and explores why both eyes are aligned. We concluded that the brain may analyze image features for binocular fusion while calculating the horizontal disparity for depth perception.

Spatial dynamics of the eggs illusion: Visual field anisotropy and peripheral vision.

The eggs illusion is a visual phenomenon in which bright circular patches located at the midpoints between the intersections of a dark grid are perceived as being elongated along the direction orthogonal to the grid line. In the four experiments we report here, we explored the spatial properties of the eggs illusion by manipulating retinal eccentricity and the location of the stimulus in the visual field. In Experiment 1, we examined whether central and peripheral configurations affected the illusory magnitude. In Experiment 2, we varied the spatial location of grid patterns and found that the eggs illusion was intensified when the pattern was presented in the horizontal, not vertical or diagonal position, relative to the fixation. In Experiment 3, we varied the retinal eccentricity of the pattern along the horizontal meridian and found that the illusion was enhanced in the retinal periphery. In Experiment 4, we manipulated the size of the stimulus and found that peripheral enhancement of the eggs illusion was more apparent for a larger pattern. The visual field anisotropy and the peripheral enhancement of the eggs illusion are discussed in relation to mechanisms underlying grid-induced illusions.

Vertical size disparity induces enhanced neural responses in good stereo observers.

Stereoscopic three-dimensional vision requires cortical processing for horizontal binocular disparity between the two eyes' retinal images. Behavioral and theoretical studies suggest that vertical size disparity is used to recover the viewing geometry and to generate the slant of a large surface. However, unlike horizontal disparity, the relation between stereopsis and neural responses to vertical disparity remains controversial. To determine the role of cortical processing for vertical size disparity in stereopsis, we measured neuromagnetic responses to disparities in people with good and poor stereopsis, using magnetoencephalography (MEG). Healthy adult participants viewed stereograms with a horizontal or vertical size disparity, and judged the perceived slant of the pattern. We assessed neural activity in response to disparities in the visual cortex and the phase locking of oscillatory responses including the alpha frequency range using MEG. For participants with good stereopsis, activity in the visual areas was significantly higher in response to vertical size disparity than to horizontal size disparity. The time-frequency analysis revealed that early neural responses to vertical size disparity were more phase-locked in good stereo participants than in poor stereo participants. These results provide neuromagnetic evidence that vertical-size disparity processing plays a role in good stereo vision.

Illusory depth induced by binocular torsional misalignment.

This study reports a new depth illusion in which a static flat pattern appears stratified stereoscopically when viewed binocularly with an elevated gaze. Three psychophysical experiments measured perceived relative depth and fixational cyclodisparity (a rotation of one eye's view relative to the other eye's view about the line of sight) when flat patterns drawn with solid or dashed curved lines were fixated at various levels of gaze elevation. Experiments 1 and 2 showed that the patterns drawn with solid lines produced illusory depth only at large gaze elevations (downward and upward). Experiment 3 showed that the magnitude of the illusory depth was correlated with that of fixational cyclodisparity. These results suggest that the illusory depth originates in the binocular torsional misalignment generated by gaze elevation.

A long-distance stereoscopic detector for partially occluding surfaces.

An external noise technique was used to investigate the stereoscopic process that generates an illusory phantom occluder from binocularly unmatched elements. Observers were required to identify the quadrant in which a binocularly defined target was presented. We had three targets: (a) two vertical binocular bars with the unmatched portions arranged to induce a stable phantom occluder (valid), (b) the same stimuli except the image for the left eye was switched with that for the right eye therefore not inducing a stable occluder (invalid), and (c) a single binocular bar with the same unmatched portion (single-bar). For each target, the luminance contrast of the signal required for 75% correct responses was measured at four levels of external interocular noise. Contrast thresholds were found to be lower for the valid target than for both the invalid and the single-bar targets. The results suggest that the visual system has a stereoscopic detector that responds to stimuli that meet a long-distance requirement for the perception of partially occluding surfaces.

Greater depth seen with phantom stereopsis is coded at the early stages of visual processing.

A visual search task was used to investigate the spatially parallel coding of depth from binocular disparity and from binocularly unmatched features. Experiment 1, using disparity noise, showed that detectability is higher for illusory phantom targets defined by unmatched features than for disparity-defined targets, although the two targets were equated as to theoretically minimum depth. Experiment 2, using binocularly unmatched noise whose width was equal to the disparity of the noise used in Experiment 1, showed that noise severely interferes with the detection of both the disparity and the phantom targets. These results are consistent with the idea that the greater depth seen with phantom stereopsis is coded at the early stages of visual processing.

Inferring the depth of 3-D objects from tactile spatial information.

Four psychophysical experiments were conducted to examine the relation between tactile spatial information and the estimated depth of partially touched 3-D objects. Human participants touched unseen, tactile grating patterns with their hand while keeping the hand shape flat. Experiment 1, by means of a production task, showed that the estimated depth of the concave part below the touched grating was well correlated with the separation between the elements of the grating, but not with the overall size of the grating, nor with the local structure of the touched parts. Experiments 2 and 3, by means of a haptic working memory task, showed that the remembered depth of a target surface was biased toward the estimated bottom position of a tactile grating distractor. Experiment 4, by means of a discrimination task, revealed that tactile grating patterns influenced speeded judgments about visual 3-D shapes. These results suggest that the haptic system uses heuristics based on spatial information to infer the depth of an untouched part of a 3-D object.

Vertical size disparity and the correction of stereo correspondence.

We examined the stage of vertical-disparity processing that produces a global stereoscopic slant. In two psychophysical experiments, we measured perceived slant about a vertical axis for two-dimensional stereoscopic patterns consisting of random dots, concentric lines, and radial lines. Binocular image differences were introduced into each pattern by vertically magnifying either the entire image for the right eye or that for the left eye. Because the continuous lines were geometrically ambiguous in local stereo correspondence, the three patterns differed from each other in the local horizontal disparity measured in retinal coordinates. The two experiments revealed that, despite the differences in the retinal horizontal disparity, the slant settings were generally similar for the three patterns, in both short and long viewing distances (25 cm and 120 cm, respectively). These results are consistent with the idea that the visual system uses vertical disparity at least when establishing local stereo correspondence. A Bayesian model is proposed to account for the results.

Illusory motion produced by dichoptic stimuli during saccades.

A new motion illusion is reported in which saccadic eye movements can produce a perceived jump of a static stimulus presented dichoptically. In three experiments, observers made saccades while viewing a stationary stimulus consisting of a disk and random dots presented separately to the two eyes. In experiments 1 and 2, by measuring the strength of the perceived motion and the velocity of binocular eye movements, we found that (a) motion ratings were high for the stimulus that contained a large interocular difference in luminance, and (b) the saccadic strategy of the observer was virtually identical across different stimulus conditions. In experiment 3, by measuring the detectability of a short temporal gap introduced into the stimulus around saccades, we found that saccadic suppression was normal in the dichoptic presentation. We discuss possible mechanisms underlying the illusory motion.

Perceived depth of curved lines in the presence of cyclovergence.

Mitsudo [Mitsudo, H. (2007). Illusory depth induced by binocular torsional misalignment. Vision Research, 47, 1303-1314] reported a new depth illusion in which a static flat pattern consisting of curved lines appears stereoscopically stratified when viewed with eccentric elevated gaze. He proposed a hypothesis that the illusory depth produced with the curved-line stereogram might originate in a failure to counteract the effect of cyclovergence (i.e., the binocular misalignment of the eyes about the lines of sight). To test this hypothesis, we measured observers' cyclovergence with a video-based eye tracker while they were making a depth judgment of the curved-line stereogram. The observers' cyclovergence was induced by the elevation of gaze (Experiment 1) and by cyclorotated random dots (Experiment 2). The results showed that the magnitude of perceived depth correlated well with the measured cyclovergence for the curved-line stereogram. In contrast, when similar stimuli contained more dot-like elements, perceived depth was relatively independent of cyclovergence. These results support Mitsudo's hypothesis and are consistent with the notion that the stereo system requires unambiguous image cues-e.g., spatially distributed dot-like elements-to counteract the retinal cyclodisparity and produce perceived depth. A computational model was proposed to account for the results.

Additivity of retinal and pursuit velocity in the perceptions of depth and rigidity from object-produced motion parallax.

Two psychophysical experiments were conducted to investigate the mechanism that generates stable depth structure from retinal motion combined with extraretinal signals from pursuit eye movements. Stimuli consisted of random dots that moved horizontally in one direction (ie stimuli had common motion on the retina), but at different speeds between adjacent rows. The stimuli were presented with different speeds of pursuit eye movements whose direction was opposite to that of the common retinal motion. Experiment 1 showed that the rows moving faster on the retina appeared closer when viewed without eye movements; however, they appeared farther when pursuit speed exceeded the speed of common retinal motion. The 'transition' speed of the pursuit eye movement was slightly, but consistently, larger than the speed of common retinal motion. Experiment 2 showed that parallax thresholds for perceiving relative motion between adjacent rows were minimum at the transition speed found in experiment 1. These results suggest that the visual system calculates head-centric velocity, by adding retinal velocity and pursuit velocity, to obtain a stable depth structure.

Evidence for the correcting-mechanism explanation of the Kanizsa amodal shrinkage.

An object phenomenally shrinks in its horizontal dimension when shown on a 2-D plane as if the central portion of the object were partially occluded by another vertical one in 3-D space (the Kanizsa amodal shrinkage). We examined the predictions of the correcting-mechanism hypothesis proposed by Ohtsuka and Ono (2002, Proceedings of SPIE 4864 167-174), which states that an inappropriate operation of the mechanism that corrects a phenomenal increase in monocularly visible areas accompanied by a stereoscopic occluder gives rise to the illusion. In this study we measured the perceived width (or height in experiment 3) of a square seen behind a rectangle, while controlling other factors which potentially influence the illusion, such as the division of space or depth stratification. The results of five experiments showed that (a) the perceived width was not influenced when the occluder had a relatively large binocular disparity, but was underestimated when the occluder did not have disparity, and (b) the shrinkage diminished when the foreground rectangle was transparent, was horizontally oriented, or contained no pictorial occlusion cues. These results support the hypothesis that the correcting mechanism, triggered by pictorial occlusion cues, contributes to the Kanizsa shrinkage.

Rapid image-segmentation and perceptual transparency share a process which utilises X-junctions generated by temporal integration in the visual system.

Perceptual transparency requires local same-polarity X-junctions, which can also be generated by temporal integration under natural dynamic conditions. In this study, segmentation performance and target appearance were measured for a uniform gray target embedded in a random-dot frame presented with a temporally adjacent mask. Although static cues for both segmentation and transparency were unavailable, transparency was observed only when collinear same-polarity edges reduced backward masking, in both the fovea and the perifovea. These results suggest that the visual system has a common underlying mechanism for rapid segmentation and transparency, which utilises same-polarity X-junctions generated by temporal integration.