Combining binocular and monocular curvature features.

A study is reported of the perception of visual surfaces in wire-frame stimuli generated by combinations of monocular surface contours and binocular disparity that provide differing information about 3-D relief. Observers vary considerably in the relative contribution made by the binocular and monocular cues to the perception of overall 3-D form. Without training, many observers may entirely fail to perceive surface curvature from the binocular disparity patterns, interpreting the form of the surface only according to the monocular information. For other observers, both cues contribute to the end percept, with the monocular interpretation dominating where the disparity information indicates planarity and with disparity dominating where disparity information suggests curvature and the monocular interpretation suggests planarity. Where stereo and monocular interpretations indicate inconsistent surface curvature features at a common location, more complex resolution strategies are suggested.     

Integration of motion parallax with binocular disparity specifying different surface shapes

We investigated the interaction between motion parallax and binocular disparity cues in the perception of surface shape and depth magnitude by the use of the random dot stimuli in which these cues specified sinusoidal depth surfaces undulating with different spatial frequencies. When ambiguous motion parallax is inconsistent with unambiguous disparity cue, the reasonable solution for the visual system is to convert the motion signal to the flow on the surface specified by disparity. Two experiments, however, found that the visual system did not always use this reasonable solution; observers often perceived the surface specified by a composite of the two cues, or the surface specified by parallax alone. In the perception of this composite of the two cues, the apparent depth magnitude increased with the increase of the depth magnitude specified by both cues. This indicates that the visual system can combine the depth magnitude information from parallax and disparity in an additive fashion. The interference with parallax by disparity implies that the parallax processing is not independent of the disparity processing.     

How is motion disparity integrated with binocular disparity in depth perception?

Two experiments presented motion disparity conflicting with binocular disparity to examine how these cues determined apparent depth order (convex, concave) and depth magnitude. In each experiment, 8 subjects estimated the depth order and depth magnitude. The first experiment showed the following. (1) The visual system used one of these cues exclusively in selecting a depth order for each display. (2) The visual system integrated the depth magnitude information from these cues by a weighted additive fashion if it selected the binocular disparity in depth order perception and if the depth magnitude specified by motion disparity was small relative to that specified by binocular disparity. (3) The visual system ignored the depth magnitude information of binocular disparity if it selected the motion disparity in depth order perception. The second experiment showed that these three points were consistent whether the subject’s head movement or object movement generated motion disparity.     

Suprathreshold stereo-depth matches as a function of contrast and spatial frequency

Thresholds for stereoscopic-depth perception increase with decreasing spatial frequency below 2.5 cycles deg-1. Despite this variation of stereo threshold, suprathreshold stereoscopic-depth perception is independent of spatial frequency down to 0.5 cycle deg-1. Below this frequency the perceived depth of crossed disparities is less than that stimulated by higher spatial frequencies which subtend the same disparities. We have investigated the effects of contrast fading upon this breakdown of stereo-depth invariance at low spatial frequencies. Suprathreshold stereopsis was investigated with spatially filtered vertical bars (difference of Gaussian luminance distribution, or DOG functions) tuned narrowly over a broad range of spatial frequencies (0.15-9.6 cycles deg-1). Disparity subtended by variable width DOGs whose physical contrast ranged from 10-100% was adjusted to match the perceived depth of a standard suprathreshold disparity (5 min visual angle) subtended by a thin black line. Greater amounts of crossed disparity were required to match broad than narrow DOGs to the apparent depth of the standard black line. The matched disparity was greater at low than at high contrast levels. When perceived contrast of all the DOGs was matched to standard contrasts ranging from 5-72%, disparity for depth matches became similar for narrow and broad DOGs. 200 ms pulsed presentations of DOGs with equal perceived contrast further reduced the disparity of low-contrast broad DOGs needed to match the standard depth. A perceived-depth bias in the uncrossed direction at low spatial frequencies was noted in these experiments. This was most pronounced for low-contrast low-spatial-frequency targets, which actually needed crossed disparities to make a depth match to an uncrossed standard. This bias was investigated further by making depth matches to a zero-disparity standard (ie the apparent fronto-parallel plane). Broad DOGs, which are composed of low spatial frequencies, were perceived behind the fixation plane when they actually subtended zero disparity. The magnitude of this low-frequency depth bias increased as contrast was reduced. The distal depth bias was also perceived monocularly, however, it was always greater when viewed binocularly. This investigation indicates that contrast fading of low-spatial-frequency stimuli changes their perceived depth and enhances a depth bias in the uncrossed direction. The depth bias has both a monocular and a binocular component.     

Stereo-motion cooperation and the use ofmotion disparity in the visual perception of 3-D structure

When an observer views a moving scene binocularly, both motion parallax and binocular disparity provide depth information. In Experiments lA-1C, we measured sensitivity to surface curvature when these depth cues were available either individually or simultaneously. When the depth cues yielded comparable sensitivity to surface curvature, we found that curvature detection was easier with the cues present simultaneously, rather than individually. For 2 of the 6 subjects, this effect was stronger when the component of frontal translation of the surface was vertical, rather than horizontal. No such anisotropy was found for the 4 other subjects. If a moving object is observed binocularly, the patterns of optic flow are different on the left and right retinae. We have suggested elsewhere (Cornilleau-Pérès & Droulez, in press) that this motion disparity might be used as avisual cue for the perception of a 3-D structure. Our model consisted in deriving binocular disparity from the left and right distributions of vertical velocities, rather than from luminous intensities, as has been done in classical studies on stereoscopic vision. The model led to some predictions concerning the detection of surface curvature from motion disparity in the presence or absence of intensity-based disparity (classically termedbinocular disparity). In a second set of experiments, we attempted to test these predictions, and we failed to validate our theoretical scheme from a physiological point of view.     

Simultaneous motion perception along multiple attributes: A new class of stimuli

This paper describes a novel class of visual stimuli that have been created to study stroboscopic motion perception along multiple attributes. The stimuli are composed of discrete elements, each of which can be characterized by an arbitrary number of attributes (color, luminance, spatial frequency, binocular disparity, etc.). There are two choices for the spatiotemporal arrangement of each attribute: to elicit either unambiguous (unidirectional) motion perception or ambiguous (bidirectional) motion perception. This affords the unique ability to elicit simultaneous motion perception in opposing directions from identical stimuli. The prevailing direction depends on the relative strength of the attributes’ contribution to the motion mechanisms (for short frame duration), or on the particular attribute being attended to (for long frame duration). Thus, this class of stimuli opens up the possibility of isolating specific motion mechanisms in the visual system.     

Effect of disparity and viewing distance on perceived depth

It is shown that veridical depth perception presupposes the processing of both the magnitude of retinal disparity and observation distance according to a square-law function specified by the underlying geometrical stimulus relations. In the present study, after testing its existence, this constancy of depth perception was investigated by measuring perceived depth as a function of retinal disparity and observation distance. In addition, the relative effectiveness of convergence and accommodation as possible indicators of distance was examined through a conflicting-cues paradigm. It was shown that in the perception of depth the visual system computes distance by taking into account the convergence parameter only, rather than that of accommodation or of both.     

Evidence for patchwork approximation of shape primitives

Investigators have proposed that qualitative shapes are the primitive information of spatial vision: They preserve an approximately one-to-one mapping between surfaces, images, and perception. Given their importance, we examined how the visual system recovers these primitives from sparse disparity fields that do not provide sufficient information for their recovery. We hypothesized that the visual system interpolates sparse disparities with planes, resulting in a patchwork approximation of the implicitly defined shapes. We presented observers with stereo displays simulating planar or smooth curved surfaces having different curvatures. The observers' task was to detect whether dots deviated from these surfaces or to discriminate planar from curved or planar from scrambled surfaces. Consistent with our hypothesis, increasing curvature had detrimental effects on observers' performance (Experiments 1-3). Importantly, this patchwork approximation leads to the recovery of the proposed shape primitives, since observers were more accurate at discriminating planar-from-curved than planar-from-scrambled surfaces with matched disparity range (Experiment 4).     

Perception of surface curvature and direction of illumination from patterns of shading

Three experiments examine the perceptual salience of shading information for the visual specification of three-dimensional form. The observers in these experiments were required to estimate the surface curvature and direction of illumination depicted in computer-synthesized images of cylindrical surfaces, both with and without texture. The results indicate that the shininess of a surface enhances the perception of curvature, but has no effect on perceived direction of illumination; and that shading is generally less effective than texture for depicting surfaces in three dimensions. These and other findings are used to evaluate the psychological validity of several mathematical analyses of shading information that have recently been proposed in the literature.     

The haptic perception of curvature

Ninety-two subjects, schoolchildren and undergraduate and postgraduate students, took part in a series of experiments on the haptic perception of curvature. A graded series of surfaces was produced using piano-convex lenses masked off to produce curved strips that could be explored without using arm movements. Thresholds were measured using the constant method and a staircase procedure. Experiments 1 and 2 yielded data on the absolute and difference thresholds for curvature. Experiment 3 demonstrated that the effective stimulus for curvature is represented by the overall gradient of a curved surface. Using this measure, it was shown that the present absolute thresholds for curvature are lower than those previously reported. In Experiment 4, absolute thresholds were compared using spherical and cylindrical curves: the results showed that, at least with the narrow strips used, the type of curvature does not exert a significant influence on performance. In Experiment 5, the subjective response to curvature was assessed using a rating procedure. Power functions are reported, although the relationship between stimuli and responses had a strong linear component. This suggests that haptically perceived curvature may be a metathetic rather than a prothetic continuum.     

Perceptual learning without feedback and the stability of stereoscopic slant estimation

Subjects were examined for practice effects in a stereoscopic slant-estimation task involving surfaces that comprised a large portion of the visual field. In most subjects slant estimation was significantly affected by practice, but only when an isolated surface (an absolute disparity gradient) was present in the visual field. When a second, unslanted, surface was visible (providing a second disparity gradient and thereby also a relative disparity gradient) none of the subjects exhibited practice effects. Apparently, stereoscopic slant estimation is more robust or stable over time in the presence of a second surface than in its absence. In order to relate the practice effects, which occurred without feedback, to perceptual learning, results are interpreted within a cue-interaction framework. In this paradigm the contribution of a cue depends on its reliability. It is suggested that normally absolute disparity gradients contribute relatively little to perceived slant and that subjects learn to increase this contribution by utilizing proprioceptive information. It is argued that--given the limited computational power of the brain--a relatively small contribution of absolute disparity gradients in perceived slant enhances the stability of stereoscopic slant perception.     

Perspective, orientation disparity, and anisotropy in stereoscopic slant perception.

Stereoscopic depth estimates are not predictable from the geometry of point disparities. The configural properties of surfaces (surface contours) may play an important role in determining, for example, slant responses to a disparity gradient, and the marked anisotropy in favour of slant around a horizontal axis. It has been argued that variation in slant magnitude are attributable to the degree of perspective conflict present and that anisotropy is attributable to orientation disparity, which varies with the axis of slant. Three experiments were conducted in which configural properties were varied to try and tease apart the respective roles of orientation disparity and conflicting perspective in determining stereoscopic slant perception and slant axis anisotropy. The results could not be accounted for by the magnitude of the orientation disparities present. Conflicting perspective cues appeared to play a role but only for slant around a vertical axis. It was concluded that there are important configural effects in stereopsis attributable neither to orientation disparity nor to perspective.     

Depth,size and distance in stereoscopic vision

Evidence is presented to show that in stereoscopic vision a constant ratio of perceived size to perceived distance corresponds to a constant visual angle (the size-distance invariance hypothesis). The functions relating the size/ distance ratio to visual angle and the depth/distance ratio to disparity are determined for three as using the methods of magnitude estimation and magnitude production. The results for each a may be represented by power functions, the depth/ distance function having the higher exponent. These scales are used to predict the outcome of an experiment in which depth is matched to size. The agreement of predictions with results is good for the combined data of the group, but significant deviations occur from curves predicted for individual as. An experiment in which an oblique line is matched to a frontal extent yields data consistent with Luneburg’s hypothesis that the intrinsic geometry of visual space is non-Euclidean. The indicated curvature is negative for two as and varying from positive to negative for the third.     

Temporal tuning and attentional gating: Two distinct attentional mechanisms on the perception of rapid serial visual events

We examined the relationship between two different attention limitations on the perception of rapid events: the attentional awakening (AA, an inability to successfully process a target when it appears early on in a rapid stream of events) and the attentional blink (AB, an inability to successfully process a second target when it appears shortly after a first target [T1]). In four experiments, we failed to find a relationship between the magnitudes of these phenomena. Furthermore, we found two manipulations that selectively modulated the magnitude of each effect without altering the magnitude of the second effect: Expected range of possible rapid serial visual presentation lengths modulated the AA (but not the AB), suggesting that the AA reflects an attentional setup cost for perceiving a protracted series of rapid events, whereas the number of possible T1 positions in the stream modulated the magnitude of the AB (but not the magnitude of the AA). Our results suggest that, despite the surface similarities between the two phenomena, different mechanisms are responsible for these two attentional limitations: Whereas the AA reflects a starting cost associated with the time required to temporally tune attention to the stimulus stream, the AB reflects a blocking of undesired stimuli, aimed at protecting consolidation of T1 processing.     

“有声有色”的触觉体验：来自多感觉通道整合的线索

虚拟现实技术通过提供视觉、听觉和触觉等信息为用户创造身临其境的感知体验, 其中触觉反馈面临诸多技术瓶颈使得虚拟现实中的自然交互受限。基于多感官错觉的伪触觉技术可以借助其他通道的信息强化和丰富触觉感受, 是目前虚拟现实环境中优化触觉体验的有效途径。本文聚焦于触觉中最重要的维度之一——粗糙度, 试图为解决虚拟现实中触觉反馈的受限问题提供新思路。探讨了粗糙度感知中, 视、听、触多感觉通道整合的关系, 分析了视觉线索(表面纹理密度、表面光影、控制显示比)和听觉线索(音调/频率、响度)如何影响触觉粗糙度感知, 总结了当下调控这些因素来改变粗糙度感知的方法。最后, 探讨了使用伪触觉反馈技术时, 虚拟现实环境中视、听、触觉信息在呈现效果、感知整合等方面与真实世界相比可能存在的差异, 提出可借鉴的改善触觉体验的适用方法和未来待研究的方向。     

Magnocellular and parvocellular pathway influences on location-based inhibition-of-return

The roles of the parvocellular (P) and magnocellular (M) retino-geniculo-cortical pathways during shifts of visual attention were investigated by creating M/dorsal-biased (eg low spatial frequency target, no objects present) and P/ventral-biased (ie high spatial frequency target, the perception of 3-D objects) stimulus conditions and measuring location-based inhibition-of-return (IOR). P/ventral-biased conditions produced the greatest IOR. M/dorsal-biased conditions produced the least IOR, in one instance eliminating it altogether. The results indicate a close relationship between IOR magnitude and relative P/ventral and M/dorsal activity with location-based IOR related more to P/ventral than to M/dorsal activity.     

Vertical disparity can alter perceived direction

It has been well established that vertical disparity is involved in perception of the three-dimensional layout of a visual scene. The goal of this paper was to examine whether vertical disparities can alter perceived direction. We dissociated the common relationship between vertical disparity and the stimulus direction by applying a vertical magnification to the image presented to one eye. We used a staircase paradigm to measure whether perceived straight-ahead depended on the amount of vertical magnification in the stimulus. Subjects judged whether a test dot was flashed to either the left or the right side of straight-ahead. We found that perceived straight-ahead did indeed depend on the amount of vertical magnification but only after subjects adapted (for 5 min) to vertical scale (and only in five out of nine subjects). We argue that vertical disparity is a factor in the calibration of the relationship between eye-position signals and perceived direction.     

Dominance of configural properties in visual form perception

Perceptual relations between configural and component properties were examined in three experiments. The pattern of performance in discrimination and classification tasks with a set of four lines that varied in curvature (Experiment 1) was compared with the pattern of performance in these tasks with sets of configuration defined on spatial relations among these lines. The configurations in the latter sets varied in their component properties (“curvature”) as well as in their configural properties (“closure” in Experiment 2 and “parallelism” in Experiment 3). The results indicated that discrimination and classification performance with the configurations was dominated by their configural properties, regardless of the discriminability of their component properties. The implications of these results for the role of configural properties in form perception are discussed.     

Hemispheric differences in the interference among components of compound gratings

The relationship between local/global and high/low spatial-frequency processing in hemispheric asymmetries was explored. Subjects were required to judge the orientation of a high- or low-spatial-frequency component of a compound grating presented in the left visual field (LVF) or right visual field (RVF). In Experiment 1, attention was focused on one or the other component. A signal detection analysis indicated that sensitivity (d′) to the high-spatial-frequency target was reduced more by the presence of the low-spatial-frequency component when both were presented in the LVF rather than in the RVF. In Experiment 2, subjects determined whether a target orientation was present, independent of spatial frequency at only a single level (i.e., at the high- or low-spatial-frequency level), as opposed to both or neither level. An RVF/LH (left hemisphere) advantage was found when the decision was based on the orientation of the high-frequency component. The asymmetrical influence of visual field of presentation and spatial frequency upon sensitivity is discussed in terms of hemispheric differences in the magnitude of inhibition between spatial-frequency channels and in the role of transient channel activity to capture and direct higher order attentional processes.