Reaching for visual cues to depth: the brain combines depth cues differently for motor control and perception

Vision provides a number of cues about the three-dimensional (3D) layout of objects in a scene that could be used for planning and controlling goal-directed behaviors such as pointing, grasping, and placing objects. An emerging consensus from the perceptual work is that the visual brain is a near-optimal Bayesian estimator of object properties, for example, by integrating cues in a way that accounts for differences in their reliability. We measured how the visuomotor system integrates binocular and monocular cues to 3D surface orientation to guide the placement of objects on a slanted surface. Subjects showed qualitatively similar results to those found in perceptual studies--they gave more weight to binocular cues at low slants and more weight to monocular cues like texture at high slants. We compared subjects' performance in the visuomotor task with their performance on matched perceptual tasks that required an observer to estimate the same 3D surface properties needed to control the motor behavior. The relative influence of binocular and monocular cues changed in qualitatively the same way across stimulus conditions in the two types of task; however, subjects gave significantly more weight to binocular cues for controlling hand movements than for making explicit perceptual judgments in these tasks. Thus, the brain changes how it integrates visual cues based not only on the information content of stimuli, but also on the task for which the information is used.     

Accommodation to multiple-focal-plane displays: Implications for improving stereoscopic displays and for accommodation control

Most stereoscopic displays present images at a single focal plane, resulting in "conflicts" between the stimuli to vergence and accommodation. Minimizing these conflicts is beneficial because they can cause distorted depth percepts, visual fatigue, and reduced stereoscopic performance. One proposed solution is to present a sum of images at multiple focal planes and to vary focal depth continuously by distributing image intensity across planes-a technique referred to as depth filtering. We evaluated this digital approximation to real-world variations in focal depth by measuring accommodation responses to depth-filtered stimuli at various simulated distances. Specifically, we determined the maximum image-plane separation that supported accurate and reliable accommodation. We used an analysis of retinal-image formation to predict when responses might be inaccurate. Accommodation to depth-filtered images was accurate and precise for image-plane separations up to ～1 diopter, suggesting that depth filtering can be used to precisely match accommodation and vergence demands in a practical display. At larger plane separations, responses broke down in a manner consistent with our analysis. We develop this approach to consider how different spatial frequencies contribute to accommodation control. The results suggest that higher spatial frequencies contribute less to the accommodation response than has previously been thought.     

Vergence-accommodation conflicts hinder visual performance and cause visual fatigue

Three-dimensional (3D) displays have become important for many applications including vision research, operation of remote devices, medical imaging, surgical training, scientific visualization, virtual prototyping, and more. In many of these applications, it is important for the graphic image to create a faithful impression of the 3D structure of the portrayed object or scene. Unfortunately, 3D displays often yield distortions in perceived 3D structure compared with the percepts of the real scenes the displays depict. A likely cause of such distortions is the fact that computer displays present images on one surface. Thus, focus cues-accommodation and blur in the retinal image-specify the depth of the display rather than the depths in the depicted scene. Additionally, the uncoupling of vergence and accommodation required by 3D displays frequently reduces one's ability to fuse the binocular stimulus and causes discomfort and fatigue for the viewer. We have developed a novel 3D display that presents focus cues that are correct or nearly correct for the depicted scene. We used this display to evaluate the influence of focus cues on perceptual distortions, fusion failures, and fatigue. We show that when focus cues are correct or nearly correct, (1) the time required to identify a stereoscopic stimulus is reduced, (2) stereoacuity in a time-limited task is increased, (3) distortions in perceived depth are reduced, and (4) viewer fatigue and discomfort are reduced. We discuss the implications of this work for vision research and the design and use of displays.     

Phantom surface captures stereopsis

A phantom surface is a stereoscopic illusory area that can be seen in depth although there is no conventional stereoscopic cues [Liu, L., Stevenson, S.B., & Schor, C.M. (1994). Quantitative stereoscopic depth without binocular correspondence. Nature, 367, 66-69; Gillam, B. & Nakayama, K. (1999). Quantitative depth for a phantom surface can be based on cyclopean occlusion cues alone. Vision Research, 39, 109-112]. The phenomenon has been explained as an example of half-occlusion processing in which the visual system uses information about cyclopean occlusion structure of the visual world. We created stereo capture stereograms in which phantom surfaces changed the perceived depth of conventionally defined binocular textures. Because conventional stereoscopic matching is strongly affected by half-occlusion processing, we suggest that half-occlusion processing is an integral part of the early stereoscopic processing and solving of the correspondence problem.     

Focus cues affect perceived depth

Depth information from focus cues--accommodation and the gradient of retinal blur--is typically incorrect in three-dimensional (3-D) displays because the light comes from a planar display surface. If the visual system incorporates information from focus cues into its calculation of 3-D scene parameters, this could cause distortions in perceived depth even when the 2-D retinal images are geometrically correct. In Experiment 1 we measured the direct contribution of focus cues to perceived slant by varying independently the physical slant of the display surface and the slant of a simulated surface specified by binocular disparity (binocular viewing) or perspective/texture (monocular viewing). In the binocular condition, slant estimates were unaffected by display slant. In the monocular condition, display slant had a systematic effect on slant estimates. Estimates were consistent with a weighted average of slant from focus cues and slant from disparity/texture, where the cue weights are determined by the reliability of each cue. In Experiment 2, we examined whether focus cues also have an indirect effect on perceived slant via the distance estimate used in disparity scaling. We varied independently the simulated distance and the focal distance to a disparity-defined 3-D stimulus. Perceived slant was systematically affected by changes in focal distance. Accordingly, depth constancy (with respect to simulated distance) was significantly reduced when focal distance was held constant compared to when it varied appropriately with the simulated distance to the stimulus. The results of both experiments show that focus cues can contribute to estimates of 3-D scene parameters. Inappropriate focus cues in typical 3-D displays may therefore contribute to distortions in perceived space.     

The role of stereo vision in visual-vestibular integration

Self-motion through an environment stimulates several sensory systems, including the visual system and the vestibular system. Recent work in heading estimation has demonstrated that visual and vestibular cues are typically integrated in a statistically optimal manner, consistent with Maximum Likelihood Estimation predictions. However, there has been some indication that cue integration may be affected by characteristics of the visual stimulus. Therefore, the current experiment evaluated whether presenting optic flow stimuli stereoscopically, or presenting both eyes with the same image (binocularly) affects combined visual-vestibular heading estimates. Participants performed a two-interval forced-choice task in which they were asked which of two presented movements was more rightward. They were presented with either visual cues alone, vestibular cues alone or both cues combined. Measures of reliability were obtained for both binocular and stereoscopic conditions. Group level analyses demonstrated that when stereoscopic information was available there was clear evidence of optimal integration, yet when only binocular information was available weaker evidence of cue integration was observed. Exploratory individual analyses demonstrated that for the stereoscopic condition 90% of participants exhibited optimal integration, whereas for the binocular condition only 60% of participants exhibited results consistent with optimal integration. Overall, these findings suggest that stereo vision may be important for self-motion perception, particularly under combined visual-vestibular conditions.     

Visual performance of subjects wearing presbyopic contact lenses.

PURPOSE: The purpose of this study is to assess the visual performance of subjects wearing gas-permeable (GP) multifocal contact lenses, soft bifocal contact lenses, GP monovision lenses and spectacles. METHODS: The study included 32 subjects between the ages of 42 and 65 years wearing GP monovision, the Acuvue Bifocal (Vistakon), the Essentials GP Multifocal (Blanchard), and progressive addition lenses (PAL; spectacles group). There were eight subjects in each of these groups who were already wearing these modalities. Binocular low (18%) and high (95%) contrast acuities were recorded using the Bailey-Lovie chart; binocular contrast sensitivity from 1.5 to 18 cycles per degree (cpd) measured with the Vistech VCTS 6500 system, and monocular glare sensitivity at three luminance settings (400, 100, and 12 foot lamberts) was measured using the brightness acuity tester (BAT). Binocular near visual task performance (a modified version of letter counting method used in previous presbyopic studies) was also assessed. RESULTS: For the contact lens-wearing groups, subjects wearing GP multifocals provided the best binocular high and low contrast acuity followed by soft bifocal wearers. There was relative parity between the binocular high and low contrast acuity with PAL and GP multifocal wearers. Monovision acuity, measured binocularly, was determined to be lower than the other three groups with this difference being most significant with high contrast acuity. Among contact lens-wearing groups, it was observed that GP multifocal lens wearers experienced the lowest amount of monocular disability glare followed by soft bifocal wearers and monovision wearers. Subjects wearing soft bifocal lenses and monovision demonstrated slightly reduced binocular contrast sensitivity at all spatial frequencies. In the contact lens groups, GP multifocal lens wearers had the highest binocular contrast sensitivity at all spatial frequencies, on parity with PAL wearers, except at the highest spatial frequency (18 cpd) at which PAL wearers had better vision. Error scores for the binocular near visual task performance between the four groups revealed subjects with GP multifocal lenses and PAL wearers to have the least errors, followed by monovision users and then soft bifocal wearers with the most errors. CONCLUSION: Subjects wearing GP multifocals, soft bifocals, monovision, and PAL spectacles have good binocular contrast sensitivity, satisfactory binocular low and high contrast acuity, and increased sensitivity to glare. Presbyopic subjects requiring the use of contact lenses under dim light levels could benefit from GP multifocal lenses. Contrast and glare sensitivity evaluations provide significant information regarding the visual performance of the presbyopic contact lenses and should be included in regular presbyopic contact lens fitting.     

Head jitter enhances three-dimensional motion perception

Motion perception is a critical function of the visual system. In a three-dimensional environment, multiple sensory cues carry information about an object''s motion trajectory. Previous work has quantified the contribution of binocular motion cues, such as interocular velocity differences and changing disparities over time, as well as monocular motion cues, such as size and density changes. However, even when these cues are presented in concert, observers will systematically misreport the direction of motion-in-depth. Although in the majority of laboratory experiments head position is held fixed using a chin or head rest, an observer''s head position is subject to involuntary small movements under real-world viewing conditions. Here, we considered the potential impact of such “head jitter” on motion-in-depth perception. We presented visual stimuli in a head-mounted virtual reality device that facilitated low latency head tracking and asked observers to judge 3D object motion. We found performance improved when we updated the visual display consistent with the small changes in head position. When we disrupted or delayed head movement–contingent updating of the visual display, the proportion of motion-in-depth misreports again increased, reflected in both a reduction in sensitivity and an increase in bias. Our findings identify a critical function of head jitter in visual motion perception, which has been obscured in most (head-fixed and non-head jitter contingent) laboratory experiments.     

Chromatic assimilation unaffected by perceived depth of inducing light

Chromatic assimilation is a shift toward the color of nearby light. Several studies conclude that a neural process contributes to assimilation but the neural locus remains in question. Some studies posit a peripheral process, such as retinal receptive-field organization, while others claim the neural mechanism follows depth perception, figure/ground segregation, or perceptual grouping. The experiments here tested whether assimilation depends on a neural process that follows stereoscopic depth perception. By introducing binocular disparity, the test field judged in color was made to appear in a different depth plane than the light that induced assimilation. The chromaticity and spatial frequency of the inducing light, and the chromaticity of the test light, were varied. Chromatic assimilation was found with all inducing-light sizes and chromaticities, but the magnitude of assimilation did not depend on the perceived relative depth planes of the test and inducing fields. We found no evidence to support the view that chromatic assimilation depends on a neural process that follows binocular combination of the two eyes' signals.     

The footprints of visual attention during search with 100% valid and 100% invalid cues

Human performance during visual search typically improves when spatial cues indicate the possible target locations. In many instances, the performance improvement is quantitatively predicted by a Bayesian or quasi-Bayesian observer in which visual attention simply selects the information at the cued locations without changing the quality of processing or sensitivity and ignores the information at the uncued locations. Aside from the general good agreement between the effect of the cue on model and human performance, there has been little independent confirmation that humans are effectively selecting the relevant information. In this study, we used the classification image technique to assess the effectiveness of spatial cues in the attentional selection of relevant locations and suppression of irrelevant locations indicated by spatial cues. Observers searched for a bright target among dimmer distractors that might appear (with 50% probability) in one of eight locations in visual white noise. The possible target location was indicated using a 100% valid box cue or seven 100% invalid box cues in which the only potential target locations was uncued. For both conditions, we found statistically significant perceptual templates shaped as differences of Gaussians at the relevant locations with no perceptual templates at the irrelevant locations. We did not find statistical significant differences between the shapes of the inferred perceptual templates for the 100% valid and 100% invalid cues conditions. The results confirm the idea that during search visual attention allows the observer to effectively select relevant information and ignore irrelevant information. The results for the 100% invalid cues condition suggests that the selection process is not drawn automatically to the cue but can be under the observers' voluntary control.     

Integration of binocular disparity and monocular cues at near threshold level

We examined the dependency of the integration of multiple depth cues upon the combined cues and upon the consistency of depth information from different cues. For each observer, depth thresholds were measured by the use of stimuli in which different depth cues (motion parallax, binocular disparity, and monocular configuration) specified the surface undulating sinusoidally with different spatial frequencies and different phases. Analysis of d(') showed that the performance was better than the prediction of probability summation only when parallax and disparity cues specified an undulation with the same spatial frequency and same phase. The probability summation model overestimated the performance for the other conditions of combination of disparity and parallax, and for all of the conditions of combination of disparity and monocular configuration. These results suggest that the improvement in depth perception caused by integration of multiple cues depends on the type of combined cues, and that the visual system possibly integrates the depth information from different cues at different stages of the visual processing.     

Temporal frequency and contrast tagging bias the type of competition in interocular switch rivalry

The nature of competition underlying perceptual alternations in binocular rivalry remains controversial. Interocular swapping of rivalrous stimuli can result in either slow irregular perceptual alternations that bridge multiple interocular switches or fast regular alternations that are time locked to the stimulus exchanges. We labeled either the inputs to the eyes or the individual rivalrous stimuli using temporal frequency and contrast tagging. Tagging of eye-of-origin signals enhanced the fast regular perceptual alternations associated with eye rivalry, while stimulus tagging shifted perception towards slow irregular alternations characteristic of stimulus rivalry. Thus, the type of competition in binocular rivalry can be biased based on additional cues in the visual inputs. The results are consistent with a model in which the brain combines information across multiple visual features to resolve ambiguities in visual inputs.     

虚拟医学技术在小儿眼科斜视教学中的应用

斜视是小儿眼科教学中的难点,随着近年来虚拟医学技术的发展,将之引入小儿斜视的教学工作中,是比较切实有效的方法。我们应用桌面式、沉浸式虚拟现实体系及增强现实体系,利用头盔、3D显示器,通过特定视觉刺激图像软件形象生动地展示斜视眼位及立体视等教学要点内容,便于学生操作与体会。这些虚拟医学技术对提高小儿斜视教学水平有着积极的意义。（眼科, 2018, 27： 395-397）     

Independent visual threshold measurements in the two eyes of freely moving rats and mice using a virtual-reality optokinetic system

Slow horizontal head and body rotation occurs in mice and rats when the visual field is rotated around them, and these optomotor movements can be produced reliably in a virtual-reality system. If one eye is closed, only motion in the temporal-to-nasal direction for the contralateral eye evokes the tracking response. When the maximal spatial frequency capable of driving the response ("acuity") was measured under monocular and binocular viewing conditions, the monocular acuity was identical to the binocular acuity measured with the same rotation direction. Thus, the visual capabilities of each eye can be measured under binocular conditions simply by changing the direction of rotation. Lesions of the visual cortex had no effect on the acuities measured with the virtual optokinetic system, whereas perceptual thresholds obtained previously with the Visual Water Task are. The optokinetic acuities were also consistently lower than acuity estimates from the Visual Water Task, but contrast sensitivities were the same or better. These data show that head-tracking in a virtual optokinetic drum is driven by subcortical, lower frequency, and contralateral pathways.     

The binocular depth stereoacuity of the pigeon and its relation to the anatomical resolving power of the eye

The stereoacuity of the pigeon was measured by finding the smallest depth difference discriminable between two arrays of elements, each array confined to one of two parallel planes in space. For both the human and the pigeon, the depth between the two planes was not discriminable under stationary monocular conditions. The mean binocular depth acuity for five human subjects was 4 arc sec, reflecting hyperacuity. In the pigeon, the binocular stereoacuity was between 0.8-1.8 arc min, allowing each eye to discriminate a visual angle of at most 0.89 min of arc. This value is less than the anatomical resolving power of the retina as predicted by sampling theory. The area of the pigeon retina involved in this binocular depth judgement is likely to be the area dorsalis, a specialised region that does not contain a fovea. The pigeon is a granivorous bird with laterally placed eyes, thus we demonstrate that lifestyle and binocular field width do not predict stereoscopic vision quality.     

立体显示中深度线索对调节反应及其微波动的影响

目的 通过评估绘图深度线索和视差深度线索对调节反应的影响,分析线索冲突对调节及其微波动的影响方法 实验研究.采用开放视野的红外自动验光仪测量30例被检者双眼注视50 cm处不同视标下右眼的调节反应.视标分为无绘图深度线索和有绘图深度线索,并且分别以2D和3D的形式呈现(视标1:无绘图深度线索无视差深度线索;视标2:仅有绘图深度线索;视标3:仅有视差深度线索;视标4:既有绘图深度线索又有视差深度线索).采用重复测量的方差分析 结果 视标1～4的调节反应均值分别为(1.35±0.24)D、(1.42±0.27)D、(1.40±0.27)D和(1.36±0.25 )D,4种视标的调节反应差异有统计学意义(F=3.48,P＜0.05).有无视差深度线索或绘图深度线索对调节反应的影响差异有统计学意义(P＜0.05),但是当两个线索组合时对调节反应的影响差异无统计学意义.结论 双眼视差深度线索和绘图深度线索在一定程度上增加了被检者的调节反应,但当两者结合同时作用时,对人眼调节反应的影响不是简单叠加作用,可　能与调节所接受的生理和心理共同作用的复杂性有关.与平面图像相比,短暂3D视觉刺激,调节微波动相对比较稳定,可能与短期内3D丰富的画面比较吸引专注注视和视觉系统积极适应有关.     

Differences in contrast sensitivity between monofocal,multifocal and accommodating intraocular lenses: long‐term results

Background: To evaluate long‐term contrast sensitivity (CS) and visual acuity following implantation of monofocal, accommodating, refractive and diffractive multifocal intraocular lenses (IOLs) in patients with unilateral cataract Methods: In this prospective non‐randomized clinical trial, 87 patients with unilateral cataract were enrolled in four groups for phacoemulsification and IOL implantation in Ophthalmology Department of Goztepe Training and Research Hospital. Twenty‐four patients had monofocal (Alcon Acrysof; group 1), 21 patients accommodating (Human Optics 1CU; group 2), 22 patients diffractive multifocal (Tecnis ZM900; group 3) and 20 patients refractive multifocal (AMO Rezoom; group 4) IOL implantations. Ages of patients were between 40 and 70. Parameters analysed at the 18th postoperative month were subjective refractions, monocular and binocular distance and near photopic CSs, visual acuities. Results: Near visual acuities were statistically better in group 3 than the other groups (P < 0.05). At low spatial frequencies, mean monocular distance CSs of group 1 and mean monocular near CSs of groups 1 and 2 were statistically higher than those of group 4 (P < 0.05). There was no significant difference between binocular CSs of group 4 and the other groups at low spatial frequencies. At high spatial frequencies, monocular and binocular CSs of groups 1 and 2 were statistically higher than those of groups 3 and 4 (P < 0.05). Near CSs was better in group 3 than group 4. Conclusions: In patients with unilateral cataract, monofocal, accommodating and partially diffractive multifocal IOL provided higher CS scores when compared with refractive multifocal IOL and in multifocal IOL groups binocular CSs were better than monocular CSs when compared with other groups.     

Binocular accommodative response with extended depth of focus under controlled convergences

Vergence and accommodation can be mismatched under virtual reality viewing conditions, and this mismatch has been thought to be one of the main causes of visual discomfort. The goal of this study was to investigate how optical conditions of the eyes affect accommodative responses to different convergence. Specifically, we hypothesized that extending the depth of focus (DoF) could weaken the control of the screen on accommodation, so that accommodation could be induced by convergence. To test this hypothesis, we extended the DoF using Zernike spherical aberrations (fourth and sixth orders) induced by a binocular adaptive optics (AO) vision simulator. Nine normal subjects between the ages of 21 and 34 (26 ± 5) years were recruited. Three optical conditions were generated: AO condition (aberration-free), monovision condition, and extended depth of focus (EDoF) condition. Binocular accommodative responses, along with binocular visual acuity and stereoacuity, were measured under all three optical conditions with varied binocular vergence levels. At 3 diopters of binocular convergence, the EDoF condition was the most efficient in inducing excessive accommodative response compared with the monovision condition and the AO condition. Visual acuity was impaired with EDoF as compared with the other two conditions. The average stereoscopic thresholds (at 0 vergence) under the EDoF condition were degraded compared with the AO condition but were superior to those of the monovision condition. Therefore, despite some compromise to visual performance, extending the DoF could allow for a more natural vergence–accommodation relationship, providing the potential for alleviating the vergence–accommodation conflict and associated visual fatigue symptoms in virtual reality.     

双眼植入多焦和单焦人工晶状体后视力及立体视比较研究

目的 比较双眼植入ReSTOR+3D非球面多焦点人工晶状体(MIOL)与传统球面单焦点人工晶状体(SIOL)后全程视力和立体视觉情况.方法 前瞻性临床研究.对2009年5月至2010年6月在上海公利医院眼科治疗的病人18例(36只眼)植入ReSTOR+3DMIOL(SN6ADI),20例(40只眼)植入传统球面SIOL(SNt0AT).以术后第二眼为标准随访3个月.测两组的远、中、近视力及近立体视锐度,进行相应统计学分析.结果 术后裸眼及最佳远、近矫正视力两组无明显差异(t值分别为1.14、1.74、0.99,P＞0.05),在25cm、30cm、33cm、40cm、50cm、60cm、70cm近中距离上,MIOL组远视力矫正下近视力均优于SIOL组(t值分别为14.02、20.28、24.52、32.74、21.91、11.66、9.76,P＜0.01).MIOL组裸眼近立体视锐度优于SIOL组(P＜0.05).双眼矫正近视力后,两组的立体视觉差异无统计学意义(X2=64.97 P＞0.05).结论 ReSTOR+3D多焦点IOL较单焦点IOL可提供更好的近、中视力和裸眼近立体锐度,实现了很高地脱镜率,良好地全程视力使其拥有广泛的应用价值.

Abstract：

Objective To compare visual acuity from far to near and stereoscopic vision in patients who underwent bilateral implantation of aspheric diffractive multifocal intraocular lens (IOLs)with monofocal IOLs. Methods This prospective study comprised patients having implantation of an aspheric diffractive multifocal ReSTOR SN6AD1 IOL with a C3.0 D add (SN6AD1) or a monofocal IOL (SN60AT). Visual acuity from far to near distances and stereoscopic vision were evaluated 3months postoperatively. Results Multifocal IOL group comprised 36 eyes of 18 patients. Monofocal IOL group comprised 40 eyes of 20 patients. The differences between groups of uncorrected and corrected distance visual acuity and corrected near visual acuity had not statistically significant (P＞0.05).The mean distance-corrected visual acuity at 25 cm, 30 cm, 33 cm, 40 cm, 50 cm, 60 cm, 70 cm were better in the multifocal group than in the monofocal group (P＜0.01). The near stereoscopic vision. in the multifocal group were better than in the monofocal group (P＜0.01). When near corrected,there were no significant difference between the stereoscopic vision for the multifocal and monofocal groups (P＞0.05). Conclusions The diffractive multifocal IOL with a low add power provides significantly better intermediate, near visual acuity than the monofocal IOL. When near uncorrected, the multifocal group has better stereoscopic vision. Spectacle independence is higher with the multifocal intraocular lens.     

Saliency in a suppressed image affects the spatial origin of perceptual alternations during binocular rivalry

During binocular rivalry, perception alternates between dichoptically presented incompatible images. With larger images, such perceptual alternations will typically start locally and then gradually spread across the image, known as traveling waves of perceptual dominance. Several image-features (such as local contrast) are known to determine where in the image a traveling wave originates. Here we investigate whether orientation contrast in the suppressed image affects these spatial origin(s) of perceptual alternations. The results show that the origins are increasingly biased towards locations of increasing orientation contrast in the suppressed image. This increase in bias is related to the efficiency of visual search for the orientation contrast, tested offline: we find large biases towards orientation contrast when visual search for it is efficient, and small biases when search for it is inefficient. Our results imply that rivalry suppression is not homogenous across the suppressed image, but is dependent on local image-features in the suppressed image. The relation between spatial bias and visual search performance suggests that spatial origins of perceptual alternations are biased to salient locations in the suppressed image. Moreover, the finding that saliency affects the spatial origin of a perceptual alternation is in agreement with the idea that saliency is represented at a monocular, unconscious level of visual processing.