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PubMed2004

Stereoscopic depth magnitude estimation: effects of stimulus spatial frequency and eccentricity.

To determine the effects of stimulus spatial frequency and retinal eccentricity on the perception of depth magnitude derived from disparity cues alone, subjects were asked to estimate the magnitude of depth of a stereoscopically viewed Gabor patch presented to the central or peripheral field with either crossed or uncrossed absolute disparity. Disparity vergence responses to the same Gabor stimuli were separately estimated subjectively by determining the offset required for dichoptic nonius alignment following presentation of the stimulus. The normalized stereoscopic magnitude estimation data generally showed that crossed disparities were perceived with greater depth than uncrossed disparities of the same magnitude, whether presented to the central or peripheral field. Asymmetries in magnitude of depth perception ranged from mild differences between depth directions to complete lack of depth perception for one direction. Disparity vergence response functions varied from (1) appropriate initiation of vergence to both directions of disparity, (2) initiation of vergence to only one direction of disparity, or (3) an attenuated initiation of vergence response to either direction of disparity. Within subjects, their asymmetries in magnitude of depth perception did not correlate with their asymmetries in vergence initiation. The similarity of the asymmetric depth magnitude estimation for a given individual at both stimulus locations tested suggests that common neural mechanisms are responsible for central and peripheral depth magnitude estimation. The lack of correlation between the perceptual and motor responses to the same stimuli suggests that the neural pathways for these responses diverge shortly after the detection of disparity in primary visual cortex.

Adult↗
PubMed1994

The effect of gap depth on the perception of whether a gap is crossable.

Four experiments were performed in order to examine the effect of gap depth on human observers' perception of whether or not a gap is crossable. Experiments 1 and 2 showed that as the gap's depth increased, observers tended to increasingly underestimate the maximum width of a gap they could step across. Experiments 3 and 4 clarified this finding: The observed covariation of perceived gap crossability and gap depth depended on the observer's direction of gaze, rather than on the physical depth of the gap. The optical relations to which observers might be attending are discussed, as well as the possibility that cognitive-affective processes might have contributed to observers' underestimation of their actual capabilities.

Depth Perception↗
PubMedDate not supplied

[Perception in space. Visual aspects of space perception].

The ability to apprehend and appropriate space is based on a series of perceptive and cognitive processes, in which vision plays a leading part. Preliminary, elementary operations first allow a subject to achieve the elaboration of a visually structured percept. Both experiences on healthy subjects, and pathological data have provided evidence for a right hemisphere superiority in tasks such as localization of stimuli and discrimination of line orientation. Depth perception rests both on indirect cues (e.g. relative size of objects, perspective and movement parallax) and on two types of specific stereoscopic processes: the so-called local and global stereoscopies, which depend respectively on the integrity of visual cortices and of inferior occipito-temporal areas (with a right-hemisphere dominance). Movement perception is integrated at the level of area V5, at the lateral occipito-temporal junction on both sides. The accuracy of reaching movements and displacements within space require the elaboration of a system of coordinates in which the position of the egocentric reference will then be taken into account. This is achieved by successive coding of eye and head position relative to the body axis, and by building the body reference from proprioceptive and vestibular afferences to the parietal cortex. Finally, attention must be both diffusely scattered throughout the whole space and voluntarily allocated as needed on a given object. Simultanagnosia and unilateral spatial neglect (USN) are examples of disruption of these attentional processes. Simultanagnosia illustrates the loss of the ability to shift attention from one point to another, but both shrinking of the attentional field and poorly sustained fixation are also possible mechanisms of this syndrome. USN probably has several components. Unilateral brain damage causes interhemispheric attentional imbalance that explains the attentional preference for the ipsilesional side of space. The predominance of neglect following right-sided lesions might be the consequence of hemispheric specialization regarding directed attention, with the left hemisphere preferentially allocating attention to objects and the right one being able to attend to the whole space. The anatomical basis of such a functional differentiation is still unclear.

Animals↗
PubMed2004

Experience in early infancy is indispensable for color perception.

Early visual experience is indispensable to shape the maturation of cortical circuits during development. Monocular deprivation in infancy, for instance, leads to an irreversible reduction of visually driven activity in the visual cortex through the deprived eye and a loss of binocular depth perception. It was tested whether or not early experience is also necessary for color perception. Infant monkeys were reared for nearly a year in a separate room where the illumination came from only monochromatic lights. After extensive training, they were able to perform color matching. But, their judgment of color similarity was quite different from that of normal animals. Furthermore, they had severe deficits in color constancy; their color vision was very much wavelength dominated, so they could not compensate for the changes in wavelength composition. These results indicate that early visual experience is also indispensable for normal color perception.

Animals↗
PubMed2003

Use of Dictionary of Occupational Titles (DOT) on formwork carpentry--a comparison between the United States and Hong Kong.

OBJECTIVES: This study aimed at investigating the utilization and applicability of the Dictionary of Occupational Titles (DOT) as a methodology to study the job profile (nature and physical demand) of formwork carpentry in the local situation. STUDY DESIGN: Thirty male formwork carpenters were recruited by convenient sampling to participate in a two-hour interview, with reference to the DOT Physical Demand Questionnaire (DOTPDQ) and the WestTool Sort Questionnaire. The information obtained was further consolidated by comparing the results from the interview to three construction sites and training guidelines from the formwork carpentry training centers. The triangulation of the data formulated a job profile of formwork carpenters. RESULTS: The results from the DOTPDQ revealed that workers' work demands were standing, walking, pushing, pulling, reaching, climbing, balancing, stooping, crouching, lifting, carrying, handling and near acuity. This produced an agreement of 84.6% with the original DOT. A discrepancy was found in the demands of kneeling, fingering, far acuity and depth perception. CONCLUSIONS: The discrepancy between the data from the United States and local appeared to be minimal. It was thus inferred that the DOT-based job profile was largely valid for describing formwork carpentry in Hong Kong. In-depth analysis should be conducted to further substantiate the validity of utilizing the DOT system for other job types and their physical demands.

Adult↗
PubMed2005

Gaze-contingent soft tissue deformation tracking for minimally invasive robotic surgery.

The introduction of surgical robots in Minimally Invasive Surgery (MIS) has allowed enhanced manual dexterity through the use of microprocessor controlled mechanical wrists. Although fully autonomous robots are attractive, both ethical and legal barriers can prohibit their practical use in surgery. The purpose of this paper is to demonstrate that it is possible to use real-time binocular eye tracking for empowering robots with human vision by using knowledge acquired in situ. By utilizing the close relationship between the horizontal disparity and the depth perception varying with the viewing distance, it is possible to use ocular vergence for recovering 3D motion and deformation of the soft tissue during MIS procedures. Both phantom and in vivo experiments were carried out to assess the potential frequency limit of the system and its intrinsic depth recovery accuracy. The potential applications of the technique include motion stabilization and intra-operative planning in the presence of large tissue deformation.

Artificial Intelligence↗
PubMed2001

Self-motion and the perception of stationary objects.

One of the ways that we perceive shape is through seeing motion. Visual motion may be actively generated (for example, in locomotion), or passively observed. In the study of the perception of three-dimensional structure from motion, the non-moving, passive observer in an environment of moving rigid objects has been used as a substitute for an active observer moving in an environment of stationary objects; this 'rigidity hypothesis' has played a central role in computational and experimental studies of structure from motion. Here we show that this is not an adequate substitution because active and passive observers can perceive three-dimensional structure differently, despite experiencing the same visual stimulus: active observers' perception of three-dimensional structure depends on extraretinal information about their own movements. The visual system thus treats objects that are stationary (in an allocentric, earth-fixed reference frame) differently from objects that are merely rigid. These results show that action makes an important contribution to depth perception, and argue for a revision of the rigidity hypothesis to incorporate the special case of stationary objects.

Form Perception↗
PubMed2003

Persisting visual hallucinations and illusions in previously drug-addicted patients.

BACKGROUND: Tetrahydrocannabinol (cannabis) and lysergic acid diethylamide (LSD) are psychomimetic agents that induce impairment of sensory perception. Illusions and hallucinations are mostly visual. Most frequently the visual phenomena occur in conjunction with drug abuse. PATIENTS AND METHODS: Three previously drug-addicted patients were examined for either persisting or spontaneously recurrent visual phenomena. Two patients complained of persisting visual illusions (vibrations, dyskinetopsia and impaired depth perception) during more than 12 months after an excessive use of cannabis. The third patient was a multiple drug abuser (LSD for 6 years) and complained of visual hallucinations and palinopsia following heavy ethanol intake, 20 years after stopping the use of any drug. RESULTS: Results from neuro-ophthalmic and neurological examinations were normal for the first two patients. The third patient presented abnormal visual fields with preserved visual acuity; electroencephalography was abnormal, suggesting an underlying toxic encephalopathy. CONCLUSIONS: Persistent visual illusions or hallucinations can occur during several months after an intake of cannabis. Flash-back phenomena are frequent amongst LSD abusers. They rarely occur at long times after the last intake (20 years in the present case); when they do so, precipitating factors are often present (ethanol, medication, anesthesia). Such phenomena reflect the cortical dysfunctions that can be induced by illegal substances.

Adolescent↗
PubMed1997

Gradients as visual primitives.

Following J. J. Gibson (1950), it is implicitly assumed in the literature that texture gradients are directly available as perceptual primitives. Yet, the depth response to compression gradients is poor compared with gradients of linear perspective. This may indicate that mechanisms for directly detecting the differential structure that constitutes a compression gradient do not exist. We tested this hypothesis outside the context of depth perception by measuring the speed with which participants could detect a gradient anomaly as a function of the number of elements in the gradient. Only in the case of linear perspective did anomalies "pop out." This was attributable to the emergent feature of alignment of the ends of the elements forming the gradient rather than the direct detection of its differential structure. It is argued that gradients are not perceptual primitives and that the poor depth response to compression in a variety of context (motion parallax, stereo, and perspective) therefore is not surprising.

Adult↗
PubMed2003

Apparent motion cues distort object localisation in egocentric space.

The visual localisation of objects in space is thought to rely on retinal information defining the environmental context and non-retinal cues from proprioception and motor commands. Here, the influence of dynamic contextual cues on the perception of egocentric space in a reaching task was investigated. Compared to performances with realistic motion or static cues, target localisation was less accurate when apparent motion was used to provide contextual information about space between the hand and the target. This effect could not be explained by the 'presence' of motion, or a bias in depth perception. Since the distortion was connected with the reaching area it was concluded that cognitive factors can unconsciously influence the perception of egocentric space, in particular distance estimation. We propose a mechanism for this whereby signals from areas MT/MST (middle temporal/medial superior temporal) create a perceptual bias through cortico-cortical connections with posterior parietal cortex.

Adult↗
PubMed1995

Visual Displays and Visual Perception in Minimal Access Surgery.

Visual perceptual processing underlies safe execution of endoscopic surgery. This review deals with the limitations of the present visual display technology used in endoscopic surgery with reference to the normal direct stereoscopic vision and pints to the research and development needed in this important technological and psychomotor aspect of endoscopic surgery. Eyeball movements (saccadic and smooth pursuit), visual cues (stereoscopic and monoscopic), accommodation, individual visual attributes, and the display technology itself are all important. Monocular depth cues are degraded by the cureent display systems, and technological advances in this are will improve perceptual processing and reduce both fatigue and human error during endoscopic interventions. Depth perception can be improved by alternative techniques to three-dimensional imaging such as the VISTRAL system and the Suspended Image System based on projection of image by parabolic mirrors and advanced beam spitter technology.

Journal Article↗
PubMedDate not supplied

Distance and shape: perception of the 3-dimensional world by weakly electric fish.

Weakly electric fish orient at night in complete darkness by employing their active electrolocation system. They emit short electric signals and perceive the consequences of these emissions with epidermal electroreceptors. Objects are detected by analyzing the electric images which they project onto the animal's electroreceptive skin surface. This process corresponds to similar processes during vision, where visual images are cast onto the retinas of eyes. Behavioral experiments have shown that electric fish can measure the distance of objects during active electrolocation, thus possessing three-dimensional depth perception of their surroundings. The fundamental mechanism for distance determination differs from stereopsis used during vision by two-eyed animals, but resembles some supplementary mechanisms for distance deduction in humans. Weakly electric fish can also perceive the three-dimensional shape of objects. The fish can learn to identify certain objects and discriminate them from all other objects. In addition, they spontaneously categorize objects according to their shapes and not according to object size or material properties. There is good evidence that some fundamental types of perceptional invariances during visual object recognition in humans are also found in electric fish during active electrolocation. These include size invariance (maybe including size constancy), rotational invariance, and translational invariance. The mechanisms of shape detection during electrolocation are still unknown, and their discoveries require additional experiments.

Animals↗
PubMedDate not supplied

Monocular aniseikonia: a motion parallax analogue of the disparity-induced effect.

Mayhew and Longuet-Higgins have recently outlined a computational model of binocular depth perception in which the small vertical disparities between the two eyes' views of a three-dimensional scene are used to determine the 'viewing parameters' of fixation distance (d) and the angle of asymmetric convergence of the eyes (g). The d/g hypothesis, as it has been called, correctly predicts that a fronto-parallel surface, viewed with a vertically magnifying lens over one eye, should appear to be rotated in depth about a vertical axis. We report here a comparable illusion for surfaces specified by monocular motion parallax information, which can be explained more simply by considering the differential invariants of the optic flow field. In addition, our observations suggest that the disparity-induced effect is not a 'whole field' phenomenon nor one limited to small magnification differences between the eyes.

Aniseikonia↗
PubMed1999

Modeling and optimization of rotational C-arm stereoscopic X-ray angiography.

Stereoscopy can be an effective method for obtaining three-dimensional (3-D) spatial information from two-dimensional (2-D) projection X-ray images, without the need for tomographic reconstruction. This much-needed information is missed in many X-ray diagnostic and interventional procedures, such as the treatment of vascular aneurysms. Fast C-arm X-ray systems can obtain multiple angle sequences of stereoscopic image pairs from a single contrast injection and a single breath hold. To advance this solution, we developed a model of stereo angiography, performed perception experiments and related results to optimal acquisition. The model described horizontal disparity for the C-arm geometry that agreed very well with measurements from a geometric phantom. The perceptual accommodation-convergence conflict and geometry limited the effective stereoscopic field of view (SFOV). For a typical large image intensifier system, it was 28 cm x 31 cm at the center of rotation (COR). In the model, blurring from finite focal-spot size and C-arm motion reduced depth resolution on the digital display. Near the COR, the predicted depth resolution was 3-11 mm for a viewing angle of 7 degrees , which agreed favorably with results from recently published studies. The model also described how acquisition parameters affected spatial warping of curves of equal apparent depth. Pincushioning and the difference between the acquisition and display geometry were found to introduce additional distortions to stereo displays. Preference studies on X-ray angiograms indicated that the ideal viewing angle should be small (1-2 degrees), which agreed with some previously published work. Perceptual studies indicated that stereo angiograms should have high artery contrast and that digital processing to increase contrast improved stereopsis. Digital subtraction angiograms, with different motion errors between the left and right-eye views, gave artifacts that confused stereopsis. The addition of background to subtracted images reduced this effect and provided other features for improved depth perception. Using the modeling results and typical clinical angiography requirements, we recommend acquisition protocols and engineering specifications that are achievable on current high-end systems.

Angiography, Digital Subtraction↗
PubMed1982

[Neurobiological principles of binocular space perception. Historical review on the development of the concept and knowledge].

Binocular vision and space perception are not readily explained on the basis of direct anatomical evidence on the visual system. Galen (2nd century A.D.) localized the mechanism underlying binocular vision into the optic chiasm. Nearly 2000 years later the famous physicist Newton (1704), undoubtedly stimulated by clinical reports on hemianopia and Descartes' (1686) hypothesis on the retinal image projections on the brain, came up with a new hypothesis on the optic chiasm which could explain how information from homonymous visual hemifields could converge from the two eyes to one and the same area in the brain. He predicted that the temporal retinal fibers of both eyes fail to cross. Irrefutable proof of the special arrangement of the optic chiasm was provided by Ramon y Cajal (1899) with the aid of the Golgi method. It took more than 50 years before binocularly oriented cells were discovered in the visual cortex by Baumgartner and colleagues (1958), and before neurons were found in that area by Hubel and Wiesel (1959) which responded to stimulation from corresponding retinal sites of the two eyes. These electrophysiological studies profited by the localization of the cortical visual center in the occipital lobe by Panizza (1856), the identification of the corpus geniculatum laterale as a relais station of the retino-cortical pathway by Monakow (1883, 1885) and the discovery of the projection of homonymous retinal halves upon alternating layers of this structure by Minkowski (1920). Wheatstone opened the road to the understanding of stereoscopic vision by inventing the stereoscope in 1838. He based his research largely upon the the investigation of Aguilonius (1613), Vieth (1818) and Müller (1926) on the horopter. The most elegant evidence on the significance of visual disparity for depth perception came recently from Julesz and his demonstration of "random-dot-stereograms" (1971). Binocular neurons which are sensitive to minimal stimulus disparities within Panum's area (1858) have only recently been observed in the striate cortex by electrophysiological recordings (Barlow et al., 1967; Poggio and Fischer, 1977). These cells were designated as "disparity or depth detectors". They displace the "cyclopean eye" of Helmholtz (1867) and Hering (1879) from its Homeric seat in the forehead to the cortical field whose striations were already noted by Gennari (1782) near the posterior end of the cranium.

Animals↗
PubMed2004

On the relationship between orbit orientation and binocular visual field overlap in mammals.

The orbital apertures of Primates are among the most convergent (i.e., facing in the same direction) among mammals. It is often assumed that orbit convergence is associated with binocular visual field overlap and stereoscopic depth perception in primates. Likewise, it is also assumed that orbit orientation reflects the shape of the visual field across mammals. To date, however, no study has demonstrated that orbit and visual field orientation are correlated, much less comparable, across mammals. In this study, data on orbit convergence were collected for a representative sample of mammals for which data on the extent of the visual field are available. Both standard and phylogenetically controlled comparisons were made. The results demonstrate that orbit convergence and binocular visual field overlap are significantly correlated and display a linear relationship. Based on orbit convergence, Primates as a group have the largest binocular visual fields among mammals.

Adaptation, Physiological↗
PubMed1988

Determinants involved in the perception of the Necker cube: an application of catastrophe theory.

The study is concerned with evaluating interactions at the organic level within the visual perception subsystem of living systems. The reported work focuses on the identification of some of the determinants of multistable perception by experimentally testing a nonlinear dynamical systems (catastrophe) model of the Necker Cube. This technique serves as an advantage over linear threshold models which cannot effectively study multivalued functional relationships. It was proposed that manipulation of two independent control parameters (bias or changing shape by continuously varying perspective lines and selective stimulus shading) was compatible with the subjective dichotomy of bistable perception of the Necker cube. One hundred and twenty naive subjects, categorized by age, sex, and optical aids, were presented with a computer-generated sequence of 63 stimuli (7 shading levels X 9 perspective levels) to which they had to respond as to whether they saw a "hollow" or "solid" image. The work revealed that bias and shading exerted their effects in opposition and that each influenced the other. Both were decisive factors involved in the perception of the cube. These findings are supported by topological and psychological evidence.

Adolescent↗