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The perceived shade of gray depends primarily on the luminance relationship between surfaces percieved to lie in the same plane and not between surfaces that are merely adjacent in the retinal image. This result implies that depth perception must precede lightness perception and that lateral inhibition cannot explain lightness constancy.
BACKGROUND: Cerebral achromatopsia is a disturbance of colour perception which may be complete or partial. CLINICAL RECORD: A 28-year-old male patient presented five months after carbon monoxide poisoning with achromatopsia. The achromatopsia was unaccompanied by an inability to recognise faces (prosopagnosia) nor was there any disorder of form or depth perception. RESULTS: Magnetic resonance imaging showed bilateral sharply defied areas of haemorrhagic infarction in the globus pallidus with extensive infarction involving temporal and occipital lobes and with apparent partial sparing of the visual cortex, presumably due to arterial insufficiency. The disturbance of central colour vision resolved spontaneously after a further period of 6 months. CONCLUSION: The symptom of achromatopsia is analysed with particular reference to the recent work of Professor Zeki on disturbance of central colour vision following CO poisoning and the unusual MRI findings.
Recent developments in microelectronics have encouraged the use of 3D data bases to create compelling volumetric renderings of graphical objects. However, even with the computational capabilities of current-generation graphical systems, real-time displays of such objects are difficult, particularly when dynamic spatial transformations are involved. In this paper we discuss a type of visual stimulus (the stereokinetic effect display) that is computationally far less complex than a true three-dimensional transformation but yields an equally compelling depth impression, often perceptually indiscriminable from the true spatial transformation. Several possible applications for this technique are discussed (e.g., animating contour maps and air traffic control displays so as to evoke accurate depth percepts).
Stereoscopic depth perception relies on binocular disparities, or small geometric differences between the retinal images of each eye. The most reliable binocular depth judgments are those that are based on relative disparities between two simultaneously visible features in a scene. Many cortical areas contain neurons that are sensitive to disparity, but it is unclear whether any areas show a specific sensitivity to relative disparity. We recorded from neurons in the early cortical visual area V2 of the awake macaque during presentation of random-dot patterns. The depth of a central region ('center'), and that of an annular surrounding region ('surround'), were manipulated independently in these stimuli. Some cells were fully selective for the resulting relative disparities. Most showed partial selectivity, which nonetheless indicated a sensitivity for the depth relationship between center and surround. Both types of neural response could support psychophysical judgments of relative depth.
The Pulfrich pendulum phenomenon, in which a pendulum swinging in the frontoparallel plane appears to swing in an ellipse when a neutral density filter is placed over one eye of the observer, was investigated in stereoblind subjects. It was found that such subjects can report the presence of the Pulfrich effect although they fail to fuse random-dot stereograms and fail to exhibit interocular transfer of the movement aftereffect. These findings suggest that 'stereoblind' subjects must retain some residual binocular mechanism for depth perception. Three possibilities are considered: (i) the stereoblind may be able to utilise contiguous temporal disparities as a cue for depth in the Pulfrich effect; (ii) they may retain some residual binocularity, sufficient to reveal the Pulfrich effect but not for other more demanding tasks for the binocular mechanisms; and (iii) they may retain some coarse magnocellular pathway disparity mechanism having lost their high-acuity parvocellular disparity system. There is little evidence to support any of these hypotheses, but the third shows promise.
For 35 to 39 days, four observers wore continuously left-right reversing spectacles which pseudoscopically reverse the order of binocular disparity and direction of convergence. In three tests, we investigated how the visual system copes with the transformation of depth and distance information due to the reversing spectacles. In stereogram observation, after a few days of wearing the spectacles. the observers sometimes perceived a depth order which was opposite to the depth order that they had perceived in the pre-spectacle-wearing period. Monocular depth cues contributed more to depth perception in the spectacle-wearing period than they did in the pre-spectacle-wearing period. While the perceived distance significantly decreased during the spectacle-wearing period, we found no evidence of adaptive change in distance perception. The results indicate that the visual system adapts itself to the transformed situation by not only changing the processing of disparity but also by changing the relative efficiency of each cue in determining apparent depth.
A new system capable of generating static and dynamic random-element stereograms is designed and implemented. The stereogram is composed of a rectangle that can be varied in size, element density, orientation, and horizontal and vertical disparities, and can be swept at several velocities. The system is based on an inexpensive personal computer (Amiga 500), designed to work as a slave system under the control of a host computer (e.g. any personal computer). A colour monitor or two black and white monitors are suitable display devices to view the stereograms. This system is used to perform psycophysical experiments in humans and neurophysiological experiments in behaving monkeys, with the purpose of studying depth perception and binocular vision.
A substantial proportion of both simple and complex neurons in the cortex subserving central vision are differentially sensitive to binocular disparity of isolated line patterns (local stereopsis), a sensitivity based on a positional disparity between the neuron's receptive fields in the two eyes. In addition, a subset of cortical neurons, nearly all complex neurons, responds to dynamic random-dot stereograms containing no depth cues other than disparity. These neurons are capable of signaling the correct binocular matches among a multitude of false matches in the stereograms (global stereopsis). The discovery of cyclopean neurons in striate cortex, at early stages of the processing neural network for stereoscopic vision provides a new insight of the basic neural mechanisms underlying binocular depth perception.
Because of the horizontal separation of both ocular globes, the projection angles are slightly different. These differences are commonly termed retinal disparities. Vertical and horizontal retinal disparities occur constantly in normal life. We have investigated the responses of single cells in cortical areas V1 and V2 of behaving Macaca mulatta monkeys to retinal disparities by using dynamic random dot stereograms. Our findings show that cortical visual cells are sensitive to both vertical and horizontal disparities. To calculate the distance between two objects in a three-dimensional space from horizontal disparities, it is necessary to know the fixation distance. It has been suggested that the horizontal gradient of vertical disparity contains information to estimate the fixation distance and therefore to scale horizontal disparities. We suggest that these cells sensitive to horizontal and vertical disparities represent a neural mechanism that provides information to the visual system in order to achieve a correct eye alignment and depth perception.
Otoliths of two species of bony fishes have been found to be piezoelectric. Hence, in theory, they constitute a mechanism for depth perception or frequency analysis of sound waves, or both.
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.
Stereoacuity was measured with an operant conditioning technique in adult cats with unilateral deafferentation of oculomotor proprioception (section of the ophthalmic branch of the fifth cranial nerve). Binocular stereoacuity was found not to exceed monocular depth sensitivity, in contrast to what is obtained in normal cats. The results confirm previous findings and indicate a role of oculomotor proprioception in binocular depth perception of the cat.
The short-term adaptation (1-4 h) to the induced effect was investigated on four subjects. A 4% meridional afocal magnifier axis 180 was worn before one eye in normal binocular conditions, which produced vertical retinal-image disparity. In certain circumstances this vertical disparity will give rise to the induced effect, i.e. a surface in the objective frontal plane will appear to be rotated around a vertical axis with the apparently closer edge on the same side as the magnifying lens. Adaptation, which is considered as a reduction in the induced effect over time, was measured with a modified Howard - Dolman apparatus. Both the maximal induced effect and the maximal short-term adaptation occurred within the first 30 min and ranged from 40 to 85%. The final level of short-term adaptation to the induced effect ranged from 20 to 45%. These results suggest a rapid recalibration of binocular depth perception by higher-level perceptual centers.
BACKGROUND: Depth perception is reduced in endoscopic surgery, although little is known about the effect this has on surgical performance. METHODS: To assess the role of depth cues, 45 subjects completed tests of depth cue reliance. Surgical skill was assessed using the Minimally Invasive Surgical Trainer-Virtual Reality, a previously validated laparoscopic simulator. RESULTS: We could demonstrate no difference in cue reliance for three depth cues--namely stereo, texture, and outline--between surgeons and medical students. Greater dominance on stereo for medical students was a positive finding and a negative finding for the surgeons when correlated with surgical performance. CONCLUSIONS: We suggest that surgeons learn to adapt to the nonstereo environment in MIS, and this is the first study to show evidence of this phenomenon. This difference in stereo reliance is a reflection of the experience that surgeons have with laparoscopy compared with medical students, who have none.
When a closer surface partially occludes a more distant surface, there exist image zones adjacent to the occluding edge on the rear surface which are visible to one eye and not the other. These half-occluded or interocularly unpaired zones do not carry explicit disparity information, yet their depth is perceived as a stable and continuous extension of the rear surface. Moreover, such zones escape binocular rivalry. In addition to these properties, we now report another special characteristic of this unpaired zone in comparison to normally paired regions. An unpaired probe dot added here escapes non-unique local Panum matching which would otherwise bestow it with a depth outside the surface. Thus paradoxically, depth of the probe is most stable in the unpaired zone. This finding indicates that what is considered to be one of the most fundamental processes for binocular depth perception, namely local matching, is subject to more global surface occlusion constraints.
This paper examines and contrasts motion-parallax analogues of the induced-size and induced-shear effects with the equivalent induced effects from binocular disparity. During lateral head motion or with binocular stereopsis, vertical-shear and vertical-size transformations produced 'induced effects' of apparent inclination and slant that are not predicted geometrically. With vertical head motion, horizontal-shear and horizontal-size transformations produced similar analogues of the disparity induced effects. Typically, the induced effects were opposite in direction and slightly smaller in size than the geometric effects. Local induced-shear and induced-size effects could be elicited from motion parallax, but not from disparity, and were most pronounced when the stimulus contained discontinuities in velocity gradient. The implications of these results are discussed in the context of models of depth perception from disparity and structure from motion.
The true receptive field of more than 90% of neurons in the middle temporal visual area (MT) extends well beyond the classical receptive field (crf), as mapped with conventional bar or spot stimuli, and includes a surrounding region that is 50 to 100 times the area of the crf. These extensive surrounds are demonstrated by simultaneously stimulating the crf and the surround with moving stimuli. The surrounds commonly have directional and velocity-selective influences that are antagonistic to the response from the crf. The crfs of MT neurons are organized in a topographic representation of the visual field. Thus MT neurons are embedded in an orderly visuotopic array, but are capable of integrating local stimulus conditions within a global context. The extensive surrounds of MT neurons may be involved in figure-ground discrimination, preattentive vision, perceptual constancies, and depth perception through motion cues.
BACKGROUND/AIMS: Blindness is a major public health problem in developing countries, even though most could be prevented by relatively simple hygienic and medical interventions. Relatively few patients use the quality health care services available, despite their low cost, due to problems of access or socio-cultural barriers. This health services research project stressed the need for measurement of subjective self-perceived health. The objectives of this study were twofold: a) To translate, adapt and integrate the cultural context found in Mali and validate two instruments for measuring, respectively, perceived vision and quality of life. b) To study the relationship between these variables and visual deficiencies by gender. METHODS: The perceived vision and quality of life questionnaires were based on a translation of the Aravind questionnaire, adapted to Mali. The resulting perceived vision questionnaire comprises 13 questions, grouped according to five subscales (global vision, visual perception, sensory adaptation, visual field and depth perception). Furthermore, the 13 questions on quality of life were grouped into four subscales (personal care, mobility, social life and psychological). For both questionnaires, a global score could be computed. These two questionnaires were administered to a representative sample of 203 subjects with impaired vision, aged over 40, in a rural area in Mali. RESULTS: The acceptability of the questionnaires was good (1% missing data). The convergent validity was adequate for all but one subscale (psychological). The discriminate validity is acceptable for three of the six subscales where measurement can be made (visual perception, personal care, mobility). The Cronbach alpha coefficients indicate good reliability for the global scores. CONCLUSIONS: Analysis of mean results confirms the validity of the International Classification of Disease (ICD) definition of blindness (seeing less than 0.05 results in a steep decrease in quality of life). Moreover, blindness affects the quality of life of women more severely than that of men; this may be related to the availability of social support.