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PubMed1997

Fixation disparity and accommodation as a function of viewing distance and prism load.

Fixation disparity was measured with dichoptically presented nonius lines at viewing distances of 20, 30, 40, 60, and 100 cm, so that both vergence and accommodation were stimulated adequately as in normal vision. As the viewing distance was shortened, mean fixation disparity changed monotonically from 1 min arc eso (i.e., the eyes converged in front of the target) to 3 min arc exo. The average standard deviation of the psychometric function of fixation disparity, which is a measure of the temporal variability of vergence, was smallest at 100 cm (when fixation disparity was eso) and increased as viewing distance decreased. Fixation disparity itself and the change of fixation disparity with distance differed reliably among subjects with normal binocular vision, but neither was related to the momentary degree of accommodation. Fixation disparity was also measured at a constant distance of 40 cm, but with prisms in front of the eyes that induced the same vergence angles as viewing distances between 20 and 100 cm. The slope of these conventional fixation disparity curves as a function of prism load was generally larger than the slope of fixation disparity as a function of viewing distance (which can be explained by accommodative vergence), but the slopes of the two types of curves were correlated (r = 0.39, P = 0.02, n = 25).

Accommodation, Ocular↗
PubMed2000

Transformation of the visual-line value in binocular vision: stimuli on corresponding points can be seen in two different directions.

We examined Wheatstone's (1838 Philosophical Transactions of the Royal Society of London 128 371-394) claim that images falling on retinally corresponding points can be seen in two different directions, in violation of Hering's law of identical visual direction. Our analyses showed that random-dot stereograms contain stimulus elements that are conceptually equivalent to the line stimuli in the stereogram from which Wheatstone made his claim. Our experiment demonstrated that two lines embedded in a random-dot stereogram appeared in two different directions when they stimulated retinally corresponding points, if the disparity gradient value of the lines was infinity relative to adjacent elements. To ensure that the two lines stimulated corresponding points, observers made vergence eye movements while maintaining the perception of the two lines in two different directions.

Convergence, Ocular↗
PubMed2000

A central mechanism of chromatic contrast.

The color appearance of a light can be altered by introducing a second, surrounding field. This phenomenon, called chromatic induction, is attenuated by chromatic variation within a remote region outside the surround [Shevell & Wei (1998). Vision Research, 38, 1561-1566]. We now consider the locus of the neural mechanism mediating the attenuation caused by the remote chromatic contrast. In the first experiment, the magnitude of chromatic variation within the remote region is changed either: (i) in the same eye that views the patch judged in color; or (ii) in only the opposite eye. The measurements are virtually the same in both cases, which implies attenuation of chromatic induction is mediated by a central, binocular mechanism. In the second experiment, the patch with its immediate inducing surround is changed in binocular disparity relative to the remote region with chromatic variation. The patch and surround, seen together in one depth plane, are perceived to be in front of, behind, or in the same plane as the remote region with chromatic variation. Attenuation of chromatic induction is strongest when the patch and surround are in the same depth plane as the remote region. This change of color appearance with disparity is consistent with a central binocular process. Overall, the color-appearance measurements are explained by monocular encoding of chromatic differences at edges, and a central binocular mechanism of chromatic-contrast gain control.

Color Perception↗
PubMed1989

[Computer-assisted determination of parameters of depth perception: studies with the 3-rod device].

Psychophysical determination of the thresholds of stereoscopic vision takes about 20 min. During this procedure, you may have the feeling that the thresholds are shifting. To verify this phenomenon, the thresholds must be determined in a period shorter than 20 min, e.g., 4-6 min. This is one of the reasons we constructed a computerized machine to detect the stereoscopic thresholds. The heart of this device is a micrometric screw driven by a stepping motor that moves the central rod of the three-rod apparatus. This motor is controlled by a microcomputer with a basic program. A special program has been designed that functions in a manner similar to that of the Octopus perimeter and detects the thresholds with in a few steps. It is thus possible to reduce the detection time for one threshold dramatically. The first results with this device are not only very precise, but also demonstrate the minute values between stereoscopic thresholds. For an observation distance of 380 mm, we were able to calculate the disparation angles down to 3 s of arc. If a number of measurements are taken, the threshold fluctuations and physiological zero position can be calculated.

Depth Perception↗
PubMed1979

Binocular single vision and perceptual processing.

Stimuli with small binocular disparities are seen as single, despite their differing visual directions for the two eyes. Such stimuli also yield stereopsis, but stereopsis and single vision can be dissociated. The occurrence of binocular single vision depends not only on the disparities of individual stimulus elements, but also on the geometrical relation of different parts of the pattern presented to each eye. A pair of vertical bars with opposite binocular disparities is seen as single if the pair is moderately widely spaced but not if it is narrow. Vertical alignment and identity in length of such bars also increase the occurrence of double vision. It is argued that these effects reflect the extraction of features of the monocular patterns, with these detected monocular features determining the binocular percept. Single and double vision of bars differing in orientation can be similarly analysed. The occurrence of relatively elaborate processing of monocular signals does not exclude the possibility that binocular interaction can occur between signals that have not been so processed. Multiple sites or types of binocular interaction are likely.

Functional Laterality↗
PubMed1997

Poor stereopsis can support size constancy in albinism.

PURPOSE: The size of a retinal image is inversely related to the distance to the object that generates the image. Normal subjects therefore exhibit size constancy, in which the perceived size of an image is scaled according to its perceived distance. Albinos usually have such poor binocular vision that they perform very poorly on clinical tests for stereopsis. To investigate the functional consequences of this poor stereopsis, we investigated whether stereopsis in these subjects could support size constancy. METHODS: The stereothresholds of 10 albinos and 12 normal control subjects were measured. The presence of absence of size constancy was investigated by having subjects equate the subjective size of stereoscopically presented images whose image disparity indicated that they were at different distances. RESULTS: Laboratory results indicated that eight albinos (including five whose clinical tests indicated a lack of stereopsis) had measurable stereopsis of several thousand are seconds or better. Of these, four also exhibited size constancy. CONCLUSIONS: Albinos who do not demonstrate stereopsis on clinical tests can have stereoscopic perception that commonly used clinical tests do not detect. Moreover, some of these patients even use this poor stereopsis in judging the size of stereoscopically presented images.

Adolescent↗
PubMed2006

The Mallett Fixation Disparity Test: influence of test instructions and relationship with symptoms.

Fixation disparity is a minute ocular misalignment under conditions of binocular single vision and is typically detected in primary eye care practices in the UK using the Mallett Unit Fixation Disparity Test. This instrument creates natural viewing conditions, when the patient's binocular system is fused using both central and peripheral fusion locks. This allows the examiner to determine the minimum prism power that eliminates the fixation disparity: the associated phoria or aligning prism. The spherical power that eliminates the fixation disparity, the aligning sphere, can also be determined. The near Mallett Unit Fixation Disparity Test has been shown to have good sensitivity and specificity for detecting symptomatic heterophoria. Cases of decompensated heterophoria tend to have a fixation disparity and the aligning prism or aligning sphere is a good indicator of the correction that will render the heterophoria compensated. The purpose of this study was, for the first time, to investigate the effect of test instructions on the results of the Mallett Unit Fixation Disparity Test. In study 1, we surveyed and observed practitioners to determine the instructions that are typically used. In study 2, we compared results obtained with this "standard" method of questioning with a more "specific" form of questioning that has been suggested in the literature. The participants for study 2 were 105 patients aged 7-70 years who were randomly selected from those attending a community optometric practice. Significantly different results were obtained with the two sets of instructions. The specific form of questioning revealed more cases of fixation disparity and the results with this method showed a better correlation with symptoms. This only held for near vision: for distance vision, symptoms were not significantly correlated with the presence of fixation disparity. This agrees with previous work with the Mallett unit, which showed a significant relationship with symptoms only at near. We also found that patients with more severe symptoms had greater degrees of aligning prism. Our study supports previous work indicating that the Mallett unit is a useful tool for detecting symptomatic heterophoria at near. However, we found that the testing method is important: patients need to be asked not just whether the nonius strips are aligned but also whether one or both of the strips ever moves. More research is needed to investigate the significance of precise test instructions in other optometric and orthoptic tests.

Adolescent↗
PubMed2003

Short-latency ocular following in humans is dependent on absolute (rather than relative) binocular disparity.

A previous study showed that the initial ocular following responses elicited by sudden motion of a large random-dot pattern were only modestly attenuated when that whole pattern was shifted out of the plane of fixation by altering its horizontal binocular disparity, but the same disparity applied to a restricted region of the dots had a much more powerful effect [Vision Research 41 (2001) 3371]. Thus, if the dots were partitioned into horizontal bands, for example, and alternate bands were moved in opposite directions to the left or right then ocular following was very weak, but if the (conditioning) dots moving in one direction were all shifted out of the plane of fixation (by applying horizontal disparity to them) then strong ocular following was now seen in the direction of motion of the (test) dots in the plane of fixation, i.e., moving images became much less effective when they were given binocular disparity. We sought to determine if the greater impact of disparity with the partitioned images was because there were additional relative disparity cues. We used a similar partitioned display and found that the dependence of ocular following on the absolute disparity of the conditioning stimulus had a Gaussian form with an x-offset that was close to zero disparity and, importantly, this offset was almost unaffected by changing the absolute disparity of the test stimulus. We conclude from this that it is the absolute--rather than the relative--disparity that is important, and that ocular following has a strong preference for moving images whose absolute disparities are close to zero. This is consistent with the idea that ocular following selectively stabilizes the retinal images of objects in and around the plane of fixation and works in harmony with disparity vergence, which uses absolute disparity to bring objects of interest into the plane of fixation [Archives of Ophthalmology 55 (1956) 848].

Eye Movements↗
PubMed2006

Apparent position in depth of stationary moving three-dimensional objects.

Motion signals contained within a stationary object projected on the fronto-parallel plane shift the object's apparent spatial position in the direction of the motion [see De Valois, R. L., & De Valois, K. K. (1991). Vernier acuity with stationary moving Gabors. Vision Research, 31(9), 1619-1626]. We report an analogous apparent position shift of three-dimensional objects that contain local elements that move in depth. Our stimulus was a transparent three-dimensional cylinder defined by 150 limited-lifetime dots, oriented such that it was end on and its tangent plane was circular. Dots moved in depth by changes in their binocular disparities. In the first experiment, observers judged the positions of the near and far ends of the cylinder, by moving marker lines in depth, for different dot speeds. The results showed that when dots moved towards the observer, the perceived location of the two ends of the cylinder appeared closer in depth. When dots moved away from the observer, the opposite effect was produced. Additionally, the amount of apparent position shift produced was dependent on dot speed, with faster speeds producing larger positional offsets. However, we found in the second experiment that when the cylinder contained randomly moving dots, or when the cylinder contained equal amounts of dots moving towards and away from the observer, positional shifts were very much reduced, or abolished. Our findings suggest that motion signals can induce a misperception of position in depth that is similar manner to that produced by motion within an object in the two-dimensional image plane.

Cues↗
PubMed1999

The effect of target proximity on the aniso-accommodative response.

Aniso-accommodation (unequal binocular accommodation) to lens-induced anisometropia has been demonstrated by subjective and objective measurement techniques (Marran and Schor, Vision Res. 38(22), 3601-3619). The gain of the response was significantly reduced for some subjects when aniso-accommodation was stimulated by a target at 1 m compared to a target at 20 cm, even when the targets viewed were matched in retinal image size, convergence levels and aniso-accommodative stimuli. The two conditions did differ in both the accommodative level and proximity of the target. Thus the higher gain of the response in the 20 cm condition could have arisen from either high proximity, high accommodative level or a combination of both. In this investigation, target proximity and accommodative level were manipulated independently while extra-retinal cues such as absolute disparity and image size were held constant. The results show that high target proximity alone rather than accommodative level or a combination of the two was responsible for the distance dependent effect demonstrated by subjects. Furthermore, accommodative level did not affect the response when target proximity was held constant. Subjects who demonstrated invariant aniso-accommodation with changes in viewing distance also demonstrated invariant aniso-accommodation with experimental manipulation of target proximity at the target distances tested, as would be expected. These results suggest that high level processing, rather than a reflex blur response, is involved in aniso-accommodation. This conclusion is strengthened by the previous finding of long reaction and response times, 11 and 15 s respectively, to step aniso-accommodative stimuli (Marran and Schor, Vision Res. 38(22), 3601-3619). Since the experimental target provided subjects with visual feedback of the relative blur of the dichoptically viewed letters, subjects had access to perceptual blur information cues. Some subjects seemed to have required both aniso-blur and proximity to exercise this volitional response. Those subjects who showed a distance invariant aniso-accommodative response may have been able to use perceived aniso-blur alone and their ability to disregard proximal cues may have resulted from greater experience with the aniso-accommodative stimuli. Alternatively, these subjects may have had a lower threshold to proximal stimuli and experienced target proximity at the more distant (1 m) viewing condition.

Accommodation, Ocular↗
PubMed2006

Capture of visual direction in dynamic vergence is reduced with flashed monocular lines.

The visual direction of a continuously presented monocular object is captured by the visual direction of a closely adjacent binocular object, which questions the reliability of nonius lines for measuring vergence. This was shown by Erkelens, C. J., and van Ee, R. (1997a,b) [Capture of the visual direction: An unexpected phenomenon in binocular vision. Vision Research, 37, 1193-1196; Capture of the visual direction of monocular objects by adjacent binocular objects. Vision Research, 37, 1735-1745] stimulating dynamic vergence by a counter phase oscillation of two square random-dot patterns (one to each eye) that contained a smaller central dot-free gap (of variable width) with a vertical monocular line oscillating in phase with the random-dot pattern of the respective eye; subjects adjusted the motion-amplitude of the line until it was perceived as (nearly) stationary. With a continuously presented monocular line, we replicated capture of visual direction provided the dot-free gap was narrow: the adjusted motion-amplitude of the line was similar as the motion-amplitude of the random-dot pattern, although large vergence errors occurred. However, when we flashed the line for 67 ms at the moments of maximal and minimal disparity of the vergence stimulus, we found that the adjusted motion-amplitude of the line was smaller; thus, the capture effect appeared to be reduced with flashed nonius lines. Accordingly, we found that the objectively measured vergence gain was significantly correlated (r=0.8) with the motion-amplitude of the flashed monocular line when the separation between the line and the fusion contour was at least 32 min arc. In conclusion, if one wishes to estimate the dynamic vergence response with psychophysical methods, effects of capture of visual direction can be reduced by using flashed nonius lines.

Convergence, Ocular↗
PubMed1994

3-D vision and figure-ground separation by visual cortex.

A neural network theory of three-dimensional (3-D) vision, called FACADE theory, is described. The theory proposes a solution of the classical figure-ground problem for biological vision. It does so by suggesting how boundary representations and surface representations are formed within a boundary contour system (BCS) and a feature contour system (FCS). The BCS and FCS interact reciprocally to form 3-D boundary and surface representations that are mutually consistent. Their interactions generate 3-D percepts wherein occluding and occluded object parts are separated, completed, and grouped. The theory clarifies how preattentive processes of 3-D perception and figure-ground separation interact reciprocally with attentive processes of spatial localization, object recognition, and visual search. A new theory of stereopsis is proposed that predicts how cells sensitive to multiple spatial frequencies, disparities, and orientations are combined by context-sensitive filtering, competition, and cooperation to form coherent BCS boundary segmentations. Several factors contribute to figure-ground pop-out, including: boundary contrast between spatially contiguous boundaries, whether due to scenic differences in luminance, color, spatial frequency, or disparity; partially ordered interactions from larger spatial scales and disparities to smaller scales and disparities; and surface filling-in restricted to regions surrounded by a connected boundary. Phenomena such as 3-D pop-out from a 2-D picture, Da Vinci stereopsis, 3-D neon color spreading, completion of partially occluded objects, and figure-ground reversals are analyzed. The BCS and FCS subsystems model aspects of how the two parvocellular cortical processing streams that join the lateral geniculate nucleus to prestriate cortical area V4 interact to generate a multiplexed representation of Form-And-Color-And-DEpth, or FACADE, within area V4. Area V4 is suggested to support figure-ground separation and to interact with cortical mechanisms of spatial attention, attentive object learning, and visual search. Adaptive resonance theory (ART) mechanisms model aspects of how prestriate visual cortex interacts reciprocally with a visual object recognition system in inferotemporal (IT) cortex for purposes of attentive object learning and categorization. Object attention mechanisms of the What cortical processing stream through IT cortex are distinguished from spatial attention mechanisms of the Where cortical processing stream through parietal cortex. Parvocellular BCS and FCS signals interact with the model What stream. Parvocellular FCS and magnocellular motion BCS signals interact with the model Where stream.(ABSTRACT TRUNCATED AT 400 WORDS)

Depth Perception↗
PubMed1987

Effect of reading in low illumination on fixation disparity.

Fixation disparity has been shown to be produced by prism stress, and it has therefore been proposed that its presence constitutes a sign that binocular vision is under stress (decompensated heterophoria). Fixation disparity can be present in the absence of any other indication of stress, and therefore may have a physiological basis. This study investigates the effect on fixation disparity and visual discomfort of placing the visual system under a different type of stress, that of requiring the subject to read under conditions of inadequate illumination. In 50 young adults, it was found that fixation disparity increased under inadequate illumination and that this was accompanied by symptoms in the form of visual discomfort. After a period of rest from any near-vision task, the fixation disparity decreased and the symptoms were reduced.

Adolescent↗
PubMed1988

Stereoscopic mechanisms in monkey visual cortex: binocular correlation and disparity selectivity.

The neural signals in visual cortex associated with positional disparity and contrast texture correlation of binocular images are the subject of this study. We have analyzed the effects of stereoscopically presented luminous bars and of dynamic random-dot patterns on the activity of single neurons in cortical visual areas V1, V2, and V3-V3A of the alert, visually trained rhesus macaque. The interpretation of the results and considerations of possible neural mechanisms led us to recognize 2 functional sets of stereoscopic neurons. (1) A set of neurons, tuned excitatory (T0) or tuned inhibitory (TI), which respond sharply to images of zero or near-zero disparity. Objects at or about the horopter drive the T0 neurons and suppress the TI, while objects nearer and farther have the opposite effects on each type, inhibition of the T0 and excitation of the TI. The activity of these neurons may provide, in a reciprocal way, the definition of the plane of fixation, and the basic reference for binocular single vision and depth discrimination. (2) A second set of neurons includes tuned excitatory at larger crossed or uncrossed disparities (TN/TF) and neurons with reciprocal excitatory and inhibitory disparity sensitivity with cross-over at the horopter (NE/FA). Binocularly uncorrelated image contrast drives these neurons to a maintained level of activity, which shifts, in response to correlated images, toward facilitation or suppression as a function of positional disparity. These neurons may operate in the neural processing leading to stereopsis, both coarse and fine, and also provide signals for the system controlling binocular vergence. These results indicate that cortical visual neurons are binocularly linked to respond to the relative position and contrast of the images over their receptive fields, and also that both these aspects of binocular stimulation may be utilized by the brain as a source of stereoscopic information.

Animals↗
PubMedDate not supplied

Integration and beyond: linking information from disparate sources and into workflow.

The vision of integrating information-from a variety of sources, into the way people work, to improve decisions and process-is one of the cornerstones of biomedical informatics. Thoughts on how this vision might be realized have evolved as improvements in information and communication technologies, together with discoveries in biomedical informatics, and have changed the art of the possible. This review identified three distinct generations of "integration" projects. First-generation projects create a database and use it for multiple purposes. Second-generation projects integrate by bringing information from various sources together through enterprise information architecture. Third-generation projects inter-relate disparate but accessible information sources to provide the appearance of integration. The review suggests that the ideas developed in the earlier generations have not been supplanted by ideas from subsequent generations. Instead, the ideas represent a continuum of progress along the three dimensions of workflow, structure, and extraction.

Computer Communication Networks↗
PubMed1985

On Pulfrich-illusion eye movements and accommodation vergence during visual pursuit.

When the Pulfrich illusion is perceived with stationary fixation, and visual pursuit of the pendulum is then initiated, rapid vergence changes occur which correspond to the illusory elliptical path. During steady-state visual tracking of the illusion, however, the eyes move along a planar path without systematic changes in vergence. These latter pursuit movements with monocular filter involve large fixation disparities relative to unobstructed vision (0.5 degree to 1 degree divergence); hence, it is proposed that the planar tracking path probably results from strong dominance of the oculomotor system by stimuli from the unobstructed eye. During visual tracking with monocular filter and a target moving along a nonillusory elliptical path in depth, appropriate changes in vergence occur, but comparable vergence changes also arise when the target is fully hidden from one eye. This response apparently represents a superposition of accommodation vergence upon smooth pursuit movements; similar responses also occur during monocular tracking of a target moving around a circular path in depth.

Accommodation, Ocular↗
PubMed1987

Brightness acuity test and outdoor visual acuity in cataract patients.

The disparity between functional outdoor vision and the acuity measured in the standard refracting lane is well known among clinicians. A simple device, the brightness acuity tester (B.A.T.), was developed to predict a patient's functional outdoor acuity. The B.A.T. has an illuminated hemispheric bowl, 60 mm in diameter, with a 12-mm aperture. Fourteen normals and 50 patients with cataracts were tested using the B.A.T. and then tested outside in bright sunlight. The B.A.T. correlated extremely well (r = +0.84, P less than .0001) with the acuities measured outside. There was no decrease in visual acuity in the 14 normal patients, but there was a one to ten line decrease in vision among the cataract patients. Upon retesting, the B.A.T. scores did not vary, while outside testing scores did change due to variable outdoor ambient light levels on sunny days. The B.A.T. is a simple, repeatable, useful test for predicting functional outdoor acuity.

Cataract↗
PubMed1998

Learning and homeostasis: drug addiction and the McCollough effect.

The contribution of conditioned responses (CRs) to homeostasis may be seen by examining seemingly disparate phenomena of color vision (aftereffects and chromatic adaptation) and drug addiction (withdrawal symptoms and tolerance). Color aftereffects may be elicited by nonchromatic stimuli previously paired with color (the McCollough effect, [ME]). Similarly, pharmacological aftereffects may be elicited by nonpharmacological stimuli previously paired with a drug (withdrawal symptoms). The authors summarize evidence indicating that both the ME and withdrawal symptoms are CRs. The chromatic CR is expressed as chromatic adaptation in the presence of color, and the ME in the absence of color. The pharmacological CR is expressed as pharmacological adaptation (tolerance) in the presence of the drug, and withdrawal symptoms in the absence of the drug. Both drug withdrawal symptoms and the ME are manifestations of the contribution of conditioning to normal homeostatic regulation. The authors discuss the implications of this conclusion for understanding regulatory processes and the evolution of behavioral mechanisms.

Color Perception↗