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PubMed1982

Nonmotor component of fusional response to vertical disparity: a second look using an afterimage method.

An afterimage method has been used to investigate the relative magnitudes of the nonmotor and motor components of the fusional response to vertical disparity in a complex visual stimulus of diameter 57 degrees consisting of 50 horizontal lines and a square of side 2.5 degrees in the middle. The largest vertical disparity that evoked a stable fusional response was found to be in the range 3-6 degrees, of which the nonmotor component amounted only to 8-15', i.e., 2-10% of the total. At these fusional amplitudes, binocular single vision was already disrupted in the foveola. When the 50 horizontal lines were omitted from the stimulus so that only the central square of side 2.5 degrees remained, the fusional amplitudes decreased by only 25% while the absolute level of the nonmotor components remained the same. The nonmotor components found here are much smaller than those (amounting to about 2 degrees, or 25-40% of the total response) reported recently in the literature.

Afterimage↗

PubMed2006

Binocular coordination of the eyes during reading.

Saccadic eye movements and fixations are the behavioral means by which we visually sample text during reading. Human oculomotor control is governed by a complex neurophysiological system involving the brain stem, superior colliculus, and several cortical areas. A very widely held belief among researchers investigating primate vision is that the oculomotor system serves to orient the visual axes of both eyes to fixate the same target point in space. It is argued that such precise positioning of the eyes is necessary to place images on corresponding retinal locations, such that on each fixation a single, nondiplopic, visual representation is perceived. Vision works actively through a continual sampling process involving saccades and fixations. Here we report that during normal reading, the eyes do not always fixate the same letter within a word. We also demonstrate that saccadic targeting is yoked and based on a unified cyclopean percept of a whole word since it is unaffected if different word parts are delivered exclusively to each eye via a dichoptic presentation technique. These two findings together suggest that the visual signal from each eye is fused at a very early stage in the visual pathway, even when the fixation disparity is greater than one character (0.29 deg), and that saccade metrics for each eye are computed on the basis of that fused signal.

Fixation, Ocular↗

PubMed1983

Binocular vision after lens implantation.

Orthoptic tests for binocular vision were performed on 107 patients with intraocular lenses 3 to 10 months after surgery. The tests carried out were: cover test, Worth-test, Lang's two-pencil-test, the TNO-test and Lang's stereo-test. The implanted lenses were of the four-loop Binkhorst type. They were all implanted after planned extracapsular surgery. Visual acuity 3 to 10 months after operation was 0.5 or more in 95% of the patients (0.8 or more in 53% of the patients). Refraction in spheric equivalents was -0.7 +/- 1.26 diopters. The calculated optical aniseikonia was 2.2% +/- 1.8%. Orthotropia as disclosed by the unilateral cover-test prevailed in 81% of the patients. Most of the patients were exotrop (14 patients for the near, 6 patients for the near and the far). The results of Lang's two-pencil-test were positive in 89%. The Worth-test (polarized, for distance) showed suppression in 23% of the patients. The random-dot-test (TNO-test) showed positive results in 68% (retinal disparities 1980 arc seconds or less), in 57% (480 arc seconds or less), and the new Lang stereotest in 58% (1200 arc seconds or less) of the patients.

Diplopia↗

PubMed1999

From elements to perception: local and global processing in visual neurons.

Gestalt psychologists in the early part of the century challenged psychophysical notions that perceptual phenomena can be understood from a punctate (atomistic) analysis of the elements present in the stimulus. Their ideas slowed later attempts to explain vision in terms of single-cell recordings from individual neurons. A rapprochement between Gestalt phenomenology and neurophysiology seemed unlikely when the first ECVP was held in Marburg, Germany, in 1978. Since that time, response properties of neurons have been discovered that invite an interpretation of visual phenomena (including illusions) in terms of neuronal processing by long-range interactions, as first proposed by Mach and Hering in the last century. This article traces a personal journey into the early days of neurophysiological vision research to illustrate the progress that has taken place from the first attempts to correlate single-cell responses with visual perceptions. Whereas initially the receptive-field properties of individual classes of cells--e.g., contrast, wavelength, orientation, motion, disparity, and spatial-frequency detectors--were used to account for relatively simple visual phenomena, nowadays complex perceptions are interpreted in terms of long-range interactions, involving many neurons. This change in paradigm from local to global processing was made possible by recent findings, in the cortex, on horizontal interactions and backward propagation (feedback loops) in addition to classical feedforward processing. These mechanisms are exemplified by studies of the tilt effect and tilt aftereffect, direction-specific motion adaptation, illusory contours, filling-in and fading, figure--ground segregation by orientation and motion contrast, and pop-out in dynamic visual-noise patterns. Major questions for future research and a discussion of their epistemological implications conclude the article.

Contrast Sensitivity↗

PubMed1985

Development of stereopsis depends upon contrast sensitivity and spatial tuning.

Various binocular functions such as disparity vergence, and stereopsis develop rapidly during the third to fifth month of life. This time frame coincides with the rapid postnatal development of visual acuity, contrast sensitivity and possibly the tuning of spatial channels. Recent studies of adult stereopsis reveal that small disparities near the stereo threshold are processed within spatial channels tuned above 2.5 cycles/deg. Sensitivity to these higher spatial frequencies develops during the 3-5 months of life. Adult studies also reveal that larger disparities are processed by spatial channels tuned to lower spatial frequencies. Tuning of these low spatial frequency channels may not be present until 3 months of age. Lack of spatial tuning prior to 3 months is predicted to be associated with a lack of disparity tuning as well as an inability to ignore false disparities in ambiguous or redundant stimuli such as random dot stereograms. These observations suggest that refractive anomalies such as anisometropia that limit high frequency spatial resolution and binocular integration can present a major obstacle to the postnatal development of binocular vision.

Adult↗

PubMed1999

Receptive fields of disparity-selective neurons in macaque striate cortex.

To identify neuronal mechanisms underlying stereopsis, we characterized interactions between inputs from the two eyes in disparity-selective neurons in macaque V1. All disparity-selective cells showed suppressive interactions between the right and left eyes, and some showed facilitatory interactions. Disparity selectivity was narrower than the receptive-field width and was constant across the receptive field. Such position-invariant disparity selectivity is also found in anesthetized cat V1. Complex cells have been suggested to inherit their disparity selectivity from simple cells with receptive fields mismatched between the two eyes. However, we found no such antecedent disparity-tuned simple cells. We did find disparity-selective cells with some simple-cell characteristics, but surprisingly, they also showed position-invariant disparity selectivity rather than simple linear binocular interactions.

Animals↗

PubMed2004

Microsaccades keep the eyes' balance during fixation.

During fixation of a stationary target, small involuntary eye movements exhibit an erratic trajectory-a random walk. Two types of these fixational eye movements are drift and microsaccades (small-amplitude saccades). We investigated fixational eye movements and binocular coordination using a statistical analysis that had previously been applied to human posture control. This random-walk analysis uncovered two different time scales in fixational eye movements and identified specific functions for microsaccades. On a short time scale, microsaccades enhanced perception by increasing fixation errors. On a long time scale, microsaccades reduced fixation errors and binocular disparity (relative to pure drift movements). Thus, our findings clarify the role of oculomotor processes during fixation.

Fixation, Ocular↗

PubMed1982

Relationship between fixation disparity curves and symptoms in monofixators.

Patients with small angle strabismus usually are asymptomatic. Occasionally, however, binocular asthenopic symptoms are reported. Two monofixators, one symptomatic and the other asymptomatic, with similar profiles on strabismic evaluation had forced vergence Fixation Disparity Curves (FDC) measured. Slope of the FDC may help to discriminate between the symptomatic and asymptomatic patient.

Adult↗

PubMed1987

Analysis of binocular visual function using tests made under binocular conditions.

Accommodation and vergence have a complex relation which occasionally breaks down, resulting in a loss of visual efficiency along with symptoms of discomfort associated with use of the eyes. Studies of accommodation/vergence interactions and tonic vergence disorders indicate that, using classical analysis techniques, separate methods are frequently necessary to determine whether existing binocular deficiencies are causing reported symptoms. The problem with current systems of binocular visual function analysis is that the vergence error which exists under binocular conditions is often not the same as that which is measured under monocular conditions. A rationale for, and technique of, analyzing binocular function using results of tests made under binocular conditions is described. This analysis incorporates the concepts of CA/C, proximal vergence (PV), and fixation disparity along with several accommodative measures (facility, lag, sustaining ability, and accuracy). By identifying relevant binocular components and the interrelations, the clinician should be better able to assess the contribution of each and examine which may be modified most easily by vision therapy, lenses, and/or prism intervention.

Accommodation, Ocular↗

PubMed2002

Blindness and visual impairment in the Americas and the Caribbean.

AIM: To summarise available data on the prevalence and causes of visual impairment and blindness in the Americas and the Caribbean. METHODS: The published literature was searched in Medline and LILACS using the following key words: blindness, visual impairment, prevalence. Articles were reviewed, and the references of the articles were also searched for relevant articles, which were also reviewed. RESULTS: Using the mortality in children under the age of 5 as an indicator, the overall prevalence of childhood blindness (in the under age 15 group) for the region was estimated at 0.45/1000, with the majority (67%) living in countries with mortality of children under age 5 above 30/1000 live births. Corneal opacities were more common in countries where the under 5 year mortality are above 30/1000 live births and retinopathy of prematurity (ROP) was an important cause in countries with intermediate death rates. For adults, overall blindness rates were not estimated because of the social, economic, and ethnic diversity in the region. The primary causes of visual loss in adults in the Americas were age related eye diseases, notably cataract and glaucoma in the African-American and Hispanic populations, and age related macular degeneration in the white population. Uncorrected refractive error was a significant cause of decreased vision across ages, ethnic groups, and countries. CONCLUSION: More data are needed on the magnitude and causes of visual loss for the Caribbean and Latin American countries. Rates of blindness and visual loss from available data within these countries are widely disparate. Prevention and control of avoidable blindness needs to be an ongoing focus in this region.

Adolescent↗

PubMed1982

The role of the vertical dimension in stereoscopic vision.

It is shown how the full three-dimensional structure of a scene may in principle be computed from a correlated pair of retinal images, when all that is known about the orientations of the two eyes is that the planes of their horizontal meridians accurately coincide. The vertical dimension plays a crucial role in the theory; visible points which lie on the horizontal meridian supply no information about the angle of convergence or the direction of gaze. (i) If the scene contains three or more nonmeridional points, not all lying in a vertical plane, then their positions in space are fully determined by the horizontal and vertical coordinates of their images on the two retinas. (ii) If just two nonmeridional points are visible, or more than two, lying in a vertical plane, then their retinal images admit, in general, just two distinct three-dimensional interpretations. One of these is usually unrealistic; but a choice between them may be perceptually difficult if the vertical plane containing the points is nearly perpendicular to the interocular axis. These results suggest that vertical disparities may play an important role in the binocular perception of absolute depth. Elsewhere (Mayhew, this issue) this suggestion is found to provide a quantitative explanation of Ogle's 'induced effect'.

Depth Perception↗

PubMed2000

Foundations of spatial vision: from retinal images to perceived shapes.

Vision is based on spatial correspondences between physically different structures--in environment, retina, brain, and perception. An examination of the correspondence between environmental surfaces and their retinal images showed that this consists of 2-dimensional 2nd-order differential structure (effectively 4th-order) associated with local surface shape, suggesting that this might be a primitive form of spatial information. Next, experiments on hyperacuities for detecting relative motion and binocular disparity among separated image features showed that spatial positions are visually specified by the surrounding optical pattern rather than by retinal coordinates, minimally affected by random image perturbations produced by 3-D object motions. Retinal image space, therefore, involves 4th-order differential structure. This primitive spatial structure constitutes information about local surface shape.

Form Perception↗

PubMed1999

Stereoscopic depth cues can segment motion information.

Can the motion system selectively process elements at a particular depth? We attempted to answer this question using global coherence tasks in which signal and noise elements could be given different disparities. In experiment 1 we found that, if all the signal elements had a disparity different from that of the noise elements, performance was far better than when they had the same disparity (at least for stereo-normal observers). In a second experiment we found that adding additional noise elements to the motion task had no effect if they had a different disparity (however, they had a marked effect for stereo-blind observers). We conclude that stereo disparity can be used as a segmentation cue by the motion system.

Color Perception↗

PubMed1999

Stored representations of three-dimensional objects in the absence of two-dimensional cues.

Object recognition was studied in human subjects to determine whether the storage of the visual objects was in a two-dimensional or a three-dimensional representation. Novel motion-based and disparity-based stimuli were generated in which three-dimensional and two-dimensional form cues could be manipulated independently. Subjects were required to generate internal representations from motion stimuli that lacked explicit two-dimensional cues. These stored internal representations were then matched against internal three-dimensional representations constructed from disparity stimuli. These new stimuli were used to confirm prior studies that indicated the primacy of two-dimensional cues for view-based object storage. However, under tightly controlled conditions for which only three-dimensional cues were available, human subjects were also able to match an internal representation derived from motion of that of disparity. This last finding suggests that there is an internal storage of an object's representations in three dimensions, a tenet that has been rejected by view-based theories. Thus, any complete theory of object recognition that is based on primate vision must incorporate three-dimensional stored representations.

Analysis of Variance↗

PubMed1985

Forced vergence fixation disparity curves at distance and near in an asymptomatic young adult population.

Forced vergence fixation disparity curves were measured at distance and near on an asymptomatic young adult population (average age 24.7 +/- 5.2 years, range 14 to 35). Approximately 60% of the 62 subject sample had the same curve types at distance and near (type I, 50%, type II, 9.7%). Unlike curves were found in 40.3% of the sample. Detailed breakdown of the unlike curve types is provided, along with a discussion of the results. Clinical implications of the necessity to establish normative data for fixation disparity curves at distance and near for different vergence states, age groups, and symptomatic patients are stressed.

Adolescent↗

PubMed2001

The effect of saccades and brief fusional stimuli on phoria adaptation.

PURPOSE: The purpose of this experiment was to determine whether brief fusional stimuli and saccades similar to those seen in the alternate cover test affect phoria disadaptation. METHODS: Three cover test conditions were performed randomly. Before each test condition, subjects fused for 2 min at an angle convergent to the subjective phoria. In one test condition, subjects viewed monocularly. In another condition, subjects alternately fixated with each eye (no binocular time). In a third condition, subjects alternately fixated with each eye, and there was a 100-ms period of binocular viewing between alternations. The ocular vergence angle was monitored using scleral search coils. RESULTS: Vergence angle was plotted against time for each condition. The area under this plot was determined using a computer program. The area reflected the rate at which ocular vergence returned to the original phoria position. The mean area for the monocular condition was 300.9, the mean area for alternate fixation with no binocular time was 300.3, and the mean area for alternate fixation with binocular time was 205.2. CONCLUSIONS: Saccadic alternations do not affect phoria adaptation. However, short periods in which binocular disparate images are viewed significantly increase the rate at which phoria adaptation declines for some subjects.

Adaptation, Ocular↗

PubMed1976

Binocular visual mechanisms in cortical areas I and II of the sheep.

1. Units were recorded in the primary and secondary visual areas (V1 and V2) of the sheep. They were stimulated binocularly, using an adjustable prism to vary the disparity. 2. Cells in V1 responded optimally to stimuli with very small or zero disparities, but cells in V2 frequently preferred disparities of several degrees crossed or uncrossed. Many cells in V2 were particularly selective to disparity, often giving no response to a monocular stimulus. 3. Cells preferring the same disparity occur in discrete columns, about 400 muM wide. Changes between columns result from a step displacement of the receptive field of one eye. The disparities encoded in successive columns seem to follow a regular sequence: crossed, zero, uncrossed, zero, etc. 4. In both V1 and V2, cells are clustered, perhaps in columns, according to their orientation preference and ocular dominance. In V2, the constant disparity columns appear to be independent of the orientation clusters.

Animals↗

PubMed1976

Steroscopic discrimination in infants.

The ability to make discrimiations of binocular disparity was investigated in 2-month-old infnats by two methods: (a)fixation preference between patterns differing in the disparity they contained, and (b)recovery from habituation of high-amplitude sucking when there was a change in disparity in the visual reinforcer. The stimuli were random-dot steregrams. The results for both mthods indicated that at least some infants of this age could perform steroscopic discriminations and that both techniques were feasible for development for longitudinal studies of steroscopic vision.

Depth Perception↗