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PubMed1992

Contribution of accommodation and disparity-vergence to transient nearwork-induced myopic shifts.

Many investigations have attempted to demonstrate a relationship between sustained near-vision and the aetiology of environmentally-induced myopia, but it remains unclear whether myopic development relates to the actions of either accommodation or vergence. The present study investigated the effect of varying the disparity-vergence demand during a near-vision task while maintaining the accommodative stimulus constant. We required 20 young subjects to perform a near task (viewing distance = 20 cm) for 20 min through base-in, zero power and base-out prisms. The magnitude of the prisms for each individual corresponded to one-third their near vergence range. Pre- and post-task measurements of refractive state were determined objectively using an infrared optometer, with post-task readings being recorded over the initial 50 s following task completion. The results indicated no significant difference between the transient far-point shift for the three disparity-vergence conditions. However, combining data across conditions, a significant, mean shift in the myopic direction of 0.14 D was observed during the 10-20 s period immediately following task completion which dissipated within 20-50 s post-task. These findings suggest that the transient post-task myopic shift was not related to the output of disparity-vergence.

Accommodation, Ocular↗
PubMed1996

Stereopsis from interocular spatial frequency differences is not robust.

Based on data obtained using one-dimensional noise patterns, Tyler & Sutter (1979). (Vision Research, 19, 859-865) concluded that stereoscopic tilt can result from an interocular spatial frequency difference in the absence of consistent horizontal disparity. We tested stereopsis using two-dimensional random-dot patterns that were bandpass filtered to contain 1.0 octave bands of spatial frequency with means that differed between the two eyes. With vertical, one-dimensional stimuli we replicated the results of Tyler and Sutter. However, stereoscopic tilt was not perceived based on spatial frequency differences alone when the monocular images contained as little as a +/- 14 deg range of orientation variation. In addition, model simulations demonstrate that the modest steroscopic performance produced by interocular spatial frequency differences in one-dimensional noise patterns are predicted by random disparity correlations at the pattern edges. The observations lead to the conclusion that stereopsis from frequency differences in the absence of pointwise disparity correlations does not reflect a special processing capability of human vision but is an artifact associated with one-dimensional stimuli. As such, it plays no role in steroscopic analysis of the natural environment.

Convergence, Ocular↗
PubMed1996

The rate of visual recovery after penetrating keratoplasty for keratoconus.

PURPOSE: We performed a retrospective study of 68 patients who underwent penetrating keratoplasty (PK) for keratoconus during the years of 1988 and 1989. The purpose of this study was to determine the speed of visual recovery, final best corrected visual acuity, and rate of rejection episodes. METHODS: The visual acuity, type of correction, and complications were recorded at 3, 6, 12, 18, 24, 36, and 48 months. RESULTS: The average final best corrected visual acuity was 20/23, and all patents achieved 20/30 or better vision (range: 20/15 to 20/30). The best vision was achieved at 16.5 +/- 8.7 months. Thirty-one percent of the patients were corrected with contact lenses. Factors associated with best corrected vision were preoperative vision, combined suture technique, and a donor/host size disparity of 0.25 mm (P < 0.005). Thirty-one percent of the patients had at least one episode of graft rejection. The use of contact lenses was not associated with an increased risk of rejection. CONCLUSIONS: PK for keratoconus yields an excellent visual result, but this study shows the need for long-term careful follow-up to achieve optimal visual rehabilitation.

Adolescent↗
PubMed1989

Differential effects of retinal target displacement, changing size and changing disparity in the control of anterior/posterior and lateral body sway.

When visual information is provided in addition to input from other sensory systems, normal body sway can be attenuated by about 50%. The essential visual cue is retinal target displacement, which increases as eye-target distance is reduced. As a result, both lateral and anterior/posterior (A-P) sway decrease as eye/target distance decreases. We have investigated the geometrical basis of lateral and fore-aft sway detection by vision, the latter provided by detection of change in disparity of the visual axis of both eyes and change in target size. Using known movement detection thresholds the largest possible distance for visual stabilization of posture can be calculated to be 34 m for lateral sway, and for fore-aft sway 1.50 m for change in disparity and 3.20 m for change in size. Visual stimuli were designed to selectively stabilize fore-aft sway either by change in size or by change in disparity. The experiments revealed that the theoretical prediction overestimates the efficacy of visual stabilization of lateral sway and underestimates the efficacy of fore-aft sway stabilization. It is proposed that microoscillations of the eye increase the threshold for detection of retinal target displacements, leading to less efficient lateral sway stabilization than expected, and that the threshold for detection of self motion in the A-P direction is lower than the threshold for object motion detection used in the calculations, leading to more efficient stabilization of A-P sway.

Form Perception↗
PubMed2006

Cortical effects of brief daily periods of unrestricted vision during early monocular form deprivation.

Experiencing daily brief periods of unrestricted vision during early monocular form deprivation prevents or reduces the degree of resulting amblyopia. To gain insight into the neural basis for these "protective" effects, we analyzed the monocular and binocular response properties of individual neurons in the primary visual cortex (V1) of macaque monkeys that received intermittent unrestricted vision. Microelectrode-recording experiments revealed significant decreases in the proportion of units that were dominated by the treated eyes, and the magnitude of this ocular dominance imbalance was correlated with the degree of amblyopia. The sensitivity of V1 neurons to interocular spatial phase disparity was significantly reduced in all treated monkeys compared with normal adults. With unrestricted vision, however, there was a small but significant increase in overall disparity sensitivity. Binocular suppression was prevalent in monkeys with constant form deprivation but significantly reduced by the daily periods of unrestricted vision. If neurons exhibited consistent responses to stimulation of the treated eye, monocular response properties obtained by stimulation of the two eyes were similar. These results suggest that the observed protective effects of brief periods of unrestricted vision are closely associated with the ability of V1 neurons to maintain their functional connections from the deprived eye and that interocular suppression in V1 may play an important role in regulating synaptic plasticity of these monkeys.

Animals↗
PubMed2000

Seeing in three dimensions: the neurophysiology of stereopsis.

From the pair of 2-D images formed on the retinas, the brain is capable of synthesizing a rich 3-D representation of our visual surroundings. The horizontal separation of the two eyes gives rise to small positional differences, called binocular disparities, between corresponding features in the two retinal images. These disparities provide a powerful source of information about 3-D scene structure, and alone are sufficient for depth perception. How do visual cortical areas of the brain extract and process these small retinal disparities, and how is this information transformed into non-retinal coordinates useful for guiding action? Although neurons selective for binocular disparity have been found in several visual areas, the brain circuits that give rise to stereoscopic vision are not very well understood. I review recent electrophysiological studies that address four issues: the encoding of disparity at the first stages of binocular processing, the organization of disparity-selective neurons into topographic maps, the contributions of specific visual areas to different stereoscopic tasks, and the integration of binocular disparity and viewing-distance information to yield egocentric distance. Some of these studies combine traditional electrophysiology with psychophysical and computational approaches, and this convergence promises substantial future gains in our understanding of stereoscopic vision.

Journal Article↗
PubMed1996

Development of identical orientation maps for two eyes without common visual experience.

In the mammalian visual cortex, many neurons are driven binocularly and response properties such as orientation preference or spatial frequency tuning are virtually identical for the two eyes. A precise match of orientation is essential in order to detect disparity and is therefore a prerequisite for stereoscopic vision. It is not clear whether this match is accomplished by activity-dependent mechanisms together with the common visual experience normally received by the eyes, or whether the visual system relies on other, perhaps even innate, cues to achieve this task. Here we test whether visual experience is responsible for the match in a reverse-suturing experiment in which kittens were raised so that both eyes were never able to see at the same time. A comparison of the layout of the two maps formed under these conditions showed them to be virtually identical. Considering that the two eyes never had common visual experience, this indicates that correlated visual input is not required for the alignment of orientation preference maps.

Animals↗
PubMed1984

Presbyopia and ocular motor balance.

Presbyopic case analysis generally involves determining the need for base-in prism. Common methods for prescribing prism, and components of convergence and the paradoxical lack of symptoms at near in presbyopic patients, and clinical measurement of fixation disparity are discussed in this context. Prism and vision therapy are treatment choices.

Accommodation, Ocular↗
PubMed1987

On the late invention of the stereoscope.

It was not until 1838, when Wheatstone published his account of the stereoscope, that stereoscopic depth perception entered into the body of binocular phenomena. It is argued that the stereoscope was not invented earlier because the phenomenon of stereopsis based on disparity had not been adequately described. This was the case despite the fact that there had been earlier descriptions of tasks that could be performed better with two eyes than with one; the perceptual deficits attendant upon the loss of one eye had been remarked upon; analyses of the projections to each eye were commonplace, and binocular disparities were accurately illustrated; moreover, binocular microscopes and telescopes had been made over a century earlier. Theories of binocular vision were generally confined to accounting for singleness of vision with two eyes, and the concepts employed to account for this were visible direction, corresponding retinal points, and union in the brain. The application of these concepts inhibited any consideration of disparities, other than for yielding diplopia. When perception of the third dimension was addressed by Berkeley at the beginning of the eighteenth century, it was in the context of monocular vision and binocular convergence. Thereafter visual direction became the province for binocular vision and it was analysed in terms of geometrical optics, whereas visual distance was examined in the context of learned associations between vision and touch. This artificial division was challenged initially with respect to visual direction and later with respect to stereopsis. An additional factor delaying the invention of the stereoscope was that experiments on binocular vision generally involved abnormal convergence on extended objects. Wheatstone's accidental observation of stereopsis was under artificial conditions in which disparity alone defined the binocular depth perceived. Once invented the stereoscope was enthusiastically embraced by students of vision. It is suggested that the ease with which retinal disparity could be manipulated in stereopairs has led to an exaggeration of its importance in space perception. "The appearances, which are by this simple instrument rendered so obvious, may be easily inferred from the established laws of perspective; for the same object in relief is, when viewed by a different eye, seen from the two points of sight at a distance from each other equal to the line joining the two eyes. Yet they seem to have escaped the attention of every philosopher and artist who has treated of the subjects of vision and perspective." (Wheatstone 1838, page 371).

Depth Perception↗
PubMed1983

Depth from dichoptic edges depends on vergence tuning.

Small but reproducible fixation disparities occur in normal subjects when they view certain types of dichoptic stimuli. During dichoptic as well as stereoptic stimulation the motor fusion process determines first the average vergence state of the eyes. The subsequent fine tuning of vergence is shown to depend on the spatial distribution of contrast edges both of the same contrast sign ('stereoptic edges') and of opposite contrast sign ('dichoptic edges'). Stereoptic edges tend to induce superposition attempts of the vergence control system and dichoptic edges tend to antagonise this process. If a single low-contrast dichoptic edge is presented with zero disparity and within a stereoptic reference frame, a fixation disparity of several minutes of arc results. This influences depth vision since dichoptic edges are perceived (as monocular edges) at the actual rather than at the intended fixation distance. The findings explain previous paradoxical results of eg Kaufman and Pitblado who reported seeing depth in opposite-contrast stereograms. Their results seemed to contradict the well-established 'same-sign rule' (SSR) which states that the stereoptic system only detects disparities of edges with the same contrast sign. It is concluded that (i) the SSR holds; (ii) dichoptic (and monocular) edges are seen at the horopter; (iii) the vergence fine tuning prevents superposition of dichoptic edges even if this causes a fixation disparity.

Depth Perception↗
PubMed1991

Discrimination of motion in depth trajectory following acute alcohol ingestion.

Evoked potentials were recorded at four scalp sites O1, O2, T5, T6; before and after the consumption of one of two alcohol dose levels, to the occurrence of disparate stimuli in dynamic random dot stereograms. Twenty young adult subjects, all of whom had vision which was normal or corrected to normal, were randomly assigned to the two dosage groups. Subjects viewed stimuli which embodied crossed disparity and followed one of three trajectories in depth. The task was to distinguish between and identify, by specific button presses, these trajectories. One half of the stimuli were presented with monocular cues to motion in depth (dark condition); in the remainder only stereoscopic depth information was available (uniform condition). Responses to both conditions showed alcohol-related reduction of evoked responses across all sites. It was concluded that the visual systems associated with the processing of motion in depth were susceptible to alcohol effects, as revealed by reductions in the amplitude of evoked responses.

Adult↗
PubMed1975

Stereopsis in normal domestic cat, Siamese cat, and cat raised with alternating monocular occlusion.

Normal cats, cats raised with alternating monocular occluders, and Siamese cats were tested for stereoscopic vision using a shadow-casting technique that produced line stereograms. Human subjects were tested with the same apparatus. 1. Normal cats had stereoscopic vision, but AO and Siamese cats did not. 2. Normal AO cats, and Siamese cats all had equal visual activity (about 6 feet of arc) and all could make accurate judgments about the depth of real objects. 3. Normal cats could fuse crossed disparities as great as 50 feet of arc and uncrossed disparities as great as 30 feet of arc. 4. Normal cats could make stereoscopic discriminations with stimulus disparities greater than 1 degree, even though they could not fuse disparities of this magnitude. 5. Both the AO and the Siamese cats exhibited a convergent squint. 6. Human subjects viewing identical stimuli could not fuse stimuli having disparities greater than 10 feet, although they could make depth judgments for crossed disparities as great as 5 degrees. 7. These results imply that animals without binocular cells in area 17 do not have stereoscopic vision, but do not determine if disparity-selective cells in the visual cortex are responsible for stereopsis.

Animals↗
PubMed1988

Effect of vision therapy on stereophotogrammetric profiling--a controlled clinical trial.

A single, masked controlled clinical trial showed that vision therapy improved both the accuracy and repeatability of stereophotogrammetric performance on the profiling task with experienced, visually normal observers. The average fixation disparity measured under working conditions decreased and stereoscopic acuity as measured with a Howard-Dolman apparatus increased. The data suggest that vision therapy is most helpful for those profiling situations in which disparity stimuli are plentiful and stereopsis is the dominant depth cue.

Clinical Trials as Topic↗
PubMed1989

[The development of stereoscopic vision in the first months of life].

Dynamic random dot patterns generated on a TV screen by a special pattern generator are proven to be adequate stimuli for testing stereopsis. The registration of transient visual evoked cortical potentials (VECP) to these random dot stimuli offers objective proof of stereopsis. Using different disparities in older people a typical amplitude behaviour in the dynamic random dot stereo VECP with maximum amplitudes to medium sized disparities was found. Compared to 20 adults and 30 children with normal vision, in 26 children with negative TNO- and Titmus-tests for stereoscopic vision treated for strabismus no dynamic random dot stereo VECPs could be recorded. The development of stereoscopic vision in the first 12 months of life was investigated in five babies. First positive components in the dynamic random dot stereo VECP could be found in the 4th to 6th months of life in all children. From these results the assumption, that stereoscopic vision develops in later childhood, must be revised in favour of a much earlier period of infancy.

Adult↗
PubMed1981

Why two eyes are better than one: the two views of binocular vision.

Despite centuries of research on the topic, the answer to the question "'Are two eyes significantly better than one, independent of stereopsis?" is still uncertain. In this investigation, steps are taken toward answering the question in a behavioral context. Three sets of experiments are reported in which human binocular and monocular performance are compared in a variety of exteroceptive and visuomotor tasks. In all of the experiments, two eyes facilitated performance. The findings suggest that the binocular system is able to detect the matching information, that is, the concordance, in the monocular optic arrays and to use that information to increase visual efficiency. Furthermore, stereopsis was not found to be important in the performance of visuomotor skills in three dimensions when the subjects were free to move their heads. Thus, the results indicate that an important ecological benefit of binocular frontal vision is having binocular concordance, rather than having binocular disparity.

Adult↗
PubMed2006

Integration of intermittent visual samples over time and between the eyes.

The authors investigated the integration of alternate disparate monocular inputs for binocular perception in 1-handed catching experiments (N = 14, 32, 22, and 15 participants, respectively in Experiments 1-4). They varied the no-vision interval between alternate monocular samples to measure catching performance, and they compared the alternating monocular conditions with binocular and monocular conditions with equal no-vision intervals. They found no evidence of a binocular advantage for one-handed catching in the alternating monocular conditions. Performance in monocular and alternating monocular conditions did not differ across no-vision intervals ranging from 0-80 ms and was particularly worse than performance in binocular viewing conditions when the no-vision interval was 40 ms or more. The authors argue that the dissimilarity between disparate monocular inputs created by the approaching object limited the integration of those inputs and subsequent binocular perception.

Adult↗