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C Schor

Publications and source records attributed to C Schor.

At least 19 recordsLinked to original sources
PubMed1990

Readily visible changes in color contrast are insufficient to stimulate accommodation.

In an earlier study (Wolfe & Owens, 1981) it was reported that humans could not accommodate to an insoluminant red-green border. However, recent masking studies (Switkes, Bradley & DeValois, 1988) have shown that, using an appropriately normalized contrast metric, contrast decrements similar to those produced by defocus are equally visible for color or luminance modulated grating patterns. We have compared accommodative responses to 1.75 c/deg gratings that consisted of either isochromatic luminance modulations or isoluminant red-green color modulations. All four observers could accommodate accurately to luminance modulated gratings over a wide range of contrasts. However, no appropriate accommodative responses were obtained even for the highest contrast color modulated gratings. These results show the changes in color contrast are ineffective as stimuli for the human accommodative response even when the changes in chromatic contrast accompanying defocus are readily perceived.

Accommodation, Ocular↗
PubMed1990

Visual function with presbyopic contact lens correction.

All forms of ophthalmic correction for presbyopia require compromises in viewing flexibility and visual function. The unique effects on vision of contact lenses used in managing presbyopia are especially intriguing and potentially problematic. Bifocal contact lenses produce unique changes in the nature and quality of the retinal image. In monovision (MV) correction, anisometropia is intentionally created by fitting one eye to see clearly at optical infinity and the other eye to see clearly at the near working distance. Our review of the literature indicates that most visual functions are affected by these departures from the conventional optical correction strategies used on nonpresbyopes. Sensory functions such as contrast sensitivity and stereoacuity are affected most, whereas motor functions such as convergence and accommodation are not noticeably impaired. MV appears to produce a more widely acceptable visual compromise than currently available bifocal contact lenses for most patients.

Adaptation, Ocular↗
PubMed1989

Binocular fusion limits are independent of contrast, luminance gradient and component phases.

Panum's binocular fusion limit has been shown to increase with the size of graded contrast targets (Schor, Wood & Ogawa, 1984). This suggests the hypothesis that the fusion limit may be controlled by the maximum luminance gradients present in the stimuli. The luminance gradient is reciprocally related to image contrast, so the hypothesis predicts that the fusion limits should also decrease with increasing contrast. To investigate this luminance gradient hypothesis we designed stimuli in which the contrast and phase of the spatial frequency components could be varied independently of the luminance gradients. Disparity limits for fusion were unaffected by variations of as much as a log unit in contrast, luminance gradient or phase of the frequency components, disconfirming the luminance gradient hypothesis. Instead, fusion limits for various compound frequency targets were well predicted by the smallest fusion range for any spatial frequency component in the image that was above its contrast detection threshold.

Contrast Sensitivity↗
PubMed1989

Interocular differences in contrast and spatial frequency: effects on stereopsis and fusion.

Anisometropia produces interocular differences in contrast and spatial frequency. The influence of these two parameters on Panum's fusional limit (PFL) and stereoscopic depth thresholds was investigated with sinusoidal gratings and one-dimensional band-pass-limited targets. Vertical fusion limits were unaffected by large interocular differences in contrast (40-10%) at two spatial frequencies (0.8 and 1.6 cpd). However, when tested with a low spatial frequency (0.8 cpd), stereothresholds increased 150% with an interocular difference in contrast as small as 50-25%. Stereoacuity was reduced less by differential contrast when tested with higher spatial frequencies (3.2 cpd). When tested with low spatial frequencies the stereothreshold was elevated more by reducing the contrast of one image than by equal contrast reductions of both ocular images. Stereothresholds appear to be elevated by binocular suppression evoked by interocular differences in contrast. Vertical as well as horizontal fusion limits decreased with increasing interocular size difference. Horizontal fusion limits fell off more gradually with increasing size difference than did vertical fusion limits, particularly at higher spatial frequencies (2.4 cpd). Similarly, stereothresholds increased with increasing interocular size differences. Changes in the fusion limit and stereothreshold that occur with interocular size differences are predicted from positional disparities between edge features rather than from differences in spatial frequency.

Contrast Sensitivity↗
PubMed1989

Adaptive disorders of accommodation and vergence in binocular dysfunction.

Disturbances of binocular vision are described clinically by the Duane-White classification in terms of the magnitude of the accommodative-convergence ratio (AC/A). Convergence excess and convergence insufficiency are assumed to result from high and low AC/A ratios respectively. It is assumed that the abnormal AC/A ratio is an independent variable that underlies abnormal phorias. However, recent studies have demonstrated that the AC/A ratio is inversely related to the adaptability of tonic accommodation (lens adaptation) and directly related to adaptability of tonic vergence (prism adaptation). We have tested whether clinical categories of convergence excess and convergence insufficiency are associated with insufficient and excessive adaptation of tonic accommodation and tonic vergence. Results demonstrate greater amplitude and duration of accommodative after-effects (lens adaptation) in the convergence insufficiency than the convergence excess group. Vergence after-effects (prism adaptation) had the reverse trend for the two groups. These results indicate that adaptive disorders of accommodation and vergence may underlie binocular disorders in symptomatic patients categorized as convergence excess and convergence insufficiency.

Accommodation, Ocular↗
PubMed1989

Effects of interocular blur suppression ability on monovision task performance.

Suppression of anisometropic blur induced by monovision contact lenses was examined in 18 presbyopic subjects. Suppression ability was quantified by reducing the contrast of a bright test target, viewed by subjects wearing a monovision correction, until the blurred image was suppressed. Subjective success with monovision was evaluated using a patient survey and no correlation to blur suppression ability was found. Objective success was evaluated in terms of performance at three near work tasks, each requiring a different level of stereoscopic localization. A significant correlation was found between card filing performance (requiring a moderate level of stereopsis) and blur suppression ability. Correcting either the dominant or non-dominant eye for near in the monovision correction did not significantly affect blur suppression ability. There was no evidence for adaptation to monovision in terms of increasing blur suppression ability over time.

Adult↗
PubMed1988

The role of eye movements and masking in monocular rivalry.

Using an image stabilization technique, we have examined the role of eye movements on the phenomenon of monocular rivalry. Cross-correlation analysis confirms that perceptual onset of the vertical grating coincides with perceptual disappearance of the horizontal (and vice versa). Vertical and horizontal retinal image motion can modulate, selectively, the visibility of the horizontal and vertical gratings. However, a stable percept was observed when constant retinal image motion was matched for both vertical and horizontal components. These results support an eye-movement and afterimage explanation for the phenomenon of monocular rivalry (Georgeson, 1984). However, in addition to enhancing the visibility of a horizontal grating, a vertical "saccadic-like" retinal image motion decreased the visibility of the vertical component. This result is consistent with rivalry. That is, the visibility of both gratings are not independent, but are inversely related to each other. Our results are inconsistent with a hypothesis for monocular rivalry based upon inherently unstable neural interactions, but they also show that unstable fixational eye movements and afterimages are not the only factors determining the perceptual fluctuations.

Afterimage↗
PubMed1988

Imbalanced adaptation of accommodation and vergence produces opposite extremes of the AC/A and CA/C ratios.

It is customary to describe abnormal interactions between accommodation and convergence according to the Duane-White classification of convergence excess and insufficiency or divergence excess and insufficiency. However, recent studies indicate that these extremely high and low categories of accommodative vergence may result from adaptive disorders of accommodation and convergence. The AC/A ratio is shown in this paper to vary inversely with adaptability of accommodation and to vary directly with adaptability of vergence. Extremely high AC/A ratios can result from an imbalance in which vergence adapts readily to prism, whereas accommodation does not adapt to minus lenses. Similarly, extremely high values for convergence accommodation (the CA/C ratio) are observed when there is robust adaptation of accommodation to lenses and little adaptation of vergence to prism. Furthermore, in these extreme cases the AC/A and CA/C ratios tend to be inversely related. For example, abnormally high AC/A ratios are often accompanied by extremely low CA/C ratios and vice versa. These observations suggest that equalizing the adaptability of accommodation and vergence with orthoptics may prove to be an effective means of restoring the AC/A and CA/C ratios to normal values.

Accommodation, Ocular↗
PubMed1988

Influence of accommodative and vergence adaptation on binocular motor disorders.

Tait described four categories of binocular disorders including convergence excess, convergence insufficiency, divergence excess, and divergence insufficiency. These disorders are defined by the distance where the largest heterophoria occurs (distance or near), and the amplitude of the accommodative vergence ratio (AC/A). Insufficiency corresponds to a low AC/A ratio, whereas excess corresponds to a high AC/A. The magnitude of the AC/A ratio, which may be influenced by the adaptability of the accommodation and vergence systems, has been shown to be reciprocally related to adaptability of accommodation. Likewise, the degree of vergence accommodation has been shown to be related reciprocally to adaptability of vergence to prism. An imbalance of adaptability of accommodation and vergence systems produces abnormal cross-coupling between the two motor systems. When accommodation is more adaptable than vergence, the AC/A ratio is low and the CA/C ratio is high. Conversely, when vergence is more adaptable than accommodation, the AC/A ratio is high and the CA/C ratio is low. A method is reported for temporarily restoring moderate amplitudes of abnormal AC/A and CA/C ratios by reducing excessive adaptation with fatigue. Finally, new clinical procedures for measuring adaptation of accommodation and the CA/C ratio are presented. Taken together with current measures of vergence adaptation and AC/A ratio these procedures will permit a more complete evaluation of mutual interactions between accommodation and vergence in patients diagnosed as having excessive and insufficient vergence.

Accommodation, Ocular↗
PubMed1988

Patterns of binocular suppression and accommodation in monovision.

The binocular depth of focus of monovision wearers was compared to the sum of the two monocularly determined depths of focus. Observers fell into three groups based upon ocular sighting dominance. Complete binocular summation of the monocular depths of focus was observed in subjects without a preferred fixating eye. Subjects who preferred to fixate with one eye had difficulty suppressing blur of that eye while the binocular target was within the depth of focus of the nonpreferred eye. A third group showed partial summation of the two monocular depths of focus. Similar patterns of accommodative response, measured objectively with the SRI optometer, were observed in subjects wearing monovision corrections. Accommodative response to sinusoidal variations in blur was controlled primarily by the dominant sighting eye. These results demonstrate the effectiveness of interocular suppression of anisometropic blur in monovision correction and the influence of ocular dominance upon this suppression process.

Accommodation, Ocular↗
PubMed1987

Ocular dominance and the interocular suppression of blur in monovision.

Presbyopic contact lens patients with monocular corrections (monovision) see clearly at all distances by virtue of an interocular suppression of anisometropic blur that occurs regionally between corresponding retinal areas. This suppression fails to occur with small high-contrast targets viewed under low luminance conditions. The effect of target size and contrast upon interocular suppression of blur was quantified by reducing contrast of a bright test spot, viewed binocularly while wearing various plus lenses monocularly, until the out-of-focus image was suppressed. The strength of interocular suppression was equivalent when the plus lens was before either eye. However, after subjects wore a plus lens over their nonsighting eye for one day, interocular suppression of blur became enhanced when the nonsighting eye was blurred, and it became reduced when the sighting eye was blurred. Successful monovision subjects suppressed blur at higher contrast levels than did unsuccessful subjects. These results suggest a possible clinical test for quantifying adaptation to monovision.

Adaptation, Physiological↗
PubMed1986

Disparity vergence dynamics and fixation disparity.

The initial velocity of the vergence response to step changes in disparity was examined in two subjects as a function of the retinal locus and spatial frequency of fusion stimuli. Vergence velocity decreased for both subjects as retinal eccentricity increased and it was unaffected by spatial frequency. The same subjects had very different amounts of fixation disparity (FD). The flat forced duction FD curve for one subject who had rapid vergence adaptation remained unaffected by these two stimulus variables. In contrast, the steep forced duction FD curve for the other subject became steeper as retinal eccentricity and target coarseness increased. These results suggest that prism adaptation overshadows the effects of disparity vergence dynamics upon FD, but in the absence of adaptation, two dynamic properties of vergence (velocity and decay during occlusion) determine the slope of the peripheral limits of the forced duction FD curves. The results also suggest that the vergence system is more sensitive to small disparities subtended by high than low spatial frequency detail. We recommend that the slope of the extreme limits of the forced duction curve be used clinically to discriminate between normal and abnormal disparity vergence dynamics.

Adaptation, Ocular↗
PubMed1986

Combined effects of spatial frequency and retinal eccentricity upon fixation disparity.

The variations in fixation disparity (FD) with spatial frequency and retinal locus of fusion locks were examined in two groups of subjects having either steep or flat forced-duction FD curves. FD for subjects with flat curves was unaffected by either spatial frequency or diameter of fusion locks. FD of observers with steep forced-duction curves increased with both the coarseness and eccentricity of fusion locks. Panum's horizontal fusion range increased markedly with target spatial period but only modestly with retinal eccentricity. These results indicate that factors other than the dimensions of Panum's fusional area (PFA) result in the increase of FD found in some observers with peripheral fusion locks.

Convergence, Ocular↗
PubMed1984

Binocular sensory fusion is limited by spatial resolution.

Binocular sensory fusion which was previously thought to have a maximum spatial extent at the fovea of 1 deg is at least 600% larger when stimulated by low spatial frequency (coarse) detail. This upper limit for sensory fusion has a constant phase disparity limit of 90 deg which corresponds to the monocular Rayleigh criterion for spatial resolution of two diffraction patterns. At low spatial frequencies the sensory fusion limit equals the upper disparity limit for stereoscopic depth perception which suggests that a common mechanism underlies these two phenomena.

Depth Perception↗
PubMed1983

Spatial characteristics of static and dynamic stereoacuity in strabismus.

The spatial and temporal organization of stereoscopic depth perception were compared in normal and strabismic observers. The minimum and maximum disparities for stimulating static and dynamic stereopsis in strabismus were examined as a function of spatial separation of disparate stimuli. Disparities and their spacing were produced by spatial modulation of two vertical lines viewed haploscopically. Most strabismics had normal upper disparity limits but elevated static and dynamic stereothresholds. Moderate stereothreshold elevations (100 arc sec) were constant for spatial separations greater than 15 arc min. Two new types of spatial crowding effects upon stereopsis were observed. The first type resulted from the constant elevation of the disparity threshold. The second type consisted of a reduced maximum disparity limit for stereopsis. In both cases, the constriction of the range of perceivable depth produced a reduction in the spatial and temporal frequency limits for depth perception. Clinical tests of stereoacuity that crowd stimuli closer than 0.25 degree underestimated the strabismic patients' potential stereoacuities by a factor of 2 to 4. Similarly, tests of dynamic stereopsis that use temporal frequencies greater than 1 Hz will underestimate optimal dynamic stereoacuity.

Depth Perception↗
PubMed1983

Rotating grating treatment of amblyopia with and without eccentric fixation.

Treatment of amblyopia with and without eccentric fixation using short term occlusion and rotating gratings was analyzed in a controlled study. A control group (n = 18) viewed a blank rotating disc for 15 minutes once a week for ten weeks while the treatment group (n = 20) observed rotating gratings for the same time period. Both groups performed visually guided tasks to maintain treatment interest. Visual acuity was analyzed before and after each treatment session using a multiple Landolt ring chart. Data were gathered in a double masked manner. Analysis did not reveal a statistically significant improvement of visual acuity with rotating grating therapy for treating amblyopia with or without eccentric fixation.

Adolescent↗
PubMed1982

Oculomotor functions in reading disability.

Oculomotor functions and information processing systems from a complex interaction in the achievement of reading skills, This study was designed to determine the relation between oculomotor functions and reading ability in fourth and sixth grade children. Four oculomotor functions were measured during the reading of digits and prose. The oculomotor functions were used independently and collectively to predict reading ability under both reading conditions. These relations were then examined with and without removal of the influence of verbal intelligence. The results indicate that when examined collectively, oculomotor functions are related to reading ability, but when oculomotor functions are examined independently, only marginally significant relations appear. Similar relations were found for both age groups. Verbal intelligence influenced some of the individual relations. The individual oculomotor functions that appeared responsible for the collective relation were frequency of fixations and lag of accommodation. A reciprocal-causation model between reading ability and oculomotor functions is proposed.

Accommodation, Ocular↗