Search PubMed⌕ Search

Biomedical subjects

C M Schor

Publications and source records attributed to C M Schor.

At least 19 recordsLinked to original sources
PubMed1992

Disparity tuning in mechanisms of human stereopsis.

The change in sensitivity across some stimulus dimension which follows adaptation to a particular stimulus can reveal a great deal about the tuning characteristics of underlying sensory/perceptual mechanisms. In this study, a psychophysical adaptation paradigm was employed to characterize the disparity tuning of perceptual mechanisms involved in stereopsis. The stimulus was a dynamic random-dot stereogram (DRDS) portraying a surface which varied in interocular correlation (IOC) and retinal disparity. Adaptation to a fully correlated DRDS surface produced an elevation in IOC threshold over a relatively narrow range of disparities, with maximum effect at the disparity of the adapting stimulus. The width of these disparity tuning functions varied from 5 arc min for adaptation at the horopter to 20 arc min for adaptation at 20 arc min disparity. Frequently, IOC sensitivity was enhanced for disparities on either side of the adapted disparity, suggesting that an opponent center-surround organization operates at an early level of disparity processing. A model of underlying channel structure consistent with these data is presented.

Adaptation, Ocular↗
PubMed1992

Failure of rivalry at low contrast: evidence of a suprathreshold binocular summation process.

Presentation of different images to the two eyes normally results in a time-varying alternation between the two images (binocular rivalry). However, we find that when orthogonal gratings are viewed dichoptically at low contrast, a stable summation between the two images is perceived in the form of a dichoptic plaid. The range of perception of the dichoptic plaid depends on spatial frequency, contrast and luminance of the gratings. This phenomenon differs from the "false fusion", a fleeting summation of different images perceived only under very brief presentation of the stimuli. The observations suggest that there exists a neural process that performs a summation of dissimilar images, and that is distinct from the competitive process of suppression and binocular rivalry.

Contrast Sensitivity↗
PubMed1992

A dynamic model of cross-coupling between accommodation and convergence: simulations of step and frequency responses.

The near triad consists of an increase in accommodation, vergence, and pupillary constriction. All three motor systems exhibit phasic and tonic responses. The tonic response adapts readily to phasic efforts of accommodation and vergence. Cross-coupling between accommodation and vergence provides a means of dynamically adjusting the tonic set points of the two motor systems to a common near or far working distance. Accommodative vergence cross-links play a dominant role in coordinating proximal changes in accommodation and convergence. The magnitude of cross-link interactions can be modified by imbalanced strength of tonic adaptation by accommodation and vergence. Reducing adaptation of tonic accommodation increases the AC/A ratio and decreases the CA/C ratio. Reducing adaptation of tonic vergence has the opposite effect. A model is able to predict these and other interactions simply by reducing the decay time constant of one of the two motor systems. For example, reducing the time constant for tonic accommodation results in an increased AC/A ratio and decreased CA/C ratio. Reducing the time constant for tonic vergence has the opposite effect. The model predicts transient step responses by accommodative vergence when the AC/A ratio is low and transient step responses of vergence accommodation when the CA/C ratio is low. It also predicts a reciprocal relationship between the AC/A and CA/C ratios. When one cross-link ratio is high the other cross-link ratio is low. Simulated frequency responses predict the low frequency roll off of low AC/A and low CA/C ratios. The step and frequency responses of cross-link ratios are shown to be the same for proximal (perceived distance) and retinal (blur and disparity) stimuli. The model suggests that physiological variations of tonic decay time constants may play an important role in determining clinically abnormal values of AC/A and CA/C ratios.

Accommodation, Ocular↗
PubMed1992

Negative feedback control model of proximal convergence and accommodation.

A comprehensive model has been developed to illustrate the interactions between the observer and the surrounding environment in the control of oculomotor responses to distance or 3-D space. Accommodation and vergence respond to both spatiotopic (body reference) proximal percepts and retinotopic (eye referenced) physical stimuli of blur and disparity. Both spatiotopic and retinotopic stimuli are derived respectively from perceptual and physical correlates of negative feedback for eye position. The spatiotopic and retinotopic stimulus errors are combined in the feedforward path and drive a common oculomotor controller which has a phasic-tonic organization. Spatiotopic and retinotopic stimuli are shown to be effective over complementary operating ranges. Perceptual spatiotopic errors of gaze provide optimal stimuli for near responses to large depth intervals whereas physical-retinotopic cues of blur and disparity provide quantitative information about small binocular fixation errors. Small dynamic variations of target distance are sensed both spatiotopically and retinotopically. Coarse and fine spatiotopic errors of gaze are processed differently. Large spatiotopic errors are sampled intermittently at the beginning of the near response, whereas small retinotopic position errors and spatiotopic velocity errors are sampled continuously throughout the near response. Former reports of empirically observed higher velocity of vergence responses to very large depth intervals is explained in terms of stimulus sampling modes rather than in terms of separate oculomotor control mechanisms. The model demonstrates a complementary function of top-down spatiotopic cues, which are used to initiate the near response, and bottom-up retinotopic cues, which are used to refine and complete the near response. Cross-couplings by vergence-accommodation and accommodative-vergence serve to coordinate the components of the near response when feedback from sensed response of one motor system (i.e. vergence) is more accurate than that of the other motor system (i.e. accommodation). The model presented here is concerned primarily with the near response mediated by accommodation and disjunctive eye movements and not by the independent vergence mediated by non-conjugate or yoked saccades of unequal amplitude.

Accommodation, Ocular↗
PubMed1991

Depth attraction and repulsion in random dot stereograms.

Previous studies of perceived attraction or repulsion of adjacent visual targets have used local targets whose positions were varied in both depth and direction. We have measured these effects in three subjects using dynamic random-dot stereograms to isolate depth-axis effects. Results show that both attraction and repulsion effects can occur for overlapping, positively correlated, random-dot surfaces. The results were quantitatively similar to those reported previously for local targets. Manipulation of interocular correlation confirmed that the effects are produced by binocular interactions. Results are explained as accurate judgments based on the stimulus at the cyclopean level.

Depth Perception↗
PubMed1991

Interocular correlation, luminance contrast and cyclopean processing.

We have investigated the nature and viability of interocular correlation as a measure of signal strength in the cyclopean domain. Thresholds for the detection of interocular correlation in dynamic random element stereograms were measured as a function of luminance contrast, a more traditional measure of stimulus strength. At high contrasts, correlation thresholds were independent of contrast. At low contrasts, correlation thresholds were inversely proportional to the square of contrast. Stereothresholds were also measured as a function of both contrast and interocular correlation. At low contrasts, stereoacuity was inversely proportional to both interocular correlation and the square of contrast. These results are consistent with an inherently multiplicative mechanism of binocular combination, such as a cross-correlation of the two eye's inputs.

Contrast Sensitivity↗
PubMed1990

Selective nonconjugate binocular adaptation of vertical saccades and pursuits.

Hering's law describes the equal and symmetrical rotation of the two eyes. It is possible to calibrate the binocular yoking of the two eyes in response to disparate size and/or motion of the two ocular images. It is unclear if the ratio of movements by the two eyes is modified by selective adaptation of each versional system (i.e. saccades and pursuits) or if there is an apparent adaptation of Hering's law that results from a single underlying process. The latter could be accomplished by vergence (prism) adaptation, which could interact with all versional systems. In this investigation, binocularly stimulated saccades and pursuits were adapted separately for 2 hr to unequal vertical target displacements. Three adaptation paradigms were used; each included a 10% binocular gradient disparity. The adapting stimulus for the pursuit system was 0.25 Hz vertical triangular motion of 20 deg, peak to peak. Two saccade adaptation paradigms included one which emphasized correcting vertical disparity during the pulse component of the saccade, the other minimized the influence of disparity prior to, during and immediately after saccades (vergence paradigm). Yoking ratios (YRs) for vertical pursuits and saccades were compared before and after adaptation. The pursuit paradigm produced marked adaptation of the pursuit YRs while it had negligible effect on saccade YRs. The pulse saccade paradigm adapted the saccade YRs twice as much as the pursuit YRs whereas the vergence paradigm resulted in little adaptation of YRs for either saccades or pursuits. Pursuits adapted to the first paradigm in 15-30 min whereas saccades adapted to the second paradigm in 1.5-2 hr. These results indicate that there is not a single common nonconjugate adaptation mechanism for vertical pursuits and saccades. Results of the vergence paradigm demonstrate that feedback during or immediately after eye movements is necessary in order to stimulate the binocular versional adaptation mechanism. Versional adaptation may be considered as a calibration of Hering's law.

Adaptation, Ocular↗
PubMed1989

Is edge information for stereoacuity spatially channeled?

Models of stereopsis generally assume that binocular correspondence is achieved through alignment of luminance edges in the two eyes. Yet the stimulus properties which constitute edge information for stereopsis have not been defined. Three experiments explored the nature of these stimulus properties. The first two experiments tested whether local luminance gradient and the relative phase of spatial components supply information about the position of edges which influences stereosensitivity. In Expt 1, stereothresholds were reduced with increased spatial frequency or contrast of sinusoidal luminance gratings, but no simple relationship between target luminance gradient and stereosensitivity was found. In Expt 2, stereothresholds were equivalent for targets having identical spatial frequency components, but differing in maximum luminance gradient and the relative spatial phase of their components. In addition, stereothresholds were lower for the target having the higher contrast in pairs of unequal-contrast targets having equal maximum luminance gradients. These results suggest that the properties of luminance gradient and relative spatial phase do not influence stereosensitivity independently of spatial frequency and contrast. Experiment 3 directly tested whether stereosensitivity depends on edge information whose disparity is detected independently at different spatial scales. Stereothresholds for IF + 5F compound targets were found to be equivalent to thresholds obtained separately with the more sensitive of the two components. Taken together with a compressive nonlinearity in the relationship between contrast and stereothreshold obtained by others (Halpern and Blake, 1989; Legge and Gu, 1989) and replicated in Expt 1, the results of Expt 3 indicate that, whatever the exact nature of the luminance discontinuity information utilized in disparity detection, it is processed independently at different spatial scales.

Contrast Sensitivity↗
PubMed1989

Hyperacuity, superresolution and gap resolution in human stereopsis.

Different types of stereoscopic acuity were studied with tasks adapted from studies of visual direction acuity. Dynamic, random-element stereograms portraying multiple surfaces in depth and a temporal 2AFC procedure were used for all measurements. The three tasks required detection of a depth offset (Hyperacuity task), a depth-axis thickening (Superresolution task), and a depth-axis gap between surfaces (Gap Resolution task). Thresholds for the three tasks were on the order of 3 sec arc, 30 sec arc and 200 sec arc of retinal disparity, respectively. These results are comparable to those for the analogous visual direction tasks on which they were patterned, suggesting that the underlying judgments involved are similar. Results are used to estimate the intrinsic noise of horizontal disparity processing.

Depth Perception↗
PubMed1989

Photographic method for Brückner and Hirschberg testing.

An apparatus for strabismus detection was designed to allow photographing fundus and corneal reflexes for Brückner and Hirschberg testing, respectively. Photographs of adult subjects with simulated strabismic deviations were viewed by two groups of observers, optometry students and optometry faculty. In general there was no significant difference in testing results between the two groups. Subject error produced unreliable Brückner results. However, Hirschberg testing was effective approximately 80% of the time in detecting a deviating eye in strabismus of magnitudes of about 5 delta. Anisometropia in Brückner testing was also investigated. Magnitudes of 2.00 D or greater significantly affected results. Clinical application of the photographic Hirschberg test may have potential value as a screening method. The photographic Brückner test is not recommended for adults; its reliability in children requires investigation.

Equipment Design↗
PubMed1989

Panum's fusional area estimated with a criterion-free technique.

It has been reported that criterion-free estimates of the upper disparity limits for fusion of line targets are small enough to be accounted for by monocular vernier sensitivity. However, targets such as lines, which contain high spatial frequencies, may ensure small fusion limits, since fusion limits obtained with criterion-dependent methods for narrow-band targets, such as sinusoids or difference-of-Gaussian luminance profiles, are proportional to target spatial periods. Experiment 1 therefore explored whether criterion-free methods give fusion limits for narrow-band targets that can be accounted for by vernier sensitivity. Vertical fusion limits were estimated by a method that forced observers to discriminate a disparate sinusoidal grating from an immediately adjacent zero-disparity grating. Fusion limits were too large to be explained by monocular vernier thresholds obtained for the same targets. In addition, fusion limits were not affected by large changes in target contrast, whereas vernier thresholds increased as contrast was decreased. The results of Experiment 1 also argued against interocular suppression as the cause of single vision, since vernier offsets that were visible when viewed monocularly were invisible under binocular viewing conditions. In Experiment 2, manual adjustment of disparities yielded fusion limits little different from those obtained with the forced-choice method of Experiment 1, demonstrating that it is possible to design adjustment methods for assessing fusion limits that are as sensitive as forced-choice methods. In Experiment 3, large reductions in target contrast, which have the effect of decreasing disparity sensitivity, did not alter fusion limits, disconfirming the idea that fusion limits estimated with discriminative procedures represent disparity-detection thresholds. In Experiment 4, disparities were adjusted until a just noticeable difference in grating contrast appeared. These disparities were larger than fusion limits, indicating that fusion limits did not represent a change in apparent contrast arising from disparity limitations of binocular summation. Together, the four experiments support the existence of binocular fusion as a unique category of sensory performance, disconfirm several nonfusional explanations of single vision, and support the use of criterion-free as well as adjustment methods in measuring fusion limits.

Attention↗
PubMed1988

Changing size (looming) as a stimulus to accommodation and vergence.

Changing size (looming) produces changes in accommodation and vergence. Dynamic responses of vergence and accommodation to sinusoidal looming of a Maltese cross were recorded with an SRI dual-Purkinje-image eyetracker and optometer. The ratio of these two motor responses was compared with the response accommodative convergence/accommodation (AC/A) ratio and convergence accommodation/convergence (CA/A) ratio determined from sinusoidal variations of blur and disparity respectively. The response to changing size was found to be more similar to the AC/A ratio than the CA/C ratio. In addition, when a changing disparity or changing blur stimulus was combined with a changing size stimulus, the response phase lags of accommodation and vergence were decreased. In addition, the CA/C ratio was increased when changing size was added to changing disparity, but the AC/A ratio was unaltered when changing size was added to changing blur. These results indicate that changing size is stimulating accommodation directly and vergence secondarily through an AC/A crosslink.

Accommodation, Ocular↗
PubMed1988

Voluntary effort as a stimulus to accommodation and vergence.

Voluntary effort can be used to elicit changes in accommodation and vergence. We investigated the potential cross-coupling interactions of accommodative vergence/accommodation (AC/A) and vergence accommodation/vergence (CA/C) in response to voluntary effort. The responses of accommodation and vergence were measured simultaneously with a dual Purkinje image eye tracker and infrared optometer while subjects viewed a Maltese cross monocularly through a pinhole pupil and made voluntary efforts to imaginary changes in target distance. The accommodation and vergence evoked by voluntary efforts occurred in a ratio that was similar to the response AC/A ratio and different from the response CA/C ratio. One exceptional subject was found who could voluntarily accommodate with one eye occluded without the usual concomitant change in vergence. These results indicate that it is primarily the focusing response that is adjusted during voluntary efforts in the near response.

Accommodation, Ocular↗
PubMed1987

The effects of optical vergence, contrast, and luminance on the accommodative response to spatially bandpass filtered targets.

The steady-state monocular accommodative response was measured using a new type of stimulus, a spatially bandpass filtered luminance distribution known as a difference of Gaussian or DOG. The independent variables were: spatial frequency, optical vergence, contrast, and mean luminance. High-contrast DOGs of varying peak spatial frequencies were presented monocularly over a range of target optical vergences. In addition, DOGs were presented at a fixed dioptric vergence (1) over a range of contrasts with a constant mean luminance, and (2) over a range of mean luminances with a constant contrast. The magnitude of the accommodative error was found to depend on the total stimulus condition, i.e. the particular level of each independent variable. Changes in retinal-image contrast from the background (zero-defocus) level, which were brought about by the errors of accommodation under the various stimulus conditions, were mathematically determined and were used to calculate the "accommodative (contrast discrimination) Weber fractions". Comparisons between accommodative and psychophysical contrast discrimination data revealed that there are limits to the degree of shared processing of contrast discrimination information between accommodation and visual perception.

Accommodation, Ocular↗
PubMed1987

Clinical method for measuring adaptation of tonic accommodation and vergence accommodation.

Previous work indicates that tonic adapters of accommodation and vergence have indirect effects on accommodative vergence and vergence accommodation and that these crosslink responses interact with one another to produce clear and single vision. Clinical measurements of tonic accommodation, tonic vergence, vergence accommodation (CA/C ratio), and accommodative vergence (AC/A ratio) are therefore of value in determining possible binocular vision abnormalities. Currently, clinical methods are only available for measuring the tonic vergence (phoria) and the AC/A ratio. We have determined a fast and accurate method for evaluating both tonic accommodation (accommodative phoria) and vergence accommodation (CA/C ratio). In both procedures, the loop of accommodation was opened with a 0.2 cpd difference of Gaussian (DOG) target. This target was found to be incapable of stimulating any reflex accommodation, yet it is localized in space and can easily stimulate binocular fixation. Our results indicate that opening the loop of accommodation with a pinhole Maxwellian view or with a 0.2 cpd DOG produces similar response values for tonic accommodative aftereffects and vergence accommodation responses. The decay rate of tonic accommodative aftereffects as well as changes in the resting focus of accommodation caused by vergence accommodation (CA/C) can be measured clinically using dynamic retinoscopy in combination with the DOG target. Clinical values of the CA/C ratio obtained with these techniques were highly correlated (r = 0.92) with laboratory measures obtained with an objective infrared optometer and eye trac monitor.

Accommodation, Ocular↗
PubMed1987

Fatigue of accommodation and vergence modifies their mutual interactions.

Aftereffects of accommodation and convergence demonstrate the adaptability of these two visual motor systems. These aftereffects were reduced after ramp tracking exercises of either accommodation or vergence, and this reduced aftereffect or fatigue was associated with an increase of accommodative vergence and vergence accommodation. When one motor system (accommodation or vergence) adapted more readily than the other, fatigue caused an increase of the cross-link interaction originating from the more adaptable motor system and a reduction of the cross-link interaction originating from the less adaptable motor system. These results suggest an inverse relationship between adaptation and the cross-link interactions between accommodation and vergence and that adaptation of tonic accommodation and tonic vergence is a process that underlies plasticity of accommodative vergence and vergence accommodation respectively.

Accommodation, Ocular↗
PubMed1986

A computational model of the error detector of human visual accommodation.

A mathematical model is proposed for the error detector of the human visual accommodative system. The model supposes that the accommodative error detector derives both the direction and the magnitude of the accommodative error from naturally-occurring oscillations of the lens and their effects on retinal-image contrast. Differential operators take the first derivatives of two time varying functions: lens power and retinal-image contrast. Directional information is obtained by comparing the signs of these two derivatives and magnitude information is obtained by comparing their amplitudes.

Accommodation, Ocular↗