BINOCULAR CONTRAST DETECTION WITH UNEQUAL MONOCULAR ILLUMINANCE

Age-related normal limits for spatial vision

PURPOSE

To establish age-related, normal limits of monocular and binocular spatial vision under photopic and mesopic conditions.

METHODS

Photopic and mesopic visual acuity (VA) and contrast thresholds (CTs) were measured with both positive and negative contrast optotypes under binocular and monocular viewing conditions using the Acuity-Plus (AP) test. The experiments were carried out on participants (age range from 10 to 86 years), who met pre-established, normal sight criteria. Mean and ± 2.5σ limits were calculated within each 5-year subgroup. A biologically meaningful model was then fitted to predict mean values and upper and lower threshold limits for VA and CT as a function of age. The best-fit model parameters describe normal aging of spatial vision for each of the 16 experimental conditions investigated.

RESULTS

Out of the 382 participants recruited for this study, 285 participants passed the selection criteria for normal aging. Log transforms were applied to ensure approximate normal distributions. Outliers were also removed for each of the 16 stimulus conditions investigated based on the ±2.5σ limit criterion. VA, CTs and the overall variability were found to be age-invariant up to ~50 years in the photopic condition. A lower, age-invariant limit of ~30 years was more appropriate for the mesopic range with a gradual, but accelerating increase in both mean thresholds and intersubject variability above this age. Binocular thresholds were smaller and much less variable when compared to the thresholds measured in either eye. Results with negative contrast optotypes were significantly better than the corresponding results measured with positive contrast (p < 0.004).

CONCLUSIONS

This project has established the expected age limits of spatial vision for monocular and binocular viewing under photopic and high mesopic lighting with both positive and negative contrast optotypes using a single test, which can be implemented either in the clinic or in an occupational setting.

A new trifocal corneal inlay for presbyopia

Corneal inlays (CIs) are the most recent surgical procedure for the treatment of presbyopia in patients who want complete independence from the use of glasses or contact lenses. Although refractive surgery in presbyopic patients is mostly performed in combination with cataract surgery, when the implantation of an intraocular lens is not necessary, the option of CIs has the advantage of being minimally invasive. Current designs of CIs are, either: small aperture devices, or refractive devices, however, both methods do not have good performance simultaneously at intermediate and near distances in eyes that are unable to accommodate. In the present study, we propose the first design of a trifocal CI, allowing good vision, at the same time, at far, intermediate and near vision in presbyopic eyes. We first demonstrate the good performance of the new inlay in comparison with a commercially available CI by using optical design software. We next confirm experimentally the image forming capabilities of our proposal employing an adaptive optics based optical simulator. This new design also has a number of parameters that can be varied to make personalized trifocal CI, opening up a new avenue for the treatment of presbyopia.

A Convenient and Robust Test to Quantify Interocular Suppression for Children With Amblyopia

Interocular suppression was quantified by the interocular luminance difference that was needed when the two eyes were balanced in discriminating a black-white stripe formed butterfly stimulus, which was dichoptically presented through polarized glasses. Stronger interocular suppression was found in amblyopes than that in controls at both the near (33 cm, 0.95 ± 1.00 vs. 0.14 ± 0.18, p < .001) and far (5 m, 2.18 ± 0.97 vs. 0.24 ± 0.16, p < .001) viewing distances. The interocular suppression in amblyopes was significantly correlated with the interocular visual acuity difference, the visual acuity of amblyopic eye, the Worth-4-Dot test, and the stereo acuity at both the near and far distances (for all cases, p < .001). Our new test enables convenient and robust measurements of interocular suppression in children with amblyopia. The measured interocular suppression is in agreement with other clinical measures.

How Ocular Dominance and Binocularity Are Reflected by the Population Receptive Field Properties

Purpose

The neural substrate of binocularity and sighting ocular dominance in humans is not clear. By utilizing the population receptive field (pRF) modeling technique, we explored whether these phenomena are associated with amplitude and pRF size differences.

Methods

The visual field maps of 13 subjects were scanned (3-T Skyra) while viewing drifting bar stimuli. Both eyes (binocular condition), the dominant eye and the nondominant eye (two monocular conditions) were stimulated in separate sessions. For each condition, pRF size and amplitude were assessed. Binocular summation ratios were calculated by dividing binocular by mean monocular values (amplitude and pRF size).

Results

No differences in pRF size were seen between the viewing conditions within each region, that is, either between binocular and monocular or between dominant and nondominant viewing conditions. Binocular amplitudes were higher than the monocular amplitudes, but similar among the dominant and nondominant eyes. Binocular summation ratios derived from amplitudes were significantly higher than one (∼1.2), while those ratios derived from pRF size were not. These effects were found in all studied areas along the visual hierarchy, starting in V1.

Conclusions

Neither the amplitude nor the pRF size show intereye difference and therefore cannot explain the different roles of the dominant and the nondominant eyes. Binocular, as compared to monocular vision, resulted in higher amplitudes, while receptive fields' sizes were similar, suggesting increased binocular response intensity as the basis for the binocular summation phenomenon. Our results could be applicable in imaging studies of monocular disease and studies that deal with nondisparity binocularity effects.

Binocular contrast summation and inhibition depends on spatial frequency, eccentricity and binocular disparity

PURPOSE

When central vision is compromised, visually-guided behaviour becomes dependent on peripheral retina, often at a preferred retinal locus (PRL). Previous studies have examined adaptation to central vision loss with monocular 2D paradigms, whereas in real tasks, patients make binocular eye movements to targets of various sizes and depth in 3D environments.

METHODS

We therefore examined monocular and binocular contrast sensitivity functions with a 26-AFC (alternate forced choice) band-pass filtered letter identification task at 2° or 6° eccentricity in observers with simulated central vision loss. Binocular stimuli were presented in corresponding or non-corresponding stereoscopic retinal locations. Gaze-contingent scotomas (0.5° radius disks of pink noise) were simulated independently in each eye with a 1000 Hz eye tracker and 120 Hz dichoptic shutter glasses.

RESULTS

Contrast sensitivity was higher for binocular than monocular conditions, but only exceeded probability summation at low-mid spatial frequencies in corresponding retinal locations. At high spatial frequencies or non-corresponding retinal locations, binocular contrast sensitivity showed evidence of interocular suppression.

CONCLUSIONS

These results suggest that binocular vision deficits may be underestimated by monocular vision tests and identify a method that can be used to select a PRL based on binocular contrast summation.

Binocular vision adaptively suppresses delayed monocular signals

A neutral density filter placed before one eye will produce a dichoptic imbalance in luminance, which attenuates responses to visual stimuli and lags neural signals from retina to cortex in the filtered eye. When stimuli are presented to both the filtered and unfiltered eye (i.e., binocularly), neural responses show little attenuation and no lag compared with their baseline counterpart. This suggests that binocular visual mechanisms must suppress the attenuated and delayed input from the filtered eye; however, the mechanisms involved remain unclear. Here, we used a Steady-State Visual Evoked Potential (SSVEP) technique to measure neural responses to monocularly and binocularly presented stimuli while observers wore an ND filter in front of their dominant eye. These data were well-described by a binocular summation model, which received the sinusoidal contrast modulation of the stimulus as input. We incorporated the influence of the ND filter with an impulse response function, which adjusted the input magnitude and phase in a biophysically plausible manner. The model captured the increase in attenuation and lag of neural signals for stimuli presented to the filtered eye as a function of filter strength, while also generating the filter phase-invariant responses from binocular presentation for EEG and psychophysical data. These results clarify how binocular visual mechanisms-specifically interocular suppression-can suppress the delayed and attenuated signals from the filtered eye and maintain normal neural signals under imbalanced luminance conditions.

Google Glass Glare: disability glare produced by a head-mounted visual display

PURPOSE

Head mounted displays are a type of wearable technology - a market that is projected to expand rapidly over the coming years. Probably the most well known example is the device Google Glass (or 'Glass'). Here we investigate the extent to which the device display can interfere with normal visual function by producing monocular disability glare.

METHODS

Contrast sensitivity was measured in two normally sighted participants, 32 and 52 years of age. Data were recorded for the right eye, the left eye and then again in a binocular condition. Measurements were taken both with and without the Glass in place, across a range of stimulus luminance levels using a two-alternative forced-choice methodology.

RESULTS

The device produced a significant reduction in contrast sensitivity in the right eye (>0.5 log units). The level of disability glare increased as stimulus luminance was reduced in a manner consistent with intraocular light scatter, resulting in a veiling retinal illuminance. Sensitivity in the left eye was unaffected. A significant reduction in binocular contrast sensitivity occurred at lower luminance levels due to a loss of binocular summation, although binocular sensitivity was not found to fall below the sensitivity of the better monocular level (binocular inhibition).

CONCLUSIONS

Head mounted displays such as Google Glass have the potential to cause significant disability glare in the eye exposed to the visual display, particularly under conditions of low luminance. They can also cause a more modest binocular reduction in sensitivity by eliminating the benefits of binocular summation.

The Unilateral Mean Luminance Alters Additive Internal Noise in Normal Vision

Luminance has been found to play a modulating role in the processes of many visual tasks. However, the mechanisms underlying the modulation role of luminance have been little studied, and the conclusions have been controversial. Here, using a dichoptic viewing paradigm by varying the luminance in one eye while measuring the contrast-detection threshold in the other eye, we studied the effect of different unilateral mean luminance values on the detectability of sine wave gratings against backgrounds of various levels of white noise in normal subjects. We found that unilateral luminance altered the additive internal noise within a perceptual template model framework, with low luminance increasing the additive internal noise and high luminance reducing it. This finding helps to reveal how luminance modulates contrast detection and its relative mechanisms.

Comparison of binocular through-focus visual acuity with monovision and a small aperture inlay

Corneal small aperture inlays provide extended depth of focus as a solution to presbyopia. As this procedure is becoming more popular, it is interesting to compare its performance with traditional approaches, such as monovision. Here, binocular visual acuity was measured as a function of object vergence in three subjects by using a binocular adaptive optics vision analyzer. Visual acuity was measured at two luminance levels (photopic and mesopic) under several optical conditions: 1) natural vision (4 mm pupils, best corrected distance vision), 2) pure-defocus monovision ( + 1.25 D add in the nondominant eye), 3) small aperture monovision (1.6 mm pupil in the nondominant eye), and 4) combined small aperture and defocus monovision (1.6 mm pupil and a + 0.75 D add in the nondominant eye). Visual simulations of a small aperture corneal inlay suggest that the device extends DOF as effectively as traditional monovision in photopic light, in both cases at the cost of binocular summation. However, individual factors, such as aperture centration or sensitivity to mesopic conditions should be considered to assure adequate visual outcomes.

The effect of unilateral mean luminance on binocular combination in normal and amblyopic vision

Luminance plays a modulating role in the processes of several visual tasks, which in turn provides significant information for the understanding of visual processing. Here, using a binocular phase combination paradigm, we studied the effect of unilateral changes in mean luminance on binocular combination in both normal and amblyopic vision. We found, in normal observers, attenuation of one eye's stimulus luminance with neutral density filters produces binocular phase combination similar to those of amblyopic subjects. Correspondingly, in amblyopic observers, reduction of the fellow eye's stimulus luminance produces binocular phase combination similar to those of normal subjects. These phenomena could be explained by an attenuated contribution of the filtered eye to the binocular phase percept due to reduced gain-control. The findings have major implications both for the study of binocular combination and for amblyopia treatment.

Monocular vs. binocular measurement of spatial vision in elders

PURPOSE

To evaluate the frequency and magnitude of binocular gain (and loss) for high-contrast acuity, various low-contrast spatial vision measures and contrast sensitivity in an elderly population.

METHODS

Ninety-five individuals over the age of 70 (mean age 80.5 years, range, 71.8 to 93.5) with acuity of 20/40 (logMAR 0.3) or better in at least one eye participated. Each individual was tested binocularly and then monocularly on high-contrast acuity, various low-contrast acuity measures, and contrast sensitivity. Binocular gain for acuity measures was defined as better performance by one line or more under binocular conditions than with the better eye alone. Binocular loss was defined as poorer performance by one line or more binocularly than with the better eye alone. For contrast sensitivity, the criterion for binocular gain or loss was one letter triplet (0.15 log unit) or more difference. For each measure, the frequency of binocular gain and loss, as well as the mean gain or loss were determined.

RESULTS

The mean difference between binocular and better eye monocular acuity was <2 letters for all measures, suggesting little gain or loss. However, the percent of individuals showing a line or more of gain or loss ranged from approximately 20% for high-contrast acuity to 49.5% for low-contrast acuity at low luminance. This indicates that for 1/5 to (1/2) of individuals, binocular vision is not well represented by monocular vision measures. All low contrast measures and contrast sensitivity show more binocular gain/loss than standard acuity. As has been previously reported, in the presence of large interocular differences, binocular gain is not seen, but binocular loss does not necessarily occur.

CONCLUSIONS

For a significant portion of this elderly population, binocular performance is not well represented by better eye monocular measures. This indicates that to get a true sense of an individual';s vision function in daily life, one must measure vision binocularly.

Binocular interaction in post-LASIK subjects with unsatisfactory outcome

PURPOSE

A shift from binocular summation to binocular inhibition has been observed when there is a significant decrease in contrast sensitivity or reduced retinal illuminance in one eye compared to the other as in cases of unilateral cataract and amblyopia. This study aims to determine how binocular function in post-LASIK subjects with unsatisfactory outcomes is influenced by differences between the two eyes in visual acuity and contrast sensitivity.

MATERIALS AND METHODS

A retrospective analysis of monocular and binocular visual acuity (VA) and contrast sensitivity (CS) results was performed for 23 LASIK subjects who previously underwent a Functional Vision Test battery at Vision Sciences Research Corp. (San Ramon, CA). Photopic VA was measured with ETDRS chart and sine-wave grating CS was measured with FACT chart.

RESULTS

The difference between binocular and better monocular VA and CS was calculated for each subject and referred to as binocular difference. The interocular difference was calculated as better eye-worse eye for VA and CS. Significant negative correlations (P < 0.01) were observed between binocular difference and interocular difference for VA (r = -0.84) and CS at each spatial frequency of 1.5 (r = -0.90), 3 (r = -0.85), 6 (r = -0.79), 12 (r = -0.68), and 18c/d (r = -0.72). The greater the difference between the eyes, the lesser the binocular summation. Linear regression predicted a 1-line decrease in binocular summation for a 4-line increase in interocular acuity difference and a 35% decrease in binocular summation for a 50% increase in interocular difference of contrast sensitivity at 3c/d (ANOVA, P < 0.01).

CONCLUSIONS

Binocular inhibition in post-LASIK subjects increases as the visual sensitivity difference between the two eyes increases. Since the peak of human contrast sensitivity function is at the middle spatial frequencies, the decrease in binocular CS at 3c/d due to reduced CS in one eye is functionally significant.

Binocular interactions in patients with age-related macular degeneration: Acuity summation and rivalry

This study examined two aspects of binocular function in patients with age-related macular degeneration (AMD): summation/inhibition of visual acuity and rivalry. The performance of 17 patients with AMD was compared with that of 17 elderly controls and 21 young people. Monocular and binocular acuities were measured using a multiple-E optotype test. Binocular ratios, defined as the better-eye acuity divided by the binocular acuity, were calculated. We also measured eye dominance during rivalry (proportion of time the participants reported perceiving the input to each eye) and rivalry rates (number of alternations per minute). The results showed that while overall binocular ratios were similar for the three groups, the frequency distributions of people who experienced inhibition, equality or summation were different for the young and AMD groups. In the rivalry test, patients experienced more piecemeal perception than the elderly and young controls, but time dominance from the better-seeing eye was comparable for the three groups. Rivalry rates decreased with age and further with pathology. Moreover, rivalry time dominance of the worse-seeing eye was negatively correlated with interocular acuity differences for the AMD group.

Binocular contrast inhibition in subjects with age-related macular degeneration

In subjects with normal vision, binocular contrast sensitivity is generally higher than monocular sensitivity, indicating summation of contrast in the two eyes. We have compared monocular and binocular contrast sensitivity and acuity for a group of 13 subjects with age-related macular degeneration (AMD). Relative to a normal control group, many of the AMD subjects showed reduced binocular contrast summation, and binocular inhibition was found for eight subjects for a narrow or an extended frequency band. A better monocular than binocular function may have practical implications for reading and orientation in AMD.

Binocular summation for grating detection and resolution in foveal and peripheral vision

Peripheral monocular grating resolution has been shown to be limited by the sampling density of the underlying retinal ganglion cells. We wanted to determine if peripheral resolution is also sampling limited binocularly; and, if so, how great is any improvement in either detection or resolution when viewing binocularly? We measured detection and resolution acuity for sinusoidal gratings in foveal and peripheral vision both monocularly and binocularly. Detection and resolution acuity were very similar in foveal vision and displayed a binocular improvement of 5% over best monocular acuity. However, in peripheral vision, while detection acuity improved by 6% binocularly, resolution acuity improved by 16%, with a subsequently smaller aliasing zone. This improvement was greater than predicted by probability summation and implies that the two monocular ganglion cell sampling arrays combine at a higher level resulting in a higher binocular sampling density.

Binocular summation in orientation discrimination depends on stimulus contrast and duration

Binocular summation, an improvement in visual performance with binocular viewing compared to monocular viewing, has been studied extensively in detection tasks. Monocular detection thresholds for stationary stimuli are typically about 40% higher than binocular thresholds. Binocular summation in discrimination tasks, however, is often lower and less consistent. A possible explanation for this difference is that saturation of responses limits the extent of binocular summation in discrimination tasks. To investigate this possibility, we used an orientation discrimination task and varied stimulus contrast and exposure duration. Monocular and binocular orientation discrimination thresholds were obtained using one-dimensional difference-of-Gaussian stimuli. For briefly exposed stimuli, binocular summation is greatest at low contrasts (e.g. 66% at 8% contrast) and is reduced systematically at higher contrasts so that monocular and binocular thresholds are approximately equal at contrasts above 15%. Binocular summation for low-contrast stimuli is greatest at a brief exposure duration (50 msec), is reduced at longer durations, and is not significant at durations of 100 msec or longer. Thus, binocular summation in orientation discrimination is greatest for relatively low-energy stimuli. These results are consistent with models of binocular energy summation and the hypothesis that saturation of responses after binocular combination can limit binocular summation in discrimination tasks.

Can second eye cataract extraction be justified?

A prospective study was carried out to test the null hypothesis that there is no additional benefit to be gained from second eye cataract extraction. Twenty-nine patients with unilateral cataract but contralateral pseudophakia completed a questionnaire enquiring into their visual disability and underwent testing of monocular and binocular visual acuity, contrast sensitivity, glare disability, fusion and stereopsis. These tests were repeated 4 months after second eye cataract extraction and lens implantation. Patients universally considered their vision to have been improved by second eye surgery and the prevalence of all symptoms were significantly reduced by this procedure. Normal binocular summation and stereopsis were restored following second eye cataract extraction. Symptomatic patients may benefit from second eye cataract extraction with lens implantation.

Binocular enhancement of visual acuity

Using a computerized test system, we compared binocular and monocular visual optotype acuity, varying both contrast and contrast disparity between the two eyes. When contrast was the same in the two eyes, binocular acuity was better than best monocular acuity by an average of 0.045 log minimum angle of resolution, or 11%. When contrast differed in the two eyes, binocular acuity in most but not all cases was still better than the monocular acuity of the eye that received the higher contrast. This binocular advantage became smaller but remained significant as contrast disparity became larger. These results are most simply explained by threshold contrast summation of high-spatial-frequency letter components.

Visual acuity and contrast sensitivity in cataract: summation and inhibition of visual performance

Patients exhibiting uniocular cataract often report an improvement in vision on closing their cataractous eye. Such qualitative evidence suggests the presence of binocular inhibition--the converse of binocular summation (that is the normal superiority of binocular over monocular vision). To quantify the extent of inhibition in cataract, binocular and monocular visual acuity and contrast sensitivity were measured in 28 patients. Twelve patients showed binocular inhibition for visual acuity whilst 11 showed inhibition for contrast sensitivity measured at four cycles per degree (c.deg-1). Contrast sensitivity for 8 c.deg-1 targets was further recorded in a subset of 14 patients in whom seven showed inhibition. In patients who demonstrated inhibition, the mean decrement in performance for visual acuity was 13%. For contrast sensitivity at 4 and 8 c.deg-1 the mean decrement was 25% and 32% respectively. The clinical significance of these findings is discussed with respect to the assessment of visual function and management of the cataract patient.

The importance of measuring binocular contrast sensitivity in unilateral cataract

Monocular and binocular contrast sensitivities were measured in patients with uniocular cataract. The cataractous eye showed a greater monocular loss at higher spatial frequencies compared to lower spatial frequencies. Binocular contrast sensitivity depended on the contrast sensitivity differences between the two eyes. At low spatial frequencies, where the monocular sensitivity difference was minimal, binocular summation was obtained. As the sensitivity difference increased at higher spatial frequencies, the binocular contrast sensitivity decreased steadily until it reached a level below the sensitivity of the cataractous eye, demonstrating binocular inhibition. The clinical implications of binocular inhibition obtained with uniocular cataract are discussed.

Temporal arteritis associated with Borrelia infection. A case report

A 71-year-old man had sudden vision loss associated with headache. A temporal artery biopsy revealed a typical picture of giant cell arteritis. Subsequent steroid therapy failed to restore sight. A later blood culture contained spirochetes compatible with Borrelia species, and a silver stain of the temporal artery biopsy specimen demonstrated a similar spirochete. Treatment with i.v. ceftriaxone sodium led to some limited return of sight. To our knowledge, this is the first case report of a spirochete compatible with Borrelia found in a temporal artery biopsy specimen.