Visual Performance as a Function of Luminance in Glaucoma: The De Vries-Rose, Weber's, and Ferry-Porter's Law

Correlation of contrast sensitivity with ganglion cell/inner plexiform layer thickness and damage location in glaucoma with varying severity

OBJECTIVES

To investigate the correlation of contrast sensitivity with macular region ganglion cell/inner plexiform layer (GC/IPL) thickness and damage location in open-angle glaucoma (OAG) of varying severity.

METHODS

Cross-sectional study with 106 patients (203 eyes) who had OAG. Contrast sensitivity of each eye evaluated by quick contrast sensitivity function test based on intelligent algorithm. The GC/IPL thickness measured with optical coherence tomography; six sectors were delineated for localization of damage area. All eyes were grouped by the healthy macular sector and divided into pre-perimetric, early, moderate, and advanced stages, according to severity of visual field impairment.

RESULTS

Mean GC/IPL thickness in the entire macular region and each sector were correlated with parameters that reflected contrast sensitivity (p < 0.01). The structure-function correlations were stronger nasally compared with temporally, and superiorly compared with inferiorly. Eyes with normal structure in inferior temporal sector had less visual field (p' = 0.024) and macular damage (p' = 0.034) compared with eyes that had healthy superior nasal sector; there was no difference in contrast sensitivity (p = 0.898). The structure-function correlations were significant in early, moderate, and advanced glaucoma (p < 0.05) but not in pre-perimetric glaucoma (p = 0.116).

CONCLUSIONS

GC/IPL thinning in all sectors of the macular region in OAG was correlated with contrast sensitivity impairment, whereas the inferior temporal sector was least affected. Contrast sensitivity was supported as a severity evaluation indicator of early, moderate, and advanced glaucoma, but not of pre-perimetric glaucoma.

The impact of visual function on staircase use performance in glaucoma

OBJECTIVES

This cross-sectional study aimed to investigate the relationship between visual function and staircase use in glaucoma.

METHODS

Overall, 181 patients with glaucoma with a best-corrected visual acuity ≥20/400 were classified into mild to moderate (mean deviation [MD] ≥ -12 dB) and advanced (MD < -12 dB) groups, according to 24-2 VF of the worse eye. Staircase use evaluation included stair descent and ascent time (SDT/SAT) and self-reported stair difficulty. Correlations between staircase use and visual function were analysed, including binocular visual acuity, integrated visual field (IVF), and binocular contrast sensitivity (CS). Linear and logistic regression adjusted by age, sex, and comorbidities inspected the effect of visual parameters on SDT/ SAT and stair difficulty.

RESULTS

Visual function best correlated with SDT among staircase use. In mild to moderate glaucoma, area under the log CS function (AULCSF) (β = -1.648, P = 0.031) was the only visual factor significant for SDT (adjusted R2 = 0.106), whereas AULCSF (β = -1.641, P = 0.048) and MD of IVFINF0-24 (β = -0.089, P = 0.013) were associated with SDT in advanced glaucoma (adjusted R2 = 0.589). The AULCSF was the only significant visual parameter related to SAT (β = -1.125, P = 0.019) and stair difficulty (adjusted odds ratio = 0.003; 95% confidence interval, 0-0.302; P = 0.013).

CONCLUSIONS

SDT provides a higher correlation with visual function than self-reported stair difficulty. Patients with impaired CS or inferior IVF defects should be advised on stair safety and referred to low-vision services.

[Awareness for glaucoma in the general population]

BACKGROUND

Glaucoma is not a rare entity but because very few symptoms occur and visual field defects are frequently first recognized at a late stage, a large proportion of glaucoma diseases remain undetected. The aim of this study was to identify the proportion of undiagnosed glaucoma in German population-based cohort studies and to contextualize them in the context of the literature.

MATERIAL AND METHODS

The prevalence of glaucoma in the Gutenberg Health Study (GHS) and the age-related investigations on health of the University of Regensburg (AugUR) was evaluated based on visual field examinations and optic disc color photography according to the ISGEO criteria. Furthermore, the self-reported glaucoma diagnoses were collected and the proportion of undiagnosed glaucoma was determined.

RESULTS

The proportion of undiagnosed glaucoma was 55% in the GHS, and 53% in the AugUR study. The results correlate with results from previous studies from other countries in which the proportion of unrecognized glaucoma ranged from 33% to 78%. In the GHS and the AugUR study the proportion of undiagnosed glaucoma was higher in younger age groups and in women.

DISCUSSION

Roughly every second case of glaucoma is undetected. As the symptoms are often nonspecific or take a long time to appear, there is a risk of advanced glaucomatous visual field defects or blindness due to a lack of glaucoma awareness. Studies have shown that a systematic screening can halve this risk.

Remote Contrast Sensitivity Testing Seems to Correlate With the Degree of Glaucomatous Macular Damage

PRCIS

Remote contrast sensitivity (CS) testing through a free downloadable home test correlates with glaucomatous macular damage measured by 10-2 visual field (VF) testing.

PURPOSE

To assess the feasibility and validity of home CS monitoring as a measure of glaucomatous damage using a free downloadable smartphone application.

METHODS

Twenty-six participants were asked to remotely use the Berkeley Contrast Squares (BCS) application, a free downloadable tool that records the user's CS for varying degrees of visual acuity. An instructional video detailing how to download and use the application was sent to the participants. Subjects were asked to send logarithmic CS results with a minimum 8-week test-retest window, and test-retest reliability was measured. Results were validated against office-based CS testing that was collected within the previous 6 months. Validity analysis was also carried out to determine whether CS as measured by BCS is a good predictor of 10-2 and 24-2 VF mean deviation (MD).

RESULTS

There was a high BCS test-retest reliability with an intraclass correlation coefficient score of 0.91 and a significant correlation between repeat test results and baseline test scores (Pearson, 0.86, P < 0.0001). There was significant agreement between unilateral CS scores as measured by BCS and office-based CS testing ( b = 0.94, P < 0.0001, 95% CI: 0.61 to 1.27). Unilateral CS as measured by BCS was significantly associated with 10-2 VF MD ( r2 = 0.27, P = 0.006, 95% CI: 3.7 to 20.6), but not with 24-2 VF MD ( P = 0.151).

CONCLUSION

This study suggests that a free, rapid home CS test correlates with glaucomatous macular damage as measured by 10-2 VF.

Atypical responses to faces during binocular rivalry in early glaucoma

Purpose

Glaucoma is a progressive optic neuropathy that damages retinal ganglion cells and a neurodegenerative disease as it affects neural structures throughout the brain. In this study, we examined binocular rivalry responses in patients with early glaucoma in order to probe the function of stimulus-specific cortical areas involved in face perception.

Methods

Participants included 14 individuals (10 females, mean age 65 ± 7 years) with early pre-perimetric glaucoma and 14 age-matched healthy controls (7 females, mean age 59 ± 11 years). The 2 groups were equivalent in visual acuity and stereo-acuity. Three binocular rivalry stimulus pairs were used: (1) real face/house, (2) synthetic face/noise patch, and (3) synthetic face/spiral. For each stimulus pair, the images were matched in size and contrast level; they were viewed dichotically, and presented centrally and eccentrically at 3 degrees in the right (RH) and in the left hemifield (LH), respectively. The outcome measures were rivalry rate (i.e., perceptual switches/min) and time of exclusive dominance of each stimulus.

Results

For the face/house stimulus pair, rivalry rate of the glaucoma group (11 ± 6 switches/min) was significantly lower than that of the control group (15 ± 5 switches/min), but only in the LH location. The face dominated longer than the house in the LH for both groups. Likewise, for the synthetic face/noise patch stimulus pair, rivalry rate of the glaucoma group (11 ± 6 switches/min) was lower than that of the control group (16 ± 7 switches/min) in the LH, but the difference failed to reach significance. Interestingly, the mixed percept dominated less in glaucoma than in the control group. For the synthetic face/spiral stimulus pair, the glaucoma group had lower rivalry rate at all 3 stimulus locations.

Conclusion

This study reveals atypical responses to faces during binocular rivalry in patients with early glaucoma. The results may be suggestive of early neurodegeneration affecting stimulus-specific neural structures involved in face processing starting in the pre-perimetric phase of the disease.

Stimulus contrast, pursuit mode, and age strongly influence tracking performance on a continuous visual tracking task

Human observers tend to naturally track moving stimuli. This tendency may be exploited towards an intuitive means of screening visual function as an impairment induced reduction in stimulus visibility will decrease tracking performance. Yet, to be able to detect subtle impairments, stimulus contrast is critical. If too high, the decrease in performance may remain undetected. Therefore, for this approach to become reliable and sensitive, we need a detailed understanding of how age, stimulus contrast, and the type of stimulus movement affect continuous tracking performance. To do so, we evaluated how well twenty younger and twenty older participants tracked a semi-randomly moving stimulus (Goldmann size III, 0.43 degrees of visual angle), presented at five contrast levels (5%-10%-20%-40%-80%). The stimulus could move smoothly only (smooth pursuit mode) or in alternation with displacements (saccadic pursuit mode). Additionally, we assessed static foveal and peripheral contrast thresholds. For all participants, tracking performance improved with increasing contrast in both pursuit modes. To reach threshold performance levels, older participants required about twice as much contrast (20% vs. 10% and 40% vs. 20% in smooth and saccadic modes respectively). Saccadic pursuit detection thresholds correlated significantly with static peripheral contrast thresholds (rho = 0.64). Smooth pursuit detection thresholds were uncorrelated with static foveal contrast thresholds (rho = 0.29). We conclude that continuous visual stimulus tracking is strongly affected by stimulus contrast, pursuit mode, and age. This provides essential insights that can be applied towards new and intuitive approaches of screening visual function.

Investigating the discrepancy between MAIA and MP-1 microperimetry results

PURPOSE

Previous work has suggested that sensitivities measured on the iCare MAIA and Nidek MP-1 microperimeters differ systematically, although it is unclear whether one or both devices are inaccurate. Here, we assess the discrepancy between these two instruments as well as with a rigorous reference standard.

METHODS

Fifteen healthy participants underwent visual field testing on the MAIA and MP-1 microperimeters. Results were compared to a reference measure of increment thresholds on a laboratory-based, calibrated computer monitor system using the same background luminance and target size. Discrepancies were assessed as a function of eccentricity along the vertical meridian. Differences in decibels (dB) due to differences in the maximum stimulus luminance between devices were accounted for mathematically.

RESULTS

The mean sensitivity measured with the MAIA was <1 dB lower than laboratory-based measures, which was statistically significant but of limited clinical importance. In contrast, the mean sensitivity measured with the MP-1 was >8 dB lower than the laboratory measures. The difference was greater for an eccentric superior retinal location, in contrast to what would be predicted if the discrepancy was due to a ceiling effect caused by the MP-1's limited dynamic range.

CONCLUSIONS

While MAIA measurements showed low bias compared with our rigorously determined reference standard, the MP-1 showed large discrepancies that could not be explained purely by the limited dynamic range of the instrument. MAIA and MP-1 sensitivity values cannot be compared directly, and caution is advised when assessing absolute sensitivities or eccentricity effects in the extensive MP-1 literature.

Intraocular Pressure Elevation Compromises Retinal Ganglion Cell Light Adaptation

Purpose

Functional adaptation to ambient light is a key characteristic of retinal ganglion cells (RGCs), but little is known about how adaptation is affected by factors that are harmful to RGC health. We explored adaptation-induced changes to RGC physiology when exposed to increased intraocular pressure (IOP), a major risk factor for glaucoma.

Methods

Wild-type mice of both sexes were subjected to 2 weeks of IOP elevation using the bead model. Retinas were assessed using a multielectrode array to record RGC responses to checkerboard white noise stimulation under both scotopic and photopic light levels. This information was used to calculate a spike-triggered average (STA) for each RGC with which to compare between lighting levels.

Results

Low but not high IOP elevation resulted in several distinct RGC functional changes: (1) diminished adaptation-dependent receptive field (RF) center-surround interactions; (2) increased likelihood of a scotopic STA; and (3) increased spontaneous firing rate. Center RF size change with lighting level varied among RGCs, and both the center and surround STA peak times were consistently increased under scotopic illumination, although none of these properties were impacted by IOP level.

Conclusions

These findings provide novel evidence that RGCs exhibit reduced light-dependent adaptation and increased excitability when IOP is elevated to low but not high levels. These results may reveal functional changes that occur early in glaucoma, which can potentially be used to identify patients with glaucoma at earlier stages when intervention is most beneficial.

Efficient measurements for the dynamic range of human lightness perception

PURPOSE

Patients with an eye disease often report nyctalopia, hemianopia, and/or photophobia. We hypothesized that such symptoms are related to the disease impacting the dynamic range of lightness perception (DRL). However, there is currently no standardized approach for measuring DRL for clinical use. We developed an efficient measurement method to estimate DRL.

STUDY DESIGN

Clinical trial METHODS: Fifty-five photophobic patients with eye disease and 46 controls participated. Each participant judged the appearance of visual stimuli, a thick bar with luminance that gradually changed from maximum to minimum was displayed on uniform background. On different trials the background luminance changed pseudo-randomly between three levels. The participants repeatedly tapped a border on the bar that divided the appearance of grayish white/black and perfect white/black. We defined the DRL as the ratio between the luminance values at the tapped point of the border between gray and white/black.

RESULTS

The mean DRL of the patients was approximately 15 dB, significantly smaller than that of the controls (20 dB). The center of each patient's DRL shift depending on background luminance, which we named index of contextual susceptibility (iCS), was significantly larger than controls. The DRL of retinitis pigmentosa was smaller than controls for every luminance condition. Only the iCS of glaucoma was significantly larger than controls.

CONCLUSIONS

This measurement technique detects an abnormality of the DRL. The results support our hypothesis that the DRL abnormality characterizes lightness-relevant symptoms that may elucidate the causes of nyctalopia, hemeralopia, and photophobia.

Threshold versus intensity functions in two-colour automated perimetry

PURPOSE

Two-colour computerised perimetry is a technique developed for assessing cone- and rod-function at fixed background luminances in retinal disease. However, the state of adaptation during testing is unknown but crucial in the interpretation of results. We therefore aimed to determine the adaptational state of rod- and cone-mechanisms in two-colour perimetry.

METHODS

Sensitivity to 480 nm (blue) and 640 nm (red) Goldmann size V targets was determined for 10 normal subjects aged 16 to 46 years at 17 locations in the central 60 degrees of the visual field under scotopic conditions and then from -1.5 log cd m^-2 to 2 log cd m^-2 (white background) in 0.5 log unit steps. Data were fitted with threshold versus intensity (tvi) functions of the form logT = logT₀ + log ((A + A₀ )/A₀ )ⁿ .

RESULTS

No clear rod-cone break was observed for 640 nm stimuli. For 480 nm stimuli, transition from rod-detection to cone-detection occurred at mesopic illumination levels, where rod adaptation approached Weber behaviour. Cone detection mechanisms did not display Weber-like adaptation until the background luminance approached 1 log cd.m^-2 . Diseases resulting in a "filter effect" - including disorders of the photoreceptors - are therefore predicted to affect sensitivity when rod function is probed with short-wavelength targets under scotopic conditions, but less so under mesopic conditions. Filter effects are similarly anticipated to affect cone function measured using long-wavelength targets under mesopic conditions (e.g., during microperimetry), but less so under photopic conditions.

CONCLUSIONS

Asymmetries in adaptation in automated two-colour perimetry are predicted to artefactually favour the detection of losses in rod sensitivity under scotopic conditions and cones under mesopic conditions.

Contrast Sensitivity and Glaucoma

Spatial contrast sensitivity decline occurs in different types and stages of glaucoma. In this short review we discuss methods of assessing contrast sensitivity, the role of contrast sensitivity in the pathogenesis of glaucoma and its practical application in clinical practice.

Binocular superior visual field areas associated with driving self-regulation in patients with primary open-angle glaucoma

BACKGROUND/AIMS

The aim of our study was to investigate the associations between driving self-regulation and glaucoma severity, and between driving self-regulation and glaucomatous visual field (VF) defect patterns.

METHODS

In 247 patients with primary open-angle glaucoma included in this prospective observational study, a battery of ophthalmic examination was performed, including visual acuity (VA) and VF. Integrated binocular VF was constructed and mean of total deviation (mTD) values in four sectors was calculated (mTDsup-peri, mTDsup-centre, mTDinf-peri and mTDinf-centre). In addition, all participants answered seven questions regarding their avoidance in driving. (1) at night, (2) in rain, (3) in fog, (4) on freeways, (5) lane changing, (6) at high speed and (7) close to the car in front. The associations between these driving behaviours and 10 variables (age, gender, best VA, worst VA, the four sectorial average TD values, years holding a driver's licence and distance driven per week) were analysed using the generalised linear model with binomial distribution, followed by the model section method using the corrected Akaike information criterion.

RESULTS

As a result of the model selection, it was suggested that deterioration of mTDsup-peri was associated with (1) avoiding driving at night and (2) avoiding driving in rain. On the other hand, mTDsup-centre was related to (3) avoiding driving in fog.

CONCLUSION

Damage in visual function was related with driving behaviours in patients with glaucoma.

Refractive Outcomes After Trabecular Micro-Bypass Stents (iStent Inject) with Cataract Extraction in Open-Angle Glaucoma

Purpose

Simultaneous cataract and glaucoma surgery has traditionally been challenging for the anterior segment surgeon. The introduction of minimally invasive glaucoma surgery (MIGS) in conjunction with cataract surgery appears safe and effective in lowering intraocular pressure. Although a significant visual impact leading from the combined procedure is unexpected, we aim to describe the refractive outcomes in a cohort of patients undergoing simultaneous cataract removal and iStent inject and discuss the potential implications of combined surgery in patients with co-existent glaucoma.

Patients and Methods

This is a retrospective consecutive case series inclusive of patients undergoing combined femtosecond laser-assisted cataract surgery and the insertion of two trabecular micro-bypass stents (iStent inject). Visual acuity, refraction and astigmatic vector analysis were collated and analysed from the preoperative and 4 weeks postoperative visits.

Results

One hundred and six eyes of 89 patients from 2 surgeons were included in the original cohort. The mean absolute difference from target refraction was 0.36 ± 0.25D. 73.9% of eyes were within ± 0.5D of the refractive target and 98.9% of eyes were within ± 1.00D. 73.8% of eyes had 0.5D or less residual refractive astigmatism following the procedure.

Conclusion

We present a novel cohort of glaucoma patients undergoing combined trabecular micro-bypass stents (iStent inject) and cataract surgery achieving excellent refractive outcomes. The results of this study indicate that this second-generation device is refractively neutral.

Retinal Contrast Gain Control and Temporal Modulation Sensitivity Across the Visual Field in Glaucoma at Photopic and Mesopic Light Conditions

Purpose

Glaucoma affects many aspects of visual performance, including adaptation, and this may depend on ambient luminance. We determine the influence of glaucoma and luminance on temporal aspects of adaptation, specifically on contrast gain control and temporal modulation sensitivity (TMS).

Methods

This case-control study included 12 glaucoma patients and 25 age-similar controls (50-70 years). Threshold perimetry was performed with a minimized testing grid (fovea and four peripheral locations). Stimuli (Goldmann size III 50 ms increment/decrement) were presented on a time-varying background with sinusoidally-modulated luminance (amplitude 60%; frequency 0-30 Hz; mean background luminance, 1 and 100 cd/m2). TMS (2.5-30 Hz) was measured in the same locations with a sinusoidally-modulated stimulus (Goldmann size IV, 334 ms) on a steady background (1 and 100 cd/m2).

Results

In healthy subjects, contrast sensitivity decreased with increasing background modulation frequency and increased again at very high frequencies, indicating contrast gain control. Minimum sensitivity was located between 2.5 and 20 Hz, depending on luminance and eccentricity. In glaucoma patients, the same frequency dependency was found (P = 0.12) but with an overall reduced sensitivity (P = 1 × 10-5), independent of luminance (P = 0.20). Decrements differentiated better between glaucoma and healthy subjects than increments (P = 0.004). TMS was reduced in glaucoma (P = 5 × 10-6) across all frequencies and luminance levels, with complete loss for high frequencies at 1 cd/m2.

Conclusions

Contrast gain control is largely unaffected in glaucoma, suggesting intact amacrine cell function. Perimetry with decrements or a high-frequency stimulus on a low-luminance background seems best to differentiate between glaucoma and healthy subjects.

How do different lighting conditions affect the vision and quality of life of people with glaucoma? A systematic review

This article is a systematic review of evidence regarding the impact of different lighting conditions on the vision and quality of life (QoL) of people with primary open-angle glaucoma (POAG). A systematic literature search was carried out using CINAHL, MEDLINE, PsycARTICLES, PsycINFO, Embase, and Ovid Nursing Database for studies: published up to April 2019; including people diagnosed with POAG; and assessing visual function or QoL in response to changing lighting/luminance levels or glare. Two researchers independently screened studies for eligibility. Data were extracted from eligible studies regarding study design, participant characteristics, outcomes, and results. Quality of included studies was critically appraised. Of 8437 studies, 56 eligible studies were included. Studies investigated the effects of lighting on the following domains among people with POAG: QoL (18/56), psychophysical measures (16/56), functional vision (10/56), activities of daily living (10/56), and qualitative findings (2/56). POAG negatively affects low-luminance contrast sensitivity, glare symptoms, and dark adaptation time and extent. In vision-related QoL questionnaires, people with POAG report problems with lighting, glare, and dark adaptation more frequently than any other domain. These problems worsen with progressing visual field loss. Early-stage POAG patients experience significantly more difficulties in low-luminance or changing lighting conditions than age-matched controls (AMCs), challenging perceptions of early-stage POAG as asymptomatic. However, performance-based studies seldom show significant differences between POAG participants and AMCs on tasks simulating daily activities under non-optimal lighting conditions. Further research with larger samples is required to optimise ambient and task-oriented lighting that can support patients' adaptation to POAG.

Dark Adaptation Survey as a Predictive Tool for Primary Open-Angle Glaucoma

PURPOSE

To study the extent to which patients with primary open-angle glaucoma (POAG) have subjective difficulties with dark (DA) adaptation and vision under low-luminance conditions and to correlate the reported difficulties with severity of disease, specifically visual field loss.

DESIGN

Prospective, comparative case series.

PARTICIPANTS

Two hundred twenty patients with and without POAG who sought treatment at an outpatient subspecialty glaucoma clinic between October 2016 and September 2018.

METHODS

A questionnaire designed to assess difficulties with DA and vision under low luminance (the DA survey) was developed and given to patients during routine clinical evaluation in a hospital-based eye clinic. Retrospective data related to ocular health and glaucoma severity were abstracted from the medical record. A multiple regression analysis was performed to create a predictive model for POAG.

MAIN OUTCOME MEASURES

Severity of glaucoma, specifically visual field loss, and classification of participants as glaucoma patients or control participants.

RESULTS

Eighty-five patients with POAG and 127 control participants completed the questionnaire. Exploratory factor analysis of the DA survey using a principal components analysis showed that the items split into 2 dimensions: tasks that involved difficulty with vision under low luminance and those that required DA. Cronbach's α showed a high degree of internal consistency reliability (α = 0.88). Increasing DA survey score correlated with binocular visual field loss among patients with mild, moderate, and advanced POAG (P < 0.001). A model used to differentiate patients with POAG from control participants that included average and intereye difference in cup-to-disc ratio, family history of glaucoma, and DA survey score showed an accuracy, sensitivity, and specificity of 96.7%, 92.9%, and 99.2%, respectively.

CONCLUSIONS

This pilot study revealed that problems with DA and vision under low luminance are commonly encountered by patients with POAG. These areas of visual disability are not assessed routinely in glaucoma care. A questionnaire assessing vision under low luminance and light-dark transitions may serve as a proxy for functional impairment in glaucoma. When paired with risk factors such as structural features of the optic nerve and family history, this survey instrument may be suitable to screen for patients with POAG.

Does eye examination order for standard automated perimetry matter?

Purpose

In spite of faster examination procedures, visual field (VF) results are potentially influenced by fatigue. We use large‐scale VF data collected from clinics to test the hypothesis that perimetric fatigue effects are greater in the eye examined second.

Methods

Series of six Humphrey Swedish Interactive Testing Algorithm (SITA) VFs from 6901 patients were retrospectively extracted from a VF database from four different glaucoma clinics. Mean deviation (MD) was compared between first and second tested eyes. A surrogate measure of longitudinal MD variability over time was estimated from errors using linear regression of MD against time then compared between first and second tested eye.

Results

Right eye VF was tested consistently first throughout in 6320 (91.6%) patients. Median (interquartile range; IQR) MD in the first tested (right) eye and second tested (left) eye was −2.57 (−6.15, −0.58) dB and −2.70 (−6.34, −0.80) dB respectively (median reduction VF sensitivity of 0.13 dB; p < 0.001). Median (IQR) increase in our surrogate measure of longitudinal MD variability in the second eye tested was 3% (−43%, 50%); this effect was not associated with patient age or rest time between examinations.

Conclusion

Statistically significant perimetric fatigue effects manifest on average in the second eye tested in routine clinics using Humphrey Field Analyzer SITA examinations. However, the average effects were very small and there was enormous variation among patients. We recommend starting with a right eye examination so that any perimetric fatigue effects, if they exist in an individual, will be as constant as possible from visit to visit.

Spatial contrast sensitivity from star- to sunlight in healthy subjects and patients with glaucoma

Glaucoma is traditionally considered an asymptomatic disease until later stages. However, questionnaire studies revealed visual complaints related to various tasks, especially under extreme luminance conditions (such as outdoor at night on an unlit road or outside in the sun). We measured contrast sensitivity (CS) over a luminance range of 6 log units spanning the scotopic to photopic range and we aimed (1) to determine whether Weber's law also holds under extremely high luminance conditions and (2) to compare CS as a function of spatial frequency and luminance between glaucoma patients and healthy subjects. We included 22 glaucoma patients and 51 controls, all with normal visual acuity. For the second aim, we used a subgroup of 22 age-similar controls. Vertically oriented sine-wave gratings were generated with a projector-based setup (stimulus size 8x5 degrees). CS was measured monocularly at 1, 3, and 10 cycles per degree (cpd); mean luminance ranged from 0.0085 to 8500 cd/m². ANOVA was used to analyze the effect of glaucoma, luminance, and spatial frequency on logCS. In controls, Weber's law held for 3 and 10 cpd; for 1 cpd, CS dropped above 1000 cd/m² (P = 0.003). The logCS versus log luminance curves did not differ grossly between patients and controls (P = 0.14; typically 0-0.2 log units); the difference became larger with decreasing luminance (P = 0.003) but did not depend clearly on spatial frequency (P = 0.27). We conclude that differences between glaucoma and healthy were relatively modest for the spatially redundant, static stimulus as used in the current study.

Mild Intraocular Pressure Elevation in Mice Reveals Distinct Retinal Ganglion Cell Functional Thresholds and Pressure-Dependent Properties

Elevation of intraocular pressure (IOP) causes retinal ganglion cell (RGC) dysfunction and death and is a major risk factor for glaucoma. We used a bead injection technique to increase IOP in mice of both genders by an average of ∼3 mmHg for 2 weeks. This level of IOP elevation was lower than that achieved in other studies, which allowed for the study of subtle IOP effects. We used multielectrode array recordings to determine the cellular responses of RGCs exposed to this mild degree of IOP elevation. We found that RGC photopic receptive field (RF) center size and whole-field RGC firing rates were unaffected by IOP elevation. In contrast, we found that the temporal properties of RGC photopic responses in the RF center were accelerated, particularly in ON sustained cells. We also detected a loss of antagonistic surround in several RGC subtypes. Finally, spontaneous firing rate, interspike interval variance, and contrast sensitivity were altered according to the magnitude of IOP exposure and also displayed an IOP-dependent effect. Together, these results suggest that individual RGC physiologic parameters have unique IOP-related functional thresholds that exist concurrently and change following IOP elevation according to specific patterns. Furthermore, even subtle IOP elevation can impart profound changes in RGC function, which in some cases may occur in an IOP-dependent manner. This system of overlapping functional thresholds likely underlies the complex effects of elevated IOP on the retina.SIGNIFICANCE STATEMENT Retinal ganglion cells (RGCs) are the obligate output neurons of the retina and are injured by elevated intraocular pressure (IOP) in diseases such as glaucoma. In this study, a subtle elevation of IOP in mice for 2 weeks revealed distinct IOP-related functional thresholds for specific RGC physiologic parameters and sometimes showed an IOP-dependent effect. These data suggest that overlapping IOP-related thresholds and response profiles exist simultaneously in RGCs and throughout the retina. These overlapping thresholds likely explain the range of RGC responses that occur following IOP elevation and highlight the wide capacity of neurons to respond in a diseased state.