Effect of Age and Glaucoma on the Detection of Darks and Lights

Utility of Light-Adapted Full-Field Electroretinogram ON and OFF Responses for Detecting Glaucomatous Functional Damage

Purpose

To compare parameters of electroretinogram (ERG) responses for their ability to detect functional loss in early stages of nonhuman primate (NHP) experimental glaucoma (EG), including photopic negative responses (PhNR) to a standard brief red flash on a blue background (R/B) and 200-ms-long R/B and white-on-white (W/W) flashes, to W/W flicker stimuli (5-50 Hz), and to a dark-adapted intensity series.

Methods

Light-adapted ERGs were recorded in 12 anesthetized monkeys with unilateral EG. Amplitudes and implicit times of the a-wave, b-wave, and d-wave were measured, as well as amplitudes of PhNRs and oscillatory potentials for flash onset and offset. Flicker ERGs were measured using peak-trough and fundamental frequency analyses. Dark-adapted ERG parameters were modeled by Naka-Rushton relationships.

Results

Only PhNR amplitudes were significantly reduced in EG eyes compared to fellow control (FC) eyes. The d-wave implicit time was delayed in EG versus FC eyes only for the W/W long flash, but in all eyes it was 10 to 20 ms slower for R/B versus the W/W condition. Flicker ERGs were <0.5 ms delayed in EG versus FC overall, but amplitudes were affected only at 5 Hz. The brief R/B PhNR amplitude had the highest sensitivity to detect EG and strongest correlation to parameters of structural damage.

Conclusions

The PhNR to the standard brief R/B stimulus was best for detecting and following early-stage functional loss in NHP EG.

Translational Relevance

These results suggest that there would be no benefit in using longer duration flashes to separate onset and offset responses for clinical management of glaucoma.

Tablet-based 'ON/OFF' pathway test can distinguish between rod- and cone-dominated diseases

INTRODUCTION

The photopic ON pathway defect is associated with nocturnal vision loss. However, the measurement of ON function to detect a rod-dominated disease (rods affected more than cones) has not been explored. We evaluated whether the psychophysical evaluation of ON/OFF pathways can be used to distinguish cone-dominated from rod-dominated diseases.

METHODS

Thirty-seven patients with inherited retinal diseases were tested using the 'EyeSpeed' [iOS application] on an iPad. The test displayed a random number (1-3) of light or dark targets on a black-and-white noise background. Participants responded on a touch screen indicating the correct number of targets displayed. The outcome variables-reaction time, accuracy and performance index (speed [1/reaction time] * accuracy) to both light and dark targets were assessed for diagnostic ability using standard receiver-operating characteristic (ROC) analysis.

RESULTS

Mean ± standard deviation age and visual acuity for the cone- and rod-dominated groups were 25.15 ± 11.74 years, 0.80 ± 0.25 logMAR and 28.3 ± 14.29 years, 0.48 ± 0.26 logMAR, respectively. The median reaction time to light targets in rod-dominated disease [interquartile range] was 5.28 s [3.17], significantly greater than for patients with cone-dominated disease (2.07 s [0.93]; Mann-Whitney U test, p < 0.001). Amongst all of the outcome variables evaluated, the reaction time to light targets (criterion of ≥2.98 s) exhibited the highest area under the ROC curve (area = 0.89 ± 0.11; p < 0.001), with a sensitivity and specificity of 82.4% and 85% respectively.

CONCLUSIONS

Reaction time to light targets using the ON/OFF pathway paradigm is a valid marker to differentiate between rod- and cone-dominated retinal dystrophies. ON pathway function measured using a tablet-based test could act as a supplemental test in the diagnosis of challenging photoreceptor-specific inherited retinal diseases.

Luminance Contrast Shifts Dominance Balance between ON and OFF Pathways in Human Vision

Human vision processes light and dark stimuli in visual scenes with separate ON and OFF neuronal pathways. In nature, stimuli lighter or darker than their local surround have different spatial properties and contrast distributions (Ratliff et al., 2010; Cooper and Norcia, 2015; Rahimi-Nasrabadi et al., 2021). Similarly, in human vision, we show that luminance contrast affects the perception of lights and darks differently. At high contrast, human subjects of both sexes locate dark stimuli faster and more accurately than light stimuli, which is consistent with a visual system dominated by the OFF pathway. However, at low contrast, they locate light stimuli faster and more accurately than dark stimuli, which is consistent with a visual system dominated by the ON pathway. Luminance contrast was strongly correlated with multiple ON/OFF dominance ratios estimated from light/dark ratios of performance errors, missed targets, or reaction times (RTs). All correlations could be demonstrated at multiple eccentricities of the central visual field with an ON-OFF perimetry test implemented in a head-mounted visual display. We conclude that high-contrast stimuli are processed faster and more accurately by OFF pathways than ON pathways. However, the OFF dominance shifts toward ON dominance when stimulus contrast decreases, as expected from the higher-contrast sensitivity of ON cortical pathways (Kremkow et al., 2014; Rahimi-Nasrabadi et al., 2021). The results highlight the importance of contrast polarity in visual field measurements and predict a loss of low-contrast vision in humans with ON pathway deficits, as demonstrated in animal models (Sarnaik et al., 2014).SIGNIFICANCE STATEMENT ON and OFF retino-thalamo-cortical pathways respond differently to luminance contrast. In both animal models and humans, low contrasts drive stronger responses from ON pathways, whereas high contrasts drive stronger responses from OFF pathways. We demonstrate that these ON-OFF pathway differences have a correlate in human vision. At low contrast, humans locate light targets faster and more accurately than dark targets but, as contrast increases, dark targets become more visible than light targets. We also demonstrate that contrast is strongly correlated with multiple light/dark ratios of visual performance in central vision. These results provide a link between neuronal physiology and human vision while emphasizing the importance of stimulus polarity in measurements of visual fields and contrast sensitivity.

Preferential Loss of Contrast Decrement Responses in Human Glaucoma

Purpose

The purpose of this study was to determine whether glaucoma in human patients produces preferential damage to OFF visual pathways, as suggested by animal experimental models, patient electroretinogram (ERG), and retinal imaging data.

Methods

Steady-state visual evoked potentials (SSVEPs) were recorded monocularly from 50 patients with glaucoma and 28 age-similar controls in response to equal Weber contrast increments and decrements presented using 2.73 hertz (Hz) sawtooth temporal waveforms.

Results

The eyes of patients with glaucoma were separated into mild (better than -6 decibel [dB] mean deviation; n = 28) and moderate to severe (worse than -6 dB mean deviation, n = 22) groups based on their Humphrey 24-2 visual field measurements. Response amplitudes and phases from the two glaucoma-severity groups were compared to controls at the group level. SSVEP amplitudes were depressed in both glaucoma groups, more so in the moderate to severe glaucoma group. The differences between controls and the moderate-severe glaucoma groups were more statistically reliable for decrements than for increments. Mean responses to decremental sawtooth stimuli were larger than those to increments in controls and in the mild glaucoma but not in the moderate to severe glaucoma group at the first harmonic. OFF/decrement responses at the first harmonic were faster in controls, but not in patients.

Conclusions

The observed pattern of preferential loss of decremental responses in human glaucoma is consistent with prior reports of selective damage to OFF retinal ganglion cells in murine models and in data from human ERG and retinal imaging. These data motivate pursuit of SSVEP as a biomarker for glaucoma progression.

Age-related decline in function of ON and OFF visual pathways

Purpose

A simple psychophysical paradigm is available as a digital application in iOS devices such as iPad to measure the function of ON and OFF visual pathways. However, an age-matched normative database is not readily available. The purpose of the study is to evaluate the response of ON and OFF visual pathways as a function of age.

Methods

158 normal healthy adults (84 males and 74 females) whose age ranged 18–80 years participated in the study. None of them had any ocular disease (except cataract of grade II or less) and visual acuity of ≤ 20/25. Monocular testing (only one eye) was performed on the ‘EyeSpeed’ application on an iPad at 40cm distance. The targets ranged between 1 to 3 light or dark squares presented randomly in a noise background and participants responded by indicating the number of squares by touching the screen as fast as possible. The main outcome variables are reaction time, accuracy and performance index (1 / speed * accuracy).

Results

The median reaction time was shorter (Median (IQR): 1.53s (0.49) [dark] Vs 1.76s (0.58) [light], p < 0.001) and accuracy was higher (97.21% (3.30) [dark] Vs 95.15% (5.10) [light], p < 0.001) for dark targets than the light targets. Performance index and reaction time for both target types significantly correlated with age (ρ = -0.41 to -0.43; p < 0.001).

Conclusions

This normative database will be useful to quantify disease-specific defects. More importantly, the ON pathway function can potentially serve as a surrogate for rod photoreceptor function.

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.

Amblyopia Affects the ON Visual Pathway More than the OFF

Visual information reaches the cerebral cortex through parallel ON and OFF pathways that signal the presence of light and dark stimuli in visual scenes. We have previously demonstrated that optical blur reduces visual salience more for light than dark stimuli because it removes the high spatial frequencies from the stimulus, and low spatial frequencies drive weaker ON than OFF cortical responses. Therefore, we hypothesized that sustained optical blur during brain development should weaken ON cortical pathways more than OFF, increasing the dominance of darks in visual perception. Here we provide support for this hypothesis in humans with anisometropic amblyopia who suffered sustained optical blur early after birth in one of the eyes. In addition, we show that the dark dominance in visual perception also increases in strabismic amblyopes that have their vision to high spatial frequencies reduced by mechanisms not associated with optical blur. Together, we show that amblyopia increases visual dark dominance by 3-10 times and that the increase in dark dominance is strongly correlated with amblyopia severity. These results can be replicated with a computational model that uses greater luminance/response saturation in ON than OFF pathways and, as a consequence, reduces more ON than OFF cortical responses to stimuli with low spatial frequencies. We conclude that amblyopia affects the ON cortical pathway more than the OFF, a finding that could have implications for future amblyopia treatments.SIGNIFICANCE STATEMENT Amblyopia is a loss of vision that affects 2-5% of children across the world and originates from a deficit in visual cortical circuitry. Current models assume that amblyopia affects similarly ON and OFF visual pathways, which signal light and dark features in visual scenes. Against this current belief, here we demonstrate that amblyopia affects the ON visual pathway more than the OFF, a finding that could have implications for new amblyopia treatments targeted at strengthening a weak ON visual pathway.

Neuronal mechanisms underlying differences in spatial resolution between darks and lights in human vision

Artists and astronomers noticed centuries ago that humans perceive dark features in an image differently from light ones; however, the neuronal mechanisms underlying these dark/light asymmetries remained unknown. Based on computational modeling of neuronal responses, we have previously proposed that such perceptual dark/light asymmetries originate from a luminance/response saturation within the ON retinal pathway. Consistent with this prediction, here we show that stimulus conditions that increase ON luminance/response saturation (e.g., dark backgrounds) or its effect on light stimuli (e.g., optical blur) impair the perceptual discrimination and salience of light targets more than dark targets in human vision. We also show that, in cat visual cortex, the magnitude of the ON luminance/response saturation remains relatively constant under a wide range of luminance conditions that are common indoors, and only shifts away from the lowest luminance contrasts under low mesopic light. Finally, we show that the ON luminance/response saturation affects visual salience mostly when the high spatial frequencies of the image are reduced by poor illumination or optical blur. Because both low luminance and optical blur are risk factors in myopia, our results suggest a possible neuronal mechanism linking myopia progression with the function of the ON visual pathway.