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

Abstract

Citation: João CAR, Scanferla L, Jansonius NM. Retinal contrast gain control and temporal modulation sensitivity across the visual field in glaucoma at photopic and mesopic light conditions. Invest Ophthalmol Vis Sci. 2019;60:4270-4276. https://doi.org/ 10.1167/iovs.19-27123 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/m 2 ). 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/m 2 ). 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 ¼ 0.12) but with an overall reduced sensitivity (P ¼ 1 3 10 À5 ), independent of luminance (P ¼ 0.20). Decrements differentiated better between glaucoma and healthy subjects than increments (P ¼ 0.004). TMS was reduced in glaucoma (P ¼ 5 3 10 À6 ) across all frequencies and luminance levels, with complete loss for high frequencies at 1 cd/m 2 .CONCLUSIONS. Contrast gain control is largely unaffected in glaucoma, suggesting intact amacrine cell function. Perimetry with decrements or a high-frequency stimulus on a lowluminance background seems best to differentiate between glaucoma and healthy subjects.

Preferential Loss of Contrast Decrement Responses in Human Glaucoma

“… 44 , 45 However, when measurements were made at 9 degrees eccentricity, decremental thresholds were found to be elevated relative to increment thresholds, suggesting more OFF-pathway damage there, and pointing to retinal eccentricity of the targets as an important factor. 45 The behavioral literature is thus mixed on whether the ON or OFF pathways are preferentially damaged in glaucoma, but our data detecting preferential loss of OFF pathway sensitivity adds to the preponderance of data on non-foveal vision, where glaucoma damage is more reliably detected in early and moderate disease. The next steps following from these studies could include additional dissection of the parafoveal and increasingly eccentric visual field to address these differences directly.…”

“…Eliciting a more robust response from the periphery may be important for detection ON/OFF differences as a previous psychophysical study in patients with glaucoma has found a greater deficit for detecting contrast decrements beyond the central 9 degrees. 45 The use of multiple frequency stimulation 66 to achieve analysis according to eccentricity may facilitate addressing this point, as discussed above. Taken together, the modifications in the current SSVEP protocol may more reliably differentiate between ON and OFF pathway responses, consistent with the larger effect of glaucoma on OFF pathways detected here.…”

Relationship Between Flicker Modulation Sensitivity and Retinal Ganglion Cell Related Layer Thicknesses

“…Previous studies have found significant FMS changes across the visual field at photopic 9 , 10 , 12 and mesopic light conditions in glaucoma when compared with controls. 16 Fluctuations in retinal illuminance can have a large effect on FMS in healthy subjects. 48 Differences in pupil size (see Table 1 ) may have contributed significantly to increased intersubject variability in retinal illuminances under both photopic and mesopic conditions employed in this study.…”

“…The small differences in FMS values between the two protocols may, however, increase significantly in older subjects and particularly in those with early stage diseases of the retina who are likely to exhibit a greater decrease in sensitivity in the mesopic range. 16 …”

“…7 – 13 In agreement with the often reduced sensitivity and greater visual discomfort at lower light levels described in patients with glaucoma 14 and healthy older subjects, 15 rapid flicker sensitivity has also been shown to be reduced across the visual field under both photopic and mesopic lighting. 16 The flicker tests used in this study have been shown to have sufficient sensitivity to reveal the effects of normal aging of rod and cone-specific pathways, both in central vision and in the parafoveal retina. 17 , 18 …”