PURPOSE. Glaucoma raises contrast detection thresholds, but our natural visual environment is dominated by high contrast that may remain suprathreshold in early to moderate glaucoma. This study investigates the effect of glaucoma on the apparent contrast of visible stimuli. METHODS. Twenty participants with glaucoma with partial visual field defects (mean age, 72 ± 7 years) and 20 age-similar healthy controls (mean age, 70 ± 7 years) took part. Contrast detection thresholds for Gabor stimuli (SD, 0.75°) of four spatial frequencies (0.5, 1.0, 2.0, and 4.0 c/deg) were first measured at 10°eccentricity, both within and outside of visual field defects for participants with glaucoma. Subsequently, the contrast of a central Gabor was matched to that of a peripheral Gabor with contrast fixed at two times or four times the detection threshold. Data were analyzed by linear mixed modelling. RESULTS. Compared with controls, detection thresholds for participants with glaucoma were raised by 0.05 ± 0.025 (Michelson units, ± SE; P = 0.12) and by 0.141 ± 0.026 (P < 0.001) outside and within visual field defects, respectively. For reference stimuli at two times the detection contrast, matched contrast ratios (matched/reference contrast) were 0.16 ± 0.039 (P < 0.001) higher outside compared with within visual field defects in participants with glaucoma. Matched contrast ratios within visual field defects were similar to controls (mean 0.033 ± 0.066 lower; P = 0.87). For reference stimuli at four times the detection contrast, matched contrast ratios were similar across all three groups (P = 0.58). Spatial frequency had a minimal effect on matched contrast ratios. CONCLUSIONS. Despite decreased contrast sensitivity, people with glaucoma perceive the contrast of visible suprathreshold stimuli similarly to healthy controls. These results suggest possible compensation for sensitivity loss in the visual system.

Blur is one of the most commonly reported visual symptoms of glaucoma, but it is not directly measured by current clinical tests. We aimed to investigate the effects of glaucoma on detection and discrimination of image blur. Methods:Participants were people with glaucoma, separated into two groups with (n = 15) or without (n = 17) central visual field defects measured by 10-2 perimetry, and an age-similar control group (n = 18). First, we measured contrast detection thresholds centrally using a 2-interval forced choice procedure. We then measured blur detection and discrimination thresholds for the same stimuli (reference blurs 0, 1 arcmin) using a 2-alternative forced choice procedure under two contrast conditions: 4× individual detection threshold for the low contrast condition; 95% contrast for the high contrast condition. The stimulus was a horizontal edge bisecting a hardedged circle of 4.5° diameter. Data were analysed by linear mixed modelling.Results: Contrast detection thresholds for the glaucoma group with central visual field defects were raised by 0.01 ± 0.004 (mean ± SE, Michelson units) (p = 0.002) and by 0.01 ± 0.004 (p = 0.03) relative to control and glaucoma without central visual field defect groups, respectively. Blur detection and discrimination thresholds were similar between groups, with small elevations in blur detection thresholds in the glaucoma groups not reaching statistical significance (detection p = 0.29, discrimination p = 0.91). The lower contrast level increased thresholds from the higher contrast level by 1.30 ± 0.10 arcmin (p < 0.001) and 1.05 ± 0.10 arcmin (p < 0.001) for blur detection and discrimination thresholds, respectively. Conclusions:Early-moderate glaucoma resulted in only minimal elevations of blur detection thresholds that did not reach statistical significance in this study. Despite the prevalence of blur as a visual symptom of glaucoma, psychophysical measurements of blur detection or discrimination may not be good candidates for development as clinical tests for glaucoma.