Suprathreshold Visual Function in Glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide but the effect of glaucoma on patients’ vision under suprathreshold conditions relevant to their natural visual environment is poorly understood. This project aimed to investigate and further understand the effects of glaucoma on three aspects of suprathreshold vision; apparent contrast of suprathreshold stimuli, detection and discrimination of image blur and crowding of peripheral vision. Psychophysical methods were employed to assess these three visual functions by measuring contrast matches of Gabor stimuli, blur detection and discrimination thresholds of edge stimuli and crowding ratios of Vernier targets. These measures were obtained from glaucoma observers tested within and outside of visual field defects and the data compared with healthy controls. Contrast matching ratios were similar between glaucoma and healthy age similar controls despite sensitivity loss in the glaucoma group. Blur detection and discrimination thresholds were similar between glaucoma observers’ tested within and outside of visual field defects and age-similar controls, though thresholds were slightly elevated for high contrast stimuli in the glaucoma visual field defect group. Crowding ratios were similar between participants with glaucoma and healthy young controls. The results demonstrate that aspects of suprathreshold visual function can be maintained in early glaucoma despite sensitivity loss at threshold. The results provide empirical evidence as to the asymptomatic nature of the disease in its early stages. It appears that in early glaucoma, there may be compensatory mechanisms at work within the visual system under suprathreshold conditions that can overcome loss of sensitivity at threshold.The College of Optometrist