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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
Effects of glaucoma on detection and discrimination of image blur
YesBlur 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.
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) participated. 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
respectively) using a 2-alternative forced choice procedure under two contrast
conditions; 4x individual detection threshold for the low contrast condition, 95%
contrast for the high contrast condition. The stimulus was a horizontal edge bisecting
a hard-edged circle of 4.5° diameter. Data were analysed by linear mixed modelling.
Contrast detection thresholds for the glaucoma group with central visual field defects
were raised by 0.014 ± 0.004 (mean ± SE, Michelson units) (p=0.002) and by 0.011
± 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 ± 0.096 arcmin (p<0.001) for blur detection and
discrimination thresholds respectively.
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 glaucomaCollege of Optometrists PhD scholarshi