Impairments of contrast discrimination and contrast adaptation in glaucoma.
OBJECTIVE
Contrast detection is commonly measured clinically; however, discrimination between contrasts is also important for natural vision. Furthermore, optimal performance requires the visual system to adapt to ambient contrast conditions. Recent studies of primate neurophysiology demonstrate significant retinal involvement in contrast adaptation. This study was conducted to investigate whether glaucoma alters contrast adaptation. Both detection and discrimination task performance were examined.


METHODS
Psychophysical contrast detection and discrimination thresholds were measured in central vision, for a vertically oriented D6 centered on 3 cyc/deg. Thresholds were measured with and without adaptation to low (15%)- and high (70%)-contrast, vertically oriented, 3-cyc/deg sinusoidal gratings. Fifteen people with glaucoma, and 15 age-similar control subjects participated. Full-contrast discrimination (dipper) functions were measured for a subset (three patients with glaucoma and three control subjects).


RESULTS
On average, the glaucoma group showed elevated detection and discrimination thresholds relative to control subjects (detection: t(28) = 2.42; P = 0.02; discrimination: F(1,28) = 6.157, P = 0.02). For the subset of additionally tested participants, normalized contrast discrimination functions were similarly shaped for all observers. Glaucoma group thresholds were less influenced by contrast adaptation than were control subjects, for discrimination (F(1,28) = 10.89, P < 0.01) but not detection (F(1,28) = 2.28; P = 0.11). Differences between groups were greatest for low-contrast stimuli (significant interaction between contrast and group: P < 0.01).


CONCLUSIONS
Glaucoma alters the effect of contrast adaptation on discrimination performance, particularly at low contrast. The study of suprathreshold aspects of vision may reveal new insights into the pathophysiology of glaucoma and possibly relate better to real-world visual performance than detection measures.
McKendrick
Allison M
AM
Department of Optometry and Vision Sciences, Royal Melbourne Hospital, The University of Melbourne, Victoria, Australia. allisonm@unimelb.edu.au
Sampson
Geoff P
GP
Walland
Mark J
MJ
Badcock
David R
DR
D000221:Adaptation, Ocular / Q000502:physiology*; D000368:Aged; D000369:Aged, 80 and over; D015350:Contrast Sensitivity / Q000502:physiology*; D005902:Glaucoma, Open-Angle / Q000503:physiopathology*; D006801:Humans; D007429:Intraocular Pressure; D008875:Middle Aged; D012684:Sensory Thresholds / Q000502:physiology; D014792:Visual Acuity