Contrast increment and decrement processing measured using visual search in established glaucoma

Visual processing of contrast increments and decrements begins with ON and OFF retinal ganglion cells (RGC) through ON and OFF pathways that signal light and dark information, respectively. In experimental glaucoma, OFF RGC are selectively vulnerable, relative to ON RGC. This study tested whether the rate of performance associated with advancing visual field severity differed between ON and OFF pathways in a cross-sectional group of people with established glaucoma.

A total of 106 participants (mean age: 67, range: 33-84 years old) with established glaucoma performed a visual search task as part of a longitudinal prospective clinical study (Individualised Perimetry Progression Observations in Glaucoma Study). Participants responded quickly and accurately to high contrast larger white or black squares superimposed within a pixelated black and white background displayed on an iPad tablet. Final increment and decrement response times were calculated from 90 stimuli per contrast fitted to an exponential-Gaussian curve. In addition, 24-2 visual fields were measured (Compass perimeter, ZEST-Fast strategy).

Slower response times were observed with greater visual field loss. Cross-sectionally, there were similar rates of decline for ON and OFF response times as a function of visual field mean deviation (bootstrapped 95% confidence intervals of the regression slopes: increments [-0.21, -0.06]; decrements [-0.12, -0.04]). Individuals with central-sparing perimetric defects were faster at searching for both increment and decrement stimuli relative to individuals with central-involving loss (ON: 3.52 s vs. 2.82 s, p < 0.001; OFF: 2.54 s vs. 1.95 s, p < 0.001).

This study does not support selective OFF RGC loss in glaucoma. People with more advanced glaucoma and central visual field loss had markedly slower response times than those with early glaucoma, with equivalent performance reduction for ON and OFF probing stimuli.

This data is collected as part of a registered clinical study: Individualised Perimetry Progression Observations in Glaucoma Study (ACTRN12623000961640).