Parallel pathways, noise masking and glaucoma detection: behavioral and electrophysiological measures

We tested the hypothesis that because of their reduced neural efficiency, glaucoma patients should have increasingly impaired thresholds as external noise is added to a stimulus.

We compared the performance of 20 normals (mean age = 39 years) with that of 15 patients with early glaucoma or at very high risk for glaucoma (mean age 45 years). All patients had normal visual acuity. Contrast thresholds were measured on two sets of tasks: (1) behavioral and (2) sweep visually evoked potentials (VEPs). Two stimuli were used (a) 7.5 Hz reversing gratings of 0.69 cpd, and (b) 5.5 cpd gratings. Noise was binary and contrast varied from 0 to 80%. Psychophysical thresholds were determined using a staircase which employed a spatial four alternative forced choice procedure (4AFC) and converged on 50% correct. Sweep VEP thresholds were determined by extrapolation to zero volts as a function of log contrast.

Differences between normal subjects and patients with early glaucoma were not significant without noise. Both the absolute size of the difference and its significance increased as noise level increased. For the behavioral thresholds these trends were clearer with the 5.5 cpd grating, while for the sweep VEPs they were more clear for the 0.69 cpd grating.

The performance deficit of glaucoma patients which may be minimal under normal testing conditions is magnified when external noise is added to the stimulus. VEPs and psychophysical thresholds show interesting differences in their sensitivity to this effect. Implications for the early detection of glaucoma are discussed.