Sources of motion-sensitivity loss in glaucoma

Purpose. Primary open angle glaucoma (POAG) is a leading cause of irreversible adult blindness and is characterized by progressive optic neuropathy and constriction of the visual field. Behavioral tests for POAG target retinal ganglion cell (RGC) classes that have reduced redundancy or that might be selectively damaged, but these tests cannot differentiate dysfunctional from nonfunctional RGC inputs to motion sensors. In this study, a signal-to-noise motion-sensitivity task was used to investigate the sources of motion-sensitivity loss in patients with POAG. Methods. An equivalent noise paradigm was used to investigate sensitivity to the direction of radial optic flow patterns across the visual field in visually normal observers and patients with POAG. Internal noise and sampling efficiency were estimated from the direction of heading contrast-discrimination thresholds as a function of the level of added external noise. Results. Contrast sensitivity to optic flow fell with retinal eccentricity for all observers, and decreased with both age and POAG. Equivalent noise analysis showed that the fall-off with eccentricity was primarily due to reduced sampling efficiency with relatively little increase in the level of internal noise and that the fall-off with age was attributable to both sources of error. Compared with age-matched control observers, patients with POAG have similar levels of internal noise but significantly lower sampling efficiency at all retinal loci. Conclusions. Motion-sensitivity losses with age, eccentricity, and POAG can arise from higher levels of internal noise and lower sampling efficiency. The central and peripheral glaucomatous neuropathy is mostly attributable to a reduction in sampling efficiency, suggesting that RGCs are nonfunctional rather than dysfunctional in this condition.