Psychophysical characterisation of early functional loss in glaucoma and ocular hypertension

Abstract

Aim: The psychophysical evaluation of selective cell loss in early glaucoma and ocular hypertension.

Methods: Contrast sensitivity was measured for the detection of luminance modulated gratings at a range of spatial (0.5, 2, 8 c/deg) and temporal (0, 16 Hz) frequency combinations in three groups of age matched patients (primary open angle glaucoma, ocular hypertension, normal controls; n=16). Stimuli of 5 degrees were presented foveally and at 15 degrees along the nasal horizontal meridian under photopic conditions.

Results: Fovea: Compared to the normal group, the thresholds for the glaucoma patients were significantly elevated at all spatial and temporal frequencies (p<0.0001), but this reduction was not significantly different at any particular spatial or temporal frequency (p>0.1). There was no difference in contrast sensitivity between the normals and OHTs (p>0.10). Periphery: The thresholds of the glaucoma patients were elevated compared to the normal controls (p<0.01). The loss of sensitivity was slightly greater at the higher spatial frequencies for both stationary and flickering patterns but this did not reach significance (p=0.09). The contrast sensitivity in normal and OHT groups was not significantly different (p>0.10).

Conclusions: In early glaucoma, the reduction in contrast sensitivity to stimuli which isolate the magnocellular pathway (0.5 c/deg, 16 Hz) was not significantly different compared with the reduction in contrast sensitivity to stimuli that isolate the parvocellular pathway. These findings are not consistent with the hypothesis that the magnocellular pathway is selectively damaged in early glaucoma.

Figure 1

(A) Contrast sensitivity function for the three patient groups—foveal viewing, stationary stimuli (0 Hz). (B) Contrast sensitivity function for the three patient groups—foveal viewing, flickering stimuli (16 Hz). Error bar (SEM).

Figure 2

(A) Contrast sensitivity function for the three patient groups—peripheral viewing, stationary stimuli (0 Hz). (B) Contrast sensitivity function for the three patient groups—peripheral viewing, flickering stimuli (16 Hz). Error bar (SEM).

Figure 3

Graphs demonstrating that non-selective loss can account for the change in contrast sensitivity ratios between the M and P cell pathways. Contrast sensitivity functions (CSF) have been plotted for stimuli that are either stationary (open circles) or counterphase flickered (solid circles) observed by a single, representative (non-glaucomatous) observer. (A) shows that with logarithmic axes, the slope for the loss of sensitivity at high spatial frequencies differs for the two stimuli. (B) shows matching slopes for the two stimuli when plotted on linear axes. (C) The change in the ratio of acuity for moving and stationary stimuli arises from the relative shape of the contrast sensitivity functions for these stimuli in the normal eye.