Comparison of three tests using the frequency doubling illusion to diagnose glaucoma.

Purpose: The introduction of the FDT perimeter prompted the comparison of three tests employing frequency doubling (FD) stimuli. These measures compared different visual field locations and contrast ranges. Frequency of seeing curves were examined for the method most similar to FDT.
Methods: For 146 eyes the following were obtained: (i) contrast matches to two suprathreshold FD stimuli (normal subjects, ocular hypertensve suspects, primary open angle glaucoma subjects); (ii) two alternative forced choice (2AFC) thresholds for horizontally versus vertically orientated FD gratings: and (iii) contrast thresholds determined by method of adjustment (MOA) for five different stimulus types.
Results: A model based on the worst of the MOA hemifield thresholds performed best. The suprathreshold contrast matching tests performed worst. Frequency of seeing curves were fitted for the 146 eyes of the 2AFC tests. Although the MOA thresholds were higher than the 2AFC thresholds (for normals mean +/- SE, 8.47 +/- 0.43 dL, P < 0.0000), the best diagnostic concordance was at lower limens (75% or 80% correct) of the fitted frequency of seeing curves.
Conclusions: There was good diagnostic concordance between the MOA and 2AFC methods although the thresholds were 1.8-fold different on a log-scale. This suggests that the same neural mechanism mediates both thresholds for rapidly flickering, spatially coarse, patterns.