Clinical VF assessment was performed on all subjects to determine a baseline for subsequent laboratory-based psychophysical testing and also to identify the region of interest for further psychophysical testing. For RP and POAG subjects, the region of interest was the border of a scotoma which lay within 30° of fixation. The reason for using scotomata within 30° was because of limitations to the size of the test screen for the laboratory-based testing (see below section). Although many cases of Riddoch syndrome were initially described to occur mostly within the peripheral VF (>30° from fixation),
4 subsequent studies reported it within the central VF (within 30°),
5,7,11,12 and, thus, we examined this area as our region of interest. For RP subjects, the border of the scotoma was defined as the SAP test location at which there was a defect on the pattern deviation map at the
P < 0.005 level. For glaucoma subjects, it was defined as the test location closest to fixation which was part of a cluster of at least three contiguous points of depression at least at the
P < 0.05 level, one of which was at least
P < 0.01, as per typical glaucomatous defect definitions.
31 Using RP as a model to determine whether there was an effect of meridian or severity of VF defect upon the isocontrast zone, when we plotted isocontrast zone width as a function of isocontrast midpoint, there was a statistically significant nonzero slope (0.059 ± 0.020,
P = 0.0032). However, there was a very poor fit to the data (
R2 = 0.060), with the majority of widths within 8° (70.8%). Thus, we chose cardinal meridians that targeted regions within 30° of fixation but not temporally, in case the physiologic blind spot was accidentally tested.
For RP subjects, clinical perimetry consisted of SAP (HFA using the SITA-Standard paradigm using 30-2 and 10-2 test grids) and Goldmann kinetic perimetry. For POAG subjects, this included only SAP, as per typical clinical guidelines (HFA 24-2 SITA-Standard). Although the 24-2 test grid uses points with 6° spacing, a coarser grid compared to the 2° spacing of the 10-2, it served as a baseline for further psychophysical testing.