Abstract
Purpose: :
Frequency Doubling Technology (FDT) uses rapid temporal modulation, for which psychophysical performance can be more susceptible than conventional perimetry to changes in retinal illumination due to pupil size and lenticular density. Contrast sensitivity perimetry (CSP) has been implemented with slow temporal presentation in order to minimize such effects. This study was conducted to quantify and compare the effects of variations in retinal illuminance on (CSP) and FDT2 (Matrix) perimetry.
Methods: :
A Zeiss-Humphrey/Welch Allyn Matrix FDT2 perimeter wasused with the threshold N-30 strategy. CSP was performed using a customized station. FDT2 presented 0.5 cycle/degree spatial frequency gratings in a 5 degree square window, CSP presented 0.4 cycle/degree Gabor patches in a two-dimensional spatial Gaussian envelope ~4 degrees in diameter. FDT2 used rapid temporal presentation (counter-phase flicker at 18 Hz), while CSP used sustained temporal presentation (Gaussian temporal envelope, ~200 msec). One eye each of 21 younger subjects (23 ± 2 yrs) and ten older subjects (49 ± 8 yrs) were tested with dilated pupils, then retested with neutral density filters of 0.6, 1.2, and 1.6 log unit in front of the subject’s eye. For each subject and neutral density filter, mean sensitivity (MS) was computed as the average sensitivity across all locations. MS versus retinal illuminance data were fit with a standard adaptation model: MS = A*(log(I/(I+K)), where I is mean retinal illuminance and A,K are free parameters.
Results: :
For FDT2, as mean luminance decreased by 1.6 log unit MS decreased by an average (in log units) of 0.64 for the younger group and 0.78 for the older group. For CSP, there was less of a decrease: 0.08 (younger) and 0.17 (older). Both datasets were fit well with the standard adaptation model, which predicted that normal between-subject differences in pupil diameter could contribute 0.29 log unit (5.8 dB) to the normal range for the FDT2 and only 0.02 log unit for CSP.
Conclusions: :
Consistent with a body of psychophysical evidence on the interaction of temporal frequency and retinal illuminance, we found that reduction in mean retinal illuminance had more dramatic effects for FDT2 perimetry versus CSP. Factors contributing to variations in retinal illuminance, such as normal between-subject variability in pupil size and lenticular density will contribute to normal between-subject variability in FDT but not CSP.
Keywords: perimetry • visual fields