The physics and psychophysics of microperimetry

To assess the influences of stimulus parameters (physics) on measures of visual field sensitivity (psychophysics).

Subjects' thresholds were measured on three different clinically available perimeters: the Humphrey Field Analyzer (HFA), the Nidek MP1 (MP1), and the Opko OCT/SLO (OSLO). On all machines, visual field testing was done with a 10-2 spatial distribution of test points, using Goldmann Size III and Size I stimuli, with a presentation time of 200 ms, and using a 4-2 threshold algorithm.

All the MP1 and OSLO data fell below the values for the corresponding points on the HFA. For the Goldmann Size III target, the HFA median threshold was 33 dB, whereas the MP1 median threshold was 19 dB and the OLSO, 18 dB. Using the increment intensity values at each dB level for each microperimeter, the data were converted to equivalent HFA dB. Using this conversion, the smallest increment displayed in the MP1 (1.27 cd/m) was equivalent to 34 HFA dB, and the brightest increment displayed by the MP1 was 14 HFA dB (127 cd/m). The smallest increment displayed in the OSLO (1.56 cd/m) was equivalent to 33.1 HFA dB, and the brightest increment displayed by the OSLO was 13.6 HFA dB (137 cd/m). There was good correspondence among these results when compared using equivalent increment threshold units. However, discrepancies in our findings made us acutely aware of the importance of evaluating the consequences of design choices made by the manufacturers.

The findings underscore the need for users to check their assumptions about what the equipment is doing and to always evaluate the psychophysical consequences of the stimuli that are used by a particular instrument.