System to measure contrast-sensitivity function with a scanning-laser ophthalmoscope

A scanning laser ophthalmoscope (SLO) employs an extremely low power laser that is focused to a 10 micron spot on the human retina and scanned across the retina at the same rate and pattern that an electron beam travels across a television screen. An image is acquired by measuring the retina's reflectance along each raster. An acousto-optic modulator, in the path of the laser beam, can be used to write targets directly onto the retina. since the SLO can project targets onto the retina while simultaneously viewing the retina, retinal function at specific locations can be unambiguously assessed. We developed software so that sine wave contrast sensitivity gratings were projected onto subjects' retinas with contrasts ranging from 0 to 63.5%. Subjects determined whether the gratings tilted 45 degree(s) to the right or 45 degree(s) to the left. A staircase protocol using 6 reversals was employed to determine contrast sensitivity at each spatial frequency. In early work with the system, 16 eyes of 11 normal subjects were tested. For 8 eyes the intraclass correlation coefficient of reliability was 0.85. This system yields reproducible measures of contrast sensitivity. It may have advantages over other methods to measure contrast sensitivity because the location of tested retina can be unambiguously assessed and illumination levels from the instrument directly measured and controlled. In addition, the small entrance pupil of the SLO can reduce the effects of cataracts measurements of contrast sensitivity.