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N. R. A. Parry, A. G. Robson; Measurement of Macular Pigment Optical Density and Distribution Using the Steady-State VEP. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2129.
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To specify isoluminance at different retinal eccentricities and characterize macular pigment optical density (OD) and distribution using the steady-state visual evoked potential (VEP).
Red/Green (R/G) and Blue/Green (B/G) gratings were generated within 2 circular stimulus fields (radius 0.45 or 1.1 degree) and within 4 annular fields (maximum radius 8 degrees) on a colour monitor. Isoluminance was determined for each stimulus using minimum flicker photometry. 15Hz onset-offset VEPs were recorded to the same stimuli, as the luminance ratio between adjacent chromatic components was changed from 0.25 to 0.85 in 11 automated steps (0.5 representing photometric isoluminance). Fourier analysis showed that the power of the first harmonic was minimised at each subject's isoluminant ratio. Relative OD was computed by comparing the isoluminant ratio at any location with that for the most eccentric annulus. To compensate for the broadband characteristics of the monitor, OD values were calibrated according to minimum flicker measurements made through known concentrations of carotenoid solution.
There was close correlation between the isoluminant ratios determined by minimum flicker and VEPs for both R/G and B/G stimulation (r=0.91, p<0.0005, slope =1). Calibrated OD values computed from VEP estimates of B/G isoluminance correlated with those derived from minimum flicker measurements (r=0.95, p<0.0005, slope = 0.85). Optical density values derived from B/G VEPs increased towards the fovea and corresponded closely with minimum flicker assessment of macular pigment distribution profiles.
The steady-state VEP can be used to determine isoluminance at different retinal eccentricities. Macular pigment optical density and distribution can be measured by steady-state VEPs to B/G stimuli.
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