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Electrophysiological measurement of macular pigment distribution using annular stimuli: implications for colour vision testing

Purpose To quantify macular pigment optical density (MPOD) and distribution using steady-state VEPs and to optimize the koniocellular selectivity of large chromatic gratings.



Methods Blue/Green (B/G) gratings were generated within 1 circular and 3 concentric annular fields (maximum radius 9°). All 4 fields were modulated simultaneously, each at a different temporal frequency. Onset-offset VEPs were recorded as the luminance ratio between adjacent chromatic components within each field changed from 0.25 to 0.85 in 11 automated steps. Fourier analysis showed that the first harmonic was minimised at each subjects’ isoluminant ratio, as verified using flicker photometry. MPOD was computed at each retinal location relative to the most eccentric annulus. The stimuli were adjusted to compensate for the MP profile, allowing generation of a B/G grating that was isoluminant over the whole of the 18° stimulus area. The optimised B/G field was used to probe koniocellular function using onset-offset VEPs and psychophysical temporal tuning characteristics.



Results Results: MPOD values computed from VEP estimates of B/G isoluminance correlated with those derived from minimum flicker measurements (r=0.94, p<0.005, slope=0.80). Large B/G gratings comprising of concentric annuli and optimised to compensate for MP, elicted large chromatic-specific onset VEPs of negative polarity. Temporal tuning characteristics to the same stimuli were low-pass, in keeping with the low temporal resolution of the koniocellular system.



Conclusion The steady-state VEP can be used to determine MPOD at different retinal eccentricities. Macular pigment profiles may be used to optimise the koniocellular selectivity of large B/G stimuli.