Chromatic VEP specification of macular pigmentation: comparison with minimum motion and minimum flicker profiles.

Abstract

Purpose: To assess macular pigment (MP) using chromatic visual evoked potentials. Methods: Isoluminant Red/Green (R/G), Blue/Green (B/G) and Tritan gratings (2cycles/deg, diameters 3–12 degrees) were used to elicit onset-offset VEPs from 13 normal subjects. Chromatic selectivity was assessed by comparing onset components with achromatic and chromatic reversal VEPs and by Fourier analysis, whereby colour-specific responses are dominated by the fundamental and achromatic VEPs by the 2nd harmonic. The degree of chromatic VEP response selectivity was also expressed as an index; onset negativity/(onset negativity + onset positivity). MP optical density (OD) profiles were measured between 0 and 7.4 degrees eccentricity for the same subjects using minimum motion and flicker photometry. Results: Onset VEPs elicited by 3-degree gratings at isoluminance were characterized by a predominant negative wave; reversal and achromatic VEPs by positive components. Extending the spatial extent of B/G gratings beyond 3-degrees diameter resulted in onset VEPs with additional achromatic components which resembled contrast reversal and achromatic VEPs and which lowered the power of the fundamental and the index of selectivity For onset VEPs elicited using 9-degree B/G gratings, the peak MP OD correlated with the power of the VEP fundamental (r=0.70) and with the index of selectivity (r=0.83). Conclusions: The loss of selectivity seen in chromatic VEPs generated by large gratings containing short-wavelength components can be related to subject-specific levels of MP.