Abstract
Purpose: :
There are large individual differences in the relative amounts of red to green light people require to minimize flicker in heterochromatic flicker photometry (HFP). This has been attributed to variation in L:M cone ratio, an idea supported by the observation, over 40 years ago, that variation in flicker photometric spectral sensitivity is correlated with the relative amounts of L to M photopigment in the living eye measured by retinal densitometry. Recently, Adaptive Optics technology (AO) combined with retinal densitometry has provided the first direct measurements of the relative numbers of L to M cones in the living human eye. Estimates of cone ratio obtained from electroretinographic 30–Hz–flicker–photometry (FP–ERG) were demonstrated to be correlated (r2=0.98) with direct measurements of foveal cone proportions from AO (Hofer et al., J. Neurosci 2005, 25:9669–79). This information offers a new opportunity to evaluate the relationship between variation in psychophysical flicker photometry and variation in the L:M cone ratio.
Methods: :
The relative L:M cone contribution to flicker photometry is reflected in the relative red:green luminance values at the subject’s equiluminance point. Red/green equiluminant points were determined for each subject on a 5.4°, foveal, 0.5 cpd, 16 Hz red/green grating using Cambridge Research Systems’ (CRS) Psycho software and a CRS VSG 2/4 video card. Equiluminant points were determined from the average of 20 settings for each subject on each of three separate days; the average deviation across days was around 1% L cones [%L=L/(L+M)]. L:M was estimated using a method modified from Gunther & Dobkins (Vis. Res. 2002, 42:1367–78). L–cone fundamentals were determined from each subject’s L–photopigment genetic sequence using the method of Carroll et al. (JOSA A 2000, 17:499–509). These results were compared with FP–ERG–derived L:M proportions determined previously in a study in which a subset of the ERG subjects had their cone ratios measured directly by AO.
Results: :
There was a highly significant correlation between FP–ERG–derived L:M proportions and L:M estimates from red/green equiluminance settings in psychophysical flicker photometry.
Conclusions: :
The correlation between cone ratios estimated from HFP using individualized L cone spectra from genetic analysis of subjects and FP–ERG reveals that about half of the inter–subject variability in HFP can be accounted for by individual differences in cone ratio.
Keywords: color vision • photoreceptors: visual performance • genetics