Variation of human cone spectral composition with eccentricity

Abstract

Purpose : The cone mosaic sets the primary limit on the fidelity of luminance and color vision. Here we describe the spectral composition of the cone mosaic in the parafovea using an adaptive optics (AO) line-scan OCT-based optoretinogram (ORG) and compare the modality against AO retinal densitometry and flicker-photometric electroretinogram (ERG).

Methods : Two subjects were imaged in an AO line-scan OCT from at 1.5 to 10 deg temporal eccentricity (ecc.) after 3-4 min. of dark adaptation. OCT volumes were recorded after a 10-20 ms 660±10 nm LED flash. The B-scan rate was 6 to 12 kHz and field of view was 1 to 1.6 deg. The stimulus-induced optical phase change was computed between individual cone inner/outer segment junctions and outer segment tips in registered and segmented volumes and converted to optical path length (OPL). The change in OPL was subjected to Gaussian mixture model clustering to distinguish the three cone types. In one subject, cone spectral identities so obtained with ORG were compared against AO retinal densitometry at 1.5 deg ecc. and the nature of disagreements explored. ERG was conducted on both subjects with a 35 deg stimulus for comparison against ORG.

Results : Depending on cone type, bleach strength and ecc., OPL amplitudes vary from 10 - 700 nm, thus offering up to ~2 log units of sensitivity for separating cone types in ORG. Thus, high probability and low uncertainty in LMS assignments were obtained for both subjects across a large population of cones at each ecc. The cone types obtained from ORG and densitometry had ~90.5 % agreement, tested over 813 cones at 1.5 deg ecc. Disagreement of spectral-type assignments included incidence of cones with normal reflectivity in one system but not the other, typed as “dysflective". Of the disagreements, ~44% of mismatches were between L vs. M, ~5% were between LM vs. S, and ~51% involve instances of dysflectivity. For subject 1, the L:M ratio was 2.2, 3.0, 3.0 and 2.4 at 1.5, 3.5, 4.75, and 10 deg ecc., respectively. For subject 2, the L:M ratio was 2.2, 2.0, 2.1 at 1.5, 4.75 and 10 deg ecc. For both subjects, % S-cones increased with ecc., from 4 – 7.6%. The L:M ratios derived from ERG were 2.2 and 2.7 respectively.

Conclusions : Cone spectral composition based on ORG aligned well with densitometry and ERG. The L:M cone ratio remained relatively uniform in the parafovea.

This is a 2021 ARVO Annual Meeting abstract.