Abstract
Purpose: :
To perform a quantitative theoretical analysis of how ocular exposure to white light stimuli having different spectral distributions will influence the visual signals that are expected to control mean ocular defocus, lens accommodation, and hence elongation of the developing human eye, i.e., to control ocular emmetropization.
Methods: :
Published data were obtained for the spectral power distribution (SPD) of 16 standard illuminants and phases of daylight and for the relative spectral sensitivity of macular middle (M) -wavelength () sensitive and long (L) - sensitive cones. Several steps were performed for each white light stimulus: 1) The SPD was multiplied by the M (or L) cone sensitivity at each to yield the relative M (or L) cone radiance as a function of . 2) The mean of the integrated area under the M (or L) cone radiance curve was determined. 3) The midway between the means of the M and L cone radiances was defined as unique yellow. 4) The relative ocular defocus level corresponding to unique yellow was assumed to serve as a signal for short term control of reflex accommodation and long term control of axial elongation of the developing eye, and it was determined. 5) The axial elongation rate was assumed to be proportional to the reciprocal of the mean ocular defocus level and was calculated. 6) A highly simplified model that assumed that the developing eye elongated at a constant rate for a given stimulus was used to estimate the final axial length expected for an eye that had been exclusively exposed to that stimulus.
Results: :
For exposure to D65 (noon daylight), unique yellow was 544 nm, mean defocus was 0.234 D, and it was assumed that elongation at 0.500 mm/yr yielded an emmetropic adult eye of 22.27 mm axial length. For exposure to the 15 other stimuli, unique yellow ranged from 532 to 564 nm, mean defocus ranged from 0.309 to 0.124 D, and elongation rates ranging from 0.379 to 0.944 mm/yr yielded adult eyes with axial lengths ranging from 20.57 to 28.48 mm.
Conclusions: :
This theoretical analysis shows that the spectral distribution of a white light stimulus to which an eye is exposed has a significant effect on M and L cone responses, on mean ocular defocus and accommodation levels, and hence on the signals proposed to control emmetropization. These findings support the LIGHT SPECTRUM HYPOTHESIS FOR OCULAR EMMETROPIZATION, which predicts that exposing a developing human eye to light stimuli biased towards short leads to a HYPEROPIC adult eye with a short axial length, to stimuli having a relatively flat spectrum leads to an EMMETROPIC adult eye with an intermediate axial length, and to stimuli biased towards long leads to a MYOPIC adult eye with a long axial length.
Keywords: emmetropization • refractive error development • accommodation