How chromatic cues can guide human eye growth to achieve good focus

Abstract

The postnatal growing eye uses visual cues to actively control its own axial elongation to achieve and maintainsharp focus, a process termed emmetropization. Theprimary visual cue may be the difference in imagesharpness as sensed by the arrays of short and long-wavelength sensitive cone photoreceptors causedby longitudinal chromatic aberration: Shorterwavelengths focus in front of longer wavelengths.However, the sparse distribution of short-wavelengthsensitive cones across the retina suggests that they donot have sufficient spatial sampling resolution for thistask. Here, we show that the spacing of theshort-wavelength sensitive cones in humans is sufficientfor them, in conjunction with the longer wavelengthcones, to use chromatic signals to detect defocus andguide emmetropization. We hypothesize that the retinalspacing of the short-wavelength sensitive cones in manymammalian species is an evolutionarily ancientadaption that allows the efficient use of chromatic cuesin emmetropization.