Adaptations of the human eye to reduce the impact of chromatic aberrations on vision

Abstract

Sharpness and contrast of the retinal image are affected by two types of optical aberrations,

monochromatic and polychromatic. Monochromatic aberrations result from imperfections in the

refracting surfaces, while polychromatic aberrations result from the dispersion of light in the

ocular media. Wavelength-dependent differences in focal lengths are referred to as longitudinal

chromatic aberration (LCA) while differences in image position and image magnification result

from transverse chromatic aberration (TCA). The primary objective of my PhD work is to further

clarify the relationship between two chromatic aberrations, longitudinal chromatic aberration

(LCA) and transverse chromatic aberration (TCA), and visual perception. I have studied the

morphological and optical adaptations of the visual system that were developed in the course of

evolution to cope with chromatic aberrations. Generally, transverse chromatic aberration (TCA)

has been less studied even though it causes more loss in retinal image contrast. While LCA is

similar in different eyes, TCA shows large inter-individual variability. It is not known which ocular

variables determine this variability. Therefore, in project 1 I have measured chromatic differences

in perceived image magnification (determined by TCA) in different subjects with a newly

established psychophysical procedure and found that a major part of the inter-individual variance

in CDM (64%) was explained by lens thickness. Since lens thickness increase with age, also TCA will

increase. This study was published in the Journal of the Optical Society of America A, 2014.

Due to longitudinal chromatic aberration (LCA), the focus of the image on the retina cannot be

equally good at all wavelengths. Human eyes are about 2 D more myopic in blue light (450 nm)

than in red (650 nm). For this reason, the retinal image in the fovea is typically in best focus for

the mid- and long-wavelengths but severely out of focus for the blue (>1D). Probably for this

reason, the short wavelength sensitive cones (the S-cones) are lacking from the foveal center,

causing a “foveal blue scotoma”. I found that the foveal blue scotoma is highly variable among

subjects but it is not known, why. Therefore, in project 2, I have studied the variables that might

influence the appearance of the foveal blue scotoma: shape of the foveal pit and distribution of

macular pigment. I found that the shape of the foveal pit is a strong predictor of the foveal blue

scotoma - the steeper the foveal slopes, the larger the blue scotoma. Macular pigment

distribution, on the other, gave rise to the percept of Maxwell’s spot, but was not correlated to

the size of the blue scotoma. This study was published in Vision Research, 2015.

My third project deals with new technology to measure LCA. In our laboratory we use routinely

eccentric infrared photorefraction to measure refractive states in human and animal eye. The use

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of infrared light has the advantage that the subject is not aware that is being measured and that

pupils remain large which increases the signal-to-noise ratio. However eccenetric photorefraction

could also provide information on LCA if it is used in white light. The differences in refractions

measured in the R, G and B channel of the video camera should provide LCA but this technique

was never established even though it would be a great advance to obtain LCA from single pictures

of the eye. Therefore, in project 3, I studied the potential of polychromatic eccentric

photorefraction in measuring LCA. I found that the calibration of photorefraction in white light is

much more variable in different subjects, than in infrared light. The major reason was the large

individual variability in fundal reflectance in visible light and less variability in the near infrared.

Fundal reflectance has a major effect on the brightness of the pupil during the measurements.

Because the technique uses a brightness slope in the pupil, and determines the gradient of pixel

values, the slope of pupil brightness depends on the absolute pixel brightness. This finding

explains a lot of the variability of photorefraction and will be of interest to researchers using this

technique. The work was submitted to BOE in June 2015.