Optical imperfections in the human eye (known as aberrations) blur the images projected on the retina. In addition to the aberrations that occur by stimuli in all colors, known as polychromatic aberrations, the refractive index of the ocular media varies with wavelength. This variation causes the focal power of the eye to vary by almost 2 diopters throughout the visible spectrum, an effect known as longitudinal chromatic aberration (LCA).

Due to this chromatic aberration, blue images become out of focus when the eye focuses in the middle of the visible spectrum, but we are not normally aware of this chromatic blur.

Despite the degradation in optical quality caused by aberrations, observers are unaware of the blurring present in their retinal images, reflecting both the sampling properties of retinal neurons and the underlying neuronal adaptation to native optical blurring. This neural adaptation is omnipresent in vision, as the visual system adapts to changes in optics and environment over time, using similar strategies in multiple stimulus domains (color, contrast, spatial frequency, or facial perception).

The blurring of the retinal image caused by the ACL of the eye is potentially high. Chromatic blur between the wavelength of the maximum sensitivity of the S and M / L cones is ~ 1.5 D, equivalent to a blur circle of ~ 26 arc minutes for a 5mm pupil. However, the visual system is sensitive to blur as little as 1 arc minute, although this depends on the context of the blur.

An important unresolved question is: why is the perception of images not severely degraded by chromatic blur? The visual impact of ACL on polychromatic image quality is reduced by the spectral sensitivity of retinal photoreceptors, but the effect of ACL blurring on monochrome targets when the eye is focused in the middle of the spectrum is large. Understanding how the visual system deals with ACL is important to many visual processes. The process of adaptation to chromatic blurring can occur during cataract development and may play a role in adaptation to replacement of the eye's lens by an intraocular lens (IOL) after cataract surgery, as the magnitude of the aberration lens and IOL chromaticity differ. With the development of new designs of diffractive multifocal IOLs, it is also possible to modulate chromatic aberration independently for far, intermediate or near foci, canceling the ACL at least at some distances. However, the visual impact of removing the ACL remains an open question.

Optical simulations showed that the combination of natural chromatic and monochromatic aberrations increased the contrast of blue stimuli when the eye was focused on green compared to an aberration-corrected eye. Thus, the aberrated optics appeared to provide the eye with partial protection against chromatic blur.

Adaptive Optics (AO) is a technique that can be used to compensate for monochromatic aberrations of the eye, therefore, Adaptive Optics allows testing the hypothesis that increased depth of focus arising from ACL and optical interactions Monochromatic aberrations can explain the relative insensitivity of human vision to chromatic blurring in the short-wavelength (blue) components of a polychromatic image. If this is not the case, other mechanisms of perception (adaptation) can play control roles. The question is important as intraocular lens (IOL) manufacturers embark on the development of new designs to reduce ACL. These developments will lead to patients equipped with intraocular lenses that alter the balance of monochromatic / polychromatic aberrations, which may be necessary to perceptually recalibrate to a new spatial / chromatic environment.

Our results reveal that, on average, out-of-focus blue images appear less blurry when native aberrations are present than when corrected, in line with optical findings. However, the perceived quality of blue images (when the eye is focused on green) is relatively high, with or without high-order monochrome aberrations. More relevant, blue images (naturally out of focus by chromatic blur) are judged psychophysically as sharper than green (or monochrome grayscale) images out of focus by the same amount of equivalent blur (-0.87 D ). The higher perceived quality of out-of-focus blue images compared to out-of-focus or grayscale green images suggests that the visual system is calibrated based on average chromaticity and the adaptation mechanism might use color information to rule out perceptual or blurring. loss of sharpness.

The work concludes that the better perceptual quality of the defocused blue stimuli is influenced by the neuronal adaptation mechanisms. The change in psychophysical score from blue (compared to the same blurring in green) may be the basis for contingent adaptation to blue and blurred images (since the blue component of images is normally out of focus). It may also suggest that observers are naturally adapted to both the blur produced by their native aberrations and the effect produced by the natural blur in blue.
Additionally, the presence of monochrome optical aberrations protects vision against chromatic blur.

Link to the paper

The work is a collaboration between the Institute of Optics, Indiana University School of Optometry and The Schepens Eye Research Institute-Massachusetts Eye and Ear Infirmary, Department of Ophthalmology, Harvard Medical School, Boston