A neural and computational model for the chromatic control of accommodation

Abstract

Accommodation is more accurate with polychromatic stimuli than with narrowband or monochromatic stimuli. The aim of this paper is to develop a computational model for how the visual system uses the extra information in polychromatic stimuli to increase the accuracy of accommodation responses. The proposed model is developed within the context of both trichromacy and also the organization of spatial and chromatic processing within the visual cortex.The refractive error present in the retinal image can be estimated by comparing image quality with and without small additional changes in refractive state. In polychromatic light, the chromatic aberration of the eye results in differences in ocular refractive power for light of different wavelengths. As a result, the refractive state of the eye can be estimated by comparing image quality in the three types of cone photoreceptor. The ability of cortical neurons to perform such comparisons on image quality with a crude form of spatial-frequency analysis is examined theoretically. It is found that spatially band-pass chromatically opponent neurons (that may correspond to double opponent neurons) can perform such calculations and that chromatic cues to accommodation are extracted most effectively by neurons responding to spatial frequencies of between 2 and 8 cycles/deg.

Red‐green opponent channel mediation of control of human ocular accommodation.

“…The R-G response function for the broadband control (Fig. 5, open circle) is similar to the one derived by Flitcroft (1990) Figure 5. Effect of defocus and bandwidth on R-G channel output…”

“…The optical transfer function varies not only with defocus (Hopkins, 1955), but when longitudinal chromatic aberration is present, also with wavelength (Flitcroft, 1990;Kruger et al 1993). The R-G response function for the broadband control (Fig.…”

Optimal defocus estimation in individual natural images

“…In this vision system, chromatic aberrations can be exploited. It has long been recognized that chromatic aberrations provide a signed cue to defocus (29,30). The human eye's refractive power changes by ∼1 diopter between 570 and 445 nm (31), the peak sensitivities of the L and S cones.…”

“…Cells with similar properties (i.e., double chromatically opponent, spatialfrequency bandpass receptive fields tuned to the same frequency) have been reported in primate early visual cortex (34,35). Such cells would be well suited to estimating defocus (30).…”

Myopia, Light and Circadian Rhythms

“…One possible explanation for the effect of wavelength on refractive outcome is based on the intrinsic chromatic aberration of the vertebrate eye (Mandelman and Sivak 1983) which could provide a sign-of-defocus signal for emmetropisation (Flitcroft 1990) as the long and short wavelength components would produce relative hyperopic and myopic defocus of the retinal image respectively. For example, in a comparison of the effects of raising guinea pigs under equiluminant short wavelength light (430 nm) or middle wavelength light (530 nm), Liu et al (Liu, Qian et al 2011) demonstrated that after 12 weeks the 530 nm group was less hyperopic due to faster vitreous elongation, while the 430 nm group was more hyperopic following slower vitreous elongation.…”