Spectral discrimination in color blind animals via chromatic aberration and pupil shape

Abstract

We present a mechanism by which organisms with only a single photoreceptor, which have a monochromatic view of the world, can achieve color discrimination. An off-axis pupil and the principle of chromatic aberration (where different wavelengths come to focus at different distances behind a lens) can combine to provide "colorblind" animals with a way to distinguish colors. As a specific example, we constructed a computer model of the visual system of cephalopods (octopus, squid, and cuttlefish) that have a single unfiltered photoreceptor type. We compute a quantitative image quality budget for this visual system and show how chromatic blurring dominates the visual acuity in these animals in shallow water. We quantitatively show, through numerical simulations, how chromatic aberration can be exploited to obtain spectral information, especially through nonaxial pupils that are characteristic of coleoid cephalopods. We have also assessed the inherent ambiguity between range and color that is a consequence of the chromatic variation of best focus with wavelength. This proposed mechanism is consistent with the extensive suite of visual/behavioral and physiological data that has been obtained from cephalopod studies and offers a possible solution to the apparent paradox of vivid chromatic behaviors in color blind animals. Moreover, this proposed mechanism has potential applicability in organisms with limited photoreceptor complements, such as spiders and dolphins.spectral discrimination | chromatic aberration | color vision | pupil shape | cephalopod W e show in this paper that, under certain conditions, organisms can determine the spectral composition of objects, even with a single photoreceptor type. Through computational modeling, we show a mechanism that provides spectral information using an important relationship: the position of sharpest focus depends on the spectral peak of detected photons. Mapping out contrast vs. focal setting (accommodation) amounts to obtaining a coarse spectrum of objects in the field of view, much as a digital camera attains best focus by maximizing contrast vs. focal length. We note that a similar phenomenon has been advanced as a possible explanation for color percepts in red/green color-blind primates (1); however, primates have not evolved the off-axis pupil shape found in nearly all shallow water cephalopods that enhances this effect.The only other known mechanism of color discrimination in organisms involves determining the spectrum of electromagnetic radiation using differential comparisons between simultaneous neural signals arising from photoreceptor channels with differing spectral acceptances. Color vision using multiple classes of photoreceptors on a 2D retinal surface comes at a cost: reduced signal to noise ratio in low-light conditions and degraded angular resolution in each spectral channel. Thus, many lineages that are or were active in low-light conditions have lost spectral channels to increase sensitivity (2).Octopus, squid, and cuttlefish (coleoid cephalopods) have l...

Hyperspectral Imaging Bioinspired by Chromatic Blur Vision in Color Blind Animals

“…The retina acts as a monochrome image sensor, when the detection plane is in a specific position, the image of a point object of different radiation wavelengths at the distance of infinity exhibits different degrees of dispersion, diagrammatic drawing of the images can be seen in Figure 1d. Through contrast analysis of chromatic blur images, spectral discrimination can be obtained under certain conditions, which is a probable perception mechanism of chromatic blur vision in color blind animals [7]. Inspired by the studies of Stubbs et al, we believe that the chromatic blur can be used to form a hyperspectral imaging technology.…”

“…With respect to a photographic imaging system with large imaging chromatic aberrations, the resulting images typically exhibit varying degrees of blur. Stubbs et al [7] further found that the semi-annular off-axis pupil or large circular pupil of the octopus can cause a high lateral chromatic aberration. Under the premise that the lens has a certain longitudinal chromatic aberration at the same retinal position, higher lateral chromatic aberration will occur with a semicircular off-axis pupil than a small on-axis circular pupil, such as the pupil of a human eye, thereby further aggravating the degree of image blur.…”

“…Existing studies further indicate that these "color blind camouflage" animals even possess spectral recognition capabilities [11][12][13]. Several studies had put forward some probable explanations [7], but the definite mechanism of visual perception in the "color blind camouflage" animals has not been revealed to date.…”

“…We used three type of targets and simulate the spectrum for different colors. Based on the eyeball model constructed by Stubbs et al [7], we first obtained chromatically blurred images under different accommodations. Next, a deconvolution method was used to deal with these blurred images, thereby restoring the spectral images at each wavelength band.…”

Chameleon Devices

“…In nature, light sensing molecules are found in colour-changing cephalopod skin [31,40]. Cephalopods have also evolved special pupils [45] to sense the colour and pattern of their background. Specialised low-cost, bio-inspired light sensors could be developed for camouflage and could be activated and actuated by the background light.…”