Abstract

Computational models that predict the spectral sensitivities of primate cone photoreceptors have focussed only on the spectral, not spatial, dimensions. On the ecologically valid task of foraging for fruit, such models predict the M-cone (“green”) peak spectral sensitivity 10–20 nm further from the L-cone (“red”) sensitivity peak than it is in nature and assume their separation is limited by other visual constraints, such as the requirement of high-acuity spatial vision for closer M and L peak sensitivities. We explore the possibility that a spatio-chromatic analysis can better predict cone spectral tuning without appealing to other visual constraints. We build a computational model of the primate retina and simulate chromatic gratings of varying spatial frequencies using measured spectra. We then implement the case study of foveal processing in routinely trichromatic primates for the task of discriminating fruit and leaf spectra. We perform an exhaustive search for the configurations of M and L cone spectral sensitivities that optimally distinguish the colour patterns within these spectral images. Under such conditions, the model suggests that: (1) a long-wavelength limit is required to constrain the L cone spectral sensitivity to its natural position; (2) the optimal M cone peak spectral sensitivity occurs at ~525 nm, close to the observed position in nature (~535 nm); (3) spatial frequency has a small effect upon the spectral tuning of the cones; (4) a selective pressure toward less correlated M and L spectral sensitivities is provided by the need to reduce noise caused by the luminance variation that occurs in natural scenes.

Highlights

  • Young’s (1802) trichromatic theory of colour vision recognises that the retina must compromise between spectral and spatial sampling of the optical image

  • Given a physiologically realistic retina model which spatio-chromatically predicts the optimal M and L cone spectral sensitivities, we perform four sets of simulations to address the following questions: (i) How does varying the set of spectra used to model the visual environment affect the predicted M and L spectral sensitivities? (ii) Is a long-wavelength limit required to constrain the L cone spectral sensitivity to its observed position? (iii) Can better predictions of the optimal spectral sensitivities be achieved by a spatio-chromatic analysis than a purely spectral analysis? (iv) Does the variation in luminance that occurs across the spatial dimensions in natural scenes affect the optimal position of the M cone spectral sensitivity?

  • To demonstrate the advantage of spatio-chromatic modelling to estimate optimal spectral sensitivity tuning, we focus upon the spectral tuning of the cones that underlie the red-green system

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Summary

Introduction

Young’s (1802) trichromatic theory of colour vision recognises that the retina must compromise between spectral and spatial sampling of the optical image. Most accounts of primate trichromacy evaluate colour discrimination for tasks such as finding food without regard to the spatial layout of the cones (Mollon, 1989; Osorio and Vorobyev, 1996; Regan et al, 1998; Sumner and Mollon, 2000a; Dominy and Lucas, 2001; Lewis and Zhaoping, 2006; Melin et al, 2013) These studies predict the optimal value of λMmax closer to λLmax than to λSmax, but, at a wavelength 10–20 nm shorter than its actual value of 535 nm. If there is a trade-off between spatial and chromatic coding in natural images, one might speculate that the observed value of λMmax corresponds to the optimal compromise to the problem recognised by Young (1802)

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