Emergence of unique hues from sparse coding of color in natural scenes

TL;DR

This study links the perception of unique hues to the statistical properties of natural scenes by demonstrating that sparse coding of cone responses naturally yields the four unique hues, offering insights into their neural basis.

Contribution

It introduces a sparse coding model adapted to natural scene color statistics that reproduces the four unique hues and explains their neural and perceptual properties.

Findings

Sparse coding converges to four unique hues plus black and white.

Nonlinear inference yields excitatory and inhibitory interactions among hues.

Natural scene color statistics are non-Gaussian with heavy tails.

Abstract

Our subjective experience of color is typically described by abstract properties such as hue, saturation, and brightness that do not directly correspond to sensory signals arising from cones in the retina. Along the hue dimension, certain colors -- red, green, blue, and yellow -- appear unique in that they are not perceived as a combination of other colors, and the pairs red-green and blue-yellow appear opposites. However, the anatomical and physiological correlates of these 'unique hues' within the brain and the reason for their existence remain a mystery. Here, we demonstrate a direct connection between these hues and the statistics of the natural visual environment. Analysis of simulated cone responses on a dataset of 503 calibrated natural images reveals a strongly non-Gaussian distribution in 3D color space, with heavy tails in distinct, asymmetrically arranged directions. A sparse coding model is then adapted to this data so as to minimize the total sum of coefficients on the basis vectors for representing the data. A six basis-vector model converges to the four unique hues in addition to black and white. Moreover, we find that the nonlinear nature of inference in the sparse coding model yields both excitatory and inhibitory interactions among latent variables; the former facilitates combining adjacent pairs of unique hues to encode intermediate hues situated between them, while the latter enforces mutual exclusivity between opposite unique hues. Together, these findings shed new light on the distribution of color in the natural environment and provide a linking principle between this structure and the phenomenology of color appearance.