Temporal dynamics of the neural mechanisms for encoding hue and luminance contrast uncovered by magnetoencephalography

Hue and luminance contrast are the most basic visual computations, and are reflected in the earliest layers of convolutional neural networks, yet the extent to which they are extracted by the same or separate circuits in the brain, and the timing of these neural computations, is unknown. Here we answer these questions using multivariate analyses of human brain responses measured with magnetoencephalography. We report three discoveries. First, hue and luminance contrast could be decoded independently, indicating these computations are somewhat separable. Hue was computed about 15-24ms after luminance contrast. Second, representations of hue showed relatively greater generalization across time and were more sustained, providing the first neural correlate of the perceptual preeminence of hue over luminance contrast in grouping objects. Finally, luminance contrast could be decoded less well for hues associated with daylight (orange and blue), suggesting that color-constancy mechanisms are adapted to natural lighting.