Human cortical neurons respond to intermediate hues, not just red/green, yellow/blue and their combinations.

The results of a basic study on the mechanisms of human color vision were published in one of the top journals on brain science: Cerebral Cortex. Our main contribution was the precise measurement of hue selectivity in each voxel (Figure 1) and the population histogram of neurons selective to various hues (Figure 2) in the human brain, for the first time. We found an abundant population of neurons selective to intermediate colors, in addition to four opponent colors. Our results gave new insights into how color information is coded in the human brain. The results are expected to help in designing displays for higher fidelity, not just to clarify the mechanisms of human visual processing.

This study was conducted with RIKEN BSI (Drs. Keiji Tanaka, Kang Cheng, Kenichi Ueno, and Pei Sun) to collaborate with their high-resolution imaging technique in functional MRI (fMRI). The color signal in human brain was believed for a long time that the combinations of neuron activities of four opponent colors (red/green and yellow/blue; that define two dimensional color space) that work as the landmarks of color perception. It was also considered that the combination of neuronal activity for the opponent colors represents all visible colors; e.g., a combination of red and blue responses makes purple. Recent physiological studies in monkeys and psychophysical studies in humans reported indirect evidence that suggests the presence of neurons selective to intermediate colors. Although purely color-selective responses are very weak and difficult to detect in the fMRI signal of human brains, we succeeded in recording the direct responses of neuron’ s activity selective to intermediate colors in the human visual cortex for the first time by using the high-resolution fMRI technique (Figures 1, 2). Our result also suggests that color appearance in human brains is coded by multiple neurons, each of which is selective to various colors, similar to the way of coding the slant of a line segment.

The color rendering fidelity of commercial displays is limited by three primary colors (red, green, and blue), and not optimized to render intermediate colors. Our study made it clear that the visual system in humans has a high sensitivity to intermediate colors, as well as to primary/opponent colors. Our results imply that the use of multiple primary colors (in addition to red, green, and blue) will lead to higher image quality/fidelity and to more efficient information displays and more realistic image reproduction.


Figure 1 ( left ).
An example distribution of each voxel’ s (size: 2 mm × 2 mm) hue selectivity, coded by the color, in the human brain. Vertical (coronal) dissection at the occipital region, including visual cortex.

Figure 2 (right).
Averaged hue-selectivity histogram in the primary visual cortex, (across participants average). Distance from the origin represent the relative population of voxels selective to the hue. Colors of the outer ring represent the approximate hue appearance. Red symbols represent averages and blue shaded area shows standard deviation. Radial dotted lines indicate hue directions that appear pure red, green, yellow, and blue (unique hues).

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