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The Human Visual System at a Glance

Different kinds of information are processed by different photosensitive cells in the retina of your eye, traveling to the brain over different nerve pathways. The brain combines signals from these visual channels to interpret the image.

Light passing through the cornea is focused by the lens onto the retina, passing first through fine blood vessels, then through neural wiring to reach the photoreceptors. A layer of neurons on top of these photoreceptors then processes, encodes, and ships off the resulting data to the brain via the optic nerve. The optic nerve terminates in the brain’s lateral geniculate nucleus, a kind of switchboard that directs data elsewhere into the brain for further processing.

The photoreceptors fall into two broad classes—rods for night vision and cones for high-resolution color vision. The rods provide high gain at the cost of low resolution, in part because each rod is connected to many cells in the optic nerve. The cones resolve fine details, because each one is connected through an intermediate layer of image-processing nerves to a single optic nerve cell.

For the sensors to work properly, the light hitting them must vary continually, so as to refresh the signal. The eye performs such refreshing, even on static images, by making slight intermittent movements, called saccades.

Half the cones are in the fovea, a 2-millimeter area in the macula. It spans 2 degrees of the visual field; of these cones, half fall in the very center, spanning 0.5 degree. Cones come in varieties sensitive to long, medium, and short wavelengths, corresponding roughly to red, green, and blue, respectively. There are roughly 30 times as many red- and green-sensitive cones as blue-sensitive cones.

IMAGE: Jireh Design

The red and green cones perceive the range of wavelengths between 500 and 700 nanometers on the visible-light spectrum. The human eye tends to focus on yellow (580 nm), because that wavelength gives the best overall image quality, exciting the green and red cones nearly equally. Far-red and far‑blue wavelengths provide little resolution.

Output from the cones undergoes preprocessing in neural layers in the retina, right above the cones. The neural layers encode an image into a luminance channel, which distinguishes black from white, and two chrominance channels, one managing red versus green, the other, yellow versus blue.

These preprocessing neural layers generate the luminance channel by combining inputs from the red cones and the green cones and effectively ignoring the blue cones. They generate the red/green chroma channel by subtracting green-cone input from red-cone input. The yellow/blue chroma channel subtracts the blue-cone inputs from those of the red and green cones.

—J.P.