Visual system

Lore & Background

The visual system comprises the eye, optic nerve, optic chiasma, optic tract, lateral geniculate body, optic radiation, visual cortex, and visual association cortex. The anterior visual pathway includes structures before the lateral geniculate nucleus; the posterior pathway includes structures after. Light enters the eye, is refracted by the cornea and lens, and projects an inverted image onto the retina. The retina contains rods (120 million per eye) for low-light vision and cones (6-7 million per eye) for color and day vision, which transduce light into electrical pulses via opsins. These pulses travel through the optic nerve, decussate at the optic chiasm, and branch to three destinations. The lateral geniculate nucleus (LGN) primarily relays and filters visual information from the retina to the visual cortex.

Reader's Guide

The visual system is significant for its role in constructing visual perception and enabling higher-order tasks such as object identification. Its neural components, including the lateral geniculate nucleus (LGN) and visual cortices V1 through V6, process information hierarchically: V1 performs edge detection and creates a saliency map; V2 handles illusory contours and depth; V3 processes global motion; V4 recognizes simple shapes; V5 integrates local motion into global motion; and V6 analyzes object motion relative to background. The inferior temporal gyrus recognizes complex shapes and faces, while the suprachiasmatic nucleus halts melatonin production at first light. The system's non-image-forming functions, such as the pupillary light reflex and circadian photoentrainment, underscore its broader physiological impact. The retina's processing includes center-surround receptive fields and lateral communication via horizontal and amacrine cells, resulting in five populations of ganglion cells that transmit image-forming and non-image-forming information to the brain.

Did You Know?

• The retina contains about 120 million rods and 6-7 million cones per human eye.
• The LGN primarily relays and filters visual information from the retina to the visual cortex, rather than gauging object range or assigning velocity tags.
• V1 is involved in edge detection and global organization, but specific timings like 40 ms and 100 ms are not established canonical facts.
• The suprachiasmatic nucleus halts production of melatonin indirectly at first light.