Summarizing Vision

Summary

Vision is the only photo responsive sense. Visible light travels in waves and is a very small slice of the electromagnetic radiation spectrum. Light waves differ based on their frequency (wavelength = hue) and amplitude (intensity = brightness).

In the vertebrate retina, there are two types of light receptors (photoreceptors): cones and rods. Cones, which are the source of color vision, exist in three forms—L, M, and S—and they are differentially sensitive to different wavelengths. Cones are located in the retina, along with the dim-light, achromatic receptors (rods). Cones are found in the fovea, the central region of the retina, whereas rods are found in the peripheral regions of the retina.

Visual signals travel from the eye over the axons of retinal ganglion cells, which make up the optic nerves. Ganglion cells come in several versions. Some ganglion cell axons carry information on form, movement, depth, and brightness, while other axons carry information on color and fine detail. Visual information is sent to the superior colliculi in the midbrain, where coordination of eye movements and integration of auditory information takes place. Visual information is also sent to the suprachiasmatic nucleus (SCN) of the hypothalamus, which plays a role in the circadian cycle.

Glossary

candela

(cd) unit of measurement of luminous intensity (brightness)

circadian

describes a time cycle about one day in length

cone

weakly photosensitive, chromatic, cone-shaped neuron in the fovea of the retina that detects bright light and is used in daytime color vision

cornea

transparent layer over the front of the eye that helps focus light waves

fovea

region in the center of the retina with a high density of photoreceptors and which is responsible for acute vision

hyperopia

(also, farsightedness) visual defect in which the image focus falls behind the retina, thereby making images in the distance clear, but close-up images blurry

iris

pigmented, circular muscle at the front of the eye that regulates the amount of light entering the eye

lens

transparent, convex structure behind the cornea that helps focus light waves on the retina

myopia

(also, nearsightedness) visual defect in which the image focus falls in front of the retina, thereby making images in the distance blurry, but close-up images clear

presbyopia

visual defect in which the image focus falls behind the retina, thereby making images in the distance clear, but close-up images blurry; caused by age-based changes in the lens

pupil

small opening though which light enters

retina

layer of photoreceptive and supporting cells on the inner surface of the back of the eye

rhodopsin

main photopigment in vertebrates

rod

strongly photosensitive, achromatic, cylindrical neuron in the outer edges of the retina that detects dim light and is used in peripheral and nighttime vision

superior colliculus

paired structure in the top of the midbrain, which manages eye movements and auditory integration

suprachiasmatic nucleus

cluster of cells in the hypothalamus that plays a role in the circadian cycle

tonic activity

in a neuron, slight continuous activity while at rest

vision

sense of sight

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