Vision: Seeing the Light

Light (Electromagnetic Radiation)

physical energy in the form of electrically charged particles

Visible Light

the small portion of the full electromagnetic radiation spectrum that gives rise to our senses in for form of colors

Cornea

a transparent covering on the eye’s surface through which light first enters

Iris

the pigmented, circular muscle in the eye that regulates the size of the pupil to adjust to changes in the level of illumination (darker = larger pupil)

Pupil

the opening inside the iris that allows light to enter the eye; light travels here second

Lens

the structure that focuses light rays on the retina using the process of accommodation; light travels here third

Accommodation

the process the lens uses to focus the visual image on the retina

Retina

the inner surface of the eye; contains photoreceptors; light travels here fourth

Photoreceptors

light-sensitive receptor cells that convert light energy into neural signals that the brain uses to create visual sensations; the two types are rods and cones; light travels here last, as it is converted to signals

Rods

the photoreceptors that are only sensitive to intensity of light + are responsible for peripheral vision

Cones

the photoreceptors that are sensitive to colors and to the fine details of objects in bright light

Bipolar Cells

the layer of interconnecting cells that the neural signals pass through first

Ganglion Cells

the layer of neurons that the neural signals pass through second

Axon

small structures that project from each ganglion cell and make up one nerve fiber within the optic nerve

Optic Nerve

the structure that transmits visual information to the brain

Blind Spot

the part of the retina where the optic nerve leaves the eye; a hole in the retina that contains no photoreceptors and, thus, no ability to images cast to the brain

Fovea

the part of the retina that corresponds to the center of our gaze; gives rise to our sharpest vision, as it contains only cones

David Hubel and Torsten Wiesel

the men who discovered feature detectors

Feature Detectors

neurons within the visual cortex that respond only to particular features, such as horizontal or vertical lines

Trichromatic Theory

proposed by Hermann von Helmholtz, based on Thomas Young’s work; poses that the ability to see different colors depends on the relative activity of three types of color receptors in the eye (red, green, and blue-violet)

Afterimages

the visual images of stimuli that remain after the stimulus is removed; the basis of Ewald Herring’s theory

Opponent-Process Theory

proposed by Ewald Herring, suggests that eyes have three types of color receptors, and that the experience of color results from opposing receptors: red-green and blue-yellow receptors, and another set of opposing receptors (black-white) is responsible for detecting differences in brightness

Color Vision

likely includes elements from both the trichromatic theory and the opponent process theory

Trichromats

people with normal color vision who can discern all colors of the visible spectrum + the various combinations they can create

Monochromats

people who have no color vision and can only see in black and white

Dichromats

people who can see some colors but not others

Red-Green Color Blindness

the most common type of color blindness; inability to discern between red and green