THE VISUAL SYSTEM SENSE OF SIGHT

THE VISUAL SYSTEM Photoreception: detection of light, perceived as sight Stimulus: visible light Sense organ: eyes Function: seeing Phototransduction: transforms light waves into electrochemical impulses that the brain can interpret

VISIBLE LIGHT

LIGHT WAVE PROPERTIES Light has 3 characteristics: Wavelength: frequency (determines color/hue) longer/wider = lower frequency = reddish shorter/narrower = higher frequency = bluish Wave amplitude: intensity (determines brightness) shorter = less intensity = duller taller = more intensity = brighter Purity: quality (determines saturation) richness of color

STRUCTURES OF THE EYE Cornea: clear front membrane; bends the light as it enters the eye Sclera: protective white outer layer; helps maintain the eye’s shape Iris: colored ring of muscle; dilates/constricts pupil Pupil: seemingly black, adjustable opening; regulates amount of light that enters the eye Lens: transparent flexible structure; changes shape to focus light on retina Vitreous humor: thick clear gel; maintains the eye’s form/shape Retina: multilayered light sensitive surface; absorbs light, processes images, and sends information to the brain contains rods and cones

ACCOMMODATION The process by which the eye’s lens changes shape (curvature) to focus far or near objects on the retina. Distant focus/far object = long, flattened lens Close focus/near object = short, rounded lens

RETINA The multilayered light-sensitive surface in the eye that records electromagnetic energy and converts it to neural impulses for processing in the brain. Rods: photoreceptor cells; peripheral vision Cones: photoreceptor cells; color vision Fovea: central focal point; cones only; visual acuity Optic disk: blind spot; hole in retina; no receptor cells; where optic nerves exits the eye Optic nerve: bundle of ganglion axons; sends neural impulses to brain

PHOTORECPTORS Rods Cones Long and thin 120 million Black, white, gray vision Night/twilight vision Sensitive to low illumination Peripheral vision Located in periphery of retina Dark adaptation: eyes become more sensitive to light in low illumination Short and fat 6 million Color vision Daylight vision Sensitive to high illumination Fine detailed vision Concentrated in center of retina Light adaptation: eyes become less sensitive to light in high illumination

FEATURE DETECTORS David Hubel and Torsten Wiesel (1963) Special neurons in the brain's visual system that respond to particular features of complex stimuli. Features such as lines, angles, sizes, shapes, and movement

PARALLEL PROCESSING The simultaneous distribution of information across different neural pathways. The brain processes color, motion, form and depth all at once.

Opponent-Process Theory COLOR VISION Trichromatic Theory Opponent-Process Theory Thomas Young and Hermann von Helmholtz (1802/1852) States that color perception is produced by three types of cone receptors in the retina that are particularly sensitive to different, but overlapping, ranges of wavelengths. Red, blue, green Supported by color blindness Edwald Hering (1878) States that cells in the visual system respond to complementary pairs. Pairs: Blue-yellow Red-green Black-white Supported by afterimages

Substractive Color Mixing TRICHROMATIC THEORY Additive Color Mixing Substractive Color Mixing If three colors of light are mixed, addition of all wavelengths result in white. If three colors of pigments are mixed, subtraction of all wavelengths result in black.

COLOR BLINDNESS A deficiency in the ability to distinguish between certain colors. Generally involves the inability to distinguish between shades of red and green Usually an inherited, genetic disorder More common in men than women True color blindness is a total lack of color vision Rare Supports trichromatic theory

OPPONENT-PROCESS THORY A given cell might be excited by red and inhibited by green, a different cell might be excited by yellow and inhibited by blue, whereas another cell might be excited by black and inhibited by white.

AFTERIMAGES A visual image that persists after a stimulus is removed. Supports opponent-process theory

VISION LOSS Nearsightedness Farsightedness Myopia: A condition in which nearby objects are seen more clearly than distant objects. Lens focuses image of distant objects in front of retina Need corrective lenses to see distant objects (puts image on retina) Hyperopia: A condition in which faraway objects are seen more clearly than near objects. Lens focuses near objects behind retina Need corrective lenses to see near objects (puts image on retina)

VISION LOSS