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Cornea—transparent, curved layer in the front of the eye that bends incoming light rays.

Iris—colored muscle surrounding the pupil that regulates the size of the pupil opening.

Pupil—small adjustable opening in the iris that is smaller in bright light and larger in darkness.

Lens—structure behind the pupil that changes shape, becoming more spherical or flatter to focus incoming rays into an image on the light-sensitive retina.

Accommodation—process of changing the curvature of the lens to focus light rays on the retina.

Retina—light-sensitive surface in the back of the eye containing rods and cones that transduce light energy. Also has layers of bipolar cells and ganglion cells that transmit visual information to the brain.

Fovea—small area of the retina in the most direct line of sight where cones are most concentrated for highest visual acuity in bright light.

Photoreceptors—modified neurons (rods and cones) that convert light energy to electrochemical neural impulses.

Rods—photoreceptors that detect black, white, and gray and that detect movement. Rods are necessary for peripheral and dim-light vision when cones do not respond. Distributed throughout the retina, except none are in the fovea.

Cones—photoreceptors that detect color and fine detail in daylight or in bright-light conditions. Most concentrated at the fovea of the retina, none are in the periphery.

Optic nerve—nerve formed by ganglion cell axons; carries the neural impulses from the eye to the thalamus of the brain.

Acuity—ability to detect fine details, sharpness of vision. Can be affected by small distortions in the shape of the eye.

Normal vision—rays of light form a clear image on the retina of the eye.

Nearsighted—too much curvature of the cornea and/or lens focuses image in front of the retina so nearby objects are seen more clearly than distant objects.

Farsighted—too little curvature of the cornea and/or lens focuses the image behind the retina, so distant objects are seen more clearly than nearby objects.

Dark adaptation—increased visual sensitivity that gradually develops when it gets dark.

Bipolar cells—second layer of neurons in the retina that transmit impulses from rods and cones to ganglion cells.

Ganglion cells—third layer of neurons in the retina, whose axons converge to form the optic nerve.

Blind spot—region of the retina where the optic nerve leaves the eye so there are no receptor cells; creates an area with no vision.

Feature detectors—individual neurons in the primary visual cortex/occipital lobes that respond to specific features of a visual stimulus.

Parallel processing—simultaneously analyzing different elements of sensory information, such as color, brightness, shape, etc.

Trichromatic theory—proposed mechanism for color vision with cones that are differentially sensitive to different wavelengths of light; each color you see results from a specific ratio of activation among the three types of receptors.

Opponent-process theory—proposed mechanism for color vision with opposing retinal processes for red–green, yellow–blue, white–black. Some retinal cells are stimulated by one of a pair and inhibited by the other.

Sensory adaptation—temporary decrease in sensitivity to a stimulus that occurs when stimulation is unchanging.

Attention—the set of processes from which you choose among the various stimuli bombarding your senses at any instant, allowing some to be further processed by your senses and brain.

Hearing and the human ear:

Audition—the sense of hearing. The loudness of a sound is determined by the amplitude or height of the sound wave.

Frequency—the number of complete wavelengths that pass a point in a given amount of time. The wavelength is inversely proportional to the frequency. Frequency or wavelength determines the hue of a light wave and the pitch of a sound.

Pitch—the highness or lowness of a sound. The shorter the wavelength, the higher the frequency, the higher the pitch. The longer the wavelength, the lower the frequency, the lower the pitch.

Timbre