3.9 CHAPTER REVIEW

KEY POINTS

• Sensation refers to the response of sensory receptors in the sense organs to stimulation and the transmission of that information to the brain. Perception refers to the process through which the brain organizes and interprets sensory information.

• When sensory receptors are stimulated by an appropriate form of energy, transduction converts the energy into neural impulses, which are transmitted to the brain.

• Each sense is specialized in terms of the type and level of energy to which it will respond. Sensory thresholds include the absolute threshold and the difference threshold. Weber’s law states that the just noticeable difference will vary depending on the strength of the original stimulus. Sensory adaptation takes place when the sensory receptor cells gradually decline in sensitivity to a constant stimulus.

• The sensory receptors for vision respond to light waves. The human eye is sensitive to a very narrow, specific range of wavelengths in the electromagnetic energy spectrum.

• Light waves enter the eye and pass through the cornea and the pupil. The iris controls how much light is allowed in. Behind the pupil is the lens, which focuses light on the retina through accommodation. The retina contains the sensory receptors for vision, the rods and cones.

• Rods are used for vision in dim light and for peripheral vision. Cones are used for color vision, for vision in bright light, and for seeing fine details. Cones are concentrated in the fovea, while rods are more prevalent in the periphery of the retina. There are no rods or cones in the optic disk, which creates a blind spot in the visual field.

• Rods and cones send information to the bipolar and ganglion cells. The ratio of cones to ganglion cells is much smaller than the ratio of rods to ganglion cells. The optic nerve fibers exit the back of each retina at the optic disk and meet at the optic chiasm, where some of the fibers cross over to the opposite side of the brain and then transmit information from the thalamus to the visual cortex. Feature detectors are highly specialized neurons in the visual cortex.

• Color is the psychological experience of different wavelengths of light and involves hue, brightness, and saturation. The color of an object is determined by the light wave it ref lects. In combination, the trichromatic theory and the opponent-process theory explain color vision. The trichromatic theory explains red–green color blindness and color processing in the retina. The opponent-process theory explains afterimages and color processing in the ganglion cells and the brain.

• The sense of hearing is called audition. The loudness, pitch, and timbre of a sound are determined by the amplitude, frequency, and complexity of a sound wave. Loudness is measured in decibels.

• Sound waves are collected in the outer ear, amplified by the eardrum in the middle ear, and transduced in the inner ear. The sensory receptors for hearing are the hair cells, which are located on the basilar membrane in the cochlea. The auditory nerve carries information to the thalamus and auditory cortex in the brain. In combination, frequency theory and place theory explain the sensation of pitch.

• The sensory receptors for smell (olfaction) and taste (gustation) are specialized to respond to chemical substances. The sensation of smell is caused by airborne molecules stimulating odor receptors on the olfactory receptor cells in the nasal lining. Olfactory information travels via the axons of the receptor cells to the olfactory bulb and is transmitted along the olfactory tract to different brain areas, including the temporal lobe and limbic system. Olfactory sensitivity varies widely and tends to decline with age. Pheromones communicate information that influences the behavior of other members of the same species.

• Taste results from the stimulation of sensory receptors in the taste buds, which are located on the tongue and the inside of the mouth. When activated by chemical substances dissolved in saliva, the taste buds send neural messages to the thalamus in the brain. There are five primary tastes: sweet, salty, sour, bitter, and umami.

• The skin includes several kinds of sensory receptors, which are unevenly distributed among the parts of the body. The Pacinian corpuscle is the skin receptor that is sensitive to pressure.

• Pain occurs when nociceptors are stimulated. The fast pain system transmits signals that produce sensations of intense but short-lived pain, while the slow pain system transmits signals that produce sensations of dull, long-lasting pain. Substance P is a neurotransmitter that increases pain messages within the spinal cord. The gate-control theory of pain helps explain how psychological and other factors influence the subjective experience of pain.

• The kinesthetic sense involves the location and position of body parts in relation to one another, which is detected by specialized neurons called proprioceptors. The vestibular sense provides information about balance, equilibrium, and orientation.

• Both bottom-up and top-down processing are involved in everyday perception. Gestalt psychology emphasized the perception of gestalts, or whole forms.

• We rely primarily on shape to identify an object. Figure–ground relationships are important in distinguishing an object from its background. The Gestalt psychologists formulated several principles of perceptual organization, including the laws of proximity, similarity, closure, good continuation, and Prägnanz.

• Depth perception involves both monocular and binocular cues. Monocular depth cues include relative size, overlap, aerial perspective, texture gradient, linear perspective, motion parallax, and accommodation. Binocular depth cues include convergence and binocular disparity.

• The perception of movement involves integrating information from the eye muscles, the retina, and the environment. The illusion of induced motion is a result of our assumption that the background is stationary. The perception of stroboscopic motion results from images being rapidly registered on the retina.

• Objects are perceived as stable despite changes in sensory input and retinal image, a concept called perceptual constancy. Size constancy and shape constancy are two important forms of perceptual constancy.

• Perceptual illusions are used to study perceptual principles. The Müller-Lyer illusion involves the principles of depth cues and size constancy. The moon illusion results from the principles of overlap and size constancy.

• Perceptual interpretations can be influenced by learning experiences, culture, and expectations. Perceptual set often determines the interpretation of an ambiguous stimulus.

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