CHAPTER 5 Sensation and Perception

• Sensation is the process by which our sensory receptors and nervous system receive information and transmit it to the brain. Perception is the process by which our brain organizes and interprets that information.

• Bottom-up processing is analysis that begins with the sensory receptors and works up to the brain. Top-down processing is information processing guided by higher-level mental processing, such as when we construct perceptions by filtering information through our experience and expectations.

• Our senses (1) receive sensory stimulation (often using specialized receptor cells); (2) transform that stimulation into neural impulses; and (3) deliver the neural information to the brain. Transduction is the process of converting one form of energy into another.

• Our absolute threshold is the minimum stimulation needed for us to be consciously aware of any stimulus 50 percent of the time. (Stimuli below that threshold are subliminal.)

• A difference threshold (also called the just noticeable difference, or jnd) is the minimum change needed to detect a difference between two stimuli 50 percent of the time.

• Weber’s law states that two stimuli must differ by a constant minimum percentage (rather than a constant minimum amount).

• We do sense some stimuli subliminally—less than 50 percent of the time—and can be affected by these sensations. But although we can be primed, subliminal stimuli have no powerful, enduring influence on behavior.

• We grow less sensitive to constant sensory input.

• This diminished sensitivity to constant or routine odors, sounds, and touches (sensory adaptation) focuses our attention on informative changes in our environment.

• Perception is influenced by our perceptual set—our mental predisposition to perceive one thing and not another.

• Our physical, emotional, and cultural context, as well as our motivation, can create expectations about what we will perceive, thus affecting those perceptions.

Vision: Sensory and Perceptual Processing

• The visible light we experience is just a thin slice of the broad spectrum of electromagnetic energy.

• The hue (blue, green, and so forth) we perceive in a light depends on its wavelength, and its brightness depends on its intensity.

• Light entering the eye through the pupil is focused by the lens on the retina—the inner surface of the eye.

• The retina’s light-sensitive rods and color-sensitive cones convert the light energy into neural impulses.

• After processing by bipolar and ganglion cells in the eyes’ retina, neural impulses travel through the optic nerve to the thalamus and on to the visual cortex.

• According to the Young-Helmholtz trichromatic (three-color) theory, the retina contains three types of color receptors. Contemporary research has found three types of cones, each most sensitive to the wavelengths of one of the three primary colors of light (red, green, or blue).

• According to the opponent-process theory, there are three additional color processes (red-versus-green, blue-versus-yellow, black-versus-white). Contemporary research has confirmed that, on the way to the brain, neurons in the retina and the thalamus code the color-related information from the cones into pairs of opponent colors.

• These two theories, and the research supporting them, show that color processing occurs in two stages.

• In the visual cortex, nerve cells called feature detectors respond to specific features of the visual stimulus, such as shape, angle, or movement.

• Through parallel processing, the brain handles many aspects of vision (color, movement, form, and depth) simultaneously. Other neural teams integrate the results, comparing them with stored information and enabling perceptions.

• Gestalt psychologists showed that the brain organizes bits of sensory information into gestalts, or meaningful forms. In pointing out that the whole may exceed the sum of its parts, they noted that we filter sensory information and construct our perceptions.

• To recognize an object, we must first perceive it as distinct (see it as a figure) from its surroundings (the ground).

• We bring order and form to sensory input by organizing it into meaningful groups, following such rules as proximity, continuity, and closure.

• Humans and many other species perceive depth at, or very soon after, birth. We transform two-dimensional retinal images into three-dimensional depth perceptions that allow us to see objects in three dimensions and to judge distance.

• Binocular cues, such as retinal disparity, are depth cues that rely on information from both eyes.

• Monocular cues (such as relative size, interposition, relative height, relative motion, linear perspective, and light and shadow) let us judge depth using information transmitted by only one eye.

• As objects move, we assume that shrinking objects are moving away and enlarging objects are approaching. The brain computes motion imperfectly, with young children especially at risk of incorrectly perceiving approaching hazards such as vehicles.

• Perceptual constancy is our ability to recognize an object regardless of the changing image it casts upon our retinas due to its changing angle, distance, or illumination.

• Color constancy is our ability to perceive consistent color in an object, even though the lighting and wavelengths shift.

• Shape constancy is our ability to perceive familiar objects (such as an opening door) as unchanging in shape. Size constancy is our ability to perceive objects as unchanging in size despite their changing retinal images. Knowing an object’s size gives us clues to its distance; knowing its distance gives clues about its size, but we sometimes misread monocular distance cues and reach the wrong conclusions, as in the Moon illusion.

• Experience guides our perceptual interpretations. Some perceptual abilities (such as color and figure-ground perception) are inborn. But people blind from birth who gain sight after surgery lack the experience to visually recognize shapes, forms, and complete faces.

• Sensory restriction research indicates that there is a critical period for some aspects of sensory and perceptual development. Without early stimulation, the brain’s neural organization does not develop normally.

• Given eyeglasses that shift the world slightly to the left or right, turn it upside down, or reverse it, people can, through perceptual adaptation, learn to move about with ease.

The Nonvisual Senses

• Sound waves vary in amplitude (perceived as loudness) and in frequency (perceived as pitch—a tone’s highness or lowness).

• Sound energy is measured in decibels.

• Through a mechanical chain of events, sound waves travel from the outer ear through the auditory canal, causing tiny vibrations in the eardrum. The bones of the middle ear transmit the vibrations to the fluid-filled cochlea in the inner ear, causing waves of movement in hair cells lining the basilar membrane. This movement triggers nerve cells to send signals along the auditory nerve to the thalamus and then to the brain’s auditory cortex.

• Small differences in the loudness and timing of the sounds received by each ear allow us to locate sounds.

• Sensorineural hearing loss (or nerve deafness) results from damage to the cochlea’s hair cells or their associated nerves. Conduction hearing loss results from damage to the mechanical system that transmits sound waves to the cochlea. Cochlear implants can restore hearing for some people.

• Our sense of touch is actually several senses—pressure, warmth, cold, and pain—that combine to produce other sensations, such as “hot.”

• Pain reflects bottom-up sensations (such as input from nociceptors, the sensory receptors that detect hurtful temperatures, pressure, or chemicals) and top-down processes (such as experience, attention, and culture).

• Pain treatments often combine physical and psychological elements, including distractions. Combining a placebo with distraction, and amplifying the effect with hypnosis (which increases our response to suggestions), can help relieve pain. Posthypnotic suggestion is used by some clinicians to help control undesired symptoms and behavior.

• Both taste and smell are chemical senses.

• Taste involves five basic sensations—sweet, sour, salty, bitter, and umami. Taste receptors in the taste buds carry messages to matching partner cells in the brain.

• There are no basic sensations for smell. Some 20 million olfactory receptor cells for smell, located at the top of each nasal cavity, send messages to the brain. These cells work together, combining their messages into patterns that vary, depending on the different odors they detect.

• Through kinesthesia, we sense the position and movement of individual body parts.

• We monitor our head’s (and therefore our body’s) position and movement, and maintain our balance, with our vestibular sense.

Sensory Interaction

• Sensory interaction is the influence of one sense on another. This occurs, for example, when the smell of a favorite food enhances its taste.

• Embodied cognition is the influence of bodily sensations, gestures, and other states on cognitive preferences and judgments.

ESP—Perception Without Sensation?

• The three most testable forms of extrasensory perception (ESP) are telepathy (mind-to-mind communication), clairvoyance (perceiving remote events), and precognition (perceiving future events).

• Researchers have not been able to replicate (reproduce) ESP effects under controlled conditions.