Chapter SummarySensing the World: Some Basic Principles
To study sensation is to study an ageless question: How does the world out there get represented in here, inside our heads? Put another way, how are the external stimuli that strike our bodies transformed into messages that our brains comprehend? Thresholds Each species comes equipped with sensitivities that enable it to survive and thrive. We sense only a portion of the sea of energy that surrounds us, but to this portion we are exquisitely sensitive. Our absolute threshold for any stimulus is the minimum stimulation necessary for us to detect it 50 percent of the time. Signal detection researchers report that our individual absolute thresholds vary with our psychological state. Experiments reveal that we can process some information from stimuli too weak to recognize. But the restricted conditions under which this occurs would not enable unscrupulous opportunists to exploit us with subliminal messages. To survive and thrive, an organism must have difference thresholds low enough to detect minute changes in important stimuli. In humans, a difference threshold (also called a just noticeable difference, or jnd) increases in proportion to the size of the stimulus—a principle known as Weber’s law. Sensory Adaptation Sensory adaptation refers to our ability to adapt to unchanging stimuli. For example, when we smell an odor in a room we’ve just entered and remain in that room for a period of time, the odor will no longer be easily detected. The phenomenon of sensory adaptation focuses our attention on informative changes in stimulation by diminishing our sensitivity to constant or routine odors, sounds, and touches. Vision Each sense receives stimulation, transduces it into neural signals, and sends these neural messages to the brain. We have glimpsed how this happens with vision. The Stimulus Input: Light Energy The energies we experience as visible light are a thin slice from the broad spectrum of electromagnetic radiation. The hue and brightness we perceive in a light depend on the wavelength and intensity. The Eye After entering the eye and being focused by a camera-like lens, light waves strike the retina. The retina’s light-sensitive rods and color-sensitive cones convert the light energy into neural impulses, which are coded by the retina before traveling along the optic nerve to the brain. Visual Information Processing In the cortex, individual neurons called feature detectors, respond to specific features of a visual stimulus, and their information is pooled for interpretation by higher-level brain cells. Sub-dimensions of vision (color, movement, depth, and form) are processed separately and simultaneously, illustrating the brain’s capacity for parallel processing. The visual pathway faithfully represents retinal stimulation, but the brain’s representation incorporates our assumptions, interests, and expectations. Color Vision Research on how we see color supports two nineteenth-century theories. First, as the Young-Helmholtz trichromatic (three-color) theory suggests, the retina contains three types of cones. Each is most sensitive to the wavelengths of one of the three primary colors of light (red, green, or blue). Second, as opponent-process theory maintains, the nervous system codes the color-related information from the cones into pairs of opponent colors, as demonstrated by the phenomenon of afterimages and as confirmed by measuring opponent processes within visual neurons of the thalamus. The phenomenon of color constancy under varying illumination shows that our brains construct our experience of color. Hearing The Stimulus Input: Sound Waves The pressure waves we experience as sound vary in frequency and amplitude, and correspondingly in perceived pitch and loudness. The Ear Through a mechanical chain of events, sound waves traveling through the auditory canal cause minuscule vibrations in the eardrum. Transmitted via the bones of the middle ear to the fluid-filled cochlea, these vibrations create movement in tiny hair cells, triggering neural messages to the brain. Research on how we hear pitch supports both the place theory, which best explains the sensation of high-pitched sounds, and frequency theory, which best explains the sensation of low-pitched sounds. We localize sound by detecting minute differences in the intensity and timing of the sounds received by each ear. Hearing Loss and Deaf Culture Hearing losses linked to conduction and nerve disorders can be caused by prolonged exposure to loud noise and by diseases and age-related disorders. Those who live with hearing loss face social challenges. Cochlear implants can enable some hearing for deaf children and most adults. But Deaf Culture advocates, noting that Sign is a complete language, question the enhancement. Additionally, deafness can lead to sensory compensation where other senses are enhanced. Advocates feel that this furthers their view that deafness is not a disability. Other Important Senses Touch Our sense of touch is actually four senses—pressure, warmth, cold, and pain—that combine to produce other sensations, such as "hot." One theory of pain is that a "gate" in the spinal cord either opens to permit pain signals traveling up small nerve fibers to reach the brain, or closes to prevent their passage. Because pain is both a physiological and a psychological phenomenon, it often can be controlled through a combination of physical and psychological treatments. Taste Taste, a chemical sense, is likewise a composite of five basic sensations—sweet, sour, salty, bitter, and umami—and of the aromas that interact with information from the taste buds. The influence of smell on our sense of taste is an example of sensory interaction. Smell Like taste, smell is a chemical sense, but there are no basic sensations for smell, as there are for touch and taste. Unlike the retina’s receptor cells that sense color by breaking it into component parts, the 5 million olfactory receptor cells with their 1000 different receptor proteins recognize individual odor molecules. Some odors trigger a combination of receptors. Like other stimuli, odors can spontaneously evoke memories and feelings. Body Position and Movement Finally, our effective functioning requires a kinesthetic sense, which notifies the brain of the position and movement of body parts, and a sense of equilibrium, which monitors the position and movement of the whole body. Key TermsSensation - the process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment.
Perception - the process of organizing and interpreting sensory information, enabling us to recognize meaningful objects and events. Bottom-Up Processing - analysis that begins with the sensory receptors and works up to the brain's integration of sensory information. Top-Down Processing - information processing guided by higher-level mental processes, as when we construct perceptions drawing on our experience and expectations. Psychophysics - The study of relationships between the physical characteristics of stimuli, such as their intensity, and our psychological experience of them. Absolute Threshold - the minimum stimulation needed to detect a particular stimulus 50 percent of the time. Signal Detection Theory - a theory predicting how and when we detect the presence of a faint stimulus ("signal") amid background stimulation ("noise"). Assumes there is no single absolute threshold and detection depends partly on a person's experience, expectations, motivation, and level of fatigue. Subliminal - below one's absolute threshold for conscious awareness. Difference Threshold - the minimum difference between two stimuli required for detection 50 percent of the time. Weber's Law - states that the difference threshold is directly proportional to the magnitude of the stimulus with which the comparison is being made. Sensory Adaptation - diminished sensitivity as a consequence of constant stimulation. Transduction - conversion of one form of energy into another. In sensation, the transforming of stimulus energies, such as sights, sounds, and smells, into neural impulses our brains can interpret. Wavelength - the distance from the peak of one light or sound wave to the peak of the next. Hue - the dimension of color that is determined by the wavelength of light. Intensity - the amount of energy in a light or sound wave, which we perceive as brightness or loudness, as determined by the wave's amplitude. Pupil - the adjustable opening in the center of the eye through which light enters. Iris - a ring of muscle tissue that forms the colored portion of the eye around the pupil and controls the size of the pupil opening. Lens - the transparent structure behind the pupil that changes shape to help focus images on the retina. Accommodation - the process by which the eye's lens changes shape to focus near or far objects on the retina. Rods - retinal receptors that detect black, white, and gray; necessary for peripheral and twilight vision, when cones don't respond. Cones - retinal receptor cells that are concentrated near the center of the retina and that function in daylight or in well-lit conditions. Detect fine detail and give rise to color sensations. Optic Nerve - the nerve that carries neural impulses from the eye to the brain. Blind Spot - the point at which the optic nerve leaves the eye, no receptor cells are located there. Fovea - the central focal point in the retina, around which the eye's cones cluster. Retina - the light-sensitive inner surface of the eye, containing the receptor rods and cones plus layers of neurons that begin the processing of visual information. Acuity - the sharpness of vision. Nearsightedness - a condition in which nearby objects are seen more clearly than distant objects because distant objects focus in front of the retina. Farsightedness - a condition in which faraway objects are seen more clearly than near objects because the image of near objects is focused behind the retina. Feature Detectors - nerve cells in the brain that respond to specific features of the stimulus, such as shape, angle, or movement. Parallel Processing - the processing of several aspects of a problem simultaneously; the brain's natural mode of information processing for many functions, including vision. Young-Helmholtz Theory - the theory that the retina contains three different color receptors - one most sensitive to red, one to green, one to blue - which when stimulated in combination can produce the perception of any color. Opponent-Process Theory - the theory that opposing retinal processes enable color vision. Color Constancy - perceiving familiar objects as having consistent color, even if changing illumination alters the wavelengths reflected by the object. Audition - the sense of hearing. Frequency - the number of waves that pass a given point in a specific time, usually one second. Pitch - a tone's experienced highness or lowness; depends on frequency. Middle Ear - the chamber between the eardrum and cochlea containing three tiny bones that concentrate the vibrations of the eardrum on the cochlea's oval window. Inner Ear - the innermost part of the ear, containing the cochlea, semicircular canals, and vestibular sacs. Cochlea - a coiled, bony, fluid-filled tube in the inner ear through which sound waves trigger nerve impulses. Place Theory - in hearing, the theory that links the pitch we hear with the place where the cochlea's membrane is stimulated. Frequency Theory - in hearing, the theory that the rate of nerve impulses traveling up the auditory nerve matches the frequency of a tone, thus enabling us to sense its pitch. Conduction Hearing Loss - hearing loss caused by damage to the mechanical system that conducts sound waves to the cochlea. Nerve Deafness - hearing loss created by damage to the hair cells or the auditory nerve fibers in the inner ear. Gate Control Theory - theory that spinal cord contains neurological gate that blocks pains signals or allows them to pass. Opened by activity of pain going up small nerve fibers & is closed by act of large fibers or by info coming from brain. Sensory Interaction - the principle that one sense may influence another, as when the smell of food influences its taste. Kinesthesis - the system for sensing the position and movement of individual body parts. Vestibular Sense - the sense of body movement and position, including the sense of balance. Sensorineural Hearing Loss - hearing loss caused by damage to the cochlea's receptor cells or to the auditory nerves; also called nerve deafness. |
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