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Psychology, 5/e Wortman, Loftus & Weaver | |||||
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Psychology, 5/e Wortman, Loftus & Weaver | |||||
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Sensation is the process whereby stimulation of the receptor cells in the sense organs results in a neural impulse to the brain. Perception is the brain’s interpretation of this sensory information. Different species have different sensory capabilities as do different members of the same species. Sensory differences can be biologically based, or can result from different experiences and motivations. Information flowing from the sense organs to the brain is called bottom-up or data-driven processing; knowledge stored in the brain which directs your attention and influences your perception is called top-down or conceptually driven processing. The study of the relationship between physical stimuli and our subjective experience of them is called psychophysics.
A stimulus is any form of energy to which an organism can respond. The organism’s response is called a sensation and can be affected by both the quality, or kind, of sensation it produces and the quantity, or amount, of stimulation.
The sensory threshold is the lowest intensity stimulus to which a person will respond 50% of the time under ideal conditions. Sensory capacity can be limited by several factors, including the presence of background noise, lack of prior information about the stimulus, and motivation to ignore weak stimuli. Signal detection theory attempts to separate these various influences.
The ability to notice changes in the intensity of a stimulus is stimulus discrimination. A just noticeable difference (JND) is the amount of change required for a difference in the stimulus to be perceived. Weber’s law postulates that a just noticeable difference in sensation (the smallest detectable change in stimulation) is always a constant proportion of the initial stimulus intensity. Although it does not hold under all circumstances, it is a useful approximation. Fechner’s law states that as stimulus intensity increases, larger and larger increases in intensity are needed to produce subjectively equivalent changes.
All sensory systems display sensory adaptation, the reduction of sensitivity due to constant stimulation, although the degree of adaptation varies from one sense to another.
Electromagnetic energy, or light, behaves as if it were composed of subatomic particles, called photons. Light travels in wavelike patterns, and the physical distance between the peak of one wave of light to the peak of the next is called its wavelength. The wavelength of light determines its color. The frequency of a light wave indicates how many waves occur in a given unit of time. Wavelength is inversely proportional to the frequency of a wave. In addition, the intensity of light determines its brightness. This intensity is measured in amplitude, which is the variation between the "peak" and the "trough" of the wave.
Light enters the eye through the transparent cornea (which covers the front of the eyeball) and goes through a hole in the iris called the pupil and through the lens, whose shape is changed by ciliary muscles in order to focus the image on the retina.
The outermost layer of the retina contains two types of receptor cells, rods and cones. Rods are long, thin receptor cells in the retina that are sensitive to light of low intensity and that function in dim light and nighttime vision. Cones are receptor cells in the retina that are sensitive to color and that are primarily used for daytime or high-light intensity vision. Rods and cones channel their impulses through bipolar cells and then through ganglion cells, which form the fibers of the optic nerve. The optic nerve carries visual information to the brain for interpretation.
Cones are highly concentrated in and near the center of the retina, an area known as the fovea ("small pit"). Rods are denser in the areas farther away from the center of the fovea, sense black and white, and work in dim light. Both rods and cones sense light by means of chemical reactions in which a pigment, such as rhodopsin which is in the rods, is bleached out. Consequently, when one comes from a light to a dark area, vision is diminished until this pigment is replenished. Night blindness occurs when rods become insensitive to dim light, and can sometimes result from an inability to store vitamin A in the body.
Fibers from the optic nerve form several pathways to the brain, the most important of which goes through the thalamus to the primary visual cortex at the back of the brain. Hubel and Wiesel’s research indicates that each of the neurons in the visual cortex is activated only by certain types of stimuli, such as a horizontal line. This suggests that these feature detector cells actually are involved in an early step in the processing of visual stimuli. More recent research suggests that beyond the primary visual cortex, in the secondary visual cortex, nerve impulses feed into at least three separate processing systems: one each for analyzing color, form, orientation, depth, and movement.
The trichromatic theory of color holds that there are only three types of cones (red, blue, and green), each maximally sensitive to a different wavelength (color) of light. The opponent-process theory explains that the three types of cones are linked to form three opponent systems in the brain: red-green, yellow-blue, and light-dark. It helps explain the occurrence of afterimages and color blindness. Both theories have validity: as the trichromatic theory suggests, there are three different kinds of cones. Beyond the cones, in the ganglion cells and neural network, cells operate as opponent-process units by increasing or decreasing their rates of firing depending on the color being sensed.
People who have cones containing all three kinds of iodopsin, a light-sensitive pigment, and see all colors are called normal trichromats. Studies of individuals with irregular color vision have helped researchers learn about normal color vision. Thus, psychologists learn about the normal by studying the abnormal (Theme 2). People with the most common color blindness are called red-green dichromats and have difficulty distinguishing red from green because they lack either the red or green visual pigment. Less common are blue-yellow dichromats and this form of color blindness is more common in men than women. Monochromatic people see no color because their visual information is transmitted by only one kind of iodopsin. Recent evidence suggests that the range of defects in color vision is quite diverse.
Sound waves are produced by vibrating molecules and are converted to neural impulses in the ear. Their frequency determines pitch; their intensity, or amplitude (measured in decibels), determines loudness. The human ear is sensitive to frequencies ranging from 20 hertz (Hz) to 20,000 Hz, and human voices range from 100 to 3,500 Hz. Normal conversation occurs at about 60 decibels; sounds above 120 decibels are painful and can cause hearing loss.
The pinna, or outer ear, funnels sound into the auditory canal, which amplifies the sound. A thin membrane called the eardrum seals off the end of this passage and vibrates in harmony with the sound waves. Three small bones--theincus, the malleus, and the stapes--amplify and conduct this information from the eardrum to the oval window, at which point the original message is amplified up to 90 times. Conduction deafness occurs when the ear’s ability to amplify waves malfunctions. The oval window transmits the message to the cochlea, the organ of hearing. In the cochlea, hair cells (receptors) are positioned between the basilar membrane and the tectorial membrane. These membranes rub against the hair cells, triggering a neural impulse that travels via the auditory nerve to the brain. Nerve deafness occurs when there is damage to the basilar membrane or the hair cells. In this c ase, cochlear implants, microelectrode implants placed in the cochlea, can restore some hearing.
The place theory of hearing argues that the pitch we perceive depends on which part of the basilar membrane is most displaced. However, it cannot explain all phenomena; for example, low-frequency sounds vibrate the membrane uniformly. The frequency theory and the volley principle suggest that loudness is perceived according to how often the fibers of the auditory nerve fire. Both theories must be used to explain how we hear: the place theory probably explains how we hear high-pitched sounds whereas the frequency theory provides a better explanation for low-pitched sounds.
The somatic senses are responsible for sensations of the skin (cutaneous senses), detecting the movement in the body (kinesthetic senses), and awareness of body and limb position (proprioceptive senses).
Several types of receptors embedded in the skin are probably responsible for skin sensations. Meissner’s corpuscles are thought to be pressure sensitive, as are receptors around the roots of hair cells. Merkel disks and Ruffini endings seem to be involved with the sensation of steady pressure. Pacinian corpuscles are very sensitive to touch and even very light pressure. Pain is probably detected by free nerve endings, but it is also influenced by external conditions such as anxiety. The gate control theory suggests that pain messages can be blocked in the spinal cord if there is high activation of L fibers relative to S fibers. This theory may explain the effects of acupuncture.
Olfaction (smell) requires that vaporized molecules of a substance enter the nasal passages, which are lined with the olfactory membranes containing the receptor cells. These receptor cells detect the chemical stimuli, sending nerve impulses along the olfactory bulbs to other regions in the brain. Our sense of taste is restricted to four basic categories: sweet and salty, which are sensed on the front of the tongue; sour, sensed on the sides; and bitter, sensed on the back. Taste is sensed by the taste buds located in the papillae (or bumps) of the tongue. Each of the four basic taste categories may operate independently of the others, or different tastes may be based on different patterns of neural activity. Human senses are integrated such that information from one sense influences how the other senses interpret information.
Perception is the process of giving order and meaning to sensations. There are at least two perspectives on perception. First, the direct perspective specifies that sensations contain sufficient information to be structured automatically into a meaningful whole. It is, therefore, related to bottom-up processing. The indirect perspective, or constructivist view, holds that sensory information must be supplemented with information from memory. In this view, mental representations, called schemas, are used to compare with sensory information and give it meaning. This process is top-down.
There are three major areas in the study of perceptual processes. First, form perception is the ability to detect unified patterns in a mass of sensory data. Second, perceptual constancy is the tendency of the brain to perceive objects with stable properties even though the visual images received are constantly changing. Third, depth perception is the ability to see the world in three dimensions and tell how far away an object is. Gestalt ("whole") psychologists, primarily interested in form perception, suggested that perceptions are more than the sensations that give rise to them. For example, we sometimes "fill in" missing aspects of a picture, a phenomenon called subjective contour. In similar fashion, we also tend to group together things that are close (proximity), things that form a continuous pattern (continuity), and things that resemble one another (similarity). These phenomena prov ide evidence in support of the idea that cognition and thought are dynamic, active processes, best considered reconstructive, not reproductive (Theme 4).
Depth perception, which arises from several sources, is the ability to tell how far away an object is. Binocular disparity, the difference in the angles at which the eyes view an object, aids in depth perception. Monocular depth cues are the cues that augment depth perception and are potentially available to one eye alone. For example, motion parallax is the aid to perceiving depth that consists of the differences in relative movement of retinal images that occur when we change position. Other monocular depth cues include: partial overlap (when one object blocks the view of an object behind it), linear perspective (produced by the apparent convergence of lines), relative size (more distant objects appear smaller), texture gradient (near objects appear larger and coarser), relative closeness to the horizon (objects closer to the horizon appear more distant), and light and shadow (where shadowed objects ap pear more distant).
A perceptual illusion is a perception not in accord with the true characteristics of an object or event. The Ames room illusion and other illusions are caused by our being fooled as we process information. By studying the cues that are and are not present in such illusions, psychologists can better understand vision (Theme 2).
The empiricist view argues that perceptual processes are largely the result of learning. The nativist view suggests that some perceptual processes are the result of inherited biological factors. Empiricist views are linked to the indirect or constructivist perspective whereas the nativist view is tied to the direct perspective. There is evidence to support both views. Most psychologists today take an interactionist perspective and believe that both heredity and environment are involved.
Perceptual sets, or expectations, involve contexts that establish expectations about what we think we should perceive. Generalized knowledge in the form of schemas influences expectations and thus perception.
Subliminal perception is the brain’s ability to register a stimulus presented so briefly or quickly that it cannot be consciously perceived. The findings of several initial studies of subliminal perception were interpreted to support the existence of such perception and even the ability of these perceptions to change behavior. However, due to a failure to control for other factors, the changes in behavior may not have been due to subliminal perception. Further research has led psychologists to believe that only under certain conditions, people may sometimes perceive without conscious awareness. However, most natural settings do not meet the conditions needed for subliminal perception. Furthermore, when subliminal stimuli are displayed in a barrage of consciously perceived stimuli (e.g., in a movie or audio message), the subliminal stimuli are not likely to be perceived. There is virtually no evidence that subliminal messages can get people to do things they would not ordinarily do. However, the capacity of subliminal perception should not be confused with the capacity of subliminal (unconscious) memory and cognition, which do seem to exert a strong influence on behavior.
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