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CHAPTER 4 4.1 Sensation is the process by which sense organs gather information about the environment and transmit it to the brain for initial processing. Perception is the related process by which the brain selects, organizes, and interprets sensations. The basic senses are visual, auditory (hearing), olfactory (smell), gustatory (taste), tactile (touch), and proprioception (the sense of the body’s position and motion). 4.2 Psychophysics is the study of the relationship between physical stimuli and the psychological experience of them. Three basic principles apply across all the senses: There is no one-to-one correspondence between physical and psychological reality; sensation and perception are active, not passive; and sensory and perceptual processes reflect the impact of adaptive pressures over the course of evolution. 4.3 All senses have these features in common: they translate information, have thresholds, require constant decision making, detect changes, and are selective. Sensation begins with an environmental stimulus; all sensory systems have specialized cells called sensory receptors that respond to environmental stimuli and typically generate action potentials in adjacent sensory neurons. The process of converting stimulus information into neural impulses is called transduction. Within each sensory modality, the brain codes sensory stimulation for intensity and quality. 4.4 The absolute threshold is the minimum amount of energy needed for an observer to sense that a stimulus is present. The difference threshold is the lowest level of stimulation required to sense that a change (just noticeable difference or “jnd”) in stimulation has occurred. According to Weber’s law, regardless of the magnitude of two stimuli, the second must differ by a constant proportion from the first for it to be perceived as different. According to Fechner’s law, the magnitude of a stimulus grows logarithmically as the subjective experience of intensity grows arithmetically so that people subjectively experience only a fraction of actual increases in stimulation. According to Stevens’s power law, subjective intensity increases in a linear fashion as actual intensity grows exponentially. Sensory adaptation is the tendency of sensory systems to respond less to stimuli that continue without change. Psychophysiologists are using cognitive augmentation (which involves detecting sensation intensity) to increase the challenge of video games and increase alertness in military personnel. 4.5 The stimuli of the visual system are light waves (measured in nms); wavelength is experienced as hue (color) and wave height as brightness. Two basic processes occur in the eyes: Light is focused on the retina by the cornea, pupil, and lens, and the retina transduces this visual image into a code that the brain can read. The fovea, in the central region of the retina, is where vision is the sharpest, except for the blind spot where the optic nerve leaves the eye. The retina includes two kinds of photoreceptors: rods (which produce sensations in black, white, and gray and are very sensitive to light) and cones (which produce sensations of color). Rods and cones excite bipolar cells, which in turn excite or inhibit ganglion cells, whose axons constitute the optic nerve which carries information to the brain. Ganglion cells, like sensory cells higher up in the nervous system, have receptive fields, areas that are excited or inhibited by the arriving sensory information. 4.6 From the optic nerve, visual information travels along two pathways simultaneously. One is to the superior colliculus in the midbrain, which in humans is particularly involved in eye movements. The other is to the lateral geniculate nucleus in the thalamus and on to the visual cortex. Feature detectors in the primary visual cortex respond only when stimulation in their receptive field matches a particular pattern or orientation. Beyond the primary visual cortex, visual information flows along two pathways, the “what” pathway (or ventral stream) is involved in determining what an object is, and the “where” pathway (or dorsal stream) is involved in locating the object in space, following its movement, and guiding movement toward it. 4.7 Positive psychology studies have focused on individuals who have shown exceptional coping after suffering debilitating harm to a sensory system, such as blindness. Resilience is correlated with positive thinking, faith, talent, and social support. The three psychological dimensions of visual perception are color (hue), saturation, and brightness. Two theories together explain what is known about color vision. According to the Young-Helmholtz, or trichromatic, theory, the eye contains three types of receptors, which are most sensitive to wavelengths experienced as red, green, or blue, and the experience of other colors are accounted for by a mixing process. According to opponent-process theory, the colors we experience (and the afterimages we perceive) reflect three antagonistic color systems—a blue-yellow, red-green, and black-white system. Both are correct: Trichromatic theory operates at the level of the retina and opponent-process theory at higher neural levels. 4.8 Sound travels in sound waves, and audition (hearing) occurs as a vibrating object sets air particles in motion. The sound wave’s frequency, which is experienced as pitch, refers to the number of times those particles oscillate per second (measured in Hz). Most sounds are actually composed of waves with many frequencies, which gives them their complexity, or timbre. The loudness of a sound reflects the height and depth, or amplitude, of the wave (measured in dB). 4.9 Sound waves enter the ear through the pinna and auditory canal of the outer ear to the eardrum, which in turn sets the ossicles of the middle ear in motion, amplifying the sound. When the stirrup (one of the ossicles) strikes the oval window of the inner ear, it creates waves of pressure in the fluid of the cochlea. Hair cells attached to the basilar membrane then detect the waves, triggering firing of the sensory neurons whose axons comprise the auditory nerve which transmits audio information to the brain. Two theories, once considered opposing, explain the psychological qualities of sound. According to place theory, which best explains transduction at high frequencies, different areas of the basilar membrane respond to different frequencies. According to frequency theory, which best explains transduction at low frequencies, the rate of vibration of the basilar membrane transforms frequency into pitch. 4.10 Sound localization—identifying the location of a sound in space— depends on binaural neurons that respond to relative differences in the loudness and timing of sensory signals transduced by the two ears. 4.11 The environmental stimuli for smell are gas molecules suspended in the air. These molecules flow from the throat and nose along the nasal passages and into the olfactory epithelium, where they are detected by hundreds of different types of auditory celia receptors. The axons of these receptor cells comprise the olfactory nerve, which transmits information to the olfactory bulbs and on to the primary olfactory cortex deep in the frontal lobes. 4.12 The environmental stimuli for smell are gas molecules suspended in the air. These molecules flow from the throat and nose along the nasal passages and into the olfactory epithelium, where they are detected by hundreds of different types of auditory celia receptors. The axons of these receptor cells comprise the olfactory nerve, which transmits information to the olfactory bulbs and on to the primary olfactory cortex deep in the frontal lobes. 4.13 Taste occurs as receptors in the taste buds transduce chemical information from molecules soluble in saliva into neural information, which is integrated with olfactory sensations in the brain. Multiple taste buds are located on papillae (bumps on the tongue) which contain the auditory receptor cells. Taste receptors stimulate neurons that project to the medulla and pons in the hindbrain. From there, the information is carried along two neural pathways, one leading to the primary gustatory cortex, which allows identification of tastes, and the other leading to the limbic system, which allows initial gut-level reactions and learned responses to tastes. The gustatory system responds to four tastes: sweet, sour, salty and bitter. A recently discovered gene for taste identifies non-tasters (double recessive) who often prefer spicy foods, supertasters (double dominant) who prefer bland food, and mixed alleles who are medium tasters. 4.14 Touch includes three senses: pressure, temperature, and pain. Sensory neurons synapse with spinal interneurons that stimulate motor neurons (producing reflexes) as well as with neurons that carry information up the spinal cord to the medulla. From there, nerve tracts cross over, and the information is conveyed through the thalamus to the somatosensory cortex, which contains a map of the body. “Phantom limb” appears to originate in stimulation of adjacent somatorysensory areas. The function of pain is to prevent tissue damage; the experience of pain is greatly affected by beliefs, expectations, arousal and distraction. 4.15 The proprioceptive senses register body position and movement. The vestibular sense provides information on the position of the body in space by sensing gravity and movement. The vestibular sense receptors are celia in the inner ear which detect head movements. Kinesthesia provides information about the movement and position of the limbs and other parts of the body relative to one another. Kinesthesia receptors are cells in the joints, muscles, and tendons. 4.16 Perception involves the organization and interpretation of sensory experience. Perceptual organization includes form, depth, and motion perception and perceptual constancy. Form perception refers to the organization of sensations into meaningful shapes and patterns (percepts). The Gestalt psychologists described several principles of form perception, including figure-ground, similarity, proximity, continuation, simplicity, and closure. More recently, a theory called recognition-by-components has argued that people perceive and categorize objects by first breaking them down into elementary units. The brain’s efforts to organize percepts can sometimes fail and thus produce perceptual illusions. 4.17 Depth perception is the organization of perception in three dimensions; it is based on binocular cues such as retinal disparity and convergence, and monocular visual cues such as interposition, elevation, texture gradient, linear perspective, shading, aerial perspective, familiar size, relative size, and motion parallax. Motion perception, the perception of movement, relies on motion detectors from the retina through the cortex. It appears to involve two systems: The first computes motion from the changing image on the retina, and the second uses information from eye muscles about the movement of the eyes. Perceptual constancy refers to the organization of changing sensations into percepts that are relatively stable. Three types of perceptual constancy are color, shape, and size constancy. 4.18 Perceptual interpretation means generating meaning from sensory experience. According to the theory of direct perception, the meaning or adaptive significance of a percept is often obvious, immediate, and innate. Trying to distinguish the relative roles of nature and nurture in perception may in some ways be asking the wrong question, because the nervous system has innate potentials that require environmental input to develop. Perception simultaneously involves bottom-up processing, which begins with raw sensory data that feed “up” to the brain, and topdown processing, which begins with higher level expectations, knowledge and motivation. 4.19 Expectations based on both the current context and enduring knowledge structures (schemas) influence the way people interpret ongoing sensory experience. Motives can also influence perception, including motives to perceive stimuli we desire and avoid perceiving stimuli with uncomfortable content.