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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.