Chapter 7

SUMMARY

7.1 Outline the three basic principles that apply to all senses.

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Sensation refers to the process by which sense organs gather information about the environment and transmit it to the brain for initial processing. Perception refers to the closely related process by which the brain selects, organises and interprets sensations.

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Three basic principles apply across all the senses. First, there is no one-to-one correspondence between physical and psychological reality. Second, sensation and perception are active, not passive. Third, sensation and perception are adaptive.

7.2 Describe the processes common to all sensory systems.

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Sensation begins with an environmental stimulus; all sensory systems have specialised cells called sensory receptors that respond to environmental stimuli and typically generate action potentials in adjacent sensory neurons. This process is called transduction. Within each sensory modality, the brain codes sensory stimulation for intensity and quality.

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The absolute threshold refers to the minimum amount of stimulation needed for an observer to notice a stimulus. The signal detection theory asserts that people make a judgement about whether a stimulus is present or absent. The difference threshold refers to the lowest level of stimulation required to sense that a change in stimulation has occurred (a just noticeable difference [jnd]).

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Weber’s law states that 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. Fechner’s law holds that the physical magnitude of a stimulus grows logarithmically as the subjective experience of intensity grows arithmetically; in other words, people only subjectively experience a small percentage of actual increases in stimulus intensity. Stevens’ power law states that subjective intensity grows as a proportion of the actual intensity raised to some power; that is, that sensation increases in a linear fashion as actual intensity grows exponentially.

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Sensory adaptation is the tendency of sensory systems to respond less to stimuli that continue without change.

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Subliminal perception is the tendency to perceive information outside our conscious awareness.

7.3 Explain how light wavelengths are transformed into vision and colour.

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The eyes are sensitive to a small portion of the electromagnetic spectrum called light. In vision, light is focused on the retina by the cornea, pupil and lens. Rods are very sensitive to light, allowing vision in dim light; cones are especially sensitive to particular wavelengths, producing the psychological experience of colour. Cones are concentrated at the fovea, the region of the retina most sensitive to detail.

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The ganglion cells of the retina transmit visual information via the optic nerve to the brain. Ganglion cells, like other neurons involved in sensation, have receptive fields, a region of stimulation to which the neuron responds. Feature detectors are specialised cells in the cortex that respond only when stimulation in their receptive field matches a particular pattern or orientation, such as horizontal or vertical lines.

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From the primary visual cortex, visual information flows along two pathways, or processing streams, called the ‘what’ and the ‘where’ pathways. The ‘what’ pathway is involved in determining what an object is; this network runs from the primary visual cortex in the occipital lobes through the lower part of the temporal lobes (the inferior temporal cortex). The second stream, the ‘where’ pathway, is involved in locating the object in space, following its movement and guiding movement towards it. This pathway runs from the primary visual cortex through the middle and upper regions of the temporal lobes and up into the parietal lobes.

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The property of light that is transduced into colour is wavelength. The Young–Helmholtz, or trichromatic, theory proposes that the eye contains three types of sensory receptors, sensitive to red, green or blue. Opponent-process theory argues for the existence of pairs of opposite primary colours linked in three systems: a blue–yellow system, a red–green system and a black–white system. Both theories appear to be involved in colour perception; trichromatic theory is operative at the level of the retina and opponent-process theory at higher neural levels.

7.4 Explain how sound waves are transformed into hearing.

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Hearing, or audition, occurs as a vibrating object sets air particles in motion. Each round of expansion and contraction of the air is known as a cycle. The number of cycles per second determines a sound wave’s frequency expressed in hertz (Hz), which corresponds to the psychological property of pitch. Amplitude, expressed in decibels (dB), refers to the height and depth of the wave and corresponds to the psychological property of loudness.

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Sound waves travel through the auditory canal to the eardrum, where they are amplified. Transduction occurs by way of hair cells attached to the basilar membrane that respond to vibrations in the fluid-filled cochlea. This mechanical process triggers action potentials in the auditory nerve, which are then transmitted to the brain.

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Two theories, once considered opposing, explain the psychological qualities of sound. Place theory, which holds that different areas of the basilar membrane respond to different frequencies, appears to be most accurate for high frequencies. Frequency theory asserts that the basilar membrane’s rate of vibration reflects the frequency with which a sound wave cycles, explains the sensation of low-frequency sounds.

7.5 Distinguish among the main functions of smell, taste, touch and the vestibular and kinaesthetic systems.

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The environmental stimuli for smell, or olfaction, are invisible molecules of gas emitted by substances and suspended in the air. As air enters the nose, it flows into the olfactory epithelium, where hundreds of different types of receptors respond to various kinds of molecules, producing complex smells. The axons of olfactory receptor cells constitute the olfactory nerve, which transmits information to the olfactory bulbs under the frontal lobes and on to the primary olfactory cortex, a primitive region of the cortex deep in the frontal lobes.

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Taste, or gustation, is sensitive to molecules soluble in saliva. Much of the experience of flavour, however, is really contributed by smell. Taste occurs as receptors in the tastebuds on the tongue and throughout the mouth transduce chemical information into neural information, which is integrated with olfactory information in the brain.

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Touch includes three senses: pressure, temperature and pain. The human body contains approximately 5 million touch receptors of at least seven different types. Sensory neurons synapse with spinal interneurons that stimulate motor neurons, allowing reflexive action. They also synapse with neurons that carry information up the spinal cord to the medulla, where nerve tracts cross over. From there, sensory information travels to the thalamus and is subsequently routed to the primary touch centre in the brain, the somatosensory cortex, which contains a map of the body.

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Pain is greatly affected by beliefs, expectations and emotional state. Gate-control theory holds that the experience of pain is heavily influenced by the central nervous system, through the action of neural fibres that can ‘close the gate’ on pain, preventing messages from other fibres getting through.

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The proprioceptive senses provide information about the body’s position and movement. The vestibular sense provides information on the position of the body in space by sensing gravity and movement. Kinaesthesia provides information about the movement and position of the limbs and other parts of the body relative to one another.

7.6 Describe the various ways in which perception is organised.

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The hallmarks of perception are organisation and interpretation. Perceptual organisation integrates sensations into meaningful units, locates them in space, tracks their movement and preserves their meaning as the perceiver observes them from different vantage points. Form perception refers to the organisation of sensations into meaningful shapes and patterns (percepts). The Gestalt psychologists described several principles of form perception, including figure–ground perception, similarity, proximity, good continuation, simplicity and closure. A more recent theory, called recognition-by-components, asserts that we perceive and categorise objects in the environment by breaking them down into component parts, much like letters in words.

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Depth perception is the organisation of perception in three dimensions. Depth perception organises two-dimensional retinal images into a three-dimensional world, primarily through binocular and monocular visual cues.

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Motion perception refers to the perception of movement. Two systems appear to be involved in motion perception. The first calculates motion from the changing image projected by the object on the retina; the second makes use of commands from the brain to the muscles in the eye that signal eye movements.

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Perceptual constancy refers to the organisation of changing sensations into percepts that are relatively stable in size, shape and colour. Three types of perceptual constancy are size, shape and colour constancy, which refer to the perception of unchanging size, shape and colour despite momentary changes in the retinal image. The processes that organise perception leave perceivers vulnerable to perceptual illusions, some of which appear to be innate and others of which depend on culture and experience.

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Perceptual interpretation involves generating meaning from sensory experience. Perceptual interpretation lies at the intersection of sensation and memory, as the brain interprets current sensations in the light of past experience. Perception is neither entirely innate nor entirely learned. The nervous system has certain innate potentials, but these potentials require environmental input to develop. Experience can alter the structure of the brain, making it more or less responsive to subsequent sensory input.

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Bottom-up processing refers to processing that begins ‘at the bottom’, with raw sensory data that feeds ‘up’ to the brain. Top-down processing starts ‘at the top’, from the observer’s expectations and knowledge. According to current thinking, perception proceeds in both directions simultaneously.

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Experience with the environment shapes perceptual interpretation by creating perceptual expectations called perceptual set. Two aspects of perceptual set are current context and enduring knowledge structures called schemas. Motives, like expectations, can influence perceptual interpretation.

KEY TERMS

absolute threshold The minimum amount of physical energy (stimulation) needed for an observer to notice a stimulus.

accommodation In vision, the changes in the shape of the lens that focus light rays; in Piaget’s theory, the modification of schemas to fit reality.

amplitude The difference between the minimum and maximum pressure levels in a sound wave, measured in decibels; amplitude corresponds to the psychological property of loudness.

audition Hearing.

auditory nerve The bundle of sensory neurons that transmit auditory information from the ear to the brain.

binocular cells Neurons that receive information from both eyes.

binocular cues Visual input integrated from two eyes that provides perception of depth.

bipolar cells Neurons in the retina that combine information from many receptors and excite ganglion cells.

blind spot The point on the retina where the optic nerve leaves the eye and which contains no receptor cells.

blindsight A phenomenon in which individuals with cortical lesions have no conscious visual awareness but can make discriminations about objects placed in front of them.

bottom-up processing Perceptual processing that starts with raw sensory data that feed ‘up’ to the brain; what is perceived is determined largely by the features of the stimuli reaching the sense organs.

closure A Gestalt rule of perception which states that people tend to perceive incomplete figures as complete.

cochlea The three-chambered tube in the inner ear in which sound is transduced.

colour constancy The tendency to perceive the colour of objects as stable despite changing illumination.

complexity The extent to which a sound wave is composed of multiple frequencies.

cones One of two types of photoreceptors, which are specialised for colour vision and allow perception of fine detail.

consciousness The subjective awareness of mental events.

cornea The tough, transparent tissue covering the front of the eyeball.

cycle A single round of expansion and contraction of the distance between molecules of air in a sound wave.

decibels (dB) Units of measure of amplitude (loudness) of a sound wave.

depth perception The organisation of perception in three dimensions; also called distance perception.

difference threshold The smallest difference in intensity between two stimuli that a person can detect.

direct perception A theory which states that sensory information intrinsically carries meaning.

eardrum The thin, flexible membrane that marks the outer boundary of the middle ear; the eardrum is set in motion by sound waves and in turn sets in motion the ossicles; also called the tympanic membrane.

feature detectors Neurons that fire only when stimulation in their receptive field matches a particular pattern or orientation.

Fechner’s law The law of psychophysics proposed by Gustav Fechner, that the subjective magnitude of a sensation grows as a proportion of the logarithm of the stimulus.

figure–ground perception A fundamental rule of perception described by Gestalt psychology that states that people inherently differentiate between figure (the object they are viewing, sound to which they are listening, etc.) and ground (background).

form perception The organisation of sensations into meaningful shapes and patterns.

fovea The central region of the retina, where light is most directly focused by the lens.

frequency In a sound wave, the number of cycles per second, expressed in hertz and responsible for subjective experience of pitch.

frequency theory The theory of pitch that asserts that perceived pitch reflects the rate of vibration of the basilar membrane.

ganglion cells Nerve cells in the retina that integrate information from multiple bipolar cells, the axons of which bundle together to form the optic nerve.

gate-control theory Theory that emphasises the role of the central nervous system in regulating pain.

good continuation A Gestalt rule of perception which states that, if possible, the brain organises stimuli into continuous lines or patterns rather than discontinuous elements.

gustation Taste.

hair cells Receptors for sound attached to the basilar membrane.

hertz (Hz) The unit of measurement of frequency of sound waves.

hue The sensory quality people normally consider colour.

hypnosis An altered state of consciousness characterised by deep relaxation and suggestibility which a person voluntarily enters through the efforts of a hypnotist.

iris The ring of pigmented tissue that gives the eye its blue, green or brown colour; its muscle fibres cause the pupil to constrict or dilate.

just noticeable difference (jnd) The smallest difference in intensity between two stimuli that a person can detect.

kinaesthesia The sense that provides information about the movement and position of the limbs and other parts of the body; receptors in joints transduce information about the position of the bones, and receptors in the tendons and muscles transmit messages about muscular tension.

lens The disc-shaped, elastic structure of the eye that focuses light.

loudness The psychological property corresponding to a sound wave’s amplitude.

Müller–Lyer illusion A perceptual illusion in which two lines of equal length appear different in size.

monocular cues Visual input from a single eye alone that contributes to depth perception.

motion detectors Ganglion cells that are particularly sensitive to movement.

motion parallax A monocular depth cue involving the relative movements of retinal images of objects; nearby objects appear to speed across the field of vision, whereas distant objects barely seem to move.

motion perception The perception of movement in objects.

olfaction Smell.

olfactory epithelium Thin pair of structures in which transduction of smell occurs.

olfactory nerve The bundle of axons from sensory receptor cells that transmits information from the nose to the brain.

opponent-process theory A theory of colour vision that proposes the existence of three antagonistic colour systems: a blue–yellow system, a red–green system and a black–white system; according to this theory, the blue–yellow and red–green systems are responsible for hue, while the black–white system contributes to perception of brightness and saturation.

optic nerve The bundle of axons of ganglion cells that carries information from the retina to the brain.

perception The process by which the brain selects, organises and interprets sensations.

percepts Meaningful perceptual units, such as images of particular objects.

perceptual constancy The organisation of changing sensations into percepts that are relatively stable in size, shape and colour.

perceptual illusions Perceptual misinterpretations produced in the course of normal perceptual processes.

perceptual interpretation The process of generating meaning from sensory experience.

perceptual organisation The process of integrating sensations into meaningful perceptual units.

phantom limbs Misleading ‘sensations’ from missing limbs.

pheromones Chemicals secreted by organisms in some species that allow communication between organisms.

pitch The psychological property corresponding to the frequency of a sound wave; the quality of a tone from low to high.

place theory A theory of pitch which proposes that different areas of the basilar membrane are maximally sensitive to different frequencies.

proprioceptive senses Senses that provide information about body position and movement; the two proprioceptive senses are kinaesthesia and vestibular sense.

proximity A Gestalt rule of perception which states that, other things being equal, the brain groups objects together that are close to each other.

psychophysics Branch of psychology that studies the relationship between attributes of the physical world and the psychological experience of them.

pupil The opening in the centre of the iris that constricts or dilates to regulate the amount of light entering the eye.

receptive field A region within which a neuron responds to appropriate stimulation.

recognition-by-components The theory that asserts that we perceive and categorise objects in our environment by breaking them down into component parts and then matching the components and the way they are arranged against similar ‘sketches’ stored in memory.

response bias In signal detection theory, the participant’s readiness to report detecting a signal when uncertain; also called decision criterion.

retina The light-sensitive layer of tissue at the back of the eye that transduces light into neural impulses.

rods One of two types of photoreceptors; allow vision in dim light.

schemas Integrated patterns of knowledge stored in memory that organise information and guide the acquisition of new information.

sensation The process by which the sense organs gather information about the environment.

sensory adaptation The tendency of sensory systems to respond less to stimuli that continue without change.

sensory receptors Specialised cells in the nervous system that transform energy in the environment into neural impulses that can be interpreted by the brain.

shape constancy The perception that an object’s shape remains constant despite the changing shape of the retinal image as the object is viewed from varying perspectives.

signal detection theory The theory that experiencing a sensation means making a judgement about whether a stimulus is present or absent.

similarity A Gestalt rule of perception which states that the brain tends to group similar elements within a perceptual field.

simplicity A Gestalt rule of perception which states that people tend to perceive the simplest pattern possible.

size constancy The perception that the shape of objects remains unchanged in spite of the fact that different impressions are made on the retina each time the object is encountered.

sound localisation Identifying the location of a sound in space.

sound wave A pulsation of acoustic energy.

Stevens’ power law A law of sensation proposed by S. S. Stevens, which states that the subjective intensity of a stimulus grows as a proportion of the actual intensity raised to some power.

subliminal perception Perception of stimuli below the threshold of consciousness.

tastebuds Structures that line the walls of the papillae of the tongue (and elsewhere in the mouth) that contain taste receptors.

timbre The psychological property corresponding to a sound wave’s complexity; the texture of a sound.

top-down processing Perceptual processing that starts with the observer’s expectations and knowledge.

transduction The process of converting physical energy into neural impulses.

trichromatic theory A theory of colour vision initially proposed by Thomas Young and modified by Herman von Helmholtz that proposes that the eye contains three types of receptors, each sensitive to wavelengths of light that produce sensations of blue, green and red; by this theory, the colours that humans see reflect blends of the three colours to which the retina is sensitive; also called the Young–Helmholtz theory.

vestibular sense The sense that provides information about the position of the body in space by sensing gravity and movement.

visual cliff A clear table with a checkerboard directly beneath it on one side and another checkerboard that appears to drop off like a cliff on the other, used especially with human infants in depth perception studies.

wavelength The distance over which a wave of energy completes a full oscillation.

Weber’s law The perceptual law described by Ernst Weber that states that for two stimuli to be perceived as differing in intensity, the second must differ from the first by a constant proportion.

‘what’ pathway The pathway running from the striate cortex in the occipital lobes through the lower part of the temporal lobes, involved in determining what an object is.

‘where’ pathway The pathway running from the striate cortex through the middle and upper regions of the temporal lobes and up into the parietal lobes, involved in locating an object in space, following its movement and guiding movement towards it.

Young–Helmholtz theory A theory of colour vision initially proposed by Young and modified by Helmholtz which proposes that the eye contains three types of receptors, each sensitive to wavelengths of light that produce sensations of blue, green and red; by this theory, the colours that humans see reflect blends of the three colours to which the retina is sensitive; also called trichromatic theory.