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Sensation

& Perception -kirja: chapter summaries

Chapter 1 Summary

INTRODUCTION

1. Sensation and perception are central to, and often precede, almost

all aspects of human behavior and thought. There are many practical

applications of our increased understanding of sensation and perception.

2. Gustav Fechner invented several clever methods for measuring the

relationship between physical changes in the world and consequent psychological

changes in observers. These methods remain in use today. Using Fechner’s

methods, researchers can measure the smallest levels of stimulus that can be

detected (absolutethreshold) and smallest differences that can be detected

(difference thresholds, or just noticeable differences).

3. A more recent development for understanding performance

signaldetection theory—permits us to simulate changes in the perceiver (e.g.,

internal noise and biases) in order to understand perceptual performance

better.

4. We learn a great deal about perception by understanding the

biological structures and processes involved. One early observation—the

doctrine of specific nerve energies—expresses the fact that people are aware

only of the activity of our nervous systems. For this reason, what matters is

which nerves are stimulated, nothow they are stimulated. The central nervous

system reflects specializations for the senses from cranial nerves to areas of

the cerebral cortex involved in perception.

5. The essential activities of all neurons, including those involved in

sensory processes, are chemical and electrochemical. Neurons communicate with

each other through neurotransmitters, molecules that cross the synapse from the

axon of one neuron to the dendrite of the next. Nerve impulses are

electrochemical; voltages change along the axon as electrically charged ions

(sodium and potassium) pass in and out of the membranes of nerve cells.

6. Recordings of individual neurons enable us to measure the lowest

level of stimulus required for a neuron to fire (absolute threshold). Both the

rate and the timing pattern of neural firing provide additional information

about how the brain encodes stimuli in the world.

7. Electroencephalography (EEG) and magnetoencephalography (MEG) measure

the activity of many neurons with great precision intiming, but not brain

location.

8. Positron emission tomography (PET) and functional magnetic resonance

imaging (fMRI) measure metabolic changes to precisely localize the activity of

many neurons in the brain, but with little precision in timing.

9. Fourier analysis is a mathematical tool that helps researchers break

down complex sounds and images in ways that permit better understanding of how

sounds and images are sensed and perceived.

Chapter 2 Summary

The

First Steps in Vision: From Light to Neural Signals

1. This chapter provided some insight into the complex journey that is

required for us to see stars and other spots of light. The path of the light

was traced from a distant star through the eyeball and to itsabsorption by

photoreceptors and its transduction into neural signals. In subsequent chapters

we’ll learn how those signals are transmitted to the brain and translated into

the experience of perception.

2. Light, on its way to becoming a sensation (a visual sensation, that

is), can be absorbed, scattered, reflected, transmitted, or refracted. It can

become a sensation only when it’s absorbed by a photoreceptor in the retina.

3. Vision begins in the retina, when light is absorbed by rods or cones.

The retina is like a minicomputer that transduces light energy into neural

energy.

4. The retina sends information to the brain via ganglion cells, neurons

whose axons make up the optic nerves. Retinal ganglion cells havecenter

surround receptive fields and are concerned with changes in contrast (the

difference in intensity between adjacent bits of the scene).

5. The visual system deals with large variations in overall light

intensity by (a) regulating the amount of light entering the eyeball, (b) using

different types of photoreceptors in different situations, and (c) effectively

throwing away photons we don’t need.

6. Age-related macular degeneration (AMD) is a disease associated with

aging that affects the macula. The leading cause of visual loss among the

elderly in the United States, AMD gradually destroys sharp central vision,

making it difficult to read, drive, and recognize faces.

7. Retinitis pigmentosa (RP) is a family of hereditary diseases

characterized by the progressive death of photoreceptors and degeneration of

the pigment epithelium. In the most common form of the disease, patients first

notice vision problems in theirperipheral vision and under low light

conditions, situations in which rods play the dominant role in collecting

light.

Chapter 3 Summary

Spatial

Vision: From Spots to Stripes

1. In this chapter we followed the path of image processing from the

eyeball to the brain. Neurons in the cerebral cortex translate the array of

stars perceived by retinal ganglion cells into the beginnings of forms and

patterns. The primary visual cortex is organized into thousands of tiny

computers, each responsible for determining the orientation, width, color, and

other characteristics of the stripes in one small portion of the visual field.

In Chapter 4 we will continue this story by seeing how other parts of the brain

combine the outputs from these minicomputers to produce a coherent

representation.

2. Perhaps the most important feature of image processing is the

remarkable transformation of information from the circularreceptive fields of

retinal ganglion cells to the elongated receptive fields of the cortex.

3. Cortical neurons are highly selective along a number of dimensions,

including stimulus orientation, size, direction of motion, and eye of origin.

4. Neurons with similar preferences are often arranged in columns in

primary visual cortex.

5. Selective adaptation provides a powerful, noninvasive tool for

learning about stimulus specificity in human vision.

6. The human visual cortex contains pattern analyzers that are specific

to spatial frequency and orientation.

7. Normal visual development requires normal visual experience. Abnormal

visual experience early in life can cause massive changes in cortical

physiology that result in a devastating and permanentloss of spatial vision.

Chapter 4 Summary

Perceiving

and Recognizing Objects

1. A series of extrastriate visual areas continue the work of visual

processing. Emerging from V1 (primary visual cortex) are two broad streams of

processing: one going into the temporal lobe and the other into the parietal lobe.

The temporal pathway seems specifically concerned with what a stimulus might

be. This chapter follows that pathway. (The parietal wherepathway will be

considered in later chapters.)

2. After early visual processes have extracted basic features from the

visual input, it is the job of middle vision to organize these features into

the regions, surfaces, and objects that can, in turn, serve as input to object

recognition and scene-understanding processes.

3. Perceptual “committees” serve as an important metaphor in this

chapter. The idea is that many semi-independentprocesses are working on the

input at the same time. Different processes may come to different conclusions

about the presence of an edge or the relationship between two elements in the

input. Under most circumstances, we see the single conclusion that the

committees settle on. The math of Bayes’ theorem is one way to formalize this

process of finding the most likely explanation for input.

4. Multiple processes seek to carve the input into regions and to define

the edges of those regions, and many rules are involved in this parsing of the

image. For example, image elements are likely to group together if they are

similar in color or shape, if they are near each other, or if they are

connected to each other. Many of these grouping principles were first

articulated by members of the Gestalt school.

5. Other, related processes seek to determine if a region is part of a

foreground figure (like this black O) or part of the background (like the white

area around the O). These rules of grouping and figure–ground assignment are

driven by an implicit understanding of the physics of the world. Thus, events

that are very unlikely to happen by chance (e.g., two contours parallel to each

other) are taken to have meaning. (Those parallel contours are likely to be

part of the same figure.)

6. The processes that divide visual input into objects and background

have to deal with many complexities. Among these are occlusion—the fact that

parts of objects may be hidden behind other objects, and the fact that objects

themselves have a structure. Is your nose an object or a part of a larger

whole? What about glasses or hair or a wig?

7. Template models of object recognition hold that an object in the

world is recognized when its image fits a particular representation in the

brain in the way that a key fits a lock. It has always been hard to see how

naïve template models could work, because of the astronomical number of

templates required: we might need one “lock” for every object in every

orientation in every position in the visual field.

8. Structure models propose that objects are recognized by the

relationship of parts. Thus, an H could be defined as two parallel lines with a

perpendicular line joining them between their centers. A cat would be more

difficult, but similar in principle. In their pure form, such models are

viewpoint-independent. The orientation of the H doesn’t matter. Object

recognition, however, is often viewpoint-dependent, suggesting that the correct

model lies between the extremes of naïve template matching and pure structural

description.

9. Faces are an interesting special case in which viewpoint is very

important. Upright faces are much easier to recognize than inverted faces.

Moreover, some regions of the brain seem to be specifically interested in

faces. They lie near other regions in the temporal lobes that are important for

recognition of other sorts of objects.

10. Recent physiological work showed very specific responses to very

specific objects (e.g., the actress Jennifer Aniston) in the human temporal

lobe. Other work showed that the first, rough acts of object recognition take

place so fast that they must be accomplished by the first, feed-forward sweep

of activity from the to the higher processing centers of the visual system.

However, routine perception of objects requires feedbackfrom higher visual

areas to those lying earlier in the pathway.

Chapter 5 Summary

The

Perception of Color

1. Probably the most important fact to know about color vision is that

lights and surfaces look colored because a particular distribution

ofwavelengths of light is being analyzed by a particular visual system. Color

is a mental phenomenon, not a physical phenomenon. Many animal species have

some form of color vision. It seems to be important for identifying possible

mates, possible rivals, and good things to eat. Color vision has evolved

several times in several different ways in the animal kingdom.

2. Rod photoreceptors are sensitive to low (scotopic) light levels.

There is only one type of rod photoreceptor. It yields one “number” for each

location in the visual field. Rods can support only a one-dimensional

representation of color from dark to light. Thus, scotopic vision is achromatic

vision.

3. There are three types of cone photoreceptors, each having a different

sensitivity to the wavelengths of light. Cones operate at brighter light levels

than rods, producing three “numbers” at each location; the pattern of activity

over the different cone types defines the color.

4. If two regions of an image produce the same response in the three

cone types, they will look identical; that is, they will be metamers. And they

will look identical even if the physical wavelengths coming from the two

regions are different.

5. In additive color mixture, two or more lights are mixed. Adding a

light that looks blue to a light that looks yellow will produce a light that

looks white (if we pick the right blue and yellow). In subtractive color

mixture, filters, paints, or other pigments that absorb some wavelengths and

reflect others are mixed. Mixing a typical blue paint and a typical yellow

paint will subtract most long and short wavelengths from the light reflected by

the mixture, and the result will look green.

6. Color blindness is typically caused by the congenital absence or

abnormality of one cone type—usually the L- or M-cone, usually inmales. Most

color-blind individuals are not blind to differences in wavelength. Rather,

their color perception is based on the outputs of two cone types instead of the

normal three.

7. A single type of cone cannot be used, by itself, to discriminate

between wavelengths of light. To enable discrimination, information from the

three cones is combined to form three cone-opponent processes. Cones sensitive

to long wavelengths (L-cones) are pitted against medium-wavelength (M) cones to

create an (L – M) process that is roughly sensitive to the redness or greenness

of a region. (L + M) cones are pitted against short-wavelength (S) cones to

create a process roughly sensitive to the blueness or yellowness of a region.

The third process is sensitive to the overall brightness of a region.

8. Color appearance is arranged around opponent colors: red versusgreen,

and blue versus yellow. This color opponency involves further reprocessing of

the cone signals from cone-opponent processes into color-opponent processes.

9. The visual system tries to disentangle the properties of surfaces in

the world (e.g., the “red” color of a strawberry) from the properties of the

illuminant (e.g., the “golden” light of evening), even though surface and

illuminant information are combined in the input to the eyes. Mechanisms of

color constancy use implicit knowledge about the world to correct for the

influence of different illuminants and to keep the strawberry looking red under

a wide range of conditions.

Chapter 6 Summary

Space

Perception and Binocular Vision

1. Reconstructing a three-dimensional world from two, non-Euclidean,

curved, two-dimensional retinal images is one basic problem faced by the brain.

2. A number of monocular cues provide information about

three-dimensional space. These include occlusion, various size and position

cues, aerial perspective, linear perspective, motion cues, accommodation, and

convergence.

3. Having two eyes is an advantage for a number of reasons, some of

which have to do with depth perception. It is important to remember, however,

that it is possible to reconstruct the three-dimensional world from a single

two-dimensional image. Two eyes have other advantages over just one: expanding

the visual field, permitting binocular summation, and providing redundancy if

one eye is damaged.

4. Having two laterally separated eyes connected to a single brain also

provides us with important information about depth through the geometry of the

small differences between the images in each eye. These differences, known as

binocular disparities, give rise to stereoscopic depth perception.

5. Random dot stereograms show that we don’t need to know what we’re

seeing before we see it in stereoscopic depth. Binocular disparity alone can

support shape perception.

6. Stereopsis has been exploited to, literally, add depth to

entertainment—from nineteenth-century photos to twenty-first-century movies. It

has also served to enhance the perception of information in military and

medical settings.

7. The difficulty of matching an image element in one eye with the

correct element in the other eye is known as the correspondence problem. The

brain uses several “tricks” to solve the problem. For example, it reduces the

initial complexity of the problem by matching large “blobs” in the

low-spatial-frequency information before trying to match every high-frequency

detail.

8. Single neurons in the primary visual cortex and beyond havereceptive

fields that cover a region in three-dimensional space, not just the

two-dimensional image plane. Some neurons neurons seem to be concerned with a

crude in-front/behind judgment. Other neurons are concerned with more precise,

metrical depth perception.

9. When the stimuli on corresponding loci in the two eyes are different,

we experience a continuous perceptual competition between the two eyes known as

binocular rivalry. Rivalry is part of the effort to make the best guess about

the current state of the world based on the current state of the input.

10. All of the various monocular and binocular depth cues are combined

(unconsciously) according to what prior knowledge tells us about the

probability of the current event. Making the wrong guess about the cause of

visual input can lead to illusions. Bayes’ theorem is the basis of one type of

formal understanding of the rules of combination.

11. Stereopsis emerges suddenly at about 4 months of age in humans, and

it can be disrupted through abnormal visual experience during acritical period

early in life.

Chapter 7 Summary

Attention

and Scene Perception

1. Attention is a vital aspect of perception because we cannot process

all of the input from our senses. The term attention refers to a large set of

selective mechanisms that enable us to focus on some stimuliat the expense of

others. Though this chapter talked almost exclusively about visual attention,

attentional mechanisms exist in all sensory domains.

2. In vision, it is possible to direct attention to one location or one

object. If something happens at an attended location, we will befaster to

respond to it. It can be useful to refer to the “spotlight” of attention,

though deployments of attention differ in important ways from movements of a

spotlight.

3. In visual search tasks, observers typically look for a target item

among a number of distractor items. If the target is defined by a salient basic

feature, such as its color or orientation, search is very efficient and the number

of distractors has little influence on the reaction time (the time required to

find the target). If no basic feature information can guide the deployment of

attention, then search is inefficient, as if each item needed to be examined

one after the other. Search can be of intermediate efficiency if some feature

information is available (e.g., if we’re looking for a red car, we don’t need

to examine the blue objects in the parking lot).

4. Search for objects in real scenes is guided by the known features of

the objects, by the salient features in the scene, and by a variety of

scene-based forms of guidance. For example, if you’re looking for your can of

soda, you will guide your attention to physically plausible locations

(horizontal surfaces) and logically sensible places (the desk or counter,

probably not the floor).

5. Attention varies over time as well as space. In the attentional-blink

paradigm, observers search for two items in a rapid stream of stimuli that

appear at fixation. Attention to the first target makes ithard to find the

second if the second appears within 200 to 500 ms of the first. When two

identical items appear in the stream of stimuli, a different phenomenon,

repetition blindness, makes it hard to detect the second instance.

6. The effects of attention manifest themselves in several different

ways in the brain. In some cases, attention is marked by a generalincrease in

neural activity. In other cases, attention to a particular attribute tunes

cells more sharply for that attribute. And in still other cases, attention to a

stimulus or location causes receptive fields to shrink so as to exclude

unattended stimuli. All of these effects might be the result of a single,

underlying physiological mechanism of attention.

7. Damage to the parietal lobe of the brain produces deficits in visual

attention. Damage to the right parietal lobe can lead to neglect, a disorder in

which it is hard to direct attention into the contralesional (in this case, the

left) visual field. Neglect patients may ignore half of an object or of their

own body. Balint syndrome is the result of bilateral damage to the parietal

lobe. Patients with this disorder may show simultagnosia, an inability to see

more than one object at a time.

8. Scene perception involves both selective and nonselective processing.

Tasks like visual search make extensive use of selectiveprocessing.

Non-selective processing allows observers to appreciate the mean and variance

of features across many objects (or proto-objects). Thus, you know the average

orientation of trees in the woods (vertical) before knowing whether any

particular tree is oriented perfectly vertically. Using spatial-frequency

information, even without segmenting the scene into regions and objects, the

nonselective pathway can provide information about the nature of a scene (e.g.,

whether it’s natural or man-made).

9. Picture memory experiments show that people can remember thousands of

images after only a second or two of exposure to each. In contrast, change

blindness experiments show that people can miss large changes in scenes if

those changes do not markedly alter the meaning of the scene.

10. Our perceptual experience of scenes consists of nonselective

processing of the layout and ensemble statistics of the scene, combined with

selective processing of a very few objects at each moment. However, the final

experience is an inference based on all of the preceding processing, not merely

the sum of that processing. Usually this theory is adequate because we can

rapidly check the world to determine whether the chair, the book, and the desk

are still there. In the lab, however, we can use phenomena like inattentional

blindness and change blindness to reveal the limits of our perception, and it

is becoming increasingly clear that those limits can have real-world

consequences.

Chapter 8 Summary

Motion

Perception

1. Like color or orientation, motion is a primary perceptual dimension

that is coded at various levels in the brain. Motion information is used to

determine where objects are going and when they’re likely to get there, and to

help us move through our environment without being hit in the head by flying

objects.

2. We can build a simple motion-detecting circuit by using linear

filters that delay and sum information (and are followed by nonlinearities).

3. V1 neurons view the world through a small window, leading to the

well-known aperture problem (that is, a V1 neuron is unable to tell which

elements correspond with one another when an object moves through its receptive

field).

4. Strong physiological and behavioral evidence that the middletemporal

area (MT) is involved in the perception of global motion.

5. Aftereffects for motion, like those for orientation or color, can

provide important insights into the underlying mechanisms in humans.

6. Luminance-defined (first-order) motion and contrast- or

texture-defined (second-order) motion appear to be analyzed by separate

systems.

7. The brain has to figure out which retinal motion arises in the world,

and which arises because of eye movements. Moreover, the brain must suppress

the motion signals generated by our eye movements, or the world will be pretty

“smeared.”

8. Motion information is critically important to us for navigating

around our world, avoiding imminent collision, and recognizing the movement of

animals and people.

Chapter 9 Summary

Hearing:

Physiology and Psychoacoustics

1. Sounds are fluctuations of pressure. Sound waves are defined by the

frequency, intensity (amplitude), and phase of fluctuations. Sound frequency

and intensity correspond to our perceptions of pitch andloudness, respectively.

2. Sound is funneled into the ear by the outer ear, made more intense by

the middle ear, and transformed into neural signals by theinner ear.

3. In the inner ear, cilia on the tops of inner hair cells are flexed by

pressure fluctuations in ways that provide information about frequency and

intensity to the auditory nerve and the brain. Auditory nerve fibers convey

information through both the rate and the timing patterns with which they fire.

4. Different characteristics of sounds are processed at multiple places

in the brain stem before information reaches the cortex. Information from both

ears is brought together very early in the chain of processing. At each stage

of auditory processing, including primary auditory cortex, neurons are

organized in relation to the frequencies of sounds (tonotopically).

5. Humans and other mammals can hear sounds across an enormousrange of

intensities. Not all sound frequencies are heard as being equally loud,

however. Hearing across such a wide range of intensities is accomplished by the

use of many auditory neurons. Different neurons respond to different levels of

intensity. In addition, more neurons overall respond when sounds are more intense.

6. Series of channels (or filters) process sounds within bands of

frequency. Depending on frequency, these channels vary in how wide (how many

frequencies) or narrow they are. Consequently, it is easier to detect

differences between some frequencies than between others. When energy from

multiple frequencies is present, lower-frequency energy makes it relatively

more difficult to hear higherfrequencies.

7. Hearing loss is caused by damage to the bones of the middle ear, to

the hair cells in the cochlea, or to the neurons in the auditory nerve.

Although hearing aids are helpful to listeners with hearing impairment, they

cannot restore hearing as well as glasses can improve vision.

Chapter 10 Summary

Hearing

in the Environment

1. Listeners use small differences, in time and intensity, across the

two ears to learn the direction in the horizontal plane (azimuth) from which a

sound comes.

2. Time and intensity differences across the two ears are not sufficient

to fully indicate the location from which a sound comes. In particular, time

and intensity differences are not sufficient to indicate whether sounds come

from the front or the back, or from higher or lower (elevation).

3. The pinna, ear canal, head, and torso alter the intensities of

different frequencies for sounds coming from different places in space, and

listeners use these changes in intensity across frequency to identify the

location from which a sound comes.

4. Perception of auditory distance is similar to perception of

visualdepth because no single characteristic of the signal can inform a

listener about how distant a sound source is. Listeners must combine intensity,

spectral composition, and relative amounts of direct and reflected energy of

sounds to estimate distance to a sound source.

5. Many natural sounds, including musical instruments and human speech,

have rich harmonic structure with energy at integer multiples of the timbre

frequency, and listeners are especially good at perceiving the pitch of

harmonic sounds.

6. Important perceptual qualities of complex sounds are timbre(conveyed

by the relative amounts of energy at different frequencies) and the onset and

offset properties of attack and decay, respectively.

7. Because all the sounds in the environment are summed into a single

waveform that reaches each ear, a major challenge for hearing is to separate

sound sources in the combined signal. This general process is known as auditory

scene analysis. Sound source segregation succeeds by using multiple

characteristics of sounds, including spatial location, similarity in frequency

and timbre, onset properties, and familiarity.

8. In everyday environments, sounds to which a person is listening often

are interrupted by other, louder sounds. Perceptualrestoration is a process by

which missing or degraded acoustic signals are perceptually replaced.

Chapter 11 Summary

Music

and Speech Perception

1. Musical pitch has two dimensions: tone height and tone chroma.

Musical notes are combined to form chords. Notes and chords vary in duration

and are combined to form melodies.

2. Melodies are learned psychological entities defined by patterns of

rising and falling musical pitches, with different durations and rhythms.

3. Rhythm is important to music, and to auditory perception more broadly.

The process of perceiving sound sequences is biased tohear rhythm.

4. Humans evolved to be able to produce an extremely wide variety of

sounds that can be used by languages. The production of speech sounds has three

basic components: respiration, phonation, and articulation. Speech sounds vary

in many dimensions, including intensity, duration, periodicity, and noisiness.

5. In terms of articulation and acoustics, speech sounds vary according

to other speech sounds that precede and follow (coarticulation). Because of

coarticulation, listeners cannot use any single acoustic feature to identify a

vowel or consonant. Instead, listeners must use multiple properties of the

speech signal.

6. In general, listeners discriminate speech sounds only as well as they

can label them. This is categorical perception, which also has been shown for

the perception of many other complex familiar auditory and visual stimuli.

7. How people perceive speech depends very much on their experience with

speech sounds within a language. This experience includes learning which of the

many acoustic features in speech tend to co-occur. Because of the role of

experience in how we hear speech, it is often difficult to perceive and produce

new speech sounds from a second language following experience with a first

language.

8. One of the ways that infants learn words is to use their experience

with the co-occurrence of speech sounds.

9. Speech sounds are processed in both hemispheres of the brain much

like other complex sounds, until they become part of the linguistic message.

Then, speech is further processed in anterior and ventral regions, mostly in

the left superior temporal cortex, but also in the superior posterior temporal

cortex.

Chapter 12 Summary

Spatial

Orientation and the Vestibular System

1. The vestibular organs are a set of sense organs located in the inner

ear that sense head motion and gravity and contribute to our sense of spatial

orientation.

2. The vestibular organs include three semicircular canals (horizontal,

anterior, and posterior) that sense angular motion, and two otolithorgans

(utricle and saccule) that sense both gravity and linear acceleration.

3. Hair cells are the mechanoreceptors that convert both orientation

with respect to gravity and head motion into signals that are sent to the

brain.

4. The vestibular organs make a predominant contribution to our sense of

spatial orientation, which includes three sensory modalities: linear motion,

angular motion, and tilt. Direction and amplitude are qualities that define

each of these three sensory modalities.

5. Our sense of spatial orientation results from a combination of

information from multiple sensory systems. The vestibular andvisual systems

make predominant contributions to our sense of spatial orientation.

6. The vestibular system contributes to our sense of spatial

orientation, but the brain processes the vestibular to yield perceptions that

differ substantially from the responses found on the afferentneurons.

7. We are exquisitely sensitive to head motion, even in the dark,

recognizing the directions of rotation, linear motion, and tilt at verylow

thresholds.

8. The vestibular system contributes to dynamic visual acuity by evoking

compensatory vestibulo-ocular reflexes (VORs).

9. The vestibular system helps maintain balance via postural reflexes.

10. No area of the cortex exclusively devoted to processing vestibular

information has been identified. Areas of the cortex that respond to vestibular

stimuli also respond to visual and other sensory stimuli.

11. Vestibular problems are widespread, and treatments are limited. For

Ménière’s syndrome patients, for example, the symptoms may become so disabling

that patients accept treatments that yield permanent disability just to be rid

of the symptoms.

Chapter 13 Summary

Touch

1. The sense of touch produces a number of distinct sensory experiences.

Each type of experience is mediated by its ownsensory receptor system(s).

Tactile receptors are responsive not only to pressure, but also to vibration,

changes in temperature, and noxious stimulation. The kinesthetic system, which

also contributes to our sense of touch, is further involved in sensing limb

position and the movement of our limbs in space. Pleasant or emotional touch is

another form of sensory specialization.

2. Four classes of pressure-sensitive (mechano-) receptors have been

found within hairless skin, and another five classes within hairy skin. The

organs used to sense limb position and movement (namely, our muscles, tendons,

and joints) are more deeply situated within the body. Thermoreceptors respond

to changes in skin temperature that occur, for example, when we contact objects

that are warmer or cooler than our bodies. Nociceptors signal tissue damage (or

its potential) and give rise to sensations of pain.

3. The pathways from touch receptors to the brain are complex. Two major

pathways have been identified: a fast one (the dorsal column–medial lemniscal

pathway) that carries information from mechanoreceptors, and a slower one (the

spinothalamic pathway) that carries thermal and nociceptive information. Only

the second pathway synapses when it first enters the spinal cord. These

pathways project to the thalamus and from there to the primary somatosensory

area, located in the parietal lobe just behind the central sulcus. This area

contains several somatotopically organized subregions, in which adjacent areas

of the body project to adjacent areas of the brain. The neural organization of

the brain for touch has been shown to be remarkably plastic, even in adults.

4. Downward pathways from the brain play an important role in the

perception of pain. According to the gate control theory, signals along these

pathways interact at the spinal cord with those from the periphery of the body.

Such interactions can block the pain signals that would otherwise be sent

forward to the brain. The sensation of pain is further moderated by areas in

the cortex.

5. Investigators have measured sensitivity to mechanical force by

applying nylon hairs of different diameters to the skin. They determine spatial

acuity of the skin by measuring the two-point touch threshold, and more

precisely by discriminating the orientation of gratings applied to the skin.

Tactile pressure sensitivity and spatial acuity vary with body site because of

varying concentrations of different types of mechanoreceptors. The minimum

depression of the skin needed to feel a stimulus vibrating at a particular rate

(frequency) provides a measure of vibration sensitivity.

6. The sense of touch is intimately related to our ability to perform

actions. Signals from the mechanoreceptors are necessary for simple actions

such as grasping and lifting an object. Conversely, our own movements determine

how touch receptors respond and, hence, which properties of the concrete world

we can feel. Touch is better adapted to feeling the material properties of

objects than it is to feeling their geometric features (e.g., shape),

particularly when an object is large enough to extend beyond the fingertip.

7. Like other sensory modalities, touch gives rise to internal

representations of the world, which convey the positions of objects using the

body as a spatial reference system. Touch-derived representations are inputs to

higher-level functions like allocation of attention and integration with

information from other modalities.

8. The psychological study of touch is useful for a number of

applications. Virtual touch environments that transmit forces to the touch

receptors can provide a basis for training people to perform remote operations

like surgery and perhaps, in the future, will convey the illusion of touched

objects over the Internet.

Chapter 14 Summary

Olfaction

1. Olfaction is one of the two chemical senses; the other is taste

(discussed in the next chapter). To be perceived as scent, a chemical must

possess certain physical properties; however, even some molecules that possess

these characteristics cannot be smelled. Olfaction has some unique

physiological properties, one of which is that only about 35% of the genes that

code for olfactory receptors in humans are functional. Another unusual feature

is that most smells also stimulate the somatosensory system via the trigeminal

nerve, and it is often impossible to distinguish the contribution of olfactory

sensation from trigeminal stimulation.

2. The dominant biochemical theory of odor perception—shape-pattern

theory—contends that the fit between a molecule and an olfactory receptor (OR)

determines which molecules are detected as scents, and that specific odorant

molecules activate arrays of ORs, producing specific patterns of neural

activation for each perceived scent. However, this theory is not universally

accepted, and alternate explanations exist (e.g., vibration theory).

3. Recently, researchers demonstrated a closer connection between

thevisual system and olfaction than has ever before been thought to exist. Two

examples are binaral rivalry and the observation that what we’re smelling

influences what we concomitantly register as seeing. There is also a difference

between active sniffing and passive inhalation of odors at both neurological

and functional levels. Active sniffing may also have therapeutic applications

for patients suffering from extreme physical disabilities.

4. Almost all odors that we encounter in the real world are mixtures,

and we appear not to be very good at analyzing the discrete chemical components

of scent mixtures. Olfaction is thus primarily a synthetic, as opposed to

analytical, sense. However, analytical olfactory ability can be developed with

training. True odor imagery is also weak (or nonexistent) for most people, but

training, as in the case of odor experts, appears to facilitate this ability.

5. The psychophysical study of smell has shown that various odorant

intensity levels and various cognitive functions are required for odor

detection, discrimination, and recognition. Identification differs from odor

recognition in that, in the former, one must come up with a name for the

olfactory sensation. It is very difficult to name even very familiar odors—an

experience known as the tip-of-the-nose phenomenon—one of several indications

that linguistic processing is highly disconnected from olfactory experience.

Unlike the case with other sensory experiences, however, we do not need to

access any semantic information about an odor in order to respond to it

appropriately, as long as it is familiar.

6. Another important discrepancy between the physical experience and the

psychological experience of odors is the difference between receptor adaptation

and cognitive habituation. Receptor adaptation occurs after continual odorant

exposure over a number of minutes, can be undone after a few minutes away from

the odorant, and is explained by a basic biochemical mechanism. By contrast,

cognitive habituation occurs after long-term exposure (e.g., in a living or

work environment) to a particular odor and takes weeks away from the odor to

undo. Psychological influences can have strong effects on both perceived odor

adaptation and habituation. At present, the physiological mechanisms

responsible for the cognitive habituation are not fully understood.

7. The most immediate and basic response we have to an odor is whether

we like it or not; this is hedonic evaluation. Odor hedonics are measured by

pleasantness, familiarity, and intensity ratings. Pleasantness and familiarity

are linearly related to odor liking; odor intensity has a more complex

relationship to hedonic perception. Substantial evidence suggests that our

hedonic responses to odors are learned and not innate, even for so-called

stenches. That we have learned to like or dislike various odors rather than

being born with hardwired responses is evolutionarily adaptive for

generalistspecies like humans. The caveats to the learned proposition are odors

that are highly trigeminally irritating (pain-inducing) and the potential

genetic variability in the number and types of receptors expressed across

individuals, which may influence olfactory sensitivity (intensity) and hence

odor hedonic perception.

8. The key to olfactory associative learning is the emotional value of

the context in which the odor is first encountered. If the emotional context is

good, the odor will be liked; if it is bad, the odor will be disliked.

Previously acquired emotional associations with odors also underlie validated

aromatherapy effects. Emotional potency further distinguishes odor-evoked

memories from memories triggered by other sensory cues. The neuroanatomy of the

olfactory and limbic systems and their neuroevolutionary development illustrate

how emotional processing and olfactory processing are uniquely and intimately

interrelated.

9. Pheromones are chemicals emitted by individuals that affect the

physiology and/or behavior of other members of the same species and may or may

not have any smell. In all mammals that have been shown to use pheromones for

communication, detection is mediated through the vomeronasal organ and

processed by the accessory olfactory bulb. Humans do not possess a functional

VNO or AOB, and evidence for human pheromones is controversial. However, human

chemosignals that are processed through the olfactory system appear to have

some influence on hormonal status and sexual arousal.

Chapter 15 Summary

Taste

1. Flavor is produced by the combination of taste and retronasal

olfaction (olfactory sensations produced when odorants in the mouth are forced

up behind the palate into the nose). Flavor sensations are localized to the

mouth, even though the retronasal olfactory sensations come from the olfactory

receptors high in the nasal cavity.

2. Taste buds are globular clusters of cells (like the segments in an

orange). The tips of some of the cells (microvilli) contain sites that interact

with taste molecules. Those sites fall into two groups: ion channels that mediate

responses to salts and acids, and G protein–coupled receptors that bind to

sweet and bitter compounds.

3. The tongue has a bumpy appearance because of structures called

papillae. Filiform papillae (most numerous) have no taste buds. Taste buds are

found in the fungiform papillae (front of the tongue), foliate papillae (rear

edges of the tongue) and circumvallate papillae (rear center of the tongue), as

well as on the roof the mouth.

4. Taste projects ipsilaterally from the tongue to the medulla, thalamus,

and cortex. It projects first to the insula in the cortex, and from there to

the orbitofrontal cortex, an area where taste can be integrated with other

sensory input (e.g., retronasal olfaction).

5. Taste and olfaction play very different roles in the perception of

foods and beverages. Taste is the true nutritional sense; taste receptors are

tuned to molecules that function as important nutrients. Bitter taste is a

poison detection system. Sweet taste enables us to respond to the sugars that

are biologically useful to us: sucrose, glucose, and fructose. Salty taste

enables us to identify sodium, a mineral crucial to survival because of its

role in nerve conduction and muscle function. Sour taste permits us to avoid

acids in concentrations that might injure tissue.

6. Umami, the taste produced by monosodium glutamate, has been suggested

as a fifth basic taste that detects protein. However, umami lacks one of the

most important properties of a basic taste: hardwired affect. Some individuals

like umami, but others do not. The presence of glutamate receptors in the gut

suggests that protein detection occurs there. Digestion breaks down proteins

into their constituent amino acids, and the glutamate released stimulates gut

glutamate receptors, leading to conditioned preferences for the sensory

properties of the foods containing the protein.

7. The importance of taste to survival requires that we be able to

recognize each taste quality independently, even when it is present in a

mixture. By coding taste quality with labeled lines in much the same way that

frequencies are coded in hearing, nature has ensured that we have this

important capability. These labeled lines are noisy. For example, acids are

able to stimulate fibers mediating saltiness, as well as those mediating

sourness. Thus, acids tend to taste both salty and sour.

8. Foods do not taste the same to everyone. The Human Genome Project

revealed that we carry about 25 genes for bitter taste. The most studied bitter

receptor responds to PROP and shows allelic variation in humans leading to the

designations “PROP nontaster” for those who taste the least bitterness and

“PROP taster” for those who taste the most. In addition, humans vary in the

number of fungiform papillae (and thus taste buds) they possess. Those with the

most taste buds are called supertasters and live in a “neon” taste world; those

with the fewest live in a “pastel” taste world. Psychologists discovered these

differences by testing people’s ability to match sensory intensities of stimuli

from different modalities. For example, the bitterness of black coffee matches

the pain of a mildheadache to nontasters but resembles a severe headache to

supertasters. The way foods taste affects palatability, which in turn affects

diet. Poor diet contributes to diseases like cancer and cardiovascular disease.

9. For taste, unlike olfaction, liking and disliking are hardwired; for

example, babies are born liking sweet and salty and disliking bitter. When we

become deficient in salt or sucrose, liking for salty and sweet tastes,

respectively, increases. Junk foods are constructed to appeal to these

preferences. Liking the burn of chili peppers, on the other hand, is acquired

and, with the exception of some pets, is essentially limited to humans. Because

taste buds are surrounded by pain fibers, supertasters perceive much greater

burn from chilis than do nontasters.