introduction to psychology, 7th edition, james w. kalat chapter 4: sensation and perception chapter...

Introduction to Psychology, 7th Edition, James W. KalatChapter 4: Sensation and Perception

Chapter 4

Sensation and Perception


Sensation and Perception

Sensation is the conversion of energy from the environment into a pattern of response by the nervous system.

Perception is the interpretation of that information.


Module 4.1

Vision


Sensing the World Around Us

Stimuli are energies in the environment that affect what we do.

Receptors are the specialized cells in our bodies that convert environmental energies into signals for the nervous system.


The Detection of Light

Light is the stimulus that the visual system is designed to detect.

Visible light is just one very small portion of the electromagnetic spectrum, which is the continuum of all the frequencies of radiated energy.

The human eye is designed to detect energy in the wavelengths from 400 to 700 nm.


Figure 4.2 The lens gets its name from Latin for lentil, referring to its shape—an appropriate choice, as this cross section of the eye shows. The names of other parts of the eye also refer to their appearance.


The Structure of the Eye

The pupil is an adjustable opening in the eye through which light enters.

The iris is the structure on the surface of the eye, surrounding the pupil, and containing the muscles that make the pupil dilate or constrict.

The iris gives your eye its characteristic color, too.



The cornea is a rigid, transparent structure on the very outer surface of the eyeball. It focuses light by directing it through the pupil.

When the light goes through the pupil, it is directed to the lens.

The lens is a flexible structure that can vary in thickness, enabling the eye to accommodate, adjusting its focus for objects at different distances.



The lens directs the light through a clear, jellylike substance called the vitreous humor to the back of the eyeball.

At the back of the eye is the retina, the structure containing the visual receptors.


Common Disorders of Vision

Presbyopia develops as humans age because the lens decreases in flexibility, resulting in a reduced ability to focus on nearby objects.

Elongated eyeballs cause myopia, so that the person can focus well on nearby objects, but not distant ones. This condition is also called nearsightedness.

Flattened eyeballs cause hyperopia, so that the person can focus well on distant objects, but not on nearby ones. This is also called farsightedness.


Figure 4.3 The flexible, transparent lens changes shape so that objects (a) far and (b) near can come into focus. The lens bends entering light rays so that they fall on the retina. In old age the lens becomes rigid, and people find it harder to focus on nearby objects


Common Disorders of Vision

Glaucoma is a condition caused by increased pressure within the eyeball, causing damage to the optic nerve and loss of peripheral vision.

A cataract is a disorder in which the lens of the eye becomes cloudy. This disorder is treated by removing and replacing the actual lens with a contact lens.


Concept Check

What happens if a person with normal vision puts on contact lenses designed for a person with myopia?

His vision, especially his near vision, will be blurry.


The Visual Receptors

The retina contains two types of specialized neurons, the rods and the cones.

Rods far outnumber cones in the human eye. About 5-10% of the visual receptors in the human retina are

cones.



The cones are utilized in color vision, daytime vision and detail vision.

The rods are adapted for vision in dim light. Species that are active at night have few cones and many

rods, giving them particularly good night vision.


TABLE 4.1 Differences between rods and cones



The fovea is the center of the human retina, and the location of the highest proportion of cones.

It is the area of the eye with the greatest acuity. Rods are more plentiful in the periphery of the retina.


Concept Check

If you see a brightly colored object in the periphery of your vision, the colors will not seem very bright at all. Why is this?

You have mostly rods in the periphery of your retina, thus a more limited ability to detect color.


Dark Adaptation

Most humans require one or two minutes to see in the dark. This process of gradual improvement is called dark adaptation.

Exposure to light causes molecules of retinaldehydes to be chemically altered and stimulate the visual receptors.


Dark Adaptation

In conditions of normal daytime light, these molecules are depleted and regenerated at about the same rate, so the amount available in the retina is balanced and level of visual sensitivity is constant.

In dim light or darkness the receptors regenerate these molecules without any subsequent depletion, and so the increase in available retinaldehydes also enhances dark adaptation.


Dark Adaptation

Cones and rods perform this regeneration at different rates. Although cones regenerate their retinaldehydes more

quickly, they are also being more heavily used in the daytime.

By the time you need enhanced night vision, the rods have fully regenerated their supply of retinaldehydes.

The relative abundance of rods, and their undisturbed regeneration of the chemical, gives them a much higher level of sensitivity to faint light


Figure 4.8 These graphs show dark adaptation to (a) a light you stare at directly, using only cones, and (b) a light in your peripheral vision, which you see with both cones and rods. (Based on E. B. Goldstein, 1989)


Concept Check:

It is said that dogs and cats can see in the dark – do you think this is really true?

Although these animals have much better vision in dim light than we do, there must be some light present for the rods to function.


Concept Check

In the daytime which predominates, the fovea or periphery of the eye?

Unless you walk into a dark room, you will be using the fovea, because cones are the receptors for daytime (well-lighted) vision.


The Visual Pathway

The visual receptors send their impulses away from the brain, toward the center of the eye. First the bipolar cells gather the impulses from the rods

and cones. Then the bipolar cells make synaptic contacts with

ganglion cells.


Figure 4.7 Because so many rods converge their input into the next layer of the visual system, known as bipolar cells, even a small amount of light falling on the rods can stimulate the bipolar cells. Thus, the periphery of the retina, with many rods, has good perception of faint light. However, because bipolars in the periphery get input from so many receptors, they have only imprecise information about the location and shape of objects.


The Visual Pathway

The axons of the ganglion cells join together to form the optic nerve, which makes a “U-turn” and exits the eye.

There are no photoreceptors at the point at which the nerve leaves the eye. This is called the blind spot.

You are not aware of your blind spot because information from the retina of each eye “fills in” the blind spot in the other eye. This integration occurs in the visual cortex.


The Visual Pathway

At the optic chiasm, half of each optic nerve crosses to go to the opposite side of the brain.

At this point the axons begin to separate, sending information to a number of locations in the brain.

The greatest number of axons goes to the occipital lobe via the thalamus.


Figure 4.9 Axons from cells in the retina depart the eye at the blind spot and form the optic nerve. In humans about half the axons in the optic nerve cross to the opposite side of the brain at the optic chiasm. Some optic nerve axons carry information to the midbrain; others carry it to the thalamus, which relays information to the cerebral cortex.


The Visual Pathway

The information from each retina is integrated in the visual cortex.

Each cell in the cortex receives input from both the left and the right retinas.

When the retinas are focused on the same point in space, the input from each side is easily integrated because the message is from each is almost the same.


The Visual Pathway

If the images conflict with each other, cortical cells will be alternately stimulated and inhibited as they try to integrate the information.

The alternation between seeing the conflicting information from each retina is called binocular rivalry.


Figure 4.11 To produce binocular rivalry, move your eyes toward the page until the two circles seem to merge. You will alternate between seeing red lines and green lines.


The Visual Pathway

The brain activity of the visual cortex is crucial for the sense of vision.

People with intact eyes but a damaged visual cortex lose the ability to imagine visual imagery.


Color Vision

Different wavelengths of electromagnetic energy correspond to different colors of light. There are three kinds of cones that respond to different

wavelengths. Cells in the visual path process the information from

these cones in terms of opposites.


Color Vision

The three types of information are: Red vs. green Yellow vs. blue White vs. black

The cells in the cerebral cortex integrate the input from the parts of the visual field to create a color experience for the objects that we see.


Color Vision

The Young-Helmholtz theory This is also known as the trichromatic theory. It proposes that our receptors respond to three primary

colors. “Color vision depends on the relative rate of response by

the three types of cones.”


Color Vision

Each type of cone is most sensitive to a specific range of electromagnetic wavelengths. Short wavelengths are seen as blue. Medium wavelengths are seen as green. Long wavelengths are seen as red.


Figure 4.13 Sensitivity of three types of cones to different wavelengths of light. (Based on data of Bowmaker & Dartnall, 1980)


Color Vision

Each wavelength induces different levels of activity in each type of cone. For example, light that stimulates the medium and long

wavelength cones about equally will be perceived as yellow.

Light that excites all three types equally is perceived as white.


Color Vision

The Opponent-Process Theory Trichromatic theory does not account for some of the

more complicated aspects of color perception. People experience four colors as primary – red, green,

blue and yellow. People also report seeing colored after-images after

staring at an object of one color. If you stare at a red object, you tend to see a green after-image when you stop staring.


Color Vision

The Opponent-Process Theory Because of these facts, Ewald Hering proposed that we

perceive color not in terms of separate categories but rather in a system of paired opposites.

Red vs. green Yellow vs. blue White vs. black


Color Vision

The Opponent-Process Theory The negative after-images that we experience after

staring at objects are results of the alternating stimulation and inhibition of neurons in the visual system.

A bipolar neuron that responds strongly to yellow will be inhibited by blue.

After you’ve stared at a yellow object, your fatigued bipolar cell will behave as if it’s been inhibited, and yield a sensation of blue.


Concept Check

A bipolar cell is stimulated by red wavelengths. You stare at a red object. What will happen when you stop staring?

You will see a negative afterimage in green.


Concept Check

Why do negative after-images that you see seem to “move around?”

Because the image is in your eye, not from any object at which you are gazing.


Color Vision

The Retinex Theory The trichromatic and opponent-process theory don’t

account for our experience of color constancy. Color constancy is the tendency of an object to appear

nearly the same color even though we see it in a variety of lighting conditions.


Color Vision

The Retinex Theory Edwin Land proposed that we perceive color because

the cerebral cortex compares various retinal patterns (thus the name retina + cortex = “retinex.”)

By comparing different patterns of light from different areas of the retina, cortical cells synthesize a color perception for each area.


Color Vision

The Retinex Theory The fact that certain types of brain damage disrupt color

constancy, causing for example an object to look orange under one level or type of lighting, and red, green, yellow or even white under other conditions, is considered to be strong evidence for the Retinex theory.


Color Vision

Colorblindness Total inability to distinguish colors is very rare except as

a result of brain damage. About 4% of all people are partly colorblind.


Color Vision

Colorblindness Colorblindness can result from the absence of one of the

three types of cones. Colorblindness can also result when one of the three

types of cones is less responsive than the other two. The color that stimulates that type of cone is seen as almost gray.


Color Vision

Colorblindness Red-green colorblindness is the most common type. There are two forms – protanopia, in which the afflicted

person lacks long-wavelength cones, and deuteranopia, in which the person lacks medium-wavelength cones.

Yellow-blue colorblindness (known as trianopia) is very rare.


How We See

Before animals could see color, there was no color. What you see is “in your brain.” Not an exact representation

of the world around you, but a construction and interpretation of many stimuli.

Sensation seems simple, but it is perhaps one of the most challenging areas of this science.


Module 4.2

The Nonvisual Senses


Hearing

The ear is designed to detect and transmit sound waves to the brain. Sound waves are vibrations in the air or other medium. Sound waves vary according to frequency and

amplitude. Frequency is measured by the number of vibrations or

cycles of the sound wave per second, referred to as hertz (Hz.)


Figure 4.20 The period (time) between the peaks of a sound wave determines the frequency of the sound; we experience frequencies as different pitches. The vertical range, or amplitude, of a wave determines the sound’s intensity and loudness.


Hearing

The perception of frequency is referred to as pitch. We perceive a high-frequency sound wave as high-pitched,

and a low-frequency wave as low-pitched. Amplitude is intensity of sound waves and is perceived as

loudness. Pitch and loudness are psychological experiences, and the

perception of these qualities does not solely depend on frequency and amplitude.


Hearing

The Ear The ear is a complex organ. It converts weak sound

waves into waves of pressure that can be transported by sensory neurons and interpreted by the brain.

The cochlea is the location of the hearing receptors. It is a spiral-shaped organ with canals containing

fluid.


Hearing

The Ear Sound waves strike the tympanic membrane, or

eardrum. The vibrations of the eardrum cause three very tiny

bones, the malleus, the incus, and the stapes, (literally the hammer, anvil and stirrup) to work to make the sound waves become stronger signals.

The stirrup causes the cochlea to vibrate. This vibration displaces hair cells along the basilar

membrane within the cochlea.


Figure 4.21 When sound waves strike the eardrum (a), they cause it to vibrate. The eardrum is connected to three tiny bones—the hammer, anvil, and stirrup—that convert the sound wave into a series of strong vibrations in the fluid-filled cochlea (b). Those vibrations displace the hair cells along the basilar membrane in the cochlea, which is aptly named after the Greek word for snail. Here the dimensions of the cochlea have been changed to make the general principles clear.


Hearing

The Ear The hair cells are connected to neurons of the auditory

nerve. The auditory nerve transmits the impulses from the

cochlea to the cerebral cortex.


Hearing

Hearing Loss There are two common forms of deafness.

Conduction deafness results when the three special bones in the ear fail to transmit sound waves properly to the cochlea.

Nerve deafness results from damage to the structures that receive and transmit the impulses - the cochlea, hair cells or auditory nerve.


Hearing

Pitch Perception Adult humans can hear sound waves approximately

between 15 and 20,000 Hz. How we hear pitch depends in part on the frequency to

which we are listening.


Hearing

Pitch Perception At low frequency (up to about 100 Hz), we hear by the

workings of the frequency principle. Sound waves passing through the fluid in the cochlea

cause all the hair cells to vibrate, producing action potentials that are synchronized with the sound waves.


Hearing

Pitch Perception At about 100-4000 Hz, we hear by the workings of the

volley principle. Fewer hair cells can fire at this pace, but those that do

respond in groups, called volleys, and produce action potentials.

Volleys are the chief mechanism for transmitting most speech and music to the brain.


Figure 4.22 The auditory system responds differently to low-, medium-, and high-frequency tones. (a) At low frequencies hair cells at many points along the basilar membrane produce impulses in synchrony with the sound waves. (b) At medium frequencies different cells produce impulses in synchrony with different sound waves, but the group as a whole still produces one or more impulses for each wave. (c) At high frequencies only one point along the basilar membrane vibrates; hair cells at other locations remain still.


Hearing

Pitch Perception Beyond 4000 Hz, we hear by the workings of the place

principle. The place principle states that the location of the hair

cells stimulated by the sound waves depends on their frequency.

The highest frequency sounds vibrate hair cells near the stirrup.

Between 100 and 4000 Hz, we are hearing due to the combined effects of the volley and place principles.


Concept Check

You are listening to your mother on the telephone. Which principle(s) of hearing are operating to help you hear her?

Volley and place


Hearing

Localization of sounds How does the auditory system determine the source of a

sound? To estimate the approximate location of origin of a

sound, the auditory system compares the messages received by the two ears.

The sound waves will arrive at the closer ear slightly sooner (if coming from the right, it arrives at the right ear just a little before it arrives at the left ear.)


Figure 4.23 The stereophonic hearing of our ears enables us to determine where a sound is coming from. The ear located closest to the sound will receive the sound waves first. A change of less than one ten-thousandth of 1 second can alter our perception of the location of a sound source.


Hearing

Localization of sounds The distance of a sound can be estimated based on

loudness and pitch. A sound that is growing louder is interpreted as

approaching. A higher frequency sound is interpreted as nearer than a

low frequency sound; a sound that is increasing in pitch is interpreted as approaching.

The only cue for absolute distance is the amount of reverberation experienced by the listener.


Concept Check

If a person who uses hearing aids in both ears is only wearing one in the right ear, what will be the effect on sound localization?

Sounds may be interpreted as coming from the right even when they aren’t.


Concept Check

Why is it hard to tell whether a sound originates in front or behind you?

Because the sounds arrive in both ears at the same time, there is no basis for comparison of the source of the sound.


The Vestibular Sense

What we generally call balance is the vestibular sense. The vestibule is a structure in the inner ear on each

side of the head. Changes in the position of the vestibule cause receptors

to be stimulated. These receptors tell the brain the direction of tilt, amount

of acceleration and position of the head with respect to gravity.

The vestibular sense plays a crucial role in maintaining balance and posture.



The structure of the vestibular system Three semicircular canals are oriented in three

directions. These canals contain a jellylike substance and are lined

with hair cells. Acceleration causes the jellylike substance to move the

hair cells, stimulating them.



The structure of the vestibular system Hair cells are also contained in two otolith organs. The otoliths are calcium carbonate particles. These particles stimulate different sets of hair cells,

depending on which way the head tilts. They are telling your brain “which way is up.”


Figure 4.24 (a) Location of and (b) structures of the vestibule. (c) Moving your head or body displaces hair cells that report the tilt of your head and the direction and acceleration of movement.


The Cutaneous Senses

Touch is actually considered to be several independent senses: Pressure Warmth and Cold Pain Vibration Movement and Stretch of Skin

These sensations depend on several different kinds of receptors.



These are most noticeable in our skin, but we do have the same receptors in our internal organs, allowing us to feel internal pain, pressure, and temperature changes.

Therefore we also refer to these senses as comprising the somatosensory system.


Figure 4.25 Cutaneous sensation is the product of many kinds of receptors, each sensitive to a particular kind of information.



The primary somatosensory cortex In certain areas, such as the fingertips and lips, there are

proportionally many more cutaneous receptors. These areas also are allotted more tissue in the parietal

lobes of the human cerebral cortex. Most humans with no impairment in these areas are very

good at identifying familiar objects by touch alone.



Pain Pain receptors are simple nerve endings that travel to

the spinal cord. The perception of pain is a complex mixture of sensation

and perception that is in part mediated by emotion. Two different areas of the brain govern sensory and

emotional interpretations. This is one reason that at least some people can be

distracted or use self-hypnosis to manage reactions to pain.



The gate theory of pain Just seeking treatment or believing that one has been

treated can result in a reduction of symptoms. The effectiveness of placebos in reducing the

experience of pain has been well supported by experimental studies.

A variety of processes can increase or decrease pain to injured areas of the body.


Figure 4.26 Pain messages from the skin are relayed from spinal cord cells to the brain. According to the gate theory of pain, those spinal cord cells serve as a gate that can block or enhance the signal. The proposed neural circuitry is simplified in this diagram. Green lines indicate axons with excitatory inputs; red lines indicate axons with inhibitory inputs.



The gate theory of pain On the basis of these observations, Metzack and Wall

(1965) proposed the gate theory of pain. This is the theory that pain messages must pass through

a “gate,” thought to be in the spinal cord. This gate can block the messages.



Neurotransmitters and pain Substance P is a neurotransmitter that the nervous

system releases for intense pains. Reactions to painful stimuli are reduced in animals that

lack substance P.


Figure 4.27 Substance P is the neurotransmitter most responsible for pain sensations. Endorphins are neurotransmitters that block the release of substance P, thereby decreasing pain sensations. Opiates decrease pain by mimicking the effects of endorphins.



Neurotransmitters and pain Endorphins, which are chemically identical to opiates,

are released by the nervous system in response to the release of substance P.

They effectively weaken pain sensations. Endorphin release can also be induced by sensory

experiences such as listening to music or sexual activity.



Neurotransmitters and pain Capsaicin is the chemical that is present in hot peppers. It stimulates receptors that respond to painful heat. It causes the release of substance P and depletes

supply of it in the nervous system. Creams containing capsaicin can be used to relieve

muscle pain.


Phantom Limbs

A fascinating phenomenon in neuroscience now under study is the experience of phantom limbs

In phantom limb phenomenon, an amputee feels a missing body as if it were still there

These were once thought to be an emotional reaction or mere irritation of the stump of the missing limb


Phantom Limbs

The sensations are now understood to be produced by activity in the neurons of areas in the somatosensory cortex adjacent to the area once belonging to the missing limb

For example, the neurons of the face area are adjacent to the hand area of the somatosensory cortex

These face neurons may occasionally produce a feeling of a phantom hand by stimulating the area that once registered only sensations from the hand


The Chemical Senses

Taste and smell are jointly referred to as the “chemical senses.” Many invertebrates rely almost entirely on these senses; other mammals use them much more heavily than do humans.


The Chemical Senses

Taste The sense of taste detects chemicals on the tongue. Its major function is to control and motivate our eating

and drinking. The taste buds are located in the folds on the surface of

the tongue. They contain the vast majority of human taste receptors.


Figure 4.29 (a) The tongue is a powerful muscle used for speaking and eating. Taste buds, which react to chemicals dissolved in saliva, are located along the edge of the tongue in adult humans but are more widely distributed in children. (b) A cross section through part of the surface of the tongue showing taste buds. (c) A cross section of one taste bud. Each taste bud has about 50 receptor cells within it.


The Chemical Senses

Taste receptors Traditionally the view from Western medicine has held

that there are four primary tastes – sweet, sour, salty and bitter.

The flavor of monosodium glutamate (MSG), a common ingredient in Asian cooking, may represent a fifth.

Researchers are using the word umami for this fifth type of taste receptor.


The Chemical Senses

Olfaction Olfaction is another term for the sense of smell. The receptors for smell are located in the mucous

membranes in the rear air passages of the nose. They detect the presence of airborne molecules of

chemicals.


Figure 4.31 Olfaction, like any other sensory system, converts physical energy into a complex pattern of brain activity.


The Chemical Senses

Olfaction We are aware now that there are at least hundreds of

types of olfactory receptors (contrast this with the number of types of visual receptors.)

Other mammals have far many more receptors than humans do.

Each type of olfactory receptor is extremely specialized for one small group of closely related chemicals.


Figure 4.30 The olfactory receptor cells lining the nasal cavity send information to the olfactory bulb in the brain. There are at least 100 types of receptors with specialized responses to airborne chemicals.


The Chemical Senses

Olfaction Smell is vital for food selection. Neurons in the prefrontal cortex receive both taste and

olfactory input, and combine them to produce the perception of flavor.

The olfactory tract also bypasses the relay system in the thalamus.

It travels to the olfactory bulb, a structure in the base of the brain that is directly in contact with the limbic system


The Chemical Senses

Olfaction Especially in nonhuman mammals, olfaction plays a vital

social role. These animals rely heavily on pheromones, chemicals

that they release into the environment. Pheromones are important for sexual communication,

acting upon the vomeronasal organ to send messages to other individuals regarding fertility and sexual receptivity.


The Chemical Senses

Olfaction Humans prefer not to rely upon the social influences of

pheromones and olfaction. But there is some evidence that they play a role anyway. In one study, it was shown that female college students

who room together tend to have synchronized menstrual cycles.


Sensory Systems

The world that is sensed by a cat, a snail, or a bat is very different that the world that is sensed by you and me.

The function of our senses is to give us the information that we need most to survive and thrive in our environment.


Module 4.3

The Interpretation of Sensory Information


Perception of Minimal Stimuli

Thresholds Early psychological researchers thought it would be

relatively simple to determine the weakest possible stimuli that humans could detect.

They were wrong.


Figure 4.33 Typical results of an experiment to measure a sensory threshold. There is no sharp boundary between stimuli that you can perceive and stimuli that you cannot perceive.



Thresholds It was soon discovered that no sharp line exists between

stimuli that a person can detect and those that they cannot.

Therefore, a sensory threshold was defined as “intensity at which a given individual can detect a stimulus 50% of the time.”

There are no guarantees however that an individual will report all the stimuli above the threshold, or fail to report all those below it.



Thresholds The environment (i.e. lighting conditions) will also

influence the individual’s thresholds. The absolute threshold has been defined as the sensory

threshold at the time of maximum sensitivity; that is, when conditions would allow for the best possible receptivity to the stimulus.


Figure 4.34 People can make two kinds of correct judgments (green backgrounds) and two kinds of errors (red backgrounds). Someone who too readily reports the stimulus present would get many hits, but also many false alarms.



Signal detection theory When trying to detect relatively weak stimuli, people can

be correct and incorrect in two different ways, respectively.

A hit is a correct detection of an actual stimulus. A correct rejection occurs when no stimulus is presented

and no detection is claimed. A miss is an incorrect rejection when a stimulus actually

is presented. A false alarm is an incorrect detection when no stimulus

is presented.



Signal detection theory Signal-detection theory is the study of people’s

tendencies to make hits, correct rejections, false alarms, and misses.

Several factors work together to influence the rates of these outcomes.

The response in each trial does depend on what the person’s senses are conveying.

But an individual’s responses may also depend on their willingness to take a risk of an incorrect response, and on the emotions that a particular stimulus might evoke.


Figure 4.35a Results of an experiment to measure a sensory threshold using two different sets of instructions (with first version of instructions.)


Figure 4.35b Results of an experiment to measure a sensory threshold using two different sets of instructions (with second version of instructions.)



Subliminal Perception The concept of subliminal perception is well known to the

general public. Subliminal perception is the idea that a stimulus can

influence behavior even when it is so weak or brief that we do not perceive it consciously.

There is concern that subliminal perception can powerfully manipulate human behavior.



What does “subliminal” mean? When the term “subliminal” is used, it refers to the

quality of being “below the (sensory) threshold.” Scientists use it to indicate that the stimulus was not

consciously detected in a given presentation. Because the only way to know if a stimulus has been

detected is to ask, it is very difficult to interpret the results of research on subliminal stimuli.



What subliminal perception cannot do Claims that subliminal stimuli in advertisements can

make people buy things are unsupportable. This claim has been tested repeatedly and no

evidence has been found. Advertisements in American culture have little need of

subliminal stimuli. They are overtly and effectively manipulative.



What subliminal perception cannot do Messages in music (recorded backwards or

superimposed) cannot make people do anything, evil or otherwise.

This claim has also been repeatedly tested under controlled conditions.

No one listening to the messages can discern these messages.

No one’s behavior has been changed after listening to music containing messages.



What subliminal perception cannot do Subliminal audiotapes just don’t work

Claims that addictions can be overcome, self-esteem can be improved, and general self-improvement can be achieved through the use of subliminal audiotapes are also unsupported.

Any results achieved through the use of these tapes can be attributed to the placebo effect or to the individual user’s motivation to improve.



What subliminal perception can do Some subtle effects on subsequent perception and

emotion have been supported “Priming” individuals to see an object in subsequent

presentations has been achieved through repeated presentations (Bar & Biederman, 1998)

Emotional states can be influenced by subliminal presentation of messages that may be perceived as emotionally loaded (Masling et al., 1991)



Subliminal perception The fact that subliminal perception can influence

behavior at all is interesting. But the effects overall are much smaller than people

hope or fear.


Perception and Recognition of Patterns

Brightness contrast There are interesting fundamental questions to answer

in the area of perception How does your brain decide how bright an object is? The apparent brightness of an object that you are

looking at can be increased or decreased by the objects around it.

This phenomenon is called brightness contrast.


Perception and Recognition of Patterns

Face recognition There are several interesting processes involved in face

recognition To some extent, we use unusual characteristics to

recognize faces. Most people recognize faces as a synthesized whole

configuration of features. There seems to be a “module” in the brain devoted to

face recognition. If this area is damaged, it is possible to lose the ability to recognize faces.

Children who have been diagnosed with autism also are much poorer than average at face recognition.


The Feature-Detector Approach

One explanation for how we analyze complex stimuli suggests that we break them down into component parts We have feature detectors, specialized neurons that

respond to the presence of certain simple features, such as angles and lines.

For example, one feature detector might be stimulated only by the presence of vertical lines, or 90 angles.

Feature detectors are essential for the first stages of analysis, but perception of complex stimuli requires other processes as well.



Hubel & Wiesel’s experiments Important evidence for the existence of feature detectors

comes from the Nobel Prize winning research of Hubel and Wiesel (1981).

They inserted thin electrodes into cells of the visual cortex in monkeys and cats and recorded activity of those cells when different light patterns were shown to the animals.


Figure 4.40 Hubel and Wiesel implanted electrodes to record the activity of neurons in the occipital cortex of a cat. Then they compared the responses evoked by various patterns of light and darkness on the retina. In most cases a neuron responded vigorously when a portion of the retina saw a bar of light oriented at a particular angle. When the angle of the bar changed, that cell became silent but another cell responded.



Hubel & Wiesel’s experiments The researchers were able to identify cells that fired only

in the presence of vertical bars of light, and others that only fired for horizontal bars.

In later experiments, they found cells that only fired in response to movement in particular directions.



The waterfall illusion experienced by humans is evidence that humans do indeed have feature detectors.

In this illusion, a person first stares at a waterfall for one minute or more.

If the person then looks at cliffs immediately after staring at the waterfall, the cliffs will appear to “flow upward”.

This suggests that the cells that detect downward motion have become fatigued from the act of staring at the waterfall.



Do feature detectors explain perception? Scientists believe that feature detectors are just a first

step in a series of complex processes that create perception.

Simple visual illusions such as the Necker cube suggest that we must also actively impose meaning on images that we see.

There is a branch of psychology that specializes in explaining how humans arrive at the integrated whole images and make meaningful interpretations of the visual world.


Gestalt Psychology

Gestalt psychology focuses on the human ability to perceive overall patterns. The word Gestalt has no true English equivalent, but is

close to synonymous with “pattern” or “configuration.” According to Gestalt psychologists, visual perception is

an active creation, not merely the adding up of lines and movement.


Gestalt Psychology

Principles of Gestalt Psychology When looking at an image, we make a distinction

between figure and ground.


Gestalt Psychology

Principles of Gestalt psychology This is a picture of a reversible figure – a stimulus that

can be perceived in more than one way. When we decide which side is the front of the object, then we will see it as a stable image. We are imposing order on an array, not just adding up small features.


Gestalt Psychology

Principles of Gestalt psychology The principle of proximity states that humans tend to

perceive objects close together as belonging to a group. The principle of similarity states that we perceive objects

that resemble each other as forming a group.


Gestalt Psychology

Principles of Gestalt psychology We may perceive continuation, and fill in gaps in lines, or

closure of familiar figures. We tend to perceive a good figure, one that is simple

and symmetrical. Gestalt visual principles have analogs in the perception

of sound.


Figure 4.46 Gestalt principles of (a) proximity, (b) similarity, (c) continuation, (d) closure, and (e) good figure.


Perception of Movement and Depth

Visual constancy Visual constancy is our tendency to perceive objects as

keeping their size, shape and color even though the image that strikes our retina changes from moment to moment.

So an automobile that is driving away looks like it is moving away, not merely shrinking, even though the image on our two retinas is growing smaller.


Figure 4.50 (a) Shape constancy: We perceive all three doors as rectangles. (b) Size constancy: We perceive all three hands as equal in size.


Perception of Movement

Motionblindness can result from damage to a small area of the temporal lobe.

This fact is further evidence that the visual system analyzes different aspects of an image via different pathways in the brain.



How do we distinguish between our own movement and the movement of objects? The vestibular system works to keep the visual system

informed of the movements of your head. We see motion when an object is moving relative to the

background. When an object is stationary and the background is

moving, we may experience induced movement, a visual illusion in which we incorrectly perceive the object as moving.



Stroboscopic movement is an illusion of movement created by a rapid succession of stationary images. Animation and motion pictures work by stroboscopic movement.

The phi effect, in which your brain creates motion from rows of adjacent lights blinking on and off sequentially, is exploited by many a nightclub and motel owner.


Depth Perception

Our retinas are two-dimensional surfaces, but they give us very good depth perception – our ability to perceive distance. There are several factors involved in creating our depth

perception. Some are binocular cues (depending on both eyes) and

others are monocular (needing only one eye.)


Depth Perception

Binocular cues One important contributor is retinal disparity, which is the

difference in apparent position of an object seen by each retina.

This discrepancy allows us to gauge distance. Convergence is the degree to which our eyes must turn

in to allow us to focus on a very close object.


Figure 4.53 Convergence of the eyes as a cue to distance. The more this viewer must converge her eyes toward each other in order to focus on an object, the closer the object must be.


Depth Perception

Monocular cues Monocular cues allow a person to judge depth and

distance accurately using only one eye. Object size can be used if we already have an idea of

the approximate size of the objects. Linear perspective is used when parallel lines are drawn

so that they converge as they approach the horizon. Detail – generally objects that are closer can be seen in

greater detail than those that are farther away.


Depth Perception

Monocular cues Interposition – nearby objects will obstruct objects that

are farther away. Texture gradient refers to the fact that clusters of objects

will seem more densely packed the farther away the clusters are.

Shadows give clues to distance depending on size and position.


Depth Perception

Monocular cues Accommodation, as you will recall, is how the lens

changes shape to focus on objects, growing thinner to focus on nearby objects and thicker to focus on close things.

Motion parallax is the principle that close objects will pass by faster than distant objects.


Optical Illusions

An optical illusion is a misinterpretation of a visual stimulus. Psychologists are attempting to find a parsimonious

explanation for these misinterpretations. Many can be explained by considering the relationship

between size perception and depth perception.


Optical Illusions

When we misjudge distance, we misjudge size as well. For example, the Ames room illusion causes us to misjudge

the heights of people standing in it using a powerfully misleading set of background cues.

We see an immensely tall and a very short person, but once we remove all the misleading cues, we realize that they are people of similar height standing at different distances in relation to us.


FIGURE 4.59b The Ames room is a study in deceptive perception, designed to be viewed through a peephole with one eye. (b) This diagram shows the positions of the people in the Ames room and demonstrates how the illusion of distance is created. (Wilson et al., 1964)


Figure 4.57 The trade-off between size and distance: A given image on the retina can indicate either a small, close object or a large, distant object.


Figure 4.61 Many optical illusions depend on misjudging distances. The jar on the right seems larger because the context makes it appear farther away.


Optical Illusions

Even a two-dimensional drawing can contain cues that lead to the erroneous perception of depth.

The drawings of M.C. Escher work by this principle.


Figure 4.60 These two-dimensional drawings puzzle us because we try to interpret them as three-dimensional objects.


Optical Illusions

Vision plays a prominent role in some auditory illusions. Visual capture effect is the tendency to identify a sound as

coming from a visually prominent source rather than its actual source. The inaccurate judgment of sound’s distance leads us also to misjudge its intensity.

Ventriloquism works using this auditory illusion.


Optical Illusions

Cross-cultural influences It is thought that how an individual sees the Muller-Lyer

illusion is partly influenced by cultural and other factors. The illusion is stronger for city dwellers and for children. This suggests that experience with buildings and with

drawings of objects may have some impact on interpretation of two-dimensional images.


Optical Illusions

The moon illusion To most people, the moon appears to be about 30%

larger when it is close to the horizon. Measuring it with navigational equipment will prove to

you that it is in fact the same size. It is hard to explain exactly why this illusion occurs, but it

probably is influenced by our tendency to use background cues for reference in judging size.


Optical Illusions

The moon illusion When the moon is at the horizon, we can compare it to

the other familiar objects and the interposed terrain, so we judge it to be very large.

When it is high in the sky, we have no basis to gauge its distance at all. We unconsciously judge the horizon moon to be more distant, and therefore larger.

This latter explanation fits with the general notion that optical illusions are a product of misjudgments of size and distance.


Visual Illusions and Perception

The moon illusion and all that we are learning about visual perception and misperception reinforce an important point.

What you are seeing is not “out there” – it’s in your brain. Vision is usually an accurate if complex reconstruction of

the world around us, but we can be very, very mistaken about what we think we see.

introduction to psychology, 7th edition, james w. kalat chapter 4: sensation and perception chapter...

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