Perception
John G. Taylor (1999) describes perception
as the process of bringing in one or two stimulus events and then constructing
they rest of the stimulus events from our knowledge of the environment. This implies that one of the functions of the
brain is to learn an environment and how to respond to it. Remember, we did not
evolve to live in our current environment but to live 150,000 years ago.
Therefore any explanation of perception must explain perception 150,000 years
ago, not perception today.
Understanding Sensation and Perception
The brain integrates and reacts to
incoming information, including that of light, sound, and pressure. Sensation is the process by which this energy is transformed into neural
activity. Thus, our sense organs let the physical world speak to the nervous
system in biological terms (Coren, Porac, & Ward, 1984). Perception is the
process by which we organize and interpret sensory information.
Types
of Sensory Systems
In this chapter we are going to cover
two-types of sensory systems. The first type is the so called “major” systems
or more accurately, the latest to evolve. The second type systems we will cover
are the “minor” or the earliest to evolve. The major systems are vision and hearing. The
minor systems are smell, taste, and touch. The first systems to be covered are
vision and hearing. These systems evolved recently and processing in the brain
of these two systems is essentially the same. The big difference between the
major and minor sensory systems is that the minor systems are generally “hard
wired”. That is they function very well from birth on and do not require any
interaction with the environment. The major systems have “critical periods”
within which they require stimulation in order to function properly, if the
stimulation is not received, then perception is distorted.
S.B.
and Vision
When S.B. was born, his eyes were
insensitive to light. An operation made this sensitivity possible. Initially,
he had sensations, in that neural activity was stimulated by light. But he had
no perception (Gregory, 1966). Until age 52, S.B. was unable to see at all. He
was, however, active and fearless in his approach to the world: He bicycled; he
built things; he always tried to imagine how the objects he touched might look.
At 52, S.B. underwent a corneal graft, an
operation that enabled him to see for the first time in his life. Initially, he
saw only blurs; objects did not take form. But in a few days, he was able to
see objects as discrete shapes. Soon he had little difficulty recognizing
objects by sight, as long as these were items he had previously touched. For
example, he could readily tell time because while blind he had developed the
habit of carrying a pocket watch with no crystal; he would feel where the hands
were.
Other visual tasks proved more difficult.
S.B. had considerable trouble perceiving distance--looking down from a window
some 30 or 40 feet above the ground, he thought he could lower himself down by
his hands. He never learned to read by sight, although he almost immediately
recognized capital letters and numbers. Again, the crucial factor seemed to be
his previous experience touching these symbols: In the school for the blind
that he had attended, prior to learning Braille, he had learned to recognize by
touch the shapes of capital letters and numbers.
You might think that S.B. was ecstatic
about his new eyesight, but this was not the case. Shortly after the operation,
he became depressed and less active, and he stayed this way until his death
three years later. Depression can be a common reaction among those who gain
sight as adults, perhaps because they realize what they have previously missed
or perhaps because their new vision does not function optimally (Gazzaniga, 1992). In any event, following their operation
some individuals--including S.B.--revert to living in the dark.
Types
of Processing
We perceive the world using processes.
First we learn how to understand what it is we perceive using bottom-up processing. After we have
learned what is in the environment and how to perceive it, we switch to top-down processing. We start our using bottom-up processing because it is
slow and laborious however it is great at teaching us what we perceive. We then
switch to top-down processing because it is fast and it helps us navigate the
world quickly.
Bottom-up processing happens when we first
start to learn the environment. We take in the stimulus and break it down into
its basic components. After each component is analyzed, the image is put back
together so that we may perceive it as a whole. Bottom-up processing is very
accurate but is very slow. It is great for learning an environment, how it
operates, and what is in it. Below is a diagram of bottom-up processing:
After we learn to perceive and understand
the environment we switch to top-down processing. Top-down processing involves
the brain telling the sensory system what it expects to see and therefore that
is what we see. Top-down processing is very fast, but can be very inaccurate.
Cells that respond only to specific
environmental characteristics like the orientation of lines are called feature detectors, and their function seems to have a
“critical period” for development. In other words, stimulation of these cells
must occur within a certain time frame of that function is either totally or
partially lost forever. Consider again the example of S.B. In one instance, he
was shown a woodworking tool in a glass case at a museum. He did not understand
what he saw. Then it was taken out of the case. S.B. closed his eyes, touched
it, and then exclaimed, "Now I can see!" Another example is an infant born with
cataracts. If the cataracts are not removed within the first 3 months, the
child will never be able to distinguish between a sphere and a cube.
All of our senses share certain
characteristics. Each is stimulated by external energy that produces neural
impulses in receptor neurons, in a process known as transduction. These neural impulses give rise to different sensations
that are coordinated into perception. Granted that all sensations arrive at the
brain via a neural route, and granted that the neurons involved are
anatomically identical, what explains the variety of sensations we eventually
experience? One answer was proposed by Johannes Müller
(1826, 1826b) in his doctrine of specific nerve
energies. According to this view, neural messages
register as different sensations because they move along specific nerves that
terminate in given areas of the brain. The doctrine of specific nerve energies
explains, for instance, why we can distinguish sound from odor: Nerves leading
from the ears and the nose end in parts of the brain which learn to hear and
respond to smells.
This theory can also explain aspects of an
unusual sensory phenomenon known as synesthesia, in which stimulation of one type of
sensory receptor gives rise to the involuntary experience of another sense
(Andrews, 1978; Cytowic & Wood, 1982; Marks,
1975). Here the individual tastes a sound, hears a color, or sees a smell.
Synesthesia is not well understood, but some theorists suggest that it is due
to unusual connections among neurons (Motluk, 1994).
A further explanation is needed, and one
popular hypothesis is that the overall pattern of neural excitation and
inhibition in the brain produces the specificity of sensations (Kandel, 1981). Synesthesia, from this perspective, is the
result of a complex integration of neural messages, perhaps in terms of their
emotional associations (Cytowic, 1989). Just where in
the brain this integration occurs is unknown, as is exactly how it takes place.
Adaptation-Level Theory
Fechner addressed the relationship between
stimulus intensity and sensation intensity without reference to context. His
focus was only on the stimuli being judged. The stimulus being judged
influences magnitude estimates, but adaptation-level
theory proposes that
scaling is additionally affected by other stimuli surrounding the stimulus being
judged. Also critical are residual stimuli, those that have been experienced in
the past (Helson, 1964).
Background stimuli and residual stimuli
create the context or adaptation level, in which scaling occurs. Height is a
typical example. We judge people as short or tall not only in terms of their
actual heights, but also in terms of other people in the vicinity and in terms
of other people we have known. A common experience is to see National
Basketball Association point guards like Allen Iverson and Jason Williams as
short, even though they are both at least several inches taller than the
average American male. They seem short because their peers are so tall.
Perception
How do we organize certain sensations so
that we perceive the poster that hangs on our wall or others so that we
perceive a symphony? The term perception describes the process by which
sensations are organized, as well as the product of this organization: an
internal representation of some external stimulus. The fundamental task of perception
is the recognition of and discrimination between different configurations of
stimuli, a process known as pattern perception.
Research converges to characterize the process and product as selective,
coherent, creative, personal, and cultural. Since we process perceptions by
meaning, all of those descriptions seem to follow.
Perception
Is Selective
Our perception is selective which means that
we do not simultaneously attend to all the stimuli occurring around us (Lavie & Tsal, 1994). In a
crowded restaurant, you listen carefully to what your friend across the table
is saying and not to conversations at other tables. On a bus, you look out the
window to see how close you are to your intended stop; you are less likely to
notice the clothing or cologne of your fellow passengers. You accord some
stimuli more weight than other stimuli. The ease with which you can do this
depends on the competing stimuli and attention processes (Kinchla,
1992). .
Perception
Is Coherent`
To say that our perception is coherent is
to say that conscious experiences are psychologically meaningful wholes.
Remember S.B., whose story began this section? When he first gained sight, all
he saw were blurs. But he quickly came to see the world in terms of discrete
(coherent) objects.
Perception
Is Creative
To say that our perception is creative
means that it is not a literal version of reality but rather something we
create. Among the raw ingredients of perception are, of course, our sensations,
which are determined by external stimuli. However, perception is also
determined by the habits, tendencies, and styles we bring to bear on our
sensations (Wehner & Stadler,
1994).
Perception
Is Personal
To say that our perception is personal is
to say that we see the world from our own point of view. Most people say we see
with our eyes, and hear with our ears. We see and hear with our brain and our
brain has determined the meaning of the stimuli that we are processing.
Therefore the stimulus has a specific meaning to us but not every one has the
same specific meaning.
Form Perception
To recognize an object, we use the
principle of the figure-ground
relationship. We group
stimuli together into a unified form called a figure and distinguish the figure
from surrounding stimuli, referred to as the ground. All perception of forms
requires the organization of figures against backgrounds. Your perception may
flip back and forth rapidly, but at any given moment you can see only one
figure against one ground.
Psychologists have specified a number of
principles that describe how we organize stimuli into coherent forms. These
principles were first proposed by the Gestalt psychologists, and so they are
called gestalt organizational
principles of form
perception. They include proximity, similarity, closure, and continuity.
Proximity is our tendency to group
together stimuli that are near each other. Similarity refers to our tendency to
group together stimuli that are similar in size, shape, color, or form. Moving
to the right, you see not vertical columns of varied shapes but horizontal rows
of dots and dashes, organized by the principle of similarity. According to the
principle of closure, when an object has gaps, like the one in the lower left
of the figure, you fill them in. What results in this case is a tiger. The
tendency to smooth out irregularities also shows continuity, as you can see in
the lower right of the figure. Current research into gestalt organizational
principles attempts to locate just where they operate in the course of
information processing. In contrast to the premise of the original Gestalt
psychologists that perception of form takes place all at once and early in
information processing, it now appears as if these principles result from
several mechanisms operating at differing stages (Peterson & Gibson, 1994a,
and 1994b). Consistent with this hypothesis is a case study of a 71-year-old
woman with cortical degeneration (Kartsounis &
Warrington, 1991). She could discriminate shapes when they were presented alone
but not when they were overlapping. Her brain damage selectively affected her
perception of forms.
Distance Perception
In everyday life, we judge distances all
the time, usually with great accuracy. How far away is that approaching truck?
How close is that fork wrapped in spaghetti? Explaining how people go about
perceiving distance has been a major concern of psychologists (Gibson, 1988).
Remember that vision involves the
projection of images onto the retina, where they stimulate neurons leading to
the brain. These images contain no information about distance or depth. The
projections of objects at differing distances might look exactly the same, if
the objects are of appropriately different sizes. Why, then, do we have no
trouble telling how far away an object is, whether it is under our nose, just
beyond our reach, or somewhere over the rainbow? Think one more time of the
example of S.B., and remember his difficulty perceiving distances. This might
suggest that there is a “critical period” for depth perception.
Visual Constancies
Consider the following familiar
experiences:
1.
You see a friend at the top of a staircase. As he walks down the stairs,
his size remains constant. This becomes intriguing when you appreciate that the
light stimulating your retina is constantly changing. In particular, the size
of the retinal image constantly changes. Why do you not experience a change in
your friend's size?
2.
You are reading this paragraph on a computer screen while slouching in a
chair. As you tilt the screen away from you, it still maintains its rectangular
shape. Again, this is intriguing. The retinal image changes from rectangular to
trapezoidal as the screen tilts. Why do you not experience a change in the
shape of the screen?
3.
You have switched to a new laundry detergent, and your pleasure knows no
bounds because your clothes have never looked so bright. You walk down the
street admiring the brightness of the sleeve of your favorite shirt. You pass
beneath a large tree that shades the entire sidewalk, you, and your sleeve. You
continue to admire how bright your clothes have become. Yet the intensity of
light reflected by your shirt was greatly reduced when you walked under the
tree. The amount of light stimulating your retina has changed. Why do you not
experience a change in your shirt's brightness?
These examples illustrate visual constancies of size, shape, and brightness: the
tendencies for visual perceptions to stay constant even as visual sensations
change. As physical stimulation changes, our perceptual experiences remain
constant. This phenomenon makes the world appear stable even though our
moment-to-moment stimulation can fluctuate wildly. We somehow take into account
factors besides the size, shape, or intensity of retinal stimulation (Wallach,
1987)?
Illusions
Perception psychologists have long been
interested in illusions, phenomena in which our perception of an
object is at odds with its actual characteristics. For example, the phi
phenomenon refers to the fact that stationary objects in different locations
are seen to move if they flash at the appropriate interval (about four to five
times per second). This phenomenon is responsible for our perception of
"motion" pictures as moving, when in fact they are composed of a
series of stationary images.
Another type of apparent movement is the autokinetic effect, the tendency of a single point of light
in a darkened room to appear to move, even when it is stationary. The autokinetic effect is caused by slight movements of our
eyes while we fixate on the point of light (Matin
& MacKinnon, 1964). We lack a context in which to locate the light, and we
are unaware that our eyes are moving. Instead, we attribute the movement to the
light itself. Apparent movement illustrates the gestalt truism that the whole
(perception) is not the same as its parts (sensations).
Illusions give us an idea of how our
sensory systems learned the environment and the price we pay for that learning.
For example, people raised in a non-straight line environment are never fooled
by straight line illusions as those raised in a straight line environment. The
opposite is also true.
Perceptual Learning
Certain ways of structuring sensations are
built into our nervous system (Hubel & Wiesel, 1962, 1979). Some believe
that infants come into the world already equipped with certain feature
detectors. For instance, Johnson, Dziurawiec, Ellis,
and Morton (1991) showed that in the first hour following birth, infants are
more likely to track a moving stimulus that looks like a face than they are to track
similar but non-face like stimuli. Some believe that this tendency reflects the
operation of an evolved mechanism, i.e. the newborn is predisposed to attend to
the most important aspect of the environment--the parent--and the parent's
attention in turn is drawn to the responsiveness of the infant. However,
research indicates that infants prefer complex visual stimuli over relatively
simple stimuli. One of the most complex visual stimuli is the human face (Langolois, 1999). Infants also prefer attractive faces to
plain faces (Langolois, 1999).
But feature detectors are merely
components in perception. If these are not used, they lose their capacity.
Blakemore and Cooper (1970) raised kittens in such a way that for the first
five months of their lives, they saw only vertical (or horizontal) stripes. As
adult cats, these animals were unable to perceive horizontal (or vertical)
stimuli. They were unresponsive to a stick waved at them unless it was
presented in the same orientation with which they had early experience: vertical
or horizontal.
With age and experience, the individual
learns to organize the basic components of perception in more sophisticated
ways. Children become increasingly able to recognize patterns (Alberti & Witryol, 1990). That young children do not perceive patterns
as readily as adults explains why lullabies across different cultures have
simpler melodies than other songs from the same cultures do (Unyk, Trehub, Trainor,
& Schellenberg, 1992). We sing lullabies to
infants in order to soothe them. If the melody is too complex, they cannot
grasp it, and they (and we) will be sleepless. Speech directed to infants is
similarly simple, in terms of rhythm, pattern, and soft phonemes.
Perceptual Sets
The second way in which perception is personal
is that psychological states and characteristics influence what and how we
perceive. We often perceive what we expect to perceive. A perceptual set (or mental set) is a predisposition to perceive a
particular stimulus in a particular context.
Perceptual sets can be created in
different ways. Perceptual sets may result from habitual experiences,
prevailing needs like hunger or thirst, emotions, personality characteristics,
or social pressures (Beck, Neeper, Baskin, &
Forehand, 1983; Logan & Goetsch, 1993; Palfai & Salovey, 1992; Toner
& Gates, 1985).
Stereotypes may produce perceptual sets,
leading us to see individuals in a stereotyped group only in terms consistent
with the stereotype. For example, Gibbons and Kassin
(1987) presented artwork to research participants and described the artists as
either retarded or nonretarded children. Although the
paintings and drawings were identical, those attributed to retarded children
were seen as less skilled.
A perceptual set is useful to the degree
that it leads us to perceive stimulus characteristics that are relevant to the
purpose at hand. In such cases, a perceptual set leads to more efficient
processing of information (Auerbach & Leventhal, 1973; Macrae, Milne,
& Bodenhausen, 1994). For instance, when I am
grading a student's test, I attend to the content of what is written and not
whether a pen or pencil was used. In other cases, a perceptual set channels
information processing in the wrong direction (Dustman et al., 1984). Again,
while grading a test, I may be looking only for the typical right answer and so
may inadvertently ignore an unusual but equally correct one.
Perception Is Cultural
Perception is also influenced by our
larger social context. We can thus characterize perception as cultural. Our
culture directs our attention to some stimuli rather than others, dictates how
we group together configurations of stimuli and distinguish them from one
another, and provides concepts with which we interpret stimuli. Accordingly, a
round red light is seen as a signal to stop in cultures with automobiles but as
something different in cultures without. As pointed out in the discussion of
perceptual sets, we are greatly influenced by our expectations. One of the
important sources of these expectations is cultural beliefs and practices. To
the degree these differ across cultures, then so to do perceptions (Deregowski, 1980; Tajfel, 1969).
Susceptibility to Illusions
Researchers have studied the
susceptibility to illusions of people in different cultures. Differences exist,
and these are interpretable in terms of the characteristic experiences provided
by culture. A well-known investigation along these lines was conducted by Segall, Campbell, and Herskovitz
(1963, 1966), who studied individuals from 15 different cultures, both European
and non-European, in terms of their responses to the Müller-Lyer
illusion. In general, Europeans were more susceptible to this visual illusion
than non-Europeans, meaning that they tended--incorrectly--to perceive the
right line as longer than the left line. In contrast, non-Europeans
tended--correctly--to see the two lines as the same length.
Segall and his colleagues interpreted this
difference in terms of the degree to which individuals were accustomed to a
"carpentered" environment: a visual setting dominated by rectangularly shaped buildings. Recall one of the
interpretations of the Müller-Lyer illusion: People
see the lines as corresponding to outside corners of rooms versus inside
corners. European individuals have more experience with rooms like these, and
so they are more prone to the illusion.
As noted, other interpretations of this
illusion are possible. Jahoda (1966), for instance,
argued that the documented cultural differences are due to variations in
experience with two-dimensional representations of three-dimensional objects.
Those in European cultures have a lifelong familiarity with such drawings and
have learned to regard certain artistic conventions as distance cues. So, they
perceive the Müller-Lyer illusion in these terms.
Those without such familiarity do not bring these expectations to bear on the
lines. Thus, they do not misinterpret what they see. Mundy-Castle (1966) showed
children from non-European cultures these pictures and asked them to answer such
questions as:
1.
What do you see?
2.
What is the man doing?
3.
Can the deer see the man?
4.
Which is closer to the man: the elephant or the deer?
Research subjects correctly identified
each item in the pictures, but they tended to interpret the pictures in
two-dimensional terms, not making use of distance cues. This does not mean that
these subjects were insensitive to distance in their everyday lives. Rather,
they were unfamiliar with the conventions of European drawings used to signify
distance.
Binocular Rivalry
Another demonstration of how culturally
provided expectations influence perception is a study by Bagby
(1957), who investigated the phenomenon of binocular rivalry. In binocular
rivalry, different visual stimuli are shown separately to an individual's two
eyes. Under appropriate circumstances, one image may dominate the other and be
the only one the individual perceives.
Bagby's research subjects were from either
Culture and Sensation
Perception is the psychological
interpretation of sensations, and so it is perhaps not surprising that culture
can influence perception. The less obviously answered question is whether
culture affects sensation. The answer seems to be that it does not. Other than
the threshold differences mentioned earlier, the biological characteristics of
people's sensory systems are essentially the same. Culture influences
attention, of course, but there is little reason to think that it influences
sensation.
Pain
Let us conclude by discussing pain,
another example of an integrative phenomenon. Pain is often described as a
sensation and specifically as a cutaneous sense.
However, this conceptualization is oversimplified. Stimuli that touch our skin
may cause pain, but the sensation of pain is not limited to the stimulation of cutaneous receptors. Pain can and does occur through any
sensory receptor. Deafening sounds, blinding lights, and overwhelming tastes
produce pain just as readily as damaged skin. Pain from different sensory
sources is readily combined in experience in a way that other sensations from
different receptors are not (Algom, 1992). For
instance, a moderately painful sound coupled with moderately painful pressure
can produce a profoundly painful experience, yet it is the pain that is
combined, not the sound and the touch.
Characterizing painful stimuli in terms of
their simple physical properties proves difficult. Although pain usually
results from intense stimuli, this generalization is not always true. Compare a
pulsating shower with the prick of a pin. The water from the shower produces
much more neural activity than the pin, but it is the pin that produces pain,
not the shower.
In short, pain is a psychological puzzle.
We can characterize it: Pain is unpleasant. We can describe its adaptive
function: Pain serves as a warning signal that our well-being is threatened. We
can note its powerful effect on behavior: Pain is something we try to avoid or
escape. But we cannot--in a definitive way--say what pain is. Perhaps the
puzzle results from the assumption that pain is just a sensation. Some
psychologists argue that pain shares more in common with motives like hunger or
thirst than it does with sensations like vision or taste (Price, 1988). Motives
are experiences that impel us to behave in certain ways. Pain fits this
definition. It has a sensory component--as do hunger and thirst--but this
component must be understood within its larger context.
There are well-documented differences
among cultural groups in their willingness and ability to tolerate intense
stimuli (Fabrega, 1989; Ots,
1990; Pugh, 1991; Thomas & Rose, 1991; Villarruel
& Ortiz de Montellano, 1992; Zborowski,
1969). For instance, in comparison to women from other ethnic groups, those
from Asian cultures report more pain when having their ears pierced. The
stereotype of Northern Europeans as stoic seems to have a basis in fact because
many of these individuals can tolerate what would seem to be highly painful
stimuli without complaint.
Similarly, Northern Europeans are less
likely than those from most other cultural groups to experience chronic pain
disorders (Hes, 1958, 1968). More than those from
other cultural groups, European Americans expect to control how they feel
(Bates & Rankin-Hill, 1994), whether or not they actually can (Peterson,
Maier, & Seligman, 1993). These expectations in turn are associated with a
decreased perception of pain (Averill, 1973; Miller, 1979; Thompson, 1981). Taken
together, these findings again show how culture affects our experience of
stimuli.
A contextualized view of pain is contained
in gate-control theory (Melzack, 1973).
According to this account, the nervous system is limited in the amount of
sensory information it can handle at a given time. When too much information is
present, cells in the spinal cord act as a gate, blocking some signals from
going to the brain, while letting others pass. The brain may send messages to
the spinal cord to open or close this hypothesized gate. Most of us have
sustained an injury in the course of an engaging activity, like playing a
sport, wherein we did not notice the gash in our leg or our chipped tooth until
the activity ended. Presumably, the pain gate had been closed.
Perhaps people troubled by chronic pain
have pain gates that stay open too long, for biological or psychological
reasons. Accordingly, psychologists can help if they can devise a way to close
the hypothesized gates. Endorphins may be involved in the gating of pain (Basbaum & Fields, 1984). Some psychologists believe
that the Chinese practice of acupuncture controls pain because it closes
particular gates (Chapman, Wilson, & Gehrig, 1976). Other strategies for
controlling pain include drugs, massage, hypnosis, relaxation, and distraction
(Caillet, 1993; Hamill
& Rowlingson, 1994).
Pleasure
It is noteworthy that psychologists have
studied pain extensively while paying relatively little attention to pleasure
(Glick & Bone, 1990). Regardless, the points just made about pain also
clarify pleasure. Pleasure has a sensory component, but it is not limited to
any given sensory receptor. We experience pleasure through touch, smell, taste,
vision, and so on. Pleasure exerts a powerful influence on our behavior.
Finally, it is difficult, if not impossible, to characterize a pleasurable
stimulus in terms of its simple physical properties. For example, we may
experience hot water as pleasurable when we are taking a shower but as painful
when we are merely touching it, even though it is the same temperature
(Hermann, Candas, Hoeft,
& Garreaud, 1994). Depending on what else is
occurring, the identical sensory stimulation can
produce pleasure, indifference, pain, or even revulsion.
Pleasure may best be regarded as a motive
behind behavior. It is influenced by a host of biological, psychological, and
social factors. In the modern Western world, for instance, most of us find a
kiss to be highly pleasurable. But kissing is not among the cultural practices
of all groups. Those in traditional Chinese and Japanese societies did not kiss
one another (Ford & Beach, 1951).
Attention
Orientation
of Attention
Orientation refers to the positioning of our sense organs to best receive
environmental stimulation. Intense stimuli demand our attention. Our prevailing
needs and goals also direct our attention. We stare at objects we want to see;
we stick our nose over objects we want to smell; we place our ears close to
objects we want to hear. These are all examples of overt orientation.
Psychologists are also concerned with covert orientation, which occurs when we
direct our attention to stimuli without physically moving our sense organs
(Posner, 1978): "Don't turn around and stare, but check out who just
walked in the room." Covert orientation implies that attention does not
occur solely through our senses but also involves top-down processes.
Divided
Attention
Divided attention describes our ability to attend to different stimuli at
the same time. As we learn to perform a task, we need to attend to its details
less and less. Performance becomes automatized, and
our attention can be deployed elsewhere. Consider driving a car. When you first
slid behind the wheel of an automobile, the amount of stimulation overwhelmed
you.
So many things competed for your
attention: steering wheel, brakes, accelerator, turn signal, speedometer,
pedestrians, school crossings, and other cars. With experience, you can do all
sorts of things while driving, including listening to the radio, carrying on a
conversation, and even watching the gas gauge.
Selective
Attention
Another topic of interest concerns selective attention (Johnston & Dark, 1986). You can tune in some
information while tuning out other information. Tuned-in information is front
and center in our conscious experience, but what about tuned-out information?
One way researchers answer this question is through studies of shadowing. They
place headphones on a research subject and deliver one message to the right ear
and another message to the left ear. The subject is asked to repeat one of the
messages out loud (to shadow the message), under the assumption that he or she
will therefore pay more attention to it. The typical finding is that subjects
completely shut out the non-shadowed message. When asked later to recall its
content, a subject can report nothing. He or she may not even know what
language was spoken (Cherry, 1953). Other studies suggest that selective
attention is more complex. If the non-shadowed message contains the subject's
name, the subject notices it at least some of the time (Moray, 1959; Wood &
Cowan, 1995).
How does Attention
Work?
People can also learn to improve their
attention (Pashler, 1992). Specific instructions and
practice are helpful, even among the elderly (Kramer, Larish,
& Strayer, 1995; Trudeau, Overbury,
& Conrod, 1990). Stoffregen
and Becklen (1989) showed, for instance, that with practice,
people can improve their ability to divide attention between two complex tasks.