I. THE PERCEPTION PARADOX
The fundamental paradox of perception is that what seems so easy for the perceiver is
exceedingly difficult for psychologists to understand and explain. In order to function so
effectively and efficiently, perceptual systems must be extremely complex.
II. THREE APPROACHES TO PERCEPTION
Constructivists argue that the perceptual system must often make a reality out of bits of sensory
information. Computationalists believe that neural activity transforms sensory stimulation into
our experience of reality. According to the ecological approach to perception, we perceive most
clues from the environment directly, without interpretation.
Psychophysics is the study of the relationship between the physical energy of the environmental
stimuli and the psychological experience that those stimuli produce.
A. Absolute Thresholds: Is Something Out There?
Absolute threshold is the minimum amount of energy that can be detected 50 percent of
the time. Stimuli that fall below this threshold and are usually not detected are referred to
as subliminal stimuli. Stimuli that are detected because they are above the threshold are
called supraliminal stimuli.
B. Thinking Critically: Can Subliminal Stimuli Influence Your Behavior?
What am I being asked to believe or accept?
There are claims that humans can be influenced or persuaded by subliminal messages,
which are detected, perceived, and overtly acted upon without conscious awareness.
What evidence is available to support the assertion?
Some research studies show that subliminal information can have an impact on behavior,
judgment, and emotion.
Are there alternative ways of interpreting the evidence?
One alternative explanation of the supposed success of subliminal stimuli is the possibility
that only research supporting the idea of subliminal perception has been reported. Another
possibility is that positive expectations of subliminal stimuli (such as with self-help tapes)
are more important than the content of the subliminal messages; therefore, the findings
may be due to the placebo effect.
What additional evidence would help to evaluate the alternatives?
Double-blind, placebo-controlled experiments would provide empirical research results
that could offer insight into the extent of the influence of subliminal perceptions.
What conclusions are most reasonable?
The evidence available to date suggests that subliminal perception does occur, but that it
has no potential for “mind control.”
C. Signal-Detection Theory
Signal-detection theory is a mathematical model that describes what determines whether a
person perceives a near-threshold stimulus. Sometimes a person perceives noise
(spontaneous random neural firing) that is always present, even without stimulation, as a
perceptual experience. Whether a person determines a perception is noise or noise-plus-
stimulus depends on sensitivity, the person’s physical ability to detect a stimulus, and on
response criterion, the person’s willingness or reluctance to say that a stimulus is present.
If a person expects a stimulus to be present, then his or her response criterion will be
lowered. In other words, as expectations of stimuli increase, the amount of stimulus energy
necessary to trigger perception is lowered. Signal-detection theory has led to improvement
in the accuracy of people who are responsible for signal-detection tasks.
D. Judging Differences: Has Anything Changed?
Just-Noticeable Difference (JND). The minimum detectable difference between two stimuli,
the difference threshold, depends on the initial magnitude or intensity of the stimuli and
on which sense is being stimulated. Weber’s law states that the just-noticeable difference
(JND), also called the difference threshold, is a fixed proportion (symbolized by the letter K)
of the intensity of the stimulus. K is different for each of the senses.
E. Magnitude Estimation: How Intense Is That?
Fechner observed that constant increases in physical energy will produce progressively
smaller increases in perceived stimulus size. Stevens’s power law describes the relative
changes in perception of size for stimuli that Fechner’s law doesn’t cover.
IV. ORGANIZING THE PERCEPTUAL WORLD
A. Basic Processes in Perceptual Organization
Perceptual organization is the process by which order is imposed on the information
received by your senses.
1. Figure-Ground Organization. Our perceptual processes actively try to assign some
stimuli to the foreground (figure) and some to the meaningless background (ground).
2. Grouping. According to Gestalt psychologists, we see a figure via principles of
grouping. These principles are proximity, similarity, continuity, closure, common
region, common fate, synchrony, and connectedness. The likelihood principle says
that we tend to perceive objects in the way that experience tells us is the most likely
arrangement. The simplicity principle says that we organize stimulus information into
the simplest possible perception.
B. Perception of Location and Distance
1. Two-Dimensional Location. This ability lets us determine whether a stimulus is coming
from the right or left, from above or below.
2. Depth Perception. Depth perception allows us to experience the world in three
dimensions. Stimulus depth cues include interposition, relative size, height in the
visual field, texture gradients, linear perspective, clarity, color, and shadows. Depth
cues from moving objects include motion parallax, the phenomenon in which near
objects seem to move faster than objects in the distance. There are also cues based
on properties of the visual system. These are accommodation, convergence, and
C. Perception of Motion
Our brain decides whether something is moving by evaluating movement cues in the
retinal image, eye and head movement, and vestibular and tactile cues.
Looming is the rapid expansion in the size of an image on the retina. You interpret this as
an approaching stimulus.
Stroboscopic motion is an illusion in which we perceive continual movement from a series
of separate still images moving across the retina. It enables us to perceive movement in
films and videos.
D. Perceptual Constancy
Perceptual constancy is the ability to perceive sameness even when the image on the
1. Size Constancy. Our perception of an object’s size is based on the size of our retinal
image and how far away we think the object is. We interpret the retinal image in
relation to its perceived distance.
2. Shape Constancy. The brain automatically puts together information about retinal
images and distance as movement occurs. Take a square object, turn it in many
different directions (movement and distance cues), and see if you perceive anything
other than a square.
3. Brightness Constancy. How bright we perceive an object to be is based on real-world
knowledge and on the brightness of that object relative to its background.
V. RECOGNIZING THE PERCEPTUAL WORLD
Perception is a result of top-down processing and bottom-up processing.
A. Bottom-up Processing
We can recognize an object because we perform feature analysis, meaning that our
sensory systems analyze stimuli into basic features before higher centers of the brain
recombine them to create a full perceptual experience. Color, motion, spatial orientation,
and patterns of light and darkness are some of the features that our visual systems analyze.
B. Top-down Processing
In top-down processing, our knowledge, motivations, and expectations influence
perception. Schemas are mental representations of our knowledge and expectations, and
can create a perceptual set, which is a predisposition to perceive a stimulus in a certain
way. Such expectancies are influenced by context and past experience. Motivation—that is,
the way we want to perceive—can also alter perceptions.
C. Network Processing
Network processing is the extensive interaction and communication among the various
feature analyzers detecting and sending sensation messages to the brain.
According to parallel distributed processing (PDP) models, recognition occurs as a result of
simultaneous (parallel) operation of connected units in the brain that are activated by
stimulus features. Other units are excited or inhibited by the connections that exist
D. Culture, Experience, and Perception
Different experiences affect perception by creating differing expectations and other top-
down processes. When people from different cultures are exposed to substantially
different visual environments, they respond differently to the same visual stimuli. People
from cultures that are not westernized often do not see depth cues in visual stimuli as
E. Linkages: Perception and Human Development
Psychologists use inborn patterns of habituation and dishabituation to study infant
perception. The study of newborns has helped psychologists identify innate perceptual
processes, such as feature analysis. At two months, infants begin to scan object perimeters
as they learn to recognize patterns and shapes. At three months, infants begin to use
binocular disparity and relative motion depth cues. They also possess the ability to use
accommodation and convergence as depth cues. However, visual experience is necessary
to develop these innate perceptual abilities.
Attention is the process of directing and focusing certain psychological resources, usually by
voluntary control, to enhance perception, performance, and mental experience. Attention
improves mental processing, requires effort, and has limited resources. Attention may be
directed overtly or covertly.
A. Focus on Research Methods: An Experiment in “Mind Reading”
Psychologists wondered whether perceptual systems might become more sensitive to
stimuli where people have covertly shifted their attention. Volunteers watched a fixation
point on a computer screen where a symbol indicated the likely location of the target.
When the fixation point cue gave correct information about the location of the stimulus,
the participants covertly directed their attention to that area and detected the target
B. Directing Attention
Selective attention is the tendency to focus on some stimuli in the environment while
ignoring others. Attention can be voluntarily guided by motivation and a knowledge of
what sources of information are critical to the task at hand. A feature of the environment
may attract our attention involuntarily.
C. Ignoring Information
Sometimes attention can be so focused that it results in inattentional blindness, a failure to
detect or identify normally noticeable stimuli. Actively ignoring stimuli can create negative
priming, which makes it more difficult to detect those stimuli for some time.
D. Divided Attention
Although attention resources are limited, people can sometimes divide their attention
between two tasks. It is easier to divide attention between two practiced or automatic
tasks (for example, tying a shoe while chewing gum) or between tasks that are
automatically processed at lower levels of the nervous system. Attention can also be
divided when different sensory systems are utilized to accomplish each task (such as
listening and using both hands).
E. Attention and Automatic Processing
Parallel processing is the ability to search a number of locations rapidly and automatically
F. Attention and the Brain
There is no single area of the brain responsible for attention. Tasks requiring attention to
multiple stimuli produce activity in more than one part of the brain. Thus damage to
specific regions of the brain produces specific types of attention deficits.
VII. APPLICATIONS OF RESEARCH ON PERCEPTION
A. Aviation Psychology
Some landing scenarios decrease a pilot’s ability to rely on top-down and bottom-up
processing. To compensate, engineers have developed cockpit displays that represent a
realistic three-dimensional image of the flight environment. In addition, visual displays
have been developed that minimize reliance on auditory cues.
B. Human-Computer Interaction
Engineering psychologists employ perception principles in designing computer displays. The
displays use depth perception cues, attention-getting stimuli, and simple visual images to
make computers easier to operate.
C. Traffic Safety
Research on divided attention is being applied to help understand the potential dangers of
driving while using various kinds of cell phones.
D. Architecture and Interior Design
Principles derived from perception research are used by architects and interior designers to
create illusions that an object (such as a building or room) is different from its actual