SENSATION
AND PERCEPTION
Thanks to the nose, ears,
eyes, tongue, and skin, we can imagine a day at the beach: glimmering blue sky,
salty water, warm sand, and crying seagulls. Our knowledge of the world depends
on the senses: vision, hearing, taste, smell, position, movement, balance, and
touch. If someone bounces a basketball, our eyes and ears pick up stimuli such
as light and sound waves and send neural signals to the brain. This process
called sensation occurs when physical energy from objects in the world or in
the body stimulates the sense organs.
However, only when the
signals come together meaningfully do we actually perceive a bouncing
basketball. Perception happens when the brain organizes and interprets sensory
information. Sensation and perception occur together, and normally we don’t
distinguish between the two separate processes. We use all five of our senses
and organize the information we get from them every day of our lives.
The Senses
Sensation is the process by which physical
energy from objects in the world or in the body stimulates the sense organs.
The brain interprets and organizes this sensory information in a process called perception. Psychophysics is the
study of how the physical properties of stimuli relate to people’s experience
of stimuli. Research in psychophysics has revealed much information about the
acuity of the senses.
Measuring the Senses
Psychologists assess the acuity of the senses in three ways:
1. Measuring the absolute threshold
2. Measuring the difference threshold
3. Applying signal detection theory
The absolute threshold is
the minimum amount of stimulation required for a person to detect the stimulus
50 percent of the time. The difference threshold is the smallest difference in
stimulation that can be detected 50 percent of the time. The difference
threshold is sometimes called the just noticeable difference (jnd), and it depends on the strength of the stimulus.
Example: If someone were comparing two
weak stimuli, such as two very slightly sweet liquids, he’d be able to detect
quite a small difference in the amount of sweetness. However, if he were
comparing two intense stimuli, such as two extremely sweet liquids, he could
detect only a much bigger difference in the amount of sweetness.
Weber’s Law
Nineteenth-century
psychologist Ernst Weber proposed a principle demonstrating the fact that we
can’t detect the difference between two stimuli unless they differ by a certain
proportion and that this proportion is constant. In other words, the just
noticeable difference for a stimulus is in a fixed proportion to the magnitude
of a stimulus. Weber’s Law holds true except in the most extreme kinds of
stimulation.
Researchers use signal detection theory to predict when a weak signal will be
detected. This theory considers the fact that the ability to detect a signal
depends not only on the strength of the signal but also on the perceiver’s
experience, motivation, expectation, and degree of alertness. Different people
respond differently to the same signal, and the same person may detect a
particular signal at one time but not another. Furthermore, people can often
detect one type of signal in a sensory modality such as hearing or vision but
be oblivious to other types of signals in the same sensory modality.
Sensory Adaptation
When people walk into a restaurant, they probably notice food smells right
away. However, as they sit in the restaurant, the smells gradually become less
noticeable. This phenomenon occurs because of sensory adaptation. Sensory
adaptation is
the decrease in sensitivity to an unchanging stimulus. The smells don’t
disappear—the people just become less sensitive to them.
Development of the Senses
Babies have
all the basic sensory abilities and many perceptual skills, but these abilities
develop and grow more sensitive over time. Babies can recognize the difference
between a human voice and other sounds, and they can locate a sound’s origin.
They can recognize the difference between smells and, very early on, can
recognize their mother’s particular smell. As for taste, they can differentiate
between sweet and salty. Babies also have fairly adept visual abilities. Soon
after birth, they can distinguish objects of different colors and sizes. When
they are just a few weeks old, they begin to differentiate among contrasts,
shadows, and patterns, and they can perceive depth after just a few months.
Sensitive Periods
Even innate
perceptual skills need the right environment to develop properly. A lack of
certain experiences during sensitive periods of development will impair a
person’s ability to perceive the world.
Example: People who
were born blind but regain their vision in adulthood usually find the visual
world confusing. Since these adults were blind in infancy, they missed the
sensory experiences necessary for their visual system to develop fully.
Vision
Researchers
have studied vision more thoroughly than the other senses.
Because people need sight to perform most daily activities, the sense of sight
has evolved to be highly sophisticated. Vision, however, would not exist
without the presence of light. Light is electromagnetic radiation that
travels in the form of waves. Light is emitted from the sun, stars, fire, and
lightbulbs. Most other objects just reflect light.
People experience light as having three features: color, brightness, and saturation. These three types of experiences come from three
corresponding characteristics of light waves:
·
The color or hue of light depends on its wavelength, the distance between the peaks of its waves.
·
The brightness of light is related to
intensity or the amount of light an object emits or reflects. Brightness
depends on light wave amplitude, the height of light
waves. Brightness is also somewhat influenced by wavelength. Yellow light tends
to look brighter than reds or blues.
·
Saturation or colorfulness depends on light complexity, the range of wavelengths in light. The color of a
single wavelength is pure spectral color. Such lights are called fully
saturated. Outside a laboratory, light is rarely pure or of a single
wavelength. Light is usually a mixture of several different wavelengths. The
greater number of spectral colors in a light, the lower the saturation. Light
of mixed wavelengths looks duller or paler than pure light.
|
Wavelength ——>
Color
|
|
Amplitude ——>
Brightness
|
|
Complexity ——>
Saturation
|
Rainbows and Lights
White light: Completely unsaturated. It is a mixture of all
wavelengths of light.
The visible spectrum: Includes the colors of the rainbow, which are red,
orange, yellow, green, blue, indigo, and violet.
Ultraviolet light: The kind of light that causes sunburns. It has a
wavelength somewhat shorter than the violet light at the end of the visible
spectrum.
Infrared radiation: Has a wavelength somewhat longer than the red light at
the other end of the visible spectrum.
The
process of vision cannot be understood without some knowledge about the structure
of the eye:
·
The cornea is the transparent, protective outer
membrane of the eye.
·
The iris, the colored part of the eye, is
a ring of muscle.
·
The iris surrounds an opening called the pupil, which can get bigger or smaller to allow different amounts
of light through the lens to the back of the eye. In bright light, the pupil
contracts to restrict light intake; in dim light, the pupil expands to increase
light intake.
·
The lens, which lies behind the pupil and
iris, can adjust its shape to focus light from objects that are near or far
away. This process is called accommodation.
·
Light passing through the cornea, pupil, and
lens falls onto the retina at the back of the eye. The retina is a thin layer
of neural tissue. The image that falls on the retina is always upside down.
·
The center of the retina, the fovea, is where vision is sharpest. This explains why people look
directly at an object they want to inspect. This causes the image to fall onto the
fovea, where vision is clearest.
Eye Trouble
Nearsightedness
is the inability to clearly see distant objects. Farsightedness is the
inability to clearly see close objects. A cataract is a lens that has become
opaque, resulting in impaired vision.
Rods and Cones
The
retina has millions of photoreceptors called rods and cones. Photoreceptors are
specialized cells that respond to light stimuli. There are many more rods than
cones. The long, narrow cells, called rods, are highly sensitive to light
and allow vision even in dim conditions. There are no rods in the fovea, which
is why vision becomes hazy in dim light. However, the area just outside the
fovea contains many rods, and these allow peripheral vision.
Because
rods are so sensitive to light, in dim lighting conditions peripheral vision is
sharper than direct vision.
Example: People can often see a star in the night sky if they look
a little to the side of the star instead of directly at it. Looking to the side
utilizes peripheral vision and makes the image of the star fall onto the
periphery of the retina, which contains most of the rods.
Cones are
cone-shaped cells that can distinguish between different wavelengths of light,
allowing people to see in color. Cones don’t work well in dim light, however,
which is why people have trouble distinguishing colors at night. The fovea has
only cones, but as the distance from the fovea increases, the number of cones
decreases.
|
Feature
|
Rods
|
Cones
|
|
Shape
|
Long and narrow
|
Cone-shaped
|
|
Sensitivity to light
|
High: help people
to see in dim light
|
Low: help people
to see in bright light
|
|
Help color vision
|
No
|
Yes
|
|
Present in fovea
|
No
|
Yes
|
|
Abundant in periphery of retina
|
Yes
|
No
|
|
Allow peripheral vision
|
Yes
|
No
|
Adaptation to Light
Dark adaptation is the process by which receptor cells sensitize to
light, allowing clearer vision in dim light. Light
adaptation is
the process by which receptor cells desensitize to light, allowing clearer
vision in bright light.
Connection to the Optic Nerve
Rods and cones
connect via synapses to bipolar neurons, which then connect to other neurons
called ganglion cells. The axons of all the ganglion cells in the retina come
together to make up the optic nerve. The optic nerve connects to the eye
at a spot in the retina called the optic disk. The optic disk is also called the
blind spot because it has no rods or cones. Any image that falls on the blind
spot disappears from view.
Transmission
of Visual Information
Visual
information travels from the eye to the brain as follows:
·
Light reflected from an object hits the
retina’s rods and cones.
·
Rods and cones send neural signals to the
bipolar cells.
·
Bipolar cells send signals to the ganglion
cells.
·
Ganglion cells send signals through the optic
nerve to the brain.
Bipolar and
ganglion cells gather and compress information from a large number of rods and
cones. The rods and cones that send information to a particular bipolar or
ganglion cell make up that cell’s receptive field.
Ganglion cell axons from the inner half of each eye cross over to the opposite
half of the brain. This means that each half of the brain receives signals from
both eyes. Signals from the eyes’ left sides go to the left side of the brain,
and signals from the eyes’ right sides go to the right side of the brain. The
diagram below illustrates this process.
Visual Processing in the Brain
After being
processed in the thalamus and different areas of the brain, visual signals
eventually reach the primary visual cortex in the occipital lobe of the brain’s
cerebrum. In the 1960s, David Hubel and Torsten Wiesel demonstrated that highly
specialized cells called feature detectors respond to these visual signals in the
primary visual cortex. Feature detectors are neurons that respond to specific
features of the environment, such as lines and edges.
From the
visual cortex, visual signals often travel on to other parts of the brain,
where more processing occurs. Cells deeper down the visual processing pathway
are even more specialized than those in the visual cortex. Psychologists
theorize that perception occurs when a large number of neurons in different
parts of the brain activate. These neurons may respond to various features of
the perceived object such as edges, angles, shapes, movement, brightness, and
texture.
Color
Vision
Objects in the
world seem to be brightly colored, but they actually have no color at all. Red
cars, green leaves, and blue sweaters certainly exist—but their color is a
psychological experience. Objects only produce or reflect light of different
wavelengths and amplitudes. Our eyes and brains then convert this light
information to experiences of color. Color vision happens because of two
different processes, which occur in sequence:
·
The first process occurs in the retina and is
explained by the trichromatic theory.
·
The second process occurs in retinal ganglion
cells and in cells in the thalamus and visual cortex. The opponent process
theory explains this process.
These two
theories are explained below.
The Trichromatic Theory
Thomas Young and Hermann von Helmholtz proposed the trichromatic
theory, or Young-Helmholtz
theory. This theory
states that the retina contains three types of cones, which respond to light of
three different wavelengths, corresponding to red, green, or blue. Activation
of these cones in different combinations and to different degrees results in
the perception of other colors.
Color Mixing
Mixing lights of different colors is called additive color mixing. This
process adds wavelengths together and results in more light. Mixing paints, on
the other hand, is called subtractive color mixing, a process that removes
wavelengths so that there is less light. If red, orange, yellow, green, blue,
indigo, and violet light were mixed, the result would be white light. If the
same color paints were mixed together, the result would be a dark, muddy color.
The
trichromatic theory also accounts for color blindness, a hereditary condition that affects
a person’s ability to distinguish between colors. Most color-blind people are dichromats, which means they are sensitive to
only two of the three wavelengths of light. Dichromats are usually insensitive
either to red or green, but sometimes they cannot see blue.
The Opponent Process Theory
Ewald Hering proposed the opponent process theory. According to this theory, the visual
system has receptors that react in opposite ways to three pairs of colors. The
three pairs of colors are red versus green, blue versus yellow, and black
versus white. Some receptors are activated by wavelengths corresponding to red
light and are turned off by wavelengths corresponding to green light. Other
receptors are activated by yellow light and turned off by blue light. Still
others respond oppositely to black and white.
Opponent
process theory explains why most people perceive four primary colors: red,
green, blue, and yellow. If trichromatic theory alone fully explained color
vision, people would perceive only three primary colors, and all other colors
would be combinations of these three colors. However, most people think of
yellow as primary rather than as a mixture of colors.
Opponent
process theory also accounts for complementary or negative afterimages. Afterimages are colors perceived after other,
complementary colors are removed.
Example: If Jack stares at a picture of a red square, wavelengths
corresponding to red will activate the matching receptors in his visual system.
For the sake of simplicity, these matching receptors can be referred to as red
receptors. Anything that makes red receptors increase firing will be seen as
red, so Jack will see the square as red. Anything that decreases the firing of
red receptors will be seen as green. If Jack stares at the square for a while,
the red receptors will get tired out and start to fire less. Then if he looks
at a blank white sheet of paper, he will see a green square. The decreased
firing of the red receptors produces an experience of a green afterimage.
Form
Perception
The ability to
see separate objects or forms is essential to daily functioning. Suppose a girl
sees a couple in the distance with their arms around each other. If she
perceived them as a four-legged, two-armed, two-headed person, she’d probably
be quite disturbed. People can make sense of the world because the visual
system makes sensible interpretations of the information the eyes pick up.
Gestalt psychology,
a school of thought that arose in Germany in the early twentieth century,
explored how people organize visual information into patterns and forms.
Gestalt psychologists noted that the perceived whole is sometimes more than the
sum of its parts. An example of this is the phi
phenomenon, or
stroboscopic movement, which is an illusion of movement that happens when a
series of images is presented very quickly, one after another.
Example: The phi phenomenon is what gives figures and objects in
movies the illusion of movement. In reality, a movie is a series of still
images presented in rapid succession.
Gestalt Principles
Gestalt
psychologists described several principles people use to make sense of what
they see. These principles include figure and ground, proximity, closure,
similarity, continuity, and simplicity:
·
Figure and ground: One of
the main ways people organize visual information is to divide what they see
into figure and ground. Figure is what stands out, and ground is the
background in which the figure stands. People may see an object as figure if it
appears larger or brighter relative to the background. They may also see an
object as figure if it differs noticeably from the background or if it moves
against a static environment.
·
Proximity: When
objects lie close together, people tend to perceive the objects as a group. For
example, in the graphic below, people would probably see these six figures as
two groups of three.
·
Closure: People
tend to interpret familiar, incomplete forms as complete by filling in gaps.
People can easily recognize the following figure as the letter k in spite of the gaps.
·
Similarity: People
tend to group similar objects together. In the next figure, people could
probably distinguish the letter T because similar dots are seen as a
group.
·
Continuity: When
people see interrupted lines and patterns, they tend to perceive them as being
continuous by filling in gaps. The next figure is seen as a circle superimposed
on a continuous line rather than two lines connected to a circle.
·
Simplicity: People
tend to perceive forms as simple, symmetrical figures rather than as irregular
ones. This figure is generally seen as one triangle superimposed on another
rather than a triangle with an angular piece attached to it.
Depth Perception
To figure out
the location of an object, people must be able to estimate their distance from
that object. Two types of cues help them to do this: binocular cues and
monocular cues.
Binocular Cues
Binocular cues are cues that require both eyes. These types of cues help
people to estimate the distance of nearby objects. There are two kinds of
binocular cues: retinal disparity and convergence.
·
Retinal disparity marks
the difference between two images. Because the eyes lie a couple of inches
apart, their retinas pick up slightly different images of objects. Retinal
disparity increases as the eyes get closer to an object. The brain uses retinal
disparity to estimate the distance between the viewer and the object being viewed.
·
Convergence is
when the eyes turn inward to look at an object close up. The closer the object,
the more the eye muscles tense to turn the eyes inward. Information sent from
the eye muscles to the brain helps to determine the distance to the object.
Monocular Cues
Monocular cues are cues that require only one eye. Several different
types of monocular cues help us to estimate the distance of objects:
interposition, motion parallax, relative size and clarity, texture gradient,
linear perspective, and light and shadow.
·
Interposition: When
one object is blocking part of another object, the viewer sees the blocked
object as being farther away.
·
Motion parallax or relative motion: When
the viewer is moving, stationary objects appear to move in different directions
and at different speeds depending on their location. Relatively close objects
appear to move backward. The closer the object, the faster it appears to move.
Distant objects appear to move forward. The further away the object, the slower
it appears to move.
·
Relative size: People
see objects that make a smaller image on the retina as farther away.
·
Relative clarity: Objects
that appear sharp, clear, and detailed are seen as closer than more hazy
objects.
·
Texture gradient: Smaller
objects that are more thickly clustered appear farther away than objects that
are spread out in space.
·
Linear perspective: Parallel
lines that converge appear far away. The more the lines converge, the greater
the perceived distance.
·
Light and shadow: Patterns
of light and shadow make objects appear three-dimensional, even though images
of objects on the retina are two-dimensional.
Creating Perspective
Artists use monocular cues to give a three-dimensional appearance to
two-dimensional pictures. For instance, if an artist wanted to paint a
landscape scene with a straight highway on it, she would show the edges of the
highway as two parallel lines gradually coming together to indicate that the
highway continues into the distance. If she wanted to paint cars on the
highway, she would paint bigger cars if she wanted them to seem closer and
smaller cars if she wanted them to seem farther away.
Perceptual
Constancy
Another
important ability that helps people make sense of the world is perceptual
constancy. Perceptual constancy is the ability to recognize that an
object remains the same even when it produces different images on the retina.
Example: When a man watches his wife walk away from him, her image
on his retina gets smaller and smaller, but he doesn’t assume she’s shrinking.
When a woman holds a book in front of her face, its image is a rectangle.
However, when she puts it down on the table, its image is a trapezoid. Yet she
knows it’s the same book.
Although
perceptual constancy relates to other senses as well, visual constancy is the
most studied phenomenon. Different kinds of visual constancies relate to shape,
color, size, brightness, and location.
·
Shape constancy: Objects
appear to have the same shape even though they make differently shaped retinal
images, depending on the viewing angle.
·
Size constancy: Objects
appear to be the same size even though their images get larger or smaller as their
distance decreases or increases. Size constancy depends to some extent on
familiarity with the object. For example, it is common knowledge that people
don’t shrink. Size constancy also depends on perceived distance. Perceived size
and perceived distance are strongly related, and each influences the other.
·
Brightness
constancy: People see objects as having the same brightness even
when they reflect different amounts of light as lighting conditions change.
·
Color constancy: Different
wavelengths of light are reflected from objects under different lighting
conditions. Outdoors, objects reflect more light in the blue range of
wavelengths, and indoors, objects reflect more light in the yellow range of
wavelengths. Despite this, people see objects as having the same color whether
they are outdoors or indoors because of two factors. One factor is that the
eyes adapt quickly to different lighting conditions. The other is that the
brain interprets the color of an object relative to the colors of nearby
objects. In effect, the brain cancels out the extra blueness outdoors and the
extra yellowness indoors.
·
Location constancy: Stationary
objects don’t appear to move even though their images on the retina shift as
the viewer moves around.
Visual
Illusions
The brain uses
Gestalt principles, depth perception cues, and perceptual constancies to make
hypotheses about the world. However, the brain sometimes misinterprets
information from the senses and makes incorrect hypotheses. The result is an
optical illusion. An illusion is a misinterpretation of a sensory
stimulus. Illusions can occur in other senses, but most research has been done
on visual illusions.
In the famous Muller-Lyer
illusion shown
here, the vertical line on the right looks longer than the line on the left, even
though the two lines are actually the same length.
This illusion is probably due to misinterpretation of depth perception
cues. Because of the attached diagonal lines, the vertical line on the left looks
like the near edge of a building, and the vertical line on the right looks like
the far edge of a room. The brain uses distance cues to estimate size. The
retinal images of both lines are the same size, but since one appears nearer,
the brain assumes that it must be smaller.
Perceptual
Set
The
Muller-Lyer illusion doesn’t fool everyone equally. Researchers have found that
people who live in cities experience a stronger illusion than people who live
in forests. In other words, city-dwelling people see the lines as more
different in size. This could be because buildings and rooms surround city
dwellers, which prepares them to see the lines as inside and outside edges of
buildings. The difference in the strength of the illusion could also be due to
variations in the amount of experience people have with making
three-dimensional interpretations of two-dimensional drawings.
Cultural
differences in the tendency to see illusions illustrate the importance of
perceptual set. Perceptual set is the readiness to see objects in a
particular way based on expectations, experiences, emotions, and assumptions.
Perceptual set influences our everyday perceptions and how we perceive reversible
figures, which are
ambiguous drawings that can be interpreted in more than one way. For example,
people might see a vase or two faces in this famous figure, depending on what
they’re led to expect.
Selective Attention
Reversible
figures also illustrate the concept of selective attention, the ability to focus on some bits of
sensory information and ignore others. When people focus on the white part of
the figure, they see a vase, and when they focus on the black part of it, they
see two faces. To use the language of Gestalt psychology, people can choose to
make the vase figure and the face ground or vice versa.
Selective
attention allows people to carry on day-to-day activities without being
overwhelmed by sensory information. Reading a book would be impossible if the
reader paid attention to not only the words on the page but also all the things
in his peripheral vision, all the sounds around him, all the smells in the air,
all the information his brain gets about his body position, air pressure,
temperature, and so on. He wouldn’t get very far with the book.
Context
Effects
Another factor
that influences perception is the context of the perceiver. People’s immediate
surroundings create expectations that make them see in particular ways.
SparkNotes Editors. (2005). SparkNote on Sensation and
Perception. Retrieved December
2016,from
http://www.sparknotes.com/psychology/psych101/sensation/
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