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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 |
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.
Structure of the Eye
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.
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.
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.
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.
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.
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.
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.
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.
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