SENSATION & PERCEPTION
Vision
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Sensation & Perception
Sensation and perception are areas that
have been of interest to psychologists for
most of the history of psychology. As we sit
here, our senses receive literally thousands
of messages. We need to make sense of
this information. Our senses take in the
information, and they do so from birth. Yet
the interpretive part -perceptionrequires knowledge.
We only use
light energy to see.
What makes up a light wave?
Wavelength
The distance from the peak of one light wave to the
peak of the next.
The wavelength distance determines the hue (color)
of the light we perceive.
Intensity (Brightness)
The amount of energy in a light wave is...
determined by the height of the wave
The higher the wave...
the more intense the LIGHT is.
Saturation
If the human eye was not responsive to differences in the purity
of light waves we would not be able to perceive differences in
saturation.
Transduction: Energy that we see as visible light
Conversion of one form of energy to another.
How is this important when
studying sensation?
Stimulus energies to neural impulses. In
the system transduction occurs when
environmental energy is transformed
into electrical or neural energy.
Receptor cells produce an electrical
change in response to a physical
stimulus.
For example:
★ Light energy to vision.
★ Chemical energy to smell and taste.
★ Sound waves to sound.
Structure of the Eye
Pupil - adjustable opening in the center of the eye
Iris - a ring of muscle that forms the colored portion of the eye
around the pupil and controls the size of the pupil opening
Lens - transparent structure behind pupil that changes shape
to focus images on the retina
Structure of the Eye
Cornea - Transparent outer covering of the eye.
Retina - Contains visual sensory receptors.
Fovea - Point of central focus. Contains most of the eye’s
cones.
Optic Nerve - Pathway to the brain’s visual cortex.
Blind Spot - Where optic nerve leaves eye - no receptors
here.
Nearsighted
FARsighted
AKA: Myopia. Nearby objects are seen more clearly
than distant objects b/c eye is elongated & image
focus before it hits the retina.
AKA: Hyperopia. Faraway objects are seen more
clearly than near objects b/c eye is shortened &
image would focus after it hit the retina.
Need for Bifocals. This issue can become more pronounced after the age of 40, when the lenses of the eyes
become progressively less flexible and have trouble accommodating to focus on objects at different
distances.
Blind Spot & Blindsight
Blind Spot occurs is the location in the
retina where the visual cortex exits to the
brain, there are no receptors there
You don't notice this blind spot in every-day
life, because your two eyes work together to
cover it up. Our brain, typically, fills in that
missing piece based on what it estimates to
be there
Blindsight—we can see things we don’t
perceive
Blind spot experiment
To find the blindspot, draw a filled-in, 1/4"-sized square and a
circle three or four inches apart on a piece of white paper.
Hold the paper at arm's length and close your left eye. Focus on the square with your
right eye, and slowly move the paper toward you. When the circle reaches your blind
spot, it will disappear! Try again to find the blind spot for your other eye. Close your
right eye and focus on the circle with your left eye. Move the paper until the square
disappears.
What happened when the circle disappeared? Did you see nothing where the
circle had been? No, when the circle disappeared, you saw a plain white background
that matched the rest of the sheet of paper.
This is because your brain "filled in" for the blind spot - your eye didn't send any
information about that part of the paper, so the brain just made the "hole" match the rest.
Try the experiment again on a piece of colored paper. When the circle disappears, the
brain will fill in whatever color matches the rest of the paper.
Fovea
Central point in the retina, around
which the eye’s cones cluster...
because of this there is little color
vision in the farthest periphery of
our vision. “Rod Free Area”
The Retina
The light-sensitive inner surface of the eye, containing receptor rods and cones plus
layers of other neurons (bipolar, ganglion cells) that process visual information.
Photoreceptor Cells
Rods
★ peripheral retina
★ detect black, white & gray
★ twilight or low light
Cones
★ near center of retina
★ fine detail & color vision
★ daylight or well-lit conditions
The retina contains two
types of photoreceptors:
rods and cones
The rods are more numerous, some
120 million, and are more sensitive
than the cones.
However, they are not sensitive to
color.
The 6 to 7 million cones provide the
eye's color sensitivity and they are
much more concentrated in the central
yellow spot known as the macula. In
the center of that region is the "fovea
centralis”, a 0.3 mm diameter rodfree area with very thin, densely
packed cones.
Photoreceptor Cells
Rods
★ peripheral retina
★ detect black, white & gray
★ twilight or low light
CONES
RODS
NUMBER
6 MILLION
120 MIL.
LOCATION
CENTER
(FOVEA)
PERIPHERY
COLOR
SENSITIVE?
YES
NO
SENSITIVITY IN DIM
LIGHT?
LOW
HIGH
ABILITY TO
DETECT DETAIL?
HIGH
LOW
NUMBER OF
BIPOLAR CELLS?
EACH HAS
OWN
SHARES ONE
WITH OTHER
RODS
Cones
★ near center of retina
★ fine detail & color vision
★ daylight or well-lit
conditions
BIPOLAR CELLS:
These are cells that rods and cones
send messages through
Bipolar & ganglion cells
Bipolar
★ Receive messages from photoreceptor
cells & transmit them to the...
Ganglion
★ which form the optic nerve (axons) &
sends messages on into the brain to the
visual cortex (occipital lobe).
Remember...
Cones each have their own bipolar
cells.
Multiple rods share one bipolar cell.
Feature Detection
Cells in the visual cortex of the
brain that respond selectively
to specific features of complex
stimuli.
Edges, Angle, Length, &
Movement
How is it possible that Frank Caliendo can possibly imitate
so many different people so convincingly?
Parallel Processing
The processing of several aspects
of a problem simultaneously.
COLOR MOTION FORM DEPTH
Perception of Color
The Eagle Uniform is
anything but red.
The uniform rejects the long
wavelengths of light that to
us are red.
So red is reflected off and
we see it.
Also, light has no real color.
It is our mind that
perceives the color.
2 color theories
Young-Helmholtz Trichromatic (3 color) Theory
According to the Young-Helmholtz theory of color vision, there are 3
receptors in the retina that are responsible for the perception of
color.
1 receptor is sensitive to the color green, another to the color blue, and a third to the color red.
These three colors can then be combined to form any visible color in the spectrum.
3 different types of receptor cells in our eyes. Together they can pick any
combination of our 7 million color variations.
Most colorblind people simply lack cone receptor cells for one or more
of these primary colors.
Principle of Additive Color Mixing
At “Showtime” you may notice that whenever the red &
green spotlights overlapped, they seemed to change to
a yellow spotlight.
Opponent-Process Theory
We cannot see certain colors together in combination.
These are antagonist/opponent colors.
white-black green-red yellow-blue
★ Trichromatic theory makes clear some of the processes
involved in how we see color, it does not explain all
aspects of color vision.
★ Opponent-process theory of color vision was developed
by Ewald Hering who noted that there are some color
combinations that we never see, such as reddish-green
or yellowish-blue.
★ Opponent-process theory suggests that color
perception is controlled by the activity of two opponent
systems; a blue-yellow mechanism and a red-green
mechanism.
Opponent-Process Theory
Opposing retinal processes
enable color vision
“OFF”
“ON”
red
green
green
blue
yellow
blue
black
white
white
black
red
yellow
Which theory is accepted?
Trichromatic or opponent process?
Most researchers agree with a combination of trichromatic and
opponent-process theory.
Individual cones appear to correspond best to the
trichromatic theory, while the opponent processes may occur
at other layers of the retina.
The important thing to remember is that both concepts are
needed to explain color vision fully.
Color Blind & Vision-Color Issues
People who suffer red-green blindness have trouble
perceiving the number within the design.
Color defects are genetically transmitted, recent
research has conclusively mapped this transmission.
Monochromats — have no or only one type of
functioning cone type and respond to light like a black
and white film—colors are records only as gradations of
intensity, likely to find daylight uncomfortable if no
functioning cones, those with one cone okay but still
can’t discriminate colors—very small number of people
have this.
Dichromats — one malfunctioning cone system,
depending on type, various colors will not be perceived,
inability to perceive blue is the rarest, in 1950 England,
found 17 people.
After
After Image
Image Experiment
Experiment
Opposite opponent colors are never perceived together.
There is no greenish-red or yellowish-blue.
You can create your own demonstration of these opponent
systems by observing the effect of afterimages.
Look at the center of the the “X” on the next screen for
approximately 30 seconds.
Then immediately look at the next white slide & blink to see the
afterimage.
After Image Experiment
Stare at the middle of the “X” for 30 seconds.
BLINK ABOUT 10 TIMES OR SO - THEN LOOK DIRECTLY INTO THE WHITE SCREEN...
DO YOU SEE THE AFTER IMAGE?
"An afterimage can retain the colors of the original stimulus
(positive afterimage,) or the colors might be reverse in the
afterimage, like a photographic negative (negative afterimage).
The conditions favoring the production of afterimages are either
brief exposures to intense or very bright stimuli, in otherwise dark
conditions (a quick glance at the setting sun), or prolonged
exposures to colored stimuli in well-lighted conditions (fixating
steadily on a colored object for 60 sec and then averting the eyes
to a gray or white background). With brief stimuli the first
afterimage is usually positive (same colors as the visual
stimulus), and when only a single stimulus is presented, the
positive afterimage is difficult to distinguish from the initial image
or sensation."
(Gustav Levine & Stanley Parkinson, Experimental Methods in
Psychology, 1994)
"After staring at the red and blue shamrock, you saw a green and yellow afterimage.
Opponent-process theory proposes that as you stared at the red and blue shamrock,
you were using the red and blue portions of the opponent-process cells. After a period
of 60 to 90 seconds of continuous staring, you expended these cells' capacity to fire
action potentials. In a sense, you temporarily "wore out" the red and blue portions of
these cells. Then you looked at a blank sheet of white paper. Under normal conditions,
the while light would excite all of the opponent-process cells. Recall that white light
contains all colors of light. But, given the exhausted state of your opponent-process
cells, only parts of them were capable of firing action potentials. In this example, the
green and yellow parts of the cells were ready to fire. The light reflected off of the white
paper could excite only the yellow and green parts of the cells, so you saw a green and
yellow shamrock."
(Ellen Pastorino & Susann Doyle-Portillo, What Is Psychology? Essentials, 2010)
In a negative afterimage, the colors you see are inverted from the original image.
For example, if you stare for a long time at a red image, you will see a green
afterimage. The appearance of negative afterimages can be explained by the
opponent process theory of color vision.
You can see an example of how the opponent-process works by staring at the red
shamrock on the next slide for about 1 minute before shifting your gaze immediately to
a white sheet of paper or a blank screen.
Stare at the center dot for 1 minute. Then when you see the next blank slide, you should have an
“afterimage” -- and some good luck!
Did it work?
In this fun optical illusion, you can see how your visual system and brain are actually able to briefly create a color
image from a negative photo.
How to Perform the Illusion
Stare at the dots located at the center of the woman's face below for about 30 seconds to a minute. Then turn
your eyes immediately to the center x of the white image on the right.
Blink quickly several times. What do you see? If you've followed the directions correctly, you should see an
image of a woman in full-color. If you are having trouble seeing the effect, try staring at the negative image a bit
longer or adjusting how far you are sitting from your computer monitor.
Explanations
How does this fascinating visual illusion work?
What you are experiencing is known as a negative afterimage. This
happens when the photoreceptors, primarily the cone cells, in your
eyes become overstimulated and fatigued causing them to lose
sensitivity. In normal everyday life, you don't notice this because tiny
movements of your eyes keep the cone cells located at the back of your
eyes from becoming overstimulated.
If, however, you look at a large image, the tiny movements in your
eyes aren't enough to reduce overstimulation. As a result, you
experience what is known as a negative afterimage. As you shift your eyes
to the white side of the image, the overstimulated cells continue to
send out only a weak signal, so the affected colors remain muted.
However, the surrounding photoreceptors are still fresh and so they
send out strong signals that are the same as if we were looking at the
opposite colors. The brain then interprets these signals as the
opposite colors, essentially creating a full-color image from a negative
photo.
According to the opponent process theory of color vision, our
perception of color is controlled by two opposing systems: a
magenta-green system and a blue-yellow system.
For example, the color red serves as an antagonistic to the color
green so that when you stare too long at a magenta image you will
then see a green afterimage. The magenta color fatigues the magenta
photoreceptors so that they produce a weaker signal. Since magenta's
opposing color is green, we then interpret the afterimage as green.