Physics 1230: Light and Color
•  Patterns and Motion perceived by the Eye
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Temporal response and afterimages
Patterns or spatial frequency
Motion
Channels
http://www.colorado.edu/physics/phys1230
Your RETINA is wired to enhance edges!!
http://micro.magnet.fsu.edu/optics/lightandcolor/vision.html
CENTER-SURROUND OF GANGLION CELLS
When light strikes
any of the rods/
cones in the
surround, the
ganglion is inhibited
(--)
When light strikes
any of the rods/
cones in the center
of the receptive
field, the ganglion is
excited (++)
++++++
++++++
+++
RESPONSE OF EYE DEPENDS ON THE PATTERN
OF THE LIGHT
RESPONSE OF EYE DEPENDS ON THE PATTERN
OF THE LIGHT
(a) When a uniform gray area is imaged on the receptive field, a small
background response occurs
(b) When bright light is imaged on the center of the receptive field, a large
response occurs
(c) When bright light is imaged on the surround of the receptive field, the
response is decreased below background
(d) When bright light is imaged on the center and surround of the receptive
field, a small background response occurs - excitation and inhibition balance
(e) When bright bars are imaged on the center and surround of the receptive
field, a very large response occurs - the dark bars reduce inhibition
(e) When closely spaced bars are imaged on the center and surround of the
receptive field, a small background response occurs - there is no net excitation
or inhibition
Stabilization causes fading
Look at the upper right figure.This experiment works better if you close one
eye. Fixate on the black dot in the center for about 10 - 15 seconds. Pay close
attention to what you see. Now do the same thing with the upper left figure.
Did you notice any difference?
http://www.yorku.ca/eye/toc-sub.htm
Stabilized fading explained
•  The visual system does not like steady state stimulation. There is sophisticated
apparatus that allows you to view a stimulus in such a way as to nullify your
natural eye movements so that the image of the stimulus remains on exactly the
same part of the retina as if there were no eye movements. Such apparatus is
called a stabilized image system.
•  Now to the disappearing disk. Most people would see the smudge in the upper
left disappear as they stared at the black dot. Most people would not see the
smudge disappear in the upper right.
•  In the upper left, the darker area slowly becomes lighter as one moves away
from the black dot. This gradual change from black to white is a poor stimulus
for sustaining visual perception. However, if you allow your eyes to freely move
over the stimulus the perception of it will be sustained. When you fixate on the
black dot and try and hold your gaze as steady as possible the smudge should
fade away and the color of the background would predominate.
•  The upper right figure is exactly the same as the upper left except for the
dark gray ring. This dark gray ring is sufficient to keep the stimulus "alive" no
matter how hard you stare.
Stabilized fading explained
•  When you fixate the black dot and try to hold your gaze as steady as possible,
your eyes are still in constant motion. True, many of these eye movements are
very tiny tremors as opposed to the large saccades or pursuit eye movements we
make. Nevertheless these small tremors can keep a stimulus "alive". When the
stimulus is one as in the upper left where there is a very gradual change from
gray to white, the change in stimulation is so slight as to approach that
encountered by the steady state condition of a stabilized image. As a result the
image fades.
•  You will undoubtedly have noticed that even when you fixated the upper left
field and the smudge disappeared, it would spontaneously reappear and then again
fade. It reappeared because you made a large enough eye movement.
•  When you stare at a white piece of paper, the center of your view is effectively
retinally stabilized. Even when your eye moves slightly, it still sees white paper.
However, the white does not fade at the center because of EDGES. Your brain
receives the information about the edges of the paper, and your brain fills in
(much as it does for your blind spot).
Negative Afterimages
The sensitivity of a given region of your retina decreases after it is exposed
to a bright light for a period of time. This is called successive lightness
contrast.
Prolonged stimulation adapts (or desensitizes) part of your retina, so that it
has a weaker response to subsequent stimulation.
Stare at the black dot on the left for some time - then look at the dot on the
right. What do you see?
Negative Afterimages
During the period of adaptation to the cat, parts of your retina are
overexposed and become desensitized.
But the parts of your retina outside the cat are not desensitized. So when you
look at the white region on the right, you see a dark cat on a white
background.
As you move your eyes around, the afterimage should move also - to
correspond to the desensitized region of your retina. (for about 30 seconds or
so).
Rapid blinking should get rid of the afterimage faster!
Eye movements and Afterimages
•  Small, involuntary, eye movements cause the image to be scanned on your retina.
•  If you look at the figures below in your text (Fig. 7.17 and 7.18), it will appear to shimmer no matter how hard you try to prevent your eye from moving.
•  In the figure on the right, as you eye scans, image and afterimage superimpose, to give a
ripple effect.
•  In the figure on the left, depending on how your eye moves, some of the afterimages
coincide with the figure itself, and some do not. So only some areas shimmer at any one time.
Temporal Response - Positive Afterimages
•  Your retina responds only if there is a change in stimulation with time
(light turning on and off, eye scanning across an edge)
•  The response of your visual system to a brief flash is both delayed
(latency), and of longer duration (persistence) than the flash itself.
•  Positive afterimages allow us to see the flash after it is over (white
where there was white, black where there was black)
•  These afterimages can last as long as 1/20 seconds at low ambient light
levels, but are shorter at high light levels.
•  The duration of afterimages can be extended by sensitizing your eyes
beforehand by closing and covering them.
Extending the Duration of Positive Afterimages
•  Face a window through which bright sky is visible.
•  Close and cover your eyes for 30 seconds.
•  Then take your hands away for 3 seconds, looking at the intersection of
the bars of the windows against the bright sky. Look at one point for all 3
seconds.
•  Then close and cover your eyes again. Time the duration of the positive
afterimage.
•  Closing and covering your eyes increases their sensitivity.
•  Returning them to the dark increases their persistence time, so you can
observe the afterimage for longer.
MOVIES AND TV
•  For TV, a sequence of images is presented - in the US at one frame every 1/30 sec
•  The persistence of vision can be as short as 1/50 seconds, so you might expect that
the screen might flicker. This problem is avoided by scanning the horizontal lines in
the TV in order 1,3,5,7….. and 2,4,6,8……so that two frames, each covering the screen,
are interleaved. The refresh is thus every 1/60 seconds, so that flicker is avoided.
•  Movie frames are projected at 1/24 seconds, but a special shutter in the projector
shows each frame 3 times. So the rate is 72 per second, and flicker is avoided.
•  Older projects did not have the special shutter, so the image sometimes flickered hence the name “the flicks”.
•  On smaller computer monitors (<14") few people notice any discomfort below 60–72Hz
refresh rates. On larger monitors (>17") most people would experience mild discomfort
unless the refresh is set to a more comfortable 85 Hz or higher. 100 Hz is
comfortable for almost any size
FRAME RATE and REFRESH RATE
The refresh rate (most commonly the "vertical refresh rate", "vertical scan
rate" for CRTs) is the number of times in a second that display hardware
(re)draws the data it is being given. This is distinct from the measure of
frame rate in that the refresh rate includes the repeated drawing of
identical frames, while frame rate measures how a video source can feed an
entire frame of new data to a display. For example, most movie projectors
advance from one frame to the next 24 times each second. But each frame is
illuminated twice or three times before the next frame is projected using a
shutter in front of its lamp. As a result, the movie projector runs at 24
frames per second, but has a 48 or 72 Hz refresh rate. On CRT displays,
increasing the refresh rate decreases flickering, thereby reducing eye
strain.
Early movies - Zoetropes
The zoetrope is one of several animation toys which were invented in the 19th
century, as people attempted to invent ways to make moving pictures. The
zoetrope appeared first in England in 1834, then France in 1860, and finally
the United States in 1867.
High Speed Photography
•  How might these photographs be taken?
Freezing motion: high-speed photography (msec)
•  Eadweard Muybridge
•  The Galloping Horse Portfolio, 1887
http://www.artcyclopedia.com/artists/muybridge_eadweard.html
Stroboscopic photography (µsec)
.30 Bullet Piercing an Apple, 1964. A microsecond exposure of a bullet
travelling 2800 feet per second. George Eastman House collection www.geh.org
Stroboscopic photography
With the advent of the electronic flash and the electronic stroboscope, and
primarily under the guidance of Harold Edgerton from the 1930's through the
1980's, the recording of subjects in motion onto film (either stationary or
moving) became almost the exclusive domain of electronic stroboscopes.
Modern photographic
stroboscopy in its simplest form
is a method whereby a subject
in motion is lit by repeating
flashes of light from the
stroboscope while the shutter
of the camera remains open for
a period of time long enough to
capture the subject in multiple
locations during the time of
exposure.
http://www.rit.edu/~andpph/text-figures/strobe-schifley-1.jpg
Stroboscopic photography
Concept Question - Stroboscopic photography
How was this photo taken?
A.  Patching many individual stroboscopic photos together
B.  Leaving the shutter open and flashing the strobe light many times
C.  Leaving the shutter open and the lights on
The visual pathway
In the diagram, we can see that the
analyses of objects in three
dimension is based on the retinal
disparity between the images formed
in the left and the right eye.
Our brain must perform a great deal
of computations to give us an
interpretation which seems so evident
to us at first sight. In humans, it is
important for binocular stereoscopic
depth perception that each of the
possibly two retinal images of a visual
field be mapped onto the same region
of the brain.
http://ligwww.epfl.ch/~fua/vision/3/misc/exam/human/2/
Perception of Motion
Assume that while you are
staring at the bird, a racing car
zooms by. The image of the car
will travel across your retina as
indicated by the dotted line with
the arrow. This image movement
will cause you to say that the car
moves from your right to your
left.
http://www.yorku.ca/eye/toc-sub.htm
Perception of Motion
This time you are following
the car by moving your eyes
from right to left. Just as
before, your percept is that
of the car moving from right
to left. This is true even
though the image remains on
the fovea during the motion
of the car and your eyes.
http://www.yorku.ca/eye/toc-sub.htm
Perception of Motion
This illustration shows that another
way to follow the racing car is to keep
the eyes steady and to move just the
head. This causes the image to project
to exactly the same retinal location at
each instant (assuming you move your
head at precisely the correct angular
velocity) as the car moves from right
to left.
Once again, the perception is of the car
moving from right to left. This
perception will be the same as the two
previous illustrations. How the brain
distinguishes these different ways of
following moving objects is the subject
of much research.
These illustrations are gross
simplifications. In point of fact, when
we follow moving objects we use various
combinations of head and eye
movements.
http://www.yorku.ca/eye/toc-sub.htm
Motion Blindness
The patient had great difficulty pouring coffee into a cup. She
could clearly see the cup's shape, color, and position on the table,
she told her doctor. She was able to pour the coffee from the
pot. But the column of fluid flowing from the spout appeared
frozen, like a waterfall turned to ice. She could not see its
motion. So the coffee would rise in the cup and spill over the
sides. More dangerous problems arose when she went outdoors.
She could not cross a street, for instance, because the motion of
cars was invisible to her: a car was up the street and then upon
her, without ever seeming to occupy the intervening space. Even
people milling through a room made her feel very uneasy, she
complained to Josef Zihl, a neuropsychologist who saw her at the
Max Planck Institute for Psychiatry in Munich, Germany, in 1980,
because "the people were suddenly here or there but I did not
see them moving."
The woman's rare motion blindness resulted from a stroke that
damaged selected areas of her brain. What she lost—the ability
to see objects move through space—is a key aspect of vision. In
animals, this ability is crucial to survival: Both predators and
their prey depend upon being able to detect motion rapidly. In
fact, frogs and some other simple vertebrates may not even see
an object unless it is moving.
Motion Blindness
While the retina of frogs can detect movement, the retina of
humans and other primates cannot.
"The dumber the animal, the smarter its retina," observes
Denis Baylor of Stanford Medical School. The large, versatile
brain of humans takes over the job, analyzing motion through a
highly specialized pathway of neural connections.
This is the pathway that was damaged in the motion-blind
patient from Munich. Compared to the complex ensemble of
regions in the visual cortex that are devoted to perceiving color
and form, this motion-perception pathway seems relatively
streamlined and simple. More than any other part of the
cortex, it has yielded to efforts to unveil "the precise
relationship between perception and the activity of a sensory
neuron somewhere in the brain," says Anthony Movshon, an
HHMI investigator at New York University.
Motion Blindness
Consider what happens when we watch a movie, suggests
Thomas Albright of the Salk Institute. Each of the 24 frames
projected per second on the theater screen is a still
photograph; nothing in a movie truly moves.
The illusion of movement is created by the motion-processing
system, which automatically fuses, for instance, the images of
legs that shift position slightly from frame to frame into the
appearance of a walking actor. The Munich patient is unable to
perform this fusion. In life or in the movie theater, she sees
the world as a series of stills.
"The motion system must match up image elements from frame
to frame, over space and time," says Albright. "It has to detect
which direction a hand is moving in, for instance, and not
confuse that hand with a head when it waves in front of
someone's face."
Motion After-effects
Reverse Spoke Illusion
http://www.michaelbach.de/ot/mot_spokes/index.html
Spatial Frequencies
http://www2.psy.uq.edu.au/local-uq/py255/jack/2B/sld004.htm
Contrast Sensitivity Function
Measure your CSF - the envelope of the visible part of the figure marks your CSF.
The horizontal axis is spatial frequency, and the vertical axis is grating or contrast.
Our eyes are not sensitive to very low or very high spatial frequencies, or to very low
contrast.
http://www2.psy.uq.edu.au/local-uq/py255/jack/2B/sld006.htm
Channels - Spatial frequency adaptation
1. Look at the two gratings on the right side of the figure. Do you see any
differences between them?
2. Now for about 60 seconds scan your gaze back and forth along the horizontal
red bar between the two gratings on the left.
3. When you are finished scanning back and forth along the horizontal red bar,
fixate on the short red bar between the gratings on the right.
4. What differences do you see between these two gratings on the right
compared to the first time you looked at these gratings?
Spatial frequency adaptation
The two gratings on the right, when you looked at them the first time, undoubtedly
appeared identical. When you scanned the red bar between the two gratings on the left you
allowed your visual system to adapt to these gratings, while avoiding standard afterimages
by moving your eye. The adaptation was not the same for the top and bottom grating
because they differ in spatial frequency. The one on the bottom has a higher spatial
frequency than the one on top. Then, after adapting to the gratings on the left for
approximately 60 seconds, the ones on the right (if viewed while fixating between them on
the red square) probably no longer appear identical. This non-identity will not last long.
Current conventional wisdom to understand this is based on spatial frequency channels.
Spatial frequency adaptation
The analogous experiment involving orientation can be done using slanted gratings.
SPATIAL FREQUENCY ADAPTATION
The five test gratings on the left each has
a low contrast, so they are barely visible.
Adapt to a high-contrast grating on the
right by allowing your eye to move around
inside one of the circles. After 5 seconds,
look at the five gratings on the left.
What do you see?
The test grating having the same spatial
frequency as the one you adapted to
becomes invisible, while the others remain
visible.
HUMAN CSF ADAPTATION
If you try to desensitize the mechanisms that respond to gratings or spatial
frequencies, you find that after adapting to a grating of a certain period, your
sensitivity is lowered only for spatial frequencies near the adaptation grating.
The normal CSF is shown as a solid
line.
The CSF after prolonged adaptation
to previous images is shown as a
dotted line.
Channels
We describe CSF and similar effects by saying that adaptation to different
spatial frequency gratings desensitizes different channels.
A channel is a subsystem of the visual system that responds preferentially to
one type of stimulus rather than another.
It is an abstraction by your visual system of some attribute of the stimulus.
There are channels associated with low, medium and high spatial frequencies.
These channels naturally result from the center-surround receptive field,
because different size receptive fields will respond best to different spatial
frequencies.
Simultaneous Size Contrast – adaptation
by Tilt Channels
How are the center gratings oriented in
each figure?
So we have channels that adapt to
whether a set of stripes is tilted or not!
Tilt Channels
(a) Vertical tilt channel
(b) 45 degree tilt channel
A 45 degree grating of the right
spatial frequency will excite the
45 degree tilt channel but not
the vertical tilt channel.
Other Channels - the
Waterfall Illusion
Motion Channels: When you look out the window of a moving train, that then stops at a
station, what happens? Answer: The platform appears to drift forward slowly.
Example: The Waterfall Illusion is another name for the motion aftereffect. The motion
aftereffect refers to the modification of motion perception following prolonged
observation of a regularly moving stimulus. Typically the motion aftereffect involves the
apparent motion of a stationary stimulus in the opposite direction to a previously observed
one, but it can also result in a change in the apparent velocity of a moving stimulus.
Explanation: It is still a matter of debate how and why the waterfall illusion happens. The
motion sensors for the opposite directions are compared to give a final output (motion
opponency). Normally the outputs of these sensors are balanced when looking at a
stationary scene, but adaptation to a motion in one direction leads to a decrease of output
in that direction, which results in a unbalance of outputs and an illusory movement in the
other direction. The decrease of the output may be simply due to fatigue, but recently it
is considered as a kind of more active calibration.
Why?:
*
*
*
Re-calibration of 'stationary' signals
Maximization of the processing effectiveness with limited sensor capacities
Error correction?
OTHER CHANNELS
LOOMING CHANNELS: The world appears to rush toward you when jogging or
driving. What happens when you stop?
ANS: The world appears to recede slowly.
Adaptation to looming channels can cause traffic accidents!
If, after driving for a long time at high speed on a freeway, you slow down to
exit, the intermediate speed seems slower (and safer) than it actually is!!!!
http://www.michaelbach.de/ot/mot_adapt/index.html
http://www.michaelbach.de/ot/mot_adaptSpiral/index.html
more Illusions – the Ouchi Illusion
Move your eyes around the image. Does the circular middle section appear to separate
from the rest of the figure? Does it appear to be at a different depth and even move?
The Ouchi Illusion is not well understood. The illusionary motion and perceived depth may
arise from the ambiguity formed at the circular contour with the adjoining vertical edges.
http://www.illusionworks.com/html/ouchi_illusion.html
Café Wall Illusion
see animation from IllusionWorks website
and
http://www.michaelbach.de/ot/ang_cafewall/index.html
CAFÉ WALL ILLUSION
Comment. This illusion demonstrates the effect of some simple image
processing occurring at the retina combined with some complex processing in
the cortical cells of the striate cortex. The incoming image is first filtered by
the center-surround operator of the retina. The apparent tilt of the mortar
lines is caused by orientation-sensitive simple cells in the striate cortex. The
cells interact with one another to interpret the diagonal bands produced by
the retina as a single continuous line, tilted in the direction of the diagonal
bands.
the original café in Bristol
http://www.michaelbach.de/ot/ang_cafewall/index.html
Hering ILLUSION
http://www.michaelbach.de/ot/ang_hering/index.html
Rotating Snake Illusion
http://www.michaelbach.de/ot/ang_hering/index.html