Physics 1230: Light and Color
Chapter 7
• Chapter 7:
• The retina causes the following
The Human Eye and Vision - II:
processes to occur in space, due
Processing the Image
to lateral inhibition by receptive
fields
• The image that falls on your
• lightness constancy
retina is not what your brain
• edge enhancement
"gets"
• simultaneous lightness contrast
• Your retina changes the image
• perception of light and dark
similar to a computer program
regions of a scene or image
like Adobe Photoshop. It
• The following processes occur
processes the image it receives
in time
• Compare to a camera;
• Negative and positive afterimages
• Involuntary eye movements
• Persistence of vision
1
Exam #2 on October 29
• Preparation:
1. Course web page
2. CULearn – practice
problems;
3. Review in Class on
Tuesday
• Extra Credit HW
assigned through
CULearn
Compare these two pictures to understand lightness
constancy
•
•
The one below is seen in less illumination •
than the one at right
The relative darkness and lightness of
different parts of the image is preserved
when there is more illumination
The lightness of the part of the dog in
the white rectangle at left is actually
almost the same as the part of the dog in
the white rectangle below!
Here is another example of how we (mis)interpret lightness.
The squares marked A and B have the same lightness.
Review and further details of the structure of the retina
• Rods and cones are (light sensitive)
photo-receptor cells which convert
light into electrical signals then sent
to nerve cells
• Cones: daylight high resolution
viewing & reading. Dominate fovea
• Rods are more sensitive than cones.
Used for night vision. Mostly
outside of fovea
• Pooling
• Many rods feed their signals into the
same detector nerve cells
• This is called pooling — leads to
greater sensitivity
• Cones are less pooled. They resolve
better by feeding into different
nerve cell detectors
• Example - cones can resolve 2 light
sources, whereas pooled rods
indicate there is 1 light source
image of 2 lights
1 nerve
cell
detector
to brain
rods
2 nerve cell
detectors
cones
to brain
image of
2 lights
How are the rods and cones connected to the nerve
cells?
•
•
•
The photoreceptors (rods & cones) are
connected to bipolar cells
These in turn are connected to ganglion
cells which send their signals to the
brain through the optic nerve
Sideways connections are made through
the horizontal cells.
•
•
Sideways connections enable pooling to
occur at various levels
They also allow lateral inhibition to
occur by means of receptive fields
Information flow from the eyes
optic pathways from each of the 4 quadrants of view
for both eyes simultaneously
• the nerves connected to the right
eye cross to the left half of the
brain
• while the nerves from the left eye)
cross to the right half of the brain.
• This allows for parts of both eyes
that attend to the right visual field
to be processed in the left visual
system in the brain, and vice versa
A receptive field is a group of photoreceptors over a region of your
retina which are connected to a (ganglion) cell.
• The ganglion cell emits
electrical signals at a medium
(ambient) level without any
stimulation.
• Depending on where the light
from an image falls on the
receptive field it can either
further excite or inhibit the
electrical signal from the cell
to the brain
• Light falling on the center of
the receptive field causes a
stronger signal from the
ganglion cell
• Light falling on the sides
(surround) of the receptive field
inhibits the ambient signal
(reduces its strength)
• Light falling partly on the
center & partly on the surround
partly stimulates & partly
inhibits the ambient signal
from the ganglion cell in
proportion to the number of
receptors it falls on
Surround
Center
Examples
• Some examples of
ganglion cell response
to light falling on parts
of receptive field
• No light. Only
ambient signal from
ganglion cell
• Light falling mainly
on the center
• increases the
signal
• Light falling mainly
on the surround
• decreases the
signal
• Light falling on
entire receptive field
• produces
cancelling
responses. Same
as no light!!
stronger signal
ambient signal
weaker signal
ambient signal
What is the result of the following light-dark patterns
falling on various receptive fields in your retina?
•
Shown below are 6 different centersurround receptive fields in the image
on your retina as you look at a dark
grey shape on a white
background. The receptive fields are
labeled A, B, C, D, E, and F. Each
receptive field is connected to a
different (ganglion) nerve cell.
•
a) F,
•
•
•
Extra-credit points for explaining
b) B,
d) D, e) F, B and D
Which receptive field inhibits its
ganglion cell to send the weakest signal
(indicating the darkest region)?
a) A, b) B, c) C, d) D. e) E
Bright light
on white region
Little light on grey region
Which receptive fields neither enhance
nor inhibit their ganglion cell's signal?
Which receptive field stimulates its
ganglion cell to send the strongest signal
(indicating the lightest region)?
a) A, b) B, c) C, d) D. e) E
There is no change in the ambient signal for B,
D and F, since there is equal illumination on
the center and the surround for these receptive
fields. A gives the smallest signal since the
ambient signal is inhibited (no light on the
center, only on the surround). C has a
stronger-than-ambient signal because no light
falls on part of its surround. E has the
strongest signal because no light falls on more
of its surround than on C's.
What is lateral inhibition and how is it
responsible for lightness constancy?
• Lateral inhibition is the ability of one part
of a receptive field to inhibit the signal
from another
• It is one form of visual processing of the
image on our retina before the image
information is sent to our brains (as
electrical signals)
• Lateral inhibition is responsible for
lightness constancy of the perceived
relative light & dark regions of an image
on our retina as the illumination changes
• More illumination on both the light and
dark parts of a scene doesn't change our
perception of what is light and dark in an
image because it affects the surround and
center of our receptive fields equally so
their is no change in response
• We perceive light and dark by using lateral
inhibition to compare neighboring regions
of an image (at edges)
Image falls on the receptive fields of the retina.
Receptive fields
You can move that image
into different positions
over your receptive fields by
looking right at it (fovea)
or looking slighly away
or looking over parts
of the image (successively
on your fovea.
If the dogs are in brighter light we can think of their image as uniform
brightness plus the darker image in dimmer light.
The retinal fields don't change their ganglion cell signal to the
brain when light covers each retinal field completely. This is
one reason why there is lightness constancy.
+
Receptive fields
=
Information:
• News: Exam #2 on
Thursday;
• Good news: No homework
assignment on Thursday;
• Exam Preparation: see web
page for information;
• Continue with material
from Chapter #7;
• Learning about illusions:
go through the slides & read
the textbook
What is edge enhancement by lateral
inhibition?
o An edge in a picture or
scene you are looking at
is a place where the light
intensity changes rapidly
from brighter to darker.
o An edge in a scene results
in an edge in your retinal
image of that scene.
o Edge enhancement
occurs when the edge is
made more noticeable by
lateral inhibition,
o Dark part of image on
your retina next to a light
part appears darker if the
light part falls on the
region of a receptive field
which inhibits the
ganglion response. (This
region is called the
surround.)
The influence of lateral inhibition on how we see
edges in art, photography and optical illusions
Picasso
• We "notice" sharp boundaries
between light and dark much more
easily than gradual boundaries
•
This is understood by many artists
•
•
•
It is used in digital photography
•
•
Picasso uses this
Seurat and El Greco use this in
another way by painting a light region
lighter and a dark region darker on
either side of an edge in order to
enhance it
We shall later see that a digital
camera image can be made sharper by
doing the same thing (using filters in
Photoshop)
There are many interesting optical
effects and illusions based on how we
see edges
French artist George Seurat used edge enhancement by
lateral inhibition to make figures stand out sharply
Lighter just before edge
Darker just before edge
Spanish artist El Greco did
the same thing
Simultaneous lightness contrast is based on
lateral inhibition
•
Simultaneous lightness contrast occurs
when the lightness of an area is influenced
by neighboring regions
•
•
•
How you frame a picture will determine
how the picture looks
•
•
•
•
•
•
Our perception of lightness is not objective
– depends on surroundings
We have seen it operate in the art examples
of edge enhancement
If you put a light frame around it it will
look darker
If you put a dark frame around it it will
look lighter
Which of the two center squares appears
lighter (simply say what you see)?
(A) Left;
(B) Right
The effect is more pronounced if you look at the
x so that the image of the squares is in your
peripheral vision (away from your fovea) since
lateral inhibition acts over greater distances there
x
The same kind of processing by comparing with neighboring
parts of an image occurs with SIZE perception
Which middle disk is larger - the one on the (A) left or the one on the (B) right?
(simply say what you see)
Here is another version of lateral inhibition
which also mixes in some edge enhancement
• Which of the disks is lighter?
• Especially when you look at X
• (A) left; (B) right;
(simply say what you see)
• They are both the same
lightness, as you can see
when each background is
used on each half of the
disk
The Hermann grid optical illusion is based
on lateral inhibition
• Do you see dark spots in the
white regions at the intersection
of the horizontal and vertical
white bands? (Especially if you
look away a bit)
• The explanation is shown below
• When the image is on the first
receptive field there is more light
falling on the surround than
• in the second position,
• so there is more suppression and
the illusion of a dark spot at the
first location
Here is a related illusion involving lateral
inhibition
• Each vertical band has
equal light intensity across
its width.
• However the left side of
each bar appears darker
than the right side due to
lateral inhibition at the
edges
• One side is next to a
lighter bar while the other
side is next to a darker
bar.
• These adjacent bars act
just like the picture
frames
less inhibition
(looks lighter)
more inhibition
(looks darker)
Another variation on how we take cues from edges in
our perception of light and dark regions in an image
Negative afterimages occur when you stare at an
image for a long time without moving your eyes
1
Conditions for negative afterimages
o Prolonged or intense stimulation by
an image on the retina desensitizes
those parts of retina.
o Those parts of the retina have a
weaker response to subsequent to
stimulation.
o Demo Fig. 7.16 - Try It in home!!!
2
Negative afterimages are a temporal
version of lateral inhibition.
o In simultaneous lightness contrast, a
signal received at a different place in
your receptive field inhibits response.
•
In successive lightness contrast, a
signal received at one time inhibits
response in the receptive field a later
time.
Demo Fig. 7.16
Stroboscope demonstrates the concept of
persistence of vision
• Strobe provides sequence of
freeze-frame "snapshots" of
motion
o Sequence of brief positive
"afterimages" which persist
• Flicker
o Strobe gives old-time movie
effect
• Strobe gives "snapshots) of
fast rotating wheel and can
be tuned to show successive
spokes "frozen" or moving.
http://www.michaelbach.de/ot/mot_biomot/index.html
Positive afterimages and persistence of vision
Response to a brief light flash or image
shows delay (latency) & longer
duration (persistence).
Positive afterimage means white where
white, black where black.
Persistence time for positive afterimage
can last as long as 1/20 sec at low light,
1/50 sec in high light.
images in rapid succession blend.
Sequence of images blend into motion.
Visual system FILLS IN.
Persistence of vision & motion
delay
after-image
brief
image
time
persistence
time
after-image 1
time
frame 1
after-image 2
time
between
frames
time
frame 2
after-image 3
time
time
frame 3
between
frames
After-images blend to
produce illusion of
motion
Television and movies rely on persistence of vision
Flicker occurs when the time
between frames is longer than
the persistence time
• Basis for movies.
• Old time movies "flickered"
because after-images didn't overlap
• In modern movies a frame with a
new image appears every 1/24 sec.
• Frame with old image is projected 3
times during that interval so
effective frame renewal rate is 1/72
sec, with many duplicate frames.
delay
after-image
brief
image
• Basis for TV. Avoid flicker
• Physics 2000 : TV
• TV broadcast at one complete
frame each 1/30 sec (screen)
• Interlacing of 2 frames (Horizontal
lines 1,3,5,...2,4,6) yields effective
1/60 sec between odd-line "field"
and-even line "field" Web Demo
time
persistence
time
http://encyclopedia.laborlawtalk.com/Animation
after-image 1
time
frame 1
after-image 2
time
between
frames
time
frame 2
after-image 3
time
between
frames
time
frame 3
In home: Go through the slides below
• Go through the slides;
• Try demonstrations;
• Read about illusions in
the textbook;
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 to nullify your natural eye movements so that theimage of the stimulus
remains on exactly the same part of the retina as if there were no eyemovements. 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.
• 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).
What is happening in Bridget Riley "Op-art“?
Involuntary eye movements and temporal
response of eye
1. Scanning by your eye
o Builds up a detailed image by scanning the
fovea across field of view.
o Is a means of producing temporal change at
fixed retinal regions, thereby keeping
sensitivity high.
2. Certain optical illusions are prevented by
eye movements
o Fig. 7.15 would not work if your eye moved
so that the image of the diffuse disk fell on
your fovea instead of the x
3. Other optical illusions are related to
involuntary eye movements (Figs. 7.17 and
7.18). See Bridget Riley painting.
4. Classes of eye movements
a) Drifts (slow smooth movements)
b) Tremors (rapid jittery motions)
c) Saccades (sharp, abrupt movements- as in
reading)
Temporal response
Retina responds more if there is a
change in stimulation in TIME
o Flickering light or eye scanning
page.
o Cat's attention to movement.
o Scanning = time-variation of
image on retina