Physics 1230: Light and Color
Exam 4 cancelled: Exam extra credit
assignment will be due Wed. at 5PM
Extra credit to improve exam scores!
Final HWs: Due today, Tuesday, 5PM
Lecture 14: The retina and brain, image signal
processing.
Reading: Chap. 9,10 Color perception.
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The remaining lectures:
We
are
here
• Ch. 7 (Retina and visual perception),
• Ch. 9 & 10 (color & color perception).
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Ch. 7 – Visual Perception
We
are
here
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Parts of the visual processing system
Lightness and brightness
Retinal processing: Lateral inhibition
Hermann grid
Receptive field
Motion illusion
Craik O‟Brien illusion &
simultaneous lightness contrast
• Other optical illusions
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The Retina: Detecting the light and
processing the images
The retina and optic nerves are recognized as actually
parts of the brain (like your olifactory bulb in the nose).
They start development IN the brain and migrate…
Has 108 nerve endings to detect image
rods, for high sensitivity (night vision)
cones, for color and detail, 7 million
optic nerve = 106 transmission lines
fovea, region of best vision (cones)
More nerves in your retina than some creatures
have in their entire brains. Processing Power.
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Rods and cones
• Rhodopsin, a photochemical, responds to light
It is destroyed and reformed.
Signal goes to a synapse, a gap between nerve
cells
• There are 3 kinds of cones for 3 colors
red, green, blue (more later).
A great deal is understood about how the
individual cells of the retina receive light, respond
to light, and transmit signals.
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Rods and cones
Example: Rhodopsin and photosensitivity
Photo-responsive membrane
protein is known in atomic detail
Light drives a change in
molecular shape.
Opens/closes membrane
We will skip Most of cellular detail BECAUSE…
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Does our understanding of the individual rods, cones,
and other cells of the retina do much to explain this?:
(A) Creitanly
(B) Myaby Not Mcuh
We need to understand how NETWORKS of
cells WORK TOGETHER to let us perceive.
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Layers of the retina
Light
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Layers of the retina
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See text fig. 7.2
Layers of the
retina are
CROSS
Connected
From the following article:
Neurobiology: Bright blue times
Russell G. Foster
Nature 433, 698-699(17 February 2005)
doi:10.1038/433698a
a, The rods (R) and cones (C) convey visual information to the ganglion cells (G) through the bipolar cells (B). Horizontal cells (H)
allow lateral connections between rods and cones. Amacrine cells (A) allow lateral connections between bipolar and ganglion cells.
The optic nerve is formed from the axons of all the ganglion cells. A subset of ganglion cells (MG cells) also detects light directly; for
this, they require the photopigment melanopsin, as now confirmed1, 2, 3. b, Light, via melanopsin, causes changes in Ca2+ levels in
MG cells9 (a fluorescent Ca2+ indicator was used here). Counterintuitively, light passes through the transparent ganglion layer to
reach the rods and cones.
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Connections and cross connections are
MOST important.
Photoreceptors: rods and cones
connected to the
bipolar cells
connected to the
ganglion cells, funnel “data” through axons into the
optic nerve
sideways connectors (these help with analysis)
horizontal cells, next to the photoreceptors
amacrine cells
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Clicker question
The arrow points to:
A. Photoreceptors
B. Horizontal cells
C. Bipolar cells
D. Amacrine cells
E. Ganglion cells
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Clicker question
The arrow points to:
A. Photoreceptors
B. Horizontal cells
C. Bipolar cells
D. Amacrine cells
E. Ganglion cells
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Optic chiasma and brain structure
Brain damage on the left side
hurts vision on the right side.
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See text fig. 7.3
Brain anatomy
Optic chiasma
Left field of view goes to right brain
Right field of view goes to left brain
from both eyes
Visual cortex is where you “see”
Brain damage at this location hurts vision.
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Clicker question
If the left side of your brain is injured, you
might lose vision in your
A. left eye
B. right eye
C. left field of view
D. right field of view
E. some loss in left and right field of view
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All this „hardware‟ allows us to
perceive the world and
function in it.
Many complicated sub-systems have
developed. Let‟s study a few to get
some insight into how vision works.
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Interesting collective behavior 1: We detect
RELATIVE Lightness, not total Brightness
Brightness: amount of light
Lightness: property of a surface
newspaper = 0.65 (reflectance)
printer paper = 0.84
photo quality paper = 0.90-0.99
Total amount of light is far less important than
the relative amount of light, particularly as
compared with nearby objects.
Demo with room lights.
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Lightness and brightness
Lightness constancy: brain and eye correct for amount of
light so that white, gray, and black look the same
independent of brightness.
Weber‟s law: we think lightness is equally spaced when
the ratios are equally spaced
Example: lightness 0.5, 0.25, 0.125 look equally spaced.
These numbers are ½, ¼, 1/8 etc.
The spacing that looks equal is not 0.9, 0.8, 0.7, etc.
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Demo: Lights on or lights off
Retinal processing that allows
Relative Lightness sensitivity:
Amacrine and horizontal cells “turn down” the signals from areas
adjacent to bright areas.
“Lateral
Inhibition”
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See text fig. 7.5
“Receptive field”
The rods/cones and local
cells are connected in a
group:
Center of group causes
nerves to fire if illuminated.
Surrounding group causes
nerves to STOP firing if they
are illuminated.
Nerve cell
fires rapidly
Nerve cell
doesn‟t fire
Nerve cell
doesn‟t fire
Nerve cell
fires only a bit
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See text fig. 7.12
Receptive field (again)
Called LATERAL INHIBITION
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The yellow is the region receiving light.
See fig. 7.11
Because of LATERAL INHIBITION,
Edge detection is enhanced
Full illumination: Not
much nerve activity.
Half illumination gives
bigger signal
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Lateral inhibition along with relative lightness
cause: Simultaneous lightness contrast
Craik O‟Brien Illusion
Contrast at the edge affects your perception of center.
Are the small gray patches below identical?
A) YES
B) NO
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See fig 7.7
Craik O‟Brien Illusion
Simultaneous lightness contrast
These are the patches without the surround.
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Simultaneous lightness contrast (again)
“Checker shadow illusion”
Which square is lighter in shade, square A or square B?
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Simultaneous lightness contrast
“Checker shadow illusion”
Slide them together and compare.
A is surrounded by light squares and B is surrounded by dark
squares in the previous slide.
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Hermann grid illusion: dark areas are from lateral inhibition
1
3
2
The red areas show the receptive field.
Lateral inhibition is greater at 1 than at 2.
The fovea has a smaller receptive field.
So the lateral inhibition is the same
everywhere in the white area.
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White space is larger
than receptive field
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It is blacker away from a corner where there is more inhibition.
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The music
A. Kitaoka
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Does the center stripe have constant lightness?
Or is the center stripe darker in the middle and at the ends?
A) Constant
B) Darker in middle and ends
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The center stripe has constant lightness.
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Clicker question
A white sheet of paper continues to look
white as the light level is reduced. We call
this effect:
A. Simultaneous lightness contrast
B. Lateral inhibition
C. Weber‟s law
D. Lightness constancy
E. Edge enhancement
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Clicker question
The bands of gray look lighter
on their right side because of:
A.Simultaneous lightness
contrast
B. Lightness constancy
C. Weber‟s law
D. Lateral inhibition
E. Both A and D
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Victor Vasarely, Zebras. The black/white boundaries outline the necks.
The artist has made use of the tendency of the eye to find lines.
The regions of color don‟t have edges, but appear to.
Picasso
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French artist George Seurat used edge enhancement by
lateral inhibition to make figures stand out sharply
Lighter just before edge
Darker just before edge
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El Greco
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The white crosses are an illusion.
Victor Vasarely, artist.
The edges of the squares seem lighter because of the dark surrounds.
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Interesting collective behavior 2: We expect a
3D world, lit from ABOVE:
Our perception of relative lightness changes
based upon Location and Shape!
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Example: Which is the darker patch, A or B?
A
B
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A
B
Previous experience effect:
Here, the eye is “fooled” into thinking the light is from above.
The panel “A” has lots of light, so it must be really dark.
But “B” must be lighter because it is in the “shade.”
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Which creature is larger?
A) The little one in front
B) The big one in back
C) They are the same size.
Previous experience in tunnels
tells us that the creature in back is
further away, and hence must be
larger.
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http://www.michaelbach.de/ot/index.html
“Previous experience” interprets these flat images as
being from 3-dimensional boxes. The shadows tell us
what is a “floor” and what is a “wall.”
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Victor Vasarely, artist
Size constancy: Are all the vertical lines the same height?
A) Look different to me
B) Look the same to me
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http://www.michaelbach.de/ot/index.html
Interesting collective behavior 3: Sensitive to a
MOVING World. Time and motion important.
Fatigue: prolonged stimulation (staring at a lamp)
causes a weaker response and a negative
afterimage.
Successive lightness contrast: a gray object looks
darker after looking at white.
Positive afterimage: We see a flash as a bright
spot after it has gone away. Over stimulated
nerves keep firing.
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Successive lightness contrast
Negative afterimage
Stare at this for 30sec., then stare at the next slide.
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Stare at this, stare at the next slide.
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Fatigue and Involuntary eye movement
Eye movement moves the image around so that
new areas are stimulated.
Without eye movement, images fade. This has
been verified by experiments that fix the image
on the retina.
Eye movement causes wavy lines to appear as
though in motion, because the afterimage
interferes with the moved image.
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The eye is moving all the time. It corrects for motion…
IF there are edges, but not if edges are absent.
The only difference between the center and
edge is the lack of any feature to “focus” on.
http://www.michaelbach.de/ot/mot_eyeJitter/index.html
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Other illusions
There are many optical illusions with varying
explanations. Many are poorly understood.
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Are the blue shades the same?
Lateral inhibition cannot explain this!
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Lateral inhibition alone does not explain this effect, the Munker-White illusion.
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http://www.newworldencyclopedia.org/entry/Muller-Lyer_illusion
Müller-Lyer illusion
Which arrow is longer?
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http://www.michaelbach.de/ot/index.html
Müller-Lyer illusion
This is the back corner of a room, it is
further away, hence it must be larger.
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Big Moon Illusion
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What you remember.
Actual
Frankfurter illusion
While focused on the background, hold your two index fingers
horizontally in front of your eyes, not touching. A piece of finger will
appear to float in space.
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http://www.michaelbach.de/ot/sze_Frankfurter/index.html
Are the lines straight?
Hering Illusion
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Does the square have straight sides?
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http://www.michaelbach.de/ot/ang_hering/index.html
Does the square have straight sides?
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Poggendorff Illusion
Are the lines
continuous and
straight “behind” the
yellow columns?
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http://www.michaelbach.de/ot/ang_poggendorff/index.html
Poggendorff Illusion
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http://www.michaelbach.de/ot/ang_poggendorff/index.html
Art that mimics 3-d.
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Motion after effect
Motion channel, keeps firing after watching a
moving object, causing motion aftereffect.
http://www.michaelbach.de/ot/mot_adaptSpiral/index.html
The following are from Akiyoshi Kitaoka
Department of Psychology, Ritsumeikan University, Kyoto, Japan:
http://www.psy.ritsumei.ac.jp/~akitaoka/saishin27e.html
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Motion illusion (A. Kitaoka)
Note that each green circle is rotated slightly from its neighbor.
As your eye jumps around, it sees the circles rotation.
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Are the ropes tangled?
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from Akiyoshi Kitaoka
Motion illusion (Kitaoka)
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Motion illusion (Kitaoka)
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Snakes - Akiyoshi Kitaoka
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Dead Snakes - Akiyoshi Kitaoka
Motion effects from Michael Bach‟s web page
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Silhouette illusion
Motion induced blindness
Motion aftereffect (Waterfall illusion)
Spiral aftereffect (motion channel activated)
Breathing square
http://www.michaelbach.de/ot/index.html
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A good place to stop today
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