physics 1230: light and color

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Physics 1230: Light and Color

Lecture 14: The retina and brain, image signal

processing.

Reading: Chap. 9,10 Color perception.

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

The remaining lectures:

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• Ch. 7 (Retina and visual perception),

• Ch. 9 & 10 (color & color perception).

We

are

here

Ch. 7 – Visual Perception

• 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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We

are

here

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The Retina: Detecting the light and processing the images

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)

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…

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 lightIt 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

We will skip Most of cellular detail BECAUSE…

Example: Rhodopsin and photosensitivity

Photo-responsive membrane

protein is known in atomic detail

Light drives a change in

molecular shape.

Opens/closes membrane

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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.

Layers of the retina

9Light

Layers of the retina

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Layers of the

retina are

CROSS

Connected

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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.

From the following article:Neurobiology: Bright blue times

Russell G. Foster

Nature 433, 698-699(17 February 2005)

doi:10.1038/433698a

See text fig. 7.2

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

Optic chiasma and brain structure

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Brain damage on the left side

hurts vision on the right side.

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

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

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Total amount of light is far less important than

the relative amount of light, particularly as

compared with nearby objects.

Demo with room lights.

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.

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See text fig. 7.5

“Lateral

Inhibition”

“Receptive field”

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Nerve cell

fires rapidly

See text fig. 7.12

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

doesn‟t fire

Nerve cell

doesn‟t fire

Nerve cell

fires only a bit

Receptive field (again)

23The yellow is the region receiving light.

See fig. 7.11

Called LATERAL INHIBITION

Because of LATERAL INHIBITION,

Edge detection is enhanced

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Half illumination gives

bigger signal

Full illumination: Not

much nerve activity.

Lateral inhibition along with relative lightness

cause: Simultaneous lightness contrast

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Craik O‟Brien Illusion

Contrast at the edge affects your perception of center.

Are the small gray patches below identical?

See fig 7.7

A) YES B) NO

Craik O‟Brien Illusion

Simultaneous lightness contrast

26These are the patches without the surround.

Simultaneous lightness contrast (again)

“Checker shadow illusion”

27Which square is lighter in shade, square A or square B?

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Slide them together and compare.

A is surrounded by light squares and B is surrounded by dark

squares in the previous slide.

Simultaneous lightness contrast

“Checker shadow illusion”

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Hermann grid illusion: dark areas are from lateral inhibition

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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.

1

2

3

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White space is larger

than receptive field

32It is blacker away from a corner where there is more inhibition.

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34The 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

Victor Vasarely, Zebras. The black/white boundaries outline the necks.

The artist has made use of the tendency of the eye to find lines.

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Picasso

The regions of color don‟t have edges, but appear to.

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Lighter just before edge

Darker just before edge

French artist George Seurat used edge enhancement by

lateral inhibition to make figures stand out sharply

42El Greco

43Victor Vasarely, artist.

The edges of the squares seem lighter because of the dark surrounds.

The white crosses are an illusion.

Interesting collective behavior 2: We expect a

3D world, lit from ABOVE:

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Our perception of relative lightness changes

based upon Location and Shape!

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A

B

Example: Which is the darker patch, A or 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.”

Which creature is larger?

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http://www.michaelbach.de/ot/index.html

Previous experience in tunnels

tells us that the creature in back is

further away, and hence must be

larger.

A) The little one in front

B) The big one in back

C) They are the same size.

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Victor Vasarely, artist

“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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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.

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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.

Successive lightness contrast

Negative afterimage

52Stare at this for 30sec., then stare at the next slide.

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54Stare at this, stare at the next slide.

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5656

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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…

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http://www.michaelbach.de/ot/mot_eyeJitter/index.html

The only difference between the center and

edge is the lack of any feature to “focus” on.

IF there are edges, but not if edges are absent.

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.

Müller-Lyer illusion

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http://www.newworldencyclopedia.org/entry/Muller-Lyer_illusion

Which arrow is longer?

Müller-Lyer illusion

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http://www.michaelbach.de/ot/index.html

This is the back corner of a room, it is

further away, hence it must be larger.

Big Moon Illusion

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What you remember. Actual

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Frankfurter illusion

http://www.michaelbach.de/ot/sze_Frankfurter/index.html

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.

Are the lines straight?

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Hering Illusion

Does the square have straight sides?

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Does the square have straight sides?

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http://www.michaelbach.de/ot/ang_hering/index.html

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Poggendorff Illusion

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http://www.michaelbach.de/ot/ang_poggendorff/index.html

Are the lines

continuous and

straight “behind” the

yellow columns?

Poggendorff Illusion

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http://www.michaelbach.de/ot/ang_poggendorff/index.html

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Art that mimics 3-d.

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.

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

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Motion effects from Michael Bach‟s web page

• Silhouette illusion

• Motion induced blindness

• Motion aftereffect (Waterfall illusion)

• Spiral aftereffect (motion channel activated)

• Breathing square

http://www.michaelbach.de/ot/index.html

A good place to stop today

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