Please pick up a group evaluation sheet All projects ... · The optic nerve is formed from the axons of all the ganglion cells. A subset of ganglion cells (MG cells) also detects

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Physics 1230: Light and Color

Chuck Rogers, [email protected] Henley, Valyria McFarland, Peter Siegfried

physicscourses.colorado.edu/phys1230

Please pick up a group evaluation sheet

All projects should include ONE group

evaluation of the project.

• Project report due Friday on D2L, 8PM.

• Online and Written HW12 due Friday, 8PM.

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Final Exam, here, next Tues. 1:30-4P:

All multiple choice, cheat sheet, ruler,

writing tool, calculator.



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Lecture 29:

Retina and subtractive processing

• Final Exam next Tues. 1:30-4PM



Last Time: Trichromacy

• You have 3 kinds of color receptors (cones)

• Wavelengths: Short Intermediate Long

4Fig. 10.3 in textbook.

We know this because we can measure the

cones firing with different light wavelengths.

Retinas add color signals together

Light comes in

from here

http://webvision.med.utah.edu/anatomy.html

How signals are added and processed is different in

different creatures.…

Human retina Cat retina


Last Time: Metamers

• Yellow (600) looks like red (650) + green (550)

• NOTE: No mixture will look like green or red!!

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Yellow stimulates the I and L

receptors equally.Equal amounts of red

and green do too.

Why blue+yellow looks white

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S+I+L stimulated the

same amount as from

white light!

How colors are perceived

• Blue excites S receptors

• Cyan excites S + I

• Green excites I mostly

• Yellow excites I + L

• Orange excites I + L

• Red excites L

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Additive and Subtractive

color mixing

Why does one figure have a black background and the other is white?

http://upload.wikimedia.org/wikipedia/commons/1/14/AdditiveColorMixing.png

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Are these colors the same?

Last Time: Ponder:

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Are these colors the same?

Ponder:

YES! But it sure didn’t look like it…

WHY??

Retinas process images in many ways

Light comes in

from here


How signals are added and processed is different in

different creatures.…

Human retina Cat retina


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

http://www.nature.com/nature/journal/v433/n7027/full/433698a.html#B1




http://www.nature.com/nature/journal/v433/n7027/full/433698a.html

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

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.

Do the small gray patches below look 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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Lateral inhibition in the retina refers to the

tendency of groups of photoreceptors to

cause their common neuron to:

A. Fire less if the lateral ganglion is illuminated

B. Fire less if the outer ring of receptors is

more illuminated than the center.

C. Fire more if the outer ring of receptors is


D. Fire more if the entire group of receptors is

uniformly illuminated.

E. None of the above.

Receptive field (again)

30The yellow is the region receiving light.

See fig. 7.11

Called LATERAL INHIBITION

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Lateral inhibition in the retina refers to the

tendency of groups of photoreceptors to

cause their common neuron to:

A. Fire less if the lateral ganglion is illuminated

B. Fire less if the outer ring of receptors is


C. Fire more if the outer ring of receptors is


D. Fire more if the entire group of receptors is

uniformly illuminated.

E. None of the above.

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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

35It is blacker away from a corner where there is more inhibition.

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37The 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

45El Greco

46Victor 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!

What do you see?

A) Craters

B) Mounds

What do you see?

A) Craters

B) Mounds

We expect a 3D world, lit from ABOVE:

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Craters? Mounds?light

light

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A

B

Example: Which appear to be 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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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

58Stare at this for 30sec., then stare at the next slide.

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60Stare at this, stare at the next slide.

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6262

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6464

6565

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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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

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

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



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A good place to stop today.

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Thanks for taking the class!

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Documents

Please pick up a group evaluation sheet All projects ... · The optic nerve is formed from the axons of all the ganglion cells. A subset of ganglion cells (MG cells) also detects