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• Chapter 7: The Human Eye and Vision - II: Processing the Image

• The image that falls on your retina is not what your brain "gets"

• Your retina changes the image similar to a computer program like Adobe Photoshop. It processes the image it receives

• Compare to a camera;

• The retina causes the following processes to occur in space, due to lateral inhibition by receptive fields• lightness constancy• edge enhancement• simultaneous lightness contrast

• perception of light and dark regions of a scene or image

• The following processes occur in time• Negative and positive afterimages• Involuntary eye movements• Persistence of vision

Physics 1230: Light and ColorChapter 7

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

rods

image of 2 lights

cones

image of2 lights1 nerve

celldetector

to brain

2 nerve cell detectors

to brain

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

CenterCenter

SurroundSurround

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!!

light

ambient signal

stronger signal weaker signal

light light

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 center-surround 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.

• Which receptive fields neither enhance nor inhibit their ganglion cell's signal?

a) F, 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• 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.

Little light on grey region

Bright lighton white region

Extra-credit points for explaining

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 fieldsReceptive fields

You can move that imageinto different positionsover your receptive fields bylooking right at it (fovea)or looking slighly awayor looking over partsof the image (successivelyon 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.

Receptive fieldsReceptive fields

The retinal fields don't change their ganglion cell signal to thebrain when light covers each retinal field completely. This isone reason why there is lightness constancy.

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

• We "notice" sharp boundaries between light and dark much more easily than gradual boundaries

• This is understood by many artists • 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

• It is used in digital photography• 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

Picasso

French artist George Seurat used edge enhancement by lateral inhibition to make figures stand out sharply

Lighter just before edgeLighter just before edge

Darker just before edgeDarker 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

• Our perception of lightness is not objective – depends on surroundings

• We have seen it operate in the art examples of edge enhancement

• How you frame a picture will determine how the picture looks

• 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

more inhibition(looks darker)

less inhibition(looks lighter)

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 afterimageso 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

• Flickero 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

timebrief image

delay

after-image

persistence time

time

time

time

frame 1

frame 2

frame 3

time between frames

time between frames

after-image 1

after-image 2

after-image 3

After-images blend to produce illusion of motion

Persistence of vision & motion

Positive afterimages and persistence of visionResponse 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.

http://encyclopedia.laborlawtalk.com/Animation

Television and movies rely on persistence of vision

• 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.

• 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

timebrief image

delay

after-image

persistence time

time

time

time

frame 1

time between frames

after-image 1

frame 2

after-image 2

frame 3

after-image 3

Flicker occurs when the time between frames is longer than the persistence time

time between frames

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 cankeep 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 eyeo 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 movementso 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 movementsa) 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