Color vision

Science of the EyeProfessional Development

Workshop

Ishara Mills-Henry, Ph.D.MIT

www.scienceoftheeye.org

Workshop OutlineI. Color Vision

– How do we see?– Where’s Waldo?

• Intro into the Retina– What can you see in the dark?– Acting it Out – Phototransduction– Central versus peripheral vision– Colorblindness

• Retinal Processing– Colored Shadows (the color wheel, mixing color versus reflecting color)

– What a weird American flag!– The “Bionic” Eye

• Group Discussion

Outline Cont’d

• II. Vision and the Brain (if time and you’re not exhausted)

• Visual Centers of the Brain (basic overview)– Blind Spot– 3-D Magic– Let’s FACE IT – Face Recognition– Fun Illusions and explanation

• Group Discussion• III. Surveys and Feedback

Workshop Concepts• Rod Photoreceptors respond primarily in dim light/ Cones

respond to colors and are important for acuity

• Photoreceptors convert light into electrical signals (Phototransduction)

Rods = Dim Light Cones = Color

Workshop Concepts• Most cones are in the macula, leading to high

quality of central vision compared to peripheral vision

• Retinal processing involves integration of signals from cones and rods.

• The retina is complex but can be compared to a web of information that is processed and sent to the brain in several channels of information

• Most visual processing takes place in the brain

Things about vision science to communicate to students

• Vision is fun!!

• Ways to teach complex concepts and perhaps address student’s misconceptions about vision

• Relate vision to our everyday lives

• Science is not static and knowledge is added all the time

If a mirror is mounted flat against the wall, how big does it have to be in order for you

to see your entire body?

What are your predictions?

Most MIT graduates and high school students were asked this question and gave similar answers!

How do we see?

Indirect light

How big does the mirror have to be?

Introducing the eye

• What is a digital camera?

• How do you think it works?

• What do you need to make a digital camera?

Digital Camera and the Eye

Observe parts of camera• Lenses – focuses light onto the sensor• Sensor (e.g. CCD) –pixels (photosensors) capture light (color and

intensity) and convert it to an electrical signal and digital signal

• Image Processing Unit - processes the information, and creates an image (e.g. jpeg)

LENSES

SENSOR

IMAGE PROCESSOR

Convert Electrical signal to Digital

Storage

Label parts of the eye that are analogous to camera

In Living Color: Constructing color vision

• What are your favorite colors?

• What is light and how is it useful?• Compare these two photos:

Visible Light Spectrum

Blu-Ray laser – 405 nm Standard DVD laser – 650nm

Visible LightSpectrum

Marsh Marigold Flower A. Visible Light (how humans see the flower)

B. UV light (how bees see the flower)

(Image from http://www.eso.org/~rfosbury/home/natural_colour/biochromes/UV_flowers/nc_bio_flower_uv.html

Some animals have UV cones

Activity: Introducing Color Vision

• Where’s Waldo?

Signals Everywhere!!

• Give an example of a signal. How do we use this signal?

• Name one example of how signals are important in your everyday life.

• What are some examples of signals in the cell or human body?

Sending Messages• Imagine you want to pass a note (without a cell phone)

how would you do it….

• Signals indicate warn direct command or act. Electrical quantity or effect that can be varied in such a way as to convey a message

Will you go out with me?Yes ___No ____ Maybe ____

The Retina

From www.howstuffworks.com

Retina

Humans are considered trichromats –

Blue, Red, and Green Cone Photorecpetors

Rod photoreceptors – are important for vision in dim light

Photoreceptors respond to different wavelengths

Photoreceptors

Outer segments

Compare the sizes of the photoreceptors outer segments, which one is bigger?

Three Main Parts:a. Outer Segmentsb. Cell bodyc. Synaptic terminal

Photosensitive Proteins

• Rhodopsin (the rod opsin) and cone opsins are situated in the membrane of the disk structures

Rhodopsin

What do you see in the dark?

• When you walk into a dark room, can you immediately see objects around you?

• Are you able to distinguish colors in dim light?

• Do you think that you can see certain colors in the dark? If so, which colors?

Activity: What can you see in the dark? (groups of four)

Three types of colored balls

Test colors by using cups of water with varying amounts of food coloringor colorful magazine pictures.

Test variables, i.e. light

Light Adaption: What is the minimum amount of light that is necessary for cones to respond?

Pirates of the Caribbean

• Separate by colorLeft Center Right

Blue Yellow Orange

• Separate Easter Eggs by numberLeft Right1 2

Count the number of errors

Three sets of Data- In the light- Immediately after light is turned off- With “pirate eye”

• Questions to ponder:

• What are your predictions?

How photoreceptors respond

1. In the dark, photoreceptors are constantly releasing neurotransmitters. They have a starting negative membrane potential. Photoreceptors respond to light through the rhodopsin and cone opsins

2. Absorption of light initiates change in protein setting off cascade of events making photoreceptor more negative

3. Change in membrane potential stops release of a signal (neurotransmitter) from terminal (faucet gets turned down)

Phototransduction = converting light into electrical signal

Outer Segment(Rod)

Na+/Ca++

Channels

Activity: Acting it out

• Phototransduction– Light (Light bulbs, electric personality)– Na+ (salt shaker, a good chef)– Membrane (disks, self-identified introverts)– Opsins (good dancers)– Ion Channel (2 people, difficult to separate in class)– cGMP (dynamic personality)– Neurotransmitter (free spirit)

Activity: Central versus Peripheral Vision

Measuring your Central Vision• Basically created a Giant Protractor

• Predict what you think you can see “out the corner of your eye”.

• Focus on the pushpin while another person moves the colored shapes toward the center of the protractor

• When you see color or shape, notify recorder

• Use protractor and ruler to measure the angle

The macula is responsible for central vision

Light

Retinal Layers

Retina

Fovea

Macula

Small sensitive region in the center of the retina

All cones and no rods

The majority of cones are present in the macula

Types of Color BlindnessMonochromacyRod or Cone – very rare, All rods or all cones are missing

Dichromacy – Two types of photoreceptors are presentProtanopia – red cones are missingDeuteranopia – green cones are missingTritanopia – blue cones are missing

Anomalous Trichromacy- All three photoreceptors are present but there is a spectral shift Protanomaly- spectral sensitivity of red cones are shiftedDeuteranomaly – spectral sensitivity of green cones are shifted Tritanomaly – spectral sensitivity of blue cones are shifted

B RG

437 nm564 nm

533 nm

NORMAL CONE SENSITIVITY CURVES(TRICHROMAT)

NORMAL CONE SENSITIVITY CURVES(TRICHROMAT)

Activity: Color blindness

B R

437 nm 564 nm

Deuteranopia(no green cones; only red and blue)

1% of Males

39

B G

437 nm 533 nm

Protanopia(no red cones; only green and blue)

1% of Males

RG

564 nm533 nm

Tritanopia(no blue cones; only green and red)

Very rare

B RG

437 nm 564 nm

Most commonDeuteranomaly

(green shifted toward red)

5% of Males

Color BlindnessLeft Right

Results

Pedigree

What is the mode of inheritance?

Gene Therapy for color blind monkeys

All male squirrel monkeys are born only able to see mostly yellows and blues. It is difficult for them to distinguish reds from greens. Female squirrel monkeys can see the

whole spectrum.

What photoreceptor(s) do you think they lack?

What advantage does color provide for these animals?

Credit: Neitz Laboratory

Colored Shadows Activity

• What color do you get when you mix all colors together?

• Remind students about subtractive and additive color mixing

Colored Shadows

• How many colors do you see?

• What are the color of the shadows?

• What are the colors of the shadows when they overlap?

• When you block one of bulbs or turn it off, what color shadows do you see (do this for all 3)?

How can you make a thousand colors out of three types of cone

photoreceptors?

If you mix equal amounts of red, green, and blue light, you will get white color

Other colors are perceived by mixing the proper ratio of red, green, and blue

Flow of Information

• Vertical flow of information – direct – photoreceptors to bipolar to ganglion cells

• Lateral flow of information –indirect - input from horizontal and amacrine cells

Stare at the white cross for 30 seconds

The theory of Color Opponency• Ever notice that some colors do not exist, Red – Green or Blue - Yellow

• Some ganglion cells sort color information into three different channels before sending signal to the brain

• The three channels (like TV) are

Red/Green = Sports

Blue/Yellow = News

Light Intensity (Degree of whiteness or blackness) = Reality TV

• You = Red, Opponent = Green

• You are stronger – RED wins• Opponent is stronger – GREEN Wins• You and Opponent have same strength – no

one wins

More Afterimages

Activity: Research Focus

• The “bionic” eye

• What would happen if you no longer had functional photoreceptors?

Retinal DiseasesDescribe how the vision of an individual with Retinitis Pigmentosa is affected?

Describe how the vision of an individual with Age-related Macular Degeneration is affected?

How it works

• Do you think that individuals with the “Bionic” eye implant will be able to perceive images immediately?

• Do you think that the “Bionic” eye implant, individuals will be able to see colors?

Bionic Eye Video

Artifical Retina• http://www.cnn.com/2009/HEALTH/12/11/bi

onic.eye/

Group Discussion

Vision and the Brain

Optical or Visual Illusions

Where’s my blind spot?

Why is it so hard to find?

Right Eye

Left Eye

3-D Magic

Face IT!!

From: http://www.youramazingbrain.org.uk/supersenses/hollow.htm

Other Illusions and Explanations

Which table is longer?

Rotating Wheels

Group Discussion

I hope today wasn’t like this

MIT’s Stata Center

It is said that MIT’s education is like drinking from a firehouse…….

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