Visual System: Shedding Light on Eye-Lecture by Imran Ahmad Sajid, ISSG, UOP, March 2012

8/2/2019 Visual System: Shedding Light on Eye-Lecture by Imran Ahmad Sajid, ISSG, UOP, March 2012

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Imran A. Sajid

Imran Ahmad Sajid,

T.A., ISSG, UOP


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Imran A. Sajid

Imran A. Sajid

Before you finish reading this sentence,

approximately one hundred billion

(100,000,000,000) operations will have been

completed inside your eyes.

However fantastic it may seem, you possess an

example (two, in fact) of the Universe's ultimate

technology. No scientist has ever come close to fully grasping

it, let alone inventing anything remotely similar.


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Imran A. Sajid

Imran A. Sajid

Whatever you have in your life is meaningful throughyour sensesvision and others.

Your family, your house, your office, your friends andeverything else in your surroundings, you quicklyidentify thanks to your vision.

Without eyes, you could never get a quick, completesense of everything that's happening around you.

Without them, you could never imagine colors, forms,scenes, human faces, or what the word beauty means.

But you do have eyes, and thanks to them, you cannow read these printed words before you.


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Imran A. Sajid

Nor does the act of vision cost you very mucheffort.

To see an object, all you have to do is to turn

your gaze at it. You don't need to bother giving "project,

capture, and analyze" orders to your eyes, the

components inside them, the optical nervesrunning to the back of your brain, nor to thebrain itself.


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Imran A. Sajid

You need only look, just like the rest of the

billions of creatures who have ever lived on ourplanet.

Without having to work out the opticalmeasurements, your eye's lens can focus onto

distant objects. Without needing to accurately compute the

precise contractions of various musclessurrounding the lens, you only desire to see, and

within a fraction of a second, that process iscarried out for you.


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Imran A. Sajid


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Imran A. SajidImran A. Sajid

Structure of the Presentation

A. Light: The Stimulus of Vision

B. Structure of the Human Eye

C. Colour Vision and Colour BlindnessD. Theories of Colour Blindness

E. Recap

F. Review Question


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Light: The Stimulus of Vision

Light is the physical energy that stimulates the

eye.

Light travels in waves.

Measured in wavelengths.

The difference in wavelength and frequency

makes different types of colours.

Light is:the energy producing a sensation of brightness that makes

seeing possible. or, energy capable of producing visual sensation.


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All light rays emit uniquewaves.

Wavelength: of a wave isthe distance between

two consecutive wavepeaks. The distance between

adjacent wave crests is

called wavelength. It is measured in

nanometers (1 billionth ofa meter)


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Frequency: the number of wavelengths that

pass by a certain point in space in a given

amount of time.

The number of times the wave oscillates

(swing/fluctuate) each second is called frequency

This value is usually measured in cycles per

second, or hertz (Hz).

Wavelength

Frequency


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Visual Spectrum: The range of wavelengths

that humans are sensitive to.


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Color, physical phenomenon oflight. or

Sensation of light induced ineye by waves of a certainfrequency. e.g., Violet: Highest Frequency,

Shortest Wavelength (400)

Red: Highest Wavelength,

Shortest Frequency (700) Green: Human eyes respond best

to green light at 550 nm, which isalso approximately the brightestcolor in sunlight at Earths surface


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Vision begins with light rays bouncing off thesurface of objects.

These reflected light rays enter the eye and

are transformed into electrical signals. Millions of signals per second leave the eye via

the optic nerve and travel to the visual area of

the brain. Brain cells then decode the signals into

images, providing us with sight.


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Cornea Transparent protective coating

over the front of the eye

Pupil Small dark hole in center of the

iris through which light entersthe eye.

The size of the pupil dependson the amount of light in theenvironment.

The dimmer the surroundings,the more the pupil opens in

order to allow more light toenter.

Iris Colored part of the eye, which

ranges in humans from a lightblue to a dark brown

Structure of the Eye:

1.From Cornea to Retina

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Lens

Located directly behind the pupil. Acts to bend the rays of light so that they are

properly Focuses onto the retina

The lens focuses light by changing its own

thickness, a process called accommodation.

Distant objects require a flat lens Close objects require a rounded lens to view

best

Retina

Ultimate destination of an image in the eye.

A thin layer of nerve cells at the back of the eyeball.

Here electromagnetic energy of light is

converted into messages that the brain can use.

Fovea

Center of the visual field. A particularly sensitive region of the retina.

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Imran A. SajidImran A. Sajid2. From Eye to Brain:

Receptor Cells are the cells in the retina thatare sensitive to light

Visual receptors are called rods and cones.

2.1. Receptor Cells

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Rods Long and cylindrical

About 120 million rods

Respond to light and dark

Related to vision in dimlylit situations

Very sensitive to light

Largely insensitive tocolour and details

Provide our night visionand peripheral vision

Not found in the verycentre of fovea

Cones

Short, thick and tapered

About 8 million cones

Respond to sharplyfocused perception of

color as well as light and

dark

Work best in bright light Found mainly in the

fovea

2.1. Receptor Cells

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Rods Cone

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Dark Adaptation

Increased sensitivity of rods and cones in darkness

The speed at which dark adaptation occurs is aresult of the rate of change in the chemical

composition of the rods and cones. Cones reach their greatest level of adaptation

in just a few minutes.

Rods take close to thirty minutes to reachmaximum level.

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Bipolar cells

Receive input from receptor

cells

Ganglion cells

Receive input from bipolarcells

Blind spot

Area where axons of

ganglion cells leave the eye

Rods Cone

Bipolar CellGanglion Cell

Optic Nerve

1. Light passes between

the ganglion cells and the

bipolar cells, reaching

rods and cones at theback of retina

3. Now, the bipolar cells

transmit this information to theganglion cells

2.2. Receptor Cells2. The to rods and cones, whichare sensitive to light, respond bytransmitting information to the

bipolar cells

4. The axons of the ganglion cellsgather together forming the

optic nerve which transmits the

messages from both eyes to the

brain, which are interpreted as

sight.

Each ganglion cell gathers information

from a group of rods and cones in a

particular area of the eye, and compares

the amount of light entering the center

of that area with the amount of light inthe area around it.

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3. Sending Message to the Brain

Optic nerve Made up of axons of

ganglion cells

carries neural messages

from each eye to brain

Optic chiasm

Point where part of each

optic nerve crosses to

the other side of the

brain

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Visual CortexPlace for Ultimate processing ofvisual images

Feature DetectionMany neurons in the cortex

are extraordinarily specialized, being activatedonly by visual stimuli of a particular shape orpattern.

Some cells are activated only by lines of a

particular width, shape, or orientation. Other cells are activated only by moving, as

opposed to stationary, stimuli (Hubel & Wiesel, 2004).

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C. Colour Vision

A person with normal colour vision is capable

of distinguishing no less than 7 million

different colours (Bruce, Green, & Georgeson, 1997).

Not all people are able to see full colours.

There are certain individuals whose ability to

perceive color is quite limitedthe color-

blind.

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1. Colour-blindness

Approximately 1 in 50 men

or 1 in 5000 women have

some form of

colorblindness

Dichromats

People who are blind to

either red-green or blue-

yellow

Monochromats People who see no color at

all, only shades of light and

dark

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look at the photos shown in Figure 6. If you have

difficulty seeing the differences among the seriesof photos, you may well be one of the 1 in 50

men or 1 in 5,000 women who are color-blind.

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For most people with color-blindness, the

world looks quite dull.

Red fire engines appear yellow, green grass

seems yellow, and the three colors of a traffic

light all look yellow.

In fact, in the most common form of color-

blindness, all red and green objects are seen

as yellow.

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D.

Theories of Colour-Blindness

Why people get colour-blind?

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2.1. Tri-chromatic Theory of Colour

Vision

There are three kinds of cones (the receptor cells) inthe retina, each of which responds primarily to aspecific range of wavelengths.

1. One is most responsive to blue-violet colours,

2. One to green, and

3. The third to yellow-red.

e.g. If we look at the blue sky, the blue cones are triggerand the others show less activity.

People get colour-blind because one of three conesystems malfunctions, and colours converted by thatrange are perceived improperly.

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Limitation of Tri-chromatic Theory

It can not explain the after-images.

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What you experienced just now was an

afterimage.

It occurs because activity in the retina

continues even when you are no longer

looking at the original picture.

However, it also demonstrates that the

trichromatic theory does not explain colour

vision completely.

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2.2. Opponent Process Theory

of Colour Vision

Receptor cells are linked in pairs, working

in opposition to each other.

Three pairs of color receptors

1. Yellow-blue2. Red-green

3. Black-white

Members of each pair work in opposition

Can explain color afterimages

Both theories of color vision are valid

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If an object reflects light that contains more

blue than yellow, it will stimulate the firing of

the cells sensitive to blue, simultaneously

discouraging or inhibiting the firing ofreceptor cells sensitive to yellowand the

object will appear blue.

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When we stare at the blue in the figure (e.g) our receptor cells of the bluecomponent of the yellow-blue pairing become fatigued and are less ableto respond to blue stimuli and vice versa.

On the other hand, the receptor cells for the yellow part are not tired,since they are not being stimulated.

Therefore, When we look at a white surface, the light reflected off it

would normally stimulate both the yellow and the blue receptors equally. But the fatigue of the blue receptors prevents this from happening.

They temporarily do not respond to the yellow, which makes the whitelight appear to be yellow.

Because the other colors in the figure do the same thing relative to theirspecific

opponents, the afterimage produces the opponent colorsfor a while.

The afterimage lasts only a short time, because the fatigue of the yellowreceptors is soon overcome, and the white light begins to be perceivedmore accurately.

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Both trichromatic mechanism and opponent

process theory are at work in allowing us to

see colours.

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E. Recap

Eyes are sensitive to light. As the light enters the eye, it passes through the

cornea, pupil, and lens and ultimately reaches theretina, where the electromagnetic energy of light is

converted into nerve impulses usable by the brain.These impulses leave the eye via the optic nerve.

The retina is composed of nerve cells called rods andcones, which play different roles in vision and areresponsible for dark adaptation.

Humans are able to distinguish about 7 million colours.Colour vision involves two process: trichromaticmechanism and an opponent-processing system.

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F. Review Questions

1. Light entering the eye first passes through the _____, a protectivecoating?

Cornea

2. The structure that converts light into usable neural messages is calledthe _____?

Retina

3. Light is focused on the rear of the eye by the iris... True or false?

False: it is focused by the lens

4. The proper sequence of structure that light passes when it enters theeye is the 1_____, 2____, 3____, and 4____.

Cornea, pupil, lens, retina

5. ____ theory states that there are three types of cones in the retina, eachof which responds primarily to a different colour.

Trichromatic Theory

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6. Match each type of visual receptor with its

function

a) Rods

b) Cones

1. Used for dim light, largely insensitive to colour

2. Detect colour, good in bright light

a-1, b-2

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Courtesy: Imran Ahmad Sajid, T.A., ISSG, University of Peshawar.

[email protected]

Imran A. Sajid

Imran A. Sajid
mailto:[email protected]:[email protected]


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Imran A. Sajid


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References

Robert S. Feldman. (1999, 2008). Understanding Psychology. [5th & 7th ed.].McGraw-Hill.

"Color." Microsoft Encarta 2009 [DVD]. Redmond, WA: MicrosoftCorporation, 2008.

Marburger, John H. "Light." Microsoft Encarta 2009 [DVD]. Redmond,WA: Microsoft Corporation, 2008.

Jones, Ira Snow. "Eye." Microsoft Encarta 2009 [DVD]. Redmond, WA:Microsoft Corporation, 2008.

Oxford English Dictionary

Harun Yahya. (2006). Miracles in the Eye. Harun Yahya.

eye, human. (2009). Encyclopdia Britannica. Encyclopdia Britannica

2009 Student and Home Edition. Chicago: Encyclopdia Britannica. light. (2009). Encyclopdia Britannica. Encyclopdia Britannica 2009

Student and Home Edition. Chicago: Encyclopdia Britannica.

Visual System: Shedding Light on Eye-Lecture by Imran Ahmad Sajid, ISSG, UOP, March 2012

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