Visual System: Shedding Light on Eye-Lecture by Imran Ahmad Sajid, ISSG, UOP, March 2012
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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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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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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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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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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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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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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.
imranahmad131@gmail.com
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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.
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