The Human Visual System Dr shabanzadeh A. Neuroscientist Department of Medicine University of Tehran

The Human Visual System

Dr shabanzadeh A.Neuroscientist

Department of MedicineUniversity of Tehran

Hardware Flow VisualizationUniversity of California, Davis

Visualization and Graphics Research Group

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Overview

• Physiology of the human eye

• The eye and how light is recognized

• Vision

• What does it mean for image synthesis?



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Physiology of the human eye



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Breakdown…



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Outer shell• Sclera is the white of your eyes

– 22mm in diameter and 1 mm thick

– Gives the eye it’s structural integrity

– Totally opaque

• Cornea is the transparent semi-spherical shell that covers the front of your eye where there is a break in the sclera– Radius of curvature of 8mm

– Refractive index of 1.37 – causes the convergence of rays of light within the eye!

• Aqueous humor is the fluid behind the cornea– Basically just water, with refractive index 1.33

– Carries nutrients to cornea



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Pupil• The iris is the the colorful part of the eye, connects with the

choroid– The opening centered within the iris is the pupil

– The iris consists of delicate muscle which dilates and contracts to allow various amounts of light through the pupil

– Pupil diameter ranges from about 2mm on a bright day to 8mm under dark conditions

– Both pupils dilate or contract at the same time regardless of whether lighting is consistent for both eyes – called the consensual pupillary reflex



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Lens• The lens is also called the crystalline lens

– Refractive index of 1.4 at center to 1.38 at the outside

– Gives fine vision adjustment known as accommodation which helps you focus objects of different distance to the eye

– This is controlled by muscles, and is why you get eye strain from focusing on things too close, and why looking far away helps relax it



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Retina• The retina is the light sensitive layer

– Purpose is to form an appropriate real image of the world

– The image which “exposes” onto the retina is actually inverted, but our brain “reverts” it

• The fovea is on the central axis of the retina, and is the area of greatest visual acuity– This is what is “exposed” when we “look at” something

– Only 0.25 mm in diameter, so represents very small portion of our field of view

• We all have a blind spot in each eye– Caused because of the optic nerve about 5mm from central axis

– Take a look at the book • Rods and cones are actual light receptors which make up retina

– Each retina has about 120 million rods and 6 to 7 million cones



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Physiology of the human eye



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More on Rods and Cones• They are about 0.05 mm long and 1 to 3 micrometers in

diameter• Necessarily small because we need resolution!• Cones are responsible for

– High light level vision (photopic)

– High resolution

– Color

• Rods are responsible for– Low light and night vision (scotopic)

• Changing between the two is called adaptation– Adaptation from light to dark can take 20 to 30 minutes

– From dark to light takes a few tenths of a second



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August 20, 2001 Hardware Flow Visualization 14University of California, Davis


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Light and dark adaptation: rhodopsin decrease



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Photon-metarhodopsin ii-transducin(A)-phosphodistrase(A)-cGMP hydrolysis and decrease-Na (dark)current decrease

Rhodoposin kinase inhibits metarhodopsin II



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Even more on Rods and Cones • Rods and cones contain visual pigment • Rods have one kind of pigment• Cones have 3 kinds of pigment• Somehow the light bleaches out a pigment when it is exposed,

this triggers a nerve impulse• The pigment in rods is much more sensitive than the kind in

cones



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Spectral Response• It has been shown that different wavelengths have different

ability to produce a brightness response

• The most effective wavelength for scotopic vision is 507 nm

• For photopic its 555 nm

• This leads to funny phenomenon called the Purkinje Effect– Blues and greens become brighter relative to reds and yellows as

light fades



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Color blindness • Protanope• Deutranope



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Light and dark adaptation: rhodopsin decrease

• Light• rhodopsin decrease• Dark• rhodopsin increase



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Temporal Response• Rods and cones wait until pigment is completely bleached

• This is a summing action, and causes a temporal effect

• Also related is critical fusion frequency or critical flicker frequency– This ranges in general from 5 to 55 Hz depending on brightness

– Can be more pronounced though and if things are very incoherent can be higher than 55 Hz (think flight simulators)

– Even though film is 24 frames per second, the shutter is flickering in excess of 60 times per second, so we’re okay



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Next please…• After the rods and cones, the signal is sent down “nerves”• The signal hits “bipolar cells”, and then “ganglion cells” which

connect to the brain• Processing is done in the bipolar cells before it is done in the

brain– Even though there are 127 million light receptors, the signals are

send over less than 1 million fibers

• The nerve bundles from each eye (optic nerves) actually cross over in what is called the optic chiasma and some of the vision from each eye goes into each side of the brain– This may be a preemptive coping strategy in case damage occurs

somewhere along the way



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Vision• Humans like to see things in 3D – our eyes are already adapted

for this– Leads to funny optical illusions



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Vision



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Vision• This is one reason why stereo is great – it helps us with our

natural depth resolution



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Color Vision• One clue that we perceive everything in terms of 3 colors (RGB)

is that we can construct any other color from combinations of them

• We even get bonus colors! White light is not in the spectrum, nor is purple, but we can get them from combinations of RGB colors

• Still not sure exactly how “color” is transmitted to the brain, though it is thought that there are three color pigments in the cones

• There are other theories with 4 color receptors – red, green, blue, and yellow

• Most scientists believe the tristimulus color theory



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What does it mean for image synthesis?

• We need to keep in mind that the world doesn’t work in RGB– We should use spectral colors to avoid losing information, and then

convert to RGB only at the end

• If we really want people to believe these images are real, stereo is important – gives depth information (unfortunately, we’re not doing this in the class)

• In the end product, we can get away with a lot– People can see the same color given an infinite number of spectral

combinations



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References• Principles of Digital Image Synthesis, Volume 1, Andrew S.

Glassner, 1995, Morgan Kauffman

• Introduction to Light; The Physics of Light, Vision, and Color, Gary Waldman, 2002, Dover Publications

Documents

The Human Visual System Dr shabanzadeh A. Neuroscientist Department of Medicine University of Tehran