The Visual Stimulus and the Eye
Chapter 3 Outline G8
Focusing Light Onto the Retina
Light: The Stimulus for Vision
The Eye
Light is Focused by the Eye
Transforming Light Into Electricity
How Does Transduction Occur
Hecht’s Psychophysical Experiment
The Physiology of Transduction
Pigments and Perception
Distribution of Rods and Cones
Dark Adaptation of the Rods and Cones
Measuring Cone Adaptation
Measuring Rod Adaptation
Visual Pigment Regeneration
Spectral Sensitivity of the Rods and Cones
Measuring Spectral Sensitivity
Rod and Cone Absorption Spectra
Neural Convergence and Perception
Why Rods Result in Greater Sensitivity Than Cones
Why We Use Our Cones to See Details
Lateral Inhibition and Perception
What the Horseshoe Crab Teaches Us About Inhibition
Lateral Inhibition and Lightness Perceptioin
The Hermann Grid: Seeing Spots at Intersections
Mach Bands: Seeing Borders More Sharply
Lateral Inhibition and Simultaneous Contrast
A Display That Can’t Be Explained by Lateral Inhibition
Something to Consider: Perception is Indirect
The coverage that follows does not exactly follow the text.
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External stimulus for vision (G8 – p. 44)
Light is a portion of the electromagnetic spectrum
Name of energy
AC Circuits
AM Radio waves
TV signals
FM signals
Radar waves
Infrared rays
Light
Ultraviolet rays
X-rays
Gamma rays
Conceptualizing electromagnetic energy – particles vs. waves
Some forms best conceptualized as waves – FM signal – AC circuits
Some forms best conceptualized as particles – Gamma rays – ultraviolet rays
Light is sometimes conceptualized as a wave phenomenon and sometimes as a particle
phenomenon.
Online article on the issue: http://www.livescience.com/24509-light-wave-particle-dualityexperiment.html
We’ll think of it as particles - particles of energy streaming from a light source
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Characteristics of Light in particle terms
Intensity – The number of particle streams emanating from the light source. The more streams,
the more intense the light. Note that this is a characteristic of a collection of streams, not of a
given stream.
Wavelength –We’ll conceptualize as the distance between particles within a given stream.
For light, wavelength is measured in nanometers (nm).
1 nm = 1 billionth of a meter: .000000001
Long wavelength – long distance between particles: .
.
.
.
.
.
Short wavelength – short distance between particles: . . . . . . . . . . . . . . . . .
Wavelengths of visible light: 350 nanometers to 700+ nanometers.
Speed of particles: Speed of light, i.e., about 300,000,000 (300 million) meters / second.
(Action potentials travel down the axon at 120 meters / second.)
186,000 miles/second.
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Common descriptions of light sources
Most sources of light emit multiple streams and the different streams have different wavelengths.
Exceptions are lasers. So most light sources – the sun, the moon, a light bulb, are composed of
many different components – like mixed breed dogs.
Characterizing the light emitted by such “mixed breed” lights.
In terms of Energy used. Wattage. This measure used to correlate highly with light intensity.
Overall intensity: The Lumen. Roughly corresponds to number of particle streams emitted.
800
15?
6000
LED lights, just now coming on the market, will provide the same light intensity in lumens with
even less energy consumption and even longer life. (We’ll bequeath our light bulbs to our heirs.)
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A graphical way of representing the wavelengths emitted by “mutt” lights:
The Light Spectrum.
A graph in which the intensity (no. of particle streams) of light at each wavelength is plotted.
Spectrum of a monochromatic light – light with only one wavelength
A monochromatic bluegreen light.
300
400
500
600
700
600
700
Wavelength in nm
Light from a regular
incandescent bulb.
300
400
500
Wavelength in nm
Light from a regular
fluorescent bulb or LED.
300
400
500
600
700
Wavelength in nm
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Light from an infrared remote control
Energy is primarily infrared.
300
400
500
600
700
Wavelength in nm
Light from a tanning bed - energy is
primarily ultraviolet
300
400
500
600
700
Wavelength in nm
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Why do we have specific receptors for light, rather than some other part of the EM
spectrum?
Why are we sensitive to light, as opposed to gamma rays, or x-rays, or radar waves, or TV
waves?
Two main reasons. (These could be test questions – multiple choice or essay.)
1. There’s much more light available to us than other kinds of energy.
Because there’s so much of it, it’s easier to sense than other less abundant energy.
So why does it’s abundance play a role?
Answer: Because receptors are organic entities that have limits.
If there is a lot of the energy out there, the individual receptors don’t have to be as sensitive.
We don’t need Hubble telescope receptors if what they’re supposed to receive is a strong signal.
Not only does each individual receptor not have to be too sensitive, but also, the need for
multiple receptors to maximize sensitivity is lessoned. We don’t need as many receptors as
there would have to be if the energy were less prevalent.
2. Light is differentially reflected and absorbed from the various objects in the
environment.
Long wavelength radiation, like radio waves, is all reflected from everything it strikes. So all
objects would reflect about the same amount of radiation and so all objects would appear about
the same. Like having a friend who likes EVERYTHING.
Short wavelength radiation, like cosmic rays, is all absorbed by everything it strikes. So all
objects would be essentially invisible. Like having a friend who hates EVERYTHING.
But with light, some objects absorb short wavelengths and reflect long wavelengths.
Other objects absorb long wavelengths and reflect short.
So some objects reflect short wavelength light to the eye. Others reflect long wavelength light to
the eye.
Like having a friend who recommends what you would like and does not recommend what you
would not like. So light is like a trusted friend.
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The eye
Fibrous tunic, also called the sclera
Averages about 1 mm in thickness
Made up of thousands of fibers parallel to surface
White, except for the front, where it is transparent due to orderliness of fibers
Front is called cornea
Vascular tunic, also called the choroid coat
Heavily pigmented – resulting in reduced light scatter.
About 0.2 mm in thickness
Contains a network of blood vessels and capillaries.
Provides nourishment to receptors
Two chambers . . .
Anterior chamber (ante = front)
Formed by gap left when vascular tunic separates from fibrous tunic.
Filled with aqueous humor – transparent. Constantly manufactured and drained. Too much
leads to glaucoma.
Vitreous chamber
Filled with a fluid called vitreous – transparent with the consistency of uncooked egg white.
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Two Functional systems in eye
The optical system of the eye
The Cornea
The Aqueous humor
The iris/pupil
The lens
The Vitreous humor
The neural system of the eye
The receptors
The rods
The cones
The bipolar cells
The ganglion cells
The horizontal cells
The amacrin cells
The optic nerve
The optical system
This lecture
Next lecture
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Focusing light in the human eye
Cornea – Performs most (2/3) of the focusing, i.e., “bending” of light rays.
But the focusing is fixed – it’s the same for light from near objects as it is from far
objects.
Problem – Only objects at one distance will be in focus. All others will be out of focus.
Correct Focusing. Objects whose distance from lens is exactly equal to focal length will be in
focus.
All light from the same
point in space is
projected to same point
on screen.
Object
Pointbeing
on
Object
viewed
being
viewed
Problems with a fixed lens, like the cornea
A fixed lens doesn’t bend light enough when near objects are viewed. Rays from an object
whose distance is too small will focus behind the receiving screen. For proper focus the light
rays would have to be bent more than the lens is constructed for.
Image blurred on
receiving screen
Point on near
object being
viewed
A fixed lens bends light too much when far objects are viewed. An object whose distance is
too large will focus in front of the receiving screen. The light rays are being bent too much by
the lens.
Image blurred on
receiving screen
Point on far
object being
viewed
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Two possible solutions to the focusing problem . . .
1) Movable lens: Move the lens either toward the object or away from the object..
This is what is done in cameras and cephalopods (such as octopi)
2) Variable thickness lens: Adjust the thickness of the lens.
Lens kept in same
position, but made
flatter for far objects.
Lens kept in same
position, but made
fatter for near objects.
The second solution is what has been adopted for the human eye.
The shape of the cornea doesn’t change.
The lens, located immediately behind the cornea does change its thickness.
This leads to a concept:
Accommodation: Automatic changes in the shape of the lens to keep objects at different
distances in focus.
The lens automatically becomes flatter when we focus on objects that are far away.
It automatically becomes fatter when we focus on objects that are close to us.
The focusing process is automatic. The only conscious control over the mechanism is to
indirectly control it by changing the distance of objects we are attending to. So if I had
kidnapped you and tied you to a chair in a basement demanding that you “Change the shape of
your lens!! Now!!!”, what could you do?
This automatic change in shape is called accommodation. It’s accomplished by muscles
attached to the lens which contract or expand under control of signals from the brain.
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The opias – (G8 p. 45-46)
Hyperopia – Far sightedness (hyper – a lot of, e.g., hyperactive)
Ability to see far objects well, but inability to focus on near objects.
The reason: Lens doesn’t bend light enough for near objects or eyeball is too short
=
Solution is glasses which “prebend” the light.
Presbyopia – Old age farsightedness
As we get older, the lens continues to add layers. Interior layers become less flexible, and lens
becomes less able to change shape – leading to farsightedness in old age.
Myopia – Near sightedness.
Ability to see near objects well, but inability to focus on far objects.
The reason: Lens bends light too much for far objects or Eyeball is too long
Solution is to wear glasses which increases the visual angle of objects,
as if they are close up..
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The iris
The iris is a ring of muscles that form a circle, like a doughnut, with a hole in the middle.
Some of the musceles are circular, around the hole. Others are radial, like the spokes on a bike
wheel.
The hole is called the pupil.
Simultaneous relaxation of circular and contraction of radial muscles makes hole bigger.
The tissue of the iris contains pigment that gives the eye its color.
The fine pattern of detail on the iris is determined by a combination of factors that are unique to
the individual eye.
No two irises are alike, not even your left and right iris.
From Daugman, J. Iris Recognition. American Scientist, 89, 326-333.
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The pupil.
2
Pupil diameter varies from 2 to 8 mm in diameter. Using the formula п*r , (that’s pi times
radius squared) this means that the area varies from 3.14*12 to 3.14*42 = 3.14 mm2 to 50 mm2,
about a 17:1 ratio. Who said a psychology major wouldn’t need math?
Thus the pupil admits about 17 times as much light when it is at its widest than it does when it is
at its narrowest.
The contraction/relaxation of the iris is an automatic process over which we have no direct and
very little indirect control. It’s controlled by signals sent to the iris from the brain.
As the ambient light increases, the iris automatically relaxes, allowing the
pupil to become smaller.
As the ambient light decreases, the iris automatically contracts, making the
pupil larger.
So if I kidnapped you and tied you to a chair in a basement demanding “Make
your pupil bigger!! Now!!” what could you do?
Why do we have a system that changes the size of the opening through which light enters?
Two possible answers . . .
1. The light-regulation hypothesis: To increase the amount of light allowed into the eye when
it’s dark outside and to protect the eye from too much light when it’s very bright outside.
2. The all-things-in-focus hypothesis: To keep the light rays that enter the eye as close to the
center of the lens as possible, making the focus of those rays on the retina as good as possible.
So the pupil is kept small if there’s enough light. It is enlarged only when there isn’t enough
light.
Test question alert!!
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The all-things-in-focus hypothesis – the consequence of a small and large pupil sizes
The following figure is from B&S, p. 44, Figure 2.12.
Note that in both figures, the flowers in the foreground are in good focus.
But through a large opening the background is out-of-focus.
This situation is called poor depth of field by photographers.
Through a small opening, the background is in focus.
This is called good depth of field.
Large pupil
Small pupil
The smaller the opening through which the light passes, the better the depth of field.
Many believe that the primary purpose of the variable-sized pupil is to maximize depth of field.
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