Transcript The Visual System: The Nature of Light

Visual System: Sight
The Stimulus Input: Light Energy
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Electromagnetic spectrum--Energy that includes radio waves, X-rays, microwaves, and
visible light
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Visible spectrum--Energy in the form of light visible to the human eye
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Light can be described as a particle and a wave that varies in hue (color) and intensity
(amplitude)
Humans can detect about
5 million different hues.
Shorter
Wavelengths
Longer
Wavelengths
The Stimulus Input: Light Energy
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Hue is the color of light as determined by the
wavelength of the light energy
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Remember ROY G. BIV --> red, orange, yellow,
green, blue, indigo and violet
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The eye can detect 7 million separate hues
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The brightness of light as determined by the height
(or amplitude) of the wave
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The taller the wave, the brighter the color
Afterimage
Stare at the red dot in the green square and count
to forty. Then stare at the white square and tell
me what you see. You should see a greenish/blue
dot in a reddish/purple background. That is called
an “afterimage”.
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The Visual Pathway
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Cool = Cornea
People = Pupil
Like = Lens
Fish = Fovea (Rods &
Cones)
Because = Bipolar Cells
Gangsters = Ganglion Cells
Never = Optic Nerve
Cheat = Optic Chiasm
The = Thalamus
Officers = Occipital Lobe
Primary Visual Pathway: Thalamus
processes info about form, color,
brightness & depth
Secondary Visual Pathway: Midbrain
processes info about the location of an
object
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Visual Information Processing
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Vision is the most complex, best developed and most important sense for humans and other
highly mobile creatures.
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Think of the eye as the brain’s camera. It gathers light, focuses it, converts it to a neural signal,
and sends these signals on for further processing.
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The eye transduces the characteristics of light into neural signals that the brain can process.
This transduction happens in the retina, the light sensitive layer of cells at the back of the eye.
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In the visual cortex, the brain begins working by transforming neural impulses into visual
sensations of color, motion, form, and depth.
• This process is called parallel processing -- the simultaneous processing of several aspects
of a problem
Visual Information Processing
Different parts of the
visual cortex are used to
identify different images
Afterimages – Sensations that
linger after the stimulus is
removed
Transduction -- The purpose of the visual system is transforming light energy into an electrochemical neural response (i.e. action potential and synaptic transmission) represented in the
characteristics of objects in our environment.
Color Vision
Despite the way the world appears, color does not exist outside the brain, because color is a
perception the brain creates based on the wavelength of light striking our eyes.
Color is created when the wavelength in a beam of light is recorded by the photoreceptors in the
form of neural impulses. It is then sent to specific regions of the brain for processing.
Color brightness depends on the intensity of the light waves. Color blindness is a visual disorder
that prevents an individual from discriminating certain colors.
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Opponent-process theory
--Sensory receptors come in pairs (red-green/
blue-yellow/black-white)
--If one color is stimulated, the other color is
inhibited
--Explains afterimages
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Young-Helmholtz trichromatic (three color) theory
-- There are three cones (red, green, blue) that
form millions of combinations of colors
-- Monochromatic and dichromatic vision
Ishihara Test (Color Blindness)
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People who suffer red-green blindness have
trouble perceiving the number within the design
They lack functioning red- or green- sensitive
cones, or sometimes both
Sensory Deprivation And Restored
Vision
• Critical period for normal sensory and perceptual
development
• Subjects blind from birth whose sight is later restored
have trouble making perceptions
– Facial recognition, perceptual constancy is lacking
Visual Problems
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Farsightedness--misshapen eye
focuses light rays from near objects
past the retina. See far!
Nearsightedness--misshapen eye focuses
light rays from a distant object in front of
the retina. See near!
Misshapen eye focuses light
rays from a distant object in
front of the retina. Can see
near but not far.
Presbyopia--form of farsightedness caused
when lens becomes brittle and inflexible
Astigmatism--uneven curvature of the cornea
causes multiple focus points/images on the
retina, resulting in blurry vision
How Do We Correct Vision?
Glasses, contact lenses, or LASIK surgery reshape
the cornea (which is also involved in bending light
to provide focus).
Parts Of The Eye
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Parts Of The Eye
Pupil --> adjustable opening in the center of the eye that controls amount of light
(bright conditions, iris expands, pupil smaller; dark conditions, iris contracts, pupil larger)
Iris --> ring of muscle that forms the colored portion of the eye around the pupil and controls the size
of the pupil opening
Cornea --> near the center of the retina that begins to focus the light by bending it; protects the eye
Lens --> transparent structure behind pupil that changes shape through accommodation to focus
images on the retina
Retina --> light-sensitive inner surface of the eye, containing receptor rods and cones plus layers of
neurons that begin the processing of visual information
Optic Nerve --> carries neural impulses from the eye to the brain
Blind Spot --> point at which the optic nerve leaves the eye, creating a “blind spot” because there are
no receptor cells located there
Fovea --> central point in the retina; highest concentration of rods and cones; area of sharpest
vision (visual acuity)
Retina’s Reaction To Light
Rods are visual receptors that detect black and white and
respond to less light.
Cones are visual receptors that detect sharp images and color
and respond to more light.
Ganglion cells are neurons that pass information from the
bipolar cells; their axons form the optic nerve.
Retina’s Reaction To Light
Photoreceptors are light-sensitive cells (neurons) in the retina
that convert light energy into neural energy. That is, light
energy strikes the rods and cones to produce chemical changes
that general neural signals.
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Optic Nerve: The bundle of
neurons that carries the visual
information from the retina to the
thalamus to the occipital lobe of the
brain.
--Where the stimulus, once
changed into a neural
impulse, gets passed onto
the brain.
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Blind Spot: The point where the
optic nerve exits the eye and where
there are no photo-receptors.
--Any stimulus that falls on
this area cannot be seen.
--We do not notice it because
each eye compensates for
the other and your brain
“fills in” for the missing
information. (top-down
process & Gestalt)
Retina’s Reaction To Light
Cover your right eye and stare
at the can as you move closer
to the screen. Notice the
spider disappear in your
peripheral vision?
The Cosmic Flower
Does this picture seem to pulsate? Because the lens of your eye is not
perfectly round some parts of what you look at are blurry. Your eyes make
micro movements to try to put this entire picture into focus, which creates
the pulsation.
The Hermann Grid
Are there gray dots between the squares? Rods in the periphery are
responsible. When you look at an area directly there is no dot because you are
using your cones but the periphery has dots because the rods are trying to do
two things, show you there is a dark area and a light area.