Sensation and Perception
Module 18
Vision
Energy=Light
• We only see a small spectrum of light rays
• 2 characteristics determine our sensory
experiences
– Wavelength: the distance from 1 peak of a wave
to another; determines color (hue)
– Intensity: height of the wavelength; influences
brightness
Parts of the Eye – Cornea
Cornea
• The clear bulge on the front of the
eyeball
• Begins to focus the light by bending it
toward a central focal point
• Protects the eye
Parts of the Eye - Iris
Iris
• A ring of muscle tissue that
forms the colored portion of
the eye; creates a hole in the
center of the iris (pupil)
• Regulates the size of the pupil
by changing its size--allowing
more or less light to enter the
eye
Parts of the Eye - Pupil
Pupil
• The adjustable opening in the center
of the eye that controls the amount of
light entering the eye (surrounded by
the iris)
• In bright conditions the iris expands,
making the pupil smaller.
• In dark conditions the iris contracts,
making the pupil larger.
Parts of the Eye - Lens
Lens
• A transparent structure behind the pupil;
focuses the image on the back of the eye
(retina)
• Muscles that change the thickness of the lens
change how the light is bent thereby focusing
the image (Accommodation)
• Glasses or
contacts correct
problems in the
lens’ ability to
focus.
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 and this creates the pulsation.
Farsightedness
Nearsightedness
Presbyopia
Astigmatism
Parts of the Eye - Retina
Retina
• Light-sensitive surface with cells that
convert light energy to nerve impulses
• At the back of the eyeball
• Made up of three layers of cells
– Receptor cells (Rods & Cones)
– Bipolar cells
– Ganglion cells
Parts of the Eye - Fovea
Rods
• Visual receptor cells located in the
retina
• Can only detect black and white
• Respond to less light than do cones
Cones
• Visual receptor cells located in the
retina
• Can detect sharp images and color
• Need more light than the rods
• Many cones are clustered in the
fovea at the center of the retina.
Rods
Cones
Watch Blue
Man Group’s
Rods &
Cones
Performance
Distribution of
Rods and Cones
• Cones—concentrated in center
of eye (fovea)
– approx. 6 million
• Rods—concentrated in periphery
– approx. 120 million
• Stare at a word and you’ll notice the others
around it become blurred. (Clear word seen with
cones, blurry area seen with Rods)
• Blind spot—region with no rods
or cones
The Hermann Grid
Are there gray dots between the squares? Rods in the periphery are responsible for this. 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.
Processing Visual Information
• Rods & Cones transduce light into neural signals.
• Bipolar cells—neurons that connect rods and cones
to the ganglion cells
• Ganglion cells—neurons that connect to the bipolar
cells, their axons form the optic nerve
• Optic chiasm—point in the brain where the optic
nerves from each eye meet and partly crossover to
opposite sides of the brain
Visual Processing in the Retina
Visual Processing in the Retina
Visual Processing in the Retina
Visual Processing in the Retina
Parts of the Eye – Optic Nerve
Optic Nerve
• The nerve that carries visual information from
the eye to the thalamus then on to the
occipital lobes of the brain
Parts of the Eye – Blind Spot
Blind Spot
• The point at which the optic nerve travels
through the retina to exit the eye (Optic Disk)
• There are no rods and cones at this point, so
there is a small blind spot in vision.
• We don’t notice our blind spot because each eye
compensates for the other or your brain “fills in”
the missing background info. (Top-down
process)
Cover your right
eye and stare at
the can as you
move closer to
the screen.
Notice the spider
disappear in your
peripheral vision?
Visual Pathway
From the eye to the brain
Light travels through…
Cornea – Pupil – Lens – Fovea (retina) –
Rods/Cones – Bipolar Cells – Ganglion cells
(movement & light /color & detail) – Optic
Nerve (blind spot) – Optic Chiasm (crossover
point) – Thalamus – Occipital Lobe (Primary
Visual Cortex)
Feature Detection
• Nerve cells in the brain fire to a very particular
stimulus…like shapes, angles, movement,
patterns.
– Pass along information to super cell clusters that
process more complex patterns
– Works with left temporal lobe
– Believe super cells come from our ancestors and
their survival—natural selection!
Parallel Processing
• Processing many stimuli at once (motion,
color, depth, etc)
– “Mrs. M” can’t perceive motion after stoke
– Blindsight: illustrate two-track mind.
Color Vision
• Young-Helmholtz- 3 primary colors: red, green,
blue
– Trichromatic color theory: based on the stimulation
of above 3 color receptors which are found on the
retina
• Color blindness: lacking one or both of these cones
• Herring: afterimages see opponent color after
staring at the one primary color
– Opponent process theory: red-green, yellow-blue,
black-white. Stimulate one color inhibits the
opposite.