Download Module 18

The Visual System:
The Nature of Light
Electromagnetic Energy
• An energy spectrum that includes X-rays, radar, and radio
waves
• A small portion of the spectrum includes light visible to the
human eye
• Light can be described as both a particle and a wave
Hue
• Hue is the color of
light as determined by
the wavelength of the
light energy
• Includes: red, orange,
yellow, green, blue,
indigo and violet
(ROY G. BIV)
• The eye can detect 7
million separate hues
Intensity/Amplitude
• The brightness of light as determined by
height of the wave
• The taller the wave, the brighter the color
The Visual System:
The Structure of the
Visual System
Purpose of the visual system
– Transduction: transform light energy into an
electro-chemical neural response (action potential
& synaptic transmission)
– represent characteristics of objects in
our environment such as size, color, shape, and
location
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.
The lens correctly focuses the
image onto the back of the eye
(retina).
The lens correctly focuses the
image onto the back of the eye
(retina).
(Myopia)
Misshapen eye causes lens to
focus light rays from a distant
object in front of the retina.
Can see near but not far.
The lens correctly focuses the image
onto the back of the eye (retina).
Misshapen eye causes lens
to focus light rays from
near objects past the retina.
Can see far but not near.
(Myopia)
Misshapen eye causes lens to
focus light rays from a distant
object in front of the retina.
Can see near but not far.
(Hyperopia)
Other Causes of Poor Vision
Astigmatism – Uneven curvature of the cornea
causes multiple focus points/images on the
retina resulting in blurry vision.
Other Causes of Poor Vision
• Presbyopia – form of
farsightedness caused when
lens becomes brittle &
inflexible. Usually starts to
happen in your early 40’s
Reading glasses
will correct this.
Nearsightedness
Farsightedness
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
Fovea
• The central focal point of the retina
• The spot where vision is best (most
detailed – visual acuity)
• Only cones are found in the Fovea
Receptor Cells
(Rods & Cones)
• These cells are present in every sensory
system to change (transduce) some other
form of energy into neural impulses.
• In sight they change light into neural
impulses the brain can understand.
• Visual system has two types of receptor
cells – rods and cones
Rods
• Visual receptor cells located in the
retina
• Can only detect black and white
• Respond to less light than do cones
• Located around the fovea. (remember
pen-top demo from class)
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. (remember
pen-top demo from class)
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)
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.
Optic Nerve/Blind Spot/Optic Disc
• The nerve that carries visual information from
the eye to the thalamus then on to the occipital
lobes of the brain
• Blind Spot has
NO rods or
cones
Parts of the Eye – Optic Nerve
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. (do demo on page 174)
• 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 & Gestalt
Theory)
• Want more Blind Spot demos? Click HERE
Cover your right
eye and stare at
the can as you
move closer to
the screen.
Notice the spider
disappear in your
peripheral vision?
Parts of the Eye – Blind Spot
Visual Processing in the Retina
Processing Visual Information
• Rods & Cones transform light into action
potential/synaptic transmission.
• 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
Bipolar Cells
• Cells that form the middle layer in the retina
• Gather information from the rods and cones and pass it on to the
ganglion cells
• Hundreds of Rods feed into 1 Bipolar Cell
• 1 to 2 Cones feed into 1 Bipolar Cell.
• This is why Cones have better visual acuity/clarity but Rods
collectively can see better in dim lighting
Ganglion Cells
• Pass the information from the bipolar cells
through their axons
• Together these cells form the optic nerve.
• The top layer of the cells in the retina
Visual Processing in the Retina
Visual Processing in the Retina
Visual Processing in the Retina
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)
How Can I Possibly Remember
All of That in Order?
Cherry = Cornea
Pie = Pupil
Loses = Lens
Flavor = Fovea (Rods & Cones)
Because = Bipolar Cells
Grandma = Ganglion Cells
Never = Optic Nerve
Cleans = Optic Chiasm
The = Thalamus
Oven = Occipital Lobe
Don’t like this one? Create Your Own!!!
Primary Visual Pathway:
Thalamus processes info about form,
color, brightness & depth
(What Pathway)
Secondary Visual Pathway:
Midbrain processes info about the
location of an object (Where Pathway)
– feature detectors respond
to things like angles, edges,
lines & movement
Parallel Processing of Vision
• Brain simultaneously processes information about Motion, Form,
Depth & Color then compares it to memories you have stored.
Visual Adaptation
(a) A projector mounted on a contact lens
makes the projected image move with the eye.
(b) Initially the person sees the stabilized
image, but soon she sees fragments fading and
reappearing.
Visual Impairment
• Play “Smart Glasses” (8:13) Segment
#9 from Scientific American
Frontiers: Video Collection for
Introductory Psychology (2nd edition).
Watch in Class