Download The Visual System: The Nature of Light

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
Electromagnetic Spectrum of Light
White
light
Prism
Visible light
400
Gamma
rays
10 -5
10 -3
X-rays
10 -1
Ultraviolet
rays
10 1
500
600
Infrared
rays
10 3
10 5
700
Broadcast
bands
Radar
10 7
10 9
10 11
AC
circuits
10 13
Wavelength in nanometers (billionths of a meter)
10 15
10 17
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
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
Vision
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 focuses
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 focuses
light rays from near
objects past the retina.
Can see far but not
near.
(Myopia)
Misshapen eye focuses 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.
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
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)
• 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 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.
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
Receptive Fields and Rod
vs. Cone Visual Acuity
• Cones—in the fovea, one cone often
synapse onto only a single ganglion cell
• Rods—the axons of many rods synapse
onto one ganglion cell
• This allows rods to be more sensitive
in dim light, but it also reduces visual
acuity
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. (do demo on
page 98)
• 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)
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)
How Can I Possibly Remember
All of That in Order?
Cool = Cornea
People = Pupil
Like = Lens
Frosties = Fovea (Rods & Cones)
Because = Bipolar Cells
Gangsters = Ganglion Cells
Never = Optic Nerve
Cheat = Optic Chiasm
The = Thalamus
Officers = Occipital Lobe
Don’t like this one? Create Your Own!!!
Primary Visual Pathway:
Thalamus processes info about
form, color, brightness & depth
Secondary Visual Pathway:
Midbrain processes info about
the location of an object
– feature detectors respond
to things like angles,
edges, lines & movement
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