Vision
By Michelle, Kim, Kayla, and Sean
Key Objectives
• Discuss sensory process including the
specific nature of energy transduction,
relevant anatomical structures and
specialized pathways in the brain for vision
• Explain common sensory disorders related
to vision
• Identify the major components of the visual
system and the function of each
Keep in Mind
• How does sensory information
impact human behavior?
• To what extend are our
perceptions of the environment
accurate?
Importance of Vision
• Detects targets
• Detect threats
• Detect environmental changes and stimuli
Basics of Vision
• Creates neural signals from light
waves
• Light waves the only form of
electromagnetic energy we can see
called visible light
• Transduction happens in the retina
• Transduction is the transformation
of one form of energy into another
• For vision this would be
transferring light waves into neural
signals
Visible Light
Properties of Light
• Wavelength – this is the distance between peaks of the wave
•
Affects perceived color
• Amplitude – this is the height of the wave
•
Affects perception of brightness
• Saturation – the amount of whiteness in a color; the less white, the more
saturated
• Hue – this is color, a sensation the brain produces from the wavelengths
of visible light
• Radiant Light – light that is emitted
•
The sun, lights, fireflies
• Reflected Light – light that is reflected
•
The color of your shift is reflected light
How Vision Works
• Light enters the transparent window of eye called the cornea
• Then through the lens
• Forms an upside down imagine on the retina behind it
• The brain is in charge of flipping the image back: accommodation
The Retina
• The retina is…
• Retina is the farthest light gets in the visual system before it is converted
• Where transduction takes place, light waves converted to neural signals
• The retina is like a the spinal cord but for vision and can process images
• The retina has a variety of cells including photoreceptors, rods, cones,
• Photoreceptors are the light sensitive neurons in the eye
• There are two types: rods and cones
The Eye
Rods
• Rod-shaped
• Sensitive to dim light
• NOT sensitive to colors
• For night vision and peripheral vision
• Outnumber cones in the peripheral area
of the retina
• Highest concentration of rods is just
outside the fovea and gradually
decreases
Cones
• Cone-shaped photoreceptors
• Sensitive to colors
• For day vision and color vision
• NOT sensitive to dim light
• They sense waves in red, blue, and green wave lengths
• Cones concentrate in area of retina called the fovea
•
This is where you have the sharpest vision
Dark Adaptation
• The process is which the eyes become more sensitive to light in low
illumination.
• This when you go from really bright light to low light and your eyes adjust
• Like when you walk from outside in the middle of summer into a dark
room
• Light adaption is the opposite
Light stimulus to Neural Impulse
• Starts as light enters the eye
• Stimulates rods and cones at the back
of the retina
• They convert the light waves into
neural impulses
• The impulses are collected by bipolar
cells
• Then sent off to ganglion cells
• The axons of the ganglion are bundled
together to form the optic nerve
• Impulses travel through the optic
nerve (out of the optic disk) to the
brain
Blind Spots
• The axons that go from your eye to your brain meet at the optic disk,
which is a whole in the retina where optic nerve fibers leave the eye
• Since there is a whole in the retina there is a blind spot in your vision
• Most people are not aware that they have a blind spot because your other
eye fills in your brain with the information you are missing
• On a piece of paper:
• draw a cross and a dot
• Close left eye, stare at cross
• Start 8 inches from face
• Move paper back
• Dot will disappear
Vision in the Brain
• Optic Chasm – the point at which the optic nerves
from the inside half of each eye cross over and then
project to the opposite half of the brain
• From there they go the thalamus – the brain’s major
relay station
• Then they are sent to different parts of the visual
cortex which is located in the occipital lobe
• All of this takes place in the forebrain
• You “look with your eyes but see with your brain”
• Brain is in charge of processing info from eyes
• Parallel Processing: simultaneously extracting
different kinds of information from the same input
• Retinal Disparity - is the difference between the visual
images formed by the different angles of the eye
• The brain connects the over lapping image and
creates one without seems
• Retinal disparity is important for depth perception
Brain Summary
Visual cortex
• Where all visual images are process
Forebrain
• The Diencephalon is an area of this region of the brain that also helps
process visual images, but from a more cognitive side.
Thalamus
• Before being able to travel to the Cortex, visual image fibers must travel
through the lateral geniculate nucleus, which is a part of the Thalamus.
The fibers synapse into the LGN at this point.
Occipital Lobe
• Area of the cerebral cortex that contains the visual cortex.
Vision Properties
• Color depends on three
properties
• Wavelength makes the hue
• Amplitude makes the
brightness
• Purity or Saturation
• Brightness – caused by the
intensity of the light
waves; also a sensation
• Acuity: the sharpness,
acuteness of vision
Monocular Cues vs. Binocular Cues
• Binocular Cues are depth cues
that require both eyes, usually
required for judging how far
away close objects are.
•
Example: hold two pens in front of
you and touch tips together, try
again with only one eye open….its
harder
• Monocular Cues are depth cues
that (interposition and linear
perspective) that are available to
both eyes
•
Trying to tell how far a car straight
ahead a far distance is
Theories of Color Sensation
•
Trichromatic theory: The idea that colors are sensed by three different types of cones
sensitive to light in the red, blue, and green wavelengths. Explains the earliest stage of
color sensation
•
Opponent Process theory: The idea that cells in the visual system process colors in
complementary pairs
•
Young-Helmholtz Theory: (Thomas Young and Hermann von Helmholtz) is a theory of
trichromatic color vision - the manner in which the photoreceptor cells in the eyes of
humans and other primates work to enable color vision.
•
Color Afterimage: A color a person will continue to see after looking at an object
Vision Disorders
• Nearsightedness – you can see close objects clearly while distant ones are
blurry
• Farsightedness – you can see distant objects clearly but close ones are
blurry
• Diplopia – or “double vision” when you see two images instead of one
• Color Deficiencies
Color deficiencies
• This means that you cannot see all of the colors
• Monochromat: A person who are completely color blind
• Dichromat: one affected with dichromatism (a defect of vision in which
the retina responds to only two of the three primary colors)