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Vision
Marisa Badamo, Shelly Elango, Mindy
Heard, Carly Niedbalski, and Arif Syed
Vision: How the Nervous
System Processes Light
• Good vision helps us detect desired targets,
threats, and changes in our physical environment
and to adapt our behavior accordingly
The Anatomy of Visual
Sensation
• The eye gathers light, focuses it, converts it to neural
signals, and sends these for processing into a visual
image
• The eye extracts information from light waves and
transduces them into neural signal the brain can
process
• The transduction happens in the retina
• Retina: The thin, light sensitive layer at the back of the
eyeball
• Retinal Disparity: The slight difference in the two
retinal images due to the angle from which the eye
views an object
• Acuity: acuteness and clearness of the vision
• Photoreceptors are light
sensitive cells that contain rods
and cones
• Rods and cones are specialized
neurons that absorb light and
create neural impulses
• Rods detect low intensities of
light at night but cannot
distinguish color
• Cones are specialized to detect
the light waves we sense as
red, blues, or green. Not
sensitive to light
• Accommodation: process by
which the eye changes optical
power to maintain a clear
image when an object’s
distance varies
• Adaptation: ability of the eye to adjust to
various levels of darkness and light
• Cones are concentrated in the fovea which is
where we have the sharpest vision
• Bipolar cells collect impulses from
photoreceptors and send them to ganglion cells
• The axons of the ganglion cells form the optic
nerve which sends information from the eye to
the brain
• The blind spot is a gap in the visual field resulting
from the point at which the optic nerve exits the
eye
• The visual cortex is where the brain transforms
neural impulses into visual sensations of color,
form, boundary, and movement
• Nearsightedness: the ability to see near things
more clearly than distant ones
• Farsightedness: seeing or able to
see to a great distant
• Parallel processing: the ability
of the cortex to perform more
than one task at one time
Demonstration Time
How the Visual System
Creates Color
• Color: also called hue. Color is not a property of
things in the external world. It is only a psychological
sensation created in the brain from information
obtained by the eyes from the wavelengths of visible
light.
• Color does not exist outside the brain, but only in the
mind of the viewer
• Color is created when the wavelength in a beam of
light is recoded by the photoreceptors in the form of
neural impulses and sent to specialized areas of the
brain for sensory processing.
• Amplitude: the physical strength of the wave.
Measurement from the top (peak) to the bottom
(valley) of the graph of a wave
• Wavelength: the distance electromagnetic energy
waves travel making one wave cycle
• The eyes detect a special
form of energy called
visible light (pure energy)
• Visible light lies on the
electromagnetic
spectrum: the entire
range of electromagnetic
energy, including radio
waves, X-rays,
microwaves, and of course
visible light
• Visible spectrum: the tiny
part of the
electromagnetic spectrum
to which our eyes are
sensitive.
• Within the visible spectrum light waves of
different lengths give rise to different colors
– Longer waves: red
– Medium waves: yellow and green
– Small waves: blue
• The wavelengths of light that the eye obtains the
information is used by the brain to construct colors
• Visual experiences of color, form, position, and depth are
all based on processing the same stream of sensory
information in different parts of the visual cortex
Two Ways of Sensing
Colors
• Color processing begins in the retina
• Trichromatic theory(Young-Helmholtz 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, such as red
or green. Explains color sensation from the bipolar cells
onward in the visual system
– Explains some cases of blindness
– Afterimages: sensations that linger after the stimulus is
removed. Most are negative which appear in reversed colors.
• Saturation: How intense a color is determined
by a combination of light intensity and how
much it is distributed across the spectrum of
different wavelengths
• Monocular cue: depth perception cue that
requires information from one eye
• Binocular cue: depth perception cue that
requires information from both eyes
Color Blindness
• Color blindness: typically a genetic disorder that
prevents an individual from discriminating certain
colors. The most common form is red-green color
blindness.
• Monochromats- complete color blindness total
inability to see distinguish color
• Those who confuse yellow and blue are very rare
• Only about 500 cases involve people seeing no
color at all
Can you see the #6?