The Eye
hole that allows
light to enter eye
transparent outer
covering of eye
muscle that controls
(pupil) how much
light enters eye
focuses light
on the retina
contains visual
sensory receptors
contains most of
the eye’s cones
Where optic nerve
leaves eye…no
receptors here
Wavelength (Hue)Hue (color):
dimension of color
determined by
wavelength of
light.
Wavelength the
distance from the
peak of one wave
to the peak of the
next.
Wavelength (Hue)
Different wavelengths of light result
in different colors.
400
nm700
nmLong wavelengthsShort wavelengths
Violet Indigo Blu
e
Gree
n
Yellow Orang
e
Re
d
Intensity (Brightness)
Intensity
Amount of
energy in a
wave
determined by
amplitude;
related to
perceived
brightness.
Intensity (Brightness)
Blue color with varying levels of intensity.
As intensity increases or decreases, blue color
looks more “washed out” or “darkened.”
The Lens
Nearsightedness: A condition in which nearby objects are seen
more clearly than distant objects b/c eye elongated & image
focus before it hits the retina
Farsightedness: A condition in which faraway objects are seen more clearly than near objects
b/c eye shortened & image would focus after it hit the retina
Retina
The light-sensitive
inner surface of the
eye, containing
receptor rods and
cones plus layers of
other neurons
(bipolar, ganglion
cells) that process
visual information.
Fovea
http://www.bergen.org
Central point in the retina, around which the eye’s
cones cluster…because of this there is little color
vision in the farthest periphery of our vision
Let’s test this fact…
Photoreceptors
E.R. Lewis, Y.Y. Zeevi, F.S Werblin,
1969
RODS CONES
CONES RODS
Number 6 million 120 million
LocationCenter
(fovea)Periphery
Color sensitive? Yes No
Sensitivity in
dim light?Low High
Ability to detect
detail?High Low
Number of
bipolar cells?Each has own
Shares one
with other
rods
Bipolar & Ganglion CellsBipolar cells receive messages from
photoreceptors and transmit them to ganglion
cells, which form the optic nerve (axons).
Remember…
Cones each have their
own bipolar cell.
Multiple rods share one
bipolar cell.
Test your Blind SpotBlind spot – no visual receptors here because of all the
information leaving the eye
To find your blind spot, close your right eye and
look at the cookie monster at eye level. Maintain
your focus on the cookie monster and slowly
move your paper from your eyes until the cookie
disappears. Under normal circumstances, you are unaware of your blind spot because one eye
sees what the other does not. WHY DO YOU SEE WHITE WHERE THE COOKIE
SHOULD BE? Even with one eye closed, you're not aware of the gap because your
brain "fills in" the missing information by making an assumption about what belongs in
the void.
Feature DetectionNerve cells in the visual cortex that respond to
specific features, like edges, angle, length and
movement.
There are even some feature detectors that are
specifically sensitive to the human face!
Visual Information Processing
Processing of several aspects of the stimulus
simultaneously is called parallel processing. The
brain divides a visual scene into subdivisions such
as color, depth, form and movement etc.Movement Aftereffects:
*To show you that we have detectors for movement we are going to
isolate that detector (and show you how our senses adapt).
Tim Bieber/ The Image
Bank
info sent
to visual cortex
from visual cortex
to assoc. areas
from assoc.
areas to memory
Theories of Color VisionTrichromatic theory: Based on behavioral
experiments, Helmholtz suggested that retina contains three receptors sensitive to red, blue
and green colors.
Blue Green Red
Medium Lo
w
Max
Standard stimulus
Comparison stimulus
PhotoreceptorsRed
Cone
s
Gree
n
Cone
s
Long
waveMedium
wave
Shor
t
wave
MacNichol, Wald
and Brown (1967)
measured directly
the absorption
spectra of visual
pigments of single
cones obtained from
the retinas of
humans.
Blue
Cone
s
Color Deficiency(blindness)
Genetic disorder which prevents individuals from
discriminating between certain colors due to a
weakness in or lack of one of the cones
Most common form of color “blindness” is difficulty
distinguishing between red and green
Complete color deficiency does exist but is very
rare…it would be like watching a black and white
movie.
Ishihara (color blindness)Test
25 6
5
42
56 8
Normal vision red/green deficiency
Normal vision red/green deficiency
Photos from: http://critiquewall.com
Above represent the art in “normal vision”…below what it would look like to a person
with red/green deficiency
http://www.vischeck.com/examples/
Opponent Process Theory
Hering, proposed that we process four primary
colors opposed in pairs of red-green, blue-
yellow, and black-white.
Cone
s
Retinal
Ganglion
Cells
Afterimages…
Gaze at the middle of the flag for about 30seconds, when it disappears, stare at the dot and report
if you see America’s flag.
Afterimages…
Gaze at the middle of the light bulb for about 30seconds, when it disappears, stare at the white space
and report what you see.
Afterimages…
Spanish castle afterimage….
Color Constancy
the color of an object will remain roughly constant
even if the lighting and wavelengths change