visual cortex

Report
Vision
• How does our body
construct our conscious
visual experience?
• How de we transform
particles of light energy to
another?
• We do this by receiving
light energy and
transforming it into
neural messages that our
brains process into what
we consciously see.
The Stimulus Input- Light Energy
• What strikes are eyes IS NOT color, but rather
electromagnetic energy that we perceive as color.
• The next slide shows the range from imperceptibility
short waves of gamma rays to the narrow band we
see as visible light, to the long waves of radio
transmission and AC circuits.
Both Photos: Thomas Eisner
The Stimulus Input: Light Energy
Visible
Spectrum
3
Wavelength (Hue)
Hue (color) is the
dimension of color
determined by the
wavelength of the
light.
Wavelength is the
distance from the
peak of one wave to
the peak of the next.
The amount of
energy in light waves
or intensity,
determined by a
wave’s amplitude, or
height, influences the
brightness of the light
4
Intensity (Brightness)
Intensity
Amount of
energy in a
wave
determined by
the amplitude.
It is related to
perceived
brightness.
5
Wavelength (Hue)
Violet
Indigo
400 nm
Short wavelengths
Blue
Green
Yellow
Orange
Red
700 nm
Long wavelengths
Different wavelengths of light result
in different colors.
6
Intensity (Brightness)
Blue color with varying levels of intensity.
As intensity increases or decreases, blue color
looks more “washed out” or “darkened.”
7
Purity (Saturation)
Saturated
Saturated
Monochromatic light added to green and red
makes them less saturated.
8
Color Solid
Represents all
three
characteristics of
light stimulus on
this model.
http://www.visionconnection.org
9
The Eye
10
Parts of the eye
1. Cornea: Transparent tissue where light enters
the eye.
2. Pupil- a small adjusting opening that allows
light to pass through
3. Iris: Muscle that expands and contracts to
change the size of the opening (pupil) for light.
4. Lens: Focuses the light rays on the retina.
5. Retina: Contains sensory receptors that process
visual information and sends it to the brain.
11
The Lens
Lens: Transparent
structure behind the
pupil that changes shape
to focus images on the
retina.
Accommodation: The
process by which the
eye’s lens changes shape
to help focus near or far
objects on the retina.
12
Retina
Retina: The lightsensitive inner
surface of the eye,
containing receptor
rods and cones in
addition to layers of
other neurons
(bipolar, ganglion
cells) that process
visual information.
13
Optic Nerve, Blind Spot & Fovea
Optic nerve: Carries neural impulses from the eye to the
brain. Blind Spot: Point where the optic nerve leaves the
eye because there are no receptor cells located there. This
creates a blind spot. Fovea: Central point in the retina
around which the eye’s cones cluster.
http://www.bergen.org
14
Photoreceptors
E.R. Lewis, Y.Y. Zeevi, F.S Werblin, 1969
15
Bipolar & Ganglion Cells
The neural signals produced in the rods and cones activate
the neighboring BIPOLAR cells, which then activate a
network of GANGLION cells. The axons of ganglion cells
converge to from the OPTIC NERVE, which carries visual
information to the BRAIN. Where the nerve leaves the
eye, there are no receptors; thus, the area is called the
BLIND SPOT
16
Test your Blind Spot
Use your textbook (E-BOOK). Close your left eye,
and fixate your right eye on the black dot. Move
the page towards your eye and away from your
eye. At some point the car on the right will
disappear due to a blind spot.
17
Fovea, Rods and Cones
• Most cones are clustered around
the retina’s point of central focus
called the FOVEA, whereas the
rods are concentrated in more
peripheral visions of the retina.
Many cones have their own
bipolar cells to communicate
with the visual cortex.
• Cones- enable color
• Rods- enable black and white
vision
• Unlike cones, rods are sensitive.
Adapting to a darkened room will
take the retina approximately 20
minutes.
Visual Information Processing
Optic nerves connect to the thalamus in the
middle of the brain, and the thalamus connects to
the visual cortex.
19
Feature Detection
Ross Kinnaird/ Allsport/ Getty Images
Hubel and Wiesel discovered that specific
features called Feature Detectors. Examples
include - respond to specific features, such as
edges, angles, and movement.
20
Feature Detection
• Feature detectors pass the
information to higher level
cells in the brain, which
respond to specific visual
scenes. Research has shown
that in monkey brains such
cells that specialize in
responding to specific
gazes, head angles, posture,
or body movements. In
may cortical areas, teams of
cells supercell clusters
respond to complex patters.
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. Other
brain damaged people may demonstrate blind sight by
responding to a stimulus that is not consciously perceived.
22
Tim Bieber/ The Image Bank
From Sensation to Recognition
23
Theories of Color Vision
Trichromatic theory: Based on behavioral experiments,
Helmholtz suggested that the retina should contain three
receptors that are sensitive to red, blue and green colors.
An object appears to be red in color because it rejects the
long wavelengths of red and because of our mental
construction of the color.
Standard stimulus
Comparison stimulus
Max
Medium
Low
Blue
Green
Red
24
Theories of Color Vision
• One out of every 50
people is color
deficient; this is usually
a male because the
defect is genetically
Subtraction of Colors
If three primary
colors (pigments) are
mixed, subtraction of
all wavelengths
occurs and the color
black is the result.
26
Addition of Colors
If three primary colors (lights) are mixed, the
wavelengths are added and the color white is the
result.
Fritz Goro, LIFE magazine, © 1971 Time Warner, Inc.
27
Photoreceptors
Blue
Cones
MacNichol, Wald
and Brown (1967)
measured directly
the absorption
spectra of visual
pigments of single
cones obtained from
the retinas of
humans.
Short
wave
Green
Cones
Medium
wave
Red
Cones
Long
wave
28
Color Blindness
Genetic disorder in which people are blind to
green or red colors. This supports the
Trichromatic theory.
Ishihara Test
29
Opponent Colors
Gaze at the middle of the flag for about 30
Seconds. When it disappears, stare at the dot and report
whether or not you see Britain's flag.
30
Opponent Process Theory
Hering proposed that we process four primary
colors combined in pairs of red-green, blueyellow, and black-white.
Cones
Retinal
Ganglion
Cells
31
Color Constancy
Color of an object remains the same under
different illuminations. However, when context
changes the color of an object may look different.
R. Beau Lotto at University College, London
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