Download Color Vision Theories

OPTO 5320
VISION SCIENCE I
Monocular Sensory Processes of Vision:
Color Vision
Color Vision Theories
Color Vision Theories
 Modern research has described many aspects of color vision
processing.
 Two major theories have guided the research on color vision.
1. the Trichromatic Theory (Young-Helmholtz theory)
2. the Opponent-Processes Theory (Hering’s theory).
 These complementary theories are important in explaining color
vision at different levels of the visual system.
Color Vision Theories
A. Requirements for a theory of color vision
B. Young-Helmholtz’s Trichromatic Theory
C. Hering’s Opponent Colors Theory
Color Vision Theories
• A viable color vision theory should be able to explain,
in a quantitative way, all of the phenomena of color.
• Especially, a color vision theory must be able to
account for the following.
1) Color mixture and Grassmann’s laws
2) Discrimination of hue, saturation, and brightness
3) The perception of spectral hues
4) Dichromatic and trichromatic color vision anomalies
5) The perception of violet
6) The appearance of after-images
7) The Bezold-Brucke effect
History – Isaac Newton
http://www.crystalinks.com/
•
1672 - Newton demonstrated the prismatic dispersion of
white light to obtain the spectral colors. He assumed that
when light struck the retina and it started to vibrate at a
frequency proportional to the wavelength, much like the
cochlea of the inner ear responds to sound.
http://www.ucolick.org/
History – Thomas Young
http://www.eoht.info/
 1801 - Young’s Bakerian Lecture, “it is almost impossible to
conceive each sensitive point on the retina to contain an
infinite number of particles each capable of vibrating in
perfect unison with every possible undulation, it becomes
necessary to suppose the number is limited, for instance to
the three principal colors - red, yellow and blue.”
 Later Young corrected the primaries to red, green and
violet.
 Young also described the color blindness of his friend,
John Dalton, as missing the particle vibrating in response
to red (protanopia). Color blindness became known as
“Daltonism.”
Young-Helmholtz’s Trichromatic
Color Vision Theory
http://120years.net/
 1866 - Helmholtz presented
a relatively complete
description of color vision
based on Young’s original
proposal of three
fundamental retinal
mechanisms.
 The essence of the theory is that three sets of sensory
mechanisms determine the quantitative basis for the various
aspects of color vision.
 Each of the mechanisms is sensitive across the entire visible
spectrum, but they each differ in their spectra response
properties.
Young-Helmholtz’s Trichromatic
Color Vision Theory
 The basic elements of the
Young-Helmholtz
Trichromatic Color Theory.
1) Each sensory mechanism
has a specific sensitivity
curve at each spectral
wavelength for each of the
three mechanisms
2) Hue is a function of the relative responses of the three
mechanisms at each wavelength.
3) Brightness is a function of the total of the responses of
the three mechanisms at each wavelength.
Hecht’s Trichromatic Color
Vision Theory
http://www.nap.edu/
 The elementary hypotheses
of a trichromatic theory do
not restrict the shape or
locations of the cone
fundamentals.
 Hecht developed a
mathematical account of
the physiological processes
of color vision based on the
three fundamental cone
response curves shown in
the figure.
Enduring Principles of Trichromatic
Color Theory
 Color information is initially encoded by three cone
mechanisms.
 The fundamental phenomena of color vision are a result of
neural processes which combine the signals from the three cone
mechanisms in various ways.
1) Color mixture and Grassman’s
laws
2) Discrimination of hue, saturation,
and brightness.
3) The perception of spectral hues,
except yellow
4) Dichromatic, but not trichromatic,
color vision anomalies
Hering’s Opponent-Colors Theory
 1872 - Hering proposed a theory of color vision to account for
Luminance intensity
perceptual or psychological aspects of color vision.
1. The perception of yellow in the 560 - 580 nm range.
2. Negative after-images are complements of induction stimuli.
3. The Bezold-Brucke effect: The perceived hues of
monochromatic stimuli change with intensity, except at
specific wavelengths (unique hues).
4. Unique hues represent the locations of cone photopigments
with approximately equal sensitivity to a single wavelength
Wavelength (nm)
Hering’s Opponent-Colors
Theory
1.


http://www.orden-pourlemerite.de/
 Hering’s (1872) theory was based on
2.


3.



fundamental color vision response
functions with negative as well as positive
values.
Three retinal substrates which respond to
light by opposite anabolic and catabolic
reactions.
1. catabolic by light - anabolic by dark
(bk/wh opponency).
2. catabolic by red light - anabolic by
green light (R/G opponency).
3. catabolic by yellow light - anabolic by
blue light (B/Y opponency).
Hering’s Opponent-Colors Theory
• Photopigments of the type postulated by Hering
do not exist.
• Light activated changes in chromophores are
identical in all photopigments.
• Opponent color responses can occur from
excitatory and inhibitory synaptic substances of
neural responses.
Opponent-Colors Theory
• There is considerable
psychophysical evidence for
opponent color responses.
• One line of evidence has
been obtained by hue
cancellation measurements the intensity of a primary of
one opponent pair to cancel
the perception of hue in the
other opponent pair, i.e., the
amount of yellow (570 nm)
light to cancel the blueness
of lights from 400 - 500 nm.
Enduring Principles of OpponentColors Theory
• Neural mechanisms at some level of the visual
pathway must combine information from three cone
types to form two opponent color vision channels, the
R/G & B/Y channels, and one non-opponent channel,
the luminance or bk/wh channel.
• Color perception is better explained by the information
in opponent channels than trichromatic color vision
theories.
Multi-stage Color Vision Theories
• The Young-Helmholtz Trichromatic theory and the Hering
Opponent-colors theory are one-stage models - all of color
information processing is accomplished by the initial
photopic sensory mechanisms.
• The data from hue-cancellation experiments demonstrated
that a two-stage model is necessary - a first stage for the
initial encoding and a second stage to develop spectrallyopponent responses.
• More recently, chromatic contrast experiments have
indicated the necessity for a third stage which responds to
hue without being confounded by brightness.
• Additional stages may be required to explain color
perception that is not correlated to wavelength (e.g., A Land
Mondrian).
A Three-stage Color Vision Theory
(DeValois & DeValois)
• Stage 1 is the
absorption of light
by three cone
photopigments - an
elementary
requirement for
trichromatic vision.
DeValois, R & DeValois K. Vis Res 33:1053-1065, 1993
A Three-stage Color Vision Theory
(DeValois & DeValois)
 Stage 2 is the formation of
spectrally opponent
responses.
 This model proposes three
color opponent
mechanisms, one for each
cone type, rather than two
from the Hering Opponentcolors theory.
 Because the output is also
proportional to intensity,
wavelength and intensity
can be confused.
DeValois, R & DeValois K. Vis Res 33:1053-1065, 1993
Synaptic Interactions in the Retina Second stage mechanisms
 Feed-forward and feed-back synapses of



retinal neurons in the inner and outer
plexiform layers provide connections for
spatial interactions between cone types.
Receptors (R) drive both bipolar (B) and
horizontal (H) cells.
Bipolar cells terminate on both retinal
ganglion (G) and amacrine (A) cells.
The receptive fields of retinal ganglion
cells are described as center-surround
opponent mechanisms, with the center
and surround being luminancenonopponent or color-opponent.
http://medical-dictionary.thefreedictionary.com/
Color Vision Theories and Models
 Color vision theories were originally


proposed to explain normal trichromacy
and the alterations caused by genetic
color vision anomalies.
Now there are considerable
psychophysical and physiological
research data to serve as the foundation
for color vision models.
The original concepts and terminology
have been retained
 Interactions at second
and third order must
be proportional to the
relative absorption of
light by the three cone
photopigments.
http://retina.umh.es/webvision/
Simple models of retinal circuits for Lresponses
M-cone
L-cone
+
+
M-cone + L-cone
output = L-response
• The outputs of M- and L-cones are
combined by neurons with
identical synapse types from both
cone types.
• The output of the neuron is the
linear sum of the responses of the
two types of cones.
Receptive Field Organization of
Non-Opponent RGCs
+L
• The spatial arrangement of
receptive fields of RGC and
LGN cells is a centersurround organization of
inputs from the various cone
types.
+M
• Monochromatic stimuli produce
the characteristic nonopponent (summation)
responses, and white light is a
very effective stimulus.
-M
-L
M+L>0
M-cone
Non-Opponent response
L-cone
Cone inputs to
the L-response
 The L-response represents
the linear, additive
combination of the
responses of M- and Lcones.
 SS = M + L at each
wavelength.
0
 The L-response has the
major characteristics of the
photopic luminosity
function.
L-cone
M-cone
Simple models of
neural circuits for
+G-R opponent
responses
 The responses of R/G
+
-
opponent cells are similar to
the C-response form of Spotentials.
 The output of the opponent
+G-R opponent
output
neuron is the linear difference
of the responses of the two
types of cones.
535 nm
565 nm
 SS = M – L
 The null point occurs at the
M-cone
wavelength of equal
chromatic sensitivity for the
two photopigments.
L-cone
520 nm
620 nm
0
Cone inputs to
+G-R RGC cells
 The M - L operation causes
the peak response points to
be displaced to a wavelength
shorter than the M-cone
response for the “green”
response and to a wavelength
longer than the L-cone for the
“red” response.
Receptive Field Organization of R/G
Opponent RGC cells
+L
• The spatial arrangement of
receptive fields of opponent
cells is a center-surround
organization of antagonistic
inputs from M- and L-cone
types.
-M
• Monochromatic stimuli
produce the characteristic
opponent responses, but white
light is not an effective
stimulus.
+M
-L
S-cone
M-cone
L-cone
Simple Models of
Neural Circuits for +Y-B
Opponent Responses
 The yellow component of the
+
-
+
+
Y/B opponent cells is the
additive combination of the
M- and L-cone responses
(the luminosity response).
 The output of the opponent
+Y-B opponent
output
neuron is the linear
difference of the responses
of S-cone’s output and the
sum of the M- and L-cones’
outputs.
Receptive Field Organization of Y/B
Opponent RGC Cells
• The scheme of the spatial
+L+M
organization of Y/B neurons is
similar to the R/G opponent cells
except for the difference in the
specific cone inputs.
-S
+S
-L-M
Models of Neural Circuits for Color Vision
S-cone
M-cone
L-cone
• Six distinct response types were
found for color processing.
+
-
+
-
+
+
Non-opponent
output
+Y-B opponent
output
+R-G opponent
output
• Four types of color
opponent neurons.
1) +R-G
2) +G-R
3) +Y-B
4) +B-Y
• Two types of nonopponent neurons.
1) Excitatory
2) Inhibitory
Non-opponent and Opponent
Mechanisms of Color Vision
Opponent mechanisms
Non-Opponent mechanisms
http://web.mit.edu/
 Opponent and non-opponent channels are represented in
different channels of the afferent visual pathway.
• Opponent channels – parvocellular pathway
• Non-opponent channels – magnocellular pathway
A Three-stage Color Vision Theory
(DeValois & DeValois)


Stage 3 is the formation of pure
chromatic contrast responses, by
further processing of the
responses from stage 2 spectral
opponency – called doubleopponent cells in the visual
cortex.
Stage 3 responses are still
wavelength dependent, but their
responses are related,
specifically, to chromatic
contrast and they are not driven
by luminance contrast.
DeValois, R & DeValois K. Vis Res 33:1053-1065, 1993
Receptive Field Organization DoubleOpponent Cortical Cells

+L-M
-M+L
Optimal Stimulus

The scheme of the spatial organization
of double-opponent cells combines
opponent cell responses of opposite
types in the center and surround areas
of the receptive fields.
The optimal stimulus has spatial and
spatial frequency properties – important
for identifying edges of colored objects.
Non-Optimal Stimulus
Segregation of Cortical Areas for
Specialized Visual Functions
1.
Color blobs
Inter-blobs
Image from: read.uconn.edu/PSYC3501/Lecture07/
Schematic representation of the
modular segregation of the
brain.
2.
The largest modules are ocular
dominance (OD) columns.
3.
Superimposed on the OD
organization are the interblobs,
blobs, and orientation columns.
Color Vision Theories
Recap
 Young-Helmholtz Trichromatic Theory
• Perception of hue and brightness
 Hering’s Opponent Colors Theory
• Perception of yellow
• Perception of negative afterimages
 Multi-stage Color Vision Theories
• Receptive field organization of opponent and
non-opponent channels.
 Opponent and Non-Opponent Channels
• Parvo- and Magno-cellular Pathways
 Double Opponent cells in V1