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 Grassman’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 - Newton
• 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.
History - Young
• 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 principle 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
 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.
Trichromatic Color Vision Theory
 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
Luminance intensity
• 1872 - Hering proposed a theory of color vision to account for
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).
Wavelength (nm)
Hering’s Opponent-Colors Theory

• Hering’s theory was based on
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.
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.
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 ganglion (G) and
amacrine (A) cells.
Color Vision Theories
and Models
• At one time, the objective of
color vision theory was to
propose hypotheses 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.
L-cone
M-cone
+
+
M-cone + L-cone
output = L-response
Simple models of
retinal circuits for
L-responses
• The outputs of M- and Lcones 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
L-cones.
• SS = M + L at each
wavelength.
0
• The L-response has the
major characteristics of
the photopic luminosity
function.
L-cone
M-cone
+
-
+G-R opponent
output
Simple models of
neural circuits for
+G-R opponent
responses
• The responses of R/G
opponent cells are
similar to the Cresponse form of Spotentials.
• The output of the
opponent neuron is the
linear difference of the
responses of the two
types of cones.
535 nm
565 nm
Cone inputs to
+G-R RGC cells
• SS = M - L
M-cone
• The null point occurs at the
wavelength of equal
chromatic sensitivity for the
two photopigments.
L-cone
520 nm
620 nm
0
• 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
+
-
+
+Y-B opponent
output
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 Mand L-cone responses (the
luminosity response).
• The output of the opponent
neuron is the linear difference
of the responses of S-cone’s
output and the sum of the Mand 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
 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.
• Stage 3 responses
are still wavelength
dependent.
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