Transcript Perception Chapter 7: Color Vision

Perception Chapter 7: Color Vision
Color Vision:
The reason why humans perceive different
colors in the environment is because of
the manner in which the visual system
processes various wavelength energy
present in the environment.
It is likely that this capacity evolved in
order to allow important discriminations
to be made, such as a ripe apple from a
rotten one, or a poisonous plant from an
eatable one, sexually receptive female
from non-receptive, etc.
The term "color" is actually an imprecise
term. Color is comprised of three distinct
components:
Components of color
Hue: the perceptual pigmentation
difference experience with
varying wavelengths, this is
what we are actually referring
to when we use terms such as
"green" or "blue."
Brightness: refers to the intensity
of the hue, varies in relation to
the amplitude of the light.
Saturation: refers to the degree of
white light mixed in with the
dominant wavelength.
Perceptually experienced as the
purity of the color.
Composite vs. Pure light
It was Sir Isaac Newton who made the
first detailed observations of the
composite nature of sunlight. By
using a prism, Newton determined
that sunlight was actually a mixture
of many different wavelengths,
each perceptually experienced as a
different hue, when isolated.
Pure light: perceptually this is light
which can be experienced as only
one hue. It cannot be broken
down into any further component
hues. Theoretically, this is light of a
single wavelength, which is very
difficult to achieve. Distinction is
important as many wavelengths
are experienced as the same hue.
Composite light: light comprised of
many different wavelengths, which
if isolated would be perceptually
experienced as different hues
Spectral vs. Non spectral colors
Originally Newton only included
spectral colors (Roy G Biv); by
combining short and long ends of
spectrum he created non-spectral
colors (purple, magenta, pink).
Also, original circle had seven
discrete colors and equal spacing.
Later revisions changed this.
Complementary colors across from
each other.
Newton’s color circle
Hues blend one
into another
Highest
saturation at
rim, decreases
as you move
inward.
Can be used to
make color
perception
predictions
(top)
Brightness and saturation
1) Perceptual brightness and saturation: the photopic absorption curve
shows the cones are maximally sensitive to wavelengths around 550
(greenish-yellowish color). It so happens that these hues also appear the
brightest and the least saturated (i.e. least amount of added white light
need to de-saturate). Even when their physical properties (intensity level,
purity) are the same as other hues. This phenomenon reflects not only
the differentially sensitivity of the cones, but also the functioning of cells
in the LGN as we shall see later.
Additive/Subtractive color mixing
Color contrast & constancy
Top: in constancy the visual system
takes overall illumination into
account. Constancy tends to occur
when illumination changes in the
character of its distribution, but
still maintains a wide-band of
wavelengths. Constancy tends to
fail if illuminating light is restricted
to a narrow band of wavelengths -hence, the strange clothing colors
seen at amusement park rides
when under blue light
Bottom: in contrast, relative reflection
of background affects perception.
Reflects opponent nature of color
processing. Most likely reason for
this effect is the presence of
double-opponent process cells in
Primary Visual cortex which
respond to the presence of a
certain color in center, and to the
presence of the complementary
color in surround.
Physiology of Color Vision:
Retinal level: Trichromacy – why three cone types?
If only one -- no differentiation between wavelengths at all, only able to
distinguish intensity differences = monochromat. Why is this so? if one cone
type with one absorption curve, then if Wavelength A is absorbed 25%, and B
is 50%, and C is 50%. No way to distinguish if C is same as B or if it is A only at
twice the intensity.
Physiology of Color Vision:
• What if two cone systems? Called dichromatic. More
differentiation between wavelengths possible as each
wavelengths is determined by the pattern of
responding across two systems. However, a number of
confusions are still possible as one wavelength, which
has hi absorption in A system and low in B, could be
mimicked by two wavelengths presented
simultaneously, one of which has mid A and no B, the
other of which has mid A and low B?
• For example:
• Wavelength 1 = 80% of A; 20% of B; confused with
• Wavelength 2 = 40% of A; 0% of B +
• Wavelength 3 = 40% of A; 20% of B
Physiology of Color Vision:
Trichromacy: seems to be natures balance
between enough cone systems to provide
adequate discriminative abilities (adequate
for survival) and the anatomical constraints
of retina. Note that some confusions still
can occur, such as failure of color constancy,
but most are only present in highly
contrived environments.
3 cone systems: Studies using
microspectrophotometry have
demonstrated the maximal absorption
curves for each of the three cone systems.
Short system: max abs. at around 440 nm.
Middle system: max abs. around 530
Long system: max abs around 570
Hence, most wavelengths will be
characterized by a unique pattern of
responding across the three cone systems
Photopic absorption curves and 3 cone systems
Apparent contradiction? remember that the peak of the photopic sensitivity curve was
around 550. Yet the average of the sensitivities of the three cone systems is not
going to be anything close to 550 (440+530+570/3=513). So how can this be? Two
reasons:
1) Frequency distribution of cone systems: S cones are the rarest of all the cone
systems. Only about 1 million of the 8 mil cones are of the S type. About 4.6 mil are
of the L type, and 2.3 mil are of the M type.
2) Geographic distribution of cone systems: S cones are not found within the fovea of
the retina, only M's and L's are found here. S cones are highly concentrated in a ring
just outside of the fovea. This is why a blue dot, centered only on the fovea will
actually take on a dark gray appearance.
So -- the photopic sens. curve reflects measurements taken from the fovea, where S
cones are not contributing to the over-all sensitivity, and where about twice as many
L as M cones are present. Thus sensitivity is shifted to longer end of spectrum.
Evolution of color vision
Human eye has four photopigments (chemicals in photoreceptors that respond to
light)
Rhodopsin (in rods)
3 different cone pigments (S, L, M)
Sometime prior to 500mybp: rhodopsin  original cone pigment (peak sens. 510570nm)
Sometime after 500mybp rhodopsin  short pigment
Around 50mybp original cone pig  L and M pigments
In primates, L,M split appears to be associated with the need to distinguish ripe
fruit from leaves and un-ripe fruit
Color blindness and deficiencies
Color blindness is linked
to X chromosome, so
more common in males
then females
From Retina to LGN and PVC
•
•
While retina tends to operate in trichromatic fashion, LGN (and PVC) tend to
operate in more opponent processes fashion.
Opponent process theory: idea first articulated by Ewald Hering in the 1800's,
who, based on evidence from afterimages and color contrast effects, suggested
that the visual system treats red-green, and blue-yellow as antagonistic pairs.
Later research by DeValois and others have confirmed this notion by showing that
cells in the LGN and PVC tend to respond vigorously to the presence of one color,
and are inhibited by the presence of another complementary color.
Note: achromatic channel sums responses of L and M cones.