Transcript Chapter 7:

Chapter 7: Perceiving Color
Overview of Questions
• Why do we perceive blue dots when a yellow
flash bulb goes off?
• What does someone who is “color-blind” see?
• What colors does a honeybee perceive?
What Are Some Functions of Color Vision?
• Color signals help us classify and identify
objects
• Color facilitates perceptual organization of
elements into objects
• Color vision may provide an evolutionary
advantage in foraging for food
Familiarity and recognition
How Can We Describe Color Experience?
• Basic colors are red, yellow, green, and blue
• Color circle shows perceptual relationship
among colors
• Colors can be changed by:
– Intensity which changes perceived
brightness
– Saturation which adds white to a color
resulting in less saturated color
Color Wheel – Known for centuries by artists
What Is the Relationship Between
Wavelength and Color Perception?
• Color perception is related to the wavelength of
light:
– 400 to 450nm appears violet
– 450 to 490nm appears blue
– 500 to 575nm appears green
– 575 to 590nm appears yellow
– 590 to 620nm appears orange
– 620 to 700nm appears red
Colors of Objects
• Colors of objects are determined by the
wavelengths that are reflected
• Reflectance curves - plots of percentage of
light reflected for specific wavelengths
• Chromatic colors or hues - objects that
preferentially reflect some wavelengths
– Called selective reflectance
• Achromatic colors - contain no hues
– White, black, and gray tones
Table 7.1 Relationship between predominant wavelengths reflected and color perceived
Color of Objects - continued
• Selective transmission:
– Transparent objects, such as liquids
selectively allow wavelengths to pass
through
• Simultaneous color contrast - background of
object can affect color perception
Trichromatic Theory of Color Vision
• Proposed by Young and Helmholtz (1800s)
– Three different receptor mechanisms are
responsible for color vision
• Behavioral evidence:
– Color-matching experiments
• Observers adjusted amounts of three
wavelengths to match a comparison field
to a test field
Color Matching Experiments
• Results showed that:
– It is possible to perform the matching task
without three colors in the S, M and L
wavelengths
– Observers with normal color vision need at
least 3 wavelengths to make the matches
– Observers with color deficiencies can
match colors by using only 2 wavelengths
• They think it looks OK!!
Physiological Evidence for the Trichromatic
Theory
• Researchers measured absorption spectra of
visual pigments in receptors (1960s)
– They found pigments that responded
maximally to:
• Short wavelengths (419nm)
• Medium wavelengths (551nm)
• Long wavelengths (558nm)
• Later researchers found genetic differences
for coding proteins for the three pigments
(1980s)
Figure 7.8 Absorption spectra of the three cone pigments. (From Dartnall, Bowmaker, and Mollon, 1983.)
Response of Cones and Color Perception
• Color perception is based on the response of
the three different types of cones
– Responses vary depending on the
wavelengths available
– Combinations of the responses across all
three cone types lead to perception of all
colors
– Color matching experiments show that
colors that are perceptually similar
(metamers) can be caused by different
physical wavelengths
• Color perception is based on the response
of the three different types of cones
Figure 7.12 The proportions of 530- and 620-nm lights in the field on the left have been adjusted so that the
mixture appears identical to the 580-nm light in the field on the right. The numbers indicate the responses
of the short-, medium-, and long-wavelength receptors. There is no difference in the responses of the two
sets of receptors so that two fields are perceptually indistinguishable.
Color Mixing
• Additive color mixture:
– Mixing lights of different wavelengths
– All wavelengths are available for the
observer to see
– Superimposing blue and yellow lights leads
to white
• Subtractive color mixture:
– Mixing paints with different pigments
– Additional pigments reflect fewer
wavelengths
– Mixing blue and yellow leads to green
Figure 7.11 Mixing blue paint and yellow paint creates a paint that appears green. This is subtractive color
mixture.
Are Three Receptor Mechanisms Necessary
for Color Perception?
• One receptor type cannot lead to color vision
because:
– Absorption of a photon causes the same
effect no matter what the wavelength is called the principle of univariance
– Any two wavelengths can cause the same
response by changing the intensity
• Two receptor types (dichromats) solves this
problem but 3 types (trichromats) allows for
perception of more colors
• Try This!
•
http://www.cs.brown.edu/exploratories/freeSoftware/catalogs/color_theory.html
Color Deficiency
• Monochromat - person who needs only one
wavelength to match any color
• Dichromat - person who needs only two
wavelengths to match any color
• Anomalous trichromat - needs three
wavelengths in different proportions than
normal trichromat
• Unilateral dichromat - trichromatic vision in
one eye and dichromatic in other
Figure 7.15 Ishihara plate for testing for color deficiency.
Color Experience for Monochromats
• Monochromats have:
– A very rare hereditary condition
– Only rods and no functioning cones
– Ability to perceive only in white, gray, and
black tones
– True color-blindness
– Poor visual acuity
– Very sensitive eyes to bright light
Color Experience for Dichromats
• There are 3 types of dichromatism:
– Protanopia affects 1% of males and .02%
of females
• Individuals see short-wavelengths as
blue
• Neutral point occurs at 492nm
• Above neutral point, they see yellow
• They are missing the long-wavelength
pigment
Color Experience for Dichromats - continued
• Deuteranopia affects 1% of males and .01%
of females
– Individuals see short-wavelengths as blue
– Neutral point occurs at 498nm
– Above neutral point, they see yellow
– They are missing the medium wavelength
pigment
Color Experience for Dichromats - continued
• Tritanopia affects .002% of males and .001%
of females
– Individuals see short wavelengths as blue
– Neutral point occurs at 570nm
– Above neutral point, they see red
– They are most probably missing the short
wavelength pigment
Opponent-Process Theory of Color Vision
• Proposed by Hering (1800s)
– Color vision is caused by opposing
responses generated by blue and yellow
and by green and red
• Behavioral evidence:
– Color afterimages and simultaneous color
contrast show the opposing pairings
– Types of color blindness are red/green and
blue/yellow
Figure 7.17 Color matrix for afterimage and simultaneous contrast demonstrations.
Opponent-Process Theory of Color Vision continued
• Opponent-process mechanism proposed by
Hering
– Three mechanisms - red/green,
blue/yellow, and white/black
– The pairs respond in an opposing fashion,
such as positive to red and negatively to
green
– These responses were believed to be the
result of chemical reactions in the retina
Figure 7.19 The three opponent mechanisms proposed by Hering.
Physiology of Opponent-Process
• Researchers performing single-cell
recordings found opponent neurons (1950s)
– Opponent neurons:
• Are located in the retina and LGN
• Respond in an excitatory manner to one
end of the spectrum and an inhibitory
manner to the other
Trichromatic and Opponent-Process
Theories Combined
• Each theory describes physiological
mechanisms in the visual system
– Trichromatic theory explains the responses
of the cones in the retina
– Opponent-process theory explains neural
response for cells connected to the cones
further in the brain
Figure 7.21 Our experience of color is shaped by physiological mechanisms, both in the receptors and in
opponent neurons.
Figure 7.22 Neural circuit showing how the blue-yellow and red-green mechanisms can be created by
excitatory and inhibitory inputs from the three types of cone receptors.
Color Processing in the Cortex
• There is no single module for color perception
– Cortical cells in V1, V2, and V4 respond to
some wavelengths or have opponent
responses
– These cells usually also respond to forms
and orientations
– Cortical cells that respond to color may
also respond to white
Perceiving Colors Under Changing
Illumination
• Color constancy - perception of colors as
relatively constant in spite of changing light
sources
– Sunlight has approximately equal amounts
of energy at all visible wavelengths
– Tungsten lighting has more energy in the
long-wavelengths
– Objects reflect different wavelengths from
these two sources
Figure 7.24 The reflectance curve of a sweater (green curve) and the wavelengths reflected from the
sweater when it is illuminated by daylight (white) and by tungsten light (yellow).
Possible Causes of Color Constancy
• Chromatic adaptation - prolonged exposure
to chromatic color leads to:
– Receptors “adapt” when the stimulus color
selectively bleaches a specific cone
pigment
– Sensitivity to the color decreases
• Adaptation occurs to light sources leading to
color constancy
Possible Causes of Color Constancy continued
• Effect of surroundings
– Color constancy works best when an
object is surrounded by many colors
• Memory and color
– Past knowledge of an object’s color can
have an impact on color perception
– Memory for color is not exact, so we don’t
notice slight changes caused by
illumination changes
Lightness Constancy
• Achromatic colors are perceived as remaining
relatively constant
– Perception of lightness:
• Is not related to the amount of light
reflected by object
• Is related to the percentage of light
reflected by object
Figure 7.27 A black-and-white checkerboard illuminated by tungsten light and by sunlight.
Possible Causes of Lightness Constancy
• The ratio principle - two areas that reflect
different amounts of light look the same if the
ratios of their intensities are the same
• This works when objects are evenly
illuminated
– Shadows cause problems
• Reflectance edges - edge where the
reflectance of two surfaces changes
• Illumination edges - edge where illumination of
two surfaces changes
Figure 7.28 This unevenly illuminated wall contains both reflectance edges and illumination edges. The
perceptual system must distinguish between these two types of edges to accurately perceive the actual
properties of the wall and other parts of the scene, as well.
Figure 7.29 The pattern created by shadows on a surface is usually interpreted as a change in the pattern
of illumination, not as a change in the material making up the surface. The fact that we see all of the bricks
on this wall as made of the same material, despite the illumination changes, is an example of lightness
constancy.
Figure 7.30 (a) A cup and its shadow. (b) The same cup and shadow with the penumbra covered by a black
border.
Creating Color Experience
• Light waves are not “colored”
• Color is a creation of our physiology
– Animals with different sensory apparatus,
such as honey bees, experience
something we cannot
• All of our sensory experiences are created by
our nervous system