Lecture 10 - Color Vision Flashcards
hierarchical manner
keep converging the activity of the receptive cells with increasing activity from the neuron
selective adaptation
basic idea: if you have cells responsive to particular features (diff patterns of light that hit the retina) if cells respond to those patterns you should get that info in different ways
psychophysical measure
we know that some neurons are specialized for a particular feature, if you fatigue any neuron, eventually it will slow down (selectively adapt) - it should either slow or become unresponsive: associate with perception: no longer able to see something in the environment
selective because it’s just because of that one orientation
What does color vision do for us?
at the computational level ?
• Color is a visual feature that helps us classify and identify objects.
• Color facilitates perceptual organization of elements into objects. It can
make some objects ‘pop-out’.
• May provide an evolutionary advantage in foraging for food – especially
calorie-packed fruits and berries.
why is color vision important?
Color takes on meaning.
• We like color and have strong
preferences.
• It affects our moods and
performance. [Important in food,
architecture, and UX design.]
• We assign it cultural significance
(but the meaning isn’t innate).
the physical basis of color
Associated with our sensitivity to particular wavelengths in the electromagnetic spectrum.
colors are not a property of light waves
Our experience of
color is a result of an interaction
with our sensory systems.
when particular wavelengths reach our eyes, they can cause _____ or ___ to be perceived
chromatic colors, hues
achromatic color
if all wavelengths are transmitted equally
selective reflectance of an object
specifies its chromatic color
for clear objects,
selective transmission species chromatic color
reflectance curve
plotting the percentage of photons reflected at each wavelength.
achromatic colors will reflect equal amounts of each ____, with the percentage of reflectance determining ______
wavelength, brightness
white
achromatic, reflecting all the wavelengths
black
achromatic,
absorbing all the wavelengths
four basic colors or psychological primaries
red, yellow, green, blue
pretty much any color out there can be described with these 4
correspond to specific wavelengths of light reflected from an object.
extra-spectral colors
don’t appear in the light color spectrum. They are the result of mixing two other colors.
e.g. brown
we can discriminate about _____ colors based purely on wavelength
200
color perception can be changed by:
intensity
saturation
simultaneous color contrast
intensity
changing the brightness: amount of total energy coming through
make a red appear brighter
saturation
Adding white (with all wavelengths) to a
specific color results in less saturated (desaturated)
color, making it appear gray. Remove the white, and you
get to a more ‘pure’ color as it has fewer competing
wavelengths.
tells you how many diff frequencies are present
a very saturated red = a very narrow range of wavelengths strongly associated with red;
de-saturated: add in white (so everything starts to come in)
Simultaneous color contrast:
putting diff colors in the background: your experience of that color object will change based on the background
The background or
surrounding colors of an object can alter the perceived
colors.
convergence in visual system: makes you experience the color object in a diff way
Big thing! = Mixing wavelenghts of diff light you’re receiving
additive vs. subtractive
changing which wavelenghths of light will reach eyes and will influence how you use that info
subtractive color mixing
– Mixing paints with different pigments.
– Additional pigments reflect fewer wavelengths, changing the selective reflectance.
– Mixing blue and yellow leads to green, because each pigment alone would reflect green.
blue and yellow absorb everything but the green
additive color mixture
– Mixing lights of different, specific wavelengths.
– All contributed wavelengths are available for the observer to see – direct combination of wavelengths
– Superimposing blue and yellow lights leads to white: because it’s reflecting all the frequencies: short, medium, and long
In clear test tubes, you mix two different color liquids to produce a third (new) color. What kind of mixing is this?
it’s subtractive color mixing because:
you’re creating a filter and you’re filtering out the light that coming through
selective transmission
adding in pigment will selectively transmit what it’s mutually absorbed
How does the change in wavelengths received in the eye alter
our perception of color?
Color is not “contained” in the wavelength, but is certainly correlated
with it.
• We need to build a physiological account, but with help from
psychophysics and algorithmic models.
Trichromatic theory of color vision
Proposed by Young and Helmholtz
(1800s)
– Suggested that three different receptor mechanisms are responsible for color vision.
• Based on psychophysical evidence
– Color-matching experiments:
Observers adjusted intensities of three wavelengths in a comparison
field to match a test field of one wavelength.
Color-matching experiments - Young and Helmholtz
Normal observers need three wavelengths of light to make the matches.
– Observers with color deficiencies can match colors by using only two wavelengths.
– Color matches based on different physical stimuli are called metamers.
– Strong evidence for the trichromatic theory and allows you to build a
model and look for physiological (implementational) causes.
metamer
when the psychological response matches but the physical stimulus does not (diff physical stimuli)
short blue smerfs
short wavelength for blue: 419 nm
long wavy red hair
long wavelength for red: 558nm
Problematic Observations with trichromatic theory
The fact that color blindness occurred in pairs of green/red and blue/yellow.
- Individuals have difficultly in visualizing combinations of red and green, or blue and yellow.
- Color afterimages have characteristic pairings: red after green adaptation, and blue after yellow adaptation. - - > trichromacy can’t explain!!
The good news for trichromatic theory…
- Color matching experiments show that colors that are perceptually
similar (metamers) can be caused by different physical wavelengths.
– Metamers are produced by different stimuli because they have
indistinguishable firing patterns from color receptors.
– Physiological account supports psychophysical experiment.
Ewald Hering (1800s) suggested an opponent-process theory of color vision:
– Three proposed mechanisms process colors in pairs: red/green,
blue/yellow, and white/black.
– The pairs respond in an opposing fashion, such as positively to red and
negatively to green.
– These responses were believed to be the result of chemical reactions in the
retina.
