Lecture 10 - Color Vision

Question 1

hierarchical manner

Answer 1

keep converging the activity of the receptive cells with increasing activity from the neuron

Question 2

selective adaptation

Answer 2

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

Question 3

What does color vision do for us?

at the computational level ?

Answer 3

• 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.

Question 4

why is color vision important?

Answer 4

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).

Question 5

the physical basis of color

Answer 5

Associated with our sensitivity to particular wavelengths in the electromagnetic spectrum.

Question 6

colors are not a property of light waves

Answer 6

Our experience of
color is a result of an interaction
with our sensory systems.

Question 7

when particular wavelengths reach our eyes, they can cause _____ or ___ to be perceived

Answer 7

chromatic colors, hues

Question 8

achromatic color

Answer 8

if all wavelengths are transmitted equally

Question 9

selective reflectance of an object

Answer 9

specifies its chromatic color

Question 10

for clear objects,

Answer 10

selective transmission species chromatic color

Question 11

reflectance curve

Answer 11

plotting the percentage of photons reflected at each wavelength.

Question 12

achromatic colors will reflect equal amounts of each ____, with the percentage of reflectance determining ______

Answer 12

wavelength, brightness

Question 13

white

Answer 13

achromatic, reflecting all the wavelengths

Question 14

black

Answer 14

achromatic,

absorbing all the wavelengths

Question 15

four basic colors or psychological primaries

Answer 15

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.

Question 16

extra-spectral colors

Answer 16

don’t appear in the light color spectrum. They are the result of mixing two other colors.

e.g. brown

Question 17

we can discriminate about _____ colors based purely on wavelength

Answer 17

200

Question 18

color perception can be changed by:

Answer 18

intensity

saturation

simultaneous color contrast

Question 19

intensity

Answer 19

changing the brightness: amount of total energy coming through

make a red appear brighter

Question 20

saturation

Answer 20

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)

Question 21

Simultaneous color contrast:

Answer 21

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

Question 22

Big thing! = Mixing wavelenghts of diff light you’re receiving

Answer 22

additive vs. subtractive

changing which wavelenghths of light will reach eyes and will influence how you use that info

Question 23

subtractive color mixing

Answer 23

– 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

Question 24

additive color mixture

Answer 24

– 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

Question 25

In clear test tubes, you mix two different color liquids to produce a third (new) color. What kind of mixing is this?

Answer 25

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

Question 26

How does the change in wavelengths received in the eye alter
our perception of color?

Answer 26

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.

Question 27

Trichromatic theory of color vision

Answer 27

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.

Question 28

Color-matching experiments - Young and Helmholtz

Answer 28

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.

Question 29

metamer

Answer 29

when the psychological response matches but the physical stimulus does not (diff physical stimuli)

Question 30

short blue smerfs

Answer 30

short wavelength for blue: 419 nm

Question 31

long wavy red hair

Answer 31

long wavelength for red: 558nm

Question 32

Problematic Observations with trichromatic theory

Answer 32

The fact that color blindness occurred in pairs of green/red and blue/yellow.

2. Individuals have difficultly in visualizing combinations of red and green, or blue and yellow.
3. Color afterimages have characteristic pairings: red after green adaptation, and blue after yellow adaptation. - - > trichromacy can’t explain!!

Question 33

The good news for trichromatic theory…

Answer 33

• 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.

Question 34

Ewald Hering (1800s) suggested an 
opponent-process theory of color vision:

Answer 34

– 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.