Lecture 15

Question 1

What is V4?

Answer 1

originally proposed as a sort of ‘colour centre’

Question 2

What is achromatopsia?

Answer 2

Colorblindness

Question 3

Why are monochromats important?

Answer 3

Monochromats have a very rare hereditary condition which could be
considered to produce ‘true’ colour-blindness
* Have no functioning cones (only rods), leading them to only perceive the world
in tones of white, grey, and black

• Poor visual acuity (no cones!)
• Very sensitive to bright light (all rods!)

Question 4

Describe dichromats

Answer 4

Dichromats are missing one of the three types of cones

• There are three types of dichromats (protanopia, deuteranopia, and
  tritanopia)

Question 5

Describe unilateral dichromats

Answer 5

• Unilateral dichromats has trichromatic vision in one eye and dichromatic in
  the other
• Provides a unique opportunity to understand how dichromats ‘see’ colour
  (because the same brain can interpret/describe perception of colour from
  both perspectives)

Question 6

Describe protanopia

Answer 6

Protanopia affects 1% of males and .02% of females
* Missing the long-wavelength pigment
* AKA a form of red-green colourblindness: red looks more green and less
bright
* See short-wavelengths as blue

Question 7

Describe deuteranopia

Answer 7

• Deuteranopia affects 1% of males and .01% of females
• Missing the medium-wavelength pigment
• AKA a form of red-green colourblindness: green look more red
• See short-wavelengths as blue

Question 8

Describe tritanopia

Answer 8

• Tritanopia affects .002% of males and .001% of females
• Missing the short-wavelength pigment
• AKA a form of blue-yellow colourblindness: difficulty separating blue and green, red and yellow
• See short-wavelengths as blue

Question 9

Describe Hering’s primary colours

Answer 9

• The colour circle on the right is like the colour solid
  shown previously, though without saturation and value,
  and can be divided up into four basic colour groups
  (Hering’s primary colours):
• Top half: colours with some degree of redness
• Bottom half: colours with some degree of greenness
• Right half: colours with some degree of yellowness
• Left half: colours with some degree of blueness

Question 10

Describe Hering’s opponent process theory

Answer 10

• Theory proposed by Hering (1800s) hypothesized that our colour vision arises from
  three primary mechanisms involving opponent-processes:

1. White/Black
2. Red/Green
3. Yellow/Blue

Question 11

Describe the hue cancellation method

Answer 11

• Hue cancellation method provides experimental evidence to support this
• Involves starting with one colour (e.g. blue) and having participants ‘add’ light from the opposing pair
  (in this case, yellow) to see how that changes the perception of colour

Question 12

Describe the difference between the trichromatic and opponent process theories

Answer 12

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

Question 13

Describe color constancy

Answer 13

perception of colours as relatively constant in spite of
changing light sources

Question 14

Describe chromatic adaptation

Answer 14

occurs with prolonged exposure to chromatic colours
* When the stimulus colour selectively bleaches a specific cone pigment over
an extended period of time, this results in a decrease in sensitivity to the
associated colour

Question 15

Describe Uchiwaka study

Answer 15

• Baseline (control): paper and observer in white light
• Observer not (chromatically) adapted: paper illuminated by red light; observer by white
• Observer (chromatically) adapted: paper and observer both exposed to red light

Question 16

Describe Uchiwaka study results

Answer 16

Baseline: green paper is seen as green
* Observer not adapted: perception of green paper is shifted toward red
* Observer adapted: perception of green paper is only slightly shifted toward
red (demonstrating partial colour constancy)

Question 17

Describe lightness constancy

Answer 17

Achromatic colours are perceived as remaining
relatively constant across different lighting conditions

Question 18

Describe lightness

Answer 18

the perception of the shade of
achromatic colour (white, grey, black)

Question 19

Describe reflectance

Answer 19

the proportion of light
reflected back by the object

Question 20

Describe the ratio principle

Answer 20

two areas reflecting different
amounts of light look the same if the ratios of their
intensities are the same (at least when evenly illuminated…)

Question 21

Describe reflectance edges

Answer 21

edges where the
amount of light reflected changes between
two surfaces (between a/c in picture), i.e.
changes in material

Question 22

Describe illumination edges

Answer 22

edges where lighting
of two surfaces changes (between a/b in
picture), i.e. changes in lighting