Eyes + Processing Colour

Question 1

From Eye to Cortex

Answer 1

(1) Reception - Absorption of physical energy – i.e. photons
(2) Transduction - Physical energy is converted into an electrochemical pattern in the neurons
(3) Coding - 1-to-1 correspondence
between aspects of the physical stimulus and aspects of the resultant nervous system activity

Question 2

In the eye - Rods

Answer 2

Photoreceptors which care about how much light there is in your environment / how many photons are hitting them
- Vision in dim light + movement

125 million in the outer regions of the retina

Question 3

In the eye - Cones

Answer 3

Photoreceptors particularly interested in processing colour
- Colour vision + sharpness of vision

6 million in the retina, most in the fovea (centre of retina)

Question 4

Perception is a constructive process

Answer 4

Sighted people don’t consciously experience things as being in
colour only when they look directly at them
- What sighted people perceive is not entirely driven by the wavelengths of light that hits the retina

Top down process influence

Question 5

Colour Vision

Answer 5

• Visible light is the part of the
  much wider electromagnetic
  spectrum that our cones and
  rods respond to

We don’t see radio waves, for example.

• The human eye is most
  sensitive to light in the green
  range.

Question 6

Trichromatic Theory - Thomas Young (1802) + Hermann von Helmholtz

Answer 6

• Found that all colours of the
  spectrum can be produced
  by mixing 3 primary colours
• Helmholtz then proposed that there must be three types of
  colour receptors in the
  the human eye, responding to
  different wavelengths of
  light

=> Was right - 3 cones for different wavelengths
=> Short=Blue Medium=Yellow-green Long=Red

Question 7

Opponent-Process Theory - Hering (1878)

Answer 7

Sighted people never perceive blueish-yellow or reddish-green, and also experience negative afterimages
- Colour perception assumed to have three
opponent processes
- Dual-process theory (Hurvich & Jameson, 1957) linked these
processes to combinations of inputs from the three cone types
- So trichromatic theory works at the level of photoreceptors and opponent-process theory works at the level of neurons

Question 8

Color Constancy

Answer 8

-The tendency for a surface to appear to have the same colour despite a change in the wavelengths contained in the light source
– Evolutionarily helpful - the color of light from the sun changes across the day so the colors of fruits and things will change across the day + need to know whether something is safe to eat regardless of time of day