37-39 Vision

Question 1

What explains the uneven distribution of S, M and L cones?

Answer 1

Humans were originally dichromats, but mutation occurred - appearance of third cone – which lent selective advantage

Question 2

Why would proximity of L and M cone peak sensitivity, allowing us to make fine distinctions between red and green, be a selective advantage?

Answer 2

It allows us to distinguish between foodstuffs - berries, fruit - which are usually on the red-green end of spectrum

Question 3

Some kinds of prawns have how many cones?

Answer 3

18 cones!!!

Question 4

What happens when you shine spot of dim light in dark room after dark adaptation?

Answer 4

See only light, not colour - even if light is coloured

Question 5

What is the principle of univariance?

Answer 5

Photoreceptors can only fire faster or slower, i.e. they vary along one dimension. The effect on a photopigment of absorbing a photon is independent of its wavelength. Thus the response of a photoreceptor is determined solely by the number of photons absorbed and is not sensitive to the wavelengths of those photons. This has the important consequence that there is no information in the response of a single photoreceptor about the wavelength of the light which affects it. A single photoreceptor with a single photopigment is incapable of distinguishing between an intense light at a wavelength to which it is relatively insensitive and a weak light at a wavelength to which it is relatively sensitive

Question 6

What are metamers?

Answer 6

Two things that are different but perceptually indistinguishable

Question 7

What are visual metamers?

Answer 7

Things that stimulate the cones in the same way (produces same firing of S, M and L cones) but are physically different

Question 8

Can trichromacy explain colour afterimages?

Answer 8

Yes! It’s just sensory adaptation– tiring out certain cone type, which leads to lower response rate, so afterimage is only firing of two rather than three cones.

Question 9

Which three opponent axes is colour organised into by the brain?

Answer 9

Blue-yellow
Red-green
Black-white (dark-light)

Question 10

What constitutes the horseshoe part of CIE colour space?

Answer 10

It’s the spectral locus - the pure spectra of light, as when white light is shined through a prism - the most saturated colour you can get

Question 11

What constitutes the straight line in CIE colour space?

Answer 11

The line of purples - mixture of long wavelength and short wavelength colours

Question 12

What happens when you mix opponent colours in CIE colour space?

Answer 12

You get grey

Question 13

What is an equiluminant slice of the CIE?

Answer 13

A cross-section of colours all with the same energy/luminance

Question 14

Can an increment - something lighter than its background - ever be matched to a decrement - something darker than its background?

Answer 14

Apparently not. You’ll never be happy with the match.

Question 15

What are the two types of receptive fields in ganglion cells?

Answer 15

On-centre off surround cells - fire more when light in centre, less when light in periphery

Off-centre on surround cells - fire less when light in centre, more when light in periphery

Question 16

How do cells with centre-surround antagonism fire when both centre and periphery is stimulated?

Answer 16

Not much - because they are receptive to difference, i.e. something actually happening.

Question 17

What’s the point of centre-surround antagonism?

Answer 17

The process enhances image contrast - the places in the image where a change in luminance occurs. This creates a picture of STRUCTURE of the world. A uniform field has no structure - so centre-surround cells ignore uniform stimulus. Brain performs compression algorithm - don’t need all neurons firing to tell you nothing’s happening.

Question 18

What percentage of metabolic energy is consumed by the brain?

Answer 18

40% of energy

Question 19

What is the function of centre-surround opponent receptive fields?

Answer 19

Redundancy reduction or contrast enhancement - they transmit only information about difference (i.e. structure) not about uniformity

Question 20

What is lightness?

Answer 20

Lightness is the perceived reflectance ofasurface.

Question 21

What is illumance?

Answer 21

The amount of light falling on a surface

Question 22

What is reflectance?

Answer 22

Proportion of light a surface reflects. For white up to 90%, for black only 5%.

Question 23

What is luminance?

Answer 23

The total amount of light reaching the eye. Luminance = illumance x reflectance

Question 24

How do you explain the checker square illusion?

Answer 24

If same amount of luminance is coming from non-shadow (A) and shadow (B) square, yet shadow square is in darkness, shadow square must be lighter. The property of illumination is discounted.

Question 25

What formula explains the checker square illusion?

Answer 25

Luminance A = Luminance B	
BUT:	
Illumination in A > Illumination	 in B
THEREFORE:	
Reflectance of	A < Reflectance of B