37-39 Vision Flashcards Preview

2911 Brain and Behaviour > 37-39 Vision > Flashcards

Flashcards in 37-39 Vision Deck (25)
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What explains the uneven distribution of S, M and L cones?

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


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

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


Some kinds of prawns have how many cones?

18 cones!!!


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

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


What is the principle of univariance?

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


What are metamers?

Two things that are different but perceptually indistinguishable


What are visual metamers?

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


Can trichromacy explain colour afterimages?

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.


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

Black-white (dark-light)


What constitutes the horseshoe part of CIE colour space?

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


What constitutes the straight line in CIE colour space?

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


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

You get grey


What is an equiluminant slice of the CIE?

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


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

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


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

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


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

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


What's the point of centre-surround antagonism?

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.


What percentage of metabolic energy is consumed by the brain?

40% of energy


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

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


What is lightness?

Lightness is the perceived reflectance of a surface.


What is illumance?

The amount of light falling on a surface


What is reflectance?

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


What is luminance?

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


How do you explain the checker square illusion?

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.


What formula explains the checker square illusion?

Luminance A = Luminance B
Illumination in A > Illumination in B
Reflectance of A < Reflectance of B