Flashcards in 37-39 Vision Deck (25)
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?
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?
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.