Colour Processing and Perception Flashcards
What did Newton discover/state in regards to colour perception and how?
Was Newton fully correct and if not why?
Newton underwent prism experiments where he split white light into its constituent parts (i.e. the rainbow). He stated: ): ‘Every surface reflects the rays of its own colour more copiously than the rest & in that reflected light has its colour’
Only the first part of this statement is correct. The second clause (in italics and underlined) is wrong: neither surfaces nor light are coloured!
What did thomas young state/deduce in regards to colour perception?
The trichromatic theory.
He found that:
The human visual system can discriminate between wavelengths
differing by only 1-2 nm over the total 300 nm of the visible
spectrum; i.e., we can detect ~200 different hues. Because of this…
He deduced:
‘It is impossible to conceive that each sensitive point on the retina contains an infinite number of (receptors), capable of detecting every visible hue, it is necessary to suppose their number is limited; to the 3 primary colours, red, green & blue.
(1802)
Why did Thomas Young pick red, blue and yellow to base his colour theory on?
Because they are unique; the only 3 that cannot be created by mixing other colours together. Hence the term primary colours.
What evidence supports Thomas Young’s theory and how did we attain this evidence?
Young’s Trichromacy Theory is supported by the existence of 3 Cone types in the human retina containing rhodopsins with different spectral sensitivities:
- S Cones: peak absorption, 420nm (blue); range ~400-530
- M Cones: peak absorption, 533nm (green); range ~450-630
- L Cones: peak absorption, 564nm (red-ish); range ~480-700
This evidence was attained using Action Spectra (obtained from
intra-cellular electrophysiological recordings) and Absorption Profiles (obtained from MicroSpectroPhotometry).
True or false - The world is in colour
False the world is not in colour it’s just our brains manipulating the way we see things by comparing wavelengths.
Based on the idea that colour perception comes from comparing outputs from different photoreceptors (i.e. L cones , M cones , S cones and rods), what was Ewald Herring’s Opponent theory?
Because 4 colours (red vs. green & blue vs. yellow) are never seen merging together at the same point in space (e.g., we have no word for ‘blueish-yellow’) Hering proposed that they are combined at higher levels of the visual system in a ‘mutually destructive’ (i.e., opponent) manner.
[This is a strange idea however is supported by pyschophysics]
What is Oswald Herring’s theory supported by?
Hering’s Theory is supported by Psychophysics: Adaptation results in perception of the opponent colours. (Basically what this means is if you focussed on a colour for a long period of time e.g. you focussed on a green square for a long time e.g. about thirty seconds, and then you looked at a white wall, you would see a fuzzy pinky/red shadow. The idea behind this is that your green photoreceptors get tired and as soon as you look away your red ones are liberated creating the effect of a fake pinky/red image.
Hering’s Opponency Theory is also supported by physiological
recordings showing 2 types of chromatically opponent RGCs:
Type I: with Red-Green Opponent, Centre-Surround RFs (these correspond to midget cells)
- One Cone type (L or M) mediates the RF Centre
- The antagonistic Cone type (M or L) mediates the RF Surround
Type II: Red-Green or Blue-Yellow spatially-overlapping Opponent RFs (these correspond to bi-stratified cells)
- Red/Green: One Cone type (L or M) mediates the ON-response, the antagonistic Cone type (M or L) mediates the OFF-response
- Blue/Yellow: S Cone type mediates the ON-response, combined inputs from both the L & M Cones mediate the overlapping OFF-response
Describe how a Type 1 red green opponent retinal ganglion cell would work?
The primary visual channel consists of two parallel Chromatic-Opponent Channels- explain what these are:
Type I: Red/Green Opponent (the Parvo System)
- Midget RGCs & LGN Parvo Cells send their axons up to:
- Area V1: where granule cells in layer 4Cb have axons of their own that connect to cells with lots of CO (CO-Blob Cells)
Type II : Blue/Yellow Opponent (the Konio System)
- Small Bistratified RGCs & LGN Konio Cells send their axons up to:
- Area V1: (bypass layer 4C) to other CO-Blob Cells
True or fasle- The vast majority of blob cells in area one are wave-selective and have circular RFs with opponent properties, similar to those of Type 1 and 2 RGCs.
True
What changes occur to chromatic information recieved in V1 before it is processed/during processing (3)?
(1)Amplification of the Blue-Yellow Channel
•Only ~5% of RGCs are B-Y opponent, but there is a greater proportion of B-Y opponent-processing cells in V1 cortex
(2) Cross-Channel Mixing
•Convergent inputs from the G-R & B-Y channels produce V1 cells with novel opponencies (e.g., turquoise ON-centre & orange OFF-surround)
(3) Within-Channel Mixing
- The majority of V1 cells in the R-G/G-R channel are double- (rather than single-) opponent (this basically means that the centre is stimulated two two things e.g it isn’t just red on in the centre and green on in the surround, it is red on AND green off in the CENTRE, and red OFF green ON in the surround.
- Which makes them maximally responsive to these chromatic contrasts, rather than being inhibited by them (as in the retina & LGN!)
[Note though that the receptors are interested in wavelenghts not colours - colours are a construct made from processing]
What is colour constancy?
Chromatically-sensitive cells at low levels of the visual system (retina, LGN, V1) do not respond to the ‘colours’ of surfaces in the scene, only to the wavelengths of light reflected from them.
However, the perceived colour of a surface is normally relatively independent of the spectral composition of the illuminating OR the reflected light. For example, in this room the incident fluorescent light is mainly short-middle wave (blue-green).
So surfaces will naturally reflect this incident light more than long-wave light: yet red surfaces here are still perceived as red!
This phenomenon is known as Colour Constancy & it must be
mediated by neurons at higher cortical than area V1.
True or false- Colour constancy is an act of judgement rather than processing.
False - colour constancy is an act of V4 processing.
Describe semir Zeki’s experiment on colour constancy and what they showed.
Semir Zeki’s experiments showed that responses of V1 cells depended on the wavelength however V4 cells were colour constant: e.g., ‘Red’ cells in V4 only responded to surfaces perceived by humans as being red & not to surfaces perceived as green or blue even when these reflected much more Long Wave than Middle or Short Wave light
What is an action spectra of a cone?
The wavelength the cone mpost responds to.
Where is area V4 found?
There’s an area in each cortex but in both cortexes it is next to the anterior part of the lingual gyrus.
What would a patient with lingual gyrus damage experience?
