Colour perception Flashcards
What are the 3 cone types present in most humans ?
- short
- middle
- long wavelength selectivity
What are retinal ganglion cells engaged in ?
wavelength opponent responses as interested in chromatic contrast
-red/green and blue/yellow
What do V1 exhibit in colour perception?
-double wavelength opponent + and mixed.
What is interested in the wavelengths ?
all of the neurons within primary visual pathways are i
Where does colour perception occur ?
in extra striate cortex /area v4- found in left and right cerebral cortices (a particular part of the occipital lobe called lingual gyrus )
What are the areas of V4 a part of ?
of the ventral stream - in v4 is colour perception and constancy.
What are the wavelengths of the visible spectrum (for humans ) ?
range form 400nm(UV) to 700nm (infrared)
What did Newton say ?
Newton (1704): ‘Every surface reflects the rays of its own colour more copiously than the rest & in that reflected light has its colour’
-He conceived colour perception as a simple process, involving ‘colour’ detectors in the eye which detect rays of light reflected from surfaces in our field of views which are reflected more than the rest.
Only the first part of this statement is correct. The second clause (in italics) is wrong: neither surfaces nor light are coloured!- the sweater is not blue-
What is colour?
it is an artificial construct of area v4 of the extra striate processing stream
What is Thomas young trichromatic theory ?
-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
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
(Royal Society London, 1802)
Why did Thomas young suggest the 3 colours red, green and blue ?
they are unique
-cannot be created by mixing colours
-unlike, for example, Yellow = (green + red), Turquoise = (green + blue)
or Purple = (blue + red)
What does Youngs trichromacy theory support ?
the existence of 3 cone types in the human retina containing rhodopsins with different spectral sensitivities:
M Cones: peak absorption, 533nm (green); range ~450-630
L Cones: peak absorption, 564nm (red-ish); range ~480-700
What are the S cones spectral sensitivity ?
S Cones (blue): peak absorption, 420nm (blue); range ~400-530
What are the M cones spectral sensitivity ?
M Cones(green cones): peak absorption, 533nm (green); range ~450-630
What are the L cones spectral sensitivity ?
L Cones(red): peak absorption, 564nm (red-ish); range ~480-700
What do the chromatic sensitivities & ‘tuning curves’ of the 3 isolated
cone types show?
similar Action Spectra (obtained from
intra-cellular electrophysiological recordings) and Absorption
Profiles (obtained from MicroSpectroPhotometry)
Why is one cone type not enough for us to perceive colour ?
an indication of this is that we cant see colours at night when the rods are only working.
Why is one cone type not enough for us to perceive colour ?
an indication of this is that we cant see colours at night when the rods are only working.
-you have to take the output of the 3 cone types to start colour perception
How are the 3 cone outputs are combined at the next step of the visual pathway ?
-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.
Red vs. Green opponency
this is the Ewald Hering (1834-1918): Opponent Theory
Why do you have opponent after images- in powerpoint example ?
- this is because all the green cones that are estimated by the green square when fixating on cross - they start to get tired- this process is called adaptation
- becasue the green cones are combined with the red cones this unleashes the red image against a white background
- same for the blue- blue cones are getting tired out adapting to short wave cones on retina- when blue is removed the yellow opponent colour is shown
What is herrings opponent theory also supported by ?
by -physiological
recordings showing 2 types of chromatically opponent RGCs:
What are the 2 subtypes of retinal ganglion cell that mediate different types of wavelength opponency ?
Type I ganglion cells : with different types Red-Green Opponent, Centre-Surround RFs
e. g One Cone type (L or M) will mediates the RF Centre and the antagonistic Cone type (M or L) mediates the RF Surround.
- medium wavelengths will mediate the centre
- the long wavelengths will mediate the surround
Type ||: Red-Green or Blue-Yellow spatially-overlapping Opponent RFs
- Red/Green: One Cone type (L or M) mediates the ON-response (short wave cones mediate the ON response), and 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
What do type | ganglion cells do ?
corresponds to the midget type of retinal ganglion cell from parvoleccular system
What do the 3 ganglion cell types mediate?
- Midgit type of cells mediate the parvocellular channel- known as Type |
- Small bi-stratified cells- mediate the konio cell channel- Type ||
- Chromatic processing - Type ||| ganglion cell which corresponds to the magnocellular channel- which are not chromatically sensitive at all.
What do Type | do? - refer to powerpoint
- middle wave cones are producing excitatory input centre of ganglion cells
- surrounding region of Long wave cones mediating the antagonist surround
What do Type || do ? - refer to powerpoint
- with short wave cones providing input that leads to a + ON response of these ganglion cells
- with combined input from middle wave cones and long wave cones coming together to mediate yellow OFF responses
What do type ||| do?
- centre of these ganglion cells are provided by both excitatory middle and long wave cone inputs
- and surrounded mediated by inhibitory input in both long and short wave cones
What are type ||| ganglion cells interested in ?
only in brightness/luminance contrast not chromatic contrast
what is an example of a response of Type | cell with red green opponent (red in centre and green surround)?
-This cell has a Red-ON, Green-OFF Centre-Surround RF organization:
it responds [b] see RF of cell being illuminated by a red spot filling up the centre (b) responds maximally due to L-wave ON-stimulation of its RF centre when there is very little or no M-wave light (OFF) in its surround
responds [c] less well to diffuse L-wave light; but [d] is inhibited by simultaneous (mutually destructive) L-wave & M-wave ON-stimulation in both its Centre & antagonistic Surround (because the excitatory L and inhibitory M cone inputs cancel each other out)
What do 90% of chromatic processing that occurs along retinal ganglion cells ?
is mediated by different types of red-green opponent with type | ganglion cells
-with different combinations ( such as red on green off, red off green on, green on red off, etc) - all possible combinations
What are the 10% of chromatic processing that occurs along retinal ganglion cells ??
are type || - of which there is 5% of total retinal ganglion cells Blue On and yellow OFF in RF
What are the 2 parallel chromatic opponent channels in the primary visual pathway ?
- Type I: Red/Green Opponent (the Parvo System)
mediated by Midget RGCs & LGN Parvo Cells- the parvocellular neurons have axons that go up to the V1 which make connections with granule cells in layer 4Cbeta & and also in some CO-Blob Cells - Type II: Blue/Yellow Opponent (the Konio System)
Small Bistratified RGCs & LGN Konio Cells which have axons that travel to
Area V1: ( which bypass layer 4C) but make connections to other CO-Blob Cells
What happens in the cells in CO blobs of area v1 ?
they are wavelength-selective ‘Blob’ cells
and have circular RFs with opponent properties, similar to those of Type I & II RGCs (& LGN neurons
What are the transformations in area v1 cortex ?
(1) Amplification of the Blue-Yellow Channel
(2) Cross-Channel Mixing
(3) Within-Channel Mixing
What is 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
What is Cross-Channel Mixing?
Convergent inputs from the G-R & B-Y channels produce V1 with cells with novel opponencies (e.g., some of the cells in the CO blobs have turquoise (green and blue combo) ON-centre & orange (combination of red and yellow to produce) OFF-surrounds)
What is Within-Channel Mixing?
The majority of V1 cells in the R-G/G-R channel are double- (rather than single-) opponents
why are these V1 cells within channel mixing really important ?
it makes these cells maximally responsive to these real chromatic contrasts, rather than being inhibited by them (as in the retina & LGN)
What is the nature of within Chanel mixing ?
to produce double wavelength opponency among many V1 cells
What do chromatic sensitive cells not respond to?
(retina, LGN, V1) do not respond to the colours of surfaces in the scene
What do chromatic sensitive cells respond to at low levels of the visual system?
only to the wavelengths of light reflected from them.
What is the perceived colour of the surface ?
normally relatively
independent of the spectral composition of the illuminating
OR the reflected light.
-For example, in this room the incident fluorescent light (Ceiling) 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.
What is the spectrum of sunlight at different times of day?
- the wavelength composition of incident sunlight varies enormously,
from mainly LW (‘red’ long wavelength) at twilight to mainly SW (‘blue’- short wavelengths ) later in the evening. Similarly, the spectrum is different under a forest canopy (mainly ‘green’) & under tungsten (mainly ‘blue’) light.
Yet an orange always looks orange!
Hering accounted for Colour Constancy by suggesting that it is ‘an act of judgement, not sensation’ since ‘surfaces known from experience are seen through the spectacles of colour memory’ - an orange always looks orange because we know its an orange is not true!!!!
because - Colour Constancy is a property of relatively early sensory processing in extrastriate cortical area V4
What is the Semir Zeki’s experiment of area v4 and colour constancy ?
- He recorded from cells in areas V1 & V4, & first determined their responses to different wavelengths of light (looked at their Action Spectra)
- Then used Multi-coloured (Mondrian) displays illuminated by 3 projectors shining different amounts of S, M & L Wave light onto the display & measured the amounts of reflected S, M & L Wave light (with a Photometer) from specific coloured regions placed in the cells’ RF
- Discovered that responses of V1 cells depended only on the reflected wavelength composition
BUT 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
he could illuminate this display exclusively with long wave light - red prioecteor- with only tiny bits of blue and green projectors- so the whole display reflected more long wavelength light to the monkey.
What is the responses that he received after recording from the cells in V1 ?
-First examined the response of this cell of shining different wavelengths of light into its RF AND CAN SEE THE ACTION SEPCTRA WITH A PEAK AT 620NM - LONG WAVELEGNTH END OF SPECTRUM -but no correlation of this cell to colour perception
What is the Mondrian display illuminated by in the experiment shown in powerpoint of Semi and what is the response he received from v1 cells?
illuminated by long wave light
-red illuminated
hint of general redness around different parts fo the display, most of the blues look purple
- if you move red area into cells receptive field - that cells fires loads of action potentials that will be about
- this long wave interested cell- interested in long wave light red rf
- if you move the green or blue in RF - the cell gives a vigorous response
- this cell is not just interested in specific r-g-b-w - it was only activated by the fact there was more long wave light being reflected from each of these surfaces compared to middle or short wave light.
- wavelength dependant and no correlation with colour perception
- the V1 cells are long wave selective
What is the responses that he received after recording from the cells in V4 ?
This V4 cell is also Long-Wave Selective (see Action Spectrum), but it ONLY responds to regions of the Mondrian perceived as RED by human observers, NOT to other non-red surfaces (,G,B,Y) reflecting more LW than MW or SW light
- THIS CELL was exhibiting colour constancy -present at the single cell level in V4
- different cells from different colours
What is the the lower bank of calacrine sulcus is produced by ?
lingual gyrus
Where do you find area v4 ?
sit on ventral pathway
anterior end of lingual gyrus
What is the functional imaging experiment ?
zeki experiment
- healthy people stood in scanner
- looked down scanner in screen and looking down at a multi coloured sepals - activity throughout their brain was recorded while looking
- next test looking at the same display but all colours removed but replaced with different shades of grey black white
- so that the luminance reflected from display was identical to colour version however no chromatic info
- the regular subtraction done Areas of brain activated looking at colour display - isoluminant greyscale display
Why is the V1 not selectively activated by the colour Mondrian ?
there are lots of neurons in the V1 in the CO blob zone that like chromatic stimuli
- but there are lots of neurons in the inter blob zones interested in greyscale and those 2 things canceled each other out.
- no selective activation in v1
- just in V4 - interested in colour
- V4 is in left and right cortices and each of those areas have a map of the opposite half of field-bigger overrepresentation of central vision in v1
Why is there over representation of central vision in V1 ?
most of our colour perception occurs in our central vision and very little in periphery
What are the 2 types of genetic/inherited colour deficiencies ?
- Dichromacy: complete loss of 1 cone pigment type
Most commonly red/protan or green/deutan
it is X-linked(common in males): ~1-2% males; only ~0.01-0.02% females
Blue/tritan rarer; autosomal recessive males=females (both 0.008%)
Anomalous Trichromacy: 1 cone pigment is abnormal
X-linked: deutan ~ 5% V 0.4%; protan 1% V 0.03% males V females
Blue/tritan: extremely rare; autosomal dominant
What is the way you can develop colour deficiency ?
Acquired
Retinal/optic pathway damage/disease: dyschromatopsia
Area V4 Cortex: cerebral achromatopsia (real colour ‘blindness’!)
What are the simple tests for colour vision ?
Ishihara Plates
Farnsworth-Munsell 100 Hue
what is the nature of the Cerebral Achromatopsia
?
damage to the lingual gyrus- which means cannot see any colours ta all, just grey
e.g Left hemi-achromatopsia: Patient with a local lesion in area V4 of the Right Lingual Gyrus-cannot see any colours on the left half but could see brightness
