1 - TRICHROMACY V OPPONENT COLOUR Flashcards
3 photoreceptors not 4
hering (opponent)
- thought it was 4
helmholtz (trichromacy)
- thought it was 3
convergent pathway through retina
- signals from photoreceptor layer travel to middle layer and then to ganglion cell layer
- axons from ganglion cell layer converge to form the optic nerve to the rest of the brain
- signals from a large number of input cells (photoreceptors) exit the retina via a smaller number of output cells (ganglion cells)
- because fewer number of ganglion cells than photoreceptor cells in a given area
- different types of cone receptor within a circular region have different effects on one ganglion cell (some excitatory, some inhibitory)
trichromacy and convergent pathway
- trichromacy would suggest that cones that connect to a particular ganglion cell would all come from the same part of the retina and be the same type
- but wrong because not all the same type
cone photoreceptor effects on ganglion cells - opponent cells (because signals from different types have opposite effects)
1 - (L-M) cells
- L-cones = excitatory effect
- M-cones = inhibitory effect
- no S-cones
2 - (M-L) cells
- L-cones = inhibitory effect
- M-cones = excitatory effect
- no S-cones
3 - (L+M)-S cells
- L- and M-cones = excitatory effect
- S-cones = inhibitory
4 - S-(L+M) cells
- L- and M-cones = inhibitory effect
- S-cones = excitatory
1 + 2 = red/green opponency
(L-M)
(M-L)
3 + 4 = blue/yellow opponency
(L+M) - S
S - (L+M)
- because M-cones are more responsive to wavelengths of light that look green (green detectors)
- L-cones are more responsive to those that look red (red detectors)
- S-cones are more responsive to light that looks blue (blue detectors)
- (L+M) is responsive to light that looks yellow (yellow detectors)
- identifying Herings detectors with the cones shows how trichromacy and opponent colours can be reconciled
- also why red and green mixed together produces yellow
how does red and green together produce yellow?
M-cone is the green detector and L-cone is the red detector
together they are the yellow detector (L+M)
simultaneous activity in L and M cones = yellow
- if no blue to cancel it out
issues with trichromacy and opponent theory based on the previous cards
- why are there (L-M) and (M-L) opponent cells?
- and why are there S - (L+M) and (L+M) - S cells?
- this is because ganglion cells do not fire at rest (no stimulation)
- herings combination unit was active at rest
- but ganglion cells aren’t (because their resting activity firing rate is 0)
solution to the previous problem
- split the mechanism into two parts
1 = increasing activity (herings original mechanism)
2 = decreasing
2 ganglion cells are needed, one to signal eg blue and another to signal eg yellow
- eg S - (L+M) to signal blue
- eg (L+M) - S to signal yellow
which parts of each theory was correct?
TRICHROMACY
- initial photoreceptor stage in colour perception is trichromatic
- light stimulates cones
OPPONENT COLOUR THEORY
- subsequent stage in which cone signals are combined is opponent
- integration of the responses of the three cones
there’s still two issues with the combined theory of them both:
1 - receptor distribution
- the theories say they’re equally distributed in numbers and areas
2 - colour sensations
- they say that colour sensation is due to stimulation of a particular location on the retina and the responses evoked by the three types of photoreceptors at that location
- opponency theory = ‘colour experiences are derived from the responses of the cones but not directly (as in trichromacy) but via an intervening opponent stage of processing’
the actual cone distribution
NOT EQUAL
total cones = over 6 million
L- cones = 55% (around 4 million)
M-cones = 35% (around 2 million)
S-cones = 10% (around over half a million)
- individual variance in distribution of L and M
- in few individuals, M-cones can outnumber L-cones or be in equal numbers, even have only 6% M-cones
L and M cones = VARIABLE
S cones = consistent
why this cone distribution doesn’t match the theories
- an unequal distribution of cones is not compatible with either theory
- L and M cones are usually more evenly distributed
- S-cones are not evenly distributed throughout
- no S-cones in foveola
- relatively few in the rest of the fovea
- most S-cones found in parafoveal and perifoveal regions of the macula
- highest density in parafovea
- means we should be unable to see blue in centre of colour vision (foveola) and have a very limited capacity to see blue outside the macula
colour constancy according to both theories
- colour constancy should not be a thing
- they suggest that when the illumination changes, the colour should also change
- because both say that the colour sensations evoked by a stimulus is completely determined by the wavelengths contained in that stimulus which then determines the cone response
- so something looks a certain colour because it’s image contains mainly light of that colour
