Colour 1 Flashcards
What gives rise to our perception of colour
wavelengths of visual light 400-700nm on the electromagnetic spectrum
long wavelengths are reddish
short wavelengths are blueish
what is the relationship between refrangibility of light and its hue
refrangibility refers to the ability to be refracted
different wavelengths of light are refracted at different angles
a given wavelength will always be refracted at the same angle and the perceived colour of the wavelength will always be the same
describe the architecture of the visual system
in the eye:
rods + cones–> horizontal, bipolar and amacrine cells –> ganglion cells –> optic nerve fibres
in the brain
retina –> LGN –> V1 –> V2 –> ventral V4, dorsal V5/MT
what discriminates the three cone types
their relative spectral sensitivities to short, medium and long wavelengths of light
what is a relative sensitivity curve
they represent the likelihood that a photoreceptor will catch a photon of a certain wavelength
why are we trichromatic
three cone cells is the ideal trade off between minimising metamerism (due to Rushton’s prinicple of univariance) and maximising spatial acuity
what did Young’s psychophysical colour matching experiments demonstrate
observers were able to match a test light to a target light by varying the intensities of three different independent matching lights
the matching lights were primaries which means it was impossible to mix two of the matching lights to produce the third light
what is a metameric match
when a test and target light appear perceptually indiscriminable
means the following conditions are met: Ltest = Lmatch
Mtest =Mmatch
Stest = Smatch
the lights may differ in spectral composition but their triplet cone signals are indiscriminable to the visual system - metamers
e.g display technology
what physical measurements can be used to estimate the cone spectral sensitivities
microspectrophotometry
suction electrode recording
artifical production
what psychophysical measurements can be used to estimate cone spectral sensitivities
colour matching functions
what is micropsectrophotometry
a measuring beam that passes tranversely though the outer segment of a cone cell’s spectral transmission is compared to a reference beam passing outside of the cone to derive the absorption spectrum of the outer segment of the cone
what is suction electrode recording
a single human/primate cone outer segment is drawn inside a small glass electrode and its electrical current response to lights of different wavelengths
(light evokes a transient outward current which will saturate with stimulation)
what is artificial production
human cone photopigments are produced from DNA and their absorbance spectra are measured in solution
what are colour matching functions
estimates derived from behavioural matches of spectral light
what is a protanope
a dichromat who is missing long wavelength cones
what is a deuteranope
a dichromat who is missing medium wavelength cones
what is a tritanope
a dichromat who is missing short wavelength cones
what are anomalous trichromats
individuals with an altered cone type
what does it mean to be protanomalous
to have altered long wavelength cone cells
what does it mean to be deutranomalous
to have altered medium wavelength cone cells
how can you simulate dichromacy (deutranope)
set one cone to 0 and compute the singals of the other two
why is it impossible to simulate anomalous trichromacy
these individuals have a colour dimension that is unique to them, making simulation impossible
they have different sets of cone pigments so there are pairs of natural stimuli that appear distinct to them but which are metameric for typical trichromats
what asymmetries are there in the trichromatic scheme
The spectral distribution of peak sensitivties is asymmetric such that long and medium cones are closer together
genetic alterations in long and medium cones is common but rare in short cones
long and medium cone colour deficits are sex linked but short cone defects are not
short wave length receptors are rare, consituting less than 5% of cones
how did trichromacy evolve
original mammals are thought to have been nocturnal and retained only two classes of cones from their reptilian ancestors
during evolution of primates the duplication of a gene on the x-chromosome yielded two pigments close together in the long/middle wave region of the spectrum
these pigments separated to give the long and middle pigments we see now
why are long and medium colour deficiencies sex linked
The L and M pigment genes are on the X Chromosome.
The X/Y chromosome determines sex: females have XX, males have XY.
The S gene is on Chromosome 7
why are long and medium colour vision deficits common
The L and M pigment genes lie very close together on the X chromosome.
Both are thought to have arisen from an ancestral pigment gene.
They remain 96% homologous.
how do genes evolve
Failure to align RNA strands perfectly is thought to generate new gene families.
Unequal crossing over allowed the ancestral pigment to evolve in the L and M pigment genes.
why is having both medium and long cones advantageous
primate ancestors are frugivorous
behavioural advantage for trichromacy in searching for ripe fruit among leaves
ability to segment an object from its background
Regan et al. 2001)
Studied the red powder monkey, taking spectral measurements in situ of the fruits they eat when they are ripe and the foliage background.
Calculated the cone signals that would be available and simulated two visual systems: real and arbitrary.
Found that the particular comparison of the long and medium code signals separates out the foliage and the fruits, reducing the variability of the foliage samples.
who carries anomalous trichromacy
Women who are carriers of anomalous trichromacy, have the genetic code for four cone pigments:
The normal S, M and L, and an anomalous pigment (either L’ or M’).
The L and M pigment genes are both coded on the X-chromosome, and are recessive.
The normal S, M and L, and an anomalous pigment (either L’ or M’).
Two random processes determine the make-up of her retina:
X-chromosome inactivation (which determines which X-chromosome is expressed in each cell)
Cone-pigment expression (which determines whether a cone will be an L or an M cone)
what behavioural test can confirm tetrachromacy
Jordan et al’s (2010) modified colour matching task where you must identify the odd one out of three stimuli
The stimuli consisted of mixtures of red and green, so that short wavelength cones were not sensitive to them.
Normal trichromats will then only be sensitive to L and M, and it follows that there should be some combination that exactly balances a yellow wavelength.
However if another cone type is present there should be no point where the yellow balances the red and green mixture so you cannot do a colour match just by manipulating red and green lights.
administered to known carriers of anomalous trichromacy
Measured discriminatin response times and accuracies and identified participant cDa29 as tetrachromatic.
summary
For colour-normal human observers, colour vision depends on the outputs of three classes of cone photoreceptor. These observers need a mixture of three lights to match a test light.
Colour vision deficiencies affecting the L- and M-cones are common, and sex-linked.
Dichromacy is a reduction of normal colour vision.
Anomalous trichromacy is an alteration of normal colour vision.
Asymmetries between the S-cones and the L- and M-cones suggest they may have evolved at different times and for different purposes.
Comparisons between the L- and M-cones provide a visual signal that is beneficial for detecting ripe fruit against a background of leaves.
Women who are carriers of anomalous trichromacy have a tetrachromatic genotype. There is some evidence that this can support behavioural tetrachromacy
