Colour 1

Question 1

What gives rise to our perception of colour

Answer 1

wavelengths of visual light 400-700nm on the electromagnetic spectrum

long wavelengths are reddish
short wavelengths are blueish

Question 2

what is the relationship between refrangibility of light and its hue

Answer 2

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

Question 3

describe the architecture of the visual system

Answer 3

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

Question 4

what discriminates the three cone types

Answer 4

their relative spectral sensitivities to short, medium and long wavelengths of light

Question 5

what is a relative sensitivity curve

Answer 5

they represent the likelihood that a photoreceptor will catch a photon of a certain wavelength

Question 6

why are we trichromatic

Answer 6

three cone cells is the ideal trade off between minimising metamerism (due to Rushton’s prinicple of univariance) and maximising spatial acuity

Question 7

what did Young’s psychophysical colour matching experiments demonstrate

Answer 7

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

Question 8

what is a metameric match

Answer 8

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

Question 9

what physical measurements can be used to estimate the cone spectral sensitivities

Answer 9

microspectrophotometry
suction electrode recording
artifical production

Question 10

what psychophysical measurements can be used to estimate cone spectral sensitivities

Answer 10

colour matching functions

Question 11

what is micropsectrophotometry

Answer 11

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

Question 12

what is suction electrode recording

Answer 12

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)

Question 13

what is artificial production

Answer 13

human cone photopigments are produced from DNA and their absorbance spectra are measured in solution

Question 14

what are colour matching functions

Answer 14

estimates derived from behavioural matches of spectral light

Question 15

what is a protanope

Answer 15

a dichromat who is missing long wavelength cones

Question 16

what is a deuteranope

Answer 16

a dichromat who is missing medium wavelength cones

Question 17

what is a tritanope

Answer 17

a dichromat who is missing short wavelength cones

Question 18

what are anomalous trichromats

Answer 18

individuals with an altered cone type

Question 19

what does it mean to be protanomalous

Answer 19

to have altered long wavelength cone cells

Question 20

what does it mean to be deutranomalous

Answer 20

to have altered medium wavelength cone cells

Question 21

how can you simulate dichromacy (deutranope)

Answer 21

set one cone to 0 and compute the singals of the other two

Question 22

why is it impossible to simulate anomalous trichromacy

Answer 22

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

Question 23

what asymmetries are there in the trichromatic scheme

Answer 23

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

Question 24

how did trichromacy evolve

Answer 24

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

Question 25

why are long and medium colour deficiencies sex linked

Answer 25

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

Question 26

why are long and medium colour vision deficits common

Answer 26

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.

Question 27

how do genes evolve

Answer 27

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.

Question 28

why is having both medium and long cones advantageous

Answer 28

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.

Question 29

who carries anomalous trichromacy

Answer 29

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)

Question 30

what behavioural test can confirm tetrachromacy

Answer 30

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.

Question 31

summary

Answer 31

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