colour vision 2

trichromatic theory

- some combination of 3 channels can explain discrimination of colour
- each channel would give rise to a specific colour sensation
- each channel would have its own spectral sensitivity function

- does not explain some aspects of discrimination
- does not explain the presence of all unique hues red, green, blue, yellow
- can't explain how dichromats can perceive yellow and white

Hering colour theory - trivariance of vision

three channels that respond in opposition
- allowed 4 unique colours
- check text book

stage colour theory

- theory combining trichromacy at the retinal receptors with further neural processes in the retina and beyond into the visual cortex

- signals from 3 cones types are processed in 3 channels
- 2 chromatic channels (colour) and 1 achromatic channel (luminance).
- brightness signal is mostly contributed through red, green and luminosity levels

colour opponent cells

- have been found in the retina and visual cortex
- these cells respond differently depending on target colour
- for retinal ganglion cells the receptive fields showed differences in the centre surround depending on colour

Colour vision deficiency can be classified as ...

congenital or acquired

congenital colour vision deficiency

present from birth
come from an inherited anomaly that affects the photopigments or rod and cone structure

acquired colour vision deficiencies

happen due to ocular disease process

colour vision deficiency statistic

8% males and 0.5 % females have a colour vision deficiency

whatare the three types of colour vision deficiencies?

1) monochromacy
2) dichromacy
3) anomalous trichromacy

dichromacy

- protanopia (lacks L cones)
- deuteranopia (lacks M cones)
- tritanopia (lacks S cones)
- first two more common in males

- person has 1 missing cone class

anomolous trichromacy

protanomoly, deutanomoly, tritanomoly

monochromacy

- the only true colourblind condition
- people with this have never experienced colour perception
- they can match any colour with a single primary colour
they do this by the luminosity channel (brightness match)
- very rare to have monochromacy

typical monochromats

- act as though they have no cones
- low visual acuity
- prefer dim light levels
- V ^ function is the same as dark adapted eye
- the cause is there is rod photopigment in their cones

blue cone monochromats

- have only blue cones and rods
- have colour perception under low light levels

dichromacy

- dichromats can match any colour using 2 primary colours
- they have one absent cone type or photopigment (S M L cones) but have same number of cones as someone who has normal vision
-

what is the selective advantage of dichromacy

dichromats can defeat camouflage designed to confuse colour - normal

what are the 3 types of dichromacy?

1) protanopia
2) deuteranopia
3) tritanopia

protanopia

- absence of L cone (red) photopigments (missing on spectral sensitivity)
- the L cone gene is replaced with a hybrid gene which has an identical spectral sensitivity to a normal M cone photopigment

deuteranopia

absnese of M cone (green) photopigment
- have no m cone or hybrid gene which has the normal spectral sensitivity of L cones photopigment gene
- deuteranopes have normal spectral sensitivity

tritanopia

absence of S cones (blue)
- happens due to genetic defects in S cone photopigment gene
-tritanopes have normal spectral sensitivity
- inherited as an autosomal dominant trait
- tritanopia is rare - male and female affected equally.

Small Field Tritanopia

- seen in everyone that has normal colour perception
- the S cones are absent from the central 20 mins of arc of the fovea
- this produced a colour vision defect for targets of 20 mins of arc or less

Neutral points in dichromacy

- dichromats possess a neutral point
- a region in the spectrum that is perceived as colourless i'e. A WL that is undistinguishable from white

What are confusion lines?

Define groups of colours confused by the dichromats and can be shown on the CIE chromaticity diagram

Confusion lines - protanopia

Protanopes can distinguish up to 18 colours including yellow, white and blue. (There should be the same number of lines)

Confusion lines - deuteranopia

Deuteranopes can distinguish up to 28 colours

Confusion lines - tritanopia

Tritanopes perceive 10-11 colours

Why are dichromats not classed a colourblind?

They can distinguish at least some colours

anomalous trichromacy

- people with this have trichromatic vision
- require a different mixture of primaries to a normal in order to obtain a colour match
- happens due to a 2/3 come spectral absorption cues being very similar


- colour vision of anomalous trichromats varies

Some may have same as normal people
Some have same as dichromats

How are anomalous trichromats checked?

With an anomoloscope
Checks indidvidual's rayleigh matches

Protanomoly

L pigment is replaced by an M like hybrid pigment
Therefore protanomals are less sensitive to long wavelengths of light
- reduced L sensitivity in spectral sensitivity curve

Deuteranomoly

M pigment is replaced with L like pigment hybrid
- deuteranomals have a normal spectral sensitivity function
- most common form of congenial colour deficiency

Red Green defects (duteranopia/protanopia)

Defects in which L or M comes are affected

X linked recessive
Gene carried by mother
Usually exhibited in boys

Tritan defects (blue yellow)

Equal in males and females
- ocular pathology should be considered as tritanopia is rare

Acquired colour vision defects

Can be a result of general ocular disease/ medications/ toxic poisoning/ head trauma

Acquired defects at photoreceptor level

Associated with tritan deficiency

Defects in inner retina and ganglion cells

Can cause green deficiency but not always

Acquired colour vision defects characteristics

1) monocular or unequal
2) recent onset
3) changing
4) other symptoms may be present
5) equal in males and females

Conventional colour vision defect characteristics

1) equal in both eyes
2) long lasting
3) constant
4) commonly red green defect
5) more common in males than females