Orthoptics I - Colour Vision

Give an example of why we see blue-yellow colour?

body clock

Give examples of why we see red-green colour?

foraging
hunting/evading

SImply, what is colour vision?

wavelength analysis

What wavelengths are in the visible light spectrum?

400nm - 700nm

Describe shorter wavelengths

blue and high energy

Describe longer wavelengths

red and lower energy

What is opsin?

a photopigment with 7 transmembrane helices

What molecule sits inside opsin?

the chromophore retinal

What happens to the retinal molecule when light hits?

11-cis retinal becomes all-trans retinal, triggering a photoreceptor response

Describe the univariance of the cone cells

cones act as photon counters and the wavelengths change the probability of the photon being absorbed - the varying sensitivities to different wavelengths allows different cones to respond to different "colours"

What are the wavelength dependencies of the cones?

short (blue), medium (green), long (red)

Why do the cones have different sensitivities despite having the same chromophore?

different opsins, which have different initiation energies

Where does retinal bind to the opsin?

lys312

How many amino acid differences are there between L and M cones?

3

What is the colour opponent theory of Hering?

red/green and blue/yellow are opposite hues because they are never simultaneously elicited by a colour stimulus

What is the magnocellular pathway?

diffuse bipolar cells synapsing multiple cones -> parasol ganglion cells
- provides summative information
- large and achromatic

What is the parvocellular pathway?

1 bipolar cell: 1 cone type
1 midget ganglion cell: 1 midget bipolar cell

What is the koniocellular pathway?

short-wave and diffuse bipolar cells, bistratified ganglion cells
- comparison of the output of the two

What are the 4 centre variant midget cells?

L-ON
L-OFF
M-ON
M-OFF

What are the 4 surround variant midget cells?

LM-OFF
LM-ON
LM-OFF
LM-ON

What do the centre midget cells do?

receive input from an individual L or M cone (in the central vision)

What do the surround midget cells do?

receives input from several cones, some of which will be opposing types

What do the bistratified ganglion cells of the koniocellular pathway do?

receive excitatory input from S-cones via S-ON and inhibitory input from LM-OFF diffuse bipolars

How do colour vision deficiencies arise?

loss or change in one or more cone types - the type of cone that is lost determines what colours can be perceived

Protanopia

lacking L-cone (red) receptors: reds and greens not very distinguishable

Deuteranopia

lacking M-cone (green) receptors, reds+greens. not as dark as L-cone loss
- dogs are deuternopic

Tritanopia

lacking S-cones (blue) receptors

Describe the genetics of CVDs

X-linked
rhodopsin - C3
S-cone opsin - C7
L and M cone opsin - X-chromosone

Why are protanopes and deuternopes more common?

L-cone and M-cone photopigment genes are in tandem array and have a high crossover possibility in meisos
- severity is dependent on where the crossover occured and the produced hybrid photopigment

What are Ishihara plates?

good and fast technique to diagnose CVD
- not great for anomalous trichromacies

What is the City University Test?

series of coloured lights - which is the odd one out or the most similar to X

What is the Farnsworth-Munsell D15 test?

ordering of colours
- can indicate type of CVD and its severity
similar to the 100 hue test

What is the Cambridge colour test?

uses Landolt C - determine orientation
slow reduction in colour intensity, how much can they discriminate

What is the Nagel Type 1 Anomaloscope?

uses a bipartite field: upper uses mid-wavelengths (yellowish) and lower uses short (green) and long (red)
- observer adjusts red:green intensity to match the yellow
- there is a ratio that is optimal for the observer but a range that can be accepted as matched