W5 Colour Vision

What range can the human eye detect?

Wavelengths between approximately 380-780nm

How many colours can an eye with normal colour vision detect?

150 different colours within the range of light

What is the Young-Helmholtz theory?

The retina has 3 types of cone cells which are each sensitive to a different range of light wavelengths

What are the 3 types of cone cells in the retina?

Short wavelength cone is sensitive to blue

Medium wavelength cone is sensitive to green

Long wavelength cone is sensitive to red

What is the Opponent colour theory?

4 primary colours (red, green, vs yellow, blue) were arranged in opponent pairs plus a luminance mechanism where white vs black

What is the Zone theory?

A series of zones along the visual pathway

What are the series of zones in the Zone theory?

Receptor level- Vision is trichromatic (3 colours) and mediated by cones (YH theory)

Ganglion cell level- colour information is coded into opponent channels

Luminance channel- derives input from the long and medium wavelength sensitive cones and a SMALL input from the short wavelength cones

What happens at the receptor level?

electrical signals from the 3 cone types are coded in the plexiform layers in the retina so the 3 electric opponent signals can be detected at the ganglion cell level

How are receptor and ganglion cells connected?

Horizontal and Muller cells connect the 2 laterally within the plexiform layers, giving rise to receptor fields

What is the difference between rods and cones?

Rods function at low levels of illumination but cones function at high levels of illumination

Rods have no directional sensitivity, in cones sensitivity diminishes if light enters eye obliquely (indirect)

What are similarities between rods and cones?

Both receptor types diminish in density towards peripheral retina

What is the proportional of cones in the central retina?

Red: Green: Blue- 40:20:1

NEAR FOVEA: 3:9:1

What happens to threshold blue sensitivity with age?

Despite red-green remaining stable, blue decreases due to the greater absorption of shorter wavelengths within the crystalline lens

Why is there individual variation in threshold blue perception?

Differences in macular pigment density and retinal lutein levels

What are congenital colour vision deficiencies?

Abnormal cone photopigments- the retina may be lacking in functional cone receptors or there may only be 1/2 photo pigments instead of 3 OR have abnormal absorption characteristics

What are examples of congenital colour deficiencies?

Trichromacy- all cones and pigments present and functioning

Dichromacy- 2 cone pigments present and functioning

Monochromacy- no cones at all/one functioning cone

How is dichromacy further categorised?

Protanope- long wavelength pigment missing (1% men, 0.01% women)

Deuteranope- medium wavelength pigment missing (1% men, 0.01% women)

Tristan one- short wavelength pigment missing (0.001% men AND women)

How can anomalous trichromacy further be categorised?

Protanomaly- abnormal long wavelength pigment (1% in men, 0.03% in women)

Deuteranomaly- abnormal medium wavelength pigment (4.9% in men, 0.35% in women)

Tritanomaly- abnormal short wavelength pigment (UNKNOWN)

What are the 3 types of acquired colour defects?

Type I: Red-Green

Type II: Red-Green

Type III: Blue

What is Type I coloured defect?

Similar to a protan defect, found in macular dystrophy

What is Type II colour defect?

Similar to a deutan defect, found in retrobulbar neuritis

What is Type III colour defect?

Similar to a tritian defect, found in many central and peripheral retinal lesions

What are features of acquired coloured defects?

Can be both dichromatic and anomalous trichromatic, shows a combination of characteristics, blue pathway is particularly vulnerable to to acquired defects (retinal lesions),

What is ishihara?

Standard test- 38 plate vision- 6 mistakes or more are caused by colour vision defects

What are examples of screening plates?

Transformation plates- normal vs abnormal see different numbers

Vanishing plates- normal sees number, abnormal does not

What is the City Univeristy colour vision test?

A central dot is surrounded by 4 dots and px must identify the most similar in colour

What is the D-15 colour vision test?

Px arranges 15 cps in order to form a smooth sequence to detect moderate to severe colour vision tests

What is the 100 Hue test?

Px arranges 85 coloured caps in correct order to detect precise colour vision- most detailed and sensitive

What is the HRR test?

A series of plates containing symbols to screen and diagnose colour vision deficiencies and children/people who can’t read numbers, testing people with red-green and blue-yellow deficiencies