Individual variation in normal vision

Question 1

Three main dimensions in which normal vision can differ?

Answer 1

1. Macular screening pigments
2. Density of photopigments
3. Polymorphisms in L&M cone genes = difference in absorption rates

Question 2

Average level of macular pigment (which selectively filters SW light) was 38% BLANK in women

Answer 2

LOWER - Hammond et al (1997)

Question 3

When supplemented with macular pigments, R-G vision got better/worse?

Answer 3

BETTER - Rodriquez-Carmona et al (2006)

Question 4

Women showed BLANK responsiveness to LWL light in colour discrimination

Answer 4

GREATER - McGuiness et al (1976)

Question 5

What is the relationship between macular pigment density and colour vision?

Answer 5

Bloody confusing

Question 6

Brainard et al (2000)?

Evidence for Yellow perception relying on more than density? (Pokorny & Smith, 1987)

Answer 6

2 PS - Their L/M cone ratio varied (1.15 & 3.79)
Measured wavelength of unique yellow for PSs
FOUND Little variation in yellow (576nm & 574nm)

In general pop, L/M ratio = 1:1-1:4
If variation in unique yellow was plotted as a function of L/M cone ratio, should see yellow spanning 500-600 nm but instead, we see 568-592 nm.

Question 7

Explanation for effect of L/M density?

Answer 7

The relative contribution of L and M cones to the R-G mechanisms is set by experience with the environment (Pokorny et al, 1987)

• Neural factors play a role in sbalizing unique yellow against variation in L/M ratio (Brainard, 2000)

Question 8

Polymorphisms in L&M cone pigment genes occurs at a _____ locus in the genone.

Caused by __/__ polymorphisms at codon __ in exon __

Answer 8

hypervariable
alanine/serine
180
3

Question 9

Dartnal et al (1983)

Answer 9

Used Microspectrophotometry (MSP) on 7 PS's eyes removed due to malignant growths
--
Data suggests subpopulations within each major class of cone 
- confirms results by Alpern & Moeller (1977): psychophysical tests performed on dichromats and anomalous trichromats suggests that spectral position of red pigment may vary  

Bimodal distributions for M&LWL cones

PS 1 & 4 showed variation in spectral location of LWL cone and differences in long-wavelength sensitivity
- Psychophysical tests still indicate normal trichromatic vision

THEREFORE spectral positions of cones aren’t fixed - bimodal distributions
- Even when difference in spectral locations in LWL cones affect psychophysical assessments, still achieve normal trichromatic vision

Question 10

Neitz and Jacobs (1986)

Answer 10

Pilot study on 2 PS: used Rayleigh Match presented in Maxwellian View (focussed on pupil)

• half of the circle made up of comparison light (yellow) and other half made up if red/green lights
• PS had to match R/G light to comparison light

Results
Small, but consistent difference in range of matching (R/R+G)
After controlling for the following factors, determined that it was not due too…
- Difference in ocular transmissivity (how much light gets in eye)
- Difference in photopigmant density
- Differences that arise from using a large test field

Large-scale study
Repeated study with 200 m&f PS (average age= 20.6, all Caucasian)

Results
Male distribution is bimodal
- those in left hump need less red, and have spectral absorption peaks of 556nm for LWL cones
- those in right hump, need more red, have peaks of 559nm

Female distribution is unimodal and seems to fill gap in male distribution

THEREFORE
Difference in colour matching is due to spectral positioning of cone pigments
- only in LWL cones

Difference between males and females suggests variation is sex linked

• X chromosome linked to polymorphisms of LWL sensitive mechanism
• LWL cone mechanism assumes at least 3 separate identities in normal human trichromats