Lecture 6 - Color Vision

What wavelength do humans have the best color discrimination at?

589 nm

also the yellow used for the anomaloscope

Which colors are humans the best, okay, and worse at discriminating?

best - blue/yellow

okay - green

worse - red/violet

What is the minimum wavelength discrimination that humans can achieve?

<2 nm

What is the X-Chrom contact lens?

fitted colored contact lens on one eye

How does the X-Chrom contact lens change the effective spectral sensitivity?

can tell the difference between green and red by sensitivity

What are enchroma lens?

a notch filter designed to increase the difference in the responses of the normal and anomalous cones

no evidence that enchroma lenses increases color perception

What percentage of the population have red-green defects?

~9%

What disorders/pathologies can induce color vision defects?

1. progressive cone dysfunction & dystrophies and some retinitis pigmentosa phenotypes

2. stargardt's disease

3. glaucoma and other optic neuropathies

4. diabetic retinopathy

5. retinal detachment

6. age-related macular degeneration

What is the difference between acquired and hereditary color vision defects?

acquired is often asymmetrical

How will acquired color defects appear on color vision tests?

may appear like a deutan/protan/tritan mixes, will need to test both eyes

What color defect is seen with inherited optic atrophy?

tritan defect

What color defect is seen with Stargardt's disease?

red/green defect

What protein stops rhodopsin from interacting with transducin?

arrestin

What is the function of arrestin?

blocks the interaction between rhodopsin & transducin

What protein inactivates rhodopsin?

rhodopsin kinase

What is the function of rhodopsin kinase?

phosphorylates rhodopsin (inactivates)

What protein inactivates transducin?

RGS9

What is the function of RGS9?

transducin inactivation by GTP hydrolysis, which is sped up by GTPase accelerating proteins like RGS9

What condition is from a defect in the RGS9 protein?

bradyopsia

What are the symptoms of bradyopsia?

1. problems playing sports

2. light adaptation problems - dark to light, vice versa

What is the pathophysiology of bradyopsia?

slow photoresponse recovery

What is the advantage in investing so heavily in the photoreceptors braking mechanism?

temporal resolution

What happens after a photoreceptor has been bleached?

chromophore is converted back to 11-cis retinal in order for the photopigment to be light-sensitive again

What are the steps of photopigment regeneration?

1. arrestin remains bound to the inactivated opsin until the used chromophore (photopigment) is removed - ABCR involved here

2. all-trans retinal is converted to all trans-retinol (vitamin A) and transported into the RPE by IRBP (interphotoreceptor retinoid binding protein)

3. in RPE, all trans retinol converted back into 11-cis retinal via RPE65

4. 11-cis retinal transported back into photoreceptor outer segment by IRBP (interphotoreceptor retinoid binding protein)

What does a retinal densitometry measure?

change in light absorbance by retina over time after exposure to bright bleaching light

How long does it take 50% and 100% of rhodopsin to be regenerated?

50 - about 7 mins

100 - about 35 mins

How long does it take 50% and 100% of cone photopigment to be regenerated?

50 - about 2 mins

100 - about 8 mins