6. Rods & Dark adaptation Flashcards
How many rods and cones do we have? Give the macular ratio of rods to cones.
Cones = around 5 million
Rods = 120million, increases in density towards the periphery, highest density at 18deg.
Macular ratio of rods to cones =
9:1
Which rods or cones are used more?
Cones are used more frequently by the visual system.
In AMD which of the photoreceptors are more susceptible to damage?
At early stages, rods.
What is the difference in visual acuity between rods and cones?
Cones = very high
Rods = Very low
What is the difference in dark adaptation (after bleach) between rods and cones?
Cones = 5 mins
Rods = 40 mins
What is photopic bleach?
When you are exposed to bright light, photopigments break down (bleach).
What is the difference in Colour vision between rods and cones?
Cones = Full trichromatic, due to 3 cones
Rods = Minimal colour discrimination
What is the advantage of rods, but how is it limited?
Can detect very low level of light, high sensitivity.
The limitation is that it takes around 40 mins to achieve the level of sensitivity.
What is night myopia? How does it happen?
Affects 15% of the population, where they get less than 1.5D of myopia.
It happens due to the spherical aberrations because due to the low light intensity our pupils dilate causing an increase in spherical aberrations. Also is because of accommodation, lens change.
Which part of the rods capture light?
Within the photopigment rhodopsin is 11-cis-retinal-opsin which captures light.
Opsin is a protein component.
11 cis retinal is a light sensitive molecule derived from Vitamin A.
What are the factors that affect dark adaptation?
Physical factors:
1. Stimuli size & location (due to rods converging on just one ganglion)
2. Bleaching exposure (Takes time to regenerate the photopigment)
3. Target wavelength (Rods more sensitive to short and medium wavelengths)
Physiological factors:
1. Aging (Bruch’s membrane thickens with age, and its hydraulic conductivity decreases.
2. Retinal diseases
In which retinal diseases/conditions would dark adaptation be abnormal?
- Systemic Vitamin A deficiency
- Macular degeneration
- Stargardt’s Macular dystrophy
- Diabetes
- Retinitis pigmentosa
- Crohn’s disease
What method do we use to assess dark adaptation ability? Describe the procedure.
CRT - based dark adaptometry.
Procedure:
1. Eye exposed to short duration of bright flash of light
2. Target presented at 8 deg eccentricity (in periphery, not at fovea)
3. Observer presses button when target detected
4. As sensitivity improves target detected at lower and lower intensity
5. Sensitivity recovery (log units) plotted against time after bleach.
What are the sensitivity and specificity values for CRT based dark adaptometry?
95% Sensitivity
90% Specificity
