Introduction to Color Vision

what is color vision?

- The ability to discriminate a light stimulus as a function of its wavelength

What are the wavelengths of visible light?

- 380 and 760

what does electromagnetic energy do to the human retina?

- causes photo reactions on human retina leading to the experience of vision

what is monochromatic light?

- colored light of a single wavelength

what creates the sensation of white?

- Remixing of all colors of light created by a prism with a convex lens, creates the sensation of white

what are the additive primary colors?

- Blue (435 nm), green (545 nm), and red (700 nm)
- add up to produce white

what are the subtractive primary colors?

- Yellow, magenta-red, and cyan-blue
- add up to produce black

what are compliment colors?

- additive mixture that forms white

what are interference colors?

- Color can result from interference on thin films
- Ex. soap bubbles

what can colors be produced by?

- scatter of white light

what causes the colors of sunrise and sunset?

- The sun's rays pass vertically through the earth's atmosphere- longer wavelengths are scattered
- At sunrise and sunset, the sun's rays - the path length is longer- this gives rise to an intense red color

what do all human visual pigments share?

- share a common chromophore, chemically related to vitamin A

what are rods and cones?

- The rod and cone cell pigments are all complexes of the same chromophore with different opsin proteins

what are the genes that encode the opsin proteins related to?

- The genes encoding the opsin proteins are all members of related receptors: G protein coupled receptors

what are g proteins in rod and cones cells called?

- transducins
- A Specific form of transducins are found in rods and cones
- BUT all 3 cone types share a common transducin form

what forms the photo-receptor pigment complex?

- 11-cis isomer of aldehyde vitamin A reacts with an opsin protein forming a photoreceptor-pigment complex

what happens in the opsin bound state?

- the chromophore has unique properties that contribute to color vision functioning

what is the retina?

- the point of contact between radiated energy, light and nervous system

what type of cells help with scotopic vision (dim)?

- rod cells

what type of cells help with photopic color vision (bright)?

- cone cells
- less sensitive that rods, but they mediate differentiation of colors

how close can humans distinguish 2 monochromatic light rays?

- Humans can distinguish 2 monochromatic light rays as close as ≈2 nm apart.
- The color palette has up to 300 distinct spectral colors

what portion of the retina has highest density of cones and no rods?

- fovea

what is the Young-Hemholtz Theory?

- Emphasizes trichromacy
- Confirmed by the discovery of 3 cone photoreceptor systems

What is opponent color theory?

- Developed by Karl Hering
- 3 oppositional color pigment pairs
- These do not actually exist, but it is true that the signals from the 3 cone types are combined at the level of neurons to produce
- opposing pairs
- Red-green
- Blue-yellow
- Black-white

what is trichromatic color vision?

- human vision is trichromatic
- the three detection systems are the three classes of photoreceptor cells (represent the three additive primary colors)
- blue or short: (S) sensitive
- Green or Middle: (M) sensitive
- Red or Long: (L) sensitive

how do the peaks of absornance of the three classes of photoreceptors differ?

- The peaks of absorbance vary of three cone types, but their absorbance spectra overlap considerably

how do cones react to energy?

- A combination of cone types will react more or less strongly to a given light stimulus

- Response of a cone cell is the same no matter what the energy is of the photon that it captures

- Only the efficiency of a photon capture varies with photon energy

- The dynamic output relates only to the rate of photon capture

what is the wavelength of the visible spectrum?

- 400-700nm

what is the peak of the photopic curve?

- 555nm

what is the peak of the scotopic curve?

- 505nm

who is mainly affected by red-green color deficiency?

- up to 10% of males in the US are affected
- dichromats are most commonly affected

what is the most common type of color vision defect?

- deuteranomaly (red-green)

what is tritanopia?

- loss of the S-sensitive wavelength gene (blue)

what is deuteranopia?

- loss of an M-sensitive wavelength gene, causing red-green defect

what is protanopia?

- loss of an L-sensitive wavelength gene, causes red-green defect

what are dichromats?

- missing one of three photopigments

what are deuteranopes?

- missing chlorolable (CD)

what are protonopes?

- missing erytholabe (PE)

What are tritanopes?

- missing cyanolabe (CT)

what happens when someone has missing photopigment?

- missing photopigment is replaced by the other available

what are anamolous trichromats?

- have photopigments but absorption spectrum of one of the pigments is displaced

what are congenital anomalies of color vision?

- inherited-non progressive nature

what are acquired anomalies of color vision?

- not inherited- secondary to disease (yellow-blue)

what is achromatopia?

- The absence of multiple cone types
- A severe loss in visual acuity results

what is the ishihara test?

- a collection of pseudoisochromatic plates is the most widely used diagnostic tool

what is used for precise follow up of ishihara test?

- anomaloscope

what can cause acquired color vision defects?

- Disease or exposure to drugs or toxins can also affect color perception
- Acquired disturbances of color vision are highly variable
- The clinical nomenclature tends to be somewhat confusing

what are x chrome contact lenses?

- contact lens to be worn by one eye that can help overcome
- reddish brown tint
- patients may suppress that eye

What is Kollner's rule?

- congential is red green defects, acquired is blue yellow defects
- WRONG

how is color vision seen as a biomarker of disease?

- Loss of color vision is a major complaint in rapidly changing disorders
- Color vision is also a biomarker of slow progressing diseases even though patients are unaware of color vision change

what percentage of cones are red? green? blue?

Red (64%)
Green (32%)
Blue (2%)

Are there more rods or cones?

- rods
- 100x fewer cones cells than rods

do changes in retinal cells and visual pathway or peripheral and visual acuity have greater impact on color vision?

- changes in retinal and visual pathway

where does color perception arise at?

- color perception (RGB) arises at the photoreceptor level

where does opponent color processing arise in?

- Opponent color processing arises in inner nuclear layer via horizontal and bipolar cells and continues at retinal ganglion cells
- Damage to any part of the retina or visual pathway should affect color vision

what is the strongest biomarker in detecting diabetes?

- Prediabetic group had measurable functional changes before diabetes

- Color vision is the strongest biomarker

- 70% of pre-diabetes failed color vision

how does diabetes affect color vision?

- 50% of diabetics have blue color vision defect before clinically visible retinopathy
- Color vision is the strongest biomarker
- Color vision loss associated with the severity of diabetic retinopathy and macular edema

what can we do for a patient with diabetes and DR?

Monitoring vision
Detection of progression
Health initiatives
Weight management
Exercise
Nutritional changes
Nutritional supplements
Medical management when needed

what is rabin cone test?

- Based in science
- Co-developed between Innova Systems and US Air Force
- Combines Cone Isolation technology and Contrast Sensitivity
- Color vision technology sensitive enough to detect subtle changes from disease

how does rabin cone compare to visual field test?

- threshold test, similar to visual field but just faster

what does rabin cone test measure?

- Symmetric versus asymmetric defects: Symmetric can be congenital

- One cone function versus multiple cone types: Multiple cone defects and asymmetric usually acquired…

- Blue cone only defects – acquired … usually… (rule of thumb)

- can detect progression in dry AMD within a 12 months.

These functional markers may be useful end points in future clinical trials that assess the effect of potential treatments for AMD

how does AMD affect color vision?

- reduces cone contrast

how would you treat a patient with AMD?

Early intervention initiatives
Nutritional changes
Nutritional supplements
Early and greater amount
Medical management when needed

how can contrast sensitivity testing help cataracts?

- Cone contrast testing can identify patients who "complain" of poor vision despite good visual acuity
- Cone contrast testing provides easy clinical quantification improvement after cataract surgery

how additional uses are there for Rabin Cone CCT?

- Pre-clinical detection of Plaquenil changes
- Pigment Epithelial detachment
- Medical management demands accurate diagnostic and monitoring equipment