216 #1 Flashcards
Measure of visual function
Threshold measurement.
Subjective vs Objective
Classification vs adjustment.
- Mostly do subjective classification
Contrast sensitivity
Least change in brightness
Temporal sensitivity
Perception of flicker
Visual field
Extent of vision
Colou vision
Discrimination of colour
(spatial) visual acuity
Smallest visible element.
Three different ways you can measure spatial visual acuity
Detection - visible (Detection VA = 2AFC e.g. yes/no).
Recognition - resolvable (What is there? n-alternate forced choice).
Location - discriminable (Something is there, can be seen as above detection threshold but what angle? how many? Continuous vs offset etc).
Localisation
Relates to hyperacuity. Smaller threshold than recognition. Also called Vernier acuity.
Cortically limited.
Visual acuity should be
the first preliminary measurement that you make.
- Unaltered state and legal protection.
Unaltered state = haven’t done anything else with the eye just ask how well they see.
VA is the most commonly used proxy for visual function.
- Compare individual VA
- Monitor disease progression
- Assess treatment success.
Visual acuity is quick to measure
and requires low skills.
VA is extremely variable in how it is measured.
Does not capture all aspects of vision.
Used medico-legally.
History of VA measurement. “early”.
Used stars to screen vision e.g. Persian army using the optical double star in the Big Dipper as a test in 964 CE.
- In the Southern hemisphere the 7 sisters, Pleiades, or Matariki. 5 in non-dark adapted state was good.
History of VA measurement. 1843
Standardised vision test types were invented by Heinrich Kuechler (German).
Initially has symbolic eye chart made from various objects cut from calendars and newspapers.
History of VA measurement. 1854
Eduard JaACger Von Jaxtthal (Austria) designed a set of reading samples to document functional vision.
He used font and print sizes from a single printer, now known as the Jaeger numbers. J1 (small) to J16 (large).
- Acc has asked for nearing (reading) acuity in Jaeger form.
History of VA measurement. 1862
Hermann Snellen (Dutch) published the first visual chart based on ‘optotypes’ following Donders early work in quantifying vision.
- He argued the need for standardised vision test and used grid-based letter designs.
- They were printed in Egyptian paragon font with serifs.
- Made 3 charts to help prevent memorisation.
Optotype
5 by 5 grid. Snellen has a massive E at the top.
History of VA measurement. 1888
Edmund Landolt (Swiss) introduced the broken ring which is now known as the Landolt ring. Later became an international standard.
1600s-1800s many people couldn’t read. Didn’t need to be literate to understand this.
1978 - Hugh Taylor used these design principles for a “Tumbling E chart’ for illiterates
History of VA measurement. 1970s - 1980s
Rick Ferris combined the LogMAR chart layout with the Sloan letters to establish a standardised method of
VA measurement for the ETDRS (early treatment of Diabetic Retinopathy Study).
Data collected from the ETDRS were used to select letter combinations that gave each line the same average difficulty without using all the letters.
Limits of Spatial Visual Acuity
Movement Glare Aberrations Illumination Adaptation State Pupil Size Glare
Aberration
Most people that can’t see as well are due to aberration = the failure of rays to converge at one focus due to defect in the lens/mirror.
Lower order aberration
The most common limitation is aberration and the most common aberration is refractive error.
- Myopia and hyperopia.
Young people can accommodate to overcome hyperopia but not myopia.
- Astigmatism.
Error of the eye varies with orientation.
Rule : 1 line per -0.25DS/-0.50DC
Use DS for hyperopia, myopia and DC for astigmatism.
- Myopia is twice as powerful as astigmatism in creating blur so we are not as sensitive to astigmatism.
Higher order aberrations
Cannot be easily fixed with glasses. - Spherical aberration. - Oblique astigmatism. - Coma. - Chromatic aberration. Can correct them but they tend to change over a lifetime and higher order aberrations have less affect on vision. Ec
Eccentricity
When you are looking at something straight on you have 0 eccentricity.
Eccentricity = deviation from centre?
VA decreases rapidly with increasing retinal eccentricity.
1. Photoreceptor density.
2. Neural convergence.
3. Paraxial vs peripheral optics.
4. Eye shape
Fovea
Area of high sensitivity that looks directly at the object. Has the best spatial VA.
Go out, VA drops because fewer photoreceptors at the periphery.
Cone spacing at the fovea is approximately 2.5um.
Most cones at fovea and no rods. Cones are for high detail vision.
Neural convergence.
Another reason why VA decreases as you move further from fixation is neural convergence.
~ 126 million photoreceptors and 1 million ganglion cells (GC) so there is about 126 times more input than output so get a loss of resolution.
Average - 120rods/GC and 6cones/GC but 1 : 1 in the fovea. (1 photoreceptor per GC).
myopia
emmetropes
Myopia = short sighted. Emmetropes = no refractive error.
Luminance
When brighter you have better VA. Brighter also pupils shrink which takes away all the aberrations thus also giving better VA.
MAR
minimum angle of resolution.
- A point light source passing through an aperture (the pupil) creates a point spread function on the retina.
- The PSF will also be modified by other optical aberrations in the eye.
- Angular measurement is required to define the limits of what we see.
Point source through aperture
creates an airy disk (shape) on the retina so can’t have point source with systems with an aperture.
Width of this depends on wavelength. Certain colours blur more.
Optotype
Ideally each optotype has equal recognisability. - Same details. - Same ink - Same complexity Under all levels of blur - Refractive blur is hard to stimulate.
Optypes
Figure or letters of different sizes.
