216 #1

Question 1

Measure of visual function

Answer 1

Threshold measurement.
Subjective vs Objective
Classification vs adjustment.
- Mostly do subjective classification

Question 2

Contrast sensitivity

Answer 2

Least change in brightness

Question 3

Temporal sensitivity

Answer 3

Perception of flicker

Question 4

Visual field

Answer 4

Extent of vision

Question 5

Colou vision

Answer 5

Discrimination of colour

Question 6

(spatial) visual acuity

Answer 6

Smallest visible element.

Question 7

Three different ways you can measure spatial visual acuity

Answer 7

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).

Question 8

Localisation

Answer 8

Relates to hyperacuity. Smaller threshold than recognition. Also called Vernier acuity.
Cortically limited.

Question 9

Visual acuity should be

Answer 9

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.

Question 10

Visual acuity is quick to measure

Answer 10

and requires low skills.
VA is extremely variable in how it is measured.
Does not capture all aspects of vision.
Used medico-legally.

Question 11

History of VA measurement. “early”.

Answer 11

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.

Question 12

History of VA measurement. 1843

Answer 12

Standardised vision test types were invented by Heinrich Kuechler (German).
Initially has symbolic eye chart made from various objects cut from calendars and newspapers.

Question 13

History of VA measurement. 1854

Answer 13

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.

Question 14

History of VA measurement. 1862

Answer 14

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.

Question 15

Optotype

Answer 15

5 by 5 grid. Snellen has a massive E at the top.

Question 16

History of VA measurement. 1888

Answer 16

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

Question 17

History of VA measurement. 1970s - 1980s

Answer 17

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.

Question 18

Limits of Spatial Visual Acuity

Answer 18

Movement
Glare
Aberrations
Illumination
Adaptation State
Pupil Size
Glare

Question 19

Aberration

Answer 19

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.

Question 20

Lower order aberration

Answer 20

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.

Question 21

Higher order aberrations

Answer 21

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

Question 22

Eccentricity

Answer 22

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

Question 23

Fovea

Answer 23

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.

Question 24

Neural convergence.

Answer 24

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).

Question 25

myopia

emmetropes

Answer 25

Myopia = short sighted.
Emmetropes = no refractive error.

Question 26

Luminance

Answer 26

When brighter you have better VA. Brighter also pupils shrink which takes away all the aberrations thus also giving better VA.

Question 27

MAR

Answer 27

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.

Question 28

Point source through aperture

Answer 28

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.

Question 29

Optotype

Answer 29

Ideally each optotype has equal recognisability.
- Same details.
- Same ink
- Same complexity
Under all levels of blur
- Refractive blur is hard to stimulate.

Question 30

Optypes

Answer 30

Figure or letters of different sizes.