Visual Acuity and Adaptation

Question 1

What are the subdivisions of VA?

Answer 1

1. Presence of a single feature or ‘minimum visible’
2. Features within a visible target ‘minimum resolvable’
3. Relative location of visible features or hyperacuity (Vernier’s acuity) minimal disciminable angle

Question 2

What is visual acuity

Answer 2

Measure of the spatial limits of vision

Question 3

What does visual acuity measure

Answer 3

The ability to discirminate two objects in space subtended by an angle of 1 minute of arc at the nodal point of the eye - the minimum angle of resolution

Question 4

What is the Snellen chart

Answer 4

Optotypes (letters) read at 6m or 20 ft

Question 4

How dos logmar chart differ from Snellen?

Answer 4

Definitive sizing an spacing with same number of letters on each line and line spacing is proportional to size of letters - this overcoms crowding

Read from 4m

Question 4

What are objective assessments of visual acuity?

Answer 4

Visual evoked potential and oculomotor responses

Pattern VEP testing - alternatign checkerbord patters of same luminance

Preferential looking in infacts - sinusoidal grating

Question 5

What optical factors affect visual acuity?

Answer 5

Pupil size - optimum is 2.8mm - compromise between reduction in retinal illuminationa nd decrease in refractive error and the increase in diffraction

Diffraction of light at an aperture causes spread of light from a point source - bright airy disk and light and dark concentric rings

Spherical aberration increases with pupil size

Question 5

What anatomical factors affect VA

Answer 5

Refractive error -
Retinal eccentricity - centre of fovea has maximal acuity - this falls off wtih distance form the fovea - better temporally than nasally

Question 5

What physiological factors affect VA?

Answer 5

Crowding phenomena - when targets are too close together VA suffers

Acuity falls with reduction in stimulus duration
Target and eye movements - movement of the retinal image or saccadic eye movements cause reduction in visual acuity

Ageing - increased intraocular scatter of light in ageing eye

Development - several months after birth for full development of pursuit eye movement

Question 6

How is contrast sensitivity measured?

Answer 6

Pt recognises dark and light stripes for grating for which the spatial frequency and contrast is varied.

Question 7

What factors affect contrast sensitivity?

Answer 7

Direction of grating lines
Luminance
Bar width and length
Grating motion

Question 8

How does contrast sensitivity change with spatial frequencies?

Answer 8

Falls off at higher and lower spatial frequencies

Pelli robinson chart

Question 9

How does light adaptation occur? What wavelegth is the light adapted eye maximally sensitve?

Answer 9

Works very quickly
Increases as background light increases
Dependent on calcium ion flux

Maximally sensitive at 555nm

Intensisty required for detection (activation of cone) increases as background intensity increases – Weber FEchner relation

Question 10

How does dark adaptation occur?

Answer 10

Musch slower than light adaptation

Dark adapted eye is maximally sensitve at about 505 nm

Question 11

Describe the dark adaptation curve

Answer 11

Biphasic

Early rapid response resulting from increase in cone sensitivity

Second slower response produced by increased rod sensitivity

Question 12

How does dark adaptation occur molcular level?

Answer 12

Amount of rhodopsin bleached and state of dark adaptation

Dark adaptation and regeneration of rhodopsin are dependent of local concentration of 11-cis retinal

Limiting factor for recovery after a bleach is the rate at which it is delivered to opsin in the bleached photoreceptors