Orthoptics I - Introduction to Visual Function

What are the 3 main visual functions?

form, shape and motion

What are photopic light conditions?

normal and bright light
luminence is 10^8 CD/m2

What are scotopic light conditions?

dim light, after dark adaptation
luminence is 10^3-6 CD/m2

What are mesopic light conditions?

combination, such as a street light
luminence is 0.01-3 CD/m2

What is the dorsal stream of the visual pathway responsible for?

spatial awareness

What is the ventral stream of the visual pathway responsible for?

object identification

What anatomical feature does the majority of the light refraction?

cornea

What is meant to "degrees of visual angle"?

distance from the fovea

How many rod cells are in the retina?

about 10 million

Describe photoreceptor distribution in the retina

cones dense at the fovea, rods gradually replace the cones towards the periphery
- no rods are present at the fovea

What is the average distance between the lens and the retina?

17 mm

What VA is 1 minute of arc equivalent to?

6/6 or 20/20, 0.00 logMAR

How many degrees of arc is the field of view equivalent to?

0.5" of arc

Define visual acuity

the ability of the eye to discriminate fine detail

How do the eyes avoid photoreceptor fatigue?

constant small displacements of fixation

What is detection?

the ability to detect a target based on threshold vision and dark adaptometry

What is discrimination?

distinguishing an object from the background - sensitivity

What is recognition?

aka form vision
identificstion of a gap in Landolt C

What is resolution?

identification of gratings/checkerboards

What does vernier acuity measure?

hyperacuity - up to 10" of arc
identification of target displacement

Define contrast sensitivity

distinguishment between finer and finer increments of light versus dark

Symptoms of reduced contrast sensitivity

problems with night driving
tired eyes
increased falls in the older population

How do we test contrast sensitivity?

pelli-robson chart
- similar to logMAR with 5 letters per line
contrast decreases relevant to background
performed at 1m

Describe the order of cortical connections?

retina -> via optic nerve -> lateral geniculate nucleus -> via optic radiations -> occipital lobe