Week 13 - Physiology of visual acuity

What is visual acuity?

Finest detail that can be seen at high contrast

How many minutes of arc are in a degree?

60

How many minutes of arc is produced when reading a 6/6 line on a snellen chart

How can you read a letter on a Snellen Chart?

If you can view each limb

What is minimum angle of resolution?

Minutes of arc required to recognise the letter

What does 6/6 mean?

Viewing distance / distance each letter subtends 5 minutes of arc

Why is a 6/12 letter bigger?

At a distance of 12 meters, that is the size for 5 minutes of arc

What is a cycle?

Ability to resolve one black bar and one white bar

What is a cycle per degree?

How many cycles are able to fit in a degree

What are the 4 types of acuity?

Detection

Resolution

Recognition

Localisation

What is detection?

Ability to see something (whether it is there or not)

What is the angle for types of acuity?

0.5-1.5 seconds per arc

How many seconds are in a minute of arc?

60

What is resolution acuity?

Smallest angle at which subjects can distinguish between critical elements

How is resolution acuity measured?

Landolts C or tumbling Es where letters are facing different directions

What is recognition acuity?

Subject must be able to detect, resolve and identify the target

E.g. Snellen

What type of relationship is between Snellen and MAR?

Linear relationship when both are logged aka logMAR

What is a bailey lovie chart?

Gives visual acuity in logMAR

Each letter is 0.02

Viewed at 6m

What is vernier acuity?

Localisation acuity

Small offset between 2 targets

What is Helmholtz sampling technique

Need at least one unstimulated neuron between 2 stimulated neurons to resolve

What would be required to get 2 different signals to the brain?

2 Different ganglion cells

What happens when we try to resolve 2 spots of light?

Some overlap, must be separate enough so peaks do not overlap

Where can we see the sharpest detail?

Centre of vision / fovea

What type of arrangement has good acuity?

Dense arrangement of receptive fields

What is the relation between cycles of degree and visual acuity?

Higher cycles of degree the better the visual acuity

Does acuity increase or decrease from rod mediated vision to cone mediated vision

Increase, as smaller receptive field in cone photoreceptors

Where is the peak VA?

5 degrees off the fovea

Is minimum angle of resolution greater with letter recognition than grating acuity?

Letter recognition

More difficult to detect, lower visual acuity

How does letter spacing affect visual acuity?

Crowding effect

Cramming them closer diminishes ability to read them

What is self crowding?

When a letter crowds itself, e.g. a W

How do smaller pupils limit VA?

Diffraction entering and then spreading

How do larger pupils decrease visual acuity?

Optical aberrations

What is the optimal pupil size

3-5mm