Visual acuity and contrast sensitivity

Question 1

Define visual acuity:

Answer 1

Measure of the ability to discriminate two stimuli separated in space

Question 2

Which photoreceptors are predominantly involved in visual acuity?

Answer 2

cones

Question 3

What factors affect visual acuity?

Answer 3

1. Luminance of test object

2. Degree of adaption of observer (i.e. light or dark adaptation)

Question 4

What is vernier acuity?

Answer 4

The smallest amount of misalignment that can be detected

Question 5

At what level, does vernier acuity take place?

Answer 5

At the level of the retinal synaptic organisation

Question 6

Is vernier acuity present in infancy?

Answer 6

No

Question 7

When does Vernier acuity become fully developed?

Answer 7

Age 14

Question 8

Give examples of Vernier acuity being used in everyday life:

Answer 8

1. Using a ruler to measure

2. Reading a mechanical clock

Question 9

When is Vernier acuity absent?

Answer 9

Strabismic amblyopia

Question 10

It is assumed that the average person can resolve two points seperated by ‘ X’ degree of arc.

Answer 10

1

Question 11

What is the ideal ration of photoreceptor and nerve cell to achieve optimum visual acuity?

Answer 11

1:1

Question 12

Where in the retina is the optimum 1:1 ration between photoreceptor and nerve cell for optimum visual acuity?

Answer 12

Fovea

Question 13

Why is spatial summation not present in the fovea?

Answer 13

Spatial summation is when a ganglion cell has a wide receptive area because it has connections with multiple bipolar cells and photoreceptors. However, in the fovea, there are 1:1:1 connections between cone photoreceptors: midget bipolar cell: midget ganglion cell to allow for maximum visual acuity

Question 14

What ‘behaviours of light’ limit visual acuity?

Answer 14

1. Diffraction of light as it passes through the pupil to stimulate more than one cone
2. Priasmatic separation of light into varying wavelengths will stimulate different types of cones

Question 15

Most of the neurones in the LGN and visual cortex are:

Answer 15

Foveal ganglion cells

Question 16

What effect does a smaller pupil size have on visual acuity?

Answer 16

Smaller pupil reduces aberration of light but increases diffraction

Question 17

At what pupil size does the effects of aberration and diffraction in a small pupil size cancel each other out?

Answer 17

Below 3mm

Question 18

What pupil size gives the best visual acuity?

Answer 18

2.5-3mm

Question 19

What is the importance in continuous fine eye movement when viewing a stimulus?

Answer 19

Ensures that it maintains the constant stimulation of photoreceptors such that the visual image is mantained

Question 20

What is the Troxlers phenomenon?

Answer 20

Image formed by peripheral retina rapidly fades when the eye is deliberately fixated in one position