Visual acuity and contrast sensitivity Flashcards
Define visual acuity:
Measure of the ability to discriminate two stimuli separated in space
Which photoreceptors are predominantly involved in visual acuity?
cones
What factors affect visual acuity?
- Luminance of test object
2. Degree of adaption of observer (i.e. light or dark adaptation)
What is vernier acuity?
The smallest amount of misalignment that can be detected
At what level, does vernier acuity take place?
At the level of the retinal synaptic organisation
Is vernier acuity present in infancy?
No
When does Vernier acuity become fully developed?
Age 14
Give examples of Vernier acuity being used in everyday life:
- Using a ruler to measure
2. Reading a mechanical clock
When is Vernier acuity absent?
Strabismic amblyopia
It is assumed that the average person can resolve two points seperated by ‘ X’ degree of arc.
1
What is the ideal ration of photoreceptor and nerve cell to achieve optimum visual acuity?
1:1
Where in the retina is the optimum 1:1 ration between photoreceptor and nerve cell for optimum visual acuity?
Fovea
Why is spatial summation not present in the fovea?
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
What ‘behaviours of light’ limit visual acuity?
- Diffraction of light as it passes through the pupil to stimulate more than one cone
- Priasmatic separation of light into varying wavelengths will stimulate different types of cones
Most of the neurones in the LGN and visual cortex are:
Foveal ganglion cells
What effect does a smaller pupil size have on visual acuity?
Smaller pupil reduces aberration of light but increases diffraction
At what pupil size does the effects of aberration and diffraction in a small pupil size cancel each other out?
Below 3mm
What pupil size gives the best visual acuity?
2.5-3mm
What is the importance in continuous fine eye movement when viewing a stimulus?
Ensures that it maintains the constant stimulation of photoreceptors such that the visual image is mantained
What is the Troxlers phenomenon?
Image formed by peripheral retina rapidly fades when the eye is deliberately fixated in one position
