Contrast Sensetivity Flashcards
What does form sense mean?
Distinguish the size and shape of objects from the position and orientation of there edges
What does contrast sensitivity mean
- the ability of the visual system to distinguish between an object and its background
- measure of functional vision
-diagnostic aid
-measured using sine wave gratings
Development of contrast sensitivity
-rapid development during first 6 months
- continues to development after 8 years
What is contrast sensitivity function CSF
It’s a curve and a way of assessing contrast
Factors affecting the shape of the normal CFS?
-Receptive field
- retinal area which a light stimulus can produce a potential difference single ganglion cell
- retinal receptive fields are circular (on-off response)
-excitatory bs inhibit
What does stimuli smaller than the centre receptive field produce
Only a partial response from the ganglion cell
What does stimuli larger than the centre receptive field stimulate
The surrounding area so that the overall response from ganglion cell is reduced
Info
CFS channel could be due to a series of ganglion cells that have receptive fields of different sizes so that they are maximally sensitive to different spatial frequencies
What do different neurons
Different receptor cells
As we increase spatial frequency we get an increased?
Response so more cycle
If we have a low spatial freq region then
Receptive field organisation affects it
If high spatial frequency?
Diffraction
Aberration
Scatter
Cone density
What does reduction in retinal luminance cause
A high frequency loss
What does reduction in retinal luminance cause
A high frequency loss
What else affects contrast sensitivity
-caterax
-light scatter- results in loss of luminance and loss in contrast
What else affects contrast sensitivity
-caterax
-light scatter- results in loss of luminance and loss in contrast
What factors affect contrast sensitivity
-luminance- natural light
-assessment- which method
-data collection- differences?
-age/ population- if child they may have not reached contrast threshold
-visual acuity/ refractive error
-ocular abnormalities
How can you assess contrast sensitivity
-low contrast Sloan chart
-vistech contrast chart
Pelli Robinson
Low contrast Sloan chart?
-Quantitative
-spatial contrast
Test distance- 3m
- age ability- 6+ knows letters confidently
Vistech contrast chart
-quantitive
- test distance- 2.5 m
-Age ability- any/ non verbal patients
Peli Robinson
-quantitative
-spatial contrast
-test distance- 1m
-age ability- 6+/knows letters confidently
Peli Robison? Description
Fixed spatial frequency, reducing contrast
Hiding Heidi- description
Fixed spatial frequency- reducing contrast
Vistech- description
Gratings test, reducing contrast across different spatial frequencies.
Lea low contrast
Reducing letter size, fixed low contrast
What cs tests has a fixed spatial frequency?
Low contrast ETDRS
