L4 Analysing Images & Human Contrast Sensitivity Flashcards
What type of information do visual images contain ?
Wavelength and luminance vary
Black and white images contain spatial variations in luminance that carry the information
How can luminance spatial scales change ?
Slow smooth changes in luminance reveal large objects in an overall shape
Abrupt sharp changes in luminance reveal the fine spatial structure of the world and detail
What is the task faced by the visual system ?
When we view a visual scene, an image of the world is formed upside down and back to front
Task is to extract information about the environment from the patterns of light
How can we represent the information in visual images ?
Measure the luminance in each spatial location (this approach is taken by cameras)
Break down the image into basic components (lines, blobs, corners etc)
Sinusoidal gratings
What are sinusoidal gratings ?
Simple 1 dimensional, periodic patterns in which luminance varies across space
Luminance varies across x but constant over y (vertical stripy pattern)
What are the defining characteristics of the sinusoidal gratings ?
Spatial frequency - spatial scale of the luminance
Contrast - intensity difference between the light and dark bars
Orientation - axis of grating bars
Spatial phase - relative position of the bars
Any pattern can have any combination of these four characteristics
Why are grating patterns so useful ?
It can be shown that it is possible to make any image, no matter how complex
This process is called fourier synthesis
You can decompress any image into sinusoidal gratings
E.g. DeValois and DeValois, 1990 recreated Einstein’s face
What relevance does sinusoidal gratings have for vision ?
Provide a universal language to precisely describe input into the visual system
Measure the visual systems response
Predict how it will respond to complex images
How do you measure the MTF of the whole system ?
Contrast is measured by the difference in luminance between light and dark sections
If vision is poor at transferring other spatial frequencies, we would require a high contrast to see them
Test contrast detection thresholds for a set of grating patterns
The result of MTF is known as contrast sensitivity function (CSF)
What does the CSF look life ?
It is a curve that shows contrast sensitivity
We are less sensitive to low levels and high spatial frequency gratings
It defines the window of visibility
Under the curve patterns are visible, above the curve patterns are invisible
What can we predict from the shape of a CSF ?
We can extrapolate from this to predict performance for more complex images
Why is there a fall off sensitivity for low SFs ?
Optical imperfections of the eye
The insensitivity to low spatial frequency gratings must be due to neural factors in the visual system
E.g. Retinal ganglion cells have receptive fields that exhibit a concentric centre-surround organisation
What does CSF look like at different light levels ?
As luminance levels decrease, the peak sensitivity shifts to gratings of lower and lower spatial frequency
At night our ability to see fine detail is lost
What happens to the CFS when gratings flicker or move ?
Sensitivity to low spatial frequencies improves when the temporal frequency is high
What are the behavioural effects of lesions to cells in the LGN ?
Cause loss in sensitivity to rapidly flickering low spatial frequency patterns
What do standard visual acuity tests measure ?
The finest spatial detail that an observer can discern
Assess spatial vision, using high contrasting sitimuli
How are the CSF and standard acuity measure related ?
Most visual acuity tests measure the smallest spatial detail, the size alone is the only factor
CSF is interested in both the size and the contrast limit
What are the advantages of using visual acuity tests ?
Acuity tests are quick and provide a rapid rate of assessment (CSF is time consuming and requires specialised equipment)
Acuity is used in clinical settings because many causes of loss in sensitivity to fine detail are optical in nature
What are the advantages of using CSF ?
Characterises sensitivity to spatial variations enabling scientists to predict the visibility of objects of known contrast in a complex scene
Ginsburg found that by using CSF he was able to test how well pilots could see objects in the air (acuity is a poor indicator for performance)
CSF also gets acuity anyways
