Analysing Images & Human Contrast Sensitivity Flashcards
Along what dimensions can reflected light vary?
Wavelength & luminance
What is wavelength used for?
Colour vision
What is luminance used for?
Brightness
What do slow/smooth changes in luminance reveal?
The coarse spatial structure of the world (eg. large objects/overall shape)
What do abrupt changes in luminance reveal?
The fine spatial structure of the world (eg. small objects/fine detail)
How can we represent the information in visual images? What are the name of these?
We could break the image down into more basic components (‘building blocks’). These are called sinusoidal gratings.
What are sinusoidal gratings?
Sinusoidal gratings are simple one-dimensional, periodic patterns in which luminance varies across space.
What runs along the X and Y axis in a sinusoidal grating?
X axis: Luminance varies according to a sinusoidal waveform
Y axis: Luminance is constant
What are the 4 defining characteristics of a sinusoidal grating? SCOS
- Spatial frequency (SF): Spatial scale of the luminance variation
- Contrast: Intensity difference between the light and dark bars (how ‘visible’ it is)
- Orientation: Axis of the grating’s bars
- Spatial phase: Relative position of the bars (e.g. does it ‘begin’ with a light or dark bar)
Why are grating patterns so useful?
It can be shown mathematically that it is possible to create any image, no matter how complex, from a set of sinusoidal grating patterns.
What is Fourier synthesis?
A number of sinusoidal gratings can build up any complex image.
What is the ‘modulation transfer function’ of the system/cell?
The extent to which each sinusoidal grating gets through (transmitted).
How do you measure the modulation transfer function of the whole system? What is this known as?
Measure the contrast detection thresholds for a set of grating patterns of various spatial frequencies (minimum contrast required to just ‘see’ each pattern).
The resulting MTF is known as the ‘Contrast Sensitivity Function’ (CSF) — This shows the entire visual system’s sensitivity to gratings.
What spatial frequencies are we most sensitive to?
Sensitivity greatest for gratings with spatial frequencies of 2 - 6 c/deg.
We are less sensitive to very ____ and very ____ spatial frequency gratings than to ____ ones.
We are less sensitive to very low and very high spatial frequency gratings than to intermediate ones.
Why does the CSF have a lack of sensitivity to very high spatial frequency gratings?
This is due mainly to optical imperfections of the eye which ‘blur’ the fine detail in an image.
Why does the CSF have a lack of sensitivity to very low spatial frequency gratings?
This must be due to neural factors in the visual system (the way it processes the information).
For example, retinal ganglion cells have receptive fields that have a center-surround organisation.
What is an on-center cell?
Light in the RF center excites the cell, but light in the surround inhibits the cell.
What are the 3 ranges of light throughout the day?
Photopic - daytime
Mesopic - dusk
Scoptic - near darkness
Why does our ability to see fine detail become worse as it gets darker?
The peak sensitivity shifts to gratings of lower and lower spatial frequency.
This is because our sensitivity to high spatial frequency gratings becomes worse.
What happens to the CSF when gratings flicker/move? Why is this?
When the CSF is measured with moving/flickering gratings, sensitivity to very low spatial frequencies improves when the temporal frequency is high.
This is thought to be because Magnocellular cells are 10 times more sensitive to low spatial frequency gratings than Parvocellular cells when patterns move/flicker at high rates.
Lesions in what type of cells can cause loss in sensitivity to rapidly moving/flickering low spatial frequency patterns?
Magnocellular cells.
What do standard visual acuity tests measure?
Visual acuity - the finest spatial detail that an observer can discern
What are the advantages of using visual acuity tests?
- Quick to perform and from a provide rapid assessment of spatial visual function
What are the advantages of measuring the CSF?
- Enables scientists to predict the visibility of objects of known contrast in any complex scene.
Ginsburg et al. (1982) used the CSF to successfully predict how well pilots see objects in the air and on the ground. Under conditions when fine detail is lost (e.g. fog), visual acuity is a poor indicator of performance.
Some patients suffering from Alzheimer’s disease (Nissen et al., 1985), or cataracts (Hess & Woo, 1978), show substantial contrast sensitivity deficits for both low frequency and high frequency patterns. Visual acuity tests would not pick this up.
- By measuring the entire CSF, we also get a measure of visual acuity anyway
