Analysing Images & Human Contrast Sensitivity

Question 1

Along what dimensions can reflected light vary?

Answer 1

Wavelength & luminance

Question 2

What is wavelength used for?

Answer 2

Colour vision

Question 3

What is luminance used for?

Answer 3

Brightness

Question 4

What do slow/smooth changes in luminance reveal?

Answer 4

The coarse spatial structure of the world (eg. large objects/overall shape)

Question 5

What do abrupt changes in luminance reveal?

Answer 5

The fine spatial structure of the world (eg. small objects/fine detail)

Question 6

How can we represent the information in visual images? What are the name of these?

Answer 6

We could break the image down into more basic components (‘building blocks’). These are called sinusoidal gratings.

Question 7

What are sinusoidal gratings?

Answer 7

Sinusoidal gratings are simple one-dimensional, periodic patterns in which luminance varies across space.

Question 8

What runs along the X and Y axis in a sinusoidal grating?

Answer 8

X axis: Luminance varies according to a sinusoidal waveform
Y axis: Luminance is constant

Question 9

What are the 4 defining characteristics of a sinusoidal grating? SCOS

Answer 9

1. Spatial frequency (SF): Spatial scale of the luminance variation
2. Contrast: Intensity difference between the light and dark bars (how ‘visible’ it is)
3. Orientation: Axis of the grating’s bars
4. Spatial phase: Relative position of the bars (e.g. does it ‘begin’ with a light or dark bar)

Question 10

Why are grating patterns so useful?

Answer 10

It can be shown mathematically that it is possible to create any image, no matter how complex, from a set of sinusoidal grating patterns.

Question 11

What is Fourier synthesis?

Answer 11

A number of sinusoidal gratings can build up any complex image.

Question 12

What is the ‘modulation transfer function’ of the system/cell?

Answer 12

The extent to which each sinusoidal grating gets through (transmitted).

Question 13

How do you measure the modulation transfer function of the whole system? What is this known as?

Answer 13

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.

Question 14

What spatial frequencies are we most sensitive to?

Answer 14

Sensitivity greatest for gratings with spatial frequencies of 2 - 6 c/deg.

Question 15

We are less sensitive to very ____ and very ____ spatial frequency gratings than to ____ ones.

Answer 15

We are less sensitive to very low and very high spatial frequency gratings than to intermediate ones.

Question 16

Why does the CSF have a lack of sensitivity to very high spatial frequency gratings?

Answer 16

This is due mainly to optical imperfections of the eye which ‘blur’ the fine detail in an image.

Question 17

Why does the CSF have a lack of sensitivity to very low spatial frequency gratings?

Answer 17

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.

Question 18

What is an on-center cell?

Answer 18

Light in the RF center excites the cell, but light in the surround inhibits the cell.

Question 19

What are the 3 ranges of light throughout the day?

Answer 19

Photopic - daytime
Mesopic - dusk
Scoptic - near darkness

Question 20

Why does our ability to see fine detail become worse as it gets darker?

Answer 20

The peak sensitivity shifts to gratings of lower and lower spatial frequency.
This is because our sensitivity to high spatial frequency gratings becomes worse.

Question 21

What happens to the CSF when gratings flicker/move? Why is this?

Answer 21

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.

Question 22

Lesions in what type of cells can cause loss in sensitivity to rapidly moving/flickering low spatial frequency patterns?

Answer 22

Magnocellular cells.

Question 23

What do standard visual acuity tests measure?

Answer 23

Visual acuity - the finest spatial detail that an observer can discern

Question 24

What are the advantages of using visual acuity tests?

Answer 24

• Quick to perform and from a provide rapid assessment of spatial visual function

Question 25

What are the advantages of measuring the CSF?

Answer 25

• 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