analysing images and human contrast sensitivity Flashcards
what happens when we view a visual scene?
- an image of the world is formed on the retina at the back of each eye (both upside-down and back to front)
- Visual system extracts useful information about the environment from the patterns of light within the retinal image
what types of information do visual images contain?
Wavelength (used for colour vision) and luminance (intensity) which determines brightness
what do slow (smooth) changes in luminance do?
reveal the coarse spatial structure of the world (e.g. large objects, overall shape)
what do abrupt (sharp) changes in luminance do?
reveal the fine spatial structure of the world (e.g. small objects, fine detail)
How Can We Represent the Information in Visual Images?
- measure luminance at each spatial location in the image
- break the image down into more basic components (‘building blocks’)
what are sinusoidal gratings?
simple 1-dimensional, periodic patterns in which luminance varies across space
- Luminance varies across the pattern (x) according to
a sinusoidal waveform
- In the other direction (y) luminance is constant
what are the 4 defining characteristics of sinusoidal gratings?
- Spatial frequency (SF): Spatial scale of the luminance variation (number of cycles in 1 degree of visual angle, because the width of the bars changes with viewing distance)
- 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)
A single grating pattern can have any combination of these 4 characteristics
what is ‘Fourier synthesis’?
- it’s possible to create any image, no matter how complex, from a set of sinusoidal grating patterns
- need to find out SF, , contrast, orientation and phase are needed for each sinusoidal grating and then add them all together
- Low SF gratings contribute coarse spatial information to the resulting image (e.g. large objects, overall shape) and high SFs contribute the fine detail (e.g. smaller objects)
images intensity at each
spatial location (x,y) → sinusoidal gratings: spatial frequency, contra, orientation and phase
opposite = Fourier analysis
what is ‘modulation transfer function’ (MTF)?
measuring the visual system’s response to simple sinusoidal grating patterns (e.g. contrast detection thresholds, cell firing rate)
- the extent to which each grating through (transmitted)
- can the predict how it will respond to more complex images, because all images can be made from a sum of simple sinusoidal gratings
- have to assume is that the visual system obeys some simple rules
how do you measure the MTF of the whole system?
- if our visual system does a good job of transferring some spatial frequencies, we should need very little contrast to see gratings of those spatial frequencies (we would be relatively ‘sensitive’ to these gratings)
- If vision is poor at transferring other spatial frequencies, we should be relatively ‘insensitive’ to them and require a high contrast to see them
- So measure psychophysically, contrast detection thresholds for a set of grating patterns of various spatial frequencies
- resulting MTF is known as the ‘Contrast Sensitivity Function’ (CSF) — This shows the entire visual system’s sensitivity to gratings
What Can We Predict From the Shape of the CSF?
predict performance for more complex images (as they can be represented as a set of simple grating patterns
What Does the CSF Look Like?
-inverted ‘U’
less sensitive to very low and very high spatial gratings than to intermediate ones
- sensitivity greatest for gratings with spatial frequencies of between 2-6 c/deg
- The VSF defines the visual system’s ‘window of visibility’
- under curve = visible
- above curve =invisible
Why is There a Fall-Off in Sensitivity for Low SFs?
- Optical imperfections of the eye only degrade high spatial frequency information
- The insensitivity to very low spatial frequency gratings must be due to neural factors in the visual system (the way it processes the information)
What Does the CSF Look Like at Different Light Levels?
luminance level is reduced from the photopic range (cf daytime), to the mesopic range (cf dusk) and scoptic range (cf near darkness)
What Happens to the CSF When Gratings Flicker or Move?
- sinusodial gratings can have a temporal frequency (TF)i ndicates how rapidly they move or flicker (measured in c/sec or Hz)
- When the CSF is measured with moving/flickering gratings (Van Nes et al., 1967; Robson, 1966; Kelly, 1979) sensitivity to very low spatial frequencies improves when the temporal frequency is high (e.g. 10 Hz)
what are the behavioural Effects of Lesions to Cells in the LGN?
loss in sensitivity to rapidly moving flickering low spatial frequency patterns (Merigan et al., 1991a, b)
What Do Standard Visual Acuity Tests Measure?
- measurement of visual accuracy
- optometrists and opthalmologists generally use acuity measures to assess spatial vision (‘Eye Test’), using very high contrast stimuli
How are the CSF and Standard Acuity Measures Related?
- Most visual acuity tests measure only the smallest spatial details that can be resolved by the visual system. Contrast is fixed and maximal, so that size alone is the factor that limits performance
- When we measure the CSF, we are interested in how both size (spatial frequency) and contrast limit vision
What are the Advantages of Using Visual Acuity Tests?
- quick to administer and form a practical point of view
- provide rapid assessment of spatial visual function
What are the Advantages of Measuring the CSF?
- sensitive to spatial variations in luminance over the entire range of spatial scales
- enables scientists to predict the visibility of objects of known contrast in any complex scene
- get a measure of visual acuity anyway
- predict how well pilots see objects in the air and on the ground. Under conditions when fine detail is lost (e.g. fog)
- Some patients suffering from Alzheimer’s disease, or cataracts, exhibit substantial contrast sensitivity deficits for both coarse (low frequency) and fine (high frequency) patterns. Visual acuity tests would not pick this up
