Psychophysics of spatial frequency selectivity

What is the importance of fourier analysis for vision?
It is a technique that can breakdown any image, no matter how complex into a number of simpler sinusoidal gratings
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What does psychophysical evidence suggest?
The human visual system may itself actually perform a crude form of fourier analysis
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What is extracted?
Useful spatial information in visual scenes by encoding the variations in luminance that occur at each spatial frequency
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How does this effect the interpretation of the CSF?
Campbell and Robson suggested the CSF does not reflect the sensitivity of a single mechanism, but the combined activity of many independent mechanisms (called ‘filters’, ‘detectors’, or ‘channels’)
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What is each mechanism?
Selectively sensitive to only a narrow range of spatial frequencies in an image, behaves as a filter in that it only sees luminance variations
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What do some filters respond to?
The coarse variations in luminance in an image (low spatial frequencies), others to fine detail (higher spatial frequencies)
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Campbell and Robson
The CSF reflects the envelope of sensitivities of multiple filters, each maximally sensitive to a narrow range of spatial frequency information - Consequently the visual system uses these spatial frequency filter
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What does the visual system use these spatial frequencies for?
To perform a type of fourier analysis on the retinal image
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How do low spatial frequencies respond?
They encode coarse luminance variations in the world
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How do high spatial frequencies respond?
filters respond to the fine spatial structure of the world
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What did they base their claim on?
evidence from psychophysical experiments measuring contrast detection thresholds for complex patterns
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In one experiment, What is claimed?
In one experiment they measured contrast detection thresholds for simple sinusoidal gratings of a given frequency and compared these to thresholds for detecting a square wave grating of periodicity
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What is square wave grating?
- It can be made up (Fourier synthesised) by summing a set of sinusoidal gratings of increasing frequency and decreasing contrast
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What is the first models of the CSF model?
Single mechanism model?
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What is the second mechanism model?
: The overall contrast of the pattern is the cruicial factor determining detection
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What is the multiple filter model?
Patterns are detected on the basis of the outputs of varous parallel, spatial frequency filters. Detection is possible when the most sensitive filter exceeds its own threshold
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What should fourier analysis show?
square-wave grating of frequency (ƒ) also contains a sinusoidal grating of frequency (ƒ) which is 1.27 higher in contrast than the overall pattern
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What should a square wave grating require?
1.27 times less overall pattern contrast to be detected than a sinusoidal grating of the same spatial frequency presented in isolation
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What did Campbell and Robson find?
Detection thresholds for square wave gratings were not the same as sinusoidal gratings Thresholds were typically 1.27 times lower for square wave gratings contrast sensitivity was 1.27 times higher than for sinusoidal gratings
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What did visual system appear to be?
breaking down the square-wave grating into its separate sinusoidal spatial frequency components using multiple filters (cf Fourier analysis)
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What did they find out about threshold sinusoidal gratings and square wave grating?
They were perceptually indiscriminable (looked the same)
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What does prolonged viewing of a high contrast pattern lead to?
subsequently viewed patterns harder to see
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What does the single mechanism model predict?
CSF reflects the sensitivity of a single detection mechanism so adaptation to any grating should lower the CSF uniformly - There will be a drop in contrast sensitivity at all spatial frequencies
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What does the multiple filter model suggest?
Adaptation will be spatial frequency selective and will only desensitise filters that respond to the adaptation pattern - Adpatations should produce a loss in sensitivity that is limited to a narrow range of spatial frequencies centred on adaption
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Blackmore and Campbell find?
• Adaptation is spatial frequency selective. It diminished sensitivity (elevated contrast detection thresholds) to that spatial frequency and neighbouring ones, leaving very different frequencies unaffected
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What does this provide?
This provides compelling evidence that the CSF reflects the existence of multiple spatial frequency selective filters in human vision
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What happens at contrast levels above the threshold?
• Adaptation to a sinusoidal grating, changes the perceived spatial frequency of other subsequently viewed test gratings (Blakemore & Sutton, 1969; Blakemore et al., 1970)
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What does test gratings of a lower spatial frequency than the adaptation pattern?
IT appears to be lower still after adaptation,
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What happens to higher spatial frequency test gratings?
shifted still higher in spatial frequency after adaptation
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When does masking occur?
When two patterns have similar spatial frequencies but not dissimilar ones
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What is the subthreshold summation?
If 2 patterns activate the same filter then the 2 patterns combined should be more detectable than either pattern alone
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When does summation occur?
only between gratings of closely spaced spatial frequencies and not between widely space ones (Sachs et al., 1971)
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How many filters are there?
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How selective are they?
? Estimates of the ratio (bandwidth) of the highest to lowest frequency they each respond to range from ~ 1.5 to 5, but on average about 2 (1 octave
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Other cards in this set

Card 2


What does psychophysical evidence suggest?


The human visual system may itself actually perform a crude form of fourier analysis

Card 3


What is extracted?


Preview of the front of card 3

Card 4


How does this effect the interpretation of the CSF?


Preview of the front of card 4

Card 5


What is each mechanism?


Preview of the front of card 5
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