Spatial Vision and Pattern Perception

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Spatial Vision and Pattern Perception
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Structuralist tradition - perceptions are
created by combining fundamental components.
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Wilhelm Wundt (1832-1920)
Edward Titchener (1867-1927)
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Scenes contain multiple spatial scales
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A contemporary approach to form perception and
processing
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A Sine Wave Grating
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Properties of sine wave gratings

• Spatial frequency- the number of pairs of bars
  imaged within a given distance on the retina.
• Contrast - related to the intensity difference
  between the light and dark bars of the grating
• Orientation - the axis of the gratings bars
• Phase - grating's position relative to some
  landmark

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(4.7.1 - 4.7.5)
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Contrast variation in sine wave
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Gratings at 3 orientations
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Phase shifts
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Fourier Analysis
According to Fourier analysis any signal can be
decomposed into its Fourier components - that is,
it can be represented as a sum of a series of
simple sine waves of appropriate frequencies,
phases and amplitudes. (in the case of spatial
patterns we also consider the orientation of the
component patterns).
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(4.7.6)
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Transfer Function of a Lens
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Contrast Sensitivity Function
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Eccentricity
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Luminance
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Species
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High Frequencies
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Medium Frequencies
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Low Frequencies
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Multichannel model of spatial vision
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Some psychophysical evidence for the multichannel
model
Selective adaptation studies
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Basic logic of selective adaptation studies 1.
Measure visual performance 2. Adapt observer 3.
Measure visual performance If results of 3
different from 1, then the adaptation has
affected the mechanism(s) underlying visual
performance
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1. Measure contrast sensitivity 2. Adapt to a
certain spatial frequency at high
contrast 3. Measure contrast sensitivity
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Pre-adaptation CSF
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Adapt to a fixed spatial frequency e.g. 4 cpd
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The Gestalt Approach
Max Wertheimer (1880-1943)
Kurt Koffka (1886-1941)
Wolfgang Kohler (1887-1967)
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(2.2)
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Discontinuities used by the visual system in
segmenting a scene into two or more regions
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(2.1.5)
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Law of closure - in figures with gaps we tend to
see a complete form
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Law of common fate - objects moving in the same
direction are seen as a unit.
(2.1.4)
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The Gestalt theorists drew attention to important
aspects of perceptual organization - how our
visual system organizes the input. This is a
lasting contribution. They did not have,
however,a good theory of the neural underpinnings
of these organizational principles.