PSYC2300 Chapter Notes - Chapter 3: Brodmann Area 20, Posterior Parietal Cortex, Temporal Lobe

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Chapter 3: Object and Face Recognition
Pattern Recognition
We spend most of our time (eg. When reading)
involved in pattern recognition, which is the
identification or categorisation of two-dimensional
patterns.
Patterns can be regarded as consisting as a set of
specific features or attributes, for example; the key
features of A are two straight lines and connected
cross-bar. This feature-based approach has the
advantage that visual stimuli varying greatly in size,
orientation and minor details can be identified as
instances of the same pattern.
Pattern recognition theories assume that pattern
recognition involves processing of specific features
followed by more global or general processing to
integrate information from the features. Global
processing often precedes more specific
processing.
Navon (1977) conducted experiments, where
observers decided whether the large letter was an
H or S, on other trials they decided whether the
small letters were Hs or Ss (Fig. 3.1). He found
performance speed with the small letters was
greatly slowed when the large letter differed from
the small letters; in contrast decision speed with the
large letter was not influenced by the nature of the
small letters; thus we often see the forest (global
structure) before the trees (features) rather than
the other way around.
Fig. 3.1 The kind of stimulus used by Navon (1977) to demonstrate
the importance of global features in perception.
Feature Detectors
If the presentation of a visual stimuli's leads to detailed processing of tis basic features, we may be able to identify
cortical cells involved in such processing. Hubel and Wiesel (1962) identified some cells responding in two different
ways to a spot of light depending on which part of the cell was affected:
1. An "on" response with an increased rate of firing when the light was on.
2. An "off" response with the light causing a decreased rate of firing.
They discovered two types of neuron in primary visual cortex: simple and complex cells.
Simple Cells have "on" and "off" regions with each region being rectangular. These cells respond most to dark
bars in a light field, light bars in a dark field, or straight edges between areas of light and dark.
Complex Cells resemble simple cells in responding maximally to straight-line stimuli in a particular orientation.
Complex Cells have large receptive fields and respond more to moving contours. Each complex cell is driven
by several simple cells having the same orientation preference and closely overlapping receptive fields.
End-Stopped Cells responsiveness depends on stimulus length.
Perpetual Organisation
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The first people to study perpetual organisation
were the German Psychologists Gestaltists who
emigrated to the US between the two world wars.
Their fundamental principle was The Law of
Pragnanz to which we typically perceive the
simplest possible organisation in the visual field.
Fg 3.3 (a) illustrates the law of proximity, according
to which visual elements close in space tend to be
grouped together. (b) shows the law of similarity,
according to which similar elements tend to be
grouped together. We see two crossing lines in (c)
because according to the law of good
continuation, we group together those elements
requiring the fewest changes or interruptions in
straight or smoothly curving lines. (d) illustrates the
law of closure; the missing parts of a figure are
filled in to complete the figure.
They also emphasised the importance of figure-
ground segregation in perception: one part of the
visual field is identified as the figure, and the
remainder is treated as less important and so forms
the ground.
The figure is seen as having a distinct form or shape
whereas ground lacks form. In addition, the figure is
perceived as being in front of the ground and the
contour separating the figure from the ground is
seen as belonging to the figure. The validity of this
claim can be checked in Fig. 3.4; when the goblet
is perceived as the figure, it seems to be in front of
a dark background. Faces are in front of a light
background when forming the figure.
Regions that are convex (curving outwards), small,
surrounded and symmetrical are more likely to be
perceived as the figure and which as the ground.
Findings
Geisler et al. (2001) shows the contours of objects
could be worked out very well by using two
principles different from those emphasised by the
Gestaltists:
Adjacent segments of any contour typically
have very similar orientations
Segments of any contour that are further
apart generally have somewhat different
orientations.
We use our extensive knowledge of real objects
when making decisions about contours.
The Bayesian approach entails a rational estimate
of the structure of the scene that combines fit to
the available image data with the mental set of
the perceiver.
Palmer & Rock (1994) proposed uniform
connectedness where any connected region
having uniform visual properties (colour, texture,
lightness) tends to be organised as a single
perpetual unit.
Figure-ground separation depends on features
such as convexity, symmetry and surroundedness
and also depends on top-down processes such as
past experience in terms of object familiarity and
thus is not totally innate.
Fig. 3.3 Examples of the Gestalt laws of perpetual organisation: (a)
the law of proximity; (b) the law of similarity; (c) the law of good
continuation; (d) the law of closure.
Fig. 3.4 An ambiguous drawing that can be seen as either two faces
or as a goblet.
Approaches to Object Recognition
Object recognition is identifiying objects in the visual field.
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Spatial Frequency
Hedge (2008)
emphasised
that visual
perception
develops over
time even
though it may
seem
instantaneous.
The visual
processing
involved in
object
recognition
typically
proceeds in a
coarse-to-fine
way, with initial
coarse or
general
processing
being followed
by fine or
detailed
processing.
Coarse-to-fine
processing
occurs in
numerous cells
in primary
visual cortex
responding to
high spatial
frequencies
and capture
fine detail in
the visual
image.
Numerous
other cells
respond to
spatial
frequencies
and capture
coarse
information in
the visual
image.
Fig. 3.6 shows
low spatial
frequencies in
visual input are
conveyed
rapidly to
higher-order
brain areas by
the fast
magnocellular
pathway
whereas high
spatial
frequencies
are conveyed
more slowly by
the
parvocellular
pathway; this
speed
Fig. 3.6 high and low spatial frequency versions of a place (building).
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Document Summary

Pattern recognition: we spend most of our time (eg. when reading) involved in pattern recognition, which is the identification or categorisation of two-dimensional patterns. Patterns can be regarded as consisting as a set of specific features or attributes, for example; the key features of a are two straight lines and connected cross-bar. This feature-based approach has the advantage that visual stimuli varying greatly in size, orientation and minor details can be identified as instances of the same pattern. Pattern recognition theories assume that pattern recognition involves processing of specific features followed by more global or general processing to integrate information from the features. Global processing often precedes more specific processing: navon (1977) conducted experiments, where observers decided whether the large letter was an. H or s, on other trials they decided whether the small letters were hs or ss (fig. 3. 1 the kind of stimulus used by navon (1977) to demonstrate the importance of global features in perception.

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