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Chapter 4

PSYB51H3 Chapter Notes - Chapter 4: Agnosia, Fox Sparrow, Hierarchical Database Model

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Matthias Niemeier

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Heteronymous hemianopia? Wat is it ? which brain structure is likely damaged? Wat are the
symptoms. half of the vision field is missing. Half relative to fovea, if it is the left visual field then
left vision not working. one is in left visual field and the other eye has visual field defect in right
visual field. In left eye, the left part is missing. In right eye the right part is missing. So
hteronymous means it is different eye. Homonymous is both eyes same. The brain structure is the
optic casism, it is the part where visual information is crossing over.
wat is CSF-contrast sensitivity function. Can u sketch wat it usually looks like wat does that reflect
in terms of perception? Humans limited to 60 cycles spatial frequency inverted U. On the vertical
axis is contrast sensitivity function. Inverted U means ?
middle (midlevel) vision loosely defined stage of visual processing that comes after basic features
have been extracted from the image, early vision, and before objet recognition and scene
understanding, high level vision
-goal of middle vision is to organize elements of a visual scene into groups that we can then recognize
as objects ie organizing the edges
-illusory contours = a contour that is perceived, even though nothing changes from one side of the
contour to the other in the image. Ex edges that are perceived despite lack of physical evidence for
-structuralism =believe that complex objects or perceptions could be understood by analysis of the
components, so they disagree with illusory of contours cuz an extended edge is seen bridging a gap
where no local atom of edgeness can be found
-gestalt=form in German. Perceptual whole could be greater than the apparent sum of the sensory
parts. Gestalt grouping rules = set of rules describing which elements in an image will appear to
group together. The original list of rules assembled by members of gestalt school of thought. Rules
guide the visual system in its interpretation of the raw retinal image
Good continuation a gestalt grouping rule, stating that 2 elements will tend to group together if
they seem to lie on the same contour
Perceptual committee = all the rules get together and voices opinions about how the stimulus ought
to be understood and reach a consensus on how to interpret the visual scene
An edge suddenly stops in an image according to the visual system is cuz there is another contour
occluding (stopping) the edge

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- texture segmentation = craving an image into regions of common texture properties, this similar
to the 2 principles below
-similarity =a gestalt grouping rule stating that the tendency of 2 features to group together will
increase as the similarity /b/ them increases . image chunks that are similar to each other will be
more likely to group together ie colour, size, orientation, aspects of form. Combinations, conjunctions,
of features do not work
-proximity gestalt grouping rule stating that the tendency of 3 features to group together will
increases as the distance /b/ them decreases, ie giving them a horizontally striped appearance
2 weaker grouping principles: parallelism (parallel contours are likely to belong to the same figure)
and symmetry (symmetrical regions are more likely to be seen as figure)
Common region gestalt grouping rule stating that 2 features will tend to group together if they
appear to be part of the same larger region
Connectedness gestalt grouping rule stating that 2 items will tend to group together if they are
-low level visual processes deliver fairly straightforward bits of info about a line here and a colour
there. Middle vision behaves like a collection of specialists each interpreting a part of the puzzle and
the goal is to have a single answer
Oliver Selfridges committee of demons, feature, cognitive, decision demons. Physical substrate of a
committee is probably a massively interconnected set of neurons
Decision made by committee need not be final. Ambiguous figure visual stimulus that gives rise to
2 or more interpretations of itz ID or structure ex necker cube an outline that is perceptually bi-
stable. Unlike the situation with most stimuli, 2 interpretations continually battle for perceptual
dominance. Another ex is duck-rabbits, these are really exceptions that prove the rule, every image
is, in theory, ambiguous, but middle vision perceptual committees almost always agree on a single
Accidental viewpt a viewing position that produces some regularity in visual image that is not
present in the world, ex the sides of 2 independent objects lining up perfectly, any shift in position
will ruin the image. It is assumed to the visual system that most images will not consider all the
accidental viewpts and it is also assumed that the visual system involves an implicit understanding
of physics of the world, implicit meaning innate, we dont have to verbalize it, we know that this rule
ie human leg not gone when behind a table. Even animals know
Images have no meaning until middle or high level visual processes dig into them
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