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Chapter 4-2

PSYB57H3 Chapter Notes - Chapter 4-2: Principles Of Grouping, Extrastriate Cortex, Illusory Contours

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George Cree

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Extrastriate cortex: the region of cortex bordering the primary visual cortex and containing
multiple areas in visual processing
These areas are named V2, V3, and so on
In the extrastriate regions just beyond V1 (like V2), receptive fields begin to show an interest in
properties that will be important for object perception.
From the extrastriate regions of the occipital lobe of the brain, visual information moves out
along two main pathways.
o One pathway heads up into the parietal lobe. Visual areas in this pathway are important
for processing information relating to location of objects in space and the actions
required to interact with them (moving hands, eyes). This pathway is sometimes called
the where pathway.
o The other pathway heads down into the temporal lobe (what pathway). This pathway is
the locus for explicit acts of object recognition. As we move down into temporal lobe,
receptive fields get much bigger.
o But, some basic object information is represented in both pathways and some where
information is encoded in the temporal lobe what pathway.
Early evidence between temporal lobe and object recognition studying monkeys with
damaged (lesioned) temporal lobes.
The monkeys could see ojets ut did’t ko hat the ere seeig, this is see i stroke
victims and is called agnosia.
The part of temporal lobe responsible is inferotemporal (IT) cortex
Neurons in triate cortex are activated by simple stimuli and respond only if their preferred
stimuli are presented in restricted portions of the visual field. In contrast, cells in the IT cortex
hae reeptie fields that a spread oer half or ore of the oke’s field of ie.
Types of stimuli that activated IT cells well were ot the usual spots ad lies, ut a oke’s
hand for some cells, monkey faces for other cells.
This led Barlow to suggest a hierarchical model of visual perception in which small receptive
fields and simple features of visual cortex combine with greater complexity as you move from
striate cortex to IT cortex.
Grandmother cell is a term that refers to any cell that seems to be selectively responsible to one
specific object.
IT cortex maintains close connections with parts of the brain involved in memory formation
(hippocampus) important because IT cells need to learn their receptive field properties.
Logothetis demonstrated that cells in IT cortex have plasticity. After training monkeys to
recognize novel objects, they found IT neurons that responded with high firing rates to those
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objects, but only when the objects were seen from viewpoints similar to those form which
the’d ee leared.
Homologous regions: Brain regions that appear to have the same function in different species.
Like monkeys, humans with lesions in temporal lobe show symptoms of agnosia.
Prosopagnosia is an inability to recognize faces.
Other agnosia type includes the ability to recognize animate objects (animals) but not inanimate
objects (tools)
If we flash a picture of an animal to an observer, the observer differentiates animal from
oaial sees ithi  s, eaig there a’t e a lot of feedak fro higher isual or
memory processes.
This suggests it must be possible to do rough object recognition on the first wave of activity as it
moves from retina to striate cortex to extrastriate cortex and beyond
feed forward process: process that carries out a computation (e.g. object recognition) one
neural step after another, without need for feedback from a later stage to an earlier stage.
middle (midlevel) vision: a loosely refined stage of visual processing that comes after basic
features have been extracted from the image (low level, or early vision) and before object
recognition and scene understanding (high level vision)
mid level vision is a process that combines features into objects.
The following part takes feature combinations given to us by middle level vision and asks how
we come to know what the object is (recognizing it by matching what we perceive now to a
memory of something perceived in the past).
Goal of middle vision is to organize the elements of a visual scene into groups that we can
recognize as objects.
The visual system knows that gaps are accidents of the lighting and fills in the contour
In the early stages of processing, the human visual system assembles evidence for the presence
of significant edges from cells with receptive fields at different scales.
o All these differet its of iforatio are the oied to ake the sste’s est
guess about the presence of a contour.
Illusory contour: otour that’s pereied ee though othig hages fro oe side of it to
the other in an image.
Structuralism: belief that complex objects or perceptions could be understood by analysis of the
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Wilhelm Wundt and Edward Titchener argued that perceptions are the sum of atoms of
sensation (bits of color, orientation etc)
In a structulist view, perception is built up of local sensations the way a crystal is built up of an
array of atoms.
Gestalt: idea that perceptual whole could be greater than the sum of the parts.
Gestalt grouping rules: describe which elements in an image will appear to group together.
The rule is that e ted to see siilarl orieted lies as part of the sae otour. “uh lines
support eah other i that  isile its of a edge ake it easier to pereie a third, olliear
segment that lies between them even if the middle segment is absent.
Good continuation: gestalt grouping rule stating that 2 elements will tend to group together if
they seem to lie on the same contour.
A host of rules, principles and good guesses contribute to our organized perception of the world.
These operate on a committee model.
Everyone gets together and tells how the stimulus ought to be understood. Eventually a
consensus view emerges and we settle on a single interpretation of the visual scene.
if an edge suddenly stops in an image, why does it stop? One reason is that something gets in
the way.
For the Kanizsa figure (4.11) example, the visual system thinks that there is another contour
occluding the vertical line, with the occluding edge oriented perpendicularly to the occluded
This combined with a guess that the notches in the circles represent contours that can be
extended leads to the inference of an illusory contour.
Texture segmentation: carving an image into regions of common texture properties
The visual system can determine the average of the features in a region without knowing much
about the individual features.
Texture segmentation is closely related to the Gestalt grouping principles.
Two of the strongest principles are similarity and proximity.
Similarity: tendency of 2 features to group together will increase as similarity between them
Proximity: tendency of 2 features to group will increase as distance between them decreases.
Texture grouping can be based on similarity in a limited number of features:
o Color, size, orientation, and aspects of form.
Coiatios ojutios of features do’t ork ell thus, teture segetatio is’t
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