PSY 4791 Lecture Notes - Torsten Wiesel, Thalamus, Stereopsis

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31 Jan 2013

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Primary function of senses is to guide behaviour
perception – a rapid, automatic, unconscious process by which we recognize
what is represented by information provided by our sense organs
if we see something ambigious and must figure out what it is, it is problem
solving, not perception
Brain Mechanisms of Visual Perception
perception takes place in brain – optic nerve sends signals to thalamus, which
relays information to primary visual cortex (located in occipital lobe), neurons
in primary visual cortex send visual information to two successive levels of
visual association cortex
first level – located in occipital lobe, surrounds primary visual cortex
second level – divided into 2 parts, one is middle of parietal lobe and one in
lower part of temporal lobe
often described as hierarchy of information processing – circuits of neurons
analyze particular aspects of visual information and send results of analysis to
another circuit, which performs further analysis – at each step successively
more complex features are analyzed and higher levels interact with memories
Primary Visual Cortex
knowledge of early stages of visual analysis comes from investigations of
activity of individual neurons in thalamus and primary visual cortex
David Hubel and Torsten Wiesel inserted microelectrodes into regions of
visual system in anasthetized cats and monkeys (unconsious but visual system
still active) to detect Aps produced by individual neurons
found place in visual field with most response and objects with most
found that surface of retina is mapped on surface of primary visual cortex –
centre of visual field is largest area
2,500 blocks of tissue .5x.7 mm in size containing 15,000 neurons each,
receiving information from same small region of retina, and thus small
region of visual field (amount seen through soda straw)
neural circuits within module analyzes various characteristics of their
own particular field (receptive field) – detected presence of lines and
signalled orientation of lines, thickness of lines, movement and its
receptive field – that portion of visual field in which presentation of
visual stimuli will produce an alteration in firing rate of particular
particular neuron responds to particular orientation (50 degrees,
Visual Association Cortex
first level of visual association cortex contains several subdividions, each of
which contain map of visual scene – each subdivision receives information from
different types of neural circuits within modules of primary visual cortex
orientation and widths of lines and edges – involved in perception o
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movement – keeps track of relative movement of objects
2 regions of association cortex put together information ghathered and
processed by subdivisions of first level
shape, movement and colour – combined in visual association cortex in
lower part of temporal lobe (3-d form perception) – form perception
information from motor system and body senses about movement of eyes,
head, and body – combined in parietal lobe (location of object), part of
system for object-directed behaviour – space perception
Effects of Brain Damage on Visual Perception
primary visual cortex damage – blind in portion of field
visual association cortex damage – difficulty in perceiving shapes and objects
or particular visual characteristics
achromatopsia – inability to discriminate among different hues
damage produces achromatopsia in contralateral field (total
achromatopsia results from bilateral damage)
difficulty to perceive movement and keep track of moving objacts (first level
Balint's syndrome – difficulty in perceiving location of objects and
reaching for them under visual guidance, difficulty keeping track of them in
visual scene, can see object when looking directly at it but cannot see
where located (bilateral parietoccipital region); deficit in spatial perception
visual agnosia – diffuclty recognizing objects (damage to visual
association cortex in temporal lobe)
prosopagnosia – form of visual agnosia where there is difficulty
recognizing faces and some other complex stimuli, can recognize categories
but not individual differences
Perception of Form
Figure and Ground
objects or background – objects have particular shape and particular locations
in space and backgrounds are formless and serve mostly to help us judge
location of objetcs we see in front of them
figure – visual stimuli perceived as self-contained object
ground – visual stimuli perceived as formless background on which objects
are seen
Organization of Elements: Gestalt Laws of Grouping
gestalt psychology – branch of psychology that asserts that perception of
object is produced by particular configurations of elements of stimuli
based on tendency to organize elements and empty space into cohesive
task of perception was to recognize objects in environment according to
organization of their elements - whole is more than sum of its parts;
emhasize relationship of elements to one another, not just elements
several principles of grouping can predict combination of these elements –
laws of organization:
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law of proximity – elements closest to each other are perceived as belonging
to same figure
law of similarity – similar elements are perceived as belonging to same
good continuation – given two or more interpretations of elements that form
outline of figure, simplest interpretation is preferred
law of closure – elements missing from outline of figure are filled in by visual
law of common fate – elements that move together give rise to perception of
particular figure
Alais, Blake, and Lee – found that law of common fate applies to changes
other than movement
visual objects stand out in background to extent that they exhibit
contrasts in brightness and colour
“common tempo”
Models of Pattern Perception
cognitive psychologists – study perception
Templates and Prototypes
templates – hypothetical pattern that resides in nervous system and is used
to perceive objects or shapes by process of comparison; too simplistic
prototype – hypothetical idealized pattern that resides in nervous system and
is used to perceive objects or shapes by process of compariosn, recognition
can occur even when exact match is not found
non-human studies suggest that familiarity with categories of objects may
lead to development of specific types of prototypes
Distinstive Features
distinctive features – physical characteristic of an object that helps
distinguish it from others, collection of important features that specify
particular object
Neisserb – supports hypothesis that perception involves analysis of distinctive
features (Z test)
some phenomena cannot be explained by distinctive features – model
suggests that perception of object consists of analysis and synthesis; visual
system first identifies component features of object and then adds up features
to determine what object is – would expect that more complex take longer but
usually speeds up process of perception
Enns and Rensink – found that orientation of wire-frame objects was
quickly detected in search but if lines weren't connected then orientation
harder to detect
Pilon and Friedman – foudn that misaligned vertices were difficult to
detect when wire frames not connected (closing figure provides sufficient
organization to make orientation more immediately perceptible, despite
added features)
Evaluating Scientific Issues: Does brain Work Like Serial Computer?
Computer has provided inspiration for models of human brain function
artificial intelligence – field of study in which computer programs are
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