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Brain Mechanisms of Visual Perception.doc

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McGill University
PSYC 561
Rick Schultz

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 response  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 direction ● receptive field – that portion of visual field in which presentation of visual stimuli will produce an alteration in firing rate of particular neuron  particular neuron responds to particular orientation (50 degrees, etc) 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  movement – keeps track of relative movement of objects  colour ● 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 damage)  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 forms  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: ● 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 figure ● 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 system ● 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 designed to stimulate human cognitive abilities; may help us understand mechanisms that underlie these abilities ● no program advanced enough to deal with even small fraction of human processes ● computer does task differenlty than human would ● serial computers work one step at a time – examines scene, stored it, refers to memory, etc. ● If brain were serial drive would process 10 steps a second (rate of neurons can fire) ● parallel processor – computing device that can perform several opertaions simultaneously; similar to brain ● neural networks – model of nervous system based on interconnected networks of elements that have some properties of neurons; developed by Hebb much earlier than invention of computer  input, synapses with inhibitory or excitatory, output  specific connections between units can be strengthened or weakened by extent to which response matches stimulus in some way – mimic plasticity ■ Hebbian rule of learning – hypothesis developed by Hebb for synaptic change ● write computer programs that mimic neural networks ● taught to recognize stimuli by reinforcing connections – even altered or partially missing ● Riesenhuber and Poggio – proposed model along these lines but connection can be additive (summing excitatory or inhibitory influences) or maximum input (MAX) where strongest input determines response Top-Down Processing: The Role of Cognition ● Palmer – showed that even more general forms of contect can aid in perception of objects  showed drawings of objects using tachistoscope  if object fit context could be identified 84% of time and if it didn't fit then could be identified 50% ● tachistoscope – device that can present visual stimuli for controlled (usually very brief) durations of time ● bottom-up processing (data driven processing) – perception based on successive analyses of details of stimuli that are present, constructed from elements ● top-down processing – perception based on information provided by context in which particular stimulus is encountered, uses big picture ● Haenny, Maunsell, and Schiller – obtained direct evidence that watching for particular stimulus can warm up neural circuits in visual system  trained monkeys to look at pattern of lines oriented at particular angle, to remember that pattern, and then to pick it out from series of different patterns presented immediately afterward  recorded neural acitivity of neurons in visual association cortex and found that watching for patern of lines having particular orientation affected responsiveness of neurons – if monkeys were watching 45degree angle, then neurons that detected lines of that orientation responded more vigorously than normal when pattern was presented again ● Haenny, Maunsell, and Schiller – found that this enhancement could even be produced by letting monkey feel orientation of pattern of grooves in metal plate they could not see; when subsequent visual pattern contained lines whose orientation matched those groves, larger neural response was seen Perception and Action: A Possible Synthesis ● Goodale and Milner – described large number of obervations fro
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