Midterm 1
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Transcript of Midterm 1
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Midterm 1
Oct. 21 in class
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Read this article by Wednesday next week!
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Visual Pathways
• Lateral Geniculate Nucleus maintains segregation:
– of M and P cells (mango and parvo)
– of left and right eyes
P cells project to layers 3 - 6
M cells project to layers 1 and 2
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Visual Pathways
W. W. Norton
• Primary cortex maintains distinct pathways – functional segregation
• M and P pathways synapse in different layers
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The Role of “Extrastriate” Areas
• Consider two plausible models:
1. System is hierarchical:– each area performs some elaboration on the input it is given
and then passes on that elaboration as input to the next “higher” area
2. System is analytic and parallel:– different areas elaborate on different features of the input
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The Role of “Extrastriate” Areas
• Different visual cortex regions contain cells with different tuning properties
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The Role of “Extrastriate” Areas
• Functional imaging (PET) investigations of motion and colour selective visual cortical areas
• Zeki et al.
• Subtractive Logic– stimulus alternates between two scenes that differ only in
the feature of interest (i.e. colour, motion, etc.)
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The Role of “Extrastriate” Areas
• Identifying colour sensitive regions
Subtract Voxel intensities during these scans… …from voxel
intensities during these scans
…etc.Time ->
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The Role of “Extrastriate” Areas
• result– voxels are identified that are preferentially selective for
colour– these tend to cluster in anterior/inferior occipital lobe
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The Role of “Extrastriate” Areas
• similar logic was used to find motion-selective areas
Subtract Voxel intensities during these scans… …from voxel
intensities during these scans
…etc.Time ->
MOVING STATIONARY MOVING STATIONARY
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The Role of “Extrastriate” Areas
• result– voxels are identified that are preferentially selective for
motion
– these tend to cluster in superior/dorsal occipital lobe near TemporoParietal Junction
– Akin to Human V5
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• Electrical response (EEG) to direction reversals of moving dots generated in (or near) V5
• This activity is absent when dots are isoluminant with background
• Tata, Mason & Sutherland (2007)
The Role of “Extrastriate” Areas
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The Role of “Extrastriate” Areas
• Thus PET studies doubly-dissociate colour and motion sensitive regions
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The Role of “Extrastriate” Areas
• V4 and V5 are doubly-dissociated in lesion literature:
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The Role of “Extrastriate” Areas
• V4 and V5 are doubly-dissociated in lesion literature:
– achromatopsia (color blindness): • there are many forms of color blindness• cortical achromatopsia arises from lesions in the area of V4• singly dissociable from motion perception deficit - patients with
V4 lesions have other visual problems, but motion perception is substantially spared
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The Role of “Extrastriate” Areas
• V4 and V5 are doubly-dissociated in lesion literature:
– akinetopsia (motion blindness): • bilateral lesions to area V5 (extremely rare)• severe impairment in judging direction and velocity of
motion - especially with fast-moving stimuli• visual world appeared to progress in still frames• similar effects occur when M-cell layers in LGN are
lesioned in monkeys
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How does the visual system represent visual information?
How does the visual system represent features of scenes?
• Vision is analytical - the system breaks down the scene into distinct kinds of features and represents them in functionally segregated pathways
• but…
• the spike timing matters too!
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Visual Neuron Responses
• Unit recordings in LGN reveal a centre/surround receptive field
• many arrangements exist, but the “classical” RF has an excitatory centre and an inhibitory surround
• these receptive fields tend to be circular - they are not orientation specific
How could the outputs of such cells be transformed into a cell with orientation specificity?
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Visual Neuron Responses
• LGN cells converge on “simple” cells in V1 imparting orientation (and location) specificity
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Visual Neuron Responses
• LGN cells converge on simple cells in V1 imparting orientation specificity
• Thus we begin to see how a simple representation - the orientation of a line in the visual scene - can be maintained in the visual system– increase in spike rate of specific neurons indicates presence of a line
with a specific orientation at a specific location on the retina
– Why should this matter?
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Visual Neuron Responses
• Edges are important because they are the boundaries between objects and the background or objects and other objects
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Visual Neuron Responses
• This conceptualization of the visual system was “static” - it did not take into account the possibility that visual cells might change their response selectivity over time
– Logic went like this: if the cell is firing, its preferred line/edge must be present and…
– if the preferred line/edge is present, the cell must be firing
• We will encounter examples in which neither of these are true!
• Representing boundaries must be more complicated than simple edge detection!
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Visual Neuron Responses
• Boundaries between objects can be defined by color rather than brightness
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Visual Neuron Responses
• Boundaries between objects can be defined by texture
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Visual Neuron Responses
• Boundaries between objects can be defined by motion and depth cues