Post on 22-Feb-2016
description
Visual Pathways
W. W. Norton
• Primary cortex maintains distinct pathways – functional segregation
• M and P pathways synapse in different layers
• Ascending (i.e. feed-forward) projections synapse in middle layers
• Descending (i.e. feed-back) projections synapse in superfical and deep layers
Visual Pathways
• Visual scene is represented:– Retinotopically thus…
– spatiotopically
= Fovea
Tootell R B H et al. PNAS 1998;95:811-817
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
Visual Neuron Responses
• The notion of a receptive field is fundamental in vision science– A neuron’s receptive field is the
region in space in which a stimulus will evoke a response from that neuron
– Receptive field properties vary widely across visual neurons and are never just “ON” or “OFF”
– Unit recordings in LGN reveal a centre/surround receptive field
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?
Visual Neuron Responses
• LGN cells converge on “simple” cells in V1 imparting orientation (and location) specificity
Visual Neuron Responses
• LGN cells converge on “simple” cells in V1 imparting orientation (and location) specificity
• Again, information is physically seperated into a “map”
Visual Neuron Responses
• LGN cells converge on simple cells in V1 imparting orientation specificity
• Thus we begin to see how a simple representation – orientations of lines - 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
– Reality is that spike rate probably is only one part of the story: information is coded in many ways e.g.
• Relative timing
• Graded potentials
The Role of “Extrastriate” Areas
• Different visual cortex regions contain cells with different tuning properties
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
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.)
The Role of “Extrastriate” Areas
• Identifying colour sensitive regions
Subtract Voxel intensities during these scans… …from voxel
intensities during these scans
…etc.Time ->
The Role of “Extrastriate” Areas
• result– voxels are identified that are preferentially selective for
colour– these tend to cluster in anterior/inferior occipital lobe
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
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
The Role of “Extrastriate” Areas
• Thus PET studies doubly-dissociate colour and motion sensitive regions
The Role of “Extrastriate” Areas
• V4 and V5 are doubly-dissociated in lesion literature:
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
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
Visual Neuron Responses
• Edges are important because they are the boundaries between objects and the background or objects and other objects
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 these don’t apply!
• Representing boundaries must be more complicated than simple edge detection!
Visual Neuron Responses
• Boundaries between objects can be defined by color rather than brightness
Visual Neuron Responses
• Boundaries between objects can be defined by texture
Visual Neuron Responses
• Boundaries between objects can be defined by motion and depth cues