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Motion detection with movement detectors
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It is a non-linear device: response to velocity a and velocity b is not equal to velocity a+b movement detection may be asymmetric (response to velocity a is not opposite to response to velocity -a) or symmetric Requirements on movement detectors a b a+b
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Asymmetric movement detector V = Δx/Δt
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Movement detector V = Δx/Δt Note: the same velocity v is detected for various Δx and Δt as long as Δx/Δt = constant
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Velocity dependence of movement detector v v response
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Relation between temporal and spatial gradients Δx
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Movement detector Step-like response functions are not very likely
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Movement detector responsie Δt
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Movement detector
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Movement detector in fly visual system Response to spatial sinewave pattern with wavelength 16 o and τ = 50 ms.
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The aperture problem
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Receptive field of movement detector Δx
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The aperture problem
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Identical responses for v=v 0 /cosα α v0v0
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The aperture problem A single cell cannot solve the aperture problem; only an ensemble of cells can under the assumption of a rigid body
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Example 1
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Example 2
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Various types of cells +-+ - - - + + End-stop cell
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Example 3
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Hypothetical solution
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Optic/flow detectors
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Divergence operator
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Rotation operator
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Deformation operator
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Schematic overview of monkey brain
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Responses of neurons in VIP to various optic-flow patterns
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Be aware : position of receptive field relative to stimulus !
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Retinal motion Due to object motion due to eye movements
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Schematic overview of monkey brain
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Responses of MST neurons to active vs passive optic flow
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Perceptual responses to moving stimuli
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Movement detector V = Δx/Δt Detectors have all orientations
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Transparent motion Percept ?
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Movement detector V = Δx/Δt Variation in v is achieved by both: larger Δx smaller Δt
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Transparent motion Percept : two different separately moving patterns
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References
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