Motion detectionwith movement detectors

• 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

Asymmetric movement detector

V = Δx/Δt

Movement detector

V = Δx/Δt

Note:• the same velocity v is detected for

various Δx and Δt as long as

Δx/Δt = constant

Velocity dependence of movement detector

v

v

response

Relation between temporal and spatial gradients

vt

x

xtxIttxI

t

txI

t

x

x

txI

dt

txdIx

txItxxI

x

txIt

txIttxI

t

txI

),(

),(

),(),(),(0

),(),(),(

),(),(),(

Δx

Δx

Movement detector

Step-like response functions are not very likely

Movement detector

responsieΔt

Movement detector

Movement detector in fly visual system

Response to spatial sinewave pattern with wavelength 16o and τ = 50 ms.

The aperture problem

The aperture problem

Receptive field of movement detector

Δx

The aperture problem

The aperture problemIdentical responses for v=v0/cosα

αv0

The aperture problem

A single cell cannot solve the aperture problem; only an ensemble of cells can under the assumption of a rigid body

Example 1

Example 2

Various types of cells

+ - + -

-

-

+

+

End-stop cell

Example 3

Hypothetical solution

Optic/flow detectors

Divergence operator

Rotation operator

Deformation operator

Schematic overview of monkey brain

Responses of neurons in VIP to various optic-flow patterns

Responses of neurons in VIP to various optic-flow patterns

Be aware : position of receptive field relative to stimulus !

Retinal motion

• Due to object motion

• due to eye movements

Schematic overview of monkey brain

Responses of MST neurons to active vs passive optic flow

Perceptual responses to moving stimuli

Movement detector

V = Δx/Δt

Detectors have all orientations

Transparent motion

Percept ?

Movement detector

V = Δx/Δt

Variation in v is achieved by both:

• larger Δx

• smaller Δt

Transparent motion

Percept : two different separately moving patterns

References