Chapter 8: Visual Motion Perception
Computation of Visual Motion
Building a motion detector
Bug moves Left to Right
Passes through A's RF then B's
Third motion detection cell (M) listens to A and B and can detect movement
If this is the case, response to two stationary bugs, one in each RF
Add a neutral cell (D) for delaying transmission
D has a fast adaption rate so it fires quickly at first but soon stops firing if the item stays at A.
B and D are connected to X, a multiplication cell
X only fires when both B and D are active
Delaying response from A @ D and multiplying it with B, we have a motion detector
PROBLEM
Only detects L to R motion
Detects motion at certain velocity only
HOW TO FIX
Motion detector circuit must string several motion detector circuits together and cover a larger area
Additional receptors detect long range motion
M Fires continually as bug moves across the fields of the 5 RF at the top of the circuit
Apparent Motion
Illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession
Correspondence Problem
The problem faced by motion detection systems: which feature in frame 2 corresponds to which feature in frame 1 when masking most of the screen - making it appear to move up and down.
Aperture
An opening that allows only partial view of an object
Aperture Problem: When moving object is viewed through an aperture, the direction of motion of a local feature or part of an object may be ambiguous
Connection?
Motion from several apertures can be combined to determine the global motion of objects
Global motion detectors
Middle temporal area (MT)
Vast majority of neurons in MT are selective for motion in a particular direction
MAE: Motion aftereffect
The illusion of motion of a stationary object that occurs after prolonged exposure to moving object.
Existence of MAE implies an opponent process system
Interocular transfer: The transfer of an effect from one eye to another
Input from both eyes is combined in area V1, so MAE must be in V1 or later
Recent studies with fMRI confirm adaptation in MT is responsible for MAE
Using Motion Information
Optic Array
The collection of light rays that interact with objects in the world in front of viewer
Some of these rays strike retina and enabling sight
Optic Flow
The changing angular position of points in a perspective image that we experience as we move through the world
Focus of Expansion (FOE)
The point in the center of the horizon from which, when we are in motion, all points in the perspective image seem to emanate
Biological Motion
The pattern of movement of all animals
Biological motion can be seen compellingly when lights attached to a moving human are viewed in total darkness
Eye Movements
Types of EMs
Smooth Pursuit: Voluntary EM in which the eyes move smoothly to follow a moving object
Saccade: Made both voluntarily and involuntarily, in which the eyes rapidly change fixation from one object or location to another
Vergence: A type of eye movement in, both voluntary and involuntary, in which two eyes move in opposite directions
Convergent eye movements: inward
Divergent: eyes turn outwards
Reflexive: Automatic and involuntary eye movement
Muscles
Superior colliculus: A structure in the midbrain that is important in initiating and guiding eye movements
Comparator: device that compares two outputs and indicates which is larger
In addition: an area of the visual system that receives one copy of the order issued by the motor system when the eyes move
Dynamic remapping of RFs
Saccade is planned but not yet executed
Neurons in parietal cortex remap their RFs relative to upcoming fixation location
Saccade is executed
RFs are already processing information from new location before eye lands there
Development of Motion Perception
Sensitivity to visual motion develops over time
Infants have some reflexive eye movements at birth
Adult like sensitivty to motion does not reach maturity until about 3-4 years old