Download Perceiving Motion

Chapter 8 –Perceiving Motion
Types of motion
1.
Actual Motion
G9 p181
a. Background stable and figure moves
across the retina.
Our experience is that person moves.
b. Figure stable on retina but background
moves as eyes track figure.
Our experience is that person moves even
though the image of the person is stable on
the retina. More on that, later.
c. Image of background moves across
retina as observer moves through the
environment, i.e., “Maria walks through the
room.”
Our experience is that room is stable. More
on that, later.
In all of these examples, something is actually moving – either the figure or the background.
There are differences between these examples, but they all share the important point that
something is actually moving.
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2. Apparent motion
Perception of motion arising from sequential stimulation at two or more nonmoving points in
visual field.
Nothing is actually moving. Lights at different places are merely being turned on and off.
This experience of movement occurs when viewing motion pictures, television motion,
and moving signs.
Simplest is the phi phenomenon, original proposed by Max Wertheimer
VL 8.8 for phi phenomenon.
Question: Are the neural mechanisms that govern our experience of motion when we are
presented with apparent motion stimulation the same as those that govern our experience of
motion when we are presented with real motion stimulation? Goldstein says, “Yes.”
Question:
What % of our motion experiences are of apparent motion.
Answer depends on how much television and movies you watch – but it’s a huge
percentage.
3.
Induced movement perception.
Continuous movement of a large object causes a smaller nearby stationary object to
appear to move.
Research example: http://psychlab1.hanover.edu/Classes/Sensation/induced/
You’re sitting in a car in a parking lot. Unbeknownst to you, the care next to you begins
to back out of its place. You experience your own car as moving.
Clouds moving past the moon. VL 8.12 (Not very convincing.)
4.
Motion aftereffects – the waterfall effect.
VL – 8.9 Click on “Done”, then watch the sinewave grating move to the right
VL 8.13 (Good),
8-14 (Good) (Rotate for 30 seconds, then click on Novel Stimulus button.
5. Drug induced perceptions of movement.
Drugs presumably cause neurons that are involved in the perception of movement to emit
action potentials, leading to perceptions of movement unassociated with actual movement.
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Explanations of Motion Perception
1. The Gibsonian / Gestalt Explanation
Motion is the direct perception of a specific stimulus configuration.
We have cells (or groups of cells) that directly respond to specific stimuli changing
configurations over time.
These output of these cells yields our experience of motion
Gestalt Explanation of the first basic real movement situation
The first example.
Moving figure creates a local
disturbance in the visual scene – shown
by the star shapes in the figure. This
disturbance is responded to by cells
“looking for” such disturbances and
experienced as motion.
The second example.
The movement of the wall relative to the
figure creates a local disturbance in the
visual scene. As above, this disturbance
is responded to by cells “looking for”
such disturbances and is experienced as
motion.
The third example.
Although the image of the wall on the
retina is moving, there is no local
disturbance, so there is no perception of
motion.
Note that this explanation of movement perception does not require that we perceive the object
that is moving separately as an object.
All is requires is the perception of local disturbances in the visual scene. It assumes that those
local disturbances are experienced as movement.
Bottom line on the Gestalt Explanation: It is not a mainstream belief, but it is still considered.
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Neural Circuit Explanation.
A Basic Neural Circuit for Motion – The Reichardt Detector - G9 p 182
The following circuit would respond to movement of an image across the retina.
A. Movement that will NOT be detected.
A excites E which ultimately inhibits F.
A short time later, B excites F.
If the inhibition initiated by A arrives at the same time as the excitation from B, F will not
respond.
The key is the time delay associated with having neuron E in the circuit.
E delays the inhibition, so that it arrives at F, just as the excitation from B arrives.
Same goes for C and D.
So neuron “I” will not respond to an object moving at the right speed from left to Right.
B. Movement that will be detected.
But it will respond to movement of an object going from right to left.
Play ..\..\..\MDBT\P312\Movement\Reichardt Detector Circuit.pptx
This circuit explains very easily, the first of the real movement examples, Figure 8a.
The above circuit would not respond to the particular movement (left to right) shown in 8a, but it
would respond to movement of a person in the opposite direction.
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Corollary Discharge Theory – Taking eye movements into account. G9 p 183
The above circuit accounts for the perception of movement of a moving object across a
stationary background – Figure 8a.
But what about perception of movement when the object that is moving is stationary on the
retina?
Figure 8b – Person perceived as moving even though his image on retina is stationary.
Why don’t we experience the wall as
moving?
Corollary DischargeTheory
According to this theory, movement perception is the result of a comparison of input from the
retina – indicating whether images are moving across the retina or not – and input from the
muscles that control eye movements – indicating whether the eyes are moving or not.
The input from the muscles is probably not actual signals from the eye muscles saying, “Hey,
I’ve just moved the eye.” Instead, that input is probably copies of signals saying “Hey, move the
eyes.” that were sent TO the eye muscles by movement-controlling neurons. That’s why the
signals regarding the eye muscles are called corollary discharge signals.
This theory assumes that our ultimate perception is based on the output of a movement
detecting comparator – a neuron or collection of neurons that compares the retinal image
movements such as the Reichardt detector above with eye movements.
Retinal Image
Signals
Comparator
Eye Muscles
Signals
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“Movement”
or
“No Movement”
Corollary Discharge Theory explanation of Figure 8 – G9 p 183
8a
Retinal Object
Movement
Eye
Movement
Yes
No
Comparator:
“Movement”
8b
Retinal Object
Movement
Eye
Movement
No
Yes
Comparator:
“Movement”
Afterimage Demonstration G9 p 184
Look at an object.
Turn off the lights.
Watch the afterimage move.
Image
Movement
Eye
Movement
No
Yes
Comparator:
“Movement”
Eye rubbing Demonstration G9 p. 185
Focus on an object.
Rub the outside of your eyelid (Fig 8.14).
Watch the world move.
0
Image
Movement
No
Comparator:
“Movement”
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Eye
Movement
No, but signals sent.
So where are these circuits for processing motion? G9 186
Area MT – Y1 p 230
An area of the brain where activity of virtually all the neurons are involved with motion.
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Three major collections of information about MT neurons . . .
1. MT Neurons detect movement in all directions.
Studies in which activity of individual MT neurons has been recorded have found that they
respond best (either by increasing their rates of activity or decreasing them) only when
something moves in their receptive field. So they detect movement.
Virtually all are tuned for direction of movement – each one responds best only when the object
moves in a specific direction across the receptive field.
2. MT Neurons apparently cause motion perception. G9 p 187
a. Monkeys viewed a collection of dots moving from left to right and responded accordingly,
b. Monkeys viewed same dots moving in same directions, but also received neural stimulation
of MT neurons tuned for downward movement. Resulting perception was a compromise.
This means that if you wanted to
create the illusion of movement, all
you would need to do was stimulate
the appropriate MT neurons.
3. Disruption of MT neurons impairs movement perception
1. Transcranial magnetic stimulation (TMS) .
A brief magnetic pulse, targeted at the MT, dramatically impairs perception of motion.
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The Aperture Problem: G9 p 188
Individual neurons view the world through small windows.
If a neuron detects only movement in 1 specific direction, say left-to-right, then . . .
Multiple directions of actual movement would cause the same response in that neuron.
See VL 8-16 Aperture Problem is a good demonstration of this.
Solution to the aperture problem
Pack and Born (2001) found that neurons in the MT of monkeys initially respond as if there is
an aperture problem, then the neurons begin responding to indicate the correct direction of
movement.
There is apparently communication among motion detecting neurons that allow them to
“conclude” that motion which appears to be horizontal is actually oblique or some other angle.
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Perceptual Organization from Biological Motion – Point-Light Walkers – G9 p 192
We experience organic form from minimal information if that information is in motion.
VL 8-20 and 8-21
The Biomotion Lab
Directed by Prof. Dr. Nikolaus Troje, the lab is located at Queen's University in
Kingston, Ontario.
Demonstrations – BML Walker demos many attributes of a walking human
http://www.biomotionlab.ca/ for an overview. Click on left arrow to get walker.
(http://www.biomotionlab.ca/Demos/BMLrunner.html) for a simpler demonstration
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Where is biological motion perceived?
Neurons controlling perception of biological motion have been found in the posterior superior
temporal sulcus (STSp) in the temporal lobe.
Evidence . . .
1. Activity recordings during viewing of biological motion suggest that this region is involved.
Neurons in this area change their rates of responding whenever the observer
perceives biological motion, but not other types of motion.
2. Transcranial magnetic stimulation (TMS) leads to difficulty in perceiving such motion.
When TMS is applied, the points of light appear to be just unrelated points of light –
not people.
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A complication: Eye movements
Why do we have eye movements?
1. To refresh the image on the retina so receptor adaptation won’t make the image disappear.
A stabilized image eventually disappears because eventually, the receptors adapt to unchanging
stimulation.
2. To speed up fixation of objects – neck and body muscles are too slow.
3. To keep objects fixated as the head moves.
Types of eye movements
1. Saccadic movements
Abrupt, rapid movements occurring about 3 times per second
Occur in localizing and reading.
1000s of times per hour. ~~ 180 times per minute ~~~ 3 per second.
2. Smooth pursuit eye movements- movements that occur when tracking a moving object or
when changing focus from one point in the visual field to another.
3. Vergence eye movements – Eye movements that occur when shifting attention from a close
object to a far object or vice versa.
Why we don’t experience movement associated with saccades?
Saccadic Suppression: The shutting down of retinal input during saccadic movements.
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Some specific movement modules in the cortex
The parietal lobe and movement
The anterior parietal lobe contains a map of sensations from the skin. “ante”= front
The posterior parietal lobe contains neurons that control movement.
Grasping
Arm reach
Eye movement
MT
The Lateral Intraparietal Area (LIP). Y1 p 247. Neurons in this area fire when an eye
movement is intended. Recordings have shown that the active neuron is the one whose
receptive field is the location to which the eyes will be moved.
The Medial intraparietal area (MIP). Y1 p 248. Neurons in this area fire when an arm reach
is intended. Recordings have shown that the active neuron is the associated with the direction of
the reach toward a specific location. Different neurons are associated with different directions.
The Anterior Intraparietal area (AIP). Neurons in this area fire indicating the type of grasping
motion the hand will make.
The neurons described above are all involved in intended movement. Hmm – if we could
monitor enough of them, we could predict what someone was going to do just before he/she did
it.
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