Download P312 Ch08_MotionPerception

Chapter 8 –Perception of Motion
Importance of motion perception. (Or is the ability to perceive motion as useless as your
appendix?)
1. Finding dinner. Many things that might hurt or help or feed us are indistinguishable from
background unless moving. We detect objects that are in motion that we couldn’t detect
otherwise.
Hawks trolling the skies above a forest look for movement to indicate presence of a
rodent. For example, the circled spot in the picture below. Is it a creature or a rock?
VL 8-2 The Dalmation Dog “Help Me 2” gives some idea of this use of motion.
2. Guidance of our own movement. Requires perception of “motion” of visual scene
across the retina helps us determine how to get where we want to go.
Mdbt\P312\Movement\M4H01017 3D cues walking down Holt hallway.MP4
S&B Chapter 9: Motion Perception - 1
3. Structure / shape from motion – we’re able to identify objects that aren’t identifiable
without movement.
Figure 9.1, p. 318. Joe Jones Demo mdbt\p312\movement\VTS_01_0.VOB
Also in VL 8-4 Shape from Movement. Note the different shapes illustrated by the
moving dots in the middle of the screen.
VL 8-5 Form and Motion. Remember to click “Parameter”, then “Generate Movie”.
4. Biological Motion: We’re able to recognize objects as being living creatures as
opposed to inanimate based only on motion. This helps when you want to know whether to kiss
something.
Point-light motion: Identification of living creatures from movement of points of light
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
VL-8. 20 is the same as the biomotionlab.ca walker.
VL- 8.21 is a side view of a walker.
S&B Chapter 9: Motion Perception - 2
Explaining motion perception . . .
Types of motion
1.
Real motion
a. Background stable and figure moves across the retina. (Figure 8.8 a)
What people most often think of when thinking of motion.
b. Image of figure on retina is stable but background moves (Figure 8.8 b)
c. Image of background moves across retina in a direction opposite of movement of
observer, i.e., “Maria walks through the room.” (Figure 8.8 c)
In all of these example, stimulation is literally moving across the retina.
2.
Apparent motion
Alternating stimulation at two or more points in visual field may yield perception of
motion. Stimulation does not “move”, it merely turns on and off at different places on the retina.
Basis for motion pictures, television motion, and moving signs.
Simplest is the phi phenomenon, original proposed by Max Wertheimer
VL 8.8 for phi phenomen.
VL 8.11 illustrates both apparent motion and motion capture – a part of the figure (dotted
background) appears to be captured and move back and forth.
Question: What % of all “movement” that we perceive is apparent?
Answer depends on how much television and movies you watch.
S&B Chapter 9: Motion Perception - 3
3.
Induced movement perception.
Continuous movement of a large object causes a smaller nearby stationary object to
appear to move.
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.
S&B Chapter 9: Motion Perception - 4
Explanations of Motion Perception
1. The Gibson Gestalt explanation
The perception of motion is an innate and immediate response to the complicated optic array –
the structure created by the surfaces, textures, and contours of the environment.
J. J. Gibson emphasized features of the external stimulus and held that our perception of
motion was a direct perception of characteristics of the external stimulus – of the optic
array.
He emphasized two aspects of the optic array that result in perception of motion . . .
a. A local disturbance in the optic array – movement of a figure across the background (Figure
8.8 a and 8.8 b)
b. Change in global optic flow – everything changing at once.
Mdbt\P312\Movement\M4H01017 3D cues walking down Holt hallway.MP4
Same example as before, but this time, focus on the changing array of 3D cues.
S&B Chapter 9: Motion Perception - 5
2. Physiologically based explanations of motion perception – today much more popular than
Gestalt based.
A theory of motion perception must account for the three types of movement illustrated in Figure
8.8
a. A stationary observer views a background as a figure moves in front of background.
Eyes are stable. Image of background does not move. Image of figure moves.
b. The eyes follow a moving figure.
Image of background moves across visual field. Image of figure does not move.
c. The observer moves.
Image of background moves in the opposite direction of the observer.
First question that must be answered: Is there any evidence of brain structures that respond to
movement.
S&B Chapter 9: Motion Perception - 6
Motion detecting neurons
For 50+ years it has been known that there are neurons in the cortex that respond to bars of light,
many of them responding best when the bars of light move in specific directions. These are the
complex V1 cells discovered by Hubel and Wiesel.
Movement of a figure across the visual field may trigger such neurons leading to the possibility
that responses of such neurons are involved in the perception of motion.
Important discovery: Sequential stimulations at adjacent retinal positions within the receptive
field of a motion detecting neurons yields the same response as does movement of a stimulus
across the receptive field. This means that most real and apparent motion phenomena may be
based on same neural structures.
The Aperture Problem: Individual neurons view the world through small windows.
Neurons see through a fairly small window onto the visual world
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 (p 186)
Pack and Born (2001) found that neurons in the monkey medial temporal lobe 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.
S&B Chapter 9: Motion Perception - 7
More evidence: Microstimulation G8 p. 188: Causing perception of movement through
neural stimulation.
More evidence of the neural basis for motion perception.
a. Monkeys viewed dots moving from left to right and responded accordingly
b. Monkeys viewed same dots moving in same direction, but also received neural stimulation
of MT neurons involved in downward movement. Response was a compromise.
S&B Chapter 9: Motion Perception - 8
Corollary Discharge Theory
The problem: Figure 8.8 b: A figure is perceived as moving even though its image remains at
the same place on the retina. The eye is tracking the figure, but only the image of the
background moves across the retina, and it’s moving backwards!!
S&B Chapter 9: Motion Perception - 9
How do we see something as moving when its image is stationary on the retina???
Solution: Suppose that the signal that is causing the eyes to move is copied to the part of the
brain that controls perception of motion.
The text calls the copy of the signal causing the eyes to move the CDS – for Corollary
Discharge Signal.
The signal associated with the object being perceived is the Object Displacement Signal –
ODS.
The theory hypothesizes a “comparator” - a neural subsystem that performs a comparison of the
Corrollary Discharge Signal with the Object Displacement Signal.
Here’s how the theory can explain our perception in the three instances illustrated in Figure 8.8.
Accounting for Figure 8.8a: The signals to and from the comparator when a person moves
across the visual field with eyes stationary. (This is an adaptation of G8’s Figure 8.19)
IDS + No CDS =
Movement of
Target
Motor Signal:
No Corollary Discharge
( No CDS)
Accounting for Figure 8.8 b: The signals to and from the comparator when the eyes follow a
moving person.
Motor Signal:
Corollary Discharge of
signals to muscles of eye
(CDS)
No IDS + CDS =
Movement of Target
Image Stationary
on retina
Eye follows Target
No IDS)
S&B Chapter 9: Motion Perception - 10
All possible combinations of Image Displacement Signal and Corollary Discharge Signal.
The decision processes involved in determining whether movement occurs.
Source
Image Displacement Signal
Corollary Discharge Signal
Type of signal
Image movement
No eye movement
Comparator Conclusion
Image Displacement Signal
Corollary Discharge Signal
No image movement
No eye movement
No movement. Stationary.
Image Displacement Signal
Corollary Discharge Signal
Image movement
Eye movement in opposite direction.
No movement. Scanning.
Image Displacement Signal
Corollary Discharge Signal
No image movement
Eye movement
Movement. Tracking figure.
Movement (Figure moving.)
Corollary Discharge is a great theory, but is there any behavioral or neurological evidence for it?
1. Behavioral Evidence for corollary discharge theory.
A. Moving afterimages (p. 190)
View an object for 60 seconds. Then go to a dark place. If you move your eyes, the afterimage
of the object will appear to move.
The image of the object remains at the same place on the retina. There is no movement of the
image on the retina.
But as the eyes move, corollary discharges are sent to the comparator.
So the perception is that of movement of the image – the last possibility in the above table.
B. Manipulating the eye (p 190-191)
“Pick a point in the environment and keep looking at it while very gently pushing back and
forth on the side of your eyelid. “
The scene will move.
This is explained by the assumption that the corollary discharges of the eye muscles attempting
to keep the eye focused on a spot again creates the situation shown in the bottom possibility in
the table above.
S&B Chapter 9: Motion Perception - 11
2. Physiological evidence.
A. The case of R.W.
Lesion in the medial superior temporal area of the cortex.
Problem: As R.W. moved his eyes, the stationary environment appeared to move.
Explanation: The Image Displacement Signal associated with movement of the image of the
environment across his retina was being sent to the comparator, but the Corollary Discharge
Signal indicating that his eyes were moving was NOT being sent. So he perceived movement.
B. Recording of neurons – Galetti & Fattori, 2003.
a. Certain neurons fired when the image of a bar moved from right to left across the retina as
the monkeys were fixating on a stationary reference point.
But the same neurons did not fire when the same image moved in the same direction across the
retina as they were tracking a moving target.
The stimulus of the bar was the same in both instances – movement from right to left across the
retina. But in one case, the eyes were stationary (no Corollary Dischage Signal) and in the other
case, the eyes were moving (Corollary Discharge Signal).
So this experiment provides evidence of neurons whose responses may indicate actual motion of
objects – which is why they have been called “real-motion neurons”.
Why do we perceive motion? Perhaps, more importantly, why do we have brains? 20:00
http://www.ted.com/talks/lang/eng/daniel_wolpert_the_real_reason_for_brains.html
(This is about movement rather than motion perception, but it contains info on feedback that is
relevant to corollary discharge theory.)
S&B Chapter 9: Motion Perception - 12
A complication: Involuntary rapid eye movements
Why do we have eye movements?
1. To refresh the image on the retinal. 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.
Saccadic movements
Abrupt, rapid movements.
Occur in localizing and reading.
1000s of times per hour. ~~ 10s of times per minute.
Saccadic Suppression: Why we don’t experience movement associated with saccades
Suppression of responses of motion selective neurons during saccades.
Probably a result of copies of signals that are sent to the muscles controlling the saccades
– corollary discharge theory.
Motion
Detector
Motor
Neuron
Experience of Movement
Eye muscles
S&B Chapter 9: Motion Perception - 13
Neural circuit for motion detection – how motion detecting neurons get their groove.
See mdbo\ . . . \P312\NeuralCircuitsForMotion.ppt
B
A
C
Apparent Motion
Responses of motion detection systems in the absence of actual moving external stimuli.
See PhiPhenomenon.ppt
in MDBO\312
S&B Chapter 9: Motion Perception - 14