PSYC10006 Chap.8 Motion Perception and Its Breakdowns
Motion Perception and Its Breakdowns
It is tempting to treat motion as something inferred by noticing that an object is somewhere else than it was. The four functions this module opens with show why that cannot be the whole story: in each case the motion signal delivers information that is not present in any individual view.
Camouflage is defeated, social relationships are read off moving shapes, three-dimensional shape emerges from rotation, and an action is identified from moving dots that mean nothing in a single frame. The module then turns the argument over.
A patient who lost motion perception with the rest of vision intact shows the system is separable; several illusions show it committing to an interpretation where the local information is genuinely insufficient; and two phenomena show motion actively preventing things from being seen.
What this chapter covers
- 01
Four functions of motion, each isolated by a demonstration a static version fails
- 02
Why moving dots carry an action when a single frame of the same display does not
- 03
Akinetopsia: seeing that something has moved without seeing it moving
- 04
The two everyday consequences the subject describes, and why they follow
- 05
Why one patient supports a claim about dedicated machinery
- 06
Apparent motion, and why a series of stills reads as one moving object
- 07
Induced motion, and motion judged relative to a surround rather than absolutely
- 08
The motion aftereffect, and adaptation between opposed detectors
- 09
Motion-induced blindness, where present objects vanish, and its unresolved mechanism
- 10
Motion-induced change blindness, and the transient account the subject does give
- 11
Contrast at the leading edge, apparent speed, and the consequence for driving in fog
- 12
The aperture problem, terminators, and the barber pole as the everyday case
Diagnose which motion-related failure a design would produce
- 1Identify what the design relies on. A colour change signalling a fault relies on the change capturing attention by itself, because nobody is deliberately monitoring sixteen tiles.
- 1Identify what the animation does to that. Motion across the panel generates transient signals everywhere, so the changing tile no longer produces a distinctive one.
- 1Name the phenomenon. This is motion-induced change blindness: the tiles all remain visible and it is the change that goes unnoticed. It is not motion-induced blindness, in which objects would disappear entirely.
- 1Predict the shape of the failure. Faults would be caught when someone happened to look at the right tile and missed otherwise, so the failure would be intermittent and would look like inattention rather than like a design fault.
- 1Give the fix the mechanism implies. Restore a distinctive signal the background does not supply, such as a change on a dimension nothing else varies on. Making the colour change larger does not help, because the problem is distinctiveness rather than magnitude.
Key terms
- Akinetopsia
- Loss of motion perception following damage to a motion-processing region, usually described as the middle temporal area. Form, colour and position are preserved, which is what makes the loss evidence about dedicated machinery.
- Apparent motion
- The percept of a single moving object produced by a series of still images shown in succession. It is mostly insensitive to colour changes, though colour can break a tie when the direction of travel is ambiguous.
- Motion aftereffect
- The illusion in which a stationary surface appears to drift in the opposite direction after prolonged viewing of steady motion. It implies motion is signalled by a balance between opposed detectors that adaptation can upset.
- Motion-induced blindness
- The disappearance and return of static peripheral targets while a moving background is present and fixation is held. The subject states that the moving background causes it and that why it occurs is not known.
- Aperture problem
- The ambiguity of a moving line's direction when its endpoints are hidden, since many real motions produce the same appearance within the opening. The perceived direction is taken from the visible endpoints instead.
- Terminators
- The points where a partly hidden line meets the edge of the aperture. Their direction of travel dictates the direction the whole line appears to move, which is why a rotating striped pole looks as though its stripes rise.
Motion Perception and Its Breakdowns FAQ
Why is one patient enough to make the point about motion?
Because of what was spared. Form, colour and position survived while motion did not, and a general visual impairment would have taken them too. That selectivity is evidence that motion is computed by dedicated machinery rather than read off the same representation that supports the rest of vision.
What is the difference between the two motion-induced failures?
Ask what disappears. In motion-induced blindness an object disappears while remaining physically present and unchanged. In motion-induced change blindness every object stays visible and it is the change that goes unnoticed. Both involve motion and failing to see, which is why the names are so easy to swap.
Can I explain motion-induced blindness with a mechanism?
Only by going beyond the subject, which explicitly says the moving background causes the disappearance and that why it happens is not known. A confident process account would be invented. The accurate answer states the phenomenon, the cause, the role of fixation, and that the mechanism is unresolved.
Why does fog make an approaching car more dangerous than reduced visibility alone?
Because it lowers the contrast between the car and its background, and lower contrast at a moving edge produces a weaker motion signal and a slower apparent speed. So the car is not only harder to see; when seen, it is judged to be approaching more slowly than it is, which biases the decision to pull out.
Assessment move
Learn the four functions with their demonstrations attached, because each demonstration is the argument and the function on its own is just a claim. The single most transferable item is the identification rule for the two motion-induced failures, so drill it as a question rather than as a pair of definitions: what disappeared, the object or the change?
For the illusions, keep track of which mechanisms the subject is willing to state and which it explicitly leaves open, since reporting an unresolved mechanism as settled is a marked error here rather than a stylistic one. The aperture material is best rehearsed as a procedure: identify which endpoints are visible, work out which way they travel, and give that as the perceived direction.
That procedure answers the barber pole and any unfamiliar variant of it.
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