Motion Perception Flashcards
Retinal velocity
Change in visual angle per unit of time
How do we categorize perceptual motion (fastest or slowest)?
threshold in terms of a velocity
e.g. threshold could be defined as the velocity at which motion is detected 50% of the time
subject-relative motion
motion of a single visible object with no background reference
object-relative motion
motion of a visible object relative to some other object or visible background.
what are the two sources of information or multiple situations that lead to perceived motion?
retinal displacement and optic pursuit
Optic pursuit
occurs when we track a moving object with our eye
- the image stays on the fovea, yet we perceive it as moving
Retinal displacement
the changing position of an object’s image on your eye
- eye stays still but image moves
Apparent motion
when images flash on and off in separate locations with certain timing relations. although nothing really moves between the flash locations, motion is seen.
e.g. watching motion on TV
Other words for apparent motion
- stroboscopic motion
- beta motion
Interstimulus Interval (ISI)
the time between the end of one flash and the start of another (to create apparent motion)
* measured in ms
How do different measurements of ISI, or time, impact apparent motion?
For a specific spatial disparity:
ISI < 60 ms, SIMULTANEITY is seen.
ISI 60-200 ms: OPTIMAL MOVEMENT is seen. (smooth and continuous)
ISI >200 ms: SUCCESSION, not movement, is seen.
Induced motion
involves an object and a surrounding reference frame. when the surround or frame moves, the object appears to move.
*induced motion may also make the observer feel they’re moving
What are the two general theories of motion perception?
1) Indirect Perception Theory
2) Direct Perception Theory
Indirect Perception Theory
Motion is not a basic perceptual quality; it is derived from other things
*in order to perceive motion, your brain must first calculate other things –> motion is indirectly calculated from those things
Direct Perception Theory
Motion is a basic perceptual quality. Your system is wired to perceive it.
What are the two experiments that ended the indirect vs direct perception debate?
Exner and Wertheimer
Exner experiment
If the processing of motion is indirect, it would have to depend on people’s processing of time
- tested:
1) temporal order tasks: how good people are at processing time; which shape comes first
2) how good at motion: how well people can tell the direction of motion
Conclusion of Exner experiment
Threshold for perceived succession:
BELOW 45 MS - can’t tell the temporal order of two successive flashes
BUT: can tell the direction of apparent motion with as low as 14 MS
Result: people are able to perceive motion even when they have no access to time information –> PROCESSING OF MOTION IS INDEPENDENT OF TIME
*proof of direct perception debate
What are the thresholds for perceived succession according to the Exner experiment
Threshold for perceived succession:
BELOW 45 MS - can’t tell the temporal order of two successive flashes
BUT: can tell the direction of apparent motion with as low as 14 MS
Objectless Motion
When there is an experience of simultaneity (small ISI), we perceive both objects stationary
HOWEVER we still appear some motion with no shape
Wertheimer’s Experiment
If we can perceive motion without telling what is moving (no spatial displacement, time, no access to delta x and t), then motion is direct and calculated with no circuitry
-objectless motion
Phi Motion
Only occurs when ISI is so small, that we perceive simultaneity
(def) at very small ISIs, one sees simultaneous lights but also sees something moving between them. indicates that motion perception mechanism is triggered independently of the perception of a single object from the two flashes
Two simultaneous percepts
– OFF AND ON IN FIXED LOCATIONS + MOTION – “OBJECTLESS” MOTION
Which perceptual theory is correct?
DIRECT Perception Theory is correct
Motion is a basic perceptual property. We are wired to perceive it from the retina (before brain)
In the ganglion cells, they are arranged to encode motion
If delay is long…
The movement must be slow in order for the delay neuron to be activated
What are Reichardt Detectors?
Direction sensitive motion mechanisms
- The basic requirement for velocity detection
- Opponent process arrangement to determine net motion
Algorithm:
Register: CHANGE AT LOCATION 1
– elapsed time –
Register: CHANGE AT LOCATION 2
Motion aftereffects
Occurs when detector gets fatigued from a certain direction of motion. When movement stops, the balance between the two directions is off, and we perceive motion in the direction of the non-fatigued, spontaneously firing neuron.
– Suggests opponent process arrangement in Reichardt Detectors
What is an example of motion aftereffects?
waterfall illusion
Explanation for stepping feet illusion
Magno-cellular pathway (for motion) does not see color, only luminance –> so stepping motion is only seen when luminance is the same for all blocks of color in the demo
