Perceiving Motion

Question 1

Motion Sensitivity

Answer 1

Depends on visual reference objects
–Empty field: 10-20arcmin/sec (“subject relative”)
–Background references: 1-2arcmin/sec (“object relative”)

Question 2

How do we perceive motion?

Answer 2

There are two kinds of information available to us:

• Retinal displacement: the changing position of an object’s image on your eye
• Optical pursuit: the movement of the eye in its socket to “track” an object –i.e., to keep its retinal image landing on the fovea

Question 3

How do we use that information? (Apparent Motion)

Answer 3

When two objects flash on and off in two separate locations, we sometimes perceive movement from the first temporal location to the second –even though no object ever occupied the intermediate locations

**Apparent motion is also called stroboscopic and beta motion

Question 4

Using apparent motion to understand motion perception

Answer 4

What variables determine whether we do or do not perceive this “apparent” motion when two objects flash on and off in different locations?

Asking just that question helped us to determine which of two theories of motion perception was correct: the indirect or the direct theory

Question 5

Indirect theory

Answer 5

–brain must first calculate change in position of X and change in time before it can find the velocity

Question 6

Direct theory

Answer 6

motion is perceived from the get go; it does not depend on other calculation

Question 7

Exner’s experiment on apparent motion Part 1:

Answer 7

Determining the briefest temporal interval for which we can recognize temporal order

–Flash two shapes in same location in rapid succession…

–How much time do we need between the presentations in order for subject to be able to judge which shape was presented first? 45ms!

Implication: prior to 45ms, we don’t have information about t!

Question 8

Exner’s experiment on Apparent Motion Part 2:

Answer 8

Determine whether we can perceive motion at intervals briefer than the 45ms temporal sensitivity limit

Results: People can perceive apparent motion with ISI’s as low as 15ms! This is so brief they don’t have access to change t

Question 9

Exner’s experiment results

Answer 9

–People require intervals of at least 45ms to tell temporal order (to know change in t)

–People can perceive apparent motion with ISIs as low as 15ms!

–We know they’re perceiving apparent motion because they can use it to correctly report which location was presented first

Therefore, we can’t be calculating velocity as
Because we perceive motion without knowing Change in t!

Meaning: The indirect theory is wrong

Question 10

Direct Theory of Motion is correct. What does this mean?

Answer 10

–Motion is a fundamental perceptual quality

–Motion is perceived independently from other qualities (such as shape, color, etc.)

–Your brain is “wired” to perceive motion (if the indirect theory were correct, it would mean that motion is inferred on the basis of other percepts)

Question 11

What two experiments support this theory? (That Direct Theory of Motion is correct)

Answer 11

– Wertheimer’s “Phi” motion

• Pure, “objectless” motion
• A compelling percept of motion that is not attributed to any object • Thus, motion and object perception are completely separable

– Exner’s experiment on apparent motion:

• Shortest ISI needed to tell the temporal order of 2 successive flashes = 45ms
• Shortest ISI needed to tell apparent motion direction = 14ms
• Thus, motion is processed BEFORE temporal order of stimuli

Question 12

What might a “direct” motion detector look like?

Answer 12

Reichardt Detector: change at location 1, lapsed time change in location 2.

Question 13

Reichardt detectors are simple mechanisms that can

Answer 13

explain how we see motion in different directions and at different speeds

– Arranged to detect in all different directions
–Evidence suggests opponent process arrangement
to determine net motion

Question 14

Opposite direction case in Reichardt detectors

Answer 14

no motion signaled
opponent process arrangement
Can explain motion aftereffects

Question 15

MAE: Waterfall Illusion

Answer 15

• Base level of response from each of the opposing detectors
– Eyes are constantly drifting around, stimulating motion detectors at least slightly

• Exposure to a downward direction of motion (waterfall)
– Fatigues motion detector for downward direction

• For a stationary rock, reduced response from the fatigued detector
– Shows net motion signal in the upward direction

Question 16

Problems in Motion Perception: Correspondence Problem

Answer 16

• The problem of matching a feature in one image to a feature in another image

–Either the left and right eye images from the same time point, or

–Images from the same eye at two different time points

Question 17

How do we solve the correspondence problem?

Answer 17

We’re not sure yet, but these two constraints help us limit the number of possible solutions:

• Uniqueness constraint: a feature in the world is represented exactly once in each retinal image
• Continuity constraint: except at the edges of objects, neighboring points in the world lie at similar distances from the viewer

Question 18

Aperture problem

Answer 18

• What is the aperture problem?

–The fact that, when a moving object is viewed through an aperture (receptive field), the direction of motion of a local feature or part of the object may be ambiguous

• What part of the brain is believed to be involved in solving this problem?

–Local information is probably integrated into global motion in area MT/V5 (possibly also LGN).

Question 19

Distinguishing self-motion from other motion: the problem

Answer 19

• When you move your body or your eyes, it produces the same translation of images across the retina as is caused by the movement of objects.
• How does the brain discriminate the motion signals caused by the movement of objects in the world from the motion signals created as a result of your own (self) movement?

Question 20

Distinguishing self-motion from other motion: a partial solution

Answer 20

• We know, at least, part of the solution to this problem
• Any time your motor system sends a motor signal to your eye, it sends an efference copy to the comparator in visual cortex
• We think the comparator then accounts for the motion signals generated by the rotation of the eye in its socket

Question 21

First Order Motion

Answer 21

• The motion of an object that is defined by luminance.

* (luminance defined objects change position over time)

Question 22

Second Order Motion

Answer 22

• Motion that is not defined by the type of luminance edges that would be picked up by V1 cells, as the average luminances across the “edge” are identical.
• Moving contour is defined by changes in contrast, texture, flicker, or some other nonluminance quality
• There is no real object motion, as there exists no stable object surface that could move

Question 23

First-and Second-Order Motion Perception

Answer 23

• There is much evidence to suggest that early processing of first-and second-order motion is carried out in separate pathways. First-and second-order signals appear to be fully combined at the level of area MT