motion perception and action

Information provided by motion

• attracts our attention

• provides information about the object’s three-dimensional shape

• provides information that helps us segregate figure from ground

• provides information that enables us to interact with the environment

A motion illusion experienced when

objects in different locations are rapidly presented one after another

Types of apparent motion

• beta motion

• Phi motion

• Induced motion

Beta motion

perception of motion produced by sequential presentation of stationary images; these images appear to move

Phi motion

perception of motion produced by quickly alternating presentation of stimuli; illusory “shadow” image the same colour as the background appears to move

Induced motion

moving object causes perception of motion in a stationary object

Apparent motion: Wertheimer (1912)

• apparent motion perception depends on timing

• important variable is interstimulus Interval

Interstimulus interval (ISI)

time between offset of frame 1 and onset of frame 2:

• < 30 ms: simultaneity

• 60 ms: continuous motion

• >200 ms: succession

Apparent motion: Wertheimer (1912): but percept also depends on:

• distance between stimuli

• frame duration

• stimulus onset asynchrony (SOA)

SOA law

changes in ISI and frame duration do not affect motion perception unless there is also a change in SOA

Motion correspondence problem

which elements in frame 1 correspond with which elements in frame 2?

• for N elements, there are N! possible interpretations

• Possible alternatives:

  
  

Nearest-neighbor principle (Dawson, 1991)

select alternative that minimizes overall “traveling distance” of elements

• solution for motion correspondence

Motion correspondence: problem

• match is made based on Intensity; not necessarily shape

• in absence of intensity differences, texture is used

Wagon wheel illusion (Finlay et al., 1984)

• occurs when a rotating spoked wheel is perceived as rotating backwards; often seen in films or on TV

• due to a discrepancy between the speed of rotation and the rate of filming (temporal aliasing)

• interestingly, this effect may also obtain in continuous light (not stroboscopically, as on film)

• this may be due to adaptation of certain motion-sensitive neurons as in motion aftereffects

• Motion after effect

Wagon wheel illusion (Finlay et al., 1984): with small angular displacement (i.e., slow rotation speed), a spoke in one frame is correctly matched to the nearest neighbour (itself) in the next frame

e.g., spoke matched to another at rate of 90 °/frame correctly appears to rotate clockwise

Wagon wheel illusion (Finlay et al., 1984): however, at higher speeds there is greater angular displacement, and the nearest neighbour in the next frame may cause misinterpretation of rotation direction

e.g., spoke matched to another at rate of 270 °/frame incorrectly appears to rotate counterclockwise

Wagon wheel illusion (Finlay et al., 1984): motion aftereffect

after adapting to motion in one direction, a stationary object is perceived to be moving in the opposite direction (illusory motion)

• suggests that motion-detecting neurons are wired in opposition to each other

• implies that motion is a separate aspect of vision

Ramachandran & Gregory (1978): does magno handle motion?

• used isoluminant stimuli

• there are no lightness differences

• stimuli were moved against the background

• difficult to perceive motion

• Conclusion: magno handles motion; parvo not responsible.

Isoluminant stimuli

contains lines and forms which differ from the background only on the basis of hue

Hassenstein-Reichardt motion-detector circuit (1956; Reichardt, 1961)

• receptors A-F are pointed at different locations in space

• cells G, I, K inhibit neighbours when motion is to the right

• cells H, J, L pass on activity when motion is to the left

• cell M signals leftward motion

Hassenstein-Reichardt motion-detector circuit (1956; Reichardt, 1961): problems

• following a moving object with your eyes (pursuit)

• sweeping your eyes across a stationary scene (Saccade)

Hassenstein-Reichardt motion-detector circuit (1956; Reichardt, 1961): solution

take movement of eyes into account

Pursuit

object is perceived as moving, even though retinal position doesn’t change

Saccade

appears to remain motionless

Corollary discharge theory (Sperry, 1950; von Holst & Mittelstaedt, 1950): Components

• image displacement signal

• Motor signal

• corollary discharge signal

• Comparator

Image displacement signal (IDS)

detects motion across retina

Motor signal (MS)

muscle-movement information sent to eye muscles

Motor signal (MS)

muscle-movement information sent to eye muscles

Comparator

determines perception of motion; occurs when:

a) IDS present, or

b) CDS present (but not both)

Motion perception with CDS

• observing afterimage in a dark room

• Pushing on eyeball but keeping eyes stationary

• pursuit: moving object, eyes move with it

  • e.g., tracking a car driving past: no IDS)

• Eyes paralyzed, try to move your eyes (Stevens et al., 1976)

motion perception with CDS: Observing afterimage in a dark room

retinal image stationary, eyes moving

Motion perception with CDS: pushing on eyeball but keeping eyes stationary

muscles push back, causing CDS

Eyes paralyzed, try to move your eyes (Stevens et al., 1976)

retinal image stationary, but CDS sent

Biological motion: Johansson (1973)

• attached 12 light bulbs to joints of a co-worker

• filmed point light walker in a dark room

• result is clearly identifiable as a person, even after only 200 ms

Biological motion: can be used

• to identify dancing couples

• to recognize one’s own walk (or that of friends)

• to determine sex, within 4.4 seconds of viewing (men have greater shoulder than hip movement; vice-versa for women)

• by cats (to detect other cats)

• even when display is masked by randomly moving dots (albeit with decreased accuracy)

Biological motion: may be due to evolution

• biological predisposition to recognize humans and animals

• is preferred by infants by 4 months of age (but usually develops between 6-9 months)

• associated with increased activity in superior temporal sulcus (Puce & Perrett, 2003)