Anthony J Greene1 MOTION & EVENT PERCEPTION. 2 Event Perception Because perception evolved to...

Anthony J Greene 1

MOTION & EVENT PERCEPTION

2

Event Perception

Because perception evolved to provide organisms with world information, our ability to perceive events is the purpose of perception

Perception across both 3-d space and time

The integration of motion, form and color perception

Includes perceptual learning and locomotion

Most treatments of event perception include information from all sense modalities

Anthony J Greene 3

Motion Perception: Aperture Problem

4

Motion Perception: Aperture Problem

• Motion detection cells - recall that complex cells in v1 and thick stripes in v2 can detect motion in a very local receptive field of the retina

Stimulus

V5

V1

From Movshon et. al., 1985

Anthony J Greene 5

Motion Perception: Aperture Problem

• V5 contains cells which are selective for motion across the entire visual field - cooperation between v1 and v5 is necessary for motion perception to occur

• Some cells are in V5 respond selectively for straight line motion in a particular direction

• Others cells respond to expansion and contraction patterns

• Still others respond to rotation (in depth and in the picture plane)

Anthony J Greene 6

The Case of Motion Blindness• Damage to V5 may result in motion blindness• Stationary objects appear normally• When an object or a person moves, it simply

disappears• When the object stops moving, it simply

reappears• What does this imply about the interactions of

V3 and V5 for normal perception of a moving object?

Anthony J Greene 7

Waterfall afterimage

Anthony J Greene 8

Spiral afterimage

http://www.exploratorium.edu/exhibits/depth_spinner/index.html

Anthony J Greene 9

Structure From Motion

Anthony J Greene 10

Biological Motion

• In less than 1/2 second, almost everyone perceives a human form walking

• Although only motion information is available, recursive connections between v3 and v5 allow a stable 3 -d form to be perceived

• The sex, approximate age, height, and weight of the walker are readily apparent

Anthony J Greene 11

3-D Necker Cube

Anthony J Greene 12

3-D Necker Cube• As in a 2-d Necker cube, the depth is ambiguous

• In this case, however, 3-d form is bound to motion and the direction of rotation is therefore also ambiguous

• This is a classic example of unconscious inference

• Gibson would admit that unconscious inference is going on, but, asserts that this sort of ambiguity would not occur in the real world - this sort of ambiguity was never present in the environment in which visual perception evolved

Anthony J Greene 13

Rotating Trapezoid

Anthony J Greene 14

Rotating Trapezoid

Anthony J Greene 15

Rotating Trapezoid

• Because there are strong depth cues (linear perspective) it is natural to interpret the shape as a rectangle in depth

• Even knowing that it is a trapezoid does not allow us to override the perception that it is a rectangle

• In order for motion information to be paired with 3-d form information we interpret the motion as oscillation instead of rotation

• Infants and depth, size and shape perception

Anthony J Greene 16

Visual Cliff

• E. Gibson (Late 1960’s)• The Test Is To See If The Infant Will Crawl Across

The Plexiglas Covered Cliff Towards Her Mother On The Left

• What Dominates The Infants’ Decision? Tactile Or Visual Information

Anthony J Greene 17

Visual Cliff

• Result: Infants 6 To 14 Months Would Not Venture Over The Cliff - Does Not Address Whether Or Not This Is Innate Or Learned (Or Both)

• Experiment Was Repeated With 2-3 Day Old Kittens Deprived Of Depth Information With The Same Result

Anthony J Greene 18

Visual Cliff

• Infants Less Than One Day Old Were Placed Both On The Plexiglas And On The Platform, And The Infants That Were Over The Cliff Showed More Anxiety (Anxiety Was Measured By Galvanic Skin Response, Heart Rate, Etc.)

• Stereopsis Was Not The Determining Factor Because The Same Result Was Obtained When One Eye Was Covered

Anthony J Greene 19

Pseudo Visual Cliff• To determine if the visual cliff effect was due to

pictorial depth cues, or dynamic depth cues, a control study was run where a visual cliff was simulated by changing texture gradients

• Result: the infants did venture out over the “cliff”• Demonstrates that dynamic monocular cues, such

as motion parallax are required for convincing depth perception