Traditional Theory of Perception · 1998-11-17 · Traditional Theory of Perception How do we...
Transcript of Traditional Theory of Perception · 1998-11-17 · Traditional Theory of Perception How do we...
Traditional Theory ofPerception
How do we perceive that an object has:- constant size,- constant form,- constant color,
despite: - different distances,- different perspectives,- different illuminations.
èè “Perceptual Constancies”• Ambiguity:
Distal stimulus : Proximal stimulus
• The same distal stimulus leads to different proximal stimulus and vice versa
Answer in Traditional (Cognitive) Theory
The correct solution is obtained from:
+ Size of retinal image+ Prior knowledge+ Proper cues for distance
==> Rules that relate retinal size, distance and cues==> Inferences==> Computations
Essential assumptions:
1) Perceptual inputs are insufficient sensory signals
2) Perceiver must interpret and construct meaning- ful information by means of experience
The World in three Dimensions:Depth Perception
Monocular cues:
- Linear perspective- Relative size- Changes in texture
Binocular cues:
- Binocular disparity
Gibson’s Theory of DirectPerception
Essential assumptions:
1) Perceptual information is detected by an activeperceiver
2) Information is specified with respect to a perceiver
3) Perceptual information is rich and does not require further inferences
==> Perception is direct
Direct Perception
What specifies (space-time) events?
=> Optic Flow
There is a characteristic pattern of change for everymovement:
Moving forward ==> outward flow pattern, expansion of optical texture
Moving backward ==> inward flow, contraction of optical texture
Rotation ==> horizontal translation
Important: Description is not confined to the layout of environment but is defined in relation to observer
Time-to-ContactWhat kind of information is actually used?
Example: An approaching ball has an expansionpattern projected on the retina
Definition: Relative rate of dilation of approachingobject specifies time to collision τ
Instead of computing variables such as distance,velocity and acceleration, time can be perceiveddirectly
Time-to-contact is defined between actor and visualinformation
Grasping Tau / Time-to-ContactExperimental Set-up
- Luminous balls in the dark- Deflating ball- Monocular vision only- Different ball sizes
Hypothesis:
If subjects rely on τ, then they would adjust handmovements to accommodate changing ball size
Variables:- Time of release- Time between initiation of grasp and ball contact- Continuous movements of the fingers (hand aperture)
Result:Hand aperture adjusts right until the last 30ms before ballcontact==> Subjects use the expansion rate of ball
Timing of Forehand Drive inTable Tennis
Task Constraints:
Ball is approaching at 17 m/s (temporal constraints)
Ball has to be propelled to a small target area
Variations in the angle of the bat can be maximally 6°(spatial constraints)
==> Initiation of swing at the right timeBat must be moved in controlled direction andwith controlled momentum
==> Total action time is less than 200 ms
What kind of visual information necessary?
- Time of initiation to run off a movement program or
- Continuous visual monitoring during execution?
Results:
- Important variable: direction of bat, not location
- Variability at initiation is less than at the endpoint
- Coupling between movement variables
Role of Visual Information for Patients
PARKINSON PATIENTS
They are able to catch a ball or intercept a soccerball without problems
Perception of a moving object enables patients toinitiate movements much easier
==> Perception of moving object provides more directinformation for actions
STROKE PATIENTS
Reaching for a stationary soccer ball and a movingsoccer ballQuality of movement of the affected arm in themoving task was faster and more quickly
==> Perception of change is primary==> Vision can drive the motor system when the
proprioceptive senses are disrupted