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