3: Perception & Action

Question 1

Intro to constructivist approach

Answer 1

Perception involves combing information from the senses with knowledge.
Perception must be constructed from incomplete sensory input.
Top down processing fills the gaps.

Question 2

Gregory’s theory of indirect perception

Answer 2

Top down processing- Image arrives on the retina incomplete; needs to be supplemented by stored knowledge, past experiences etc.
Recognition driven- Purpose is to identify what we’re looking at.

• Perception is an active process.
• Requires interaction between sensory input and knowledge.
• Much information from the eye is lost before reaching the brain.
• Perception involves hypothesis testing.
• Incorrect hypothesis leads to perceptual errors.

Hypothesis testing- If told what an image depicts it resolves; sensory info hasn’t changed, knowledge has.

Question 3

Depth cues: Evidence supporting constructivism

Answer 3

Top down processing:
Depth cues- perceptual system needs to transform 2D retina image to 3D interpretation.

Monocular cues:

• Gives perceptual system impression of 3D; seen with one or two eyes.
• Linear perspective (converging lines gives impression of depth)- top down knowledge reminds lines are parallel.
• Aerial perspective (light scattered, distant objects hazy)- knowledge tells us image is 3D.
• Interposition (nearer object hides distant one).
• Shading- knowledge infers where light source is.
• Relative/familiar size- comparing size of image with knowledge; can judge how far an object it.

Also ocuhomotor cues (based on muscle movement) and binocular cues (need both eyes).

Question 4

Limitations of Gregory’s view

Answer 4

Vague areas- hypothesis testing.
How are they generated?
Which hypothesis do we accept?

Question 5

Purpose of perception: Evidence for two perceptual systems

Answer 5

Ventral stream- concerned with recognition.
Dorsal stream- Drives visually guided behaviours; processes motion.

Purpose of vision?
Dual process account (Norman)- suggests perception for action and recognition map onto these streams.
Action system- associated with dorsal pathway and Gibson’s theory.
Recognition system- associated with ventral pathway and Gregory’s view.

Evidence from illusions:
- Haart, Carey, Milne (1999)- 3D version of Muller-Lyer illusion; two conditions.
Matching task- indicate length using hands.
Grasping task- grasped target figure with thumb and index finger.
Found strong illusion effect when matching; not when grasping.

Question 6

Purpose of perception: Evidence from neuropsychology

Answer 6

Two systems:
Double dissociation following brain damage.
- Perenin & Vighetto- optic ataxia patiens; damage to dorsal pathway; impaired on visually guided movements.
- James, Cullham, Humphrey, Milner, Goodale (2003)- visual agnosia patients; damage to ventral system; impaired at object recognition.