Intro to constructivist approach
Perception involves combing information from the senses with knowledge.
Perception must be constructed from incomplete sensory input.
Top down processing fills the gaps.
Gregory’s theory of indirect perception
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.
Depth cues: Evidence supporting constructivism
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).
Limitations of Gregory’s view
Vague areas- hypothesis testing.
How are they generated?
Which hypothesis do we accept?
Purpose of perception: Evidence for two perceptual systems
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.
Purpose of perception: Evidence from neuropsychology
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.
