What is the process of perception?
Obtains information about distal stimuli from proximal stimuli for behaviour and experience
What is Distal Stimuli?
Objects in the environment
What is Proximal Stimuli?
Patterns of energy detected by sensory organs
Two approaches to visual perception
Direct Perception & Indirect Perception
What is Direct Perception?
- naïve realism
- ecological optics (Gibson)
- what you see is what you get
What is Indirect Perception?
- constructivism (Gregory & von Helmholtz)
- what you see is not what you get
What is Naïve Realism?
- objects are as they appear to be (3D)
- “common sense” intuition
- there is nothing to perception
4 reasons why naïve realism is wrong
Perceptual phenomena:
- ambiguous stimuli
- context illusions
- visual illusions
- impossible objects
Ambiguous Stimuli
- 1 stimulus causes 2 percepts - so perception can’t be direct - e.g. necker cube, Schroeder staircase
- figure-ground ambiguity - e.g. rubin vase, Jastrow’s duck-rabbit (1899)
- ambiguous motion - e.g. bi-stable quartet
Context Illusions
- contextual info - ‘surrounds a stimulus’
- 1 ambiguous stimulus in 2 different contexts
- causes 2 percepts - so perception can’t be direct
Examples: - Titchener-Ebbinghaus illusion
- Muller-Lyer illusion - lines perceived as a whole rather than as individual lines
- Ponzo illusion
- Hallway illusion
- moon illusion
- Zöllner illusion
- Orbison illusion
- Poggendorf illusion
- Fraser illusion
- café wall illusion
- colour context illusion - von Helmholtz & White’s illusion
Visual Illusions
- stimulus casues a percept that is at odds with reality
- so perception can’t be direct
- subjective contours - e.g. Kanizsa Triangle, neon colour spreading illusion, ouchi illusion
Impossible Objects
- they can’t exist but they don’t look obviously impossible
- e.g. M C Escher
Gibson: Direct Perception
- perception = ‘keeping in touch with the enviro’
- his theoretical approach is ecological
- proximal stimulus signals important info about distal stimulus - e.g. centre of explansion
- processing unimportant: info ‘picked up’, mechanical resonance (e.g. radio), no processing required
Gibson’s Direct Theory (1950, 1966, 1979):
* optic array = pattern of light reaching eye which contains visual info
* optic array provides unambiguous/invariant info about objects’ layouts - in many forms: texture gradients, optic-flow patterns & affordances
* optic flow = changes in light pattern created when observer or parts of enviro moves
* optic flow - provides useful info about the direction of heading
* focus of expansion = the point the observer is moving towards - doesn’t appear to move but visual enviro moves away from it
* invariants = proporites of optic array that remain constant despite other aspects varying
* affordances = uses of objects which are perceived directly
Evaluation: Gibson’s Direct Theory
Strengths:
+provides good explanation of why most people perceive the same thing very similarly (this suggests that** individual interpretation** of something doesn’t happen)
+the theory also gives support to why people are able to perceive and process information very quickly (they don’t have to think or retrieve memories, as their perception is instant or direct)
Weaknesses:
-underestimated complexity of visual processing & object recognition
-oversimplified effetcs of motion & perception
-eco validity isn’t as important for perception than it is for social psych
-but perception of invariants is more complex than Gibson implies: requires processing, knowledge, radios can’t hear themselves
-reductionist: only takes into account the person’s environment and instincts, when other theories suggest that memory is also involved in perception
-cannot explain why some perceptions are incorrect: if we see things directly the way they are, then misinterpretation should not happen
-theory cannot account for visual illusions – these should not occur if people are only seeing precisely what is in front of them
Indirect Perception: Constructivism
- perception is more than direct registration of sensations (indirect)
von Helmholtz & Gregory: - unconscious inferences
- expectations help shape what we see
- fast & unconscious
. - objects are not as they appear to be
- internal - representations
von Helmholtz (1821-1894) - proximal stimulus lacks info - depth, colour
- therefore percepts are constructed to replace missing info
Impoverished Proximal Stimulus
- 2D retina
- 3D world - size vs distance, shape
- ambiguous - many percepts possible
- Ames projection - distorted room
Bottom-up Processing
- processing that’s directly influenced by environmental stimuli
- direct perception
- stimulus -> info arrives at retina -> processing
Top-down Processing
- stimulus processing that’s influenced by factors such as past experience & expectation
- expectations & previous experiences -> processing
What did Gregory say about bottom-up & top-down processing?
Perception involves both bottom-up and top-down processing - there’s an intelligence to perception
The process of Vision
- cones (colour vision) & rods (motor detection) in retina
- retina-geniculate-striate pathway between eye & cortex is divided into partially separate P & M pathways
- dorsal stream (M pathway) terminates in parietal cortex
- ventral stream (P pathway) terminates in inferotemporal cortex
Zeki’s Functional Specialisation Theory - with Evaluation
- different parts of cortex are specialised for different visual functions
- support: patients with selective visual deficits (e.g. achromatopsia, akinetopsia)
- criticism: less specialisation than claimed, several binding problems
Two Visual Systems - with Evaluation
Milner & Goodale (1992):
1. vision-for-perception system based on ventral stream
2. vision-for-action system based on dorsal stream
* predicted double dissociations found in patients with optic ataxia (damage to dorsal stream) & visual form agnosia (damage to ventral stream)
* the ventral stream is involved in the perception of information about objects (vision for perception) and the dorsal stream processes information to guide actions (vision for action)
* criticism: 2 systems interact & combine with each other more than implied
* criticism: visually guided action relies more on ventral stream than implied
Colour Vision
- helps to detect objects & make discrimations among them
Dual-Process Theory (Hurvich & Jameson, 1957): - 3 types of cone receptors
- 3 types of opponent processes (green-red, blue-yellow, white-black)
- theory explains negative afterimages (illusory perception of complementary colour to the one that’s fixated - e.g. green to red) & colour deficiency
- factors producing levels of colour constancy: local & global colour contrast; familiarity of object colour; chromatic adaptation; cone-excitation ratios
- cells demonstrating colpur constancy found in area V4
Common type of Colour Deficiency
Dichromacy
* deficiency in colour vision
* 1/3 cone classes missing
