Lecture 6: Perception Flashcards
What is perception?
Perception is how we recognise, organise and make sense of ewhat we hear, smell etc
Perception involves manipulation of sensory infomation but it also involves linking these sensations to representations
What is the relationship between perception and reality?
Do we percieve the world as our eyes really see it or does our mind interpret the world?
Is perception the passive receipt of signals in the nervous system or is it actively shaped by learning, memory and expectation
Gibson’s (1966,1976)
Bottom up
Distal object –> Informational medium –> Proximal stimulation –> Perceptual object
How to study anomalies of perception
Psychologists study anomalies of perception to understand how we make sense of our sensations
- When do sensations become representation?
- What are the (physical) requirements for sensations to form a mental representation?
- The point of recognition is called a percept- the point at which the mind forms a mental representation
Perceptual illusions
- Perceptual illusions suggests that what we sense (with our sensory organs) is not necessarily what we percieve (with our minds)
- Our minds must be manipulating infomation to create mental representations
- Artists and Architects use optical illusions to construct realities
Gestalts theory of visual perception
- The world and it’s objects provide all the necessary information we need to perceive things
- Anti-cognitive theory→ we don’t need higher cognitive processes to mediate between our sensory experience and our perception
- Distal object stimulate eyes via the informational medium, this consistent experience causes a perceptual object
Flows only one way to the bottom up
Gibson argues that in the real world all we need is sufficient contextual information (built up over time) to make perceptual judgement - Developed in Germany around the 1920s
- How we organise complex visual array into groups
law of Pragnanz
- we tend to percieve visual arrays in the simplist way
- we tend to organise disparate elements into a stable and coherent whole
Gestalt principles
Figure ground: some aspects of a scene stand out (figure) while other recede (ground)
- Grouping of objects according to proximity and simularity
- Emergence: is the process of complex pattern formation from simpler rules
- Continuity: prefer continous representation over discontinous
- closure: tendency to close up object that are not actually closed
- Multisability: is the tendency of ambiguous perceptual experiences to pop back and forth unstably between two or more alternative interpretations
- Reification is the constructive or generative aspect of perception, by which the experienced percept contains more explicit spatial infomation than the sensory stimulus on which it is based
Invariance is the property of perception whereby simple geometrical objects are recognised independent of ration, translation and scale; as well as several other variations such as elastic deformations, different lighting and different component features
Cognitive perspcective
From a cognitive psychological perspective, perception is not ‘seeing’ a cat
- Rather, it is the subjective experience of our mental representations of the cat produced by our (conscious) brain
- Therefore, symbols, concepts, percepts and other kinds of mental (e.g. of cats) are neural correlates of what we perceive as real
Everything we see is perceived by sensory information, which evokes a mental representation → becoming a persept
There are two different views on how we percieve the world
- bottom up theories
- Top down theories
Bottom up processing: Gibson
- Also known as Ecological perception
- Opposed to asscociationism
- Perception is based on proximal stimulus only
- We do not need higher cognitive processes to mediate between our sensory experience and our perceptions (externally driven attention)
- The real world provides sufficient infomation to make percptuaol judgement
Optic flow patterns provide important infomation about movement
- Flow of the optic array provides infomation about the perceiver’s movement either towards or away form a particular point
Invariant features offer important cues about the enviroment
- Textures expand and contract according to whether you are moving towards or away from an object
- The flow of texture and perspective is invariant, ie let always occurs in the same way as you move around an object
- Texture gradient gives the apperance of depth
- Linear perspective- parallel lines appear to converge as they recede into the distance
Gibson’s 3 main components
- Optic flow patterns
- Invariant features
- Affordances
Gibson’s centeral concept: Affordances
What an object means for us is determinded by its physical characteristic
- The quality of an object ‘affords’ certain actions
- Affordances are cues in the enviroment that aid perceptions
What are the two cues from the enviroment that aid perceptions?
- Perceptual constancy
- Depth cues
Perceptual constancy
- Perception of object remains the same even if sensory information relating to the distal object changes (Gillan 2000)
- Shape constancy
- We perceive objects as constant in size or shape even though they appear different
- Size constancy is the perception that an object remains the same size of the proximal stimulus changes
Depth cues
Distance from a surface (often our own bodies)
- Monocular and binocular depth cues
–> Monocular can be represented in 2D ]
–> Binocular depth cues use the relative positioning of your eyes
examples:
- Texture gradient: texture enable a sense of depth, if there is less tecture we can infer that the coverered parts are further away than whats nearer
- Interposition/occulsion
- Linear pespective and relative size: Convergence of the lines towards a vanishing point
–> Decreasing size of the columns plus size constancy increase the effect - Motion parallax
Binocular depth cues
- Binocular disparity: Is where both eyes send increasingly different infomation as the object approaches
- Binocular convergence: is where both eyes increasingly turn inward as the object approaches, brain interprets muscular movement as indications of distance
Duality of perception
Bottom-up processing
- Known as ‘ecological’ or ‘direct’ perception
Top-down processing
- Known as constructive processing or ‘intelligent perception’
- Driven by higher-level cognitive processes ie. expectations and knowledge
Bottom up processing
The enviroment provides perceptual infomation eg. depth cues
-Texture gradient
- interposition/occlusion
- Linear perspective
- Relate size of objects
- binocular convergence
-binocular disparity
Top-down processing
Driven by higher-level cognitive processes, ie expectations and pre-exisiting knowledge
- Perceptions are based on three things
–> What we sense (sensory)
–> what we know (knowledge)
–> what we infer (high-level)
Knowledge is stored in memory
Evidence of context effects : top down processing
The surrounding enviroment affects the speed and accuracy of our perceptual processes
- Palmer (1975)
- Corell, Park, Judd and Wittenbrink (2002)
- Gilbert and Hixon (1991)
Show image of kicthen followed by one of the other images
Appropriate items recognised more quickly
Activation of the construct ‘kitchen’ in the memory makes it quicker to recognise subsequent objects
- Assimilation of infomation from a number of sources: not just the objects themselves
Correll, Park, Judd and Wittenbrink (2002) ‘shoot/don’t shoot’
- Participants were more likely to shoot an unarmed African American compared to a European American
- Perception of the object was influenced by context
Context effects: Gilbert and Hixon (1991)
Last study
- Watch a video tape in which someone holds up the cards with the word fragments
- What could the solutions to these words be?
- European American or Asian American
Shy, short, rice, polite
- Participants in the Asian
American condition made more stereotypical word completions
Classic context effect of bringing a particular set of representations to mind
Two theories of top down processing
- Prototype theory
- Feature matching theories
Prototype Theory
- Detailed representations stored in memory
- Group by similiarity in features
- ‘Average’ model based on characteristic features (Malt and Smith 1984)
- Perceptions involves matching the stimulus with the prototype
eg. recognise cat becaiuse it has 4 legs, whisters pointy ears and long tail
Feature matching theories
- Pandemonium model ( Selfridge 1959)
- Feature integration theory (Triesman)
Pandemonium model (selfridge 1969)
Pandemonium model = chaotic and noisy place in hell > taken from Milton’s Paradise Lost
- Metaphorical ‘demons’ organised into a hierarchy which analyse features
- Matching features to what is known in memory And the one with the strongest ‘shout’ is heard by the decision
So there is a combination of bottom up and top down
If there is some damage in the system/interruption there is other ways around
Feature Integration Theory (Treisman 1986)
Object–> preattentive stage (features seperated) –> Focused attention stage (Features combined) –> Perception
Evidence of feature matching:
Visual object agnosia:
- recognition of features but inability as a whole
–> Kolb and Whishaw
- Hubel and Wiesel (1979) measured the response of individual neurons to visual stimuli in the visual cortex
- They found specific neurons respond to specific stimuli
- Cells that recognise complex objects are called ‘gnostic units’ or grandmother cells’
Facial recognition
- Development evidence for facial recognition (Farah 2000)
- People have two systems for recognising faces
–> feature analysis system
–> Configuration system - Evidence that facial recognition is something specific
What is prospagonsia
The inability to recognise faces otherwise known as ‘face blindness’
Recognition of emotion in faces
- we seem to process faces differently according to their facial expression
- Pop-out effect of angry faces (Hansen and Hansen 1988)
- We seem to process faces differently according to their facial expression
Combination of top down and bottom up approach: Connectionist neural net
- Parallel distributed processing (PDP)
- Processing takes place simultaneously
- items of infomation are not stored at specific sites but distributed throughout whole net
–> Cognitive processing is not sequential
Connectionism and perception
Parallel processing of sensory infomation and mental representations
- These inputs could be sensory inputs
- But they could also be spreading activation from another part of the system eg. memories or learned associations triggered by the context
- Cascading waves of activation are strengthened or weakened by activity in other parts of the network
Conclusion
- Top-down theories argue that sensory infomation is organised by higher cognitive processes
- Bottom-up theories do not adequately explain context effects, how the configurations of objects affects perceptual processing
- people activate schemas, templates and stereotypes to process sensory infomation
- There are two theories of topdown processing: protoype and feature matching
- Brain injuries and neural imaging studies also support top-down theories
- Top down and bottom up approaches constitute the duality of human perception, which might be integrated by a connectionist model of the mind
