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Cognitive ProcessesCS187
Dr Andre Gruning
Department of ComputingUniversity of Surrey
Email: [email protected]
SS 2008
Dr Andre Gruning Cognitive Processes
Imaging Part
Part
Part
Dr Andre Gruning Cognitive Processes
Overview
Organisation of world knowledge in terms of categories
(Why categories?)
Membership in a category? Typicality?
How similar are two categories?
What are the relations between categories? / Organisation ofcategories.
Theories of Categorisation
Dr Andre Gruning Cognitive Processes
Typicality
Sentence verification
In order to probe world knowledge and the relations between itsdifferent pieces, subjects are asked to tell whether sentence like thefollowing are true:
1 A canary is a bird.
2 An ostrich is a bird.
3 A potato is a tree
4 A rifle is a tree
Interesting? Yes, at least when you record error rates and reactiontimes.
Dr Andre Gruning Cognitive Processes
Typicality
Sentence Verification – Results
There is a Typicality Effect:
Question (1) (“Canary”) is on average answered faster thenquestion (2) (“Ostrich”)
There is a Category Similarity Effect:
It takes longer to reject (3) (“potato”) as untrue than (4)(“rifle”)
⇒ World knowledge is somehow structured. It takes differenttimes to access different pieces in different contexts
Dr Andre Gruning Cognitive Processes
Organisation of Categories
with the help attributesfrom Alan J. Parkin, Essential Cognitive Psychology, Psychology Press, Hove, 2005, p. 159.
Dr Andre Gruning Cognitive Processes
Organisation of Categories
Means of transportation.
On the black board.
Dr Andre Gruning Cognitive Processes
Relations between Categories
Relations between Categories
Superordinate: “animal” contains “fish”
Subordinate: “canary” is a “bird”
Siblings: “Shark” and “Salmon” are both members of “fish”
Default Inheritance: a category inherits attributes from itssuperordinate by default: “canary” can fly (from “bird”),breathes (from “animal”)
Overwriting Attributes: an ostrich cannot fly, though a “bird”can in general”
First approach to similarity: how many nodes to travel fromone category to the next.
Doesn’t predict the reaction times right.
Can’t explain typicality effects
Dr Andre Gruning Cognitive Processes
Attribute-based Categories
Attribute-based Categories and Similarity
there are core attributes (“must have” ones), (e.g. “horse” isfour-legged), defining in a strict sense
there are prototypical ones (“typical ones), (e.g. a “bird” canfly”, those typical one that spring to mind first.
for comparing categories:
check how many prototypical attributes agrees,if overlap not sufficient: check only on core attributes.
Problems:
What number of common attributes?Inheritance: what number of overwrite to allow?
Different definition of categories than by attributes?
Dr Andre Gruning Cognitive Processes
Categories
What is a “cup”?
Some attributes:
one can drink liquids from it
mainly hot liquids like tea, coffee, hot chocolate
round
has a handle
made of ceramics
not too big
Dr Andre Gruning Cognitive Processes
When is a cup a cup?
A psychophysical experiment: Subjects are asked how they wouldname the objects shown.taken from reproduction in Ellis and Hunt: Fundamentals of Cognitive Psychology, McGraw Hill, Boston, 1993, p.
210.
Dr Andre Gruning Cognitive Processes
Context dependence of Categories
Context dependence
Subjects who saw the picture on the slide before were either askedto imagine the objects to be filled with
with liquids (solid lines)
with food (dashed lines)
taken from reproduction in Ellis and Hunt: Fundamentals of Cognitive Psychology, McGraw Hill, Boston, 1993, p.211.]
Dr Andre Gruning Cognitive Processes
Context dependence of Categories
Results from Experimental Cognitive Science
Categories boundaries depend on context.
Categories are fuzzy.
Categories membership can be gradual.
Dr Andre Gruning Cognitive Processes
Categories
Theories of Categorisation
Exemplar Theory
Attribute Theory
Prototype Theory
. . .
Dr Andre Gruning Cognitive Processes
Categories
Attribute Theory
A category is defined through a lists of attributes and features
You have to live with exceptions:
Category “bird”Attribute “can fly”Instance “penguin”: attribute: “cannot fly”
works a bit like default inheritance in OO programming
subcategories/instances can overwrite defaults fromsupercategory
to test categories membership, just check the features. It’s adiscrete theory.
difficulties with “goodness-of-example” or “typicality effects”:
when all features/attributes are met, then recognition of“not-so-typical” instance should be as fast as very “central”instances: this is not the case!
Dr Andre Gruning Cognitive Processes
Categories
Prototype theory
A category is defined through a prototypeA prototype is like a quintessence or summary of all instances of thecategory you have ever encountered (or imagined)It is like an “ideal” member of the category, not necessary one thatexists in the real world.think of an ideal tree or ideal chair!There is some measure of the distance of an instance to theprototype: it’s a metric theory.The smaller the distance the more typical the instanceThink of the mountain landscape of a Hopfield network: distance isthe time it takes the ball (representing the instance) to roll down tothe valley (the prototype)Can deal easier with reaction times and typicality effects.Can deal better with fuzzy category membership.Fits better in with the “generation, reconstruction” paradigm ofmemory retrieval (⇒Ian Wells, last week)Smooth transition from exemplar based to prototype based categoriesin development (think of the irregular verbs!)
Dr Andre Gruning Cognitive Processes
Categories
Exemplar theory
all instances are stored
allows you to assess also the variability of the categories
no abstraction/generalisation over instances takes place
needs a lot of storage: memory is overloaded, so doesn’tsupport one main advantage of categorisation: datacompression.
maybe employed when you are building up a new category andso far have only a few instances for it.
Dr Andre Gruning Cognitive Processes
Summary
World knowledge as expressed in categories
Categories are organised in a (single?) hierarchy
Categories can be defined in terms of attributes or prototypes.
However there are often too many exceptions to allow forstrict and hard rules or clear category boundaries.
Dr Andre Gruning Cognitive Processes
Dr Andre Gruning Cognitive Processes