Cognitive ProcessesPSY 334

Chapter 5 – Meaning-Based Knowledge Representation

July 24, 2003

Propositional Representations

Notation – a method for describing the meaning that remains once details have been abstracted away.

Propositional representation – uses concepts from logic and linguistics to describe meaning.

Proposition – the smallest unit of knowledge that can be judged as true or false.

Propositional Analysis

A complex sentence consists of smaller units of meaning (propositions).

If any of the propositions are untrue, the entire sentence cannot be true.

The meaning of primitive assertions is preserved, but not the exact wording.

Kintsch’s Notation

Each proposition is a list containing a relation plus arguments:

(relation, arguments) Relation – organizes the arguments.

Verbs, adjectives, other relational terms. Arguments – particular times, places,

people, objects. Nouns

Relations connect arguments.

Psychological Reality

Psychological reality -- do propositions really exist mentally?

Bransford & Franks: Presented 12 sentences with the same 2

sets of 4 propositions. Tested on 3 kinds of sentences. Old

(previously viewed), new (containing same propositions), noncase (new and containing different propositions).

Able to identify noncase, but not old/new

Propositional Networks

Propositional network – another way of representing propositions (the structure of meaning).

Nodes – the propositions, including relations and arguments.

Links – labeled arrows connecting the nodes.

Spatial location of nodes is arbitrary. Can show hierarchies of meaning.

Associations Between Ideas

Weisberg – demonstrated that ideas are associated in the ways shown in a propositional network. Subjects memorized sentences. Given a word from the sentence, subjects

were asked to say the first word that came to mind.

Subjects cued with “slow” said “children” and almost never “bread”.

Conceptual Knowledge

Concept -- an abstraction formed from multiple experiences. Propositions – eliminate perceptual details

but keep relationships among elements. Categories – eliminate perceptual details

but keep general properties of a class of experiences. Used to make predictions. Two kinds: semantic networks, schemas

Semantic Networks

Quillian – information about categories stored in a network hierarchy. Nodes are categories. Isa links related categories to each other. Nodes have properties associated with

them. Properties of higher level nodes are also

true of lower level nodes linked to them. Categories are used to make inferences.

Psychological Reality of Networks Collins & Quillian – asked subjects to

judge the truth value of sentences: Canaries can sing – 1310 ms Canaries have feathers – 1380 ms Canaries have skin – 1470 ms

Frequently used facts also verified faster, so stored with node: Apples are eaten Apples have dark seeds

Schemas

Schema – stores specific knowledge about a category, not just properties: Uses a slot structure mixing propositional

and perceptual information. Slots specify default values for what is

generally or typically true. Isa statement makes a schema part of a

generalization hierarchy. Part hierarchy.

Psychological Reality of Schemas

Brewer & Treyens – subjects left in a room for 35 sec, then asked to list what they saw there: Good recall for items in schema False recall for items typically in schema

but missing from this room. 29/30 recalled chair, desk; 8 recalled skull 9 recalled books when there were none

Degrees of Category Membership Members of categories can vary

depending on whether their features satisfy schema constraints: Gradation from least typical to most typical.

Rosch – rated typicality of birds from 1-7: Robin = 1.1 Chicken = 3.8.

Faster judgments of pictures of typical items, higher sentence-frame ratings.

Disagreements at Category Boundaries

McCloskey & Glucksberg – subjects disagree about whether atypical items belong in a category: 30/30 apple is a fruit, chicken is not a fruit 16/30 pumpkin is a fruit Subjects change their minds when tested

later. Labov – boundaries for cups and bowls

change with context.

Event Concepts (Scripts)

Schank & Abelson – stereotypic sequences of actions called scripts.

Bower, Black & Turner – script for going to a restaurant.

Scripts affect memory for stories: Story elements included in script well

remembered, atypical elements not recalled, false recognition of script items.

Items out of order put back in typical order.

Two Theories

What happens mentally when we categorize? Two theories are being debated.

Abstraction theory -- we abstract and store the general properties of instances. Prototype theory.

Instance theory -- we store the multiple instances themselves and then compare average distances among them.

Neural Nets for Learning Schemas Gluck & Bower – designed a neural net

that abstracts central tendencies without storing instances. Patients with four symptoms classified into

two hypothetical diseases. One disease 3 times more frequent than

the other. Error correction changes the strength of

associations in the network (delta rule). Model predicted subject decisions well.

Evidence From Neuroscience

People with temporal lobe deficits selectively impaired in recognizing natural categories but not artifacts (tools)

People with frontoparietal lesions unaffected for biological categories but cannot recognize artifacts (tools).

Artifacts may be organized by what we do with them whereas biological categories are identified by shape.

Bartlett’s War of the Ghosts

Demo