Memory: Computation, Genetics, Physiology, and Behavior, Study notes of Animal Anatomy and Physiology

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Memory: Computation, Genetics,

Physiology, and Behavior

James L. McClellandStanford University

A Playwright’s Take on Memory

“What interests me a great deal is themistiness of the past”

Harold Pinter, Conversation prior tothe opening of

Old Times,

^1971

Your memories are in yourconnections •^

An experience produces a patternof activation over many neurons.

-^

The

memory trace

is adjustments

to connections among the neurons.

-^

The

memory-as-recalled

is a

pattern of activation reconstructedwith the help of the affectedconnections.

-^

Connections are affected by manyexperiences, so ‘recall’ is alwayssubject to influence from traces ofother experiences.

-^

Remembering is thus always aprocess of reconstruction.

Outline

What is “a memory”?

The core of contemporary thinking abouthow memories are formed and retrieved.

The Complementary Learning Systemsframework^ •

McClelland, McNaughton, and O’Reilly, 1995

Some physiological and genetic tests of thetheory

An Alternative Approach

Complementary and Cooperating BrainSystems^ –

Memory task performance depends on multipleinterconnected brain systems.

The contribution of each system to overall memoryperformance depends on its neuro-mechanisticproperties.

Systems work together so that overall performancemay be better than the sum of the independentcontributions of the parts.

The Complementary Learning

Systems Theory

(McClelland, McNaughton & O’Reilly, 1995)

Neuropsychological motivation

The basic theory

Neurophysiology consistent withthe account

Tests relying on geneticmanipulations

Time from experience to lesion in days

Control groups^ Lesioned groups

The Neuro-Mechanistic Theory:

Processing and Learning in Neocortex

•^

An input and a response toit result in activationdistributed across manyareas in the neocortex.

-^

Small connection weightchanges occur as a result,producing^ –

Item-specific effects

Gradual skill acquisition

•^

These small changes arenot sufficient to supportrapid acquisition ofarbitrary new associations.

Supporting Neurophysiological

Evidence

•^

The necessary pathwaysexist.

-^

Anatomy and physiology ofthe hippocampus support itsrole in fast learning.

-^

Reactivation of hippocampalrepresentations duringsleep.

Place Cells on a Triangular Track

Examples of neurons found in entorhinal cortex and hippocampal area CA3, consistent with the idea that the

hippocampus but not cortex uses sparse conjunctive

coding

Recording was made while animal traversed an eight-arm radial maze.

A simple pattern associator network

illustrating the problem of overlap

-^

Willshaw (1969) studiedsimple associative nets, inwhich connections could beeither on or off.

-^

Connections are switchedon to store associationsbetween designated inputand output patterns.

-^

When patterns overlap, thesynapses they use alsooverlap, producinginterference in memory.

-^

The probability that thepatterns will overlap goesdown if they are sparser(smaller fraction of unitsactive).