biologically-inspired control in problem solving · biologically-inspired control in problem...

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Biologically-Inspired Control in Problem Solving

Thad A. Polk, Patrick Simen,

Richard L. Lewis, & Eric Freedman

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Computational Models of Control

� Challenge: Develop computationally explicit theories of control deficits in complex problem solving– Where control deficits are often most apparent

� Symbolic models (production systems)– E.g., ACT-R, Soar, Epic, ... – Natural model of flexible, goal-driven behavior and therefore easier

to apply to complex problem solving

– But harder to map onto the brain and patients

� Neural networks– E.g., Cohen, Levine, Dehaene, Braver, O’Reilly, ...

– Neural mechanism for control (modulation) and therefore easier to map onto the brain and patients

– But harder to apply to complex problem solving

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Major Points

1. Natural & explicit mapping from goal-driven production systems onto neural computation

– Makes it possible to build plausible neural models of complex problem solving

2. Mapping leads to explicit hypothesis about the role of DLPFC in problem solving:

– Represents internally generated subgoals that modulate among choices

3. Applying to TOL accurately simulates human behavior– Intact model simulates normals, even on hardest problems

– Lesioning subgoal net simulates prefrontal deficits

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Plan

� Symbolic models of control

� A simple model of neural computation

� Mapping symbolic control onto neural nets

� Network model of Tower of London– Intact behavior– Damaged behavior

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Goal-Driven Production Systems

� Almost all models of complex cognition based on production systems (ACT-R, Soar, Epic)– Set of symbolic IF-THEN rules that match against memory

and take actions and/or change memory:

� Production systems proposed as control theory (Newell, 1973):– Allow behavior to be flexible, opportunistic, interruptible– Next step chosen dynamically based on what’s in

memory/world– Goals/subgoals modulate/control decisions

Example

Initial

Goal

•(External) Goal-driven control: Pick move to achieve ‘blue’ goal

•And so on...

Decompose problem intothree component goals

Set ‘blue’ goal

•Set goal to switch green & red

•Set (internal) subgoal: get ‘red’ off

•Achieve ‘get red off’subgoal

•Data-driven productions: Consider both legal moves

•Data-driven productions: Consider all legal moves

•(Internal) Goal-driven control: Pick move to ‘get red off’

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Key Features of Symbolic Control

� Data-driven production rules– Asymmetric associations between symbols

– Allows flexible behavior that reacts to current state

– E.g., Recognizing legal moves in current TOL state

� Top-down control from current goal/subgoal– Constrains data-driven processing– Both external goals and internally generated subgoals can control

– E.g., preferring moves that satisfy current subgoal over others

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Plan





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Neural Computation: Assumptionsfor Present Model

1. Neural processing is recurrent

2. Neural representations are distributed

3. Neural learning is correlation-based (Hebbian)

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Emergent Property:Attractors (Discrete Stable States)(Hopfield, 1982; 1984)

If distributed patternsoccur frequently, theybecome discrete stablestates for the network...

…and the network willconverge on them givenany similar patterns...

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If these patterns occurfrequently, then theybecome discrete stablestates for the network.

And the network willconverge on them givenany similar patterns...

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13




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15




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These “attractors” arediscrete & stable likesymbols.

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Plan





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Mapping Productions Onto Attractors

� Attributes = attractor nets/layers� Values = attractor patterns in the specified layer� Productions = associations between layers

IFLetter T

THENNumber 7

IFLetter L

THENNumber 1 Letter Number

Blue Production

Red Production

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IFLetter T

THENNumber 7

IFLetter L


Blue Production

Red Production


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IFLetter T

THENNumber 7

IFLetter L


Blue Production

Red Production


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IFLetter T

THENNumber 7

IFLetter L


Blue Production

Red Production


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IFLetter T

THENNumber 7

IFLetter L


Blue Production

Red Production


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IFLetter T

THENNumber 7

IFLetter L


Blue Production

Red Production


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Goal-Driven Control → Attractors

� Goals bias competition among attractors– A kind of conflict resolution

IFLetter1 T

THENResponse 7

IFLetter2 L

THENResponse 1

Letter2

Letter1

Initially, allpossibilitiesactivated…

Response

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IFLetter1 T

THENResponse 7

IFLetter2 L

THENResponse 1

Letter2

Letter1

…but attractorscompete…

Response

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IFLetter1 T

THENResponse 7

IFLetter2 L

THENResponse 1

Letter2Response

Letter1

…and eventuallyone wins.

Winner dependson associationstrengths & noise

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IFLetter1 T

THENResponse 7

IFLetter2 L

THENResponse 1

Letter2

Letter1

But a goal canbias competitionfor/against certainattractors...

Response

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IFLetter1 T

THENResponse 7

IFLetter2 L

THENResponse 1

Letter2

Letter1


Response

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IFLetter1 T

THENResponse 7

IFLetter2 L

THENResponse 1

Letter2

Letter1


Response

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Mapping Productions To Attractors


� We’ve implemented this mapping in Lisp/Perl:• Input: Simplified production rules• Output: Matlab code that implements attractor nets that

(often!) behave like the production system

� Can thus use the attractor architecture for some higher cognitive tasks

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Plan





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Modeling Tower of London

Move

Goal(External)

Subgoal(Internal)

Current State

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Move

Goal(External)

Subgoal(Internal)

Current State

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Move

Goal(External)

Subgoal(Internal)

Current State

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Move

Goal(External)

Subgoal(Internal)

Current State

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Move

Goal(External)

Subgoal(Internal)

Current State

Knowledge

Legal Moves to consider

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Move

Goal(External)

Subgoal(Internal)

Current State

Knowledge

Legal Moves to considerTry to achieve goals

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Move

Goal(External)

Subgoal(Internal)

Current State

Knowledge

Legal Moves to considerTry to achieve goalsSet subgoals to remove blockers

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Move

Goal(External)

Subgoal(Internal)

Current State

Knowledge

Legal Moves to considerTry to achieve goalsSet subgoals to remove blockersTry to achieve subgoals

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Move

Goal(External)

Subgoal(Internal)

DLPFC

Hypothesis: DLPFC represents internal subgoals– Modulates competition among posterior attractors– Note: Not needed for externally provided goals

Current State

The Role of DLPFC

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The Function of DLPFC SubgoalsCurrent Goal

100 120 140 160 180 200 2200

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

% M

axim

al A

ctva

tion

Simulation Cycle

MoveCurrent

Goal(External)

Subgoal(Internal)

Time

Act

ivat

ion

When subgoal net damaged,noise can overwhelm weakinput from subgoal leadingwrong move to be chosen.

“Correct” subgoal move(e.g., Red to peg 3) loses

“Incorrect” legal move(e.g., Red to peg 1) wins

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Human Data

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8

Problem Complexity(Min # Moves)

MinimumHuman MeanHuman Median

42 undergraduates (Intact?)18 TOL problems varying in minimum number of moves

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Intact Simulation

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8

Problem Complexity(Min # Moves)

MinimumHuman AvgHuman MedianPrediction

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Prefrontal Data

0

1

2

3

2 3 4 5

FrontalsControls

2

4

6

8

2 to 3 4 to 5

Owen et al. (1990) Carlin et al. (2000)

Problem Complexity(Minimum Number of Moves)

Group x Difficulty interaction:Prefrontal deficit is more apparent on harder problems

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Prefrontal Simulations

0

1

2

3

4

5

6

7

8

1 2 3 4 5

Intact20% Lesion40% Lesion60% Lesion

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Problems with Prepotent Responses

0

2

4

6

8

10

12

14

16

Prepotent Correct Prepotent Incorrect

MinimumIntact20% Damage30% Damage

Initial Goal GoalInitial

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Issues Raised by the Mapping� A number of features of production systems DON’T

map easily:– Variables– Independence/modularity of different productions– All-or-none matching

� Possible responses:– Neural nets lack critical functionality for modeling cognition

• Need to work on increasing their functionality

– Production systems have too much functionality• Might use less powerful production systems that map more

naturally

– Both are probably reasonable

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Summary

1. Natural & explicit mapping from goal-driven production systems onto neural computation

– Makes it possible to build plausible neural models of complex problem solving

2. Mapping leads to explicit hypothesis about the role of DLPFC in problem solving:

– Represents internally generated subgoals that modulate among choices

3. Applying to TOL accurately simulates human behavior– Intact model simulates normals, even on hardest problems

– Lesioning subgoal net simulates prefrontal deficits

biologically-inspired control in problem solving · biologically-inspired control in problem...

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