Neuroscience of Decision Making Sam uel McClure Psycholog y - - PowerPoint PPT Presentation

Neuroscience of Decision Making

Sam uel McClure Psycholog y Departm ent 1 5 May 2 0 0 8

The Ultim atum Gam e

• $ 1 0 to divide between two players
• “Proposer” chooses a division
• “Receiver” can either

– Accept: both receive proposed am ounts – Reject: both receive nothing

• How m uch should the Proposer offer?

The Trolley Problem

8 5 %

The Footbridge Problem

1 2 %

Intertem poral choice

• Choice involving rewards available at

different tim e delays

Example

W ould you rather have $ 1 0 today or $ 1 1 tom orrow? W ould you rather have $ 1 0 in a year or $ 1 1 in a year and a day?

Can behavior be captured by form al m odels?

The astonishing hypothesis

• “You, your joys and your sorrows, your

m em ories and your am bitions, your sense

• f personal identity and free will, are in

fact no m ore than the behavior of a vast assem bly of nerve cells and their associated m olecules” (Crick)

• W ith m echanistic / com putational m odels
• f the brain we m ay be able to derive

laws of behavior

Introduction to the brain

• Approxim ately 1 0 0 billion neurons
• Each with up to 1 0 ,0 0 0 connections

with other neurons

• Most com plicated com putation device

known in the universe

• Com m unication via electrical im pulses

and propag ated by chem ical synapses

Brain Recording Methods

Brain Stim ulation Reward

A neural substrate that replaces external reward

Midbrain dopam ine system

Single-unit Recording

• Encode a

reward prediction error

• For a stim ulus

that predicts reward, dopam ine f i ring indicates value

• f reward

(Schultz et al., 1 9 9 7 )

Role for the prefrontal cortex

• Greatly expanded in people com pared to other anim als
• Supports higher cognitive processes (working m em ory)
• Critical for decision-m aking (e.g. shopping experim ent)

Recording hum an brain activity: Functional Mag netic Resonance Im ag ing

• Allows m easurem ent of brain

activity at the scale of individual brain structures (m m )

• Tem poral resolution is on the
• rder of m any cognitive processes

(sec)

• Non-invasive

→ For the f i rst tim e we can m easure close to the root cause of decision m aking

Hypothesis

Dopam ine-related reward system s discount reward at a higher rate than does the prefrontal cortex. tim e discount value prefrontal cortex DA-related system s 0 .0 1 .0

2 s 12 s Free Response

d d'-d (R'-R)/R ∈ { Today, 2 weeks, 1 month } ∈ { 2 weeks, 1 month } ∈ {1%, 3%, 5%, 10%, 15%, 25%, 35%, 50%}

10

T13

A

d = Today d = 2 weeks d = 1 month

B

VStr MOFC MPFC PCC % Signal Change

• 0.2

0.0 0.2 0.4 Time (s)

• 4

4 8

z = -4mm

MOFC

y = 8mm

VStr

x = 4mm

MPFC PCC

Reg ions that respond preferentially to im m ediately available rewards

x = 44mm x = 0mm

d = Today d = 2 weeks d = 1 month

A B

10

T13

VCtx PMA RPar DLPFC VLPFC LOFC Time (s)

• 4

4 8 % Signal Change 0.0 0.4 0.8 1.2 RPar DLPFC LOFC VCtx PMA SMA

Reg ions that respond equally to rewards at all delays

Choose Early Choose Late 0.0

• 0.05

0.05 dopamine prefrontal Normalized Signal Change

Relative activity by choice

Contrast Value

2 0 1 0 0 4 0 6 0

Accept Reject

Unfair Offers

Prefrontal Cortex Insular Cortex

Am ygdala and the Ultim atum Gam e

Sanfey et al. (2 0 0 3 )

Medial Frontal Cortex Dorsolateral Prefrontal Cortex (PFC) Angular Gyrus Posterior Cingulate/ Precuneus

fMRI Results

Greene et al. (2001)

Moral Personal Dilem m as Moral Im personal Dilem m as Non-m oral Dilem m as Emotional Areas “Cognitive” Areas

Brain Activity

VMPFC and m orality

People are utilitarian without VMPFC

Transcranial Mag netic Stim ulation (TMS)

Stim ulating DLPFC increases acceptance of unfair offers in the Ultim atum Gam e

1 6 /4 offers

Are people generally rational?

• “Autom atic” brain responses

create biases and heuristics that are probably g enerally correct

• W e also have a prefrontal cortex

to enable f l exible behavior and correct decision errors (som etim es)