Reading electric minds:

How blood reveals the spark

Michelle Rigozzi

Integrating the evoked response potential (ERP) with the functional magnetic resonance imaging blood oxygen level dependent (fMRI BOLD) response

In the beginning…• Stimulus

• Neurons fire

• Oxygen deficiency.

Increase in dHb

• Blood rushes in

• Oxygen excess. dHb

drops

ERP

• Evoked response potential reads neuronal activity

• Advantage:– High temporal

resolution

fMRI

BOLD

• Functional magnetic resonance imaging

• Blood oxygen level dependent reads dHb levels

• Advantage: High spatial

resolution

What we know

Yet we are still to uncover…

…the gap• Is one really just a slow version

of the other?

• How is the underlying neuronal activity (ERP) manifested through the haemodynamic BOLD response?

• New insights for future models?

Super models

• Robinson & Rennie – neuronal model– P. A. Robinson, C.J. Rennie et al., 2002.

Neurophysical Modeling of Brain Dynamics. Phys. Rev. E.

• Friston – haemodynamic model– Friston, K.J., 2003. Dynamic Causal

Modelling. NeuroImage 0.

Real sparky

Friston

Robinson & Rennie

Neuronal networksElectricalResponse(Activity)

input BOLD

∫ing models…

Testing, testing…

• General behaviour, reliability and limits of equations

• ERP v BOLD response when changing:– Overall amplitude of a biphasic ERP – ERP peak ratio– Width of ERP

Changing ERP amplitude

• Fix peak ratio

ERP = BOLD• Hypothesis

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Increasing ERP amplitude -->

BO

LD

am

pli

tud

e

Changing ERP peak ratios

• Vary height of 2nd peak

• Hypothesis:

– Area of ERP BOLD

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35

Height of 2nd peak

BO

LD

am

pli

tud

e

Changing ERP width

• Amplitude same, width stretched

• Hypothesis:ERP = BOLD

0.001

0.01

0.1

1

0.01 0.1 1

Decreasing ERP width -->

BO

LD

am

pli

tud

e0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Decreasing width of ERP -->

BO

LD

am

pli

tud

e

Log-log

Linear-linear

What the future holds…

• We now know:– The peak amplitudes are linearly related– BOLD reflects integral of ERP– Linearity is lost once stimulation

lengthens

• Shape of ERP is critical –The relationship still has much to reveal!

Got you thinking?