Single-Trial Analysis of Bioelectromagnetic Signals: The Quest for Hidden Information - Modeling in Computational Biology and Biomedicine - page 228

Chapter 7
Single-Trial Analysis of Bioelectromagnetic
Signals: The Quest for Hidden Information

Maureen Clerc, Theodore Papadopoulo, and Christian Benar

7.1 Introduction

7.1.1 Electric Activity in the Brain

Electric signals propagate within the brain along nerve fibers (axons) as a series of
action potentials (APs). The corresponding primary current can be approximated
by a pair of opposite current dipoles, one at the depolarization and one at the
repolarization front, and this quadrupolar source moves along the axon as the
activation propagates. The separation of the two dipoles depends on the duration
of the AP and on the conduction velocity of the fiber. For a cortical axon with a
conduction speed of 5 m/s, the opposite dipoles would be about 5 mm apart.
In synapses, the chemical transmitter molecules change the ion permeabilities
of the postsynaptic membrane and a postsynaptic potential (PSP) and current
are generated. In contrast to the currents associated with an action potential, the
postsynaptic current can be adequately described by a single current dipole oriented
along the dendrite. The electric and magnetic fields of a current dipole fall off more
slowly with distance (as /r 2 ) than the fields associated with the quadrupolar AP 1

/r 3 ). Local field potentials (LFPs) correspond to a spatial average
of the individual activities: because summation of currents flowing in neighboring
fibers is more effective for synaptic currents (lasting up to tens of milliseconds),
than for APs (about 1 ms-long), LFPs mostly correspond to summations of PSPs.

currents (as 1

M. Clerc ( ) · T. Papadopoulo
Inria Sophia Antipolis Mediterranee, Athena project-team, 2004 Route des Lucioles, 06902,
Sophia Antipolis, France
e-mail: [e-mail] ; [e-mail]
C. Benar
Institut des Neurosciences des Systemes -INS, UMR 1106 INSERM, Aix-Marseille Universite,
Facult´edeMedecine La Timone, 27 Bd Jean Moulin, 13385 Marseille Cedex 05
e-mail: [e-mail]