Quantitative analysis of EEG signals: Time-frequency methods and ...

1 Outline of Neurophysiology: Brain signals
This chapter presents some basic topics of neurophysiology necessary for understanding
the experiments and results to be described in the following chapters. In this context,
the concepts exposed and the detail of their treatment are not expected to provide
a complete background on neurophysiology. On the contrary, this chapter is focused
on describing the electroencephalogram and event-related potentials (ERPs), especially
applied to the study of epilepsy and brain oscillations. Despite the wide application
of these issues, some fundamental points are still controversial and due to the complex
behavior of these signals, they are dicult to resolve with traditional approaches, thus
being ideal candidates to be studied with new quantitative methods.
1.1 Electroencephalogram (EEG)
The EEG was originally developed as a method for investigating mental processes. Clinical
applications soon became visible, most notably in epilepsy, and it was only with the
introduction of ERP recordings that EEG correlates of sensory and cognitive processes
nally became popular. The rst recordings of brain electrical activity were reported by
Caton in 1875 in exposed brains of rabbits and monkeys, but it was not until 1929 that
Hans Berger (Berger, 1929) reported the rst measurement of brain electrical activity in
humans. EEG visual patterns were correlated with functions, dysfunctions and diseases
of the central nervous system, then emerging as one of the most important diagnostical
tools of neurophysiology.
The electroencephalogram (EEG) can be roughly dened as the mean electrical activity
of the brain in dierent sites of the head. More specically, it is the sum of the
extracellular current ows of a large group of neurons. Since the generation of the EEG
from the action potentials of the neurons is beyond the scope of this thesis, for further
details I suggest the comprehensive works of Steriade et al. (1990), Lopes da Silva
(1991), Steriade (1993), Speckermann and Elger (1993), Pedley and Traub (1990) and
Basar (1980).
EEG recordings are achieved by placing electrodes of high conductivity(impedance <
5000) in dierent locations of the head. Measures of the electric potentials can be
recorded between pairs of active electrodes (bipolar recordings) or with respect to a
supposed passive electrode called reference (monopolar recordings). These measures are
mainly performed on the surface of the head (scalp EEG) orby using special electrodes
placed in the brain after a surgical operation (intracranial EEG).
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