Clinical Applications of Brain Mapping in Epilepsy

*Sandro Misciagna*

## **Abstract**

EEG brain mapping is a neurophysiological technique based on computer-assisted analysis of conventional EEG. This technique, generally consisting in quantitative analysis of EEG (QEEG), includes topographic displays of frequency or voltage, statistical comparison to normal values and discriminant analysis. QEEG assessment still remains controversy about its clinical role. QEEG topographic analysis could be useful in many neurological diseases: in cerebrovascular disease EEG analysis is useful since EEG parameters are highly correlates with regional blood and metabolism; in degenerative disease (as dementia or encephalopathies) quantitative EEG frequency analysis could suggest an organic base of the disorder even if it is not able to distinguish between the types of dementia. QEEG techniques are also potentially useful in identifying anomalies in patients with cerebral trauma or in children with cognitive disorders. In the field of epilepsy EEG brain mapping could help clinics to detect spikes, locate an epileptic focus and suggest the type of epilepsy. In this chapter author describes principles of EEG brain mapping and its potential applications in particular in the epileptic field.

**Keywords:** brain mapping, epilepsy, quantitative EEG, QEEG, brain maps, digital EEG, EEG spatial analysis, spike detection, spike analysis, seizure detection, epileptic focus, focus localization

#### **1. Introduction**

EEG was first described as a promise to provide a "window into the brain" in 1929 by Hans Berger [1]. In spite of recent advances, the analytic potential of EEG has not been fully employed. On the other hand, brain function studies and neuroimaging methods have been deeply improved, severely discrediting EEG use. However, it is important to insist that EEG can give relevant information about topography of cerebral activity, even if it is difficult to have topographic information with a conventional EEG recording.

EEG recording is based on two-dimensional representation of potential differences between two electrodes in function of time and topographic information is based on integration of information across different channels [2].

Introduction of digital EEG techniques not only displays the EEG tracing but can provide additional measurement with quantitative EEG (QEEG), also called "EEG brain mapping".

The use of EEG brain mapping is based on visualisation of coloured brain maps generated by digital analysis of cerebral electrical activity. These maps display many features that can be instantaneous of an averaged period of time. The cerebral maps include topographic displays of voltage, frequencies, power and statistical analysis with comparison with a normal reference population.

Still nowadays the clinical utility of QEEG techniques remains a controversial matter so that it could be considered as a useful tool, but also as a dangerous toy.

### **2. Short history of EEG spatial analysis**

EEG is traditionally analysed in terms of temporal waveforms at different channels, looking at power of rhythms in terms of frequency, latency of peaks or presence of particular grapho-elements. This type of traditional EEG analysis provides important insights about brain functioning in health subjects and diseases that interfere with electric brain activity, even if it cannot be considered as an imaging method.

Numerical analysis of cerebral activity was started as early as the 1930s by Dietsch [3], followed by Grass and Gibbs [4] and Drohocki [5] who applied Fourier analysis to disassemble EEG signal. Successively, in 1943, Walter [6] described an automatic analogue frequency analyser and later in 1951 Walter and Shipton [7] developed an automated topographic display called "toposcope".

With the development of microcomputers with colour graphic Duffy [8, 9] and many other researchers improved techniques for brain electrical activity mapping, EEG quantification and topographic analysis. Researchers as Lehmann directed their studies to the analysis of specific EEG spike–wave patterns, analysis of topography of a particular EEG feature at an instant time or the average of a recurring event [10].

#### **3. General principles of EEG brain maps**

EEG brain maps are produced using from 16 to 32 electrodes arranged in a grid pattern of human scalp, giving a spatial resolution of about 6 cm [11] (see **Figure 1**).

Brain maps are sensitive to the quality of data acquisition of the EEG in terms of montages, references, control settings or biological factors (such as medications, clinical problems or level of awareness), which must be considered in every cases before interpreting the data.

In fact, as Duffy himself has written "brain map without the EEG is blind" [12].

Cerebral maps are produced by a process of interpolation between the electrode sites. There are several methods of interpolation and still nowadays it is an object of controversy.

The use of EEG quantification, for example by spectral analysis, gives the possibility to reduce data and describe a long record by few numerical data. These data may be subjected to statistical analyses including visual EEG interpretation or clinical decision-making.

Recent development in the quantitative analysis of complex networks by using computer graphics has increased the availability of brain mapping, contributing to a renewed interest in quantitative investigations of EEG and it has been rapidly translated to studies of brain network organisation [13]. This is a welcome development, but the problem of mapping lies not so much in the method itself, particularly by uninformed users who can see cerebral maps as a neuroimaging technique.

But, instead, mapping systems must be operated by EEG certified neurologist expertise in the use of brain mapping [14].


**Figure 1.**

*Example of brain mapping in a traditional EEG showing absence of anomalies.*
