Interseccionalidad de género

Recording surface EEG is a non-invasive way to access the electric field of the brain through electrodes placed on the scalp. Following Nunez and Srinivasan [68], the electric potential on

the surface is a mixture of signals from different regions, dominated by the neocortex. The typical scale of EEG signals is 20-50µV, which is about a factor of 103 below the scale of an electrocardiogram (ECG). The surface EEG has a low spatial resolution (several cm), but a high resolution in the time domain (in the range of ms). However, high frequencies tend to be contaminated more strongly by noise.

Apart from the scalp EEG considered in this work, the EEG can be recorded with elec- trodes that are placed on the surface or even deep into the brain. This intracranial EEG can accurately measure local effects, at the price of severe surgical procedures. For ethical reasons, this approach is limited to patients with severe neurological pathologies. It is typically used preparatory for epilepsy surgery or tumor excision.

Measured Features

The background activity or spontaneous EEG (see [68]) is the part of the EEG that is not related to specific cognitive tasks. A straightforward description of such a time series is its power spectrum. In the context of EEG analysis, there is an established division of the fre- quency range in frequency bands. The power of different frequency bands is a commonly used observable, which can sometimes be associated with certain kinds of brain activity (see for instance Nunez [68]), and changes under the influence of anesthetics (Murphy [66]). Note that the conventions for the definition of the bands differ slightly for different sources. The back- ground activity will be studied in the view of a symbolic description in section 5.3. See also Jordan et al. [44] for a summary of different approaches to measure anesthetic depth using order pattern analysis of the EEG background activity.

In addition to the background activity, so calledgraphoelements are local patterns in the EEG data, which can be an indication for certain pathologies. In the clinical situation, specially trained neurologists visually inspect EEG recordings for occurrences of known patterns. See Weinmann [107] for a catalog of defined graphoelements including numerous samples of EEG data. Note that the classification of a pattern as normal or pathologic is highly age-dependent. Symbolic representation are a promising method to describe such local patterns. However, graphoelements will not be considered here, because they are rather sporadic indications for certain properties of an individual. The subject of this chapter is the monitoring of the (highly variable) state of consciousness based on short samples of EEG data.

The expressions evoked and event-related potentials (EP/ERP) refer to the electrical ac- tivity on the scalp that corresponds to the processing of certain stimuli. They are small in amplitude, but can be accessed by repetition of similar stimuli, and can give valuable informa- tion because the activity can be related to certain cognitive functions. Section 5.4 will study the auditory evoked potentials (AEP) caused by binaural clicking stimuli.

Artifacts

Artifacts are features in the measured data that are not related to the processes to be observed, but originate in an interference from other processes or a disturbance of the measuring appara- tus. There are many issues and effects in the course of an EEG measurement that can lead to artifacts. Due to individual differences in skin properties and effects like sweating, the surface conditions on the skin can vary spatially, between individuals, and in time. Hence, calibration is difficult and the scaling of the data must be considered with care. The measurement is done using an electrode cap, therefore any movement of the head or the cap is critical. Changes in

the impedance between the electrode and the skin can cause disruptions, or fluctuations in the scaling of the signals. Since the measured potential differences of the EEG are small, the whole measuring apparatus is vulnerable to electro-magnetic perturbations (the most obvious ones being 50 Hz-perturbations from the power supply). Furthermore, there can be influences from the electrocardiogram (ECG).

Muscle activity is accompanied by electrical signals, the electromyogram (EMG), which typically have magnitudes dominating the EEG signal. Since muscular signals mainly affect the high frequency ranges, it is common to use a lowpass filter before evaluating the EEG data, commonly used values for the upper limit range from 30−50 Hz (see Bonhomme and Hans [16]). Whitham [108] even reports observation of myogenic signals in frequencies down to 20 Hz. In studies of evoked potentials this limitation is usually bypassed by trigger-synchronized averaging over multipleepochs, i. e. samples of EEG data recorded after similar stimuli.

Reference Potential

Even though one is mainly interested in electric potentials, only potential differences can be observed, hence the potential at each electrode is measured with respect to a reference. Since there is no canonic global potential that qualifies as a ’mass’, the reference potential has to be defined from the electrode values. Especially for the question of synchronization this can be an issue. Suppose several signals are measured with respect to a common reference. If the signals are independent and a perturbation is added to the reference, it shows up (with opposite sign) in all of the time series. This can lead to positive correlation coefficients, which could be spuriously interpreted as synchronization behavior.

For the surface EEG, there are numerous so called reference montages in use, like bipolar recordings (differences between closely spaced electrodes), or using common reference elec- trodes. One can also use an average over all or the neighboring electrodes as a reference, using simple averaging or more sophisticated methods like spline interpolation, see [67] –[69], [85] for details and comparative studies on some commonly used methods. In the following, the reference used was the average over all electrodes.