GUIDELINES FOR EEG DATA ACQUISITION
EEG data acquisition is a fundamental and important activity of EEG- based research as the findings of experimental studies are highly dependent on the quality of data. In addition, if the intention is to record EEG during certain cognitive tasks such as working memory test then the experiment design plays a critical role. To ensure high quality of EEG data, the following guidelines need to be taken into consideration.
General Data Acquisition Setup
The general acquisition setup should be planned and previous literature should be consulted. The important factors are the number of EEG channels and their configuration (montage, bipolar, or referential), EEG reference selection, sampling rate, filtering, and recording of supplementary data.
Number of Electrodes
The main advantage of the EEG technique is the temporal resolution on millisecond scale as compared to other noninvasive brain mapping techniques such as fMRI. It is obvious that in multichannel EEG recordings, a certain number of electrodes are used for data acquisition, e.g., the standard international 10-20 system has 19 electrodes. However, a common question that may arise is, how many electrodes are needed? Should low-density EEG (such as 8 electrodes, 16 electrodes, or 24 electrodes) be used or high-density EEG (such as 64, 128, 256, and 512 electrodes)? The answer may depend on the objectives of the experiment, type of task to be performed, and the nature of the cognitive process to be investigated. Nevertheless, there are pros and cons of using either few electrodes or maximum electrodes. A high-density EEG system improves the spatial resolution but may take more time during setup. If the number of channels is 21 then a standard 10-20 montage can be selected.
- • Dense electrodes create the opportunity for both biological artifacts (eye movement, blinks) and nonbiological artifacts (electromagnetic interference).
- • Most EEG systems use electrolyte conductivity material; this electrolyte material may spread to the neighboring electrodes in the case of high-density EEG recordings.
- • Low-density EEG requires less time to setup the electrode cap and reduce the impedance of the electrodes while high-density EEG systems need more time to set the electrode cap for recordings.
- • High-density array EEG provides small interelectrode distance, which is highly recommended for highly localized signal sources.
- • High-density EEG has high spatial resolution along with high temporal resolution. It means that more data will be acquired, which will definitely require large storage media, faster processing, and efficient memory chips, consequently resulting in greater expense compared to low-density EEG systems.
It is noted that the low-density EEG systems are easy in operations but not recommended for exploration of brain functions and source localization. In this context, a few studies have highlighted the issue of number of electrodes to be used in EEG recording for a certain purpose; e.g., Lantz et al.18 investigated source localization for epileptic patients with different number of electrodes, i.e., 31, 63, and 123, and concluded that 123 electrodes would constitute a significant enhancement on the localization accuracy for epileptic patients.