4.5: Common Electroencephalography Methods

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Electroencephalography (EEG) and Even-Related Potential (ERP)

Electroencephalography (EEG) is one of the most common non-invasive BCI’s to measure and record brain activity. An individual is attached to an EEG machine with an electrode cup, the electrodes are placed on the user’s scalp to measure neurophysiological signals. The EEG devise is able to measure severe voltage (10-100 mV) alterations of nerve cells. It only provides information about the general mental state, such as sleep, awareness, and alertness. However, it does not detect minor alteration (1-10 mV) of nerve cells. For this reason, Event-Related Potential (ERP) is used to measure minor fluctuation. ERP is elicited either during a cognitive task that requires an individual to distinguish target stimulus from non-target stimuli (endogenous potential) or after presenting a sensory stimulus (exogenous potential). If an external stimulus is presented, it causes a decrease of electrical potential 100 ms later (N100), while if a participant is asked to distinguish one stimulus from other stimuli, it causes an increase of electrical potential 300 ms later (P300) (Müller-Putz, Scherer, & Pfurtscheller, 2007).

EEG Channel Selection Examples

To elicit P300, Piccione et al (2006) used four EEG channels and one EOG channel; Sellers and Donchin (2006) used three EEG channels. In these studies, low communication rates were attributed to the number of few channel selections (Hoffman et. al, 2008). In order to increase communication rates, Hoffman et al. (2008) assessed four different electrode configurations (4,8,16,32 electrodes). For testing classification, they used Bayesian Linear Discriminant Analysis and Fisher’s Linear Discriminant Analysis. For both BLDA and FLDA, there was a significant increase between the four electrode configuration and the eight electrode configuration. However, using more than eight electrodes provides a modicum increase for BLDA. The usage of 16 and 32 electrodes even decreased the performance for FLDA. They concluded the best electrode configuration includes 8 electrodes. It could be considered more user-friendly compared to 16 and 32 electrodes placed and provides better accuracy compared to four electrodes placed (Hoffman et. al, 2008).

Takano et al. (2011) and Kansaku (2011) examined eight channels by comparing posterior - anterior channels set and middle - lateral channels set. According to results, the EEG recording from the posterior set and lateral sets gave better accuracy.

Electromyography (EMG) and Electrooculography (EOG)

EMG and EOG are considered reliable sources of signal acquisition (Fatourechi, Bashashati, Ward & Birch, 2007). Blum, Stauder, Euler, and Navab (2012) used Neural Impulse Actuator (NIA) to see the potential of the combination of BCI and a gaze tracker. NIA is able to read alpha brain waves, beta brain waves, electromyographic signals elicited by skeletal muscles, and electrooculographic signals triggered by eye movement. With acquired signals from EMG, Blum et al. (2012) achieved to create a user interface (UI) by switching normal vision to X-ray vision on a phantom patient.