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Brain–Computer Interfaces - Index of

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180 G.R. Müller-Putz et al.<br />

Fig. 5 (a) Average LDA output for the classifier used during the grasp test (16 triggered grasp<br />

phases). (b) Corresponding averaged logarithmic band power features from electrode positions Cz<br />

and C4 during the grasp test (modified from [17])<br />

EEG. In the case <strong>of</strong> the neuroprosthesis with surface electrodes, the EEG was<br />

recorded in a bipolar manner so that synchronized interference could effectively<br />

be suppressed. Since every stimulation channel had a bipolar design, the influence<br />

on EEG recordings was minimized. Compared to the use <strong>of</strong> surface electrodes there<br />

was a much higher degree <strong>of</strong> interferences apparent in the EEG using the Freehand<br />

system.<br />

This stimulation device consists <strong>of</strong> eight monopolar stimulation electrodes and<br />

only one common anode, which is the stimulator implanted in the chest. To avoid<br />

contamination by stimulation artifacts, spectral regions with these interferences<br />

were not included in the frequency bands chosen for band power calculation. When<br />

the analyzed frequency bands <strong>of</strong> the BCI and stimulation frequency <strong>of</strong> stimulation<br />

device overlap, the repetition rate <strong>of</strong> the stimulation pulses can be shifted towards<br />

a frequency outside <strong>of</strong> the reactive EEG bands. Techniques such as Laplacian EEG<br />

derivations or regression methods with additional artifact recording channels would<br />

inevitably lead to more electrodes and therefore be a severe limitation for the<br />

application <strong>of</strong> a EEG-based brain-switch in everyday life (Fig. 6).<br />

2.5 Evaluation <strong>of</strong> the Overall Performance <strong>of</strong> the BCI Controlled<br />

Neuroprostheses<br />

For evaluation <strong>of</strong> the whole system performance in patient HK, a part <strong>of</strong> the internationally<br />

accepted grasp-and-release test [33] was carried out. The task was to<br />

repetitively move one paperweight from one place to another within a time interval<br />

<strong>of</strong> 3 min (see Fig. 7 picture 3 and 4). For the completion <strong>of</strong> one grasp sequence,<br />

3 classified left hand MI were necessary. During this 3-min interval, the paperweight<br />

was moved 5 times with a sixth grasp sequence initiated. The mean duration <strong>of</strong> the<br />

16 performed switches was 10.7 s ± 8.3 s [14].

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