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Brain–Computer Interfaces - Index of

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Dynamics <strong>of</strong> Sensorimotor Oscillations in a Motor Task 51<br />

Fig. 3 Mean ERD/ERS values (i.e. mean and standard error) obtained for left hand (left side)versus<br />

right hand imagery (right side) for the 8–10 Hz (upper row) and 10–12 Hz frequency bands<br />

(lower row). Data were obtained from the left (C3, white bar) versus right (C4, black bar) sensorimotor<br />

derivation and are shown separately for 4 BCI training sessions (S1–S4) performed with<br />

each <strong>of</strong> the (N=10) participants. This figure is modified from [9]<br />

the case <strong>of</strong> foot motor imagery, was weak and not found in every subject. The reason<br />

for this may be that the foot representation area is hidden within the mesial wall. The<br />

mesial wall refers to the tissue between the left and right hemispheres <strong>of</strong> the brain.<br />

This area is difficult to detect with electrode caps outside the head, since electrode<br />

caps are best at measuring signals near the outer surface <strong>of</strong> the brain just underneath<br />

the skull.<br />

Noteworthily, both foot and tongue motor imagery enhanced the hand area mu<br />

rhythm. In addition, tongue motor imagery induced mu oscillations in or close to<br />

the foot representation area. This antagonistic pattern <strong>of</strong> a focal ERD accompanied<br />

by a lateral (surrounding) ERS will be described in more detail in the following<br />

sections.<br />

4 Interpretation <strong>of</strong> ERD and ERS<br />

Connections between the thalamus and cortex are called thalamo-cortical systems.<br />

Increased cellular excitability (that is, an increase in the likelihood that neurons<br />

will fire) in thalamo-cortical systems results in a low amplitude desynchronized<br />

EEG [40]. Therefore, ERD can be understood as an electrophysiological correlate<br />

<strong>of</strong> activated cortical areas involved in processing sensory or cognitive information<br />

or producing motor behavior [5]. An increased and/or more widespread ERD<br />

could result from a larger neural network or more cell assemblies in information<br />

processing.<br />

Explicit learning <strong>of</strong> a movement sequence, e.g. key pressing with different fingers,<br />

is accompanied by an enhancement <strong>of</strong> the ERD over the contralateral central

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