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Brain–Computer Interfaces - Index of

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A Simple, Spectral-Change Based, Electrocorticographic Brain–Computer Interface 251<br />

P(t), to the movement <strong>of</strong> a cursor in a given direction, according to the simple linear<br />

relation ˙y = g(P(t)−P0), where P0, is a power level somewhere between movement<br />

imagery and rest, determined from a screening task (as in Fig. 4). This difference is<br />

then coupled to the movement <strong>of</strong> a cursor, with speed determined by the parameter<br />

g, which can be adjusted in the beginning <strong>of</strong> the task to be comfortable to the subject.<br />

The subject views changes in the cursor trajectory, and the subject then modifies the<br />

nature <strong>of</strong> the imagery to cause the cursor to move toward their target choice. After<br />

8–10 min, in the 1-D task, subjects will abstract away from the imagery and are<br />

able to imagine the movement <strong>of</strong> the cursor itself and produce appropriate spectral<br />

changes.<br />

The power in this identified feature, P0, is then linked to the velocity, ˙y,<br />

<strong>of</strong> a cursor on the computer screen, using the simple translation algorithm<br />

˙y = g(P(t)−P0), where P0 is a power level somewhere between movement imagery<br />

and rest, and g is a velocity parameter. This difference is then coupled to the move-<br />

Fig. 6 Feature reassessment after feedback during a speech imagery task. (a) Two different feedback<br />

sites. An initial feedback site was chosen at site “1”, and, upon reassessment, a secondary<br />

feedback site was chosen at site “2”. (b) The feature map from a speech imagery screen identified<br />

a frontal site for control (labeled “1” in (a)), which was coupled to a cursor control task. The<br />

subject was not able to accurately control the cursor in the task, obtaining only 45% correct. (c) A<br />

feature map from this unsuccessful feedback run demonstrated that, while there was no significant<br />

difference at the feedback site, there was at a different site (labeled “2” in (A)). (d) The feature<br />

map following feature reassessment. The subject was able to rapidly attain 100% target accuracy<br />

with the new feature, and the most significant change was in the reassessed electrode

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