Brain–Computer Interfaces - Index of

254 K.J. Miller and J.G. Ojemann
In a one dimensional experiment, the learning process is rapid (3–24 min,
see [8]). Though not always highly focal, we have observed cortical activity that
increases with improved performance (Fig. 7b). Anecdotally, this is associated
with a psychological “de-linking” of the screening behavior (e.g., hand movement
imagery) and the cursor control, where the cursor gains some kind of abstract, toollike,
representation with the subject. They simply will the cursor to move to the
left or the right, and there is an increase in power in the appropriate feature. Recent
results (an example is illustrated in Fig. 7c) have demonstrated efficacy using ECoG
for two-dimensional control [10]. This requires two independent signals to drive the
two degrees (typically up-down and left-right cursor position, as shown in Fig. 7c).
The motor system allows for some intuitive, separable features, using, for example,
hand and mouth, which are distinct when high-frequency ECoG features are used
[6, 36, 37]. Despite success, simultaneous 2D control has been less robust than for
one dimension. One reason for this is that it requires a good deal more mental coordination
by the subject. For example, in this simple form of a 2D target task BCI,
there is fundamental uncertainty in the null condition. When increased power in one
channel drives the cursor up and a lack of power down, with increased power in
another channel driving the cursor to the left, then weak power in both channels
could be ambiguous when targeting a down or right sided target. It takes practice
to coordinate the two null directions. Even if the signals are not overlapping
during the screening for the two different tasks, when combined to achieve twodimensional
cursor control, they may have some initial interdependence. Through
feedback, however, these can be segregated by some individuals [37, 38].
6 Case Study
We conclude this chapter with a step-by-step example of successful implementation
of electrocorticographic BCI.
Background: The patient is an 18 year old right handed woman with intractable
seizures related to a left frontal lesion. Electrodes were placed to map seizures and
localize speech function prior to resective surgery, and the array covered speech
areas and primary, peri-central, sensorimotor cortex. This excellent coverage of
motor cortex made her an ideal candidate for multiple degree of freedom cursor
control. Signals were split outside of the patient, with a common ground and
reference to the clinical system. The experiments were conducted in three stages
spanning two sessions in the same day. The subject was positioned comfortably
in her hospital bed throughout the experimental process, and the experimental setting
is illustrated in Fig. 5. The electrode localization and cortical plotting were
performed using the LOC package [16]. The three steps that the experiment was
constructed upon were cue-based screening for feature identification, independent
one-dimensional cursor-to-target feedback, and combined two-dimensional
cursor-to-target feedback.
Figure 7a, Screening for feature identification: In response to 3 s cues, she repetitively
moved either her hand or her tongue in repetition, resting for 3 s in between