Brain–Computer Interfaces - Index of

224 J.E. Huggins
(rest) periods in EEG and in ECoG that were recorded under similar paradigms
but with different subjects. Approximately 150 self-paced finger movements were
performed by 6 normal subjects while EEG was recorded and by 6 epilepsy surgery
patients while ECoG was recorded. Classification results on ECoG always exceeded
classification results on EEG. Spatial prefiltering brought the results on EEG to a
level equivalent to that found on ECoG without spatial prefiltering. However, spatial
prefiltering of the ECoG achieved a similar improvement in classification results.
Thus, in all cases, ECoG proved superior to EEG for classification of brain activity
as event or idle periods.
The electrode implantation required for ECoG, although seen by some as a drawback,
may in fact be another advantage of ECoG because the use of implanted
electrodes should greatly minimize the set-up and maintenance requirements of
a BCI. Setup for an EEG-based BCI requires an electrode cap (or the application
of individually placed electrodes) and the application of electrode gel under
each electrode. Once the subject is done using the BCI, the electrode cap must be
washed and subjects may want to have their hair washed to remove the electrode
gel. While research on electrodes that do not require electrode gel is underway [e.g.
[11]], developing electrodes capable of high signal quality without using electrode
gel is a serious technical challenge and would still require daily electrode placement
with inherent small location variations. Setup for an ECoG-based BCI would
require initial surgical implantation of electrodes, ideally with a wireless transmitter.
After recovery, however, setup would likely be reduced to the time required to
don a simple headset and maintenance to charging a battery pack. Further, instead
of the appearance of an EEG electrode cap (which resembles a swim cap studded
with electrodes), the BCI could be a relatively low visibility device, with a possible
appearance similar to a behind-the-ear hearing aid. So, from the user’s perspective,
as well as from the algorithm development perspective, ECoG provides many
advantages over EEG.
In summary, the advantages of ECoG over EEG include:
• Improved signal quality
• Insulation from electrical noise sources
• Improved resolution for localization of signal origin
• Broader signal bandwidth
• Larger signal amplitude
• Reduced setup time
• Reduced maintenance
• Consistent electrode placement
• Improved cosmesis
3.2 Advantages over Microelectrodes
ECoG may also have advantages over recordings from microelectrodes. Microelectrodes
that are intended to record the activity from single cells are implanted