Brain–Computer Interfaces - Index of

226 J.E. Huggins
300−6 kHz range [5] could reduce this difference, when multiplied by the number
of individual electrodes whose data is required for a functional BCI, it remains a
distinct technical advantage.
In summary, the advantages of ECoG over microelectrodes include:
• Ensemble activity recording
• Stable electrode placement
• Preservation of brain tissue
• Lower sampling rates
3.3 Everything Affects the Brain
Despite the advantages of ECoG over EEG and microelectrodes, it should be noted
that wherever brain activity is recorded from, it will be impossible to avoid recording
irrelevant brain activity, which must be ignored during the signal analysis. While
ECoG is less susceptible to noise from muscle artifact or electrical noise in the
environment, it is still subject to the extraneous signals from the other activities that
the brain is involved in. Indeed, any list of factors that can be expected to affect brain
activity quickly becomes lengthy, since any sensory experience that the subjects
have will affect activity in some part of the brain, as may the movement of any part
of their body, their past history, their expectations and attitude toward the experiment
and even their perception of the experimenter’s reaction to their performance. So,
while using ECoG may simplify the signal analysis challenges of detecting brain
activity related to a particular task, the detection is still a significant challenge.
4 Disadvantages of ECoG
The advantages of ECoG also come with distinct disadvantages. A primary disadvantage
is limited patient access and limited control of experimental setup. Subjects
for ECoG experiments cannot be recruited off the street in the same way as subjects
for EEG experiments. ECoG is only available through clinical programs such
as those that use ECoG for pre-surgical monitoring for epilepsy surgery. Epilepsy
surgery involves the removal of the portion of the brain where seizures begin. While
removing this area of the brain can potentially allow someone to be seizure-free, any
surgery that involves removing part of the brain must be carefully planned. Epilepsy
surgery planning therefore frequently involves the temporary placement of ECoG
electrodes for observing seizure onset and for mapping the function of specific areas
of the brain. This placement of ECoG electrodes for clinical purposes also provides
an opportunity for ECoG-based BCI experiments with minimal additional risk to
the patient. While this is an excellent opportunity, it also results in serious limitations
on experimental design. BCI researchers working with ECoG typically have
no control over the placement of the ECoG electrodes. While they could exclude
subjects based on unfavorable electrode locations, they generally do not have the