Introduction
An activity in a normal human brain can generate various responses including electrical, magnetic, and metabolic responses. These signals can be detected by appropriate sensors and they can be used for controlling a BCI system. For example, brain activity can produce magnetic fields that can be recorded using magentoencephalographic (MEG) activity. Brain activity can also result in some metabolic consequences in terms of changes in the blood flow and metabolism. Imaging methods such as functional magnetic resonance imaging (fMRI) can image these activities. At present, because of the cost and physical dimensions, methods that measure the electrical activities of the brain are more favored [VAU03].
There are various ways to record the electrical activities of the brain. Non-invasive BCI approaches mostly use the EEG signals as the source of information. EEG signals are recorded by means of electrodes placed on the scalp. Invasive approaches, on the other hand, use electrocorticography (ECoG) signals recorded from the surface of the brain or action potentials of single neurons in the cerebral cortices, using implanted microelectrodes.
EEG signals have good temporal resolution, but their spatial resolution is not good compared to other recording technology methods [VAU03]. A recent study showed that only 12% of published BCI studies use implanted electrodes, 5% use microelectrode arrays, and more than 80% use EEG signals [MAS07]. The main reason is that the EEG recording equipment is commercially produced and their cost is lower than other brain signal recording technologies. Also, since no surgery is necessary for placing electrodes, more individuals are willing to participate in such BCI experiments.
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References
[VAU03] T. Vaughan, W. J. Heetderks, L. J. Trejo, W. Z. Rymer, M. Wienrich, M. M. Moore, A. Kubler, B. H. Dobkin, N. Birbaumer, E. Donchin, E. W. Wolpaw and J. W. R, "Brain-computer interface technology: a review of the second international meeting", IEEE Trans. Neural Syst. Rehab. Eng., vol. 11, no.2, pp. 94-109, 2003.
[MAS07] S. G. Mason, A. Bashashati, M. Fatourechi, K. F. Navarro and G. E. Birch, "A comprehensive survey of brain interface technology designs", Ann. Biomed. Eng., vol. 35, no.2, pp. 137-169, Feb.2007.
