Functional neuroimaging

     Invasively or noninvasively brain activity is recorded either from recording electrical activity through electrodes (EEG, Electrocorticography (ECoG) or from single-neuron recordings within the brain), recording magnetic fields using magnetoencephalography (MEG)), or recording metabolic activity reflected in changes in blood flow (positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and functional Near Infrared (fNIR)). Despite the fact that MEG, PET, fMRI and fNIR have shown success for BCI applications these techniques are still technically demanding and expensive technologies that require sophisticated equipment that can be operated only in special facilities. Furthermore, PET, fMRI and fNIR techniques depend on metabolic processes, such as blood flow, having long latencies and thus less suitable for the control of BCIs. On the other hand, the non-invasive EEG and the invasive ECoG and single neuron recordings , are methods that have relative low costs, are simpler to use and have higher temporal resolutions, making them more practical to the use with BCIs.

 
    Invasive techniques such single-neuron recording and ECoG take recordings over the cortex; while single-neuron recording records the activity within the cortex, ECoG records the activity over the cortical surface of the brain. Single-neuron recordings and ECoG does not record single neuron activity but records activities over small regions of the brain giving them a high spatial resolution, and as it is implanted directly over the cortex, they have a high bandwidth, high SNR and high amplitude. Since ECoG electrodes do not penetrate the cortex, recorded signals are also not subjected as heavily to immune response, possess lower risk to implant as well. Furthermore, maintaining long term reliable recording with implantable electrodes is difficult.
   Although, ECoG has a higher spatial resolution compared to EEG (i.e. 1.25 - 1.4mm vs centimeters) higher frequency bandwidth ([19, 11] (i.e. 0−500Hz vs. 0−40Hz), have higher signal amplitude (50 − 100µV maximum vs. 10 − 20µV maximum), and being less susceptible to artifacts (i.e. EMG, EOG or electrical devices), EEG has become the most common source for brain activity due to its none invasiveness (requiring no craniotomy (surgical incision of the skull)), being more practical for everyday situations. EEG measures the potential over the scalp, reflecting the collective activity over large population of neurons located underneath the sensor position.
References:
1]  Lawrence A. Farwell, DrewC. Richardson, and GrahamM. Richardson. Brain finger‐ printing field studies comparing p300-mermer and p300 brainwave responses in the detection of concealed information. Cognitive Neurodynamics, pages 1–37, 2012

2]  Charles M. Gaona, Mohit Sharma, Zachary V. Freudenburg, Jonathan D. Breshears, David T. Bundy, Jarod Roland, Dennis L. Barbour, Gerwin Schalk, and Eric C. Leu‐ thardt. Nonuniform high-gamma (60 –500 hz) power changes dissociate cognitive task and anatomy in human cortex. The Journal of Neuroscience, 31(6):2091–2100, February 2011.

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