I have learned this while my research -
What are the types of BCI's
There are many different techniques to measure brain signals.
We can divide them into Non-Invasive, Semi-invasive and Invasive.
Non-invasive: the EEG signal is taken placing electrodes on the scalp, so on the most external part.
Semi-invasive: the ECoG signal is taken from electrodes placed in the dura or in the arachnoid.
Invasive: the Intraparenchymal signal is taken directly implanting electrodes in the cortex.
Invasive types of BCI are implanted directly into the brain during a neurosurgery. There are single unit BCIs, which detect the signal from a single area of brain cells, and multiunit BCIs which detect from multiple areas. Electrodes have different lengths, for example, up to 1.5 mm (Utah, Blackrock Microsystems) or 10 mm (FMA, MicroProbes) in a MEA (1) . The quality of the signal is the highest, but the procedure has several problematics, as for example the risk of forming scar tissues. The body reacts to the foreign object and builds the scar around the electrodes, which cause deterioration in the signal. Because neurosurgery can be a risky and expensive process, the target of invasive BCI are mainly blind and paralyzed patients.
Electrocorticography uses electrodes placed on the exposed surface of the brain to measure electrical activity from the cerebral cortex. It has been used for the first time in the 1950s at the Montreal Neurological Institute. It is called semi-invasive but it still requires a craniotomy to implant the electrodes. For this reason it is used only when surgery is necessary for medical reasons (epilepsy for example).
What do we mean by spatial resolution? We can take, as an analogy, the clarity of an image. A picture with a higher spatial resolution is "clearer"; in other words, it looks more precise because it is composed of more pixels per inch, showing more details. A picture with lower spatial resolution appears less clear, or more blurry because it is composed of fewer pixels per inch. Basically I have learned these to develop a web app development, which can help me. Better spatial resolution allows us to understand with better precision where the signal is coming from. In the case of EEG, when the electrical signal travels through the skull, it is attenuated due to the low conductivity of the bones. ------------------------------------------------------------------------------
Resistance to noise
ECoG signal is not impacted by noise and artifacts as for example EMG (electromyographic - caused by muscles movements) and EOG (electrooculographic - caused by eyes movements)
Lower clinical risk
The electrode arrays doesn't need to penetrate into the cortex, which makes it safer than invasive recording (4)
ECoG recordings are 50-100 µV maximum versus 10-20 µV
There have been different studies about the use of ECoG in BCI, but they are all limited to cases where surgery was needed to remove an epileptic focus.
In one study (5) for example, the researcher used ECoG to control a computer cursor in two dimensions. Five patients, in preparation of surgery for epilepsy, had a subdural array of electrodes implanted for 7-14 days. After a short training of less than 30 minutes, the patients have been able to control a cursor in two dimensions, with an average success rate of 53-73%.
In the following section we will review briefly the main non-invasive techniques. There are several non-invasive techniques used to study the brain, where EEG is the most common used because of the cost and hardware portability.
PET positron emission tomography
fMRI functional magnetic resonance imaging
fNIRS near-infrared spectroscopy
In the following image is possible to see the different brain imaging techniques, compared by spatial and temporal resolution:
What is it?
From Wikipedia "is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers."
How does it work?
MEG measures the magnetic field caused by the currents in the brain, and it offers a better spatial resolution compared to EEG. Why? Because magnetic fields suffer far less than electric fields from the spatial blurring effect of the skull and intracerebral fluid. "MEG is maximally sensitive to tangential sources and has low sensitivity to radial sources" "MEG is better than EEG at detecting high-frequency activity (e.g., above 60 Hz). This is because magnetic fields pass through the skull and scalp, whereas the electrical fields are volume conducted through these tissues, which decreases signal-to-noise ratio at higher frequencies."
positron emission tomography
What is it? PET is a nuclear imaging technique used in medicine to observe different processes, such as blood flow, metabolism, neurotransmitters, happening in the body.
How does it work?
A small amount of radioactive material, called radiotracer, is injected in the bloodstream to reach the brain. In the case of the brain, the radiotracer get attached to the glucose and creates a radionuclide called fluorodeoxyglucose (FDG) (10). The brain uses glucose and it will show different levels based on the level activity of the different regions. The images of the PET scan are multicolored, where areas with more activities are in warmer colors as yellow and red. PET scans of the brain are used often to detect illnesses as cancer or others.
Hope this will also help.