BCI Tech
BCIs can utilize a multitude of sensors that measure for different things. This will describe two in detail: electroencephalograms and magnetoencephalographies. However other sensors can be used, like the PET scan or fMRI. BCI sensors can be invasive, semi-invasive, or non-invasive. Invasive sensors are in the brain itself, semi-invasive sensors are on the surface of the brain, and non-invasive sensors are on the scalp. The more invasive the sensor, the more accurate it is. BCI devices can also stimulate the brain, with invasivity also affecting their effectiveness.
EEG and iEEG
Electroencephalograms, or EEGs, are arguably the most popular technique to read brain activity due to their low cost. It measures the electrical activity of the brain. EEGs rely on electrodes placed around the scalp or brain. The electrodes are made of conductive metal and can be either wet, semi-dry, or dry. Wet electrodes utilize gel to attach to the brain and conduct signals better. Semi-dry electrodes slowly release water or some other liquid to increase conductivity. Dry electrodes do not have anything special but are able to be applied quicker. There are also MEMS (micro electric mechanical system) electrodes that punctures the scalp or brain. The more electrodes the merrier, as it allows the BCI to determine the location of the brain activity better. Unfortunately the spatial resolution of EEGs tend to be weak, but the temporal resolution of EEGs tend to be strong. EEGs also have problems reading deep brain signals because of tissue and skull interference, which can be fixed by implanting electrodes into the brain itself.
These are known as intracranial EEGs (iEEG). This category includes electrocorticographies (ECoGs) and stereoelectroencephalographies (sEEGs). ECoGs are basically EEGs but semi-invasive, meaning that they lie in a grid pattern on the surface of the brain. Because of this, they have higher temporal and spatial resolution as they are not blocked by the skull and scalp like non-invasive EEGs are. On the other hand, sEEGs are fully invasive. They are strands of electrodes put together in a cylindrical shape, and are implanted deep into the brain.
MEGs
Magnetoencephalographies, or MEGs, measure magnetic fields, which are not blocked by the scalp or skull. This allows MEGs to read magnetic fields from the brain quite well without having to be surgically implanted. Where EEGs have electrodes, MEGs use either magnetometers or gradiometers that measure magnetic fields. Magnetometers are great at reading magnetic fields, however because of this they are more susceptible to external magnetic fields. Gradiometers are better at filtering out conflicting fields. However unlike EEGs, MEGs are unable to read radial sources.
How Do BCIs Interact with the Body?
There are many BCI devices that interact with the body. Through Functional Electrical Stimulation (FES), electrodes stimulate paralyzed muscles to cause them to contract and move. There is also something called ‘neural dust.’ Neural dust is about a millimeter long and utilizes ultrasound-sensitive piezoelectric crystals. When these crystals vibrate, they are able to generate electricity. BCIs can also stimulate the brain itself. This can be done using electrodes in or out of the brain. There is also Transcranial Magnetic Stimulation, where coils are placed on the scalp and magnetic pulses are sent to specific brain areas. This can help to solve brain related problems like epilepsy or depression. For example, Responsive Neurostimulation aims to detect a seizure and send a shock to end that seizure.