Scientists love to research for the first mention of their invention. For example, I saw an article where it was seriously claimed that the first experiments on electrical stimulation of the brain were carried out in Ancient Rome, when someone would even get shocked by electric eel.
One way or another, usually, the history of electrophysiology is usually calculated from the experiments of Luigi Galvani (XVIII century). In this series of articles, we will try to tell a small part of what science has learned over the past 300 years about the electrical activity of the human brain, about what profits can be extracted from all this.
Where does brain electrical activity come from?
The brain is made up of neurons and glia. Neurons exhibit electrical activity, glia can also do this, but in a different way [1], [2], however, today we will not pay attention to it.
The electrical activity of neurons consists of pumping sodium, potassium and chlorine ions between the cell and the environment. Between neurons, signals are transmitted using chemical mediators.
When a mediator secreted by one neuron enters a suitable receptor of another neuron, it can open chemically activated ion channels and let a small amount of ions into the cell. As a result, the cell changes its charge a little. If enough ions have entered the cell.
(For example, a signal has arrived at several synapses at the same time), other ion channels depending on the voltage (there are more) open, and the cell is activated in a matter of milliseconds on the basis of an “all or nothing” principle, and then returns to previous condition.
The best way to record the activity of individual cells is to stick an electrode into the cortex. It can be one wire, it can be a matrix with several tens of channels, it can be a pin with several hundred, or it can be a flexible board with several thousand.
It looks impressive, but in the near future, such methods will not come to every district clinic, and especially to healthy people. Firstly, it is very expensive - the cost of the procedure for each patient is measured in hundreds of thousands of dollars.
Secondly, implantation of electrodes into the cortex is still a serious neurosurgical operation with all possible complications and damage to the nervous tissue around the implant.
Thirdly, the technology itself is imperfect - it is not clear what to do with tissue compatibility of implants, and how to prevent their fouling with glia, as a result of which the desired signal ceases to be recorded over time. In addition, teaching each patient how to use an implant can take more than a year of daily training.
You can not stick the wires deep into the bark, but gently put it on it - you get an electrocorticogram. Here the signal of individual neurons can no longer be registered, but you can see the activity of very small areas (the general rule is, the farther from the neurons, the worse the spatial resolution of the method).
The invasiveness level is lower, but you still need to open the skull, so this method is mainly used for monitoring during operations. You can put wires not even on the cortex, but on the dura (the thin skull that is between the brain and the real skull).
Here the level of invasiveness and possible complications is even lower, but the signal is still quite high quality. Get epidural EEG. The method is good for everyone, however, an operation is still needed here.
Finally, a minimally invasive method for studying the electrical activity of the brain is an electroencephalogram, namely, recording using electrodes that are located on the surface of the head.
The method is the most widespread, relatively cheap (top-end devices cost no more than several tens of thousands of dollars, and most are several times cheaper, consumables are practically free), and has the highest time resolution of non-invasive methods - you can study the processes of perception, which take a few milliseconds.
Disadvantages - low spatial resolution and noisy signal, which, however, contains enough information for some medical and neuro-interface purposes. In the picture with action potential, it can be seen that the curve has two main parts - in fact, the action potential (large peak) and the synaptic potential (small amplitude change in front of the large peak).
It would be logical to assume that what we register on the surface of the head is the sum of the action potentials of individual neurons.It would be logical to assume that what we register on the surface of the head is the sum of the action potentials of individual neurons.
However, in reality, everything works the other way around - the action potential takes about 1 millisecond and, despite its high amplitude, does not pass through the skull and soft tissues, but the synaptic potentials due to their longer duration are well summed up and recorded on the surface of the skull.
This has been proven by simultaneous recording by invasive and non-invasive methods. It is also important that the activity of not every neuron can be recorded using EEG (more details here).
Neurocomputer interfaces - an evolving technology, looks like magic, especially for an unprepared person. However, in reality, this is a method in which there are many unobvious difficulties.
The secret here, as with any technology, is to take into account all the limitations and find areas of its application in which these restrictions do not interfere with the work.