Written by Travis M. Moore
Last edited 10-Jun-2020
You now have a basic understanding of how neurons fire and pass signals to each other. We could stop there if we could measure individual neurons in humans, but we can't. Electrophysiological tests in humans must be non-invasive. Single unit (neuron) recording is reserved for animal work, where very small electrodes can be inserted into neural tissue and record directly from single neurons. With human patients, we have to settle for recording the voltage of huge groups of neurons all the way from the scalp. This offers several challenges that we discuss below.
Single unit recordings are referred to as near field because the electrodes are placed as close as possible the neuron. The next step removed is called local field, and involves placing the electrode in the general vicinity of a group of neurons. Local field potentials are still recorded from within neural tissue, just not directly from a single neuron. The final step removed is the far field. We exclusively measure far field potentials in the audiology clinic. It is possible to get near and local field recordings during brain surgery (depending on what you want to measure and where the surgery is focused), but that is well outside the audiology scope of practice.
The major factors that influence what neural signals look like at the scalp are listed below. We will address each in turn.
Perhaps the most intuitive factor influencing the voltage that reaches the scalp is that electrical current traveling through the volume of the brain gets weaker the further it travels. In fact, the relationship between the distance traveled and the strength of the signal follows a law that should sound familiar from acoustics: the inverse square law (content not yet available online). In this case, instead of sound waves, the law describes the propagation of electromagnetic fields (neural firing). In brief refresher, in inverse square law states tha as current travels away from the neuron, it gets exponentially weaker. In electrophysiological terms, the distance from the neuron to the scalp is very far so there is a huge reduction in the voltage that is recordable from the scalp.
The volume under discussion is the brain, cerebrospinal fluid, skull and scalp. As the electromagnetic field of the action potential travels to the surface of the scalp, it must pass through the components above, which all have their own impedances. For instance, the skull does not conduct electricity very well, and has a high impedance. The same is true of the scalp. But even the cerebrospinal fluid (the fluid surrounding the brain) and the brain tissue itself creates additional impedance the current must overcome. Figure 2 illustrates the concepts of signal dissipation and impedance of the volumes inside and on the surface of the head. The circle represents an action potential, and its opacity represents its strength.
Another obstacle posed by volume conduction is called dispersion. Brain tissue is a complex mass of charges and cells. Some areas of tissue will allow the action potential current to pass easily, while other areas will completely block the electrical flow. This creates several winding paths from neuron to scalp. The action potential current divides to take whatever paths it can find, which leads to voltage at several different places on the scalp. If the electrodes are not positioned properly, they will not record the voltage. Figure 3 animates the process of dispersion using three different "areas" of brain tissue. The red areas stop current, the green arrows pass current, and the yellow areas hinder the current but lets some of it through.
It should be pretty obvious by now that it is impossible to measure a single action potential from a single neuron at the scalp. That little current on its own would never make it to the scalp. Our electrodes are only sensitive to voltage created by thousands upon thousands of neurons. All those currents can sum together to make something strong enough to produce measurable voltage at the scalp. However, the number of neurons firing is not enough.
Just like adding sine waves together, adding electrical potentials can result in constructive and destructive interference. That means in order to get a measurable voltage at the scalp (and not something that cancels out), neurons must be firing in synchrony. The number of neurons firing becomes irrelevant if they aren't also firing at the same time so their voltages can sum constructively.
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1Dumitru, D. and Jewett, D.L. (1993), Far‐field potentials. Muscle Nerve, 16: 237-254. doi:10.1002/mus.880160302