Here we collect background on electrophysiology documentation.
Electroencephalogram (EEG) and Magnetoencephalogram (MEG)
Excerpt of transcript from a lecture by Prof. Robert Oostenveld (slides contributed by Dr. Stephen Whitmarsh) entitled "Introduction to EEG/MEG and introduction to the FieldTrip toolbox". This and other lectures are available on the Fieldtrip YouTube channel and on Stephen's website.
(Start of transcript at time point 1.20 of the video.)
"We know that we have neurons in the brain and if you look at the presynaptic neuron, the signal arrives at the dendritic tree, it arrives at the synapse, the dendritic tree of a postsynaptic neuron. And what happens is that if we have a presynaptic action potential, the signal is transmitted to the postsynaptic neuron. And that causes a postsynaptic potential."
"If we look in detail, the presynaptic action potential will release neurotransmitters and these neurotransmitters will cause small currents to flow over the cell membrane of the postsynaptic neuron. And these small currents cause a postsynaptic potential. This should all be familiar to you."
"There are different types of potentials that can occur in a postsynaptic neuron. First of all, if the release of neurotransmitter is large enough, then that might trigger an action potential. The action potentials are very short lived. They are typically only a few milliseconds and have a biphasic pattern. So if you look at the action potentials from many neurons, it is very unlikely that they will be aligned exactly in time; so we might have one neuron producing an action potential here and a neighboring neuron producing an action potential a little bit later. So that means that with action potentials, we actually don’t really expect to see them at the scalp level, because they tend to cancel out, because they are poorly synchronized."
"There is some indication that we can see broadband activity; and broadband activity that we are observing with EEG and MEG might actually relate to action potentials. But at this moment the main motion is that we are mainly sensitive with EEG and MEG in picking up excitatory or inhibitory postsynaptic potentials."
"So basically, the activity at the synapse is not large enough to elicit an action potential in the postsynaptic neuron but a little bit of current will flow so the post synaptic neuron will depolarize or will hyperpolarize a little bit. This causes some current to flow and that current is relatively long-lived. And this relatively long-lived current allows for easy summation, spatial summation of all the neurons in the vicinity of the neuron that we are looking at."
"So if we now look at the brain and if we look at a very small section of the brain, let’s say this part. This is with a specific cell staining, the type of structure that we see. What we see is that we have different layers in the brain, the different layers in the cortical sheet and in layer IV and layer V, that is where we have the cell bodies of the pyramidal cells. These pyramidal cells have very long dendritic trees that extend towards the surface. And these pyramidal cells have a very nice topological arrangement. They are nicely aligned, which means that if multiple pyramidal cells receive input, currents start flowing in the same direction."
"So here you can see these pyramidal cells. What happens is that we will have electric currents flowing along the dendritic tree of the pyramidal cells and that produces EEG in the form of this electric current and MEG in the form of the magnetic fields."
"Of course these are very local phenomena, so what you should think of is that you have a small pool of neurons. A small current is created in this pool of neurons and what will happen is that this current will passively flow through the rest of the tissue because the brain as a whole is electrically conductive; so that means that ions will just flow through the tissue, will flow through white matter, but also through the scull, through the skin and if we apply electrodes to the skin we can record these currents as potential differences between these electrodes. So on the one hand we have propagation of action potentials and propagation of postsynaptic potentials which is an active process that involves the neurons, on the other hand we have the propagation of the current which is just a passive process. It is the same process that we tap into if we do transcranial direct current stimulation. We just have current flowing through the tissue. And that is the current that we are recording with the EEG."
"If we consider Maxwell equations; and we know that with each current there is an associated magnetic field. And very important for you to remember is the right hand rule. So if we have a current that is flowing in this direction, then with the right hand rule you can see which direction the magnetic field is flowing. And that is really useful; it is something that really helps you to understand the space topology. So like at this side of the field, it is going out and on the other side of the field, it is going in. The current is curving around. And that means that if we have small amount of current that is generated by a small pool of neurons that will produce a magnetic field which can be picked up outside the brain. Tomorrow and in the first modelling lecture I will explain that it is not only the primary current, not only the current inside the neurons but also the secondary currents that are flowing though the volume that contribute to the magnetic fields (end of transcript at time point 7.07 of lecture)."
(excerpt at time point 22.30 of video) "Especially in cognitive experiments, we will have activity in many brain regions at the same time. So if we consider activity at red dot (added comment), then that activity will be most pronounced over occipital electrodes. If we simultaneously also have a more frontal process, for example one which has "a much higher frequency content" cf green dot (added comment) then that is going to be most pronounced over frontal electrodes, frontal sensors. We will have an overlap of activity. So electrodes that are in between will see both sources. And here I am exaggerating a little bit, because if you just consider what these electrodes are going to see and depending on the choice of your reference electrode but also depending on how deep the sources are, all channels are going to see the activity from all tissue in the brain. So this frontal channel also sees as little bit of this occipital activity and this occipital channel also sees a little bit of this frontal activity. So that means that in order to analyse EEG and MEG data you always have to be aware of the fact that we have a superposition of source activity. And this entangling and superposition is basically the important challenge that you have to deal with using data analysis."