Hello, my name is Arnaud Delorme, and I'm the main software designer for EEGLAB
which is a software to process EEG data.
This is the second part of the EEGLAB introduction about the origin of the EEG signal.
So when you record EEG at the surface of the scalp (the yellow electrodes are representing electrodes at the surface of the scalp)
you actually record field from regions of the cortex.
Here we have a local patch of cortex activated, which can be far away
you see the electric field extends far away.
And when you have several of these patches which are active simultaneously
you can see that most electrodes record a combination of the activity of the two regions.
So this is why it's actually very different to record from the surface of the cortex compared to the electrodes at the surface of the scalp.
And in the subsequent lecture, we'll see how we can try to use the activity at the electrode level to reduce(?) source reconstruction of EEG data.
So scalp signals are different from source signals.
So when we have a potential at the surface of the scalp, so for instance a positive and a negative potential
what this represents is the activity of a dipole, which can be represented as a battery.
So the battery has a specific electric field, which is recorded at the surface of the scalp.
Where does this electric potential come from?
It's generally accepted that it comes from pyramidal cells.
So here there's two types of cells which are represented.
Stellate cells, which basically have a radial electric field, so no net potential that can be recorded at the surface of the scalp
and pyramidal cells, which are oriented radially to the surface of the cortex, and have a net positive potential.
On top is the electric potential, and on the bottom the magnetic potential.
And usually having just one of these cells is not enough to record signal in the surface of the scalp.
So if you have radially symmetric or randomly oriented pyramidal cell, you're not going to record some potential at the surface of the scalp.
If they are parallel but they're activated asynchronously (at different times), you're not going to record potential at the surface of the scalp.
You're only going to record a potential when they're active, they're parallel, and they're also activated simultaneously.
But it's not that simple, because there [are] a lot of cells interacting with these pyramidal cells in the cortex.
So here you can see I, II, III, IV, V, VI. These are the different layers of the cortex.
So even though the cortex is just a few millimeters thick, there are several layers of interest, with different types of cells in the different layers.
For instance, these pyramidal cells receive excitatory and inhibitory input.
They receive input from other cortical areas, they receive input from the tenements
and they also receive input from glial cells, which are not neurons but they have gap junctions with neurons, so they are also communicating with neurons.
And also there is all the neuromodulation of these cells.
So here we show a pyramidal cell.
And you can see at the surface of the cortex you have some pattern of activity that represents the activity of the electric field, but you also have a pattern in depth.
So there actually has been recording of this pattern across the different layers.
This is one of these recordings. So this is time on the x-axis, and the color represents the activity in the theta band.
So, in black here at the beginning of the recording, you have a pyramidal cell
and we can see that the activity of this neuron along the axon seems to oscillate in the theta band
with some positive activity, followed by negative activity, etc.
And there have been models trying to understand what this means.
For instance at the peak of the pyramidal theta, what this is interpreted as acquiring information, so the neurons integrate information
and then at the trough of the pyramidal theta, this has been interpreted as interpreting the information.
So there's a cycle, and this is coordinated by inhibitory basket cells and also cell from the thalamus.
The OLM cells are from the thalamus.
So I invite you to look at the reference if you're interested in these models.
It's just a schematic. There [are] other models of generation of the EEG.
It's just to show you that we actually know (?)a little bit how the EEG is generated, and what it means in terms of activation of neurons.
But historically, in brain electrophysiology, there was on one side the ERP people
and on the other side, the researchers which were interested in spike detection.
And basically in the middle there wasn't much.
So what we (and others) have been trying to do is to reconcile these two fields.
For instance, for ERP average we can see what it means in terms of EEG activity
and the spike can be correlated with the local field potential, which is the potential at the surface of the cortex.
And then the LFP can also be correlated with EEG.
So [we're trying] to reconcile these fields and push the field of brain electrophysiology to really study, with EEG at the scalp, what's happening in the brain.