What do the foldings of the cortical surface do to the brain signals that are recorded by an EEG electrode at the scalp?
EEGs will pick up potential differences generated by action potentials from the pyramidal neurons in the gyri (folds) and sulci (valleys). I say pyramidal neurons specifically because the EEG measures the sum of all signals in a given volume, and pyramidal neurons are the ones whose signals do not cancel each other out. Distance here from the surface is not as much of a factor as you would think, though it certainly does affect an electric field as a general rule!
The location of an active signal--we'll say in a seizure--in the gyri or sulci determines which electrodes in the EEG will have increased potential relative to the others. What you see on the readout is spiking towards the seizure focus. For example, if you had 3 electrodes (A, B, and C) in a row measuring two potentials (`V_(AB)` and `V_(BC)`) and a seizure focus at B, you would see two things:
- Spikes pointing downward in `V_(AB)`
- Spikes pointing upward in `V_(BC)` `<br>`
What this would show on a strip (and I wish I had one to show you!) is a readout where you see the spikes effectively pointing at the electrode with the potential seizure focus.
EEGs are conducted from electrodes placed on the scalp that pick up on neural activity in the brain. Different activities activate different regions of the brain (i.e. there is a specific location in the brain for face processing and a separate location for object processing). Because the outer surface of the brain, or cerebral cortex, is folded, some parts are located closer to the scalp than others. Therefore, brain activity located deeper in the foldings of the cortical surface may be more difficult for an EEG to pick up on.