From EEG

It’s not what it looks like: High Frequency Oscillations

Following on from the last post on baseline shifts, this is the second post on a few things we don’t normally consider when talking about standard EEG measurements. I had the idea for this one after a few excellent talks by Liset Menendez de la Prida at ISWP7 earlier this year.

Fast and furious (or is it)

Fast stuff on the EEG is difficult to see for a number of reasons: i) We usually filter raw EEG signal to make it look neater and often exclude high-gamma range signal. ii) The signal we measure on the scalp itself is already attenuated by passing through different tissues, making fast activity appear less sharp and prominent. This is true even for ECOG when compared to direct LFP recordings (which is becoming more relevant now that microelectrodes are being used more and more in patients with epilepsy). iii) Higher frequencies have a lower power – usually fast fluctuations are a lot smaller than bigger shifts on the EEG and seem to pale in comparison, when visually analysing the EEG.

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It’s not what it looks like: Baseline Shifts

Cook et al. 2013
The implantable device from Cook et al. 2013

I’ve just come back from the fantastic IWSP7: Epilepsy Mechanisms, Prediction and Control conference in Melbourne. Having apparently outgrown the initial meetings’ focus on seizure prediction, this year covered all aspects from computational models, intracranial devices, to imaging in epilepsy. For those who don’t know – Melbourne is a great place for such a conference, since Mark Cook and colleagues have managed a couple of years ago to pull off a clinical trial of implantable intracranial recording devices designed for long-term ambulatory recordings, in addition to the potential for responsive neuromodulation. The set-up can be seen on the right (an image the conference conveners seemed to love), and was a first in the world of seizure prediction. [1]

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