This will pull the average down to 7 out of 19 total channels. But these channels are getting averaged into the front channels which are fairly flat-lined in alpha. So let us assume they were 10 uv roughly on average. In this case, the back 7-channels (parietal, temporal and occipital channels) are fairly similar. This too, distorts the real activity and generates fake EEG activity as well. Whole-head Average: This montage basically takes the average of all the channels and makes it the “reference” electrode through software. The frontal channels show activity because both the beta activity and the EMG are not phase or frequency-similar and therefore stand out against their neighboring channels. So, in this case, the high alpha channels look similar to themselves and therefore cancel out and show a flat line for activity. Laplacian: Laplacian looks at the difference of the site of interest and the difference between it and the average of the electrodes surrounding it. So, I will interpret that the EEG activity is true and not caused from a corrupted reference electrode. In this example using a referential montage, there is next to no alpha showing up in the frontal channels, including the midline. Other montages can make up fake frontal alpha – in this example – as a result of vector addition (especially full-head average as seen below). This is a primarily important assessment to make on a referential montage. Given volume conduction, the sites furthest from the reference are least similar to the reference and therefore show the most fake EEG. The other thing to watch for is if there is a maximal phase-locked rhythm along the midline (FZ, CZ & PZ), out of phase with posterior channels (in this example - alpha) then some of the real alpha signal from the posterior channels leaked in through the reference. If there is some mis-alignment, then I assume the activity is real. Then use the ruler and look at the peaks and troughs for alignment. First, are the waveforms phase-locked? To see this may require zooming into just a few seconds across the entire computer screen. Referential: When there are several channels of hyper-coherent activity as is often seen with alpha, we have to consider some things. In this case, the clinician will notice the obvious delta and assume that it is real. The streetlight effect is where a clinician or researcher only sees what is under the light – or what they expect to see. Given the clinician is expecting to see delta as coincident with a TBI, that clinician will succumb to the streetlight effect. There are often neck issues and muscle spasms in those with concussion and a muscle spasm would throw fake delta into the EEG. I’m still waiting for muscle verification from the clinician on that. It is my thought that a “vibrating” sterno-mastoid or neck muscle was throwing in the fake delta via the reference. I had a case recently where high delta appeared along the vertex. Whenever in doubt, the clinician should be zooming in to the raw and use the ruler function to see just how phase-locked the different channels appear to be. But in certain situations, these metrics are very misleading and will lead to a complete misdiagnosis, given the way they generate their “reference.” Please see the example below.īelow are several montages showing that when there are several channels of fairly phase-locked activity, both Laplacian and whole-head averaging can really skew the results. So, many clinicians are using Laplacian or whole-head average montages. There has been criticism of a referential linked-ears montage for processing EEG. Real is in quotes because there technically isn’t actually real brain activity that is measured as it is always the perceived activity as measured against some sort of reference. Surprising as it may sound, occasionally, a fair bit of brain activity can get into the ear lobe, cancelling out “real” brain activity while injecting fake activity into other recording channels. It the reference electrode is on the ear lobe, then the brain wave activity seen will be compared with the activity in the ear lobe. By “relative” this means that the recording electrode determines brain wave activity in comparison to a reference electrode. ApEEG Montages in Multi-channel Hyper-coherent AlphaĮEG recordings are always relative recordings.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |