The Stress Resilient Mind Blog
Making Sense of EEG Assessment
Publication date: 11 December 2011
In my last post I made the case for EEG assessment as a useful and powerful tool in therapy and coaching. In this post I hope to put some flesh on those bones by explaining just what EEG is and what it can tell us.
EEG (short for electroencephalography) is simply an oscillating voltage measured from the scalp, and deriving from activity in the brain's cerebral cortex. We can analyse the EEG to make sense of how the brain is functioning.
To sum up very briefly, the oscillation can be described in terms of amplitude (or size of the waves), and frequency (number of cycles per second). EEG frequencies vary from around 1 cycle per second (or Hertz) up to around 40 Hz. Much of the useful information that EEG gives us derives from a computational process called frequency analysis (also known as spectral analysis and Fourier analysis). The idea is that we can decompose a complex oscillation into a number of component frequencies. Frequency bands have names – for example alpha covers a range of around 8-12 Hz, while theta covers around 4-7 Hz. The other major bands are delta (1-4 Hz) and beta (~13-36 Hz).
The value of EEG assessment comes from the fact that these bands, and their relative amplitudes, bear some relationship to subjective experience, and to symptoms in disorders such as depression, anxiety and ADHD.
For example a normal pattern is high alpha at the back of the head when the eyes are closed. Not everyone shows this pattern, and it seems that its absence predisposes you to problems such as inability to relax, even alcoholism. Furthermore, there is evidence it correlates with trauma, and that it changing it seems to relate to the resolution of that trauma.
Note I've introduced another idea here – that the EEG varies across the scalp. Many of the useful EEG markers are actually relationships between one part of the head and another (e.g. front-back or left-right).
Broadly speaking we can say that each band relates to different subjective states – e.g. delta waves are seen in deep sleep. Beta activity is seen when the brain is engaged and busy with processing, while higher delta, theta and alpha suggest (in some sense) inactivity or disengagement. (This is covered in more detail in this article on EEG.)
More EEG Markers of Interest
In my last post I mentioned Professor Richard Davidson's discovery that depressed patients show a left-right frontal imbalance. The normal pattern is that the left and right sides are either roughly in balance or there is a slight dominance of beta on the left side and alpha on the right. (This means that the left tends to be more active.) In depressed patients the opposite was the case – the left side showed more alpha (which is a kind of inactivity). Not all depressed people show this pattern (not all depression is the same) and furthermore some non-depressed people do show the pattern, meaning that it is a somewhat loose correlation rather than a definitive link. No-one really knows if depression causes the EEG pattern or the EEG pattern causes depression.
ADHD sufferers tend to have an underactive frontal brain. (The front part of the brain is highly involved in focus and concentration). This can be seen in the EEG as a relatively high theta to beta ratio. Again not every ADHD patient shows this pattern. Some people who have trouble focusing have the opposite pattern – the brain is too active (the theta to beta ratio is too low) at the back of the head. In this case it seems the brain has difficulty quieting itself. A lot of anxiety sufferers also show this pattern.
If you want to learn more about EEG markers like this, I'd recommend 'Biofeedback for the Brain' by Paul Swingle. Dr Swingle, based in Canada, is one of the world's leading neurofeedback practitioners.
In my next post, I'd like to develop this theme by (speculatively) attempting to relate EEG findings to neurochemical imbalances connected with disorders such as depression and anxiety. I'll be talking about neurotransmitters such as dopamine and GABA. Mainstream psychiatry is largely about influencing these important substances using drugs, and to some extent nutritional therapy attempts to do the same. But how do you know which one to target?
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