Asymmetry of the brainwaves

A-symmertry is the difference in power between left and right hemisphere. For example between alpha power on electrode AF8 and alpha power on AF7. An index is used to express the relative difference between right and left. For example frontal-alpha-index = alpha power on electrode F8 devided by alpha power on electrode F7.

Asymmetry and emotions

Frontal asymmetry
Frontal asymmetry can be associated with the emotional respons.
Neuropsychologists have consistently found that higher engagement of the left frontal brain relative to the right frontal brain is related to positive feelings and higher motivation. Engagement in this case is expressed as relative power over the left and right frontal regions, on the electrodes (channels) F3/F7 and F4/F8. People with relative more frontal-right activity rapport more negative affective responses to situations than people with relative more frontal-left. People with more frontal-right give less positive reactions to a movie compared to others. So frontal asymmetry can be associated with the emotional respons

Approch versus withdrawal motivation
Latest findings indicate that asymmetry is more about motivation than about affection.
Reflecting whether or not someone is drawn towards or away from something or someone.
* More activity on the left means more approach motivation ( much engagement).
* More activity on the rigt is withdrawal motivation (or negative engagement).
* The approach motivation can be positive (enthousiasm) or negative (anger)
* Relative much activity left can mean emotional arousal and exitement (sensations)
* Relative much activity right can mean under-arousal and/or shyness.

Gamma asymmetry
Some neurologist state that Gamma is associated with a state of focus. Left-right frontal gamma asymmetry is related to negative or positive thought, depending on the direction of the asymmetry.

Alpha asymmetry and depression
Hageman e.a. found that people with increased left-frontal alpha were found to process information in a rather positive way, whereas right-lateralization indicated a more negative processing mode (aversion, sad, fear).
In an article in magazine for Neuropsychobiology (2000) Stefan Debener asks: Is anterior EEG alpha asymmetry a trait marker for depression?
Depressive people often have hypo(low) activity left and hyper (high) frontal right.
They have a general (overall) high alpha on frontal and parietal.
They have a hypo (low) alpha on left-mid-frontal side.
Depression and increased right-pariental activity might indicate to much emotionality.

Research about alpha asymmetry
frontal-asymmetry-motivation-emotions-eeg
Research by Harmon-jones(PDF file)

Calculations

There there are several ways to calculate and express the (a)symmertry index.
p.s. F7 and F8 are the frontal left-right positions on the Muse headband

a) Index = alpha power F8/alpha power F7
Suppost the alpha power on AF8 (right) = 4 dB and on AF7 (left) 5 db
The simplest way is devide R by L so 4/5 = 0.8
If AF8= 5dB and AF7 = 4dB it comes to 5/4 = 1,25
So > 1 means more power Right and < 1 means more power Left

b) Index = log (alpha power F8/alpha power F7)
If you take the logaritm of the division you get positive and negative numbers
For example log(0.8) is -0.22 zero and log(1.25) is +0.22 ...
If log(..) > 0 there is more power Right and log(..) < 0 there is more power Left

Other calculation that are sometimes used :

c) index = log(F8)-log(f7) / log(F8) + log(f7)
d) study of Rosenfeld et al., 1996; Baehr and Baehr : asymmetry = (R - L)/(R + L)
e) studies Henriques and Davidson: index = log(Right-F4) - log(Left-F3)
d) and e) are expressed in microvolts

The above calculation are not right or wrong. It are different ways of showing the difference.
So be aware, if on internet you find that a index in alpha-power (a)symmertry less than -0.5 means you have many negative thoughts, find out ‘ what calculation did they use ’ .