Brain Symmetry Index (BSI)

The Brain Symmetry Index (BSI)^[1] is a measure of the degree of symmetry between the left and right hemispheres of the brain and used to monitor possible brain ischemia in carotid surgery.

Calculation

It is calculated by taking the average of the absolute differences between the mean power of the left and right hemispheres in a specific frequency range (1 to 25 Hz). This measurement serves as an indicator of the amount of ischemic damage that may have occurred.

BSI(t) = \frac{1}{k} \sum^{k}_{n=1}{\Bigg| \frac{R^*_n(t)-L^*_n(t)}{R^*_n(t)+L^*_n(t)} \Bigg|}

with k as the number of discrete frequencies, and

R^*_n(t) = \frac{1}{m} \sum^{m}_{ch=1}{a^2_n(ch,t)}

R^*_n(t) is for the channels on the right hemisphere, L^*_n(t) for the left accordingly. a^2_n(ch,t) is the Fourier coefficient with index n of channel ch, at time t, corresponding to a particular event epoch [t-T, t].

Limitation

BSI is considered a useful tool to study brain asymmetry, as it is non-invasive, easy to implement and it can be calculated from widely available imaging modalities. However, it is important to note that BSI is a measure of brain asymmetry and it does not necessarily reflect any specific cognitive or behavioral function. It is just a tool to study the brain structure and it should be used in combination with other assessment methods to understand the underlying neural mechanisms.