2.6.2. Pairwise Derived Brain Symmetry Index

The pairwise derived brain symmetry index (pdBSI) is a biomarker for hemispheric stroke and is calculated as suggested by Sheoralpanday [22] and Van putten [23]. Hemispheric stroke results in a lack of cortical power balance between the two hemispheres. As pdBSI is the indicator of hemispheric power asymmetry, it may be used for early stroke prediction [3,13]. pdBSI ranges between zero (no asymmetry) and one (total asymmetry). The pdBSI was defined in Equation (4).

with Rt_ij and Lt_ij being the power spectral density of the waveform of a right and a left node of a homologous EEG channel pair (with i = 1,2, ..., p) at frequency j (with j =1,2, ..., q). For this study, p was limited to 2, and pdBSI was measured for the frequency bands (q) 0.5–4 Hz, 4–8 Hz, 8–13 Hz, and 13–30 Hz.