Nonlinear analysis of EEG complexity in episode and remission phase of recurrent depression

Biomarkers of Major Depressive Disorder(MDD), its phases and forms have long been sought. Research indicates that the complexity measures of the cortical electrical activity (EEG) might be candidates for this role. To examine whether the complexity of EEG activity, measured by Higuchi fractal dimension (HFD) and sample entropy (SampEn), differs between healthy subjects, patients in remission and episode phase of the recurrent depression and whether the changes are differentially distributed between hemispheres and cortical regions. Resting state EEG with eyes closed was recorded from 26 patients suffering from recurrent depression and 20 age and sex-matched healthy control subjects. Artefact-free EEG epochs were analyzed by in-house developed programs running HFD and SampEn algorithms. Depressed patients had higher HFD and SampEn complexity compared to healthy subjects. Surprisingly, the complexity was even higher in patients who were in remission than in those in the episode. Altered complexity was present in the frontal and centro-parietal regions when compared to the control group. The complexity in frontal and parietal regions differed between the two phases of depressive disorder. SampEn manifested higher sensitivity than HFD in some cortical areas. Complexity measures of EEG distinguish between the three groups. Further studies are needed to establish whether these measures carry the potential to aid clinically relevant decisions about depression.