Ephaptic coupling and power fluctuations in depression.

The initial therapeutic exposure to Deep Brain Stimulation (DBS) during implantation surgery has reproducible acute behavioral effects that carry over without further stimulation. We analyzed local field potential (LFP) data from the first month following brief therapeutic intraoperative DBS. Data were recorded from the subcallosal cingulate cortex. During this month, no further stimulation was applied. Recent studies have identified beta power fluctuations in LFP data as an acute putative depression biomarker of this exposure. However, a detailed description of neural dynamics underlying brain power fluctuations is missing. Here, we consider how these fluctuations are related to brain itinerancy, that is, neural activity changes between stable and unstable states. We also provide a proof of principle study that these dynamics can be described using 2 new dynamical systems measures: instability frequency and relative wandering time. These capture interactions between neural activity and the mesoscale oscillatory electric fields generated by it. The 2 measures seem to split low vs. high Hamilton Depression Rating Scale scores within a small patient cohort. They are motivated by the cytoelectric coupling hypothesis, which suggests that efficient information processing results from mesoscale electric fields and that the re-emergence of depression symptoms might result from altered electric fields. Whether the new measures reflect general mechanisms of rapid antidepressant action remains to be tested.