In vitro modulation of endogenous rhythms by AC electric fields: Syncing with clinical brain stimulation

Abstract

Humans are exposed to an increasing prevalence of weak and strong AC electric fields, as part of daily life in the modern world. Further, electric fields are being deployed to modulate brain function for research and clinical applications. A single electric field pulse, applied via transcranial electrical or magnetic stimulation, can transiently excite or disrupt activity in neural circuits. In contrast, extended exposure to steady electric fields or pulse trains can result in long-term effects on neural activity including potentiation or depression. In addition, it has been demonstrated that brain stimulation with electric fields can improve cognitive performance in normal subjects (Marshall et al. 2006). The impact of electric fields on brain function has motivated the development of therapies to treat a wide range of psychiatric and neurological diseases using transcranial electrical or magnetic stimulation (Wassermann & Lisanby, 2001), as well as deep brain stimulation with implanted electrodes. The mechanisms by which electric fields affect brain function have not been fully elaborated. A recent report in The Journal of Physiology by Deans et al. (2007) presents new data on the interaction of AC electric fields down to a cellular level as well as with neuronal population dynamics. The report reveals a frequency-specific ability of AC electric fields to rhythmically polarize pyramidal neurons of the CA3 region of the hippocampus and demonstrates the ability of AC fields to alter pharmacologically induced endogenous oscillations in the hippocampus. These data have important implications for understanding the effect of environmental AC fields and therapeutic stimulation on the activity of neuronal ensembles. A central finding of this study is that an AC electric field, applied in vitro to a brain slice set to oscillate in the gamma frequency range through bath application of kainate, shifts the ongoing oscillation to centre on the applied field frequency or a subharmonic of that frequency. This review of Deans et al. (2007) is intended to elucidate the connection between their work and other recent in vitro findings concerning electric fields in the brain with some recent clinical findings pertaining to cognitive function and electric fields.