Pitfalls in the interpretation of multielectrode data: on the infeasibility of the neuronal current-source monopoles

TO THE EDITOR: Riera et al. (2012) recently presented a comprehensive multielectrode study of the barrel field of anesthetized rat. Notably, they reported that the estimated instantaneous current source density (CSD) from the recorded extracellular potentials does not sum to zero over the volume of the barrel column, indicating the presence of cortical current source monopoles on a mesoscopic (cell population) scale. The authors concluded that current source monopoles must be included in the interpretation of electrophysiological recordings (including EEG). Together with an accompanying commentary (Destexhe and Bedard 2012), it was further speculated that the estimated current monopoles are real, i.e., that single neurons may act as true neuronal current monopoles, and that the traditional cable equation may be invalid for describing bioelectrical neuronal phenomena. However, these claims are in contradiction with well-established models of electrophysiology, including Hodgkin and Huxley’s mathematical description of axonal action potentials and the multicompartmental models of dendritic signal integration in neurons. In fact, the existence of current monopoles would be in direct violation of the Ampere-Maxwell’s law for macroscopic media (Griffiths 1999) H Jf D t (1) which relates the magnetic field H to the electric current density of free charges Jf and the change in the displacement field D. By applying the divergence operator to this equation, and noting that the divergence of a curl is zero, it follows that