The present special issue invites experimental and theoretical work that can shed light on electric phenomena in extracellular space, with an emphasis on extracellular potentials and endogenous electric field interactions. The contributions solicited are original research papers and comprehensive reviews on methodological aspects.

1. The nature of extracellular potentials

   - What is nature of the electric currents, and what is the velocity (distribution) of the ions involved?  Since membrane currents underlie the extracellular currents, what are the relative contributions of synaptic versus voltage-gated channels, excitatory versus inhibitory synapses, capacitive currents? How important are Lorenz forces between currents (due to the generated magnetic fields) and is there a contribution of polarization currents?

    - Is extracellular space purely resistive, or is a capacitive component involved, and what factors (such as ionic concentration, Debye shielding, etc.) contribute to its resistivity?

 2. Modeling of extracellular potentials and electric field interactions

- What level of detail is needed to model extracellular electrical phenomena, and which formalism can be employed? How valid is the cable equation, or need slow concentration changes of the major ionic components involved be modeled explicitly? How can LFP calculations be incorporated in simulation software?

-  Can volume transmission be simulated adequately as volume conductors? Are ephaptic coupling effects adequately simulated in one dimension?

3. Neuronal inference from measured extracellular potentials

   - What is the importance of the electrode characteristics for the proper interpretation of the measured signals?

   - What are the underlying assumptions, and limitations, of current-source-density analysis? How valid is the dipole approximation? Why are recent experiments being interpreted in terms of monopoles, and can single-compartmental model neurons generate local field potentials at all?

   - What can be learned from the power density distribution of signal frequencies?

4. Neurostimulation

  - What is the effect on neurons of an externally applied current or potential, and what mechanism (or rationale) underlies deep brain stimulation?

5. Physiological roles of extracellular potentials and endogenous electrical fields

- Do extracellular potentials contribute to neuron-neuron and neuron-glia communication? Can there be a role for extracellular potentials in the homeostasis, clearance, and volume regulation of extracellular space, and in the control of blood flow and of the blood-brain-barrier.

  - What is the integrative role of endogenous electrical fields in brain function?