A novel framework for AC field-effects on action potential coherence and phase

Small electric fields will polarize neurons by only a small amount for this reason small electric fields have previously been suggested to have no physiologically relevant effects. We propose a mechanism whereby AC extracellular fields incrementally polarize a neuron's membrane and thus modulate the coherence and phase of synaptically driven action potentials. Knowing that a membrane polarizes in proportion to field strength (DeltaV = cE), and that spike timing changes linearly with increasing steady-state field strength (Deltat prop E), we make a number of quantitative predictions on the effects of AC extracellular fields on a neuron's spike timing oscillating fields will shift firing times with mean falling within or the oscillatory cycle (the rising edge). This mean firing time advances with increasing field strength and decreasing injected ramp slope, i.e. it increases with cE/Vdot. This effect is proportional to the inverse of the driving synaptic membrane potential slope Deltat=DeltaV/Vdot dot cE/Vdot. The strength of coherence as measured by Rayleigh coefficient (vector strength) also increases with cE/Vdot. The predictions were verified in rat hippocampal A pyramidal neurons.