Neural element activation (sparse distributed recruitment)

The biophysical account of which cells a microelectrode actually excites. The activating function of extracellular stimulation depends on the second spatial derivative of the potential along a fibre, and axons and axon terminals have lower thresholds than cell bodies, so ICMS preferentially recruits passing and terminating fibres rather than the somata nearest the tip. The consequence is that stimulation does not fill a compact sphere: it lights up a sparse, spatially distributed set of neurons interleaved with unactivated ones, extending well beyond the immediate vicinity of the electrode.

Recruited density does grow with current as the field spreads, but the pattern remains patchy and includes distant neurons whose axons happen to pass nearby. This overturns the intuitive picture of ICMS as writing a focal pixel and constrains how precisely a stimulus pattern can be sculpted into a percept; it also means that two nearby electrodes can activate partially overlapping, non-adjacent populations.