Intracortical microstimulation (ICMS)

ICMS delivers small, charge-balanced biphasic current pulses (typically a few to roughly 100 microamperes per phase) through a penetrating microelectrode positioned within the cortical grey matter, exciting neural elements within a radius of a few hundred micrometers to evoke an artificial percept or bias behaviour. Because the electrode's exposed area is tiny (thousands of square micrometers), the same physiological effect is achieved with currents about three orders of magnitude smaller than surface or deep-brain macro-stimulation.

The lowest-threshold elements near a microelectrode are axons and axon terminals rather than cell bodies, so ICMS does not simply light up a compact ball of tissue; it recruits a sparse, spatially distributed population of neurons whose recruited density grows with current but remains patchy. This biophysical fact governs how faithfully a stimulus pattern can be mapped onto a percept and is a recurring source of surprise for those who picture ICMS as writing a focal pixel.

ICMS is the principal write channel of an invasive bidirectional BCI: while motor or premotor cortex is read to drive an effector, somatosensory cortex is written to return touch, closing the sensorimotor loop.