Subthreshold intracortical microstimulation of human somatosensory cortex enhances tactile sensitivity

Intracortical microstimulation (ICMS) of the somatosensory cortex activates neurons around the stimulating electrodes and can elicit tactile sensations. However, it is not clear how the direct activation of cortical neurons influences their ability to process additional tactile inputs originating from the skin. In a human implanted with chronic microelectrode arrays in both left and right somatosensory cortices, we presented mechanical vibration to the skin while simultaneously delivering ICMS and quantified the effects of combined mechanical and electrical stimulation on tactile perception. We found that subthreshold ICMS enhanced sensitivity to touch on the skin, as evidenced by a reduction in vibrotactile detection thresholds (median: -1.5 dB), but subthreshold vibration did not systematically impact the detectability of ICMS. Suprathreshold vibration led to an increase in ICMS thresholds (median: 2.4 dB) but suprathreshold ICMS had little impact on vibrotactile thresholds. The ICMS-induced enhancement of vibrotactile sensitivity was location dependent with the effect size decreasing as the projected field of the stimulating electrode and the locus of vibratory stimulation became farther apart. These results demonstrate that targeted microstimulation of cortex alone can focally enhance tactile sensitivity, potentially enabling restoration or strengthening of retained tactile sensations after injury.