Cutaneomotor integration in humans is somatotopically organized at variouslevels of the nervous system and is task dependent

Integration of tactile afferent signals with motor commands is crucial for the performance of purposeful movements such as during manipulation of an o bject in the hand. To study the somatotopic organization of sensorimotor in tegration we applied electrical peripheral conditioning stimuli to a digit located near (homotopic stimulation) or distant from (heterotopic stimulati on) relaxed or isometrically contracted intrinsic hand muscles at variable time intervals prior to transcranial magnetic stimulation (TMS). Cutaneous stimulation has previously been shown to modulate the amplitude of the moto r evoked potential (MEP) and to shorten the duration of the silent period ( SP) evoked by TMS. In relaxed target muscles the time-dependent modulation of TMS-evoked motor responses by homotopic conditioning stimulation differe d from modulation by heterotopic stimulation. Similar differences in the mo dulation pattern evoked by homotopic and heterotopic conditioning stimulati on were observed for two distinct target muscles of the hand (abductor digi ti minimi, abductor pollicis brevis muscle). Differences in modulation were maximal when the conditioning stimulation was applied 25-30 ms and 150-200 ms prior to TMS. Comparison of the modulation of the amplitudes of MEPs ev oked by transcranial electrical stimulation (TES) and the modulation of tho se evoked by TMS suggests that differences between homo-topic and heterotop ic stimulation originate subcortically at 25- to 30-ms and, at least partia lly, cortically at 150-to 200-ms interstimulus intervals. In isometrically contracted intrinsic hand muscles the degree to which the SP was shortened reflected the location and the timing of the conditioning stimulus, Shorten ing was maximal when the conditioning stimulus was applied nearest to the c ontracted target muscle and 20 ms prior to the test stimulus. In contrast t o the SP duration, the MEP size in voluntarily contracted target muscles wa s unaffected by the location of the conditioning stimulus. The somatotopic gradient of SP shortening was abolished when the two target muscles were si multaneously activated isometrically. Together, our findings suggest that s omatotopy of input-output relationships is implemented at both a spinal and a cortical level in the human central nervous system and may also depend o n the motor task involved.