title: Interhemispheric sensorimotor integration; an upper limb phenomenon?
abstract: Somatosensory information from the limbs reaches the contralateral Primary Sensory Cortex (S1) with a delay of 23ms for finger, and 40ms for leg (somatosensory N20/N40). Upon arrival of this input in the cortex, motor evoked potentials (MEPs) elicited by Transcranial Magnetic Stimulation (TMS) are momentarily inhibited. This phenomenon is called 'short latency afferent inhibition (SAI)' and can be used as a tool for investigating sensorimotor interactions in the brain. We used SAI to investigate the process of sensorimotor integration in the hemisphere ipsilateral to the stimulated limb. We hypothesized that ipsilateral SAI would occur with a delay following the onset of contralateral SAI, to allow for transcallosal conduction of the signal. We electrically stimulated the limb either contralateral or ipsilateral to the hemisphere receiving TMS, using a range of different interstimulus intervals (ISI). We tested the First Dorsal Interosseous (FDI) muscle in the hand, and Tibialis Anterior (TA) in the lower leg, in three separate experiments. Ipsilateral SAI was elicited in the upper limb (FDI) at all ISIs that were greater than N20+18ms (all p<.05) but never at any earlier timepoint. No ipsilateral SAI was detected in the lower limb (TA) at any of the tested ISIs. The delayed onset timing of ipsilateral SAI suggests that transcallosal communication mediates this inhibitory process for the upper limb. The complete absence of ipsilateral SAI in the lower limb warrants consideration of the potential limb-specific differences in demands for bilateral sensorimotor integration.
authors.affiliation: Neural Control of Movement Lab, ETH Zurich, Switzerland. Electronic address: Kathy.ruddy@hest.ethz.ch.
authors.forename: Kathy L
authors.initials: KL
authors.lastname: Ruddy
authors.affiliation: Neural Control of Movement Lab, ETH Zurich, Switzerland. Electronic address: Ellen.Jaspers@hest.ethz.ch.
authors.forename: Ellen
authors.initials: E
authors.lastname: Jaspers
authors.affiliation: Movement and Sport Science, Department of Medicine, University of Fribourg, Switzerland. Electronic address: martin.keller@unifr.ch.
authors.forename: Martin
authors.initials: M
authors.lastname: Keller
authors.affiliation: Neural Control of Movement Lab, ETH Zurich, Switzerland; KU Leuven, Movement Control & Neuroplasticity Research Group, Belgium. Electronic address: nicole.wenderoth@hest.ethz.ch.
authors.forename: Nicole
authors.initials: N
authors.lastname: Wenderoth
mesh_terms: D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D035002:Lower Extremity / Q000502:physiology*; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology*; D055698:Touch Perception / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation; D034941:Upper Extremity / Q000502:physiology*; D055815:Young Adult
keywords: inhibition; ipsilateral; sensorimotor integration; short latency afferent inhibition; somatosensory