Face–hand sensorimotor interactions revealed by afferent inhibition

Abstract

Reorganization of the sensorimotor cortex following permanent (e.g., amputation) or temporary (e.g., local anaesthesia) deafferentation of the hand has revealed large‐scale plastic changes between the hand and face representations that are accompanied by perceptual correlates. The physiological mechanisms underlying this reorganization remain poorly understood. The aim of this study was to investigate sensorimotor interactions between the face and hand using an afferent inhibition transcranial magnetic stimulation protocol in which the motor evoked potential elicited by the magnetic pulse is inhibited when it is preceded by an afferent stimulus. We hypothesized that if face and hand representations in the sensorimotor cortex are functionally coupled, then electrocutaneous stimulation of the face would inhibit hand muscle motor responses. In two separate experiments, we delivered an electrocutaneous stimulus to either the skin over the right upper lip (Experiment 1) or right cheek (Experiment 2) and recorded muscular activity from the right first dorsal interosseous. Both lip and cheek stimulation inhibited right first dorsal interosseous motor evoked potentials. To investigate the specificity of this effect, we conducted two additional experiments in which electrocutaneous stimulation was applied to either the right forearm (Experiment 3) or right upper arm (Experiment 4). Forearm and upper arm stimulation also significantly inhibited the right first dorsal interosseous motor evoked potentials, but this inhibition was less robust than the inhibition associated with face stimulation. These findings provide the first evidence for face‐to‐hand afferent inhibition.

Plasticity of the face-hand sensorimotor circuits after a traumatic brachial plexus injury

“…Peripheral electrical stimulation intensity (pESI) for the finger and the face: the electrical stimulation intensity applied during the experiments was set at 3xpET for the finger and 2xpET for the face, thus ensuring a stimulus below the nociceptive threshold. This choice was based on AI protocols typically used (Tokimura et al, 2000; Tamburin et al, 2002; Tamburin et al, 2005; Bikmullina et al, 2009; Tamè et al, 2015) and on previous experiments from the LABNER research group (Ramalho et al, 2022), showing that a stimulation intensity of 3xpET for the face was painful for most participants.…”

“…ISIs were 15, 25, 35, 45, 55 and 65 ms for SAI and 100, 200, 300 and 400 ms for LAI. Significant FDI SAI has been reported with ISIs between 25 and 50 ms and facehand inhibition with ISIs between 45 and 65 ms (Helmich et al, 2005;Tamburin et al, 2005;Bikmullina et al, 2009;Ramalho et al, 2022). ISIs were then chosen based on the premise that SAI and LAI would occur at longer intervals for TPBI participants, considering the nerve injury.…”

“…Ramalho et al (2022) investigated sensorimotor interactions between the face and hand, hypothesizing that if face and hand representations are functionally coupled, then the electrical stimulation of the face would inhibit hand muscle motor responses. They found that the delivery of a peripheral electrical stimulus to either the skin over the right upper lip or the right cheek inhibited muscular activity from the right first dorsal interosseous (Ramalho et al, 2022). These findings provided the first evidence of face-to-hand afferent inhibition.…”

“…(c) Motor threshold to elicit 1 mV responses (1 mV/MT): defined as the minimum stimulation intensity required to obtain MEPs with at least 1 mV peak-to-peak amplitude in at least 5 out of 10 consecutively applied stimuli. The 1 mV target was chosen to obtain MEPs sufficiently large to allow the observation of the inhibition phenomenon (Turco et al, 2018;Ramalho et al, 2022).…”

“…SAI has also been demonstrated between different regions of the upper limb, such as between the skin of the hand and the forearm and arm muscles (Helmich et al, 2005; Tamburin et al, 2005; Bikmullina et al, 2009), and even between an electrical stimulus in one hand and a muscle in the contralateral hand (Ruddy et al, 2016). Ramalho et al (2022) investigated sensorimotor interactions between the face and hand, hypothesizing that if face and hand representations are functionally coupled, then the electrical stimulation of the face would inhibit hand muscle motor responses. They found that the delivery of a peripheral electrical stimulus to either the skin over the right upper lip or the right cheek inhibited muscular activity from the right first dorsal interosseous (Ramalho et al, 2022).…”