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The corticospinal system and local spinal circuits control human bipedal locomotion. The primary motor cortex is phase-dependently activated during gait; this cortical input is critical for foot flexor activity during the swing phase. We investigated whether gait-combined rhythmic brain stimulation can induce neuroplasticity in the foot area of the motor cortex and alter gait parameters. Twenty-one healthy subjects participated in the single-blinded, cross-over study. Each subject received anodal transcranial patterned direct current stimulation over the foot area of the right motor cortex during gait, sham stimulation during gait, and anodal transcranial patterned direct current stimulation during rest in a random order. Six subjects were excluded due to a failure in the experimental recording procedure. Complete-case analysis was performed using the data from the remaining 15 subjects. Self-paced gait speed and left leg stride length were significantly increased after the stimulation during gait, but not after the sham stimulation during gait or the stimulation during rest. In addition, a significant increase was found in the excitability of the corticospinal pathway of the left tibialis anterior muscle 30 min after stimulation during gait. Anodal transcranial patterned direct current stimulation during gait entrained the gait cycle to enhance motor cortical activity in some subjects. These findings suggest that the stimulation during gait induced neuroplasticity in corticospinal pathways driving flexor muscles during gait.

Neurofeedback has been a powerful method for self-regulating brain activities to elicit potential ability of human mind. GABA is a major inhibitory neurotransmitter in the central nervous system. Transcranial magnetic stimulation (TMS) is a tool that can evaluate the GABAergic system within the primary motor cortex (M1) using paired-pulse stimuli, short intracortical inhibition (SICI). Herein we investigated whether neurofeedback learning using SICI enabled us to control the GABAergic system within the M1 area. Forty-five healthy subjects were randomly divided into two groups: those receiving SICI neurofeedback learning or those receiving no neurofeedback (control) learning. During both learning periods, subjects made attempts to change the size of a circle, which was altered according to the degree of SICI in the SICI neurofeedback learning group, and which was altered independent of the degree of SICI in the control learning group. Results demonstrated that the SICI neurofeedback learning group showed a significant enhancement in SICI. Moreover, this group showed a significant reduction in choice reaction time compared to the control group. Our findings indicate that humans can intrinsically control the intracortical GABAergic system within M1 and can thus improve motor behaviors by SICI neurofeedback learning. SICI neurofeedback learning is a novel and promising approach to control our neural system and potentially represents a new therapy for patients with abnormal motor symptoms caused by CNS disorders.

The tongue plays important roles in a variety of critical human oral functions, including speech production, swallowing, mastication and respiration. These sophisticated tongue movements are in part finely regulated by cortical entrainment. Many studies have examined sensorimotor processing in the limbs using magnetoencephalography (MEG), which has high spatiotemporal resolution. Such studies have employed multiple methods of analysis, including somatosensory evoked fields (SEFs), movement-related cortical fields (MRCFs), event-related desynchronization/synchronization (ERD/ERS) associated with somatosensory stimulation or movement and cortico-muscular coherence (CMC) during sustained movement. However, the cortical mechanisms underlying the sensorimotor functions of the tongue remain unclear, as contamination artifacts induced by stimulation and/or muscle activity within the orofacial region complicates MEG analysis in the oral region. Recently, several studies have obtained MEG recordings from the tongue region using improved stimulation methods and movement tasks. In the present review, we provide a detailed overview of tongue sensorimotor processing in humans, based on the findings of recent MEG studies. In addition, we review the clinical applications of MEG for sensory disturbances of the tongue caused by damage to the lingual nerve. Increased knowledge of the physiological and pathophysiological mechanisms underlying tongue sensorimotor processing may improve our understanding of the cortical entrainment of human oral functions.

Sophisticated tongue movements are coordinated finely via cortical control. We elucidated the cortical processes associated with voluntary tongue movement. Movement-related cortical fields were investigated during self-paced repetitive tongue protrusion. Surface tongue electromyograms were recorded to determine movement onset. To identify the location of the primary somatosensory cortex (S1), tongue somatosensory evoked fields were measured. The readiness fields (RFs) over both hemispheres began prior to movement onset and culminated in the motor fields (MFs) around movement onset. These signals were followed by transient movement evoked fields (MEFs) after movement onset. The MF and MEF peak latencies and magnitudes were not different between the hemispheres. The MF current sources were located in the precentral gyrus, suggesting they were located in the primary motor cortex (M1); this was contrary to the MEF sources, which were located in S1. We conclude that the RFs and MFs mainly reflect the cortical processes for the preparation and execution of tongue movement in the bilateral M1, without hemispheric dominance. Moreover, the MEFs may represent proprioceptive feedback from the tongue to bilateral S1. Such cortical processing related to the efferent and afferent information may aid in the coordination of sophisticated tongue movements.

Sophisticated tongue movements are coordinated finely via cortical control. We elucidated the cortical processes associated with voluntary tongue movement. Movement-related cortical fields were investigated during self-paced repetitive tongue protrusion. Surface tongue electromyograms were recorded to determine movement onset. To identify the location of the primary somatosensory cortex (S1), tongue somatosensory evoked fields were measured. The readiness fields (RFs) over both hemispheres began prior to movement onset and culminated in the motor fields (MFs) around movement onset. These signals were followed by transient movement evoked fields (MEFs) after movement onset. The MF and MEF peak latencies and magnitudes were not different between the hemispheres. The MF current sources were located in the precentral gyrus, suggesting they were located in the primary motor cortex (M1); this was contrary to the MEF sources, which were located in S1. We conclude that the RFs and MFs mainly reflect the cortical processes for the preparation and execution of tongue movement in the bilateral M1, without hemispheric dominance. Moreover, the MEFs may represent proprioceptive feedback from the tongue to bilateral S1. Such cortical processing related to the efferent and afferent information may aid in the coordination of sophisticated tongue movements.

The tongue plays important roles in a variety of critical human oral functions, including speech production, swallowing, mastication and respiration. These sophisticated tongue movements are in part finely regulated by cortical entrainment. Many studies have examined sensorimotor processing in the limbs using magnetoencephalography (MEG), which has high spatiotemporal resolution. Such studies have employed multiple methods of analysis, including somatosensory evoked fields (SEFs), movement-related cortical fields (MRCFs), event-related desynchronization/synchronization (ERD/ERS) associated with somatosensory stimulation or movement and cortico-muscular coherence (CMC) during sustained movement. However, the cortical mechanisms underlying the sensorimotor functions of the tongue remain unclear, as contamination artifacts induced by stimulation and/or muscle activity within the orofacial region complicates MEG analysis in the oral region. Recently, several studies have obtained MEG recordings from the tongue region using improved stimulation methods and movement tasks. In the present review, we provide a detailed overview of tongue sensorimotor processing in humans, based on the findings of recent MEG studies. In addition, we review the clinical applications of MEG for sensory disturbances of the tongue caused by damage to the lingual nerve. Increased knowledge of the physiological and pathophysiological mechanisms underlying tongue sensorimotor processing may improve our understanding of the cortical entrainment of human oral functions.

Modulation of 20-Hz activity in the primary sensorimotor cortex (SM1) may be important for oral functions. Here, we show that 20-Hz event-related desynchronization/synchronization (20-Hz ERD/ERS) is modulated by sensory input and motor output in the oral region.

Somatosensory evoked fields (SEFs) induced by tongue stimulation can be useful as an objective parameter to assess sensory disturbances in the tongue. However, whether tongue SEFs can be useful as a clinical, objective follow-up assessment method of tongue sensation following oral surgery is unknown. Here we describe two cases in which tongue SEFs were successfully used in clinical assessment. Two patients with unilateral tongue sensory deficits caused by lingual nerve injury during lower third molar extraction were recruited. Both patients underwent surgery to repair the damaged nerve, and all tongue sensory evaluations were made once before and once after surgery. SEFs were recorded by stimulating the affected and unaffected sides of the tongue separately, and cortical activity was evaluated over the contralateral hemisphere. The unilaterality of the deficit was also assessed. In both patients, stimulation of the unaffected side evoked reproducible cortical responses pre- and post-surgery. Both patients also recovered some sensation following surgery, as pre-surgery stimulation of the affected side failed to evoke cortical activity, whereas post-surgery stimulation evoked cortical activity on both sides. Sensation was initially highly lateralized in both patients, but was restored to approximately normal in the post-surgery evaluation. Finally, both patients’ rated their subjective tongue sensations on the affected side over 50% better after the surgical intervention. These cases indicate that tongue SEFs may have a clinical use as an objective parameter for assessing the course of tongue sensory recovery.

Somatosensory evoked fields (SEFs) induced by tongue stimulation can be useful as an objective parameter to assess sensory disturbances in the tongue. However, whether tongue SEFs can be useful as a clinical, objective follow-up assessment method of tongue sensation following oral surgery is unknown. Here we describe two cases in which tongue SEFs were successfully used in clinical assessment. Two patients with unilateral tongue sensory deficits caused by lingual nerve injury during lower third molar extraction were recruited. Both patients underwent surgery to repair the damaged nerve, and all tongue sensory evaluations were made once before and once after surgery. SEFs were recorded by stimulating the affected and unaffected sides of the tongue separately, and cortical activity was evaluated over the contralateral hemisphere. The unilaterality of the deficit was also assessed. In both patients, stimulation of the unaffected side evoked reproducible cortical responses pre- and post-surgery. Both patients also recovered some sensation following surgery, as pre-surgery stimulation of the affected side failed to evoke cortical activity, whereas post-surgery stimulation evoked cortical activity on both sides. Sensation was initially highly lateralized in both patients, but was restored to approximately normal in the post-surgery evaluation. Finally, both patients’ rated their subjective tongue sensations on the affected side over 50% better after the surgical intervention. These cases indicate that tongue SEFs may have a clinical use as an objective parameter for assessing the course of tongue sensory recovery.

Objective: To evaluate lip sensory dysfunction in patients with inferior alveolar nerve injury by lip-stimulated somatosensory evoked fields (SEFs). Methods: SEFs were recorded following electrical lip stimulation in 6 patients with unilateral lip sensory disturbance and 10 healthy volunteers. Lip stimulation was applied non-invasively to each side of the lip with the same intensity using pin electrodes. Results: All healthy volunteers showed the earliest response clearly and consistently at around 25 ms (P25m) and at least one of the following components, P45m, P60m, or P80m, over the contralateral hemisphere. The ranges of the peak latencies were 23-33, 42-50, 56-67, and 72-98 ms for right-side stimulation and 23-34, 46-49, 52-68, and 71-90 ms for left-side stimulation. Affected-side stimulation did not evoke P25m component in any patients, but invoked traceable responses in 5 patients whose latencies were 57, 89, 65, 53, and 54 ms. Unaffected-side stimulation induced P25m in 2 patients at 27 and 25 ms, but not in the other 4 patients. Conclusion: The P25m component of lip SEFs can be an effective parameter to indicate lip sensory abnormality. Significance: Lip sensory dysfunction can be objectively evaluated using magnetoencephalography.

Sophisticated tongue movements contribute to speech and mastication. These movements are regulated by communication between the bilateral cortex and each tongue side. The functional connection between the cortex and tongue was investigated using oscillatory interactions between whole-head magnetoencephalographic (MEG) signals and electromyographic (EMG) signals from both tongue sides during human tongue protrusion compared to thumb data. MEG-EMG coherence was observed at 14-36Hz and 2-10Hz over both hemispheres for each tongue side. EMG-EMG coherence between tongue sides was also detected at the same frequency bands. Thumb coherence was detected at 15-33Hz over the contralateral hemisphere. Tongue coherence at 14-36Hz was larger over the contralateral vs. ipsilateral hemisphere for both tongue sides. Tongue cortical sources were located in the lower part of the central sulcus and were anterior and inferior to the thumb areas, agreeing with the classical homunculus. Cross-correlogram analysis showed the MEG signal preceded the EMG signal. The cortex-tongue time lag was shorter than the cortex-thumb time lag. The cortex-muscle time lag decreased systematically with distance. These results suggest that during tongue protrusions, descending motor commands are modulated by bilateral cortical oscillations, and each tongue side is dominated by the contralateral hemisphere.

Although oral sensory feedback is essential for mastication, whether the cortical activity elicited by oral stimulation is associated with the preferred chewing side (PCS) is unclear. Somatosensory evoked fields were measured in 12 healthy volunteers (6 with the right side as the PCS and 6 with the left side as the PCS) following tongue and hard palate stimulation. Three components were identified over the contralateral (P40m, P60m, and P80m) and ipsilateral [P40m(I), P60m(I), and P80m(I)] hemispheres. Since no component was consistently detected across subjects, we evaluated the cortical activity over each hemisphere using the activated root-mean-square (aRMS), which was the mean amplitude of the 18-channel RMS between 10 and 150ms. For tongue stimulation, the aRMS for each hemisphere was 8.23 ± 1.55 (contralateral, mean ± SEM) and 4.67 ± 0.88 (ipsilateral)fT/cm for the PCS, and 5.11 ± 1.10 (contralateral) and 4.03 ± 0.82 (ipsilateral)fT/cm for the non-PCS. For palate stimulation, the aRMS was 5.35 ± 0.58 (contralateral) and 4.62 ± 0.67 (ipsilateral)fT/cm for the PCS, and 4.63 ± 0.56 (contralateral) and 4.14 ± 0.60 (ipsilateral)fT/cm for the non-PCS. For hard palate stimulation, the aRMS did not differ between the PCS and non-PCS, whereas for tongue stimulation, the contralateral hemisphere aRMS was significantly greater for the PCS than for the non-PCS. Thus, our results show that lateralized cortical activation was associated with the PCS for tongue, but not hard palate, stimulation; a potential reason for this may be the different sensory-inputs between these two areas, specifically the presence or absence of fine motor function.

Although oral sensory feedback is essential for mastication, whether the cortical activity elicited by oral stimulation is associated with the preferred chewing side (PCS) is unclear. Somatosensory evoked fields were measured in 12 healthy volunteers (6 with the right side as the PCS and 6 with the left side as the PCS) following tongue and hard palate stimulation. Three components were identified over the contralateral (P40m, P60m, and P80m) and ipsilateral [P40m(I), P60m(I), and P80m(I)] hemispheres. Since no component was consistently detected across subjects, we evaluated the cortical activity over each hemisphere using the activated root-mean-square (aRMS), which was the mean amplitude of the 18-channel RMS between 10 and 150ms. For tongue stimulation, the aRMS for each hemisphere was 8.23 ± 1.55 (contralateral, mean ± SEM) and 4.67 ± 0.88 (ipsilateral)fT/cm for the PCS, and 5.11 ± 1.10 (contralateral) and 4.03 ± 0.82 (ipsilateral)fT/cm for the non-PCS. For palate stimulation, the aRMS was 5.35 ± 0.58 (contralateral) and 4.62 ± 0.67 (ipsilateral)fT/cm for the PCS, and 4.63 ± 0.56 (contralateral) and 4.14 ± 0.60 (ipsilateral)fT/cm for the non-PCS. For hard palate stimulation, the aRMS did not differ between the PCS and non-PCS, whereas for tongue stimulation, the contralateral hemisphere aRMS was significantly greater for the PCS than for the non-PCS. Thus, our results show that lateralized cortical activation was associated with the PCS for tongue, but not hard palate, stimulation; a potential reason for this may be the different sensory-inputs between these two areas, specifically the presence or absence of fine motor function.