Region-Specific Effects of 10-Hz Transcranial Alternate Current Stimulation Over the Left Posterior Parietal Cortex and Primary Somatosensory Area on Tactile Two-Point Discrimination Threshold

Beta resting-state functional connectivity predicts tactile spatial acuity

Tactile perception is a complex phenomenon that is processed by multiple cortical regions via the primary somatosensory cortex (S1). Although somatosensory gating in the S1 using paired-pulse stimulation can predict tactile performance, the functional relevance of cortico-cortical connections to tactile perception remains unclear. We investigated the mechanisms by which corticocortical and local networks predict tactile spatial acuity in 42 adults using magnetoencephalography (MEG). Resting-state MEG was recorded with the eyes open, whereas evoked responses were assessed using single- and paired-pulse electrical stimulation. Source data were used to estimate the S1-seed resting-state functional connectivity (rs-FC) in the whole brain and the evoked response in the S1. Two-point discrimination threshold was assessed using a custom-made device. The beta rs-FC revealed a negative correlation between the discrimination threshold and S1-superior parietal lobule, S1-inferior parietal lobule, and S1-superior temporal gyrus connection (all P < 0.049); strong connectivity was associated with better performance. Somatosensory gating of N20m was also negatively correlated with the discrimination threshold (P = 0.015), with weak gating associated with better performance. This is the first study to demonstrate that specific beta corticocortical networks functionally support tactile spatial acuity as well as the local inhibitory network.

Association between somatosensory sensitivity and regional gray matter volume in healthy young volunteers: a voxel-based morphometry study

Two-point discrimination (2PD) test reflects somatosensory spatial discrimination ability, but evidence on the relationship between 2PD and cortical gray matter (GM) volume is limited. This study aimed to analyze the relationship between cortical GM volume and 2PD threshold in young healthy individuals and to clarify the characteristics of brain structure reflecting the individual differences in somatosensory function. 2PD was measured in 42 healthy (20 females) volunteers aged 20-32 years using a custom-made test system that can be controlled by a personal computer. The 2PD of the right index finger measured with this device has been confirmed to show good reproducibility. T1-weighted images were acquired using a 3-T magnetic resonance imaging scanner for voxel-based morphometry analysis. The mean 2PD threshold was 2.58 ± 0.54 mm. Whole-brain multiple regression analysis of the relationship between 2PD and GM volume showed that a lower 2PD threshold (i.e. better somatosensory function) significantly correlated with decreased GM volume from the middle temporal gyrus to the inferior parietal lobule (IPL) in the contralateral hemisphere. In conclusion, a lower GM volume in the middle temporal gyrus and IPL correlates with better somatosensory function. Thus, cortical GM volume may be a biomarker of somatosensory function.

Therapeutic benefits of noninvasive somatosensory cortex stimulation on cortical plasticity and somatosensory function: a systematic review

Optimal limb coordination requires efficient transmission of somatosensory information to the sensorimotor cortex. The primary somatosensory cortex (S1) is frequently damaged by stroke, resulting in both somatosensory and motor impairments. Noninvasive brain stimulation (NIBS) to the primary motor cortex is thought to induce neural plasticity that facilitates neurorehabilitation. Several studies have also examined if NIBS to the S1 can enhance somatosensory processing as assessed by somatosensory-evoked potentials (SEPs) and improve behavioral task performance, but it remains uncertain if NIBS can reliably modulate S1 plasticity or even whether SEPs can reflect this plasticity. This systematic review revealed that NIBS has relatively minor effects on SEPs or somatosensory task performance, but larger early SEP changes after NIBS can still predict improved performance. Similarly, decreased paired-pulse inhibition in S1 post-NIBS is associated with improved somatosensory performance. However, several studies still debate the role of inhibitory function in somatosensory performance after NIBS in terms of the direction of the change (that, disinhibition or inhibition). Altogether, early SEP and paired-pulse inhibition (particularly inter-stimulus intervals of 30-100 ms) may become useful biomarkers for somatosensory deficits, but improved NIBS protocols are required for therapeutic applications.