Weak but Critical Links between Primary Somatosensory Centers and Motor Cortex during Movement

Abstract

Motor performance is improved by stimulation of the agonist muscle during movement. However, related brain mechanisms remain unknown. In this work, we perform a functional magnetic resonance imaging (fMRI) study in 21 healthy subjects under three different conditions: (1) movement of right ankle alone; (2) movement and simultaneous stimulation of the agonist muscle; or (3) movement and simultaneous stimulation of a control area. We constructed weighted brain networks for each condition by using functional connectivity. Network features were analyzed using graph theoretical approaches. We found that: (1) the second condition evokes the strongest and most widespread brain activations (5147 vs. 4419 and 2320 activated voxels); and (2) this condition also induces a unique network layout and changes hubs and the modular structure of the brain motor network by activating the most “silent” links between primary somatosensory centers and the motor cortex, particularly weak links from the thalamus to the left primary motor cortex (M1). Significant statistical differences were found when the strength values of the right cerebellum (P < 0.001) or the left thalamus (P = 0.006) were compared among the three conditions. Over the years, studies reported a small number of projections from the thalamus to the motor cortex. This is the first work to present functions of these pathways. These findings reveal mechanisms for enhancing motor function with somatosensory stimulation, and suggest that network function cannot be thoroughly understood when weak ties are disregarded.

Effects of Transcranial Direct Current Stimulation on GABA and Glx in Children: A pilot study

“…This suggests lateralization of motor learning in the left dominant cortex as previously described by Schambra et al [63]. The impact of transcallosal inhibition is also seen in pediatric studies applying tDCS contralateral to stroke lesions in an effort to augment motor learning of the affected hemisphere [42,64]. According to pediatric models of anodal tDCS, the current appears to travel through the motor fibers of the corpus callosum into the contralateral hemisphere [42].…”

“…While many use 2 mA stimulation parameters, 1 mA is also commonly used [4,24,38,40,41]. Additionally, current modeling investigations report variations in electric fields between adults and adolescents associated with developmental differences in skull thickness and grey and white matter, suggesting 1mA is more appropriate for children [42].…”

“…The impact of transcallosal inhibition is also seen in pediatric studies applying tDCS contralateral to stroke lesions in an effort to augment motor learning of the affected hemisphere [42,64]. According to pediatric models of anodal tDCS, the current appears to travel through the motor fibers of the corpus callosum into the contralateral hemisphere [42]. However, the same mechanism is not expected to be true for HD-tDCS which has a more focal current.…”

Effects of incidental physical activity on morphosyntactic processing in aging

“…Although language is regarded as a crystallized cognitive process [6], important changes in sentence processing have been reported in older people, particularly in the comprehension of embedded syntactic structures [8,9] and in morphosyntactic processing [10,11]; this latter process involves agreement rules between lexical units (e.g., in the phrase "He runs," the pronoun "He" inherits the number agreement to the verb "run" and the suffix "s" is added). The features used to compute agreement are the marking of gender, number, person or case [12].…”

“…For instance, a study showed a more asymmetric and frontal topographic distribution of the P600 in older adults than in young participants when number agreement was manipulated [46], while only one study performed in aging adults reported the effect of high WM load in morphosyntactic processing using gender-agreement manipulation [10]. The authors described that healthy older adults showed a smaller LAN effect than young participants as well as smaller amplitudes in P600a and P600b effects in high WM load condition compared with those in low WM load condition, a finding that was not observed in young participants [10]. These findings were interpreted as a greater neural cost to compute morphosyntactic processing as well as to integrate the information associated with the previous sentence context and to generate a generalized sentence mapping.…”