Stimulation Of Posterior Parietal Cortex Research Articles

Transcranial Direct Current Stimulation over the Posterior Parietal Cortex Increases Nontarget Retrieval during Visual Working Memory.

Visual working memory (VWM) requires precise feature binding. Previous studies have revealed a close relationship between the posterior parietal cortex (PPC) and feature binding during VWM; this study further examined their causal relationship through three transcranial direct current stimulation (tDCS) experiments. In Experiment 1 (N = 57), participants underwent three sessions of tDCS separately, including PPC stimulation, occipital cortex stimulation, and sham stimulation, and completed delayed estimation tasks for orientations before and after stimulation. Results showed that tDCS over PPC selectively prolonged recall response time (RT) and increased the probability of nontarget responses (a.k.a. failure of feature binding, pNT). In Experiment 2 (N = 29), combining metacognition estimation, we further investigated whether the effects of PPC stimulation were attributed to misbinding (i.e., participants self-reported "remembered" in nontarget responses) or informed guessing trials (participants self-reported "forgotten" in nontarget responses). We replicated the main findings in Experiment 1 and observed greater tDCS effects of PPC on RT in informed guessing trials while there are comparable effects on pNT in these two types of trials. In Experiment 3 (N = 28), we then examined whether the tDCS effects over PPC specifically influenced the memory retrieval process by using a change detection task. We found that PPC stimulation did not influence the recognition RT or accuracy. Together, this study provided direct causal evidence supporting the specific involvement of PPC in feature binding during VWM retrieval, from both aspects of speed and response preference, expanding our understanding of the neural basis of feature binding in VWM.

Interaction between BDNF Val66Met polymorphism and mismatch negativity for working memory capacity in schizophrenia

Both the brain-derived neurotrophic factor (BDNF) valine (Val)/methionine (Met) polymorphism and mismatch negativity (MMN) amplitude are reportedly linked to working memory impairments in schizophrenia. However, there is evident scarcity of research aimed at exploring the relationships among the three factors. In this secondary analysis of a randomized, controlled, double-blind trial, we investigated these relationships. The trial assessed the efficacy of transcranial direct current stimulation for enhancing working memory in clinically stable schizophrenia patients, who were randomly divided into three groups: dorsolateral prefrontal cortex stimulation, posterior parietal cortex stimulation, and sham stimulation groups. Transcranial direct current stimulation was administered concurrently with a working memory task over five days. We assessed the BDNF genotype, MMN amplitude, working memory capacity, and interference control subdomains. These assessments were conducted at baseline with 54 patients and followed up post-intervention with 48 patients. Compared to BDNF Met-carriers, Val homozygotes exhibited fewer positive and general symptoms and increased working memory capacity at baseline. A correlation between MMN amplitude and working memory capacity was noted only in BDNF Val homozygotes. The correlations were significantly different in the two BDNF genotype groups. Furthermore, in the intervention group that showed significant improvement in MMN amplitude, BDNF Val homozygotes exhibited greater enhancement in working memory capacity than Met-carriers. This study provides in vivo evidence for the interaction between MMN and BDNF Val/Met polymorphism for working memory capacity. As MMN has been considered a biomarker of N-methyl-D-aspartate receptor (NMDAR) function, these data shed light on the complex interactions between BDNF and NMDAR in terms of working memory in schizophrenia.

Stimulating the posterior parietal cortex reduces self-reported risk-taking propensity in people with tobacco use disorder

Previous research has identified a replicable role for the posterior parietal cortex (PPC) in risk behaviors, but it is unclear whether this relationship is causal. Here, we used a targeted neuromodulation protocol leveraging a single-session of 10-Hz rTMS to the PPC versus a control region in the visual cortex (V5), as well as two active comparison regions [superior frontal gyrus (SFG), dorsolateral prefrontal cortex (DLPFC)] (within-person, randomized order), to examine within-session changes in a comprehensive measure of self-reported risk-taking propensity (the Domain-Specific Risk-Taking or DOSPERT scale). Individuals with tobacco use disorder were selected as sample participants who present with clinically relevant risk-taking propensity (N = 50; 14 women, mean [SE] age=33.1 [1.04] years). Results indicated that stimulating the PPC (versus V5) resulted in trend-level reductions in self-reported risk-taking propensity (region-by-time interaction, P = 0.065). A similar pattern emerged comparing PPC stimulation to effects of stimulating either the DLPFC (P = 0.080) or SFG (P = 0.032). PPC-related reductions in risk-taking propensity were domain general and potentially driven by changes in risk perception rather than perceptions of expected benefits of risk-taking. These findings support a possible causal role of the PPC in risk behaviors that warrants further consideration for therapeutic indications in conditions like tobacco use disorder.

TDCS over PPC or DLPFC does not improve visual working memory capacity

Non-invasive brain stimulation has been highlighted as a possible intervention to induce cognitive benefits, including on visual working memory (VWM). However, findings are inconsistent, possibly due to methodological issues. A recent high-profile study by Wang et al.1 reported that anodal transcranial direct current stimulation (tDCS) over posterior parietal cortex (PPC), but not over dorsolateral prefrontal cortex (DLPFC), selectively improved VWM capacity but not precision, especially at a high VWM load. Thus, in the current pre-registered conceptual replication study, we accounted for the key potential methodological issues in the original study and tested an adequate number of participants required to demonstrate the previously reported effects (n = 48 compared to n = 20). Participants underwent counterbalanced PPC, DLPFC and sham stimulation before completing 360 trials of a continuous orientation-reproduction task with a slight variation of task stimuli and setup. We found no evidence for the selective effect of PPC stimulation. Instead, our results showed that tDCS effects were absent regardless of stimulation region and VWM load, which was largely supported by substantial to strong Bayesian evidence. Therefore, our results challenge previously reported benefits of single-session anodal PPC-tDCS on VWM.

Disentangling Cerebellar and Parietal Contributions to Gait and Body Schema: A Repetitive Transcranial Magnetic Stimulation Study

The overlap between motor and cognitive signs resulting from posterior parietal cortex (PPC) and cerebellar lesions can mask their relative contribution in the sensorimotor integration process. This study aimed to identify distinguishing motor and cognitive features to disentangle PPC and cerebellar involvement in two sensorimotor-related functions: gait and body schema representation. Thirty healthy volunteers were enrolled and randomly assigned to PPC or cerebellar stimulation. Sham stimulation and 1 Hz-repetitive-Transcranial-Magnetic-Stimulation were delivered over P3 or cerebellum before a balance and a walking distance estimation task. Each trial was repeated with eyes open (EO) and closed (EC). Eight inertial measurement units recorded spatiotemporal and kinematic variables of gait. Instability increased in both groups after real stimulation: PPC inhibition resulted in increased instability in EC conditions, as evidenced by increased ellipse area and range of movement in medio-lateral and anterior–posterior (ROMap) directions. Cerebellar inhibition affected both EC (increased ROMap) and EO stability (greater displacement of the center of mass). Inhibitory stimulation (EC vs. EO) affected also gait spatiotemporal variability, with a high variability of ankle and knee angles plus different patterns in the two groups (cerebellar vs parietal). Lastly, PPC group overestimates distances after real stimulation (EC condition) compared to the cerebellar group. Stability, gait variability, and distance estimation parameters may be useful clinical parameters to disentangle cerebellar and PPC sensorimotor integration deficits. Clinical differential diagnosis efficiency can benefit from this methodological approach.

Effects of transcranial electrical stimulation of the right posterior parietal cortex on physical control responses

The posterior parietal cortex plays an important role in postural stability by adapting to changes in input from the visual, vestibular, and proprioceptive systems. However, little is known regarding whether transcranial electrical stimulation of the posterior parietal cortex affects reactive postural responses. This study aimed to investigate changes in physical control responses to anodal and cathodal transcranial direct current stimulation and transcranial random noise stimulation of the right posterior parietal cortex using a simultaneous inertial measurement unit. The joint movements of the lower limb of 33 healthy volunteers were measured while standing on a soft-foam surface with eyes closed during various stimulation modalities. These modalities included anodal, cathodal transcranial direct current stimulation, and sham stimulation in Experiment 1, and transcranial random noise and sham stimulations in Experiment 2. The results showed that cathodal stimulation significantly decreased the joint angular velocity in the hip rotation, ankle inversion-eversion, and abduction–adduction directions compared to anodal or sham stimulation in Experiment 1. In contrast, there were no significant differences in physical control responses with transcranial random noise stimulation coeducation in Experiment 2. These findings suggest that transcranial electrical stimulation of the right posterior parietal cortex may modulate physical control responses; however, the effect depends on the stimulus modality.

Standard Non-Personalized Electric Field Modeling of Twenty Typical tDCS Electrode Configurations via the Computational Finite Element Method: Contributions and Limitations of Two Different Approaches.

Simple SummaryThe magnitude, distribution, and characteristics of the electric field induced in the brain by application of transcranial electric stimulation depend on the electrode configuration used and the specific parameters of stimulation. An approach to calculate the generated electric field is the computational finite element method. This method enables simulation of the current spread and electric field strength according to the electrode configuration and stimulation parameters used. However, current approaches have several limitations, which constrain the application of tDCS in empirical research and clinical practice. In this study, we provide examples of standard model-based electric field simulations corresponding to motor, dorsolateral prefrontal, and posterior parietal cortex stimulation using twenty typical electrode configurations. Two different current flow-modeling tools were used to compare the results and determine possible differences between both procedures regarding the specificity, as well as the reliability of the estimates. The results were rather consistent between both simulations. Some modest differences of the simulated distribution and intensity of the electric fields between the results of the respective modeling approaches were identified, which might have functional significance and reveal the need to empirically validate these models. The non-availability of quantitative data about the precise electric field distribution beyond the cortical target is a common limitation of both methods, which limits the determination of the spatial specificity of the intervention. These findings help to define future directions of research that allow to exploit the full potential of standard simulation approaches in the field of non-invasive brain stimulation.Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation procedure to modulate cortical excitability and related brain functions. tDCS can effectively alter multiple brain functions in healthy humans and is suggested as a therapeutic tool in several neurological and psychiatric diseases. However, variability of results is an important limitation of this method. This variability may be due to multiple factors, including age, head and brain anatomy (including skull, skin, CSF and meninges), cognitive reserve and baseline performance level, specific task demands, as well as comorbidities in clinical settings. Different electrode montages are a further source of variability between tDCS studies. A procedure to estimate the electric field generated by specific tDCS electrode configurations, which can be helpful to adapt stimulation protocols, is the computational finite element method. This approach is useful to provide a priori modeling of the current spread and electric field intensity that will be generated according to the implemented electrode montage. Here, we present standard, non-personalized model-based electric field simulations for motor, dorsolateral prefrontal, and posterior parietal cortex stimulation according to twenty typical tDCS electrode configurations using two different current flow modeling software packages. The resulting simulated maximum intensity of the electric field, focality, and current spread were similar, but not identical, between models. The advantages and limitations of both mathematical simulations of the electric field are presented and discussed systematically, including aspects that, at present, prevent more widespread application of respective simulation approaches in the field of non-invasive brain stimulation.

Non-invasive Brain Stimulation of the Posterior Parietal Cortex Alters Postural Adaptation.

Effective central sensory integration of visual, vestibular, and proprioceptive information is required to promote adaptability in response to changes in the environment during postural control. Patients with a lesion in the posterior parietal cortex (PPC) have an impaired ability to form an internal representation of body position, an important factor for postural control and adaptation. Suppression of PPC excitability has also been shown to decrease postural stability in some contexts. As of yet, it is unknown whether stimulation of the PPC may influence postural adaptation. This investigation aimed to identify whether transcranial direct current stimulation (tDCS) of the bilateral PPC could modulate postural adaptation in response to a bipedal incline postural adaptation task. Using young, healthy subjects, we delivered tDCS over bilateral PPC followed by bouts of inclined stance (incline-interventions). Analysis of postural after-effects identified differences between stimulation conditions for maximum lean after-effect (LAE; p = 0.005) as well as a significant interaction between condition and measurement period for the average position (p = 0.03). We identified impaired postural adaptability following both active stimulation conditions. Results reinforce the notion that the PPC is involved in motor adaptation and extend this line of research to the realm of standing posture. The results further highlight the role of the bilateral PPC in utilizing sensory feedback to update one’s internal representation of verticality and demonstrates the diffuse regions of the brain that are involved in postural control and adaptation. This information improves our understanding of the role of the cortex in postural control, highlighting the potential for the PPC as a target for sensorimotor rehabilitation.

Effects of transcranial direct current stimulation on cortex modulation by stimulation of the primary motor cortex and parietal cortex in humans

Aim of the study Transcranial magnetic stimulation (TMS) is used to measure corticospinal excitability (CSE) from the primary motor cortex (M1) in humans through motor-evoked potentials (MEPs). The variability of CSE responses to transcranial direct current stimulation (tDCS) protocols is high and needs to be reproduced in the healthy population. The M1 and posterior parietal cortex (PPC) are anatomically and functionally connected and could play a role in understanding the variability in CSE responses. We tested the individual MEPs following a common cathodal (ctDCS) protocol over the M1 and PPC. Materials and methods Twenty-eight healthy subjects were randomized for a ctDCS stimulation over the left M1 and PPC for 20 min on a separate days. The first dorsal interosseous muscle (FDI) contralateral stimulation of the left M1 was used as the resting motor threshold (RMT), while 15 single pulses 4–8 s apart at an intensity of 120% RMT were used to determine the baseline MEP amplitude and at T0, 5, 10, 20, 30, 40, 50, and 60 min after ctDCS stimulation in both sessions. Results A 20 min duration of ctDCS stimulation significantly deceased the CSE only at T0 (p = 0.046 at M1, p = 0.010 at PPC). Conclusion Our results suggested that PPC stimulation can modulate M1 excitability and PPC–M1 connectivity, but a significant effect is only observed immediately post ctDCS. The tDCS showed variability in response to the tDCS protocol is consistent with other non-invasive brain stimulation studies.

Anodal High-definition Transcranial Direct Current Stimulation over the Posterior Parietal Cortex Modulates Approximate Mental Arithmetic.

The representation and processing of numerosity is a crucial cognitive capacity. Converging evidence points to the posterior parietal cortex (PPC) as primary "number" region. However, the exact role of the left and right PPC for different types of numerical and arithmetic tasks remains controversial. In this study, we used high-definition transcranial direct current stimulation (HD-tDCS) to further investigate the causal involvement of the PPC during approximative, nonsymbolic mental arithmetic. Eighteen healthy participants received three sessions of anodal HD-tDCS at 1-week intervals in counterbalanced order: left PPC, right PPC, and sham stimulation. Results showed an improved performance during online parietal HD-tDCS (vs. sham) for subtraction problems. Specifically, the general tendency to underestimate the results of subtraction problems (i.e., the "operational momentum effect") was reduced during online parietal HD-tDCS. There was no difference between left and right stimulation. This study thus provides new evidence for a causal involvement of the left and right PPC for approximate nonsymbolic arithmetic and advances the promising use of noninvasive brain stimulation in increasing cognitive functions.

Effects of tDCS in modulating the after-effect of prismatic lenses training in stroke patients with neglect

Neglect treatment with prismatic lenses (PL) has been demonstrated to be effective in improving hemispatial neglect. This study aimed to compare the effect of cathodal transcranial direct current stimulation (c-tDCS), anodal tDCS (a-tDCS) or sham stimulation in modulating the PL effect in stroke patients. Fifteen subacute stroke patients with pathological performance on the Behavioral Inattention Test (BIT) battery, indicative of neglect, were included in this study. Patients underwent to a-tDCS over the right posterior parietal cortex (PPC), c-tDCS over the left PPC and sham stimulation during three PL training sessions (with PL of 10? rightward shift), in random sequence and separated by at least 1 day. Thirty “invisible” pointing (visual target not visible) were performed before and after each session to evaluate the after-effect of the PL training. Leftward shift after PL training was significantly reduced by a-tDCS over the ipsilasional PPC in comparison with c-tDCS over the contralesional PPC (p = 0.019) and sham stimulation (p = 0.047). No difference in the after-effect was obtained comparing the c-tDCS and sham stimulation. Error correction during the PL training do not significantly differed among the three sessions. A-tDCS over the ipsilesional PPC reduced the leftward deviation possibly interfering with the network involved in visuo-motor adaptation to PL training.

Cathodal tDCS of the Left Posterior Parietal Cortex Increases Proprioceptive Drift

ABSTRACTIn aiming movements the limb position drifts away from the defined target after some trials without visual feedback, a phenomenon defined as proprioceptive drift (PD). There are no studies investigating the association between the posterior parietal cortex (PPC) and PD in aiming movements. Therefore, cathodal and sham transcranial direct current stimulation (tDCS) were applied to the left PPC concomitantly with the performance of movements with or without vision. Cathodal tDCS applied without vision produced a higher level of PD and higher rates of drift accumulation while it decreased peak velocity and maintained the number of error corrections, not affecting movement amplitude. The proprioceptive information seems to produce an effective reference to movement, but with PPC stimulation it causes a negative impact on position.

Cathodal electrical stimulation of frontoparietal cortex disrupts statistical learning of visual configural information

Attentional performance is facilitated by exploiting regularities and redundancies in the environment by way of incidental statistical learning. For example, during visual search, response times to a target are reduced by repeating distractor configurations–a phenomenon known as contextual cueing (Chun & Jiang, 1998). A range of neuroscientific methods have provided evidence that incidental statistical learning relies on subcortical neural structures associated with long-term memory, such as the hippocampus. Functional neuroimaging studies have also implicated the prefrontal cortex (PFC) and posterior parietal cortex (PPC) in contextual cueing. However, the extent to which these cortical regions are causally involved in statistical learning remains unclear. Here, we delivered anodal, cathodal, or sham transcranial direct current stimulation (tDCS) to the left PFC and left PPC online while participants performed a contextual cueing task. Cathodal stimulation of both PFC and PPC disrupted the early cuing effect, relative to sham and anodal stimulation. These findings causally implicate frontoparietal regions in incidental statistical learning that acts on visual configural information. We speculate that contextual cueing may rely on the availability of cognitive control resources in frontal and parietal regions.

P296 Spike timing dependent plasticity in the human parietal-prefrontal network

Question Learning may occur through associative changes in the synaptic strength of neural connections that can be induced by coupling of activity of interconnected neurons with precise timing (spike-timing dependent plasticity, STDP). It is debated if this model can be applied in the context of large-scale cortical networks in humans. Methods We combined transcranial magnetic stimulation with concurrent electroencephalography to directly investigate if precise repeated activation of pre and post-synaptic large-scale cortical inputs produces STDP effects within the parietal-prefrontal network of 15 healthy volunteers. Results We found direct evidence for bidirectional STDP after effects over the DLPFC ruled by the PPC pre-synaptic inputs. Specifically, when the pre-synaptic input (PPC) precedes repeatedly the post-synaptic activation (DLPFC) by 10 ms a LTD-like effect of TMS-evoked activity is induced within the DLPFC. When the temporal order of the two stimulated areas is reverted, i.e. the pre-synaptic input (PPC) follows the post-synpatic input (DLPFC), we observed an LTP-like effect. Interestingly, such changes were accompained by PAS-depedendent modulations of the natural frequency of oscillation of the DLPFC, i.e. gamma activity. Conclusions We provide novel evidence that paired associative stimulation of posterior parietal cortex (PPC) and dorsolateral prefrontal cortex (DLPFC) induces bidirectional STDP effects in the post synaptic area (DLPFC) both in the time and in the time/frequency domain. Download : Download high-res image (811KB) Download : Download full-size image Download : Download high-res image (407KB) Download : Download full-size image

P071 tDCS effects over the left DLPFC versus the right PPC in multiple sclerosis fatigue

Background Fatigue is a frequent and debilitating symptom in patients with multiple sclerosis (MS). Its classical treatments are still faced with limited benefits and numerous side effects. Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique currently investigated in treating MS related symptoms. Objective The primary objective of this study was to evaluate the tDCS effects on MS fatigue. The secondary outcomes included the assessment of its effects on mood and attention symptoms, commonly encountered in MS. Methods 10 fatigued MS patients were enrolled in a double-blind, sham-controlled, and cross-over study. Each patient randomly received three anodal tDCS blocks: active stimulation over the left dorsolateral prefrontal cortex (DLPFC), active stimulation of the right posterior parietal cortex (PPC), and sham stimulation over either cortical site. To note, both cortical targets are key components in MS fatigue networks. tDCS blocks consisted of five consecutive daily sessions and were held apart by a washout interval of three weeks. Results Only active left DLPFC stimulation significantly ameliorated fatigue. Mood improvement was exclusively obtained following active right PPC stimulation. Neither intervention had effects on attention. Conclusion This small-scale study supports the role of left prefrontal anodal tDCS in treating MS fatigue. The lack of effects on attention might be related to the heterogeneity of the studied cohort, the relatively small sample size, the adapted stimulation parameters and the short protocol duration.

P147 Effects of cTBS on the integration of haptic light fingertip contact and body sway

Question In the present study we aimed to investigate the effects of continuous theta burst stimulation (cTBS) over the posterior parietal cortex (PPC) and how it effects body sway and the integration of light touch. Methods In right-handed young adults, cTBS with an intensity of 80% of the passive motor threshold was applied for 60 s over the left and right Posterior Partial Cortex. Additionally, a sham condition was applied. Target locations were identified using real-time neuronavigation. Participants were tested blindfolded in quiet upright Tandem-Romberg stance before and after each stimulation interval. During these tests they were instructed to actively initiate and cease finger contact with an earth-fixed referent in response to an acoustic signal. Testing was carried out on two non-consecutive sessions. Body sway was recorded in terms of Centre of Pressure (CoP) and trunk kinematics in addition to forces and torques at the contact point. Results The results of 13 healthy right handed participants reveal that cTBS over the right PPC affected postural control. The overall level of body sway was decreased after inhibition, while the effect of Light Touch was still present. Inhibition of the left PPC or sham stimulation did not decrease the overall level of body sway. Conclusion The cause of this effect might be strategies of the motor control system that lead to a co-contraction and thus reducing body sway. However, this does not mean that participants were more stable. Another possible explanation might be that inhibition of the right PPC disrupts processes for state estimation of body sway in spatial reference frames or mechanisms of stability self-exploration (i.e. Zatsiorsky and Duarte, Motor Control, 1999; Ehrenfried et al., Brain Res Cogn Brain Res, 2003; Kiemel et al., J Neurophysiol, 2006).

Effects of left DLPFC versus right PPC tDCS on multiple sclerosis fatigue

Background and objectiveFatigue is a frequent and debilitating symptom in patients with multiple sclerosis (MS). Its classical treatments are still faced with limited benefits and numerous side effects. Hence, we aimed to evaluate the effects of transcranial direct current stimulation (tDCS), a noninvasive brain stimulation technique, on such a challenging symptom. Our secondary outcomes included the assessment of tDCS impact on mood and attentional performance. MethodsTen fatigued MS patients were enrolled in a double-blind, sham-controlled, and cross-over study. Each patient randomly received three anodal tDCS blocks: active stimulation over the left dorsolateral prefrontal cortex (DLPFC), active stimulation over the right posterior parietal cortex (PPC), and sham stimulation over either cortical site. Both cortical targets are key components in the MS fatigue networks. The blocks consisted of five consecutive daily sessions and were held apart by a washout interval of three weeks. ResultsOnly active left DLPFC stimulation significantly ameliorated fatigue. Mood improvement was exclusively obtained following active right PPC stimulation. Neither intervention had effects on attention. ConclusionOur study supports the role of anodal tDCS over the left prefrontal in treating MS fatigue. The lack of tDCS effects on attention might be related to the heterogeneity of the studied cohort, the relatively small sample size, the protocol design and duration. Modifying these variables and coupling tDCS with neuroimaging might improve the clinical outcomes and enhance our understanding of the tDCS mechanism of actions.

Stimulation of PPC Affects the Mapping between Motion and Force Signals for Stiffness Perception But Not Motion Control.

When selecting an object such as a ripe fruit or sofa, we need to assess the object's stiffness. Because we lack dedicated stiffness sensors, we rely on an as yet unknown mechanism that generates stiffness percepts by combining position and force signals. Here, we found that the posterior parietal cortex (PPC) contributes to combining position and force signals for stiffness estimation. This finding challenges the classical view about the role of the PPC in regulating position signals only for motion control because we highlight a key role of the PPC in perception that is disassociated from action. Altogether this sheds light on brain mechanisms underlying the interaction between action and perception and may help in the development of better teleoperation systems and rehabilitation of patients with sensory impairments.

Cathodal transcranial direct current stimulation of the posterior parietal cortex reduces steady-state postural stability during the effect of light touch.

Touching a stable object with a fingertip using slight force (<1 N) stabilizes standing posture independent of mechanical support, which is referred to as the effect of light touch (LT). In the neural mechanism of the effect of LT, the specific contribution of the cortical brain activity toward the effect of LT remains undefined, particularly the contribution toward steady-state postural sway. The aim of the present study was to investigate the cortical region responsible for the reduction of postural sway in response to the effect of LT. Active LT was applied with the right fingertip and transcranial direct current stimulation (sham or cathodal) was applied to the left primary sensorimotor cortex or the left posterior parietal cortex in the two groups. The experiments were conducted using a single-blind sham-controlled crossover design. Steady-state postural sway was compared with the factors of transcranial direct current stimulation (sham or cathodal) and time (pre or post). In the results, the effect of LT reduced postural stability in the mediolateral direction after cathodal transcranial direct current stimulation of the left posterior parietal cortex. No effect was observed after stimulation of the left primary sensorimotor cortex. This indicates that the left posterior parietal cortex is partly responsible for the effect of LT when touching a fixed point with the right fingertip during suprapostural tasks, where posture is adjusted according to the precision requirements. Cortical processing of sensory integration for voluntary postural orientation in response to touch occurs in the posterior parietal cortex.

Variation in left posterior parietal-motor cortex interhemispheric facilitation following right parietal continuous theta-burst stimulation in healthy adults

Spatial neglect is modeled on an imbalance of interhemispheric inhibition (IHI); however evidence is emerging that it may not explain neglect in all cases. The aim of this study was to investigate the IHI imbalance model of visual neglect in healthy adults, using paired pulse transcranial magnetic stimulation to probe excitability of projections from posterior parietal cortex (PPC) to contralateral primary motor cortex (M1) bilaterally. Motor-evoked potentials (MEPs) were recorded from the first dorsal interossei and facilitation was determined as ratio of conditioned to non-conditioned MEP amplitude. A laterality index reflecting the balance of excitability between the two hemispheres was calculated. A temporal order judgment task (TOJ) assessed visual attention. Continuous theta-burst stimulation was used to transiently suppress right parietal cortex activity and the effect on laterality and judgment task measured, along with associations between baseline and post stimulation measures. Stimulation had conflicting results on laterality, with most participants demonstrating an effect in the negative direction with no decrement in the TOJ task. Correlation analysis suggests a strong association between laterality direction and degree of facilitation of left PPC-to right M1 following stimulation (r=.902), with larger MEP facilitation at baseline demonstrating greater reduction (r=−.908). Findings indicate there was relative balance between the cortices at baseline but right PPC suppression did not evoke left PPC facilitation in most participants, contrary to the IHI imbalance model. Left M1 facilitation prior to stimulation may predict an individual’s response to continuous theta-burst stimulation of right PPC.