The effects of galvanic vestibular stimulation (GVS) on ulnar nerve-induced facilitation of motor-evoked potentials in a single subject.

<p>Full-wave rectified and averaged electromyograms (EMGs) in the biceps brachii (BB) muscle after separate ulnar nerve stimulation (NERVE) at 1.0 × the motor threshold of the first dorsal interosseous muscle (<b>A</b>, <b>E</b>, <b>I</b>), separate transcranial magnetic stimulation (TMS) over the contralateral primary motor cortex at 1.1 × the active motor threshold of the BB (<b>B</b>, <b>F</b>, <b>J</b>), and the combination (COMB) of NERVE and TMS (<b>C</b>, <b>G</b>, <b>K</b>). These waveforms were obtained without GVS (left panels), during anodal GVS (middle panels), and during cathodal GVS (right panels) at 2.0 × the perceptual threshold of the head sway. The grey waveforms in <b>C</b>, <b>G</b>, and <b>K</b> represent the summation (SUM) of the averaged EMG waveforms after separate TMS and NERVE. The waveforms in <b>D</b>, <b>H</b>, and <b>L</b> represent the COMB waveforms with the SUM waveforms subtracted.</p

Schematic of the methodology and potential premotoneuronal pathways in the cervical cord.

<p>The wiring pattern is oversimplified for better understanding. Black and red solid lines represent direct (monosynaptic) connections, whereas purple and green dashed lines represent indirect (non-monosynaptic) connections no matter whether its effect is facilitatory or inhibitory. The pyramidal tract volleys (a red arrow) that are produced by transcranial magnetic stimulation (TMS) over the contralateral motor cortex and the afferent volleys (a green arrow) that are produced by electrical stimulation of the ipsilateral ulnar nerve (NERVE) at the wrist converge onto a common cervical interneuron (IN) that projects to motoneurons (MNs) of the biceps brachii (BB) muscle, which results in extra facilitation of motor-evoked potentials (MEPs) in the BB. The TMS and NERVE are timed so that the pyramidal tract and afferent volleys simultaneously arrive at the upper cervical cord. The inputs produced by galvanic vestibular stimulation (GVS) through the bilateral mastoid processes also converge on the IN pool. FDI: first dorsal interosseous muscle, EMG: electromyogram.</p

The effects of galvanic vestibular stimulation (GVS) on the firing probability of single motor units (MUs) after combined motor cortex and ulnar nerve stimulation.

<p><b>A</b>–<b>H</b> show the peristimulus time histograms (PSTHs) of a single MU in the biceps brachii (BB) muscle after separate ulnar nerve stimulation (NERVE) at 1.0 × motor threshold (MT) of the first dorsal interosseous muscle (<b>A</b>, <b>E</b>), separate transcranial magnetic stimulation (TMS) (<b>B</b>, <b>F</b>) over the contralateral primary motor cortex at 1.25 × active motor threshold of the BB, and combined stimulation (COMB) (<b>C</b>, <b>G</b>) in the control (<b>A</b>–<b>D</b>) and anodal GVS conditions (<b>E</b>–<b>H</b>). Each PSTH was obtained after 50 stimuli. The counts in these PSTHs were subtracted by the mean counts during a 50 ms prestimulus period. <b>D</b> and <b>H</b> show differential PSTHs after subtraction of the summed PSTHs after separate stimuli from the PSTHs of the COMB. The number of counts in each bin was normalized by the number of triggers. The vertical dashed line represents the onset of the excitatory peak in the PSTH after separate TMS. The superimposed waveforms in the upper right corner of each PSTH show the MU action potentials (n = 50) obtained from each stimulus trial. <b>I</b>–<b>L</b> indicate the peak counts of the MU firings in the differential PSTHs in the control and anodal GVS conditions obtained from 31 MUs that were investigated with the NERVE set at 1.0 × MT (<b>I</b>, <b>J</b>) and 20 MUs that were investigated with the NERVE set at 0.75 × MT (<b>K</b>, <b>L</b>). The error bars represent 1 standard deviation. The analysis window was set at a predefined period (1.0 ms duration) that started 1.0 ms after the onset of the TMS-induced excitatory peak in the PSTH. **<i>p</i> < 0.01.</p

The pooled data for the effects of galvanic vestibular stimulation (GVS) on the ulnar nerve-induced facilitation of motor-evoked potentials (MEPs) across subjects.

<p>The pooled data for the mean areas of the MEPs in the biceps brachii muscle after separate ulnar nerve stimulation (NERVE), separate transcranial magnetic stimulation (TMS) over the contralateral primary motor cortex, and the combination (COMB) of NERVE and TMS in the control (<b>A</b>, <b>E</b>), anodal GVS (<b>B</b>, <b>F</b>), and cathodal GVS conditions (<b>C</b>, <b>G</b>). <b>D</b> and <b>H</b> show the average of the amount of spatial facilitation in the control and GVS conditions. The areas of the MEPs after the combined stimuli were normalized by the summation of the areas of the MEPs recorded after separate TMS and NERVE. <b>A</b>–<b>D</b> and <b>E</b>–<b>H</b> illustrate the data from experiments involving ulnar nerve stimulation at 1.0 and 0.75 × the motor threshold (MT) of the first dorsal interosseous muscle, respectively. The error bars represent standard errors of the mean. The asterisks indicate a statistically significant difference between conditions (*<i>p</i> < 0.05, **<i>p</i> < 0.01).</p

Experimental tasks in the present study.

<p><b>A</b>–<b>D</b> show the number of subjects, number of test sequences, and stimulus conditions in each experiment. Each block indicates the test sequence used to assess the spatial facilitation effects. The stimulus intensities of the transcranial magnetic stimulation (TMS), ulnar nerve stimulation (NERVE), and galvanic vestibular stimulation (GVS) are indicated within the block. The order of the test sequences was randomly selected. MEP: motor-evoked potential, MT: motor threshold of the first dorsal interosseous muscle, PT: perceptual threshold of GVS-induced head sway, MU: motor unit, AMT: active motor threshold of the biceps brachii muscle.</p

The Bimanual Target-Chasing Task and Flexible Role Assignment of the Hands

<div><p>(A) Mappings between applied forces and cursor movements (top graph). With the left-hand map, the cursor moved horizontally in the direction of the longitudinal force applied by the left hand (solid purple arrows). A counter-clockwise twist force applied between the handles (as if unscrewing the lid of a jar) moved the cursor upward (solid green arrows). With the right-hand map the cursor moved in the opposite directions and thus in the direction of the forces of the right hand.</p> <p>(B and C) Performance under each mapping rule shown for a complete first session with 602 hits and from the last 100 hits of a second session. Superimposed thin lines show hit time and path index for each participant as a function of target number (data median-filtered over a ± 10-s period around each hit). Solid curve give medians across participants. Inserts in (C) exemplify cursor trajectories with a median path index of 4.6 and 1.4 across 10 target transitions. The targets, distributed about uniformly over the screen, were located 5.1 ± 2.1° (mean ± 1 SD) visual angle from its center, which corresponded to 2.2 ± 0.3 N force applied tangentially to the surfaces of the handles.</p> <p>(D and E) Hand-asymmetry indices computed for a sliding ± 10-s time window. Horizontal lines give the upper and lower 95% confidence limit of the index, postulating that hand selection would have occurred randomly. A significant positive and negative index indicates left and right-hand primarily acting, respectively.</p></div

Influence of Mapping Rule on Tool Movements during Performance with Left-Hand and Right-Hand Maps

<p>Superimposed time traces of longitudinal force and lateral tool movement (upper panels) and of twist force and rotational tool movement (lower panels) from a single participant during the last 20 s of target chasing in the first experiment; <i>r_LO</i> and <i>r_TW</i> indicate hand-asymmetry indices for longitudinal and twist forces. Bottom trace represents instances of target hits (spikes). For the last 20 s of runs by all participants and mapping rules, the slope coefficients of the linear regressions indicated that the tool moved 0.84 (0.28–1.23) mm/N longitudinal force and rotated 0.84 (0.36–1.42) °/N twist force (median and 25th–75th percentile). The corresponding values for the 30-s periods of target chasing for which fMRI data were analyzed (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040158#pbio-0040158-g005" target="_blank">Figure 5</a>A and <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040158#pbio-0040158-g005" target="_blank">5</a>B), where the wrists of the participants were strapped, were 0.28 (0.19–0.68) mm/N and 0.63 (0.42–1.04) °/N. </p

Premotor Cortical Areas with Increased BOLD Responses with Both the Left- and the Right-Hand Map Overlaid on Coronal Slices of the MNI T1-Weighted Brain Template

<p>For the left hemisphere (L), significant activations ( <i>p</i> < 0.01, FWE-corrected) occurred in one cluster (443 voxels) with two maxima in precentral gyrus (#2 and #4; BA 6), in one single-peak cluster (153 voxels) in superior frontal gyrus (#3; BA 6), in one cluster (281 voxels) with two maxima in medial frontal gyrus (#5 and #6; BA 6), and in one small single-peak cluster (29 voxels) in left inferior frontal gyrus (#1; BA 44). In the right hemisphere (R), there was one cluster (303 voxels) with three maxima, one of which was located in the precentral gyrus outside the cluster delineated by left-hand-map > right-hand-map contrast (#7; BA 6). Solid black lines in the left and right hemisphere outline the clusters identified with the right-hand-map > left-hand-map and left-hand-map > right-hand-map contrasts, respectively (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040158#pbio-0040158-g004" target="_blank">Figure 4</a>C). Histograms give percent BOLD signal change relative to mean of session for the local maxima of identified clusters. Red and blue columns refer to left- and right-hand maps, respectively. Column height gives data averaged across participants and error bar ± 1SEM ( <i>n</i> = 16). Coordinates (X, Y, Z in MNI stereotaxic space) and <i>t</i>₍₃₀₎ values for the maxima are presented below each histogram. </p