Mar Cortes^a,b, Alejandra  Climent^a, Laura Dubreuil Vall^c, Giulio Ruffini^c  Douglas Labar^a, Dylan Edwards^a

a. Non invasive Brain Stimulation and  Human Motor Control Laboratory, Burke Medical Research Institute, Weill Medical  College of Cornell University, 785 Mamaroneck Avenue, 10605, White Plains, NY,  USA.

b. Universitat de Barcelona,  Barcelona, Spain.

c. Starlab Barcelona SL, Av.  Tibidabo 47 bis, 08035, Barcelona, Spain.

Corresponding author:

Mar Cortes

Non invasive Brain Stimulation and  Human Motor Control Laboratory, Burke Medical Research Institute, Weill Medical  College of Cornell University, 785 Mamaroneck Avenue, 10605, White Plains, NY,  USA.

[email protected]

Phone: +1 914 368 3181

Abstract:  (250)[MC1] 

Background: Existing strategies to  enhance motor function following Spinal Cord Injury (SCI) are suboptimal  leaving patients with considerable disability. Available evidence suggests that  transcranial direct current stimulation (tDCS) is a promising method to improve  motor dysfunction. How tDCS affects resting brain activity monitored by EEG is  little explored.

 Objective:

Methods: We conducted a  randomized, single blind, sham-controlled, cross-over study in seven chronic  SCI subjects with cervical lesions. We investigated the effects of 20-minute  anodal tDCS applied over the left primary motor cortex (M1) on  electroencephalography (EEG) power spectrum density, coherence and frequency  band power. Subjects were randomized to receive either 1mA or sham  stimulation.  The EEG data acquisition  pre and post stimulation comprised 5-minute takes of 24 bit, 500 S/s 8-channel  EEG using StarStim Ag/AgCl EEG electrodes (at F3, F4, Cz, C4, P3 and P4; and Pi  Ag/AgCl electrodes at C3, anode, AF8, return).

Results: In comparison to sham  stimulation, 20-minutes of active 1mA tDCS induced a pattern of faster activity  around the anodal stimulating electrode, and slowing activity near the return  electrode in the frequency (full band) and mean power domain (gamma band). In  addition, tDCS increased coherence in the fastest bands (gamma, beta 2) and  decreased coherence in slower frequency bands (theta, SMR), with no relation  with brain topography or the stimulation electrode polarity.

Conclusions: These findings show  that tDCS is capable of inducing modulation of ongoing oscillatory brain  rhythms captured by EEG, in spinal cord injury patients. The combined use of  EEG and tDCS sets the stage for optimizing tDCS protocols targeting motor  cortex and may have application in treatment of motor dysfunction and chronic  pain.

Key words: (6)

Abbreviations:

Introduction[JC2] 

Transcranial direct current stimulation (TDCS)  delivered over primary motor cortex (M1) can increase or decrease corticomotor  excitability as determined by the amplitude of the motor evoked potential (MEP)  from stimulating M1 with supra-threshold transcranial magnetic stimulation  (TMS). The effect on MEP amplitude depends on the duration and intensity of  stimulation and the polarity and spatial arrangement of the TDCS electrodes,  and these effects can persist for up to an hour after TDCS is ceased. It is  assumed that the TDCS after-effect results from some form(s) of neuromodulation  at the cortical level such as synaptic potentiation or depression, or effects  on ion channels. However, further insight into the cortical mechanisms in the  human is limited experimentally.

The electroencephalogram (EEG) measures far-field  potentials from synchronized neural populations over cortical regions that  extend beyond M1, and so offers a method for measuring the effect of TDCS on  neural activity across the cortex.  In particular, such measurements do  not depend on spinal cord conduction and alpha-motoneuron activation, which is  the case for MEP recordings. This could have advantages for determining the  effect of TDCS at the cortical level in spinal cord injury (SCI). While most  SCI studies focus on the level of injury, there is increasing interest in the  compensatory changes that might occur in the cortex, and whether they might  facilitate or impede recovery.

In the present study we measured  the effect of EEG after 20 min of anodal or sham TDCS applied to the dominant  M1 in chronic cervical SCI. We show that there are changes in EEG power,  frequency and coherence that are spatially-related to the TDCS electrode  configuration, but that spread widely across the hemispheres.

[MC3] 

Materials and methods

Participants and study design

The randomized, single blind, sham  controlled, cross over study was conducted on seven chronic SCI subjects, all males with an average age of 51.14 ± 10.57 years (mean ±SD, range 34  to 65 years). Participants presented traumatic SCI at the cervical level (C4-C8);  some degree of motor function in wrist extension (score 1-5 over 5 on the  Medical Research Council (MRC) scale for motor strength in the right extensor  carpi radialis (ECR) muscle); chronic injury (>9 months after injury); and  tolerance to sitting upright for at least one hour (see Table 1 for baseline  characteristics). Patients were excluded if they presented with: progressive  neurodegenerative disorder; concomitant traumatic brain injury or stroke;  clinically significant cognitive impairment; medically unstable; change in  medication during the study; or presented contraindications to brain  stimulation (history of seizures/epilepsy, presence of metallic implants in the  brain, pacemaker, pregnancy).

The subjects were randomized  in two groups depending on whether they were receiving stimulation or not:  a) Sham or control group, b) 1mA or active  group.

We investigated the effects of  20-minute anodal t-DCS applied over the primary motor cortex (M1) on: a)  quantitative electroencephalography (EEG) power spectrum density, coherence and  frequency band power. At Baseline we  collected: clinical and functional evaluations were  performed prior the brain stimulation intervention, and included the upper extremity  motor score (UEMS), American Spinal Injury Association impairment scale (AIS),  spinal cord independence measure (SCIM III) and visual analog scale (VAS) pain  questionnaires. [DE4] 

The study was approved by the Burke Medical  Rehabilitation Institutional Review Board and conformed to the standards set out by the 1964 Declaration of  Helsinki.

AddTable 1

Transcranial direct current stimulation (t-DCS)  intervention

Participants remained seated in their own wheelchair or were  provided with a comfortable chair. The StarStim^NE noninvasive  wireless t-DCS/EEG neurostimulator (NE Neuroelectrics®, Barcelona, Spain) was  used to both record EEG data and deliver the direct current sequentially. The  StarStim^NE neurostimulator included a wireless neoprene cap, based on  the International 10-20 system, which was placed on the participants’ heads by  aligning the central CZ electrode position with the vertex (intersection of  nasion-inion and inter-aural line mid-point).

Small Ag/AgCl gelled electrodes, with a  surface contact area of 3.14 cm2 specific to the StarStim^NE device  (Pi electrodes, Neuroelectrics®), were placed over the left M1 at C3 (anode) and  contralateral supraorbital area, (AF8; cathode) (Figure 1). The electrodes were  connected to a control box device, which was wirelessly connected to a computer  and communicated with the NIC software (version 1.2, Neuroelectrics®).

Add Figure 1

During anodal stimulation, direct current was  delivered from a current-control circuit in the battery-driven stimulator  within the control box device. The current was set at 1mA intensity and applied for 20 minutes.  [DE5] For the sham stimulation, electrodes were placed in the same  position and participants received a short ramp (30sec total up / down[DE6] ) at the beginning and end of the stimulation period.

Electroencephalography (EEG)

The StarStim^NE multichannel wireless device (NE  Neuroelectrics®, Barcelona, Spain), which allows for simultaneous electroencephalography (EEG) and tDCS[DE7] , was used to record EEG data. The EEG data consisted of 5-minute  takes (pre and post stimulation) with 24 bit resolution, 500 S/s 8-channel EEG  collected with StarStim^NE Ag/AgCl EEG electrodes (geltrodes,  NE022; positions at F3, F4, Cz, C4, P3 and P4; Pi stimulating electrodes at C3  and AF8), based on the International 10-20 system, with the cap  aligned to the central CZ electrode position (vertex).[DE8]  The electrodes were connected to a control box device, which was  wirelessly connected to a computer and communicated with the NIC software  (version 1.2, Neuroelectrics®).

Transcranial magnetic stimulation (TMS)

Electromyography (EMG): A bipolar surface EMG  electrode (1 cm diameter, 2 cm inter-pole distance; Biometrics Ltd, UK) was  placed over the right ECR muscle, with the forearm relaxed in a pronated  position and supported by a cushion. The EMG activity was amplified and  filtered on site (x1000 gain, band-pass filter 20-460 Hz; SX230-1000),  digitized at 2 kHz (CED 1401, Cambridge Electronic Design, Cambridge, UK) and  stored for offline analysis using Spike 2.6 software. Measurements were  performed at rest. During the experiment, free running EMG was continuously  monitored with visual feedback of EMG to ensure complete muscle relaxation.

The right ECR muscle was selected for clinical  relevance; where restoration of motor function in  this text muscle can help increase  independence in quadriplegic subjects with activities of daily living.

Transcranial Magnetic Stimulation (TMS): A  figure-of-eight coil (Model DB-80, Tonika Elektronik A/S, Farum, Denmark),  connected to a MagPro X100 series (MagVenture A/S, Farum, Denmark) magnetic  stimulator, was placed congruent with the head and the handle rotated 45°  lateral from mid-sagittal so as to induce currents in the brain approximately  perpendicular to the central sulcus. Resting motor threshold (RMT) was established  at C3, and was defined as the minimum TMS intensity required to elicit a  reliable MEP in the contralateral ECR amplitude of >50 µV in at least 50% of  consecutive trials.[DE9] 

Data analysis

Resting quantitative EEG and corticospinal  excitability (transcranial magnetic stimulation; MEP) were recorded before (PRE)  and at the end (POST) of each intervention..Quantitative EEG measures included:  power spectrum density (PSD), mean frequency and coherence. The PSD refers to  the percentage change between PRE and POST average power in each frequency band,  for each electrode. Mean frequency is [DE10] the percentage change between PRE and POST for  mean frequency in each band and for each electrode, defined as the mean  frequency of the power spectrum weighted over that band.

Mean  coherence is the  percentage  change (PRE and POST) in each  band and each electrode, defined as the average coherence  of this electrode with all the other ones.[DE11] 

The frequency bands were defined as: Theta= [4 8]  Hz, Alpha-1=[8 10] Hz, Alpha-2=[10 12] Hz, SMR=[12 16] Hz, Beta-1=[16 25] Hz,  Beta-2=[25 35] Hz, Gamma=[35 40] Hz. (ref???)

The EEG data were  referenced to the average of all 8 electrodes. An automated quality check of  the data was then carried out using 8 sec epochs. Epochs have been rejected if  they do not meet quality criteria (too high mean power at full band or line  noise, or motion artifacts as detected by the built in accelerometer). PSD changes  and coherence analysis was carried out for each quality passed epoch. Average  PSD’s and coherence have then been computed for each subject.

EEG data were  discarded from analysis for channels presenting bad signal quality during the  entire 5-minute recording. Those subjects that did not have more than 5 valid  channels were completely discarded. After the discarding process, in order to  have the same subjects in all the conditions, 7 subjects were analyzed for Sham  and 1mA.

Resting  MEP amplitude (peak-to-peak; ECR muscle) was measured following  single-pulse TMS (10/12) set at 120% [DE12] of the RMT over the C3[DE13] . Raw and normalized values were used for  analysis[DE14] . Results are presented as mean ± standard deviation (SD), and standard  error of the mean (SEM)[DE15] .

Changes are  considered statistically significant when p < 0.05[JC16] . Since all the features are measured in 8  channels at the same time, the Bonferroni correction has been applied to the significance of all features. [DE17] 

                                          PLEASE  ADD THE STUDY DATA ANALYSIS – GIULIO / LAURA

Results

Effects of tDCS on EEG

Normalized pre-post EEG power for  1mA compared to sham shows; a)  significant increase of the mean power domain in the Gamma frequency  band under C3, the anodal stimulating electrode; b) a significant increment of faster activity around the anode (stimulating)  electrode (C3, F3) with a decreased mean frequency in the Alpha band near the  return electrode (P4); and c) significantly  increased mean coherence in the fastest frequency bands (Beta2, Gamma) and SMR  under the stimulating electrode (C3), the symmetrical location in the other  hemisphere (C4) and the vertex (Cz). The lower map provides the p-value (Wilcoxon  test) of Sham vs. active stimulation.

LAURA – ADD THE TYPE OF TEST USED FOR THE RESULTS AND THE P VALUES  OBTAINED

                                                                                          Figure 2

Effects of tDCS on Corticospinal excitability

Baseline values for resting MEP  amplitude were similar between interventions (average: 0.37±0.05 mV; mean±SEM).  No significant changes were observed for 1 mA a-tDCS or sham. The stimulation  intensities used to obtain the RMT were not significantly different between 1  mA (64±17% MSO) and sham (66±16% MSO). All participants presented MEP  responses.

Discussion

1. tdcs does modulate EEG in SCI  population.

2. The effect of the TDCS in EEG  is location specific and band specific.

3. The fastest bands are more  affected. Augmented power and frequency of fast bands (Gamma and B2) under the  anode electrode - there is more fast activity under the anodal electrode.  Why is this?

3.1. Anodal  stimulation leads to increased spontaneous neuronal firing, increase motor  cortex excitability. So we would expect more fast activity and higher  amplitudes under the anode electrode.

3.2. Brain  activity changes (increase fast band) are related to motor learning – changes  in Mu rhythm (preparation for movement, getting ready for movement). This is  shown in the coherence increase of SMR band under the stimulating electrode.

MAR – ALL OF THE ABOVE

3.2.1.  Is there are connection between C3 and P4 in resting state brain? Why is alpha  decreased significantly in P4 (cortex parietal) in the contralateral hemisphere. 

CONECTIONS BETWEEN MOTOR CORTEX  AND SENSORY MOTOR CORTEX?? – JANA

3.3. Anodal  tDCS makes C3 to be more related to the rest of the head, by increasing the  connectivity of the SMR under the stimulating electrode.

INTERPRETANTION OF THIS POINT TO GIULIO/LAURA,  ANYTHING PUBLISHED BEFORE?

3.4. This  is an excitability independent change (no TMS change associated). Can EEG pick  up changes in the brain than TMS cannot?

                                                      LABAR?

Conclusion

These findings show that t-DCS is  capable of inducing modulation of ongoing oscillatory brain rhythms captured by  EEG, in spinal cord injury patients.

The combined use of EEG and t-DCS  sets the stage for optimizing t-DCS protocols targeting motor cortex and may  have application in treatment of motor dysfunction and chronic pain.

Figures

Figure  1: a) Starstim  wireless t DCS device. b) Electric field induced by montage +  C3, - AF8 using “Pi” electrodes (3.14 cm² Ag/AgCl electrodes).

Figure  2: Results for the normalized Pre-Post Power changes of 1mA vs sham show  a) significant increase of the mean power domain in Gamma frequency band  under C3, the anodal stimulating electrode. b)  A significant increment of faster  activity around the anodal stimulating electrode (C3, F3) while a decreased  mean frequency in the  Alpha band near  the return electrode (P4) and c) significant increased  mean coherence in the fastest frequency bands  (Beta2, Gamma) and SMR under the stimulating electrode (C3), the symmetrical  location in the other hemisphere (C4) and the vertex (Cz). The bottom map  provides the p-value (Wilkoxon test) of Sham vs. active stimulation.

Tables

Table 1: Patient  characteristics

References

Cortes M, Thickbroom GW, Valls-Sole J,  Pascual-Leone A, Edwards DJ. Spinal associative stimulation: a non-invasive  stimulation paradigm to modulate spinal excitability. Clin Neurophysiol 2011;  122(11): 2254-9.

Edwards DJ, Cortes M, Thickbroom GW, Rykman  A, Pascual-Leone A, Volpe BT. Preserved corticospinal conduction without  voluntary movement after spinal cord injury. Spinal Cord 2013; 51(10): 765-7.