Cerebellar direct current stimulation modulates pain perception in humans

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Restorative Neurology and Neuroscience xx (20xx) xndashxxDOI 103233RNN-140453IOS Press

1

Cerebellar direct current stimulationmodulates pain perception in humans

1

2

Tommaso Bocciab Enrica Santarcangeloc Beatrice Vanninia Antonio Torzinibd Giancarlo CarlibRoberta Ferruccie Alberto Priorie Massimiliano Valerianifg and Ferdinando Sartucciadlowast

3

4

aDepartment of Clinical and Experimental Medicine Unit of Neurology Pisa University Medical School PisaItaly

5

6

bDepartment of Medical and Surgical Sciences and Neuroscience University of Siena Siena Italy7

cDepartment of Translational Research and New Technologies in Medicine and Surgery University of Pisa PisaItaly

8

9

dDepartment of Clinical and Experimental Medicine Cisanello Neurology Unit Pisa University Medical SchoolPisa Italy

10

11

eDepartment of Neurological Sciences University of Milan Fondazione IRCCS Ospedale Maggiore PoliclinicoMilan Italy

12

13

f Division of Neurology Ospedale Bambino Gesu IRCCS Rome Italy14

gCenter for Sensory-Motor Interaction Aalborg University Aalborg Denmark15

Abstract16

Purpose The cerebellum is involved in a wide number of integrative functions but its role in pain experience and in thenociceptive information processing is poorly understood In healthy volunteers we evaluated the effects of transcranial cerebellardirect current stimulation (tcDCS) by studying the changes in the perceptive threshold pain intensity at given stimulationintensities (VAS0-10) and laser evoked potentials (LEPs) variables (N1 and N2P2 amplitudes and latencies)

17

18

19

20

Methods Fifteen normal subjects were studied before and after anodal cathodal and sham tcDCS LEPs were obtained using aneodymiumyttriumndashaluminiumndashperovskite (NdYAP) laser and recorded from the dorsum of the left hand VAS was evaluatedby delivering laser pulses at two different intensities respectively two and three times the perceptive threshold

21

22

23

Results Cathodal polarization dampened significantly the perceptive threshold and increased the VAS score while the anodalone had opposite effects Cathodal tcDCS increased significantly the N1 and N2P2 amplitudes and decreased their latencieswhereas anodal tcDCS elicited opposite effects Motor thresholds assessed through transcranial magnetic stimulation were notaffected by cerebellar stimulation

24

25

26

27

Conclusions tcDCS modulates pain perception and its cortical correlates Since it is effective on both N1 and N2P2 componentswe speculate that the cerebellum engagement in pain processing modulates the activity of both somatosensory and cingulatecortices Present findings prompt investigation of the cerebellar direct current polarization as a possible novel and safe therapeutictool in chronic pain patients

28

29

30

31

Keywords Pain cerebellum cerebellar direct current stimulation tDCS laser evoked potentials pain modulation32

lowastCorresponding author Prof Ferdinando Sartucci MD Asso-ciate Professor of Neurology Pisa University Medical SchoolNeuroscience Department Neurology - Neurophysiology Units ViaRoma n 67 I 56126 Pisa Italy Tel +39 050 992176 (direct) Fax+39 050 550563 992405 E-mail fsartuccineuromedunipiit

1 Introduction 33

The cerebellum is involved in a wide number of 34

integrative functions ranging from working memory 35

and associative learning to motor control (Schmah- 36

mann 1991 Ito 2006 Stoodley amp Schmahmann 37

0922-602815$3500 copy 2015 ndash IOS Press and the authors All rights reserved

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2 T Bocci et al Cerebellum and pain A tcDCS study

2009 Strick et al 2009 Balsters et al 2013) It is38

also involved in the sensory cognitive (Borsook et39

al 2008) and affective dimensions of pain (Ploghaus40

et al 1999) In addition the cerebellum plays a role41

in the sensory-motor integration aimed at antinocicep-42

tive behaviour (Bingel et al 2002 Strigo et al 200343

Borsook et al 2008) as well as in salience-related44

affective and behavioral responses to nociceptive stim-45

ulation (Duerden amp Albanese 2013) In fact although46

it is not known how nociceptive information is encoded47

in the cerebellum it has been proposed that the cerebel-48

lum may integrate multiple effector systems including49

affective processing pain modulation and sensorimo-50

tor control51

Afferent inputs from nociceptors reach the cerebel-52

lum through two different and segregated pathways the53

spino-ponto-cerebellar and the spino-olivo-cerebellar54

route (Ekerot et al 1987a 1987b Ekerot et al55

1991a) and the cerebellar influence on pain process-56

ing closely resembles the inhibitory tone exerted by57

Purkinje cells over the primary motor cortex (M1) a58

phenomenon referred as cerebellum-brain inhibition59

(Kelly amp Strick 2003)60

Non-invasive brain stimulation (NIBS) techniques61

such as repetitive Transcranial Magnetic Stimulation62

(rTMS) and transcranial Direct Current Stimulation63

(tDCS) have recently emerged as interesting effective64

and promising tools for modulating pain experience65

(Antal amp Paulus 2010 Zaghi et al 2011) In fact66

a sufficient body of evidence shows analgesic effects67

of high-frequency rTMS of the primary motor cor-68

tex (M1) (Lefaucheur et al 2014) with effects likely69

arising from the restoration of defective intracortical70

inhibitory processes (Lefaucheur et al 2006) Among71

NIBS technique tDCS applied either over the motor72

(Fregni et al 2007 Mendonca et al 2011 Dasilva73

et al 2012 Reidler et al 2012) or the prefrontal cor-74

tex (Boggio et al 2008 2009 Mylius et al 2012) was75

also effective in pain modulation76

Only one study has assessed the effects of cerebellar77

rTMS suggesting that changes in pain perception were78

not specific for cerebellar stimulation (Zunhammer79

et al 2011) However no study has investigated to date80

the role of transcranial cerebellar direct current stim-81

ulation (tcDCS) a new and well-tolerated technique82

for modulating cerebellar excitability in modifying83

pain perception in humans (Ferrucci et al 2008 201284

Galea et al 2009 2011 Grimaldi et al 2014 Priori et85

al 2014) Notably despite some inter-individual dif-86

ferences recent modelling researches have revealed87

that during tcDCS the current spread to other struc- 88

tures outside the cerebellum is negligible and unlikely 89

to produce functional effects (Fig 1) (Parazzini et al 90

2013 2014a 2014b) 91

The aim of our study was to evaluate the effects 92

of tcDCS on pain perception and on its cortical cor- 93

relates We studied the changes in pain scores and in 94

laser evoked potentials (LEPs) variables (perceptive 95

threshold N1 and N2P2 amplitudes and latencies) 96

in participants undergoing direct current polarization 97

applied over the cerebellum 98

2 Materials and methods 99

21 Subjects 100

Fifteen healthy volunteers (mean age plusmn SD 101

258 plusmn 59 years 7 women) with no history of neuro- 102

logical disorders were enrolled in the study Women 103

were studied in the second week after their last menses 104

(Smith et al 1999) No subject had been under 105

medication in the month preceding the experimental 106

session which was scheduled at least 48 hours after 107

the last alcohol and caffeine consumption Written 108

informed consent was obtained from all participants 109

before enrollment in the study which was approved 110

by the local ethical Committee and followed the tenets 111

of the Declaration of Helsinki 112

22 Experimental design 113

As shown in Fig 2 at the beginning of each session 114

before cerebellar tDCS and immediately afterwards 115

the laser Perceptive Threshold (PT) corresponding to 116

the lowest intensity at which subjects perceived at least 117

50 of the stimuli (Cruccu et al 1999 Agostino et al 118

2000) was determined In order to minimize the num- 119

ber of nociceptive stimuli the nociceptive perception 120

threshold was not assessed A range of 10ndash40 stimuli 121

(mean SD 25 plusmn 5) was used to assess the perceptive 122

threshold before and after transcranial cerebellar stim- 123

ulation Less than 10 minutes were spent to determine 124

PT in line with previous reports (Truini et al 2011) 125

After the PT assessment participants were 126

instructed to pay attention to incoming laser noci- 127

ceptive stimuli in order to verbally rate the perceived 128

intensity about 2-3 seconds after each laser stimulation 129

which was performed before tcDCS (T0) immediately 130

after its termination (T1) and 60 min later (T2) 131

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T Bocci et al Cerebellum and pain A tcDCS study 3

A

B

1

08

06

04

02

0

Fig 1 - Current density generated by cerebellar transcranial directcurrent stimulation (cerebellar tDCS) in humans A Top panel shows(viewed from the back) the electrode positions for cerebellar tDCSB Examples of segmented tissues in two human realistic VirtualFamily models (Ella and Duke) undergoing cerebellar tDCS Simu-lations were conducted using the simulation platform SEMCAD X(by SPEAG Schmid amp Partner Engineering AG Zurich Switzer-land) a lateral view of cerebellum pons midbrain medulla blateral view of the skull c back view of the cerebellum d and elateral and inferior views of normalized current density amplitudefield distributions over cortical subcortical and brainstem regionsf back view of normalized current density amplitude field distribu-tions over the cerebellum Values are normalized with respect to themaximum of the current density amplitude in the cerebellum Thespread of the current density (J) over the occipital cortex - quantifiedas the percentage of occipital volume where the amplitude of J-fieldis greater than 70 of the peak of J in the cerebellum - was only 4for ldquoDukerdquo and much less than 1 for ldquoEllardquo (modified from Prioriet al (2014) with permission)

Participants were blinded to the tcDCS polarity 132

anodal cathodal and sham tcDCS stimulations were 133

administered in three different sessions and separated 134

by at least 1 week to avoid possible carry-over effects 135

The order of interventions was randomized and bal- 136

anced across subjects Laser stimuli of intensity two 137

and three times the PT intensity (I1 I2) were delivered 138

by an experimenter (AT) whereas the evaluation of 139

electrophysiological parameters was done by FS both 140

blinded to the tcDCS polarity B V settled the tcDCS 141

polarity 142

221 Subjective experience 143

The perceived sensation was rated on the 0ndash10 144

Visual Analogue Scale (where 0 = no sensation and 145

10 = unbearable pain the intermediate levels being 146

1 = barely perceived 2 = lightly pricking not painful 147

3 = clearly pricking not painful 4 = barely painful 148

5 = painful prompting to rub the skin 6 = very painful 149

and distressing 7 and more strongly unpleasant pain) 150

VAS was studied in each subject after 10 nociceptive 151

laser I1 and I2 stimuli (VAS 1 VAS2) In each partic- 152

ipant individual VAS values were averaged for each 153

Time 154

Laser Evoked Potentials were obtained by stim- 155

uli corresponding to two times the Perceptive value 156

according with previous literature and guidelines 157

(Truini et al 2005 2010) 158

23 Procedures 159

231 Laser evoked potentials (LEPs) 160

The methods used for laser stimulation are 161

reported in detail elsewhere (Truini et al 2005 162

2010) A neodymiumyttriumndashaluminiumndashperovskite 163

(NdYAP) laser was used (wavelength 104 m pulse 164

duration 2ndash20 ms maximum energy 7 J) The laser 165

beam was transmitted from the generator to the stim- 166

ulating probe via a 10 m length optical fibre signals 167

were then amplified band pass filtered (01ndash200 Hz 168

time analysis 1000 ms) and fed to a computer for stor- 169

age and analysis (Cruccu et al 2008) The dorsum of 170

the left hand was stimulated by laser pulses (individ- 171

ual variability 389ndash1575 Jcm2) with short duration 172

(5 ms) and small diameter spots (5 mm Valeriani et al 173

2012) Ten stimuli whose intensity was established 174

on the basis of the Perceptive Threshold assessed for 175

each subject at T0 T1 and T2 were delivered and 176

the laser beam was shifted slightly between consec- 177

utive pulses to avoid skin lesions and reduce fatigue 178

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4 T Bocci et al Cerebellum and pain A tcDCS study

Fig 2 ndash Experimental protocol Psychophysical and electrophysiological variables evaluated at baseline (T0) and at two different time points(T1 T2) following anodal cathodal and sham tcDCS

of peripheral nociceptors (Truini et al 2005) The179

inter-stimulus interval was varied randomly (10ndash15 s)180

Participants were reclined on a couch and wore protec-181

tive goggles They were instructed to keep their eyes182

open and gaze slightly downwards since the N2P2183

amplitude is enhanced by attention (Lorenz amp Garcia-184

Larrea 2003 Truini et al 2005) they were requested185

to mentally count the number of stimuli The main186

Aδ-LEP vertex complex N2ndashP2 and the lateralised187

N1 component were recorded through standard disc188

non-polarizable AgAgCl surface electrodes (diameter189

10 mm Biomedreg Florence Italy) N2 and P2 compo-190

nents were recorded from the vertex (Cz) referenced191

to the earlobes the N1 component was recorded from192

the temporal leads (T4) referenced to Fz (Cruccu et al193

2008) Blinks and saccades were recorded with an EOG194

electrode placed on the supero-lateral right canthus195

connected to the system reference Ground was placed196

on the mid-forehead Skin impedance was kept below197

5 k198

232 Cerebellar transcutaneous direct current199

stimulation (tcDCS)200

tDCS was applied using a battery-driven constant201

current stimulator (HDCStim Newronika Italy) and202

a pair of electrodes in two saline-soaked synthetic203

sponges with a surface area of 25 cm2 For cathodal204

stimulation the cathode was centered on the median205

line 2 cm below the inion with its lateral borders about206

1 cm medially to the mastoid apophysis and the anode 207

over the right shoulder (Ferrucci et al 2008 2012 208

2013) For anodal stimulation the current flow was 209

reversed In the real tcDCS conditions direct current 210

was transcranially applied for 20 minutes with an inten- 211

sity of 20 mA and constant current flow was measured 212

by an ampere meter (current density asymp 008 mAcm2) 213

These values are similar to those previously reported 214

for cerebellar stimulation (Ferrucci et al 2008 2013) 215

are considered to be safe (Iyer et al 2005) and are 216

far below the threshold for tissue damage (Nitsche 217

et al 2003) Apart from occasional and short-lasting 218

tingling and burning sensations below the electrodes 219

direct current stimulation strength remained below the 220

sensory threshold throughout the experimental session 221

At the offset of tDCS the current was decreased in a 222

ramp-like manner a method shown to achieve a good 223

level of blinding among sessions (Gandiga et al 2006 224

Galea et al 2009) For a sham tDCS the current was 225

turned on only for 5 seconds at the beginning of the 226

sham session and then it was turned off in a ramp- 227

shaped fashion which induces initial skin sensations 228

indistinguishable from real tDCS 229

For all the electrophysiological recordings we chose 230

the left side to avoid interference from the return 231

electrode placed over the contralateral shoulder At 232

experimental debriefing subjects were not able to dis- 233

criminate between the applied anodal cathodal and 234

sham tDCS 235

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T Bocci et al Cerebellum and pain A tcDCS study 5

Table 1

Row data (expressed as mean value plusmn 1 standard deviation a = anodal stimulation c = cathodal stimulation sh = sham condition)Both psychophysical and electrophysiological data for each subject are fully available as supplementary electronic material at

httpwwwenricasantarcangelocompublications

aT0 aT1 aT2 cT0 cT1 cT2 shT0 shT1 shT2

PT mean 462 607 609 485 376 368 472 466 489SD 080 095 092 086 062 067 098 062 081

VAS I1 mean 389 255 265 367 493 467 387 393 387SD 084 057 062 082 096 082 074 070 092

VAS I2 mean 540 402 403 524 673 665 533 549 530SD 063 082 071 048 047 049 078 069 064

N1 amplitude (V) mean 1292 848 801 1104 1496 1494 1101 1111 1121SD 318 298 258 265 258 333 250 267 283

N1 latency (ms) mean 12419 16146 15710 12704 10715 10405 12817 12867 13066SD 1090 1338 1368 1075 675 912 1320 1271 1209

N2P2 amplitude(V) mean 1114 738 757 1052 1453 1375 1114 1125 1147SD 262 237 233 265 296 329 272 269 216

N2P2 latency (ms) mean 15157 18932 18726 14878 12673 13230 15390 15108 15551SD 1312 1749 2139 2201 1849 1870 1433 1507 1675

233 Transcranial magnetic stimulation (TMS)236

Changes in Resting Motor Threshold (RMT) were237

evaluated at different intervals before and after the238

completion of tcDCS A Magstim Super Rapid Tran-239

scranial Magnetic Stimulator (Magstim Company240

Dyfed UK 22 T maximum field output) connected241

to a standard eight-shaped focal coil with wing diame-242

ters of 70 mm was used The handle of the eight-shaped243

focal coil was pointed backwards and rotated about 45244

deg to the mid-sagittal line to induce a tissue current245

perpendicular to the motor strip in the precentral sul-246

cus (Rossi et al 2009 Groppa et al 2012) RMT247

was defined as the minimum stimulator output that248

induces motor evoked potentials (MEPs) of more than249

50 V in at least five out of 10 trials when first dig-250

ital interosseus (FDI) muscle was completely relaxed251

(Ni et al 2007) The motor ldquohot spotrdquo for the targeted252

muscle was identified by single pulses of TMS deliv-253

ered at a slightly suprathreshold stimulus intensity and254

the magnetic stimuli induced monophasic pulses The255

coil was placed over the right motor cortex (centered on256

C4 according with the 10ndash20 EEG International Sys-257

tem) and electromyographic recordings were made by258

two standard non-polarizable AgAgCl surface elec-259

trodes (diameter 10 mm Biomedreg Florence Italy)260

one placed over the belly of the contralateral FDI261

muscle and the other on the skin overlying the first262

metacarpophalangeal joint of the first finger of the left263

hand RMT was evaluated to exclude possible cere-264

bellar stimulation spread out inducing motor cortex265

activation266

24 Variables and statistical analysis 267

We studied the subjective experience - percep- 268

tive threshold (PT) and pain intensity perceived 269

after laser I1 and I2 (VAS1 VAS2) - and elec- 270

trophysiological variables that is the peak-to-peak 271

amplitude of the N1 wave and N2P2 complex 272

the peak latency of N1 and N2 as reported in 273

previous papers using NdYAG laser (Lefaucheur 274

et al 2001 2002)275

Analyses were performed through SPSS15 sta- 276

tistical Package Psychophysical (PT VAS1 VAS2) 277

and electrophysiological variables (mean values of 278

ten traces N1amplitude and latency N2P2 ampli- 279

tude and latency) as well as Resting Motor Thresholds 280

(RMT) were analysed following a 3 Stimulation con- 281

ditions (anodal cathodal sham) times 3 Times (T0 T1 282

T2) design The Greenhouse-Geisser correction for 283

non sphericity was applied when necessary Con- 284

trast analysis between Times (F values) and paired 285

t tests between stimulations were alternatively used 286

for post-hoc comparisons when appropriate After 287

Bonferroni correction significance level was set at 288

p lt 0007289

The changes of all variable in T1 and T2 290

were expressed as ratio between post and pre 291

stimulation values (T1T0 T2T0) and compared 292

between each other according to a 2 Stimula- 293

tion (anodal cathodal) times 2 Times (T1To T2T0) 294

design 295

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6 T Bocci et al Cerebellum and pain A tcDCS study

Table 2

Contrast analyses all comparisons were highly significant(p lt 00001)

anodal cathodal shamPT time df F = 4430 F = 1867 ns

T0 vs Tl 228 F = 77669) F = 27523T0 vs T2 114 F = 78745 F = 27827Tl vs T2 114 ns ns

anodal vs sham cathodal vs shamT0 114 ns nsTl 114 t = 5069 t = 6991T2 114 t = 3709 t = 5849

VAS anodal cathodal sham

time 228 F = 41954 F = 31448 nsT0 vs Tl 114 F = 56968 F = 48596T0 vs T2 114 F = 52289 F = 525Tl vs T2 ns ns

anodal vs sham cathodal vs shamT0 114 ns nsTl 114 t = 644 t = 5916T2 114 t = 5294 t = 582

3 Results296

Row data (mean SD) are shown in Table 1 Base-297

line values were in line with those reported by earlier298

studies performed by using NdYAG laser (Lefaucheur299

et al 2001) Indeed only one study described a longer300

latency of the N2 wave (Cruccu et al 2008) The301

sham stimulation did not modulate any psychophys-302

ical and electrophysiological variable (Table 2) Since303

no pre-post difference was found for sham polarity this304

condition was not included in the comparison between305

Stimulations and Times306

31 Psychophysics307

PT exhibited a significant Stimulation effect308

(F(228) = 35055 p lt 00001 η2 = 0715) and a signifi-309

cant Stimulation times Time interaction (F(456) = 39464310

p lt 00001 η2 = 0738) Decomposition of the latter311

(Table 2) revealed that a) PT was higher for the anodal312

and lower for the cathodal stimulation conditions com-313

pared with the sham stimulation for both T1 and T2314

b) with respect to T0 PT increased in T1 and T2 in the315

anodal condition and decreased in the cathodal con-316

dition while no significant difference was observed317

between T1 and T2 (Fig 3A)318

Significantly different VAS1 and VAS2 were319

observed for the two stimulation intensities (F(114)320

= 54262 p lt 00001) and the three Stimulation con-321

ditions (F(218) = 88882 p lt 00001) Decomposition322

of the significant Stimulation times Time interaction323

(F(456) = 11596 p lt 00001) revealed that the reported 324

pain intensity for both stimulation intensities (VAS1 325

and VAS2) was higher for the cathodal and lower for 326

the anodal stimulation compared to the sham stimula- 327

tion (Table 2) It increased in T1 and decreased in T2 328

with respect to T0 whereas no significant difference 329

was found between T1 and T2 (Fig 3-B) 330

32 Laser evoked potentials 331

Figure 4-A shows the LEPs recorded in all experi- 332

mental conditions in a representative subject Both N1 333

and N2P2 amplitude (N1 F(456) = 10695 p lt 00001 334

η2 = 0884 N2P2 F(456) = 86864 p lt 00001 η2 = 335

0861) and latency (N1 F(456) = 110869 p lt 00001 336

η2 = 0888 N2P2 F(456) = 3660 p lt 00001 337

η2 = 0723) exhibited a significant Time times Stimulation 338

interaction Its decomposition (Table 3) showed 339

that both amplitudes increased and both latencies 340

decreased for cathodal stimulation in T1 and T2 with 341

respect to T0 the opposite occurred for the anodal 342

stimulation Both stimulations induced responses 343

significantly different from the sham condition 344

(Fig 4-B) The responses obtained after cathodal 345

stimulation were significantly improved (higher 346

amplitudes lower latencies) than those produced 347

by the anodal one (N1 amplitude F(114) = 41345 348

p lt 00001 N1 latency F(114) = 496228 p lt 00001 349

N2P2 amplitude (F(114) = 44537 p lt 00001 N2P2 350

latency F(114) = 119056 p lt 00001) 351

33 Resting motor thresholds 352

RMT values at baseline did not differ among exper- 353

imental conditions (mean plusmn SD sham 508 plusmn 83 354

anodal 491 plusmn 62 cathodal 503 plusmn 63) ANOVA 355

did not reveal any significant Stimulation (F(228) = 356

0882 p = 0425 η2 = 0059) Time (F(228) = 0212 357

p = 0810 η2 = 0015) and Stimulation times Time 358

effect (F(456) = 0339 p = 0851 η2 = 0024) for RMT 359

(Fig 5) 360

4 Discussion 361

Our study shows that cerebellar direct current 362

polarization modulates nociceptive perception and its 363

cortical correlates in healthy humans Specifically 364

cathodal tcDCS increases pain perception increases 365

amplitudes and decreases LEPs latencies likely though 366

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T Bocci et al Cerebellum and pain A tcDCS study 7

A

B

Fig 3 - A Perceptive Threshold Changes (mean plusmn SD) at T1 and T2 with respect to baseline values (T1T0 T2T0) following sham (black)anodal (white) and cathodal (grey) tcDCS (lowastlowastp lt 0001 lowastlowastlowastp lt 00001) B Changes in visual analogue scale (VAS) scores over time VAS scoresat two different stimulus intensity respectively two (A left) and three (B right) times higher than the PT (lowastlowastp lt 0001 lowastlowastlowastp lt 00001)

A B

Fig 4 ndash A LEPs grand averaging traces were recorded at baseline (T0 black) and immediately after cerebellar polarization (T1 red) due tosham (left column) anodal (middle) and cathodal (right) tcDCS B Histograms showing LEPs variables and VAS scores changes (mean plusmn SD)after sham (black) anodal (white) or cathodal (grey) tcDCS with respect to baseline Top panels changes in N1 variables (amplitude and latency)over time bottom panels changes in N2P2 complex (lowastlowastp lt 0001 lowastlowastlowastp lt 00001)

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8 T Bocci et al Cerebellum and pain A tcDCS study

Table 3

LEPs post-hoc analyses p lt 00001 for all comparison except when explicitly indicated lowastlowastp lt 0002 lowast p lt 0005

N1 amplitude latency N2P2 amplitude latency

anodal cathodal shamdf

time 228 F = 67152 F = 96489 F = 134912 F = 34946 nsT0 vs T1 114 F = 109178 F = 18815 F = 165953 F = 64281T0 vs T2 114 F = 75143 F = 167697 F = 145125 F = 37818T1 vs T2 114 ns ns ns ns

anodal cathodaltime 228 F = 102281 F = 98717 F = 6577 F = 20918 nsT0 vs T1 114 F = 511186 F = 104027 F = 144112 F = 103864T0 vs T2 114 F = 96329 F = 116841 F = 105183 F = 14012lowastlowastT1 vs T2 114 ns ns ns ns ns

anodal vs sham anodal vs shamT0 114 ns ns ns nsT1 114 ns t = 925 t = 601 6262T2 114 ns t = 8128 t = 6731 5236

cathodal cathodal vs shamT0 114 ns ns t = 3281lowast nsT1 114 t = 16594 t = 8029 t = 8262 t = 520T2 114 t = 7309 t = 12669 t = 5048 t = 5301

Fig 5 - Resting Motor Thresholds Changes (mean plusmn SD) in Resting Motor threshold (RMT) expressed as percentage of the maximumstimulator output after sham (black) anodal (white) and cathodal (grey) tcDCS with respect to baseline marked as dotted line (lowastlowastp lt 0001lowastlowastlowastp lt 00001)

reduction of the inhibitory tone exerted by the cere-367

bellum on brain targets Anodal polarization elicits368

opposite effects producing analgesia Both findings369

support the role of the cerebellum in pain control370

it is noticeable that cathodal cerebellar stimulation371

induces hyperalgesia as occurs in patients with cere-372

bellar infarction (Ruscheweyh et al 2014)373

We would like to underline that in the present study374

LEPs were obtained at laser intensities depending on375

the perceptive threshold which varied as a function376

of anodal and cathodal stimulation This means that 377

the cerebellar stimulation has not a selective analgesic 378

effect as it influences both non nociceptive and noci- 379

ceptive perception A pre-eminent analgesic cannot be 380

assessed because the nociceptive threshold was not 381

evaluated 382

As tcDCS was effective on the modulation of 383

both N1 and N2P2 components and these responses 384

are generated by parallel and partially segregated 385

spinal pathways reaching different cortical targets 386

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T Bocci et al Cerebellum and pain A tcDCS study 9

(Valeriani et al 2007) we may suggest that the cere-387

bellum is engaged in pain processing by modulating388

the activity of both somatosensory and cingulate cor-389

tices Indeed the cerebellum is involved in both the390

sensory-discriminative and emotional dimension of391

pain (Singer et al 2004 Moriguchi et al 2007) and392

non-invasive cerebellar current stimulation may modu-393

late pain experience and the associated cortical activity394

through many not alternative mechanisms In partic-395

ular changes in N1 reflects the modulation of the396

sensory component of pain while the vertex N2P2397

represents the neural correlate of affective aspects of398

pain experience (Garcia-Larrea et al 1997 Valeriani399

et al 2007) Notably tcDCS may act not only on spinal400

nociceptive neurons but also on wide-range cortical401

networks of the pain matrix (Singer et al 2004) thus402

influencing LEPs and pain experience through both403

top-down and bottom-up mechanisms404

The present study does not allow to hypothesize405

how and where tcDCS influences the cerebellar activ-406

ity A main role of Purkinje cells has been suggested407

as their activity modulation may affect the cerebellar408

inhibitory control of the cerebral cortex (Galea et al409

2009) This would be in line with the effects elicited410

by tDCS in the cerebral cortex which are observ-411

able after both short and long term delay likely also412

interfering with long-term potentiation (LTP)-like phe-413

nomena (Hamada et al 2012 Priori et al 2014)414

Moreover prolonged spiking activity in the cerebellar415

Golgi inhibitory neurons modulates the activity of the416

Purkinje cells and could partly account for the tcDCS417

after-effects (Hull et al 2013)418

The lack of changes in RMT indicates that the anal-419

gesic effects of anodal tcDCS are due to a specific420

modulation of the cerebellar activity and not to motor421

activation On the other hand tcDCS-induced cerebel-422

lar modulation (Purpura amp McMurtry 1965) could be423

not sufficient per se to activate the cerebello ndash thalamo424

- cortical motor pathway (Galea et al 2009) thus425

the reported analgesia and its cortical correlates can-426

not be sustained by the motor cortex activation This427

view is supported by the absence of any association428

between motor symptoms and pain perception in cere-429

bellar patients (Ruscheweyh et al 2014) In the same430

line in healthy subjects it has been recently shown431

that motor task-induced increased cortical excitability432

and analgesia are not associated (Volz et al 2012)433

Indeed RMT is a highly sensitive marker of motor434

tract excitability as it reflects activation of a small435

low-threshold and slow-conducting core of pyramidal436

neurons (Hess et al 1987 Rossini amp Rossi 2007) 437

although RMT may reflect changes in the activity of 438

different central nervous system structures it has been 439

satisfactorily used to assess motor cortex excitability 440

also in cerebellar patients (Battaglia et al 2006) 441

Another critical point is the possibility to modulate 442

with tcDCS both neural correlates underlying nocicep- 443

tive processing and pain perception Previous studies 444

using tDCS over motor cortex were inconsistent among 445

each other some works suggested that tDCS is able to 446

modify pain perception (Boggio et al 2008) while 447

others showed divergent effects on psychophysical 448

and neurophysiological outcome parameters (Luedtke 449

et al 2012 Ihle et al 2014) likely due to a possi- 450

ble overestimation of the role of motor areas on pain 451

processing (Antal et al 2008) 452

Our findings cannot be compared to the results 453

obtained by other Authors In fact the unique study 454

focused on the analgesic effects of non-invasive cere- 455

bellar stimulation reported till now (Zunhammer et al 456

2011) considered only subjective pain thresholds 457

In addition it described similar analgesic effects of 458

cerebellar and neck structures repetitive transcranial 459

magnetic stimulation (rTMS) thus denying any cere- 460

bellar specificity in the observed effects and suggesting 461

that the peripheral information passing through the 462

cerebellum may be responsible for analgesia The 463

main difference between the two studies possibly 464

accounting for different results consists of the neu- 465

romodulation techniques used 466

41 Limitations of the study 467

The present study has a few limitations First our 468

findings do not allow any hypothesis on the role of 469

the cerebellum in chronic pain The observations on 470

patients with cerebellar damage (Ruscheweyh et al 471

2014) suggest that their impaired inhibitory control 472

mechanisms may be not associated with the devel- 473

opment of chronic pain Second we cannot exclude 474

the possibility that tcDCS could modulate not only the 475

cerebellum but also surrounding areas such as the peri- 476

aqueductal gray However recent modelling researches 477

have revealed that during tcDCS the current spread 478

to other structures outside the cerebellum is negligi- 479

ble (Parazzini et al 2013 2014) Moreover several 480

studies have proved that in humans pain processing 481

is encoded within posterior areas of each cerebellar 482

hemisphere specifically in the hemispheric lobule VI 483

Crus I and VIIb (Moulton et al 2011) where the 484

Unc

orre

cted

Aut

hor P

roof

10 T Bocci et al Cerebellum and pain A tcDCS study

tcDCS-induced electrical field is strongly concentrated485

(Parazzini et al 2013) A further limitation is that we486

did not study the contribution of C-fibers the main487

component of spino-ponto-cerebellar and spino-olivo-488

cerebellar pathways In fact ultra-late LEPs related489

to C-fibers activations have not yet been standard-490

ized for clinical application and their occurrence could491

be markedly influenced by high order cognitive pro-492

cesses as they seem to be more affected by the level493

of consciousness and attention than A-delta responses494

(Qiu et al 2002 Opsommer et al 2003 Mouraux amp495

Plaghki 2006) Finally we wish to emphasize that in496

neuropathic patients the effects of the cerebellar stim-497

ulation could be quite different from those described498

here as both anatomical and functional connectivity499

are different from those observed in healthy partici-500

pants (Rocca et al 2010 Riedl et al 2011 Absinta et501

al 2012 Longo et al 2012 Ceko et al 2013)502

5 Conclusions503

Our findings indicate a cerebellar effect on pain504

experience and on its cortical correlates and prompt505

further investigation aimed at assessing whether the506

cerebellar direct current polarization could be used as a507

novel and safe therapeutic tool in chronic pain patients508

Acknowledgments509

We gratefully acknowledge the participation of all510

subjects as well as Mr D Barloscio and Mr C511

Orsini for their excellent technical assistance The512

paper was supported in part by the Italian operating513

and development MIUR PRIN grant year 2006 n514

2006062332 002515

RF and AP are founders and shareholders of516

Newronika srl Milan Italy517

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Truini A Panuccio G Galeotti F Maluccio MR Sartucci F 805

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cted

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inhibits nociceptive spinal pathway conduction and increases812

pain tolerance in humans Eur J Pain 15(10) 1023-1027813

Valeriani M Le Pera D Restuccia D de Armas L Mili-814

ucci R Betti V Vigevano F amp Tonali P (2007) Parallel815

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Valeriani M Pazzaglia C Cruccu G amp Truini A (2012) Clinical819

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Wager TD Rilling JK Smith EE Sokolik A Casey KL 826

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and experience of pain Science 303(5661) 1162-1167 829

Zaghi S Thiele B Pimentel D Pimentel T amp Fregni 830

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invasive cortical stimulation Restor Neurol Neurosci 29(6) 832

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