An intra-neural microstimulation system for ultra-high field magnetic resonance imaging and magnetoencephalography

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An intra-neural microstimulation system for ultra-high field magnetic resonance imaging and

magnetoencephalography

Paul Glover, Roger H. Watkins, George C. O’neill, Rochelle Ackerley, Rosa Sanchez-Panchuelo, Matthew J. Brookes, Johan Wessberg, Susan T. Francis,

Francis Mcglone

To cite this version:

Paul Glover, Roger H. Watkins, George C. O’neill, Rochelle Ackerley, Rosa Sanchez-Panchuelo, et al.. An intra-neural microstimulation system for ultra-high field magnetic resonance imaging and magnetoencephalography. Journal of Neuroscience Methods, Elsevier, 2017, 290, pp.69 - 78.

�10.1016/j.jneumeth.2017.07.016�. �hal-01599604�

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JournalofNeuroscienceMethods290(2017)69–78

ContentslistsavailableatScienceDirect

Journal of Neuroscience Methods

j ou rn a l h om epa g e : w w w . e l s e v i e r . c o m / l o c a t e / j n e u m e t h

Research Paper

An intra-neural microstimulation system for ultra-high ﬁeld magnetic resonance imaging and magnetoencephalography

Paul M. Glover

^a,∗

, Roger H. Watkins

^b

, George C. O’Neill

^a

, Rochelle Ackerley

^b,c

,

Rosa Sanchez-Panchuelo

^a

, Francis McGlone

^d,e

, Matthew J. Brookes

^a

, Johan Wessberg

^b

, Susan T. Francis

^a

aSirPeterMansﬁeldImagingCentre,UniversityofNottingham,Nottingham,NG72RD,UK

bDepartmentofPhysiology,UniversityofGothenburg,Gothenburg,40530,Sweden

cLaboratoiredeNeurosciencesIntégrativesetAdaptatives(UMR7260),Aix-MarseilleUniversité−CNRS,13331MarseilleCEDEX03,France

dSchoolofNaturalSciencesandPsychology,LiverpoolJohnMooresUniversity,Liverpool,L33AF,UK

eInstituteofPsychology,Health&Society,LiverpoolUniversity,L35DA,UK

h i g h l i g h t s

•Weproposeanintra-neuralmicrostimulationsystemfor7TfMRIandMEG.

•Thiscustom-builtsystemremovesissueswithexistingequipment.

•Itprovidesefﬁcientwork-ﬂowandimprovedparticipantcomfortandsafety.

•Stimulatingsinglemechanoreceptorsevokesactivityin7TfMRIandMEG.

•ResponsestounitarystimulationareshownfortheﬁrsttimeinMEG.

a r t i c l e i n f o

Articlehistory:

Received1March2017

Receivedinrevisedform19June2017 Accepted19July2017

Availableonline23July2017

Keywords:

Instrumentation Stimulusgeneration Low-noiseampliﬁer Nervestimulation Magnetoencephalography

Functionalmagneticresonanceimaging Ultra-highmagneticﬁeld

Human

Microneurography Tactile

Touch

Low-thresholdmechanoreceptor

a b s t r a c t

Background:Intra-neuralmicrostimulation(INMS)isatechniquethatallowstheprecisedeliveryof low-currentelectricalpulsesintohumanperipheralnerves.SingleunitINMScanbeusedtostimulate individualafferentnerveﬁbresduringmicroneurography.Combiningthiswithneuroimagingallows theuniquemonitoringofcentralnervoussystemactivationinresponsetounitary,controlledtactile input,withfunctionalmagneticresonanceimaging(fMRI)providingexquisitespatiallocalisationofbrain activityandmagnetoencephalography(MEG)hightemporalresolution.

Newmethod:INMSsystemssuitableforusewithinelectrophysiologylaboratorieshavebeenavailable formanyyears.WedescribeanINMSsystemspeciﬁcallydesignedtoprovidecompatibilitywithboth ultra-highﬁeld(7T)fMRIandMEG.Numeroustechnicalandsafetyissuesareaddressed.Thesystemis fullyanalogue,allowingforarbitraryfrequencyandamplitudeINMSstimulation.

Results:UnitaryrecordingsobtainedwithinboththeMRIandMEGscreened-roomenvironmentsare comparablewiththoseobtainedin‘clean’electrophysiologyrecordingenvironments.SingleunitINMS (current<7␮A,200␮spulses)ofindividualmechanoreceptiveafferentsproducesappropriateandrobust responsesduringfMRIandMEG.

Comparisonwithexistingmethod(s):Thiscustom-builtMRI-andMEG-compatiblestimulatorovercomes issueswithexistingINMSapproaches;itallowswell-controlledswitchingbetweenrecordingandstim- ulusmode,preventselectricalshocksbecauseoflongcablelengths,permitsunlimitedpatternsof stimulation,andprovidesasystemwithimprovedwork-ﬂowandparticipantcomfort.

Conclusions:WedemonstratethattherequirementsforanINMS-integratedsystem,whichcanbeused withbothfMRIandMEGimagingsystems,havebeenfullymet.

∗ Correspondingauthorat:TheSirPeterMansﬁeldImagingCentre,Schoolof PhysicsandAstronomy,UniversityofNottingham,Nottingham.NG72RD.UK.

E-mailaddress:[email protected](P.M.Glover).

1. Introduction

Intra-neuralmicrostimulation(INMS)isatechniquebywhich sensory nerve ﬁbres can be stimulated electrically by deliv- ering microamperes of current through an electrode inserted http://dx.doi.org/10.1016/j.jneumeth.2017.07.016

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intoa peripheralnerve duringmicroneurography(Ochoa,2010;

Torebjörketal.,1987).Itistypicallyperformedinconscioushuman participantstoevokesyntheticpercepts,andsingleunitINMScanbe usedtostimulateindividualafferents(OchoaandTorebjörk,1983;

TorebjörkandOchoa,1980;Vallbo,1981;Vallboetal.,1984).Here, asinglemechanoreceptiveafferentcanbeexploredandcharac- terisedphysiologically,andthenstimulatedelectrically.Currents of∼1–3␮Aproduceacleartactilesensation(e.g.avibrationsen- sationfroma fast-adaptingtype 1(FAI)afferent anda pushing sensationfroma slowly-adaptingtype 1(SAI)afferent)and this syntheticsensoryprojectedfieldcorrespondswelltothephysiolog- icalreceptivefield(Vallboetal.,1984).Sinceperceptuallydistinct, conscioussensationscanbeelicitedfromindividualmechanore- ceptiveneuronesinisolation(singleunit),thecontributionofthe differentmechanoreceptorclassestotactilesensationcanbestud- iedindependentlyina‘quantal’manner.Itispossibletocombine singleunitINMSwithneuroimagingtoexploreandcontrastthese quantalsignals.Theapproachfirstinvolvesmicroneurography,the recordingofimpulsetrafficinasingleprimaryafferent,resulting frommechanicalstimuliappliedtoitsreceptivefield.Thisisfol- lowedbyINMS,toselectivelyactivatethesameafferent.Avariety ofelectricalinputpatternscanthenbeusedtoprobesubsequent centralnervoussystemresponsesduringneuroimaging.

SingleunitINMSisinstarkcontrasttotranscutaneouselectri- calstimulationofperipheralnerves,duringwhichlargenumbers ofdifferentafferenttypesarenon-selectivelyrecruited,thuspro- ducingthesensationofparaesthesia(Burke,1993;Johnson,2007).

CombiningsingleunitINMSwithneuroimagingallowstheprecise assessmentofthebrain’sresponsetotactilestimulationinavery controlledmanner.WehavepreviouslyusedINMStostimulatesin- gleunitsinconjunctionwithfMRI(SanchezPanchueloetal.,2016;

Trulssonetal.,2001)andincombinationwithelectroencephalog- raphy(EEG)(Kellyetal.,1997)toassessfunctionalcentralnervous systemresponses.SanchezPanchueloetal.(2016)performedINMS during7TfMRI,butnostudytodatehasprogressedtousingmore complexorpatternedstimuli,norhavetheypresentedasufﬁcient samplesizeforseparateanalysisbyreceptorclass.

TheapproachofINMSwithneuroimagingpresentsseveraltech- nical problems in terms of compatibility, the ability to collect sufficientlylownoiserecordingsfromneurones,andparticipant safety.InfMRIrecordings,theswitchingofmagneticfieldgradients maybesufficienttogeneratecurrentsinthelongcablesrequiredto connecttheparticipanttonon-MR-compatiblestimulationequip- ment (McGlone et al., 2002; Sanchez Panchuelo et al., 2016;

Trulssonetal.,2001).Ourpreviousresearchusedacommercially- availablestimulatorsystem(ADInstruments,CastleHill,Australia) forINMSintheMRscanner.However,sincethissystemwasnot designedspeciﬁcallyforthispurpose,itprovidedlimitedfunction- alityintheboththelevelofstimulationprecisionwhichcouldbe presented(stepsof1␮A,ratherthantherequired0.1␮Aprecision) andinthepulseswhichcouldbegenerated(onlypre-programmed, simplepulsetrainscanbedelivered).Additionally,manualswitch- ingbetweenmicroneurographyrecording and INMSintroduced problems,includinglongwaittimeswhenswitchingbacktounit recordings(duetoampliﬁeroverloadafterstimulation),andaddi- tionalpreventativechanges totheequipmentwererequiredto ensurethatnoextraneouscurrentswerepassedthroughtheelec- trodetotheparticipant(SanchezPanchueloetal.,2016).InEEG andMEGrecordings,reed-relaysemployedtodeliverstimulation inconventional INMS equipment,producestimulation artefacts whichlimitstheiruse(personalobservations).Todate,nostudy hasdemonstratedtheuseofMEGtostudyINMSinducedresponses.

Here,wedesignedanINMSsystemthatwouldbecompatible foruseinarangeofneuroimagingmethods,speciﬁcallyincluding 7TfMRIandMEG.Thissystemovercomespreviousissuesbyallow- ingeaseinswitchingbetweenrecordingandstimulating,prevents

extraneouselectricalcurrentdischarge,andallowsthedeliveryof customisablepatternsofstimulation.

2. Methods

Thissectionis dividedintotwo parts.First, wedescribethe designspeciﬁcationoftheINMSsystem,andfollowthiswiththe detaileddesign.

2.1. Designspeciﬁcation

TheprimaryspecificationforanINMSsystemwasthatitshould provide a safe connectionto humanparticipants whilstin use withinthescanners.Itmustconformto,orexceed,specificationsfor anInternallyPoweredMedicalEquipment,AppliedPart(AP)classi- fied,typeBF(floatingpatientconnection)asoutlinedinIEC60101-1 (Medicalelectricalequipment−Part1:Generalrequirementsfor basicsafetyandessentialperformance).Inaddition,particularand collateralrequirementsforsafeoperationofMedicalEquipment ispertinenttothedesignoftheINMSsystemsuchasIEC60101- 2-33(MagneticResonanceEquipment),IEC60101-2-26(EEG)and IEC60101-2-10(NerveandMuscleStimulators).TheINMSsystem wasnotrequiredtobesuitableforusewithanyotherequipment requiringelectricalconnection,suchasEMG,electrocardiography, orotherstimulusdevices.

The INMS system was designed to be capable of operating safely in the presence of high static magnetic fields (7T MRI), largeswitchedmagneticfields,andhighradiofrequency(RF)fields withinthemagnetbore.Conversely,thesystemrequiredthatINMS shouldhavenodetrimentaleffectonthequalityoftheimageand functionaldatarecordedbyMRIandMEGscanners.ForbothMRI andMEGtherewasarequirementthatthereshouldbeno‘magnetic signature’,whilstrelated,thisrequiresdifferentdesignfeaturesfor thetwoneuroimagingmodalities.Inaddition,althoughtheMEG environmentimposesnoparticularsafetyconditionsontheINMS system,theMRscannerhasthepotentialforsevereelectricalinter- ferenceeffects,whichshouldbeaddressed.

Considering MR safety, the INMS system was designed for useprimarily withinthe Philips7T Achieva MR scanner(Best, Netherlands)equippedwithaNovaMedicalheadvolumetrans- mitcoil(Wilmington,MA). Thisset-upreducesthelevelsofRF presentcomparedtoaconventional3TMRscanner,asthehead- stageamplifierunitismountedontheforearm,whichisnotwithin theRFcoilat7T,unlikea3TMRscannerwhichhasabuilt-inbody RFcoil.Thehead-stageamplifier requiresstandardcomponents withminimalferrouscontent(i.e.usingsurfacemountdevices), sothatthereisnoeffectonscanquality.Ifthosepartsofthesys- teminproximitytotheborearenotconsideredmagneticwhen subjectedtothestandarddeflectiontest,theINMSsystemcanbe designatedasMR-compatible.

Connectionoftheamplifiertothepatientviatheelectrodescre- atesacircuitalloop,which cangenerateanelectromotiveforce (EMF)inthepresenceofswitchedmagneticfields(audioorradio- frequencies).Lowfrequencymagneticfieldsareusedintheimaging process should not induce voltages in the loop which disturb thestimulator,potentiallyproducingfalse stimulior shocks.RF (300MHzat7T)couldgeneratelocalheatingduetoantennaeffects oftheelectrodesandleads.LocalSpecificAbsorptionRate(SAR)lev- elsinthelimbshouldnotbeexceededwiththedeviceconnected.

Inadditionthereshouldbeprotectionagainstlocalheatingofthe tissueroundtheelectrodeinthepresenceofhighRFpower.

FortheMEGenvironment,therearenofurtherparticipant-or equipment-relatedsafetyissuesnotalreadycoveredinthedesign forMRIcompatibility.TheINMSsystemwastestedinaCTF275 channelMEG scannerequippedwithDSQ3500acquisitionelec-

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tronics(Coquitlam,BC,Canada).IntheMEG environment,there isarequirementfornoparticipantmovement,andcareistakento supportthearmtoensureanyisminimised.Therefore,anyminimal ferrouscontentoftheampliﬁerhead-stagecircuitboardisnotan issueforMEGrecordings.Itisdesirablethatthecurrentloopgen- eratedbytheINMSstimuluscurrents(<10␮A)enteringthewrist throughtheelectrodesdoesnotgenerateamagneticdipoleofsufﬁ- cientintensitytoeitherbedetectedbytheMEGsystemorcancelled bygradiometersandlocalisationmethods.

Thereweretwoprincipalfunctionaldesignobjectivesforthe INMSsystem:(1)toperformmicroneurography,enablingplace- ment of the electrodes in the median nerve and recording of unitaryneuronalactivityinthe‘Amplify’mode;(2)todeliverapro- grammedsequenceofpulsestimulisynchronouswiththescanner acquisitionwheninthe‘Stimulate’mode.Inaddition,aspeciﬁc requirementwastheabilitytoswitchbetweenthesetwomodes ofoperationsimplyandquicklyduringmechanoreceptiveafferent testing.Itwasessentialthattheswitch-overshouldnotproduce anyelectricalshocksandcouldbecontrolledremotelybytheoper- ator even whenthe participantis within thescanner.Previous attemptsatthisexperimentwithcommercialequipmentrequired theuseofmechanicalswitches,whichincombinationwithlarge cablecapacitancescouldcauseadischargewheninsidethenerve, producinganunpleasant sensationfortheparticipant(personal observations).Furthermore,theampliﬁerinthecommercialsys- temwasoverloadedbytheswitchingprocess,thustheexperiment wasdelayedwhenswitchingbackto‘Amplify’mode.Thekeyfunc- tionsoftheequipmentwerethatitshouldbecontrollablebythe operatorviaa remoteMRcompatibleboxforeaseandspeedof operationduringanexperiment.

Theampliﬁerhead-stagewasdesignedtohavehighimpedance and low-noisewhen connectedtotheelectrodes (having resis- tiveandcapacitiveimpedancesof100–500k).Thenervesignals neededtoberecordableandbefedbacktotheoperator,bothviaan audiochannelandvisuallyonascreen.Thesystemwasdesignedto allowabasiclevelofspikeanalysisinordertoassistinidentifying afferentﬁbretypes(e.g.fast-orslowly-adapting).Thehead-stage wasdesignedtobesmallandlightenoughtomountonabodysite neartheelectroderecordingsite,inordertomakeconnectionsand cablestotheelectrodesasshortaspossible.Theelectronicswere screenedfromRFandotherelectromagneticinterference,withthe screennotcausinganymovementofthehead-stageelectronics duetoinducedcurrentscausedbygradientswitching,asmove- mentcoulddislodgetheelectrode,resultingintherecordingfrom theafferentbeinglost.

Thestimulatorwasdesignedtobeananaloguecurrentampliﬁer inordertodeliveracompletelyﬂexiblepatternofstimulipulses.

Thepulseswereunipolar(switchablepolarity),bipolarorD.C.bal- anced.Thestimuluspulsewastypically200␮sdurationwithpeak currentsofupto200␮Aand0.1␮Aresolution.Themaximumcom- pliancewassettobeoforder30Vinordertodrivehighimpedance electrodes.Inpractice,lessthan10␮Aofcurrentwasused,witha voltageoftheorderof1–2V.Thesystemwasdesignedtomonitor thecurrentandvoltagedeliveredtotheelectrodesforbothtest purposesandforimpedanceestimation.

2.2. Designindetail

Fig.1showsthefunctionalblocksoftheINMSsystemdevel- opedtoachievethedesignobjectives.Theﬁrstblockcomprisesthe computercontrollerdevices:scannercomputers(forMEGorfMRI);

acomputertogeneratesynchronisationpulsesfromthescanner andprovideatriggertotheINMSsystemtoinitiateitsstimulation sequence(usingPresentationsoftware;NeurobehavioralSystems, Berkeley,CA);andahostcomputerfortheINMScontrolandsignal recording.Thesedevicesareallpositionedoutsideofthescreened

roomoftheMRIorMEGscanner,haveacommonground,andare notisolated.

Opticalﬁbrespassthroughtothescanroomsandallpiecesof equipmentareisolatedfromground.Theseprovidetheparticipant isolationandallequipmentinsidethescanroomrunsoffrecharge- ablebatteries.TheUSBisolationisprovidedbyaCorning(Hickory, NC)USB3.0opticalcable,whichwasmodiﬁedtoremovethecable

“phantom”powersupplytotheisolatedend,suchthatthepower totheUSBreceiverelectronicswasreplacedbyabatteryderived supply.Thisprovided>10Gisolationatatested500Vforalliso- latedunits.TheinterfaceunitresidedinsidetheMRscreenedroom asfarfromthemagnetaspossible.

Within the interfaceunit a USBhub (TS-HUB3K,Transcend InformationInc.,Taipei,Taiwan)providescommunicationsfora numberofdevices:aNationalInstruments(Austin,TX)USB6216 providesallanalogue and digitalinput andoutput functions;a PIC32USBAudiointerface(MicrochiptechnologyInc.,Chandler, AZ);andanArduinoUnoR3(www.arduino.cc)dedicatedtothe controloftheremotedisplay.A10mcablerunsfromtheinter- faceboxtothescannerbed(lengthdefinedbythelayoutofthe MRIscanroom),withanin-linestimulatorunit2mbeforetheunit containingthehead-stageamplifier.Asthestimulatorunitcon- tainsnomagneticorelectromechanicalpartsitcanbeplacednear theMRImagnetorMEG.Thislocationallowsthereductionofthe cablelengthbetweenstimulatorandelectrodestoaminimum,and thuspreservespulseshapeandminimiseslikelihoodofanyresid- ualcharge.Onlythestimulatorandhead-stageunitsaredefinedas MRI-compatiblewithintheboreofthemagnet.Theremotecontrol boxandpiezoloudspeakerarenominallynon-magneticandcan beusedoutsidetheboreofthemagnetduringmicroneurography.

Theseareplacedfortheeaseofuseoftheoperator.

Thekeydesignobjectivesandnoveltyofthisworkarerealised withthedesignoftheampliﬁerandstimulatorunits.Thesuccess ofthesystemreliesontheabilitytoswitchbetweenrecording andstimulationfunctionssafelyandquickly,withoutcompromis- ingthe performanceof either.Theswitching functionrelies on theadoption of opto-isolated FET analogueswitches (H11F1M, FairchildSemiconductorCorp.,Phoenix,AZ).Thesedeviceshave alowon-resistance(200)andahighoff-resistance(>100M), which result in a smooth transitionbetweenthe two withthe appropriatelyshapedLEDcurrentdrive.Thisensuredanyresid- ualcurrentsweredissipatedslowlyduringswitchoverbetween functions.Forsafety,thecurrentcarryingcapacitywaslimitedto 500␮A,stillwellabovethemaximumvaluerequiredfortheINMS application.

Fig.2showsasimplifiedcircuitdiagramoftheamplifierand stimulatorunits.Theopto-FETswitchesareshownwithouttheir LED drives for simplicity.The switches werearranged in three groups:amplifierswitch(AS)whichconnectstheamplifierfront- endtotheelectrodes;stimulatorswitch(SS)whichconnectsthe stimulatordrivetotheelectrodes;andshortswitches(SH)which shortcircuitthestimulator.ASandSSswitches arenever onat thesame timeand theSHswitches areonin amplifymodeto reducepotentialnoisebeinginjectedintothefrontend.Notethat the‘ground’electrodeisonlyconnectedthroughR_Gtothe‘ground’

oftheelectronicsattimeofampliﬁcation.Thiskeyfeatureensures thatreturncurrentsduringstimulusdriveallpassthroughtheref- erenceelectrode,andthusallowsadifferentialdrivestimulusto beused.Hence,a30Vcompliancecanbeachievedwithonly15V powerrails.

Thefront-endoftheamplifieruseslow-noise(6.5nV/√Hzand 0.8fA/√Hzatf=1kHz)JFEToperationalamplifiers(OPA2141,Texas Instruments,Dallas,TX)arrangedasadifferentialamplifier.The inputsare D.C.coupledwithanintegratorfeedbackstage(time constant=0.1s)arrangedtogiveafastreturntozeromeanoutput.

Thegainofsubsequentstagesisarrangedtogiveanoverallhead-

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Fig.1.INMSsystemdiagram.

Allequipmentwithinthescreenedroomisopticallyisolatedforparticipantsafety.Onlythestimulatorandhead-stageunitsareapprovedforusewithintheboreofthe7T magnet.Thepiezoloudspeakerandremotecontrolunitarenon-magneticandcanbeusednearthemagnetduringparticipantpreparation.Theinterfaceunitandbattery, whilstnotprojectilehazards,arenotdesignedtoworkinhighmagneticﬁelds,andaresituatedattheedgeofthescreenedroomarea.Asimilarset-upisemployedwithin theMEGsuite.Theaudiochannelmay,ifpreferred,betakenfromthehostcontrolcomputerandplayedthroughspeakersplacedatthedoorsofthescanroom.

stageampliﬁergainof200withabandwidthfrom10Hzto10kHz.

Theanalogueoutputoftheampliﬁerunitissampleddirectlybythe USB6216atarateof40kHz.Thepowersupplyrailstotheampliﬁer front-endareswitchedononlywhenthesystemisinamplifymode.

Thestimulatorelectronicsusestandardoperationalamplifier deviceswhicharecapableofdriving200␮Aoutputsintoawide rangeofimpedances.Thedriveramplifieremploysacurrentsense resistor (R_S) and positive feedback to the positive terminal of adifferentialamplifiertoachieve atrans-conductanceamplifier function. The drive potential across the electrodes is differen- tialandthisisachievedbyaninvertingvoltagefollowerwitha seriesimpedanceofRS.Auseablebandwidthof50kHzintotyp- icalelectrode impedancesis obtainedwith theamplifier being unconditionallystable underallloadconditions.The SHswitch isusedtotestthecurrentdrivebeforeconnectionsareroutedto theparticipant.Thecurrentandvoltageappliedtotheelectrodes aremonitoredandsampledfortestpurposes.Whilstitwouldbe idealtomonitortheseascloseaspossibletotheelectrodesthey aremonitoredonthestimulatordriverboard.InINMS,theactual currentusedisdeterminedbythethresholdofinducedsensation, andthesemeasurementsareonlyusedasaguidesoabsolutepreci- sionisnotrequired.Theimpedanceoftheelectrodesiscalculated anddisplayedasaguidetotheoperatorasrequired.Thepower supplytothestimulatorunitcanbeswitchedintohighorlowvolt- agesdependingonselectionofhighorlowcompliancemodes,or offwhennotrequired.Switchingthefront-endcircuitsoffwhen notrequiredreducesthechanceofinstabilityoroscillations,plus savesbatterylifetime.Powerfortheamplifierandstimulatorunits isderivedfromfour9V300mAhPP3NiMHrechargeablebatteries

arrangedtogiveupto±18Vsupplies.Thesegiveatleasttwodays intensiveuseoftheINMSsystem.

ToensurecompatibilitywiththeMRIscanner,theamplifierand stimulatorunitshavesuitablescreeningandRFprotection.5k resistorsareplacedin-linewiththeelectrodeconnectionsinternal tothehead-stageinordertoreducethemagnitudeofRFinduced currentscirculatingthroughtheelectrodes,participant,andampli- fier.Inaddition100pFRFtrapsarepresentontheconnectionsto theboardinordertopreventRFsignalsfromaffectingtheoper- ationofthestimulator.Hence,the5kresistorsandcapacitors formalow-passfilterasseenbyanyRFpickupontheelectrode wires.Theadditionalcapacitancedoescompromisethedelivery ofcurrenttotheelectrodeslightlybutcouldberemovedifMRI usagewasnotrequired.RFtrapsareusedattheinterfaceendof thecables toreducethelikelihoodofcommon-modeRFsignals passingdownthecable.TheinternalshieldingofthePVCampli- fierandstimulatorhousingsisachievedbyusingacoppercoated non-wovenpolymerfabric.Thisgivesgood RFperformancebut hasarelativelylowconductanceatlow-frequencies.Eddycurrents inducedbymagneticfieldswitchingormovementsaresubstan- tiallyreducedcomparedtouseofcopperfilmscreens.Inaddition thegroundplaneoftheamplifierboardis carefullydesignedto minimiseeddycurrenteffects,whichcouldleadtovibrations.

Alldevices are connected through theisolated USB and are controlled by software custom-written in MATLAB 2015a (The Mathworks, Natick,MA),speciﬁcallyusing theDataAcquisition Toolboxforcontrolofthe6216NiDAQ.Agraphicaluserinterface based front-end (Fig. 3) allows control of all functions, auto- matically ensuring the correct sequence of power supply and

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Fig.2. Ampliﬁerandstimulatorcircuitelementsshowingswitching.

Theampliﬁerswitch(AS),stimulatorswitch(SS),andshortswitch(SH)areopto-coupledFETanalogueswitches.Thehead-stage,stimulatorandinterfaceblocksareshown inonediagramhereforsimplicity,butareinseparatephysicalunits,asdescribedinFig.1.

Fig.3. Softwarefrontpanel.

Functioncontrolbuttonsareontheleft.Stimuluscontrolandfunctionsareatthetop,andamplifierfunctionsareinthelowerhalfofthepanel.Ascrollingdisplayshows thelatest2sofacquiredsignalwhichcanbedirectedtotheaudiochanneland/orbufferedtoarecordingfile.Thedatashownillustrateslow-noiserecordingsfromtypical electrodesinsalinewithinthebottlephantom.Inthisgraphicthedisplayissettoauto-scalewhereasthescaleisusuallyfixedto±50␮Vduringexperiments.Aslider controlsthesetpeakcurrentvaluewhichcanbeupdatedfromthecomputerorcontrolledbyarotaryencoderontheremotecontrolunit.Otherfunctionsandpanelsare describedinthetext.

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Fig.4.DetectionofunitsinMEG.

(A)ThesubjectisseatedinthechairslightlyloweredbelowthenormalMEGscanposition.Thisisfortheircomfortduringset-upoftheelectrodesandcharacterisationof theunitsfound.(B)Theamplifierhead-stageisshowntapedtotheforearmwiththeelectrodesvisible.Theupperelectrode(redterminal)isthemediannerveelectrode withthereference(currentreturn)underneathplacedsubcutaneously.Theblackwireisattachedtotheunseen‘ground’electrode.Thestimulatorboxisnotshowninthese photographsbutispositionedonthefloorclosetothesubject’sfeet.Theoperatorislocatingtheafferentbystrokingortouchingtheskinwithamonofilamentwhilstlistening totheaudioandobservingthetraces,asshowninFig.5.Thesubjectiswearingtheirindividualhead-cast.Thewiresintotheheadcastareactivelydrivenlocationmarkers forheadregistration.ForfMRIthecablesgoingtotheamplifierboxareroutedsuchthattheycannottouchthesubjectandcreateanRFantennaloopwhichcouplestothe body.

opto-switchswitching.Acquisitiondataissampledat40kHz,ﬁl- teredwithinaselectedband(usually300–5000Hz),recorded(if required),bufferedtoarollingchartdisplay,andstreamedtothe hostcomputer’saudiodevice.Thelatterisuseful,astheoperator canthenroutetheaudiofeedtoeitherthePC’sowninternalaudio, externalspeakersorbackthroughtheUSBconnectiontothePIC32 audiodriverintheinterfaceunitandonwardstopiezospeaker(or operatorearpiece)asrequired.

Aspikeanalysismodulecanbebuiltintotheacquisitiondata streamifrequiredorappliedtorecordeddataretrospectively.The PCused(L540Lenovo,China)takes10mstoacquire,ﬁlter,record anddisplaytheequivalentof50msofdata,soanyfurtheranalysis shouldﬁteasilywithintheremainingtime-frame.

In‘Stimulate’modepowerisappliedtothestimulatordriver withtheelectrodeamplifierpowereddown.Theopto-FETsSSare closedwithSHandASswitchesleftopen.Hencestimuluscurrentis routedtoconnectthedrivertotheelectrodes.Software-generated pulsesequencesareproducedat50kHzsamplingrate,fromadata filewhichdefinestheoverallprotocol.Thereiscompleteflexibility inthewaveformsgeneratedandthisopensthepossibilityofapply- inganypatternofINMS(e.g.onethatmimicsthenaturalfiringof mechanoreceptiveafferents).Theremote-controlboxallowsthe operatortocontroltheimportantfunctionsofthesoftware,such astheAmplify/Stimulateswitchingfunction,controllingapplied current,andinitiatingburstsofpulsesfordeterminingthecurrent thresholdforaperceivedsensation.Toassisttheoperator,avisual displayofthescreenoftheINMScontrolcomputerisprojectedinto thescanroom(astandardfeatureofmostMEGandfMRIlabora- tories),aswellasanalphanumericdisplayontheremotecontrol boxgivinginformationincludingtheelectricalcurrentleveland impedance.ThelatterdisplayisachievedusingtheSPIportofthe Arduinoboard.SoftwareiswrittenfortheArduinotointerpreta stringoftextsenttoanattachedCOMportfromMATLAB,andto putitonthescreen.

Two15mmtungstenelectrodes(oneinsulatedandoneuninsu- lated)(FHC,Bowdoin,ME,USA)wereinsertedthroughandglued tothecapofapolythenebottle(200ml,50mmdiameter).Asil- verplated‘ground’electrodewirewasalsoattachedtothecap.

Thebottlewasﬁlledwithsaline(0.5%byweightNaCl)suchthat theelectrodeswerecovered.Leadswereattachedinordertoplug

intotheampliﬁerhead-stage.Thisbottlephantomwasusedfor ampliﬁernoiseandstimulatortestspriortoexperimentstaking place.Inadditionthe‘ground’electrodewasconnectedtocopper tapecontactsontheoutsideofthebottle.Hencefortestpurposes onlyanoperatorcouldholdthebottlephantomasareliabletestfor electromagneticinterferenceinthescannerenvironments.Inaddi- tion,thesetestscansalsoallowedtheassessmentofsignal-to-noise measureswheninsideandoutsideofthescanner.

3. Experimental

Ethics for experiments on human participants using this equipment and associated imaging protocols was granted by TheUniversityof NottinghamMedicalSchoolEthics Committee (E09022012,ExperimentalProtocol,ParticipantInformationSheet, informedConsentFormandMRIsafetyquestionnaire).Allpartic- ipantsweregivendetailedinformationabouttheprocedureand signedawrittenconsentform.Allprocedureswereconductedin linewiththeDeclarationofHelsinki.

Foursubjectsparticipatedin sessionsinvolvingcharacterisa- tionofasinglemechanoreceptiveunitfollowedbyINMSofthis unitduringeither7TfMRIorMEGacquisition.ForthefMRIexperi- ment,theparticipantlayonthescannerbedoutsidetheboreofthe magnet,whilefortheMEGexperimenttheparticipantwasseated justbelowthelevelofthesensorhelmetandworeacustom-ﬁtted head-cast(ChalkStudios,London,UK)topreventheadmovement (Liuzzietal.,2016;Meyeretal.,2017),asshowninFig.4.Inour MEGsystem,therewasaninterferencesignalconductedthrough theMEGearthwhichdoeselevatethenoiseﬂoorasmeasuredby theINMS.Bysettingupthesubjectwiththeirheadremovedfrom theMEGhelmetandswitchingoffthepowertothebedandgantry controller,thisnoisewasreducedtoanacceptablelevel.

Aninsulatedtungstenelectrode (FHC,Bowdoin, ME;length:

15mm) was inserted percutaneously into the median nerve, approximately3cmproximalfromthewristfold,tostimulateand recordfromsinglemechanoreceptiveafferentunits.Asimilar,un- insulatedelectrodewasinsertedjust undertheskin4cmaway.

Onceasingleunitrecordingwasidentifiedinamplifiermode,it wascharacterisedbaseduponitsresponsecharacteristicstostim- ulationwithamonofilamentasfast-adaptingtype1(FAI)ortype2

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(FAII),orasslowly-adaptingtype1(SAI)ortype2(SAII)(Vallboand Johansson,1984).Unitarydatawerebandpassfiltered(0.3-4.5kHz) forvisualisationonlineandrecorded,usingcustomsoftwarewrit- teninMATLAB.Nervedatawereonlyregisteredwhenthesubject waspositionedoutsidetheboreofthe7Tscannerorjustbelowthe sensorhelmetintheMEG.Offline,thedatawereprocessedwitha bandpasssmoothingfilterinMATLAB(0.16-2.5kHz)toremove highfrequencyartefacts fromMEGrecordings,and exportedto Spike2software(CED,Cambridge,UK)foridentificationofspikes basedonacombinationofamplitudeandspikeshape.TheINMS systemwasthenswitchedto‘Stimulate’modeandpositivecur- rentpulsetrainsof60Hzfor1sweremanuallydeliveredbythe operator,incrementingthecurrentuntilasensationwasreported bytheparticipant,oruntil8␮A.Iftheperceivedlocationofthe electricallyelicitedsensationexactlymatchedthelocationwhere mechanicalstimulationoftheskingeneratedaresponse(Torebjörk etal.,1987),thestimulationprotocolwasperformed.Atthispoint theparticipantwasmovedeitherintotheboreoftheMRscanner, orintotheMEGsensorhelmet.Aftermovingtheparticipant,INMS wasre-evaluatedtoensurethattheperceptualresponsehadnot changedorwaslost,andthestimulationcurrentwasadjustedif necessary.

7TfMRIscanningprotocolsandINMSpulsedeﬁnitionsareiden- ticaltothosereportedinSanchezPanchueloetal.(2016),except fromthespatialresolutionwhichwashereincreasedto1.25mm isotropic.Aburstof 60Hz 200␮scurrent pulsesof1s(0.5son and0.5soff)wererepeated8timesina sequence,followedby arestperiodof23s.Thiswasrepeatedforatotalof8cycles.The MEGprotocolconsistedofa1sburstof60Hz200␮scurrentpulses followedbya10–10.5(arandomlyselecteddelayperiod)second restperiod,repeatedfor80cyclesinblocksof10cycles.MEGdata wererecordedat1200Hzusinga275channelMEGsystem(CTF, Coquitlam,BC),withthereferencearraysettoasynthetic3rdorder conﬁguration.

AftercompletinganfMRIstimulationcycle,oroneblockofMEG stimulation,thesensationwascheckedbyaskingtheparticipantif thesensationhadchangedinintensityorquality.Ifthesensation hadfadedduringstimulationduetominutedislodgementsofthe electrode,thestimulationcurrentwasadjustedtogiveacompara- bleintensityofstimulation,withthesamequality(Torebjörketal., 1987).

fMRIimagedatawereanalysedusingaGeneralLinearModel inmrTools(http://www.cns.nyu.edu/heegerlab).Statisticalmaps wereformedbythresholding(Z>3.08)afterfalsediscoveryrate correctionandprojectedontoaﬂattenedrepresentationofthecon- tralateralcentralsulcustocomparethespatiallocalisationwith previouslyacquiredﬁngersomatotopyinprimarysomatosensory cortex(formoredetails,see(SanchezPanchueloetal.,2016)).

MEGdatawerevisuallyinspectedforartefacts,suchasSQUID resets,andmagnetomyographicormagnetooculargraphiccontam- ination,wheretrialscontainingexcessiveartefactswereremoved.

Toallowforthereconstructionofsensordataatthesourcelevel, threeheadpositionindicators(placedonthenasion,andleftand rightpre-auricularregionsoftheparticipant’sface)wereperiod- icallyenergisedtolocatetheparticipant’sheadwithinthedome.

Thecoils’positionsrelativetothebrainweredeterminedduring themanufactureoftheheadcasts.Datawerefrequencyﬁlteredinto thebetaband(13–30Hz)andreconstructedinsourcespaceusinga beamformer(RobinsonandVrba,1999;VanVeenetal.,1997),lead ﬁeldswerecomputedusingadipoleapproximation(Sarvas,1987) inconjunctionwithamultiplelocalsphereheadmodel(Huang etal.,1999).Dipoleorientationwasdeterminedusinganon-linear searchforoptimumsignaltonoiseratio(SNR).Tolocatethesource, a pseudo-T-statistical imagewas generated acrossa 2mm iso- topicgridspanningtherighthemisphere’spre-andpost-central gyri(activewindow:0–1safterstimulusonset;controlwindow:

8–9safterstimulusonset).Afterdeterminingthesourcelocation byﬁndingthemaximumabsolutepseudo-Tscore,1–150Hzﬁltered sensordataweresourcereconstructedatthetargetlocationanda time-frequencyspectrogram(TFS)generated.

4. Results

A number ofexperiments and measurementsweremade to ensureanddemonstratethesafeworkingoftheequipmentbefore usewithhumans.Theresultsindetail,methodsanddiscussioncan befoundintheDatainBriefdocumentaccompanyingthispaper.

Insummary,duringfMRIsequences,notemperatureriselocalto theelectrodesduetoSARheatingeffectswasdiscerned(±0.1K).In addition,measurementsshowedalevelofcurrentinjectiondueto magneticﬁeldgradientswitchingwaslessthan10nA,wellbelow thelevelwhichwouldinﬂuenceappliedstimulationpatternsor causepain.

Test recordings from electrodes with known impedances demonstrated an acceptable low-levelof noise, with noundue noiseattributabletotheextraseriesimpedancesoftheswitches andRFprotection.Alow-noiserecordingfromthebottlephantom isshowninFig.3.ThisnoiserecordinghasanRMSamplitudeof 3.3␮V,equivalentto48nV/√

Hz,whichisin-linewithanexpected valueof41nV/√Hzfora100kresistoratroomtemperature.The lattervalueassumesanidealﬁlterandafrequencyinvariantsource impedance−neitherofwhicharetrueforthistypeofelectrode.

However,similarbaselinenoiselevelsareobservedinpractice.

Fig.3showsa4␮Acurrentpulsebeingdeliveredtothebot- tlephantomelectrodes,whichisofnegativepolaritywithrespect tothereferenceelectrode.Thepulsehasanequalisationreverse polarityof1/10peakamplitudewith10timesduration.Theasso- ciatedelectrodevoltage(topright)andimpedanceasafunction offrequency(middlerightgraph)canbeseen,withthemeasured impedanceat1kHz(printedas118.7,−59.22jk).Periodicmea- surementofelectrodeimpedancegivestheoperatoraninsightinto theconditionoftheelectrode.

Fig.4showstheprocessofsettingupthesubjectintheMEG system.Theamplifierhead-stageandelectrodearrangementcan beseen.Theoperatorisfindingthenervesignalsatthispointby lightlytouchingthehandandfingers.Whenaunithasbeenfoundit canbecharacterisedbyitsresponsetotouchorpressure.Thesetup phaseforfMRIissimilarexceptthesubjectislyingdownandno head-castisused.Fig.5showsexamplerecordingsfromboththe 7TfMRI(Fig.5Aand5D)andMEG(Fig.5Band5C)environments.

Physiologicalsingleunitrecordingsareshownfromeachtypeof mechanoreceptiveafferentfoundintheglabroushandskin.Hence, thisdemonstratesthatitispossibletoobtainclearrecordingsfrom allmechanoreceptortypesinbothMRIandMEGenvironments.

The recordingshavea low noise-ﬂoor, withvery littleexternal interferenceandeasilydistinguishableneuronalspikes.

Fig.6showsanexampleoffMRIactivationfromthestimula- tionofasingleFAIunitatthebaseoftheindexfinger,using60Hz pulses.The fMRIdatashowstatisticalactivationmaps (Z>3.08, FDR-corrected)toINMSoftheunitoverlaidontoasurfacerecon- structionofthecontralateral(right)hemisphereandonaflattened patchofthecentralsulcus.INMSactivationpatternsincontralat- eralprimarysomatosensorycortex(S1)areaareconsistentwiththe expectedspatiallocalisationfromfingersomatotopyderivedfrom atravellingwavevibrotactileparadigm(Sanchez-Panchueloetal., 2012;SanchezPanchueloetal.,2016).NodifferenceinimageSNR wasdemonstratedonphantomdata,andnosignificantdifference inthetemporalsignal-to-noiseratio(tSNR)ofthefMRIdatawas foundbetweentheINMSstimulationONperiod(tSNRgreymatter:

46±4)andOFFperiod(tSNRgreymatter:44±5)ortSNRwithno

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Fig.5.Examplesofrecordingsfromindividualmechanoreceptiveafferentsinthe7TfMRIandMEGenvironments.

RecordingsfromindividualSAI(A)andFAII(D)mechanoreceptiveafferentsinthe7TfMRIenvironment.TheSAIrecordingshowstheresponsetolong-lastingindentation witha2gmonofilamentandtheFAIIrecordingshowstheresponsetolightlyblowingontoitsreceptivefield.Therecordingsaremadewhiletheparticipantislyingonthe 7Tscannerbed,withthehead-stagepositionedontheparticipant’sarmatthebore-endofthescanner.Themagneticfieldexperiencedbytheamplifierhead-stageis∼0.5T.

(B)RecordingsfromindividualFAI(C)andSAI(B)mechanoreceptiveafferentsintheMEGenvironment.TheFAIrecordingshowsitsresponsetorepeatedlymovingawooden stickacrossitsreceptivefieldandtheSAIIshowsitsresponsetoasustainedindentationwithabluntwoodenstick.TheMEGrecordingsaremadewiththeparticipant positionedjustoutsideofthehelmetintheMEGscanner.Inallrecordings,thebarabovethetraceindicatesthetimingoftherespectivereceptivefieldstimulation.Inboth environments,amplifiernoisewaslessthan5␮VRMSwitha300–5000Hzfilterbandwidth.NotethehighernoisefloorintheMEGrecordings.

Fig.6.Example7TfMRIactivationinresponsetoINMSstimulation(60Hz)ofanFAIunitlocatedonthebaseoftheindexfinger.Dataisshownonaninflatedbrainand activitycanbeseentolocalisewithincontralateralS1(notethattheactivationposteriorofthethumbregion−redoutline-isduetoalargedrainingvein).Ontheexpanded maptheINMS-inducedactivityisshownindetailalongwithcolouredlinesindicatingthebordersofeachfingerrepresentationobtainedfromaprevioustravellingwave somatotopyexperimentinthesameparticipant.

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Fig.7.ResultsfromasingleINMSexperimentinMEG,inwhichstimulationofanSAIunitlocatedonpalmofthehandoccurred.(A)Thelocalisationoftask-inducedchanges inpower,projectedontoaninﬂatedcorticalsurface,showingthelocationofthesourcetobewithinthedorsolateralpostcentralgyrus.Theimageisthresholdat80%ofthe maximumvalueforclarity.(B)Trial-averagedtimefrequencyspectrogram(TFS)ofavirtualelectrodeplacedatthesourcelocation,derivedfromthemaximumabsolute pseudo-Tscore.

INMSsysteminplace(tSNRgreymatter:43±4),demonstrating thattheINMSsystemdoesnotalterMRimagequality.

Fig.7Ademonstrateslocalisationoftheneuronalactivityforsin- gleunitINMSofanSAIafferentinMEGlocatedinthepalmofthe hand.Here,toaidvisualisationofthesourcelocation,thepseudo-T statisticalimagehasbeenprojectedontoaninﬂatedcorticalsur- faceoftheparticipantusingtheFreesurferanalysissuite(http://

freesurfer.net).Thesourceimageshowsthatthemaximalchange inbetapowerduringthestimulationwaslocatedwithinthedor- solateralpostcentralgyrus.Thetime-frequencyspectrumplotin Fig.7Bshowstheaveragechangeinspectralpoweracrossthe80 trials.Withinthe15–30Hzband,thecharacteristicevent-related corticaldesynchronisationduringthetimeofthestimulus(0–1s) andresynchronisationaftercessationoftheINMSisseen.Note, at60Hznoclearpowermodulationspeciﬁctothetrialisseen, suggestingthatwiththeapplicationsofappropriateenvironmen- talnoisecancellationandspatialﬁlteringmethods,artefactsfrom passingcurrentfromtheINMSsystemintothenervearenegligible.

5. Discussion

Theseresultsdemonstratethesuperiorlevelofmeasurement using this specially-designed INMS system, and show that its performance is comparable to equipment used in a dedicated microneurographylaboratory.Further,thesystemprovidesoverall easeofuseinsettingup,switchingoffunction,andworkﬂow,meet- ingalloftherequirementsintermsofsafetyandusage.Speciﬁcally, low-noiserecordingsfromindividualmechanoreceptiveafferents wererecordedinboththe7TMRandMEGscanners.Thismadeit possibletosearchfor,identify,andrecordfromsingleunits,which werethensubjecttoINMS.Thesystemallowedpreciseelectrical pulsestobesentbackdowntheelectrode,toexciteasingleaffer- entandproduce aquantalsensation(cf.Torebjörket al.,1987;

Vallboet al.,1984).Thisclearperceivedsensationcontinuedto beartiﬁcially-inducedonre-stimulationoncetheparticipanthad enteredthe7TmagnetboreorMEGsensorhelmet,allowingthe combinationofsingleunitINMSwithconcurrentneuroimaging.

From a safety point of view, noincidental electrical micro- shockswere produced (cf. theprevious commercially-available systemusedinSanchezPanchueloetal.(2016))andthepresent systemprovidescompleteusercontrol,withadedicatedcontrol boxnearthemicroneurographer,aswellasfullcomputercontrol

outside thescanner.Our resultsshow thepotentialincombin- ingsingleunitINMSwith7TfMRIandMEG.Althoughnotshown here,itislikelythatsuchasystemcanbecombinedwithother neuroimaging methods, suchas EEG, electrocorticography, and functionalnear-infraredspectroscopy,aswellasotherneurophys- iologicalmonitoring(e.g.electrocardiography,eyemovements).

ThepresentsystemcanbeusedtoperformINMSduringneu- roimaging,whichgreatlyfacilitatesthestudyof,primarilycortical, responsestocontrolledinputfromsomatosensoryafferentnerves.

WeprovidetheﬁrstdemonstrationofsuchanapplicationinMEG.

Thissystemwillallowthecontributionofdifferentafferentclasses totheprocessingofsomatosensory informationthroughout the braintobeevaluated.Usinghighspatialresolution7TfMRI,itis hopedthattheactivitywithin-andbetweencorticallayersinpri- marysomatosensorycortexcanbevisualised.Similarly,usinghigh temporalresolutionMEG,theprecisecorticaldynamicsovertime inresponsetoINMSofdifferentafferenttypescanbeassessed.

Thecompatibilityofthesystemwithmultipleneuroimagingtech- niqueswillpotentiallyallowforthecombination ofdataacross methodologies,tofurtherelucidatethespatio-temporaldynamics ofcorticalsomatosensoryresponses.Resultsfromseparateexper- imentsinthesamesubjectinMEGand7TfMRI,maybecombined toaidintheunderstandingoftherelationshipbetweenlocations oftheneurovascularresponse(fMRI)andMEGinverselocalisation.

UsingtheprecisesignalgeneratedduringsingleunitINMSshould aidinelucidatinglocalisationaccuracybetweentechniques,aswell asunderstandingdetailedcorticalsignals.Presumedcompatibility withadditionalneuroimagingtechniqueswillallowsimultaneous acquisitionofneuroimagingdatasuchasEEG/fMRI,EEG/MEG,or EEG/functionalnearinfra-redspectroscopy.Furthermore,theINMS systemallowsunconstrained electricalpatternstobedelivered;

henceavarietyofdifferentfrequenciescanbetested,withthepos- sibilityofdeliveringothermorevariablepatterns,suchasthose derivedfromthenaturalﬁringofmechanoreceptors.Thesestudies willaidinunderstandingthefundamentalworkingsandconnec- tivityinsomatosensorycircuits.

6. Conclusions

Weshowtheimplementationandapplicationofadedicated systemforsingleunitINMSofmechanoreceptiveafferents,dur- ingcombined7TfMRIorcombinedMEGacquisition.Thisisthe

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first demonstration of the feasibility of performing single unit INMSmeasureswhilerecording MEGsignals. TheINMSsystem goesbeyondprevioussystems,whereitprovidessafeandeffective operation,andprovidescomparableneuronalrecordingandstim- ulationtoequivalentproceduresindedicatedmicroneurography laboratories.Usingthissystemwillenablethesmoothandefficient collectionofhighspatialandtemporalresolutiondatafromsingle unitINMSduringneuroimaging.Duetotheunconstrainedstimu- lationcapabilitiesofthesystem,furtherexperimentswillprobea varietyofartificialinputINMSpatternsandexaminethedetailed andprecisecentralresponsesgenerated.

Acknowledgements

ThisworkwassupportedbytheMedicalResearchCouncil[grant numberMR/M022722/1].ThisworkwasalsosupportedbyaRoyal SocietyInternationalExchangeProgrammeaward.We acknowl- edgethevaluableassistanceof AndrewStuartand AlanDorkes intheUniversityofNottinghamelectronicsandmechanicalwork- shops.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion, athttp://dx.doi.org/10.1016/j.jneumeth.2017.

07.016.

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