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! ! ! ! ! ! ! ! ! !!!!!!!!!!!!!!!!! ! ! UNIVERSITE!CATHOLIQUE!DE!LOUVAIN! ! ! !!!!!!!!!!!!UNIVERSITE!DE!MONTREAL! BELGIUM! ! ! ! ! ! ! ! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!CANADA! ! ! ! ! EXPLORING!THE!NEURAL!ENTRAINMENT!TO!MUSICAL!RHYTHMS!AND!METER:! A!STEADY8STATE!EVOKED!POTENTIAL!APPROACH! ! ! Sylvie!NOZARADAN,!MD! ! ! ! Promotors!:!Dr.!André!Mouraux!(UCL)! ! !!!!!!!!!!Dr.!Isabelle!Peretz!(UdeM)! ! ! Dissertation!submitted!for!the!degree!of!Doctor!of!Philosophy,!Faculty!of!Medicine! March!2013! ! 2! ! ! ! 3! ! ! Ces$années$de$thèse$font$sans$doute$partie$de$mes$meilleurs$moments$de$vie.$Elles$ont$été$pour$moi$ l’occasion$de$découvrir$une$passion$pour$la$recherche,$et$de$combiner$mes$centres$d’intérêts,$a$priori$ éloignés.$Mais$cela$n’aurait$pas$été$possible$sans$l’encadrement$optimal$dont$j’ai$bénéficié$durant$ces$ années$ Tout$d’abord,$c’est$toujours$une$rare$chance$de$croiser$sur$sa$route$un$mentors.$Je$mesure$la$chance$ que$j’ai$eue$d’en$recontrer$dans$le$cadre$de$ma$thèse.$Mes$remerciements$vont$donc$en$premier$à$ André,$ mon$ superviseur$ à$ l’UCL.$ Etre$ sa$ première$ doctorante$ est$ un$ grand$ honneur,$ dont$ j’espère$ avoir$été$à$la$hauteur.$Je$n’exagère$pas$en$affirmant$qu’il$a$été,$et$restera$sûrement,$un$modèle,$tant$ au$niveau$de$ses$très$nombreux$talents$dans$chaque$étape$du$travail$de$chercheur$qu’au$niveau$de$ ses$qualités$humaines.$Associée$dans$cette$supervision$en$équipe,$je$dois$beaucoup$à$Isabelle,$mon$ superviseur$à$Montréal,$de$m’avoir$permis$de$découvrir$au$Brams,$déjà$avant$la$fin$de$mes$études$de$ médecine,$la$richesse$de$cette$thématique$de$recherche.$Je$la$remercie$chaleureusement$pour$son$ soutien,$ses$encouragements$et$sa$confiance$depuis$le$début.$ Je$ remercie$ vivement$ les$ membres$ de$ mon$ comité$ d’accompagnement$ et$ du$ jury,$ Pascal$ KienlenM Campard,$ Philippe$ Lefèvre,$ Marcus$ Missal,$ Alexandre$ Zénon,$ Charles$ Schroeder$ et$ Marc$ Schoenwiesner,$pour$leur$lecture$attentive$de$la$thèse,$ainsi$que$pour$leurs$commentaires$et$conseils$ tout$ au$ long$ du$ parcours.$ Je$ remercie$ aussi$ Etienne,$ Julie$ et$ Alex,$ ainsi$ que$ leur$ équipe,$ pour$ leur$ écoute$et$commentaires$lors$des$journal$clubs$auxquels$j’ai$pu$presenter$mes$travaux$dès$le$début$de$ la$thèse,$ainsi$que$leur$bons$conseils.$ Je$ne$garderais$sûrement$pas$un$aussi$bon$souvenir$de$ces$années$si$je$n’avais$eu$la$chance$de$réaliser$ cette$thèse$dans$une$si$bonne$ambiance.$Je$remercie$donc$les$collègues$de$Nocions$(dont$Elisabeth,$la$ plus$ agréable$ collègue$ de$ bureau$ qui$ soit!),$ et$ de$ COSY,$ dont$ le$ sourire$ et$ la$ bonne$ humeur$ sont$ autant$de$cadeaux$que$j’ai$reçus$chaque$jour$et$qui$vont$me$manquer.$Que$de$bons$souvenirs$des$ conférences$SfN,$des$verres$du$vendredi,$de$la$campagne$de$vol…$Merci$également$d’avoir$été$les$ meilleurs$sujets$d’expérience$qui$soient.$Un$grand$merci$aussi$à$l’équipe$administrative$et$technique$ de$COSY$pour$leur$soutien$et$leur$efficacité.$J’ai$eu$la$chance$de$trouver$du$côté$Brams$une$ambiance$ tout$ aussi$ chaleureuse,$ qui$ aura$ rendu$ Montréal$ particulièrement$ difficile$ à$ quitter$ chaque$ fois.$ J’espère$avoir$encore$de$nombreuses$occasions$d’y$séjourner$et$de$collaborer$avec$le$Brams.$ Merci$aussi$à$Vincent$(aussi$un$de$mes$sujets$d’expérience$favoris!),$Arnould,$Leon$van$Noorden$et$le$ groupe$Rhythm$de$Bruxelles,$ainsi$qu’aux$chercheurs$de$la$communauté$Rhythm$(RPPW$et$PoRT)$pour$ ces$échanges$inspirants$dans$une$ambiance$amicale.$La$tonalité$de$ces$rencontres$est$à$cultiver,$et$a$ déjà$débouché$sur$de$belles$amitiés.$A$propos$d’amitié,$je$remercie$bien$sûr$aussi$Jessica$PhillipsMSilver$ et$Coralie$de$Hemptinne,$qui$toutes$deux$ont$fait$preuve$de$patience$pour$m’accompagner$dans$mes$ premiers$pas$au$labo,$au$Brams$et$à$l’UCL.$Je$remercie$aussi$chaleureusement$Marc$Crommelinck,$ éternel$ créateur$ d’enthousiasme,$ qui$ m’a$ mis$ entre$ les$ mains,$ dès$ la$ 2ème$ candi,$ des$ lectures$ de$ neuroscience$passionnantes$(que$je$ne$finis$pas$d’essayer$de$comprendre,$alias$Edelman!),$et$Bruno,$ qui$ après$ Marc$ a$ essayé$ de$ me$ les$ faire$ comprendre$ à$ son$ tour.$ Son$ enthousiasme$ ne$ cesse$ de$ m’inspirer$depuis.$ Ces$réflexions$sur$la$musique$ne$me$sont$pas$tombées$du$ciel.$Je$remercie$donc$très$chaleureusement$ ici$mes$mentors$du$côté$musique,$Françoise,$Burkard,$JeanMClaude,$Santi$et$Caroline,$qui$continuent$ de$m’inspirer$un$modèle$de$rigueur$au$travail,$de$curiosité$et$de$créativité,$qui$s’applique$si$bien$tant$ au$métier$d’artiste$qu’à$celui$de$chercheur.$ Et$bien$sûr,$un$énorme$merci$à$mon$copain$et$ma$famille$de$m’avoir$supporté$(au$sens$français$et$ anglais$ du$ terme!)$ dans$ ce$ cheminement,$ et$ pour$ toute$ la$ force$ qu’ils$ me$ donnent$ chaque$ jour. ! 4! ! ! ! 5! ! ! SUMMARY ..................................................................................................................................................... 9! FOREWORD...................................................................................................................................................11! I.!ENTRAINMENT!IN!NEURAL!SYSTEMS ..........................................................................................................13! I.1.!ENDOGENOUS!OSCILLATIONS .................................................................................................................... 13! I.1.1.$Spontaneous$neural$oscillations......................................................................................................... 14! I.1.2.$Frequency$tuning$function.................................................................................................................. 16! I.1.3.$Synchronization.................................................................................................................................. 17! I.1.4.$A$link$between$synchronization$of$oscillatory$activities$and$brain$function....................................... 17! I.2.!ENTRAINMENT!TO!OSCILLATORY!INPUTS................................................................................................... 20! I.2.1.$Synchronous$oscillation$to$sound$envelope$in$the$auditory$system ................................................... 20! I.2.1.1.!What!is!sound!envelope? ..............................................................................................................................20! I.2.1.2.!Frequency!decomposition!of!the!sound........................................................................................................21! I.2.1.3.!Temporal!coding!versus!rate!coding. ............................................................................................................23! I.2.2.$The$steadyMstate$evoked$potential$approach ..................................................................................... 27! I.2.2.1.!Nature!of!the!steadyPstate!evoked!potentials...............................................................................................28! I.2.2.2.!Frequency!tagging. ........................................................................................................................................29! I.2.2.3.!Methodological!considerations.....................................................................................................................30! II.!ENTRAINMENT!TO!MUSICAL!RHYTHMS .....................................................................................................35! II.1.!BEAT!IN!MUSIC:!A!UNIVERSAL!HUMAN!ABILITY?....................................................................................... 35! II.1.1.$Are$beat$and$meter$induced$in$all$musical$styles? ............................................................................ 35! II.1.2.$Language$and$music$rhythms............................................................................................................ 37! II.1.2.1.!Rhythm!in!speech.........................................................................................................................................37! II.1.2.2.!Music!mimicking!speech!rhythms!and!vice$versa.........................................................................................38! II.1.3.$Cultural$differences ........................................................................................................................... 39! II.1.3.1.!The!Groove!as!a!cultural!specificity..............................................................................................................40! II.1.3.2.!Meter,!binary!bias!and!integer!ratios!across!cultures..................................................................................40! II.1.4.$Human$development......................................................................................................................... 48! II.1.4.1.!Production....................................................................................................................................................48! II.1.4.2.!Perception. ...................................................................................................................................................49! II.1.4.3.!Enculturation................................................................................................................................................52! II.1.5.$Evolutionary$perspective ................................................................................................................... 54! ! 6! ! II.1.5.1.!Innateness. ...................................................................................................................................................55! II.1.5.2.!DomainPspecificity!and!interPindividual!differences.....................................................................................55! II.1.5.3.!Human!specificity.........................................................................................................................................58! II.2.!SENSORIMOTOR!COUPLING ...................................................................................................................... 64! II.2.1.$Production$of$periodic$signals ........................................................................................................... 64! II.2.2.$Perception$of$periodic$signals............................................................................................................ 67! II.2.2.1.!Models!and!empirical!evidence!on!time!processing. ...................................................................................67! II.2.2.2.!Neural!correlates!of!time!processing. ..........................................................................................................69! II.2.2.3.!Rhythm!processing!models. .........................................................................................................................74! II.2.2.4.!Beat!induction. .............................................................................................................................................81! II.2.2.5.!Neural!correlates!of!rhythm!processing.......................................................................................................86! II.2.2.6.!Tagging!the!neural!entrainment!to!beat!and!meter:!our!novel!approach .................................................103! II.2.2.6.1.!Study!1:!TAGGING!THE!NEURONAL!ENTRAINMENT!TO!BEAT!AND!METER!(Sylvie!Nozaradan,! Isabelle!Peretz,!Marcus!Missal,!André!Mouraux)........................................................................................103! II.2.2.6.2.!Study!2:!SELECTIVE!NEURONAL!ENTRAINMENT!TO!THE!BEAT!AND!METER!EMBEDDED!IN!A! MUSICAL!RHYTHM!(Sylvie!Nozaradan,!Isabelle!Peretz,!André!Mouraux)....................................................124! II.2.3.$Coupling$of$sensorimotor$periodic$signals....................................................................................... 156! II.2.3.1.!Characteristics!of!sensorimotor!synchronization!to!the!beat. ...................................................................156! II.2.3.2.!A!frequency!tuning!function!for!beat!perception!and!synchronization.....................................................160! II.2.3.3.!Neural!correlates!of!sensorimotor!synchronization...................................................................................170! II.2.3.4.!Study!3:!CAPTURING!WITH!EEG!THE!NEURONAL!ENTRAINMENT!AND!COUPLING!UNDERLYING! SENSORIMOTOR!SYNCHRONIZATION!TO!THE!BEAT!(Sylvie!Nozaradan,!Younes!Zerouali,!Isabelle!Peretz,!André! Mouraux) .......................................................................................................................................................179! II.2.3.5.!Perception!leading!to!movement...............................................................................................................212! II.2.3.6.!Movement!influencing!perception.............................................................................................................214! II.2.3.7.!Auditory!prominence!for!sensorimotor!coupling.......................................................................................216! II.2.3.8.!Study!4:!STEADY8STATE!EVOKED!POTENTIALS!AS!AN!INDEX!OF!MULTISENSORY!TEMPORAL!BINDING! (Sylvie!Nozaradan,!Isabelle!Peretz,!André!Mouraux).......................................................................................220! III.!DISCUSSION!AND!PERSPECTIVES ............................................................................................................245! III.1.!STUDY!1:!TAGGING!THE!NEURONAL!ENTRAINMENT!TO!BEAT!AND!METER ............................................. 245! III.1.1.$Making$a$bridge$between$beat$and$meterMrelated$SSMEPs,$transient$ERPs$and$ongoing$oscillatory$ activities. ................................................................................................................................................... 246! ! 7! ! III.1.2.$Some$remarks$on$the$frequency$domain$analysis$of$SSMEPs........................................................... 248! III.1.3.$Musicians$versus$nonmusicians...................................................................................................... 248! III.1.4.$Beat$and$meterMrelated$SSMEPs$and$head$movement$artifacts....................................................... 249! III.2.!STUDY!2:!SELECTIVE!NEURONAL!ENTRAINMENT!TO!THE!BEAT!AND!METER!EMBEDDED!IN!A!MUSICAL! RHYTHM......................................................................................................................................................... 250! III.2.1.$The$neural$sources$of$beat$and$meterMrelated$SSMEPs.................................................................... 251! III.2.2.$Similarities$between$pitch$and$meter$processing........................................................................... 253! III.2.3.$Retrieving$time$resolution$and$phase$from$SSMEPs......................................................................... 254! III.3.!STUDY!3:!CAPTURING!WITH!EEG!THE!NEURONAL!ENTRAINMENT!AND!COUPLING!UNDERLYING! SENSORIMOTOR!SYNCHRONIZATION!TO!THE!BEAT ....................................................................................... 256! III.4.!STUDY!4:!STEADYPSTATE!EVOKED!POTENTIALS!AS!AN!INDEX!OF!MULTISENSORY!TEMPORAL!BINDING .. 257! III.4.1.$CrossMfrequency$modulation$studied$using$frequency$tagging. ..................................................... 258! III.4.2.$Is$periodicity$special?...................................................................................................................... 258! IV.!GLOSSARY..............................................................................................................................................261! IV.1.!ENTRAINMENT!IN!PHYSICS,!BIOLOGICAL!SYSTEMS!AND!NEUROSCIENCE ................................................................... 261! IV.2.!MUSICAL!TERMS ......................................................................................................................................... 265! V.!REFERENCES............................................................................................................................................269! ! 8! ! ! ! 9! ! ! SUMMARY! The!ability!to!perceive!a!regular!beat!in!music!and!synchronize!to!it!is!a!widespread!human! skill.! Fundamental! to! musical! behavior,! beat! and! meter! refer! to! the! perception! of! periodicities! while! listening! to! musical! rhythms,! and! usually! involve! spontaneous! entrainment! to! move! on! these! periodicities.! However,! the! neural! mechanisms! underlying! entrainment!to!beat!and!meter!in!Humans!remain!unclear.!The!present!work!tests!a!novel! experimental!approach,!inspired!by!the!steadyPstate!evoked!potential!method,!to!explore! the! neural! dynamics! supporting! the! perception! of! rhythmic! inputs.! Using! human! electroencephalography!(EEG),!neural!responses!to!beat!and!meter!were!recorded!in!various! contexts:!(1)!mental!imagery!of!meter,!(2)!spontaneous!induction!of!a!beat!from!rhythmic! patterns,! (3)! multisensory! integration,! and! (4)! sensorimotor! synchronization.! Our! results! support!the!view!that!entrainment!and!resonance!phenomena!subtend!the!processing!of! musical! rhythms! in! the! human! brain.! Furthermore,! our! results! suggest! that! this! novel! approach!could!help!investigating!the!link!between!the!phenomenology!of!musical!beat!and! meter!and!neurophysiological!evidence!of!a!bias!towards!periodicities!arising!under!certain! circumstances! in! the! nervous! system.! Hence,! entrainment! to! music! provides! an! original! framework!to!explore!general!entrainment!phenomena!occurring!at!various!levels,!from!the! interPneural!to!the!interPindividual!level.! ! 10! ! ! ! 11! ! ! FOREWORD! One!of!the!richest!features!of!music!is!its!temporal!structure.!In!particular,!the!beat,!which! usually!refers!to!the!perception!of!periodicities!while!listening!to!music,!can!be!considered!as! a!cornerstone!of!music!and!dance!behaviors.!Even!when!the!music!is!not!strictly!periodic,! humans! perceive! periodicities! and! are! spontaneously! entrained! to! move! on! these! periodicities.!Moreover,!the!beat!can!be!grouped!or!subdivided!in!meters,!which!correspond! to!harmonics!or!subharmonics!of!the!beat!frequency!(as!in!a!waltz,!which!is!a!threePbeats! meter)!(see!also!Glossary).!! Getting!entrained!to!music!is!an!extremely!common!human!activity,!shared!by!humans!of!all! cultures.! It! is! a! highly! complex! activity,! which! involves! auditory! (and! also! visual,! proprioceptive! and! vestibular)! perception,! attentional! capacities,! as! well! as! motor! synchronization,! performance! and! coordination.! Hence,! it! is! not! surprising! that! a! large! network!of!brain!structures!is!involved!in!entrainment!to!music,!and!that!there!is!a!growing! interest! in! understanding! the! functional! and! neural! mechanisms! of! the! entrainment! to! music.!! One!of!the!major!goals!of!this!dissertation!was!to!narrow!the!gap!between!scientific!studies! on!neural!entrainment!on!the!one!hand!and!entrainment!to!musical!rhythms!on!the!other! hand.!In!both,!entrainment!processes!and!biases!towards!periodicity!have!been!described!as! fundamental! functional! characteristics.! Considering! this,! we! tested! whether! periodicities! induced!by!musical!rhythms!could!entrain!neural!activities!at!frequencies!corresponding!to! these!periodicities.!! In!the!present!work,!we!have!used!the!electroencephalogram!(EEG),!a!technique!particularly! well! suited! to! study! a! system! that! changes! dynamically! over! short! periods! of! times.! We! ! 12! ! developed! an! original! EEG! approach! to! capture! the! processing! of! beat! and! meter! periodicities.!This!approach!is!based!on!the!longPstanding!observation!that!when!the!brain!is! stimulated! periodically,! it! synchronizes! its! activity! to! the! inputs! and! produces! periodic! output!(Lunel!&!Van!der!Tweel,!1965!;!Regan,!1966).!This!neural!activity!can!be!captured! objectively!in!the!form!of!a!steadyPstate!evoked!potential!(SSPEP)!identified!by!analyzing!the! EEG!in!the!frequency!domain.! Our! experiments! show! the! interest! of! this! approach! to! study! various! aspects! of! beat! perception! in! normal! individuals:! elicited! by! mental! imagery! paced! onto! periodic! sounds! (Nozaradan! et! al.,! 2011),! emerging! spontaneously! when! listening! to! rhythmic! patterns! (Nozaradan!et!al.,!2012),!elicited!by!sensorimotor!synchronization!to!the!beat!(Nozaradan!et! al.,!in!revision),!and!finally,!elicited!by!simultaneous!auditory!and!visual!beats!which!were! temporally!congruent!or!not!(Nozaradan!et!al.,!2012).! ! Several! terms,! either! from! the! neural! oscillation! or! the! musical! rhythm! literatures,! are! recurrent!in!the!present!work.!For!a!definition!of!these!terms,!as!well!as!a!description!of! some! important! concepts! related! to! these! terms,! the! reader! is! referred! to! the! section! Glossary.! The!present!work!is!attached!with!several!media!files.!These!are!the!stimuli!of!Studies!1!to!4,! and! also! audio! tracks! which! illustrate! some! of! the! musical! aspects! addressed! in! the! theoretical!parts.! ! 13! ! ! I.!ENTRAINMENT!IN!NEURAL!SYSTEMS! Part!I!of!the!present!thesis!reviews!the!evidence!supporting!the!view!that!our!neural!system! is! biased! towards! periodicity,! under! certain! circumstances! at! least,! and! can! act! in! some! contexts!as!multiple!coupled!oscillators.!This!question!is!of!particular!interest!in!regard!to! research!on!rhythm,!pulse!and!meter!perception.!Indeed,!in!order!to!explain!the!underlying! mechanisms!that!lead!to!this!ubiquitous!human!ability,!a!theoretical!model!of!resonance!for! pulse!and!meter!(see!Section!II.2.2.3.3.)!has!proposed!to!link!the!phenomenology!of!pulse! and!meter!with!the!concepts!of!neural!oscillation!(Large!and!Kolen,!1994;!Large,!2008).!The! basic!idea!of!this!model!is!that!some!neural!oscillations,!possibly!dispersed!across!cortical! and! subcortical! areas! and! spanning! a! range! of! natural! frequencies! for! beat! and! meter! induction!in!music,!entrain!to!the!rhythm!of!the!auditory!sequence.! Following!a!review!of!the!possible!tendencies!towards!periodicity!and!entrainment!in!the! activity!of!neurons!(Section!I.1),!we!will!review!the!evidence!of!neural!entrainment!in!the! particular!case!of!synchronization!to!oscillatory!inputs!(Section!I.2).!We!will!then!discuss!the! neurophysiological!evidence!for!entrainment!in!the!auditory!system!(Section!I.2.1),!whose! stimulation! forms! can! be! seen! as! oscillatory! in! nature.! Finally,! we! will! focus! on! SSPEPs! (Section! I.2.2),! an! electrophysiological! method! making! a! specific! use! of! periodic! repeated! stimulation!to!tag!brain!activity!and!which!inspired!the!present!experimental!work.! I.1.!ENDOGENOUS!OSCILLATIONS!! There! is! a! large! amount! of! evidence! for! rhythmic! neural! activities.! To! characterize! these! dynamic! patterns,! the! term! “oscillation”! was! first! mentioned! by! Hans! Berger! in! 1929,! to! describe!cyclical!fluctuations!of!the!electrical!currents!of!the!human!scalp!at!approximately! ! 14! ! 10! Hz.! This! electrical! activity! was! enhanced! when! participants! closed! their! eyes! and! constituted!the!first!description!of!the!alpha!band,!an!ongoing!neural!activity!between!8!and! 12!Hz!typically!enhanced!when!the!eyes!are!closed!(see!e.g.,!Klimesch,!1999,!for!a!review).! Since!this!seminal!observation,!numerous!studies!have!explored!the!relationship!between! dynamic!patterns!recorded!with!EEG!or!other!techniques,!and!behavioral!states.!! The!human!brain,!with!its!numerous!connections!between!areas,!displays!lowPfrequency!and! fast!rhythmic!patterns!grouped!within!complex!wavePsequences!(Steriade,!2006).!Some!of! these!oscillations!are!due!to!intrinsic!neuronal!properties,!while!others!arise!from!the!large! interconnections! of! neurons! across! distant! brain! areas.! From! this! perspective,! the! mechanisms! underlying! oscillatory! activities,! synchronization! across! neurons! and! the! emergence! of! a! frequency! tuning! function! within! one! neuronal! population! may! be! interpreted!as!different!aspects!of!a!common!phenomenon.! I.1.1.! Spontaneous! neural! oscillations.! Two! mechanisms! at! least! have! been! proposed! to! explain! the! oscillatory! behavior! of! neuronal! discharge:! (1)! the! mutual! interconnection! between! an! excitatory! neuron! and! an! inhibitory! interneuron,! or! between! two! inhibitory! interneurons,!and!(2)!the!pacemaker!neuron.! The!hypothesis!of!an!oscillatory!activity!emerging!from!a!mutual!interconnection!of!at!least! two!cells!including!an!inhibitory!neuron!was!proposed!for!the!first!time!to!account!for!the! fast!oscillations!(between!6!and!10!Hz)!observed!in!the!rat!hippocampus!(Wang!and!Buzsaki,! 1996).! Such! network! models! of! oscillatory! activity! were! further! studied! in! the! context! of! central!pattern!generators.!In!many!animal!species,!functional!units!of!a!few!cells,!located!in! the!spinal!cord,!have!been!shown!to!generate!continuous!periodic!activity!responsible!for! automatic! movements! such! as! locomotion! in! many! animal! species! (Marder! and! Bucher,! 2001).!In!these!network!models,!two!neurons!reciprocally!inhibit!each!other.!When!isolated,! ! 15! ! these! neurons! do! not! fire! in! repetitive! bursts.! However,! when! they! are! coupled,! they! produce! alternating! patterns! of! activity! (Fig.! I.1.1.).! The! transition! between! activated! and! inhibited! states! occurs! via! various! mechanisms.! For! instance,! if! the! neuron! shows! spikeP frequency!adaptation,!the!active!neuron!may!slow!down!or!stop!firing,!thus!releasing!the! other!neuron!from!inhibition.!Alternatively,!the!inhibited!neuron!may!escape!from!inhibition! due!to!its!intrinsic!membrane!properties!and,!in!turn,!activate!or!inhibit!the!first!inhibiting! neuron.!This!postinhibitory!rebound!has!been!shown!to!be!crucial!for!the!timing!of!firing!of! the! central! pattern! generator! unit! (Marder! and! Bucher,! 2001;! Calabrese,! 1998).! By! extension,! similar! mechanisms! have! been! described! to! explain! the! oscillatory! activity! of! thalamic!neurons!as!well!as!neurons!in!the!globus!pallidus!for!instance!(Bevan!et!al.,!2002),! based!on!the!interplay!between!lowPthreshold!excitatory!calcium!current!and!burst!of!GABAP mediated!inhibition.! According!to!the!second!kind!of!mechanism,!some!neurons!are!intrinsically!rhythmic,!and! fire!either!endogenously!or!in!response,!for!instance,!to!neuromodulatory!substances!such! as!neurotransmitters.!Examples!of!such!neurons!have!been!observed!in!the!inferior!olive.! Neurons!of!this!structure!exhibit!sustained!oscillatory!activity!that!are!generally!observed! between!4!and!10!Hz!and!are!explained!by!the!interplay!between!various!ionic!currents!and! their! particular! dynamics! across! the! membrane! (Bal! and! McCormick,! 1997).! Hence,! when! they!receive!stimulation,!the!dynamic!of!their!responses!lie!within!a!narrow!frequency!range! that!coincides!with!their!natural!frequency!of!resonance,!such!that!the!transient!response!to! the! transient! input! takes! the! form! of! a! transient! oscillation.! Neurons! that! are! strongly! oscillatory!can!provide!important!timing!inputs!for!neuronal!networks,!by!driving!neurons! that!are!not!themselves!intrinsically!rhythmic!(Fig.!I.1.1.).!However,!they!are!more!difficult!to! entrain!or!reset,!except!within!a!small!frequency!range.! ! 16! ! ! ! Figure$I.1.1.$From$Marder$and$Bucher$(2001).$Upper$panel.$Rhythmic$network$based$on$the$coupling$ between$a$pacemaker$neuron$(in$red)$and$a$non$pacemaker$neuron.$Bottom$panel.$Rhythmic$network$ based$on$the$reciprocal$inhibition$between$two$non$rhythmic$neurons.$ ! I.1.2.! Frequency! tuning! function.! The! frequency! range! in! which! sustained! oscillatory! activities!are!observed!is!determined!by!structural!aspects,!acting!as!bandpass!filters,!at!the! level!of!the!single!neuron!and!the!network.!! LowPpass! and! highPpass! filtering! is! mainly! constituted! by! timing! constraints! due! to! the! conductance! (which! can! be! defined! as! the! ease! at! which! an! electric! current! crosses! the! membrane)!and!capacitance!(which!can!be!defined!as!the!ability!of!the!neuron!to!store!an! electric!charge)!of!the!neuronal!membrane!(Hutcheon!and!Yarom,!2000).!The!combination! between! lowPpass! and! highPpass! filtering! properties! determines! the! frequency! tuning! function!of!individual!neurons.!If!the!dynamic!activity!is!limited!to!a!narrow!frequency!range,! it! results! in! an! almost! periodic! activity,! as! found! in! pacemaker! neurons! for! instance.! ! 17! ! Moreover,! this! leads! to! cases! where! neurons! would! show! a! quasiPperiodic! discharge! in! response!to!nonPperiodic!input!such!as!white!noise!(Joris!et!al.,!2004).! In!addition,!the!frequency!tuning!function!of!neural!oscillatory!behaviors!can!also!result!from! the! physical! architecture! of! neuronal! networks! and! the! limited! speed! of! neuronal! communication! due! to! axon! conduction! and! synaptic! transmission! (Buzsaki! and! Draguhn,! 2004).!That!is,!the!size!of!the!synchronous!group!also!influences!the!period!of!oscillation.! Higher! frequency! oscillations! can! involve! a! small! neuronal! space,! whereas! very! large! networks! are! only! able! to! synchronize! to! slow! oscillations.! Hence,! the! frequency! tuning! function!of!the!network!is!determined!by!both!the!properties!of!the!individual!neuron,!and! the!properties!of!its!interconnections.! I.1.3.! Synchronization.! It! is! generally! assumed! that! integration! of! information! requires! synchrony,!or!coincidence,!of!convergent!inputs!(Buzsaki!and!Draguhn,!2004).!Synchrony!is! defined!as!the!simultaneous!occurrence!of!activity!in!two!or!more!cells!(see!the!Glossary!for! more!details!on!the!concept!of!synchrony).!OscillationPbased!synchrony!is!thought!to!be!the! most! efficient! physical! mechanism! for! temporal! coordination! (Pikowski! et! al.,! 2001).! Oscillatory! synchronization,! as! a! synonym! with! entrainment,! can! be! achieved! through! networks!that!include!pacemaker!or!inhibitory!processes!(Fig.!I.1.1.),!thus!emerging!from!the! synaptic!connections!and!their!intrinsic!properties.! I.1.4.!A!link!between!synchronization!of!oscillatory!activities!and!brain!function.!Given!the! diversity! of! the! voltagePdependent! channels! and! the! intrinsic! properties! of! the! cellular! membrane!within!the!whole!brain,!it!is!likely!that!every!neuron!has!a!resonance!curve!and! the!potential!to!exhibit!oscillatory!activity!under!certain!circumstances.!Whether!resonance! and!oscillatory!synchrony!in!neurons!are!simply!epiphenomena!or!whether!they!are!used!to! integrate! and! communicate! information! is! still! debated! (Hutcheon! and! Yarom,! 2000).! ! 18! ! However,!several!mechanisms!can!be!proposed!to!explain!the!advantages!for!neural!systems! to!act!as!coupled!oscillators.!! First,!oscillatory!behaviors!in!neuronal!groups!may!influence!the!response!chronometry!of! the!oscillating!neurons,!because!their!excitability!becomes!phase!dependent!(Llinas,!1988,! Hutcheon!and!Yarom,!2000;!Engel!et!al.,!2001).!The!oscillatory!fluctuation!of!the!membrane! potential!of!a!given!neuron!creates!predictable!time!windows!during!which!the!neuron!is! more! likely! to! respond! to! external! input.! If! the! input! occurs! at! an! inappropriate! time! according!to!the!excitatory!phase,!the!neuronal!response!is!dampened!and/or!delayed.! Second,!oscillatory!synchronization!may!act!as!a!filter!and!amplificator!of!the!inputs.!The! amplification! can! be! explained! by! the! beacon! effect.! At! equal! input! strength! of! each! upstream!neuron,!the!impact!of!the!inputs!ensemble!is!greater!on!the!target!cell!when!the! inputs!are!synchronous.!The!filtering!of!the!inputs!is!achieved!based!on!the!excitatory!phase! of!the!target!neuron,!but!also!based!on!the!frequency!of!the!stimulation.!The!inputs!are!thus! selected! when! they! fall! within! the! frequency! preference! of! the! neuron,! according! to! its! intrinsic!resonant!oscillatory!features!or!the!resonant!properties!of!the!network!to!which!the! neuron! is! interconnected.! This! would! thus! determine! the! “sampling! rate”! of! the! neural! network!for!a!given!input!(see!also!Section!I.2.2.1.).! Third,!oscillatory!synchronization!could!serve!as!a!mean!to!bind!cell!assemblies.!This!is!based! on! the! assumption! that! information! in! the! brain! is! processed,! transferred! and! stored! by! flexible!cell!assemblies.!These!assemblies!are!defined!as!neuronal!groups!that!are!transiently! synchronized!(Edelman,!1978;!1989).!Indeed,!the!binding!of!the!neurons!may!depend!on!the! coupling! strength,! itself! influenced! by! the! distribution! of! the! resonant! frequencies! of! the! individual!neurons!of!the!group.!As!long!as!the!frequencies!of!the!coupled!oscillators!remain! similar,! synchrony! can! be! sustained! even! with! very! weak! synaptic! links! (Buzsaki! and! ! 19! ! Draguhn,!2004;!Engel!et!al.,!2001;!Varela!et!al.,!2001).!This!flexibility!based!on!oscillatory! synchronization! is! hypothesized! to! play! a! role! in! learning.! In! the! rat! hippocampus,! a! structure!thought!to!play!a!crucial!role!in!memory,!brief!pulse!trains!delivered!at!the!peak!of! the!neuronal!oscillations!induce!longPterm!potentiation,!whereas!the!same!train!applied!outP ofPphase!weaken!the!previously!strengthened!inputs!(Csicsvari!et!al.,!2003).!! ! 20! ! ! I.2.!ENTRAINMENT!TO!OSCILLATORY!INPUTS! In! the! previous! section,! we! briefly! reviewed! the! neurophysiological! bases! of! the! natural! propensity!of!neurons!to!generate!oscillatory!activity,!and!the!possible!role!of!this!oscillatory! behavior! in! brain! function.! ! The! present! section! addresses! the! question! of! how! neurons! behave! in! contact! to! external! inputs! that! are! themselves! oscillatory! (auditory! stimuli).! Finally,!we!will!examine!how!such!repeated!stimuli!can!be!used!to!“tag”!neural!processes! using!electrophysiology!(steadyPstate!evoked!potential!approach).! I.2.1.!Synchronous!oscillation!to!sound!envelope!in!the!auditory!system! I.2.1.1.! What! is! sound! envelope?! Acoustic! stimuli! contain! multiple! temporal! dimensions.! They! can! be! summarized! in! at! least! two! components,! the! “fine! structure”! and! the! “envelope”,!which!are!usual!terms!to!describe!waveforms!in!physics.!In!acoustic,!the!fine! structure!is!determined!by!the!fast!pressure!variations!corresponding!to!the!spectral!content! of!the!sound.!The!processing!of!fine!structure!is!involved!in!pitch!perception,!which!can!be! defined!as!the!perceptual!phenomenon!of!sounds!organized!within!a!scale!from!low!to!high! tones!(Schnupp!et!al.,!2010).!The!fine!structure!is!itself!modulated!in!amplitude,!and!the! dynamic! of! this! amplitude! modulation! constitutes! the! sound! envelope.! In! humans,! amplitude! modulations! produce! various! hearing! sensations! depending! on! the! modulation! frequency.!Rhythms!and!fluttering,!as!well!as!most!amplitude!modulation!frequencies!found! in!ordinary!speech!for!instance,!correspond!to!envelope!frequencies!up!to!20!Hz!whereas! roughness!and!pitch!correspond!to!amplitude!modulation!frequencies!above!20!Hz.!As!we! will!see,!it!has!been!proposed!that!this!perceptual!boundary!may!be!related!to!a!change!in! the!coding!form!of!these!sound!inputs!(Eggermont,!2001).! ! 21! ! Whether!envelope!processing!is!embedded!in!the!auditory!system!is!an!important!question,! uploads/Geographie/ nozaradan-thesis-final.pdf

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