LETTERS. Hippocampal remapping and grid realignment in entorhinal cortex

Transcription

1 Vol Mrh 27 oi:.38/ntur56 Hippompl rmpping n gri rlignmnt in ntorhinl ortx Mrinn Fyhn *, Torkl Hting *, Alssnro Trvs,2, My-ritt Mosr & Evr I. Mosr A unmntl proprty o mny ssoitiv mmory ntworks is th ility to orrlt ovrlpping input pttrns or inormtion is stor 5. In th hippompus, this nuronl pttrn sprtion is xprss s th tnny o nsmls o pl lls 6 to unrgo xtnsiv rmpping in rspons to hngs in th snsory or motivtionl inputs to th hippompus 7 3. Rmpping is xprss unr som onitions s hng o iring rts in th prsn o stl pl o ( rt rmpping ) 4, n unr othr onitions s omplt rorgniztion o th hippompl pl o in whih oth pl n rt o iring tk sttistilly inpnnt vlus ( glol rmpping ) 4. Hr w show tht th ntur o hippompl rmpping n prit y nsml ynmis in pl-sltiv gri lls in th mil ntorhinl ortx 5,6, on synps upstrm o th hippompus. Whrs rt rmpping is ssoit with stl gri ils, glol rmpping is lwys ompni y oorint shit in th iring vrtis o th gri lls. Gri ils o o-loliz mil ntorhinl ortx lls mov n rott in onrt uring this rlignmnt. In ontrst to th multipl nvironmnt-spii rprsnttions o y pl lls in th hippompus, lol nsmls o gri lls thus mintin onstnt sptil phs strutur, llowing position to rprsnt n upt y th sm trnsltion mhnism in ll nvironmnts nountr y th niml. Hippompl pl rprsnttions my riv rom mtri rprsnttion o sp in th mil ntorhinl ortx (MEC) 5 2. A ky ll typ o th MEC is th gri ll, th sptil rptiv ils orm rgulrly tssllting tringulr pttrn tht spns th omplt nvironmnt ovr y th niml 6. Th prioi iring ils o nighouring gri lls r ost rltiv to h othr in n pprntly rnom mnnr, suh tht ll lotions within th rpting unit o th tringulr ltti r rprsnt y ny suiintly lrg lol ll nsml 6. To trmin how nurl tivity in gri lls is orgniz t th nsml lvl n how thir orgniztion might inlun th ormtion o rprsnttions ownstrm in th hippompus, w ror th simultnous tivity o multipl gri lls in lyr II o MEC in rts (Rttus norvgius) unr onitions in whih pl lls in th hippompus unrgo glol rmpping or rt rmpping 4 (Fig., Supplmntry Figs n 2, Supplmntry Mthos n Supplmntry Tls 3). Clls wr ror simultnously rom MEC n r CA3 in 5 out o 5 rts with MEC ltros. Glol rmpping ws inu y two irnt prours, on in whih th rt ws tst in th sm lotion in ithr squr or irulr nlosur 7,9,2,22 (Fig. ) n nothr in whih th niml ltrnt twn similr squr oxs in two irnt rooms 6,3,4 (Fig. ). Ths protools inu pronoun hngs in iring rt n iring lotion throughout th pl ll popultion in ll xprimnts in whih CA3 lls wr ror (7 rts; 5 xprimnts; Fig. g). Rt rmpping ws inu y hnging th wlls o squr roring nlosur rom thr lk n on whit onigurtion to on lk n thr whit onigurtion 4 (Fig. ). This inu onsistnt rt rmpping in ll svn xprimnts in whih CA3 lls wr ror (iv rts; Fig.,, h). With istint protools or inuing glol rmpping n rt rmpping in th hippompus, w xmin th olltiv nuronl iring pttrns o gri lls in MEC in 2 xprimnts with 5 or mor gri lls ror simultnously rom th orsoul prt o MEC. Popultion ynmis in ntorhinl gri lls ws oun to strongly pritiv o th typ o rmpping inu in th hippompus. Whn glol rmpping ws vok y rpling th squr ox with th irulr ox, th ross-orrlogrms or iniviul gri lls (Fig. 2, top) s wll s simultnously ror nsmls (Fig. 2, ottom) orm tringulr gri-lik pttrns, suggsting tht gri sping, gri orinttion n sptil phs istriution wr prsrv twn th onitions (Fig. 2, ). Howvr, in ll svn xprimnts using this protool, th pk o th ntorhinl ross-orrlogrm ws ost rom th origin whn trils in irnt nvironmnts wr ompr. Th ost ws similr in ll simultnously ror gri lls (Supplmntry Figs 3 5). twn xprimnts, th isplmnt rng rom. to 23.8 m, with th uppr prts o th rng orrsponing to nrly on-hl o th gri sping t th roring lotion (r vtors in Fig. 2; Supplmntry Tl 2 n Supplmntry Figs 3 n 6). Th isplmnt ws signiintly lrgr thn twn rpt trils in th sm ox (rng. 2.3 m; lu vtors in Fig. 2; Z 5 2.4, P,., on-til Wiloxon sign-rnks tst). Th irtion o th ost ws uniormly istriut twn xprimnts (Fig. 2; r 5.34, P..5, Ryligh tst or rnomnss). Whn lls wr ror rom th sm lotion ovr svrl ys, th istn n th irtion o th gri shit rmin onstnt (Supplmntry Fig. 3). In thr out o svn nimls, th gri lso show wk xpnsion rom th squr to th irl. Th xpnsion ws ohrnt mong simultnously ror lls (Supplmntry Fig. 5 n Supplmntry Tl 2). Cross-orrlogrms or simultnously ror CA3 lls wr ssntilly lt (Fig. 2 n Supplmntry Fig. 6), s woul xpt i oth iring lotion n iring rt r orthogonliz in CA3. Ths osrvtions init tht uring glol rmpping in th hippompus, th mp o gri lls in th MEC rligns with hngs in th nvironmnt without losing its intrinsi sptil phs strutur. To trmin whthr nsmls o gri lls rmin ohrnt with mor sustntil hngs in th xtrnl input, w ompr th sptil phs rltionship o simultnously ror gri lls uring orging in rooms with irnt istl lnmrks (rooms A n ). Although th gri strutur o iniviul lls ws mintin ross rooms, th prioiity ws not immitly visil in Cntr or th iology o Mmory, Norwgin Univrsity o Sin n Thnology, NO-7489 Tronhim, Norwy. 2 Cognitiv Nurosin Stor, SISSA Intrntionl Shool or Avn Stuis, I-344 Trist, Itly. *Ths uthors ontriut qully to this work Ntur Pulishing Group

2 NATURE Vol Mrh 27 th popultion ross-orrlogrms (Fig. 3 ). Howvr, pks m pprnt whn on stk ws rott, in stps o 3u, rltiv to th othr (Fig. 3,, n Supplmntry Figs 3 g, 4 n 7). Mximl orrltion ws otin or rottions rnging rom 66u lokwis to 78u ountrlokwis. With optiml rottion, th isplmnt o th popultion gri rng rom 5.7 to 27.5 m (Fig. 3, r vtors). Th irtion o th ost ws uniormly istriut (r 5.6, P..5, Ryligh tst or rnomnss). On rpt trils in th sm room (A vrsus A9), th pk o th ross-orrlogrm rmin in th ntr (rng o isplmnts 2.3 m; Fig. 3, lu vtors). Th shit twn room A n room, with optiml rottion, ws signiintly lrgr thn tht twn rpt trils in th sm room (Z 5 2.5, P,.). In iv out o ight nsmls, th gri sping ws slightly irnt twn rooms A n (Supplmntry Tl 2); in thos ss, th sling ws ohrnt twn simultnously ror nurons (Supplmntry Fig. 5). Cross-orrlogrms or nsmls o CA3 lls in rooms A n wr lt (Fig. 3 n Supplmntry Fig. 7). Ths osrvtions suggst tht gri ils mov n rott, n somtims xpn or ontrt, whn th rt ws mov to nw lotion, ut th intrinsi sptil phs strutur o th gri lls ws mintin. This irs rom th orthogonliz rprsnttions o CA3 n inits possil pttrn sprtion mhnism in th ntorhinl inputs to th hippompus. W vrii ths rsults y xmining whthr nsml tivity in MEC ws ohrnt in th tmporl omin. Using only rts with simultnous rorings rom MEC n CA3, w ompr th istriution o tmporl popultion vtors, with popultion vtors in s lists o lls tht ir togthr ross ins o 5 ms throughout th tril 3. In MEC, th mn Shnnon mutul inormtion twn tril n popultion vtors ws no lrgr or omprisons o trils in irnt gomtri shps thn or omprisons o t t2 t t2 t4 t t46 A A A 3 Hz 27 Hz 9 Hz 56 Hz 4 Hz 6 Hz 5 Hz 8 Hz 2 Hz Figur Prours or inuing glol rmpping n rt rmpping in hippompl r CA3., Firing ils o thr simultnously ror CA3 lls uring glol rmpping (irnt oxs in onstnt lotion () or similr oxs in irnt rooms ()) or rt rmpping (irnt wll olours ()). For omplt ll smpls s Supplmntry Fig. 2. Eh row shows t rom on tiv nuron (t, ttro;, ll). Eh pir o olumns shows, or h nuron, th lotion o spiks (r) on th trjtory o th rt (lt) n olour-o mp o th iring rt istriution (right; rk lu is zro, r is mximum; pk rt is init)., Chng in iring rts stimt y rt ovrlp (mns 6 s..m.). R lins init hn lvls. lu rs, sm nvironmnt (A A9); grn rs, irnt nvironmnts (A )., Sptil orrltion or lls with tivity in oth nvironmnts (mns 6 s..m.; st rottion). h, n sptil orrltion in lls with tivity in oth nvironmnts., g h Ntur Pulishing Group oxs rooms Colour rvrsl Sptil orrltion Proportion Sptil orrltion Proportion Proportion oxs rooms Colour rvrsl Sptil orrltion oxs (s in ); g, irnt rooms (s in ); h, irnt olours (s in ). Sttr plots show rt ovrlp s untion o sptil orrltion or iniviul CA3 lls (opn irls, A vrsus A9; ill irls, A vrsus ). Frquny istriutions show sptil orrltions (grn rs) ompr with istriutions or shul ll pirs (purpl rs) rom th sm xprimnts. In th irnt-oxs tsk, 2 out o 23 sptil orrltions x th uppr limit o th 95% onin intrvl o th shul istriution. In th irnt-rooms tsk, 3 out o 2 orrltions wr outsi th 95% onin intrvl. In th rt rmpping tsk, 25 out o 26 lls wr outsi th 95% onin intrvl. Th rt ovrlp twn A n is signiintly highr in h thn in us th sttr plots in h o not inlu th lrg numr o lls with low rts in on o th nvironmnts. 9

3 NATURE Vol Mrh 27 rpt trils in th sm shp, n no lrgr or irnt rooms thn or rpt trils in th sm room (Supplmntry Fig. 8; t,., P..5, on-til pir t-tsts), initing tht gri lls tht r otiv in on nvironmnt my rmin otiv in othr nvironmnts too. In CA3, inormtion out tril typ ws ov.5 its or squr vrsus irl trils s wll s trils in irnt rooms, signiintly ov th vlus otin or rpt tsts in th sm nvironmnt (Supplmntry Fig. 8; squr vrsus irl: t(7) 5 5.6, P,.; two rooms: t(4) 5 3.8, P,.). Th irntition o otivity pttrns in CA3 urthr rinors th viw tht inoming snsory input is orrlt s it ntrs th hippompus uring glol rmpping. In striking ontrst with th oorint shit uring glol rmpping, gri vrtis rmin stl whn pl lls xhiit rt rmpping, suh s whn th ox olours wr rvrs. Th pk o th ross-orrlogrm or trils with irnt olour onigurtions rmin in th ntr or six out o svn xprimnts (isplmnt 2.7 m) n ws slightly ost (7. m) in on xprimnt (Fig. 4,, n Supplmntry Figs 3h, i n 9). Th pk isplmnt tr olour rvrsl ws not signiintly irnt rom th isplmnt osrv in th sm lls on tril pirs with intil olours (rng.3 m; Z 5.4, P..5). Thr ws no hng in th sl o th gri twn lk n whit trils (Supplmntry Tl 2). Thr ws lso no signiint hng in th lotion o th ross-orrltion pk in CA3 (min shit o 6.3 m tr olour rvrsl, in ontrst with 4.9 m with similr olours; Z 5.7, not signiint; Fig. 4). Rt rmpping in th hippompus ws not ssoit with signiint rt hngs twn th iniviul gri ils o th ntorhinl nurons (Supplmntry Fig. ). W inlly xmin whthr glol rmpping in th hippompus n gri rlignmnt in th ntorhinl ortx r oinint, us strong tmporl ontiguity woul xpt i th phnomn wr mhnistilly rlt. Thr rts, two o whih h ltros in oth MEC n CA3, wr trin in squr nlosur with lights on n o on ltrnting isrt sssions. Th prour inu glol rmpping twn rk n light trils uring th trining phs in oth rts with hippompl ltros (Supplmntry Fig., ) 23. Th simultnously ror ntorhinl rprsnttion ws ohrnt ut ispl n rott twn th trils. To vok instntnous glol rmpping, th room lights wr turn on tr in o running in th rk onition on th tst y. In on niml this us sun rvrsion to th originl hippompl mp ssoit with th light onition. This instn o glol hippompl rmpping oini with n qully st rlignmnt o th ntorhinl mp, xprss s loss o sptil orrltion twn gri ils ror or n tr th light ws turn on (Supplmntry Fig., ). In th son rt, ll hippompl n ntorhinl nurons mintin thir iring ils whn th rknss ws trmint 23, prsumly rlting ontinu inlun o sl-motion inormtion on th lotion o iring in gri lls n pl lls 6. To rst th pth intgrtor, th rt ws lit up on pstl or in. Th tmporry isruption o running us n immit rvrsion to th originl hippompl mp, ssoit with th light onition, whn th rt ws rturn to th ox. At th sm tim, ohrnt shit n rottion o th popultion gri ws A 64 2 Hz 8 Hz 27 Hz 64 A A A A t52 t52 64 Hz 2 Hz 22 Hz t6 t lls 4 Hz 2 Hz 4 Hz t63 t lls Iniviul lls Fix Ensmls A Fix PV 2 3 n x x y PV 2 3 Figur 2 Rlignmnt o ntorhinl gri ils uring hippompl glol rmpping twn irnt oxs in th sm lotion., Colour-o iring rt mps or thr rprsnttiv simultnously ror MEC lls ( omplt ll smpl is givn in Supplmntry Fig. 3). Fiv-igit numrs (or xmpl 64) rr to rt intitis., Shmti illustrtion o th prour or ross-orrltion o rt mps s on iniviul rt mps (top) or stks o rt mps (ottom). Cross-orrltion mtris wr trmin y shiting th two mps rltiv to on nothr in stps o 5 m in oth th x n y irtions within th ommon stippl r. PV, 92 y n Ntur Pulishing Group A A lls A A A popultion vtor (in or h m 2 in).,, Colour-o rossorrltion mtris or rprsnttiv iniviul lls (; sm lls s in ) n ll nsmls () uring rpt tsting in th sm ox (A vrsus A9) or in irnt oxs (A vrsus ) (or ll othr xprimnts s Supplmntry Fig. 3)., Vtor igrm showing istriution ross xprimnts o th irtion n th istn o th gri isplmnt, msur rom th origin to th nrst pk o th ross-orrlogrm., Rprsnttiv ross-orrlogrm or CA3 nsmls.

4 NATURE Vol Mrh A 3 Hz 9 Hz 9 Hz A A A Rott t lls Hz 22 Hz 22 Hz t lls 78 5 Hz 7 Hz 7 Hz 343-R 8 lls L lls A A A A Rott 57 Figur 3 Rlignmnt o ntorhinl gri ils uring hippompl glol rmpping twn two rooms., Rt mps or rprsnttiv simultnously ror MEC lls ( omplt ll smpl is givn in Supplmntry Fig. 3).,, Cross-orrltion mtris or rprsnttiv gri ll nsmls., Rpt trils in room A (A A9);, room A vrsus room. Th right olumn in shows ross-orrlogrms or th rottion tht gv mximl gri strutur (lt olumn, no rottion). For ll othr xprimnts s Supplmntry Fig. 3, g., Cross-orrlogrm or inrmntl mounts o rottion in on xprimnt (lu, mximl orrltion)., Vtor igrm showing istriution ross xprimnts o irtion n istn o gri isplmnt tr optiml rottion., Rprsnttiv ross-orrlogrms or simultnously ror CA3 pyrmil lls. 5 t t2 t3 A A A 5 Hz 5 Hz 5 Hz 9 Hz 9 Hz 7 Hz 7 Hz 7 Hz 6 Hz 2 2 A A A A osrv in MEC. Th oinin o th hngs in ntorhinl n hippompl popultion ynmis in th two nimls inits tht th hngs my prt o singl intgrt pross. First, ths rsults init tht lol nsmls o gri lls my hv rigi sptil phs rltionship. Th rptition o th sm squn o tiv units s n niml ollows similr rout in irnt rns supports th hypothsis tht th gri ntwork is univrsl mtri or pth-intgrtion-s nvigtion 6,9, Son, th ssoition twn hippompl glol rmpping n ntorhinl gri rlignmnt implis ntwork ynmis in th MEC s possil trminnt o orthogonliztion prosss in hippompl pl lls (Supplmntry Fig. 2). Th stility o ntorhinl gri lls uring rt rmpping in th hippompus riss th possiility tht th rt orthogonliztion origints in th ntt gyrus n CA3 (r. 27), prhps s onsqun o onvrgn twn Figur 4 Unltr lignmnt o ntorhinl gri ils uring rt rmpping in CA3., Rt mps or rprsnttiv simultnously ror gri lls in th irnt-olours tsk ( omplt ll smpl is givn in Supplmntry Fig. 3h)., Cross-orrltion mps or rprsnttiv gri ll nsmls (or ll othr xprimnts s Supplmntry Fig. 3i). Lt olumn, trils with similr wll olours (A A9); right, trils with irnt wll olours (A )., Vtor igrm showing istriution ross xprimnts o irtion n istn o gri isplmnt., Rprsnttiv ross-orrlogrm or CA3 nsmls. 27 Ntur Pulishing Group 93

5 NATURE Vol Mrh 27 sptil signls rom gri lls in MEC n nonsptil signls rom nurons in th ltrl ntorhinl ortx 28,29. Th possil roing o sptil inormtion rom singl, univrsl nurl rprsnttion in th ntorhinl ortx onto sttistilly inpnnt ontxt-snsitiv ll nsmls in high-pity ntworks o th hippompus 3 is proly ruil or th sussul storg o pisoi mmory (s Supplmntry Disussion). METHODS Nuronl nsml tivity ws ror with ttros in 9 Long Evns rts implnt with mirorivs in orsoul MEC n/or orsl CA3 o th hippompus. Spik-triggr tivity ws smpl in loks o or 2 min whil rts hs sot oo rums in milir nlosurs o irnt shps, txturs n olours. Glol rmpping ws inu y ltrnting twn squr n irulr roring nlosurs t onstnt lotion within th sm room, y tsting th rt in similr squr oxs in two rooms with istint kgroun us 3, or y trining th rt on sprt trils in th sm nvironmnt in light n rknss 23. Rt rmpping ws inu y trining th nimls in squr nlosur with xhngl wlls o irnt olour 4. Dtil xprimntl n nlytil prours r provi in Supplmntry Mthos. Riv 4 July 26; pt 5 Jnury 27. Pulish onlin 25 Frury 27.. Mrr, D. Simpl mmory: thory o rhiortx. Phil. Trns. R. So. Lon. 262, 23 8 (97). 2. MNughton,. L. & Nl, L. in Nurosin n Conntionist Thory (s Gluk, M. A. & Rumlhrt, D. E.) 63 (Erlum, Hillsl, NJ, 989). 3. Trvs, A. & Rolls, E. T. Computtionl onstrints suggst th n or two istint input systms to th hippompl CA3 ntwork. Hippompus 2, (992). 4. Ksnr, R. P., Gilrt, P. E. & Wllnstin, G. V. Tsting nurl ntwork mols o mmory with hviorl xprimnts. Curr. Opin. Nuroiol., (2). 5. Lutg, S., Lutg, J. K., Mosr, M.-. & Mosr, E. I. Pl lls, sptil mps n th popultion o or mmory. Curr. Opin. Nuroiol. 5, (25). 6. O K, J. & Nl, L. Th Hippompus s Cognitiv Mp (Clrnon, Oxor, 978). 7. ostok, E., Mullr, R. U. & Kui, J. L. Exprin-pnnt moiitions o hippompl pl ll iring. Hippompus, (99). 8. Mrkus, E. J. t l. Intrtions twn lotion n tsk t th sptil n irtionl iring o hippompl nurons. J. Nurosi. 5, (995). 9. Kntros, C. t l. Aolition o long-trm stility o nw hippompl pl ll mps y NMDA rptor lok. Sin 28, (998).. Woo, E. R., Duhnko, P. A., Roitsk, R. J. & Eihnum, H. Hippompl nurons no inormtion out irnt typs o mmory pisos ourring in th sm lotion. Nuron 27, (2).. Frnk, L. M., rown, E. N. & Wilson, M. Trjtory noing in th hippompus n ntorhinl ortx. Nuron 27, (2). 2. Lvr, C., Wills, T., Cui, F., urgss, N. & O K, J. Long-trm plstiity in hippompl pl-ll rprsnttion o nvironmntl gomtry. Ntur 46, 9 94 (22). 3. Lutg, S., Lutg, J. K., Trvs, A., Mosr, M.-. & Mosr, E. I. Distint nsml os in hippompl rs CA3 n CA. Sin 35, (24). 4. Lutg, S. t l. Inpnnt os or sptil n pisoi mmory in th hippompus. Sin 39, (25). 5. Fyhn, M., Moln, S., Wittr, M. P., Mosr, E. I. & Mosr, M.-. Sptil rprsnttion in th ntorhinl ortx. Sin 35, (24). 6. Hting, T., Fyhn, M., Moln, S., Mosr, M.-. & Mosr, E. I. Mirostrutur o sptil mp in th ntorhinl ortx. Ntur 436, 8 86 (25). 7. Stnh, H.-A., Wittr, M. P., Mosr, M.-. & Mosr, E. I. Sptil mmory in th rt rquirs th orsoltrl n o th ntorhinl ortx. Nuron 45, 3 33 (25). 8. Srgolini, F. t l. Conjuntiv rprsnttion o position, irtion n vloity in ntorhinl ortx. Sin 32, (26). 9. MNughton,. L., ttgli, F. P., Jnsn, O., Mosr, E. I. & Mosr, M.-. Pth intgrtion n th nurl sis o th ognitiv mp. Ntur Rv. Nurosi. 7, (26). 2. Solst, T., Mosr, E. I. & Einvoll, G. T. From gri lls to pl lls: mthmtil mol. Hippompus 6, 26 3 (26). 2. Wills, T. J., Lvr, C., Cui, F., urgss, N. & O K, J. Attrtor ynmis in th hippompl rprsnttion o th lol nvironmnt. Sin 38, (25). 22. Anrson, M. I. & Jry, K. J. Htrognous moultion o pl ll iring y hngs in ontxt. J. Nurosi. 23, (23). 23. Quirk, G. J., Mullr, R. U. & Kui, J. L. Th iring o hippompl pl lls in th rk pns on th rt s rnt xprin. J. Nurosi., (99). 24. Rish, A. D. & Tourtzky, D. S. Cognitiv mps yon th hippompus. Hippompus 7, 5 35 (997). 25. Shrp, P. E. Complimntry rols or hippompl vrsus suiulr/ntorhinl pl lls in oing pl, ontxt, n vnts. Hippompus 9, (999). 26. Fuhs, M. C. & Tourtzky, D. S. A spin glss mol o pth intgrtion in rt mil ntorhinl ortx. J. Nurosi. 26, (26). 27. Lutg, J. K., Lutg, S., Mosr, M.-. & Mosr, E. I. Pttrn sprtion in ntt gyrus n CA3 o th hippompus. Sin 35, oi:.26/ sin.358 (6 Frury 27) 28. Hrgrvs, E. L., Ro, G., L, I. & Knirim, J. J. Mjor issoition twn mil n ltrl ntorhinl input to th orsl hippompus. Sin 38, (25). 29. Nr, P. A., Cllro-l, M., Jorritsm-yhm,. & Wittr, M. P. Prlll input to th hippompl mmory systm through pri- n postrhinl ortis. Nurorport 8, (997). 3. ttgli, F. P. & Trvs, A. Attrtor nurl ntworks storing multipl sp rprsnttions: mol or hippompl pl ils. Phys. Rv. E Stt. Phys. Plsms Fluis Rlt. Intrisip. Top. 58, (998). Supplmntry Inormtion is link to th onlin vrsion o th ppr t Aknowlgmnts W thnk S. Lutg, T. Solst,. L. MNughton n M. P. Wittr or isussion, R. Skjrpng or progrmming, n I. Hmmr, K. Hugn, K. Jnssn n H. W or thnil ssistn. This work ws support y Cntr o Exlln grnt rom th Norwgin Rsrh Counil. Author Contriutions M.F., T.H., M.-.M. n E.I.M. plnn xprimnts n nlyss. M.F., T.H. n M.-.M. prorm th xprimnts n nlys th t. A.T. gv vi on nlyss, n E.I.M. wrot th ppr. All uthors isuss th rsults n ontriut to th mnusript. Author Inormtion Rprints n prmissions inormtion is vill t Th uthors lr no ompting innil intrsts. Corrsponn n rqusts or mtrils shoul rss to E.I.M. (vr.mosr@ntnu.no) Ntur Pulishing Group