Jeannerod, M. (1995). Mental Imagery in the Motor Context. Neuropsychologia, 33(11), 1419-1432

7/30/2019 Jeannerod, M. (1995). Mental Imagery in the Motor Context. Neuropsychologia, 33(11), 1419-1432.

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~ P e r g a m o nNeuropsychologia,VoI . 33, No. I1, pp . 1419--1432,1995Copyright 1995E lsevierScienceLtdPrinted m Gre at Britain.All rights eserved0028-3932]9559.50+0.00

0028-3932(95)00073-9M E N T A L I M A G E R Y I N T H E M O T O R C O N T E X T

M . J E A N N E R O DV i s i o n e t M o t r i c i t r , U . 9 4 d e I' I .N . S . E . R . M . , 1 6, a v e n u e D o y e n L r p i n e , 6 9 50 0 B r o n , F r a n c e

(Received 25 June 1994; accepted 20 October 1994)A l m r a e t - - T h e w o r k i n g h y p o t h es i s o f t h e p a p e r i s t h a t m o t o r i m a g es a r e e n d o w e d w i t h t h e s a m ep r o p e r t i e s a s t h o s e o f t h e ( c o r r e s p o n d i n g ) m o t o r r e p r e s e n t a t i o n s , a n d t h e r e f o r e h a v e t h e s a m ef u n c t i o n a l r e l a t i o n s h i p to t h e i m a g i n e d o r r e p r e s e n t e d m o v e m e n t a n d t h e s a m e c a u s a l r o l e i n t h eg e n e r a t i o n o f t h i s m o v e m e n t . T h e f a c t t h a t t h e t i m i n g o f s i m u l a t e d m o v e m e n t s f o ll o w s t h e s a m ec o n s t r a i n t s a s t h a t o f a c t u a l l y e x e c u te d m o v e m e n t s i s c o n s i s te n t w i t h t h i s h y p o t h e s is . A c c o r d i n g l y ,m a n y n e u r a l m e c h a n i s m s a r e a c t i v a te d d u r i n g m o t o r i m a g e r y , a s r e v e a le d b y a s h a r p i n c r e a s e i nt e n d i n o u s r ef le x e s i n t h e l i m b i m a g i n e d t o m o v e , a n d b y v e g e t a t iv e ch a n g e s w h i c h c o r r e l a t e w i t h t h eleve l o f me n ta l e f fo rt . A t the c o r t i c a l l eve l , a s pec if ic p a t t e rn o f a c t iva t ion , t h a t c lo s e ly re semble st h a t o f a c t i o n e x e c u t io n , i s o b s e rv e d i n a r e a s d e v o t ed t o m o t o r c o n t r o l . T h i s a c t i v a t i o n m i g h t b e t h es u b s t r a t e f o r t h e e f f ec ts o f m e n t a l t r a i n in g . A h i e r a r c h ic a l m o d e l o f t h e o r g a n i z a t i o n o f a c t i o n i sp r o p o s e d : t h i s m o d e l i m p l i e s a s h o r t - t e r m m e m o r y s t o r a g e o f a ' c o p y ' o f t h e v a r i o u sr e p r e s e n t a t i o n a l st e p s. T h e s e m e m o r i e s a r e e r a s ed w h e n a n a c t i o n c o r r e s p o n d i n g t o t h e r e p r e s e n t e dgoa l t ak e s p lace . B y con t ra s t , i f t he ac t ion i s i ncomple te ly o r no t execu ted , t he w ho le s ys t emr e m a i n s a c t i v a t e d , a n d t h e c o n t e n t o f t h e r e p r e s e n t a t i o n is r e h ea r s ed . T h i s m e c h a n i s m w o u l d b e t h es u b s t r a t e f o r c o n s c i o u s ac c e ss t o t h i s c o n t e n t d u r i n g m o t o r i m a g e r y a n d m e n t a l t r a i n i n g .K e y W o r d s : m e n t a l i m a g e ry ; l e ar n i n g ; m o t o r c o n t r o l .

I N T R O D U C T I O NThe a im o f th i s pape r i s t o i nves ti ga t e t he concep t o f men t a l image ry i n t he con t ex t o fm o t o r a c ti o n s. T h e g e n e ra l id e a is th a t m o t o r i m a g e r y i s p a r t o f a b r o a d e r p h e n o m e n o n( the mo to r r ep r e sen t a t i on ) r e l a t ed t o i n t end ing and p r epa r ing movemen t s . The p roces s o fmo to r r ep r e sen t a t i on , a no rma l ly non -consc ious p roces s , c an be acce s sed consc ious lyund e r c e r t a i n cond i t i ons : a m o to r image is a consc ious m o to r r ep r e sen t a t i on . A cco rd ing t oth i s de f i n i t i on , mo to r images a r e endowed wi th t he s ame p rope r t i e s a s t hose o f t he( co r r e spond ing ) mo to r r ep r e sen t a t i on , t ha t i s , t hey have t he s ame func t i ona l r e l a t i onsh ipt o t h e i m a g i n e d o r r e p r e se n t e d m o v e m e n t a n d t h e s a m e c a u s a l ro l e i n t h e g e n e r a ti o n o ft h i s move m en t . T h i s i dea ha s b een fu l l y deve lop ed e ls ewhe re [39 ]. The a im o f t he p r e sen tpap e r i s t o exam ine i t s imp l i ca t ions fo r a mo de l o f a c t i on gene ra t i on .A p o s s i b l e w a y o f d if fe r en t ia t in g m o t o r i m a g e s f ro m o t h e r, b e t t e r k n o w n , t y p e s o fima gery (e .g . v i sua l ) i s to de te rm ine the sub jec t ive 'd i s tance ' be tw een the se l f and h is ownimag ina l expe r ience . M en ta l images can be expe r i enced by t he s e l f a s a spec t a to r w a t ch inga v i sua l s cene i n wh ich an ac t i on i s pe r fo rm ed b y t he r ep r e sen t ing sub j ec t h imse l f ( the so -ca l l ed ' ex t e rna l ' im age ry o f spo r t p sycho log i s t s , s ee [45 ]) . Images can a l so be exp e r i encedf rom wi th in , a s t he r e su l t o f t he ' f ir s t pe r son ' p roces s i nvo lv ing mos t l y a k ine s the t i cr ep re sen t a t i on o f t he ac t i on ( ' in t e rna l ' im age ry [45 ]) . M o t o r im age ry wou ld t hus be long t othe l a t t e r t ype . I t w ou ld imp ly t ha t t he sub j ec t f ee l s h imse l f execu t i ng a g iven ac t i on ,

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whether i t involves the whole body (e .g . running) , o r i t i s l imi ted to a body par t (e .g .wr i t i ng , po in t i ng t o a t a rge t o r ho ld ing p r e s su re aga in s t an obs t ac l e ) . Mo to r image ry ,t he r e fo r e , r equ ir e s a r ep r e sen t a t i on o f t he bod y a s the gene ra to r o f a c ti ng fo r ce s , and no ton ly o f the e f f ect s o f t he se fo r ce s on t he ex t e rna l wor ld . A num ber o f eve ryd ay s i tua t i onsco r r e spond t o t h i s de f i n i t i on : wa t ch ing somebody ' s a c t i on w i th t he de s i r e t o do i t ,an t i c i pa ti ng t he e f fec ts o f an ac t ion , p r epa r ing o r i n t end ing t o mov e , r e f ra in ing f rommoving , r emember ing an ac t i on , e t c . , c an be cons ide r ed a s pu t a t i ve mo to r images . Theses i t ua t ions have been ex t ens ive ly u sed i n t he con t ex t o f men t a l p r ac t i c e ( t h is po in t w i ll bedeveloped in a la te r sec t ion) .

The re a r e a t l e a s t two way s o f dea l ing w i th m o to r images . On e i s t he cogn i t i veapp ro ach , u s ing p sycho log i ca l m e tho do lo gy fo r d i s c lo s ing t he s t r uc tu r e and co n t en t o f them e n t a l p h e n o m e n o n . T h e o t h e r o n e i s m o r e p h y s io l o g ic a l, b a s e d o n o b s e r v a t i o n o f b o d i l yco r r e l a te s o f m o to r imag e ry and even tua l l y on i n f e r ri ng i ts s tr uc tu r e an d c on t en t f r omthese obse rva t i ons . These app roaches w i l l be de sc r i bed i n t he nex t two s ec t i ons .

A C C E S S IN G T H E C O N T E N T O F M O T O R I M A G E SAn in f luen t ia l t h eo ry ( the dua l cod ing t heo ry [50 ]) conce rn ing t he cogn i ti ve s t a t u s o f

men ta l image ry cons i s t s o f sp l i t t i ng t he cogn i t i ve appa ra tu s i n to two d i f f e r en t modes o fgene ra t i on , t he ve rba l and t he non -ve rba l modes . Imag e ry wou ld pe r t a in t o t he l a t te rmo de . Henc e , images wo u ld be gene ra t ed u s ing moda l i t y - speci f ic un i t s (Pa iv io ' s i m a g e n s ) .I m a g e n s co r r e s pon d t o na tu r a l ob j ec t s o r ob j ec t pa r t s , o r g roup ings o f ob j ec t s , wh ich canbeco m e s imu l t aneous ly ava i l ab l e t o bu i l d a men ta l image ( o f some bo dy ' s fa ce , o f m y l iv ingroom , o f an ac t i on , e t c .) .Th i s t heo ry imp l i e s t ha t t he ve rba l and t he non -ve rba l ( image ry ) sy s t ems a r efunc t i ona l l y i ndependen t , i n t he s ense t ha t t hey can be ac t i va t ed s epa ra t e ly : ye t t heyrema in i n t e r connec t ed , so tha t i t is pos s ib l e t o gene ra t e men ta l images t h roug h ve rba li n s t ruc t i ons , and t o de sc r i be men ta l images ve rbal l y . I wo u ld l i ke t o a rgue t ha t one o f thed i ff e r ences be tween m o to r and o the r t ypes o f image ry l ie s i n t he s t r eng th o f t h isconnec t i on be tween t he ve rba l and t he non -ve rba l sy s t ems . Common expe r i ence showstha t v i sua l ima ges a re eas i ly descr ibe d verba l ly . Shape , c o lor , s ize of represented obje c tscan be de sc r i bed i n g r ea t de t a il . I n con t r a s t , m o to r images a r e d if f icu l t t o t r ans f e r i n to ave rba l code . I t i s di ff icul t, i f no t imposs ib l e t o de sc r i be the c oo rd ina t i on o f m ovem en t sneeded fo r sw imming , f o r example . I n sp i t e o f be ing poo r ly co ded ve rba l l y , howeve r ,m o to r r ep r e sen t a t i ons a r e cogn i t ive ly acce ss ib le , u s ing ind i r ec t me tho ds such a s ma t ch ingt h e im a g e d m o v e m e n t w i t h a re a l m o v e m e n t o r m e a s u ri n g m e n t a l m o v e m e n t t im e . A s a nexamp le o f t h is app roac h , K la t zk y and he r co l le agues have n i ce ly dem ons t r a t e d t h a t som easpec t s o f mo vem en t s pe r fo rm ed du r ing i n t e r ac ti ons w i th ob j ec ts a r e consc ious lyacces s ib l e , and t ha t such knowledge i s r e f l e c t ed i n mo to r pe r fo rmance . Sub j ec t s c anre l i ab ly i nd i ca t e wh ich hand shape t hey wou ld u se when i n t e r ac t i ng w i th a g iven ob j ec tand , conv e r se ly , when a hand shape i s de s igna t ed , wh ich ca t ego ry o f ob j ec t s wou ld e l ic i t i t[ 41 ]. M ore r ecen t ly , K la t zky e t a l . [ 42 ] s t ud i ed ho w func t i ona l i n t e r ac t ions w i th ob j ec t s a r er ep re sen t ed . Th ey fou nd t ha t sub j ec ts c an de t e rmine cogn i t ive ly wh ich ac t i ons a r e e f fec t edby p rox im a l e f f ec to r s ( t he a rm) o r d i s ta l e f f ecto r s ( t he hand ) , o r wh ich ha nd con f igu ra t i onwou ld have t o be u sed fo r a g iven ac t i on . These r ep r e sen t a t i ona l d i s t i nc t i ons co r r e spondto t he mo to r pa t t e rn wh ich i s obse rved when r ea l ob j ec t -o r i en t ed ac t i ons a r e pe r fo rmed(e.g. [36]).


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MOTORIMAGERY 1421The p os s ib i l i ty o f a cce s sing t he con t en t o f m o to r images t h rough a ch rono m e t r i c

ana ly s i s o f men t a l l y s imu la t ed ac t i ons ha s been i nves t i ga t ed by u s ing a r eac t i on t imep a r a d i g m . G e o r g o p o u l o s a n d M a s s e y [2 6 ] m e a s u r e d r e a c ti o n t i m e in s u b j e c ts w h o w e r ea s k e d t o m o v e a m a n i p u l a n d u m a t a g i v e n a n g l e f r o m t h e d i r e c t i o n s h o w n b y a v i s u a lt a rge t . The d u ra t i o n o f t he r eac t ion t ime was foun d t o i nc r ea se w i th r e spec t t o t hemovemen t s no rma l ly d i r ec t ed a t t he v i sua l t a rge t . Fu r the rmore , t h i s i nc r ea se i n r eac t i ont ime was a f u nc t i on o f t he am p l i t ude o f t he ang le . The au th o r s i n t e rp r e t ed t h i s f i nd ing byhypo thes i z ing t ha t sub j ec t s men t a l l y r o t a t ed t he movemen t vec to r un t i l i t r e ached t hedes i r ed ang l e , and t ha t t he t ime t aken fo r t h i s r o t a t i on was g r ea t e r f o r l a rge r ang l e s .Reac t i on t ime , cons ide r ed a s men ta l movemen t t ime , i n f ac t co r r e l a t ed l i nea r l y w i thme n ta l m ove m en t d i f f icu l ty ( ca l cu la t ed f rom the am p l i t ude o f t he ang le s ). Th i s b ehav io rc o n f o r m s t o t h e c la s si ca l F i tt s ' l a w , w h i c h a c c o u n t s f o r t h e d u r a t i o n o f m o v e m e n t s u n d e ra c c u r a c y c o n s t r a in t s [ 1 8 ]. H e n c e G e o r g o p o u l o s a n d M a s s e y ' s c o n c l u s io n t h a t , b e c a u s eF i t t s' l a w h o l d s i n t h is c o n d i t io n , " b o t h r e al a n d i m a g i n e d m o v e m e n t s m i g h t b e g o v e r n e dby s imi l a r am p l i t ude -accu racy r e l a t ions" ( [26 ] , p . 361) . Th is f i nd ing subs t an t i a t e s t he w aypa ram e te r s o f mov em en t execu t i on ( i n t h is c a se, d ir ec t ion ) a r e cod ed cen t r a l l y du r ingm oto r p r ep a ra t i on , and p rov ides a phys io log i ca l r a t i ona l e f o r t he exp re s sion o f such aun ive r sa l mo to r r u l e a s F i t t s ' l aw .A t t e m p t s w e r e m a d e i n o u r l a b o r a t o r y t o m e a s u r e t h e d u r a t i o n o f a p u re l y m e n t a ll ype r fo rmed ac t i on . Two ma in r e su l t s we re ob t a ined i n t he se expe r imen t s . F i r s t , t hedu ra t i on o f a men t a l l y pe r fo rm ed ac t i on ( e.g . wa lk ing men ta l ly t o a p r ev ious ly me mo r i zedt a rge t ) was fo un d t o be s imi l ar t o t ha t o f the s am e execu t ed ac t i on [12 ] . Second , t hedu ra t i on a l so i nc r ea sed a s a f unc t i on o f the ' d if f icu l ty ' o f t he t a sk . Sub j ec t s we re i n s t ruc t ede i t he r t o a c tua l l y wa lk o r imag ine t hemse lves wa lk ing on beams t ha t had t he s ame l eng thbu t va r i ed i n w id th . The bea m wid th w as a s sum ed t o be a f ac to r o f d if f icu l ty , i .e . t hena r ro w er t he bea ms , t he m ore d i ff icu l t t he t a sk . A c l ea r ef f ec t o f t a sk d i f f icu l ty was foun din bo t h ac tu a l and men ta l mo vem en t t imes [13 ]. Th i s r e su l t c an be i n t e rp r e t ed w i th in t hea b o v e f r a m e w o r k o f th e s p e e d - a c c u r a c y t r ad e - of f .

A new expe r imen t was unde r t aken t o ve r i f y t he va l i d i t y o f F i t t s ' l aw in pu re ly men ta lac t i ons . No rm a l sub j ec t s we re in s t ruc t ed t o wa lk men ta l l y t h rough ga t e s o f a g ivenapp a ren t w id th pos i t i oned a t d i f f e ren t appa ren t d i s tances . The ga t e s we re p r e sen t ed t o t hesub j ec t w i th a 3 -D v i sua l d i sp l ay ( a v i r t ua l r e a l i t y he lme t ) wh ich i nvo lved no ca l i b r a t i onwi th ex t e rna l cues and no pos s ib i l i t y f o r t he sub j ec t t o r e f e r t o a known env i ronmen t .Sub j ec t s had t o i nd i ca t e t he t ime t hey s t a r t ed wa lk ing , and t he t ime t hey pa s sed t h roughthe ga t e . Aga in , men t a l wa lk ing t ime was found t o i nc r ea se w i th i nc r ea s ing ga t e d i s t anceand dec r ea s ing ga t e w id th . Thus , i t t ook t he sub j ec t s l onge r t o wa lk men ta l l y t h rough ana r row ga t e t han t o wa lk t h rough a l a rge r ga t e p l aced a t t he s ame d i s t ance [10 ] .

The ques t i on r a i s ed by t he se expe r imen t s i s t o unde r s t and a t wh ich l eve l o f a c t i ongene ra t i on t he de sc r i p t i on o f t he r ep r e sen t a t ion i s made . One o f the cu r r en t i n t e rp r e t a t i onsg iven to F i t t s l aw ho lds t ha t t he i nc rea se in mov em en t du ra t i on i s due t o an i nc r ea se i n t hea m o u n t o f f e e d b a c k i n f o rm a t i o n t o b e p r o c e s se d f o r hi g h er a c c u r a c y d e m a n d s . I f th i si n t e rp r e t a t i on we re co r r ec t , i t w ou ld imp ly t ha t t he sub j ec t imag ines m ove m en t execu t i ona n d t h a t h e u s e s s o m e i m p li ci t k n o w l ed g e a b o u t p e r f o rm i n g m o v e m e n t s o f th e s a m edeg ree o f d i f fi cu lt y [ 54 ], f o r example , t ha t o ne ha s t o s l ow dow n w hen go ing t h ro ugh ana r row doo r . Thus , i t wou ld no t be su rp r i s i ng t o obse rve s imi l a r t im ing i n bo th execu t i onand s imu la t i on . A l thoug h t he is sue o f imp l ic i t know ledge canno t be ru l ed ou t comple t e ly ,ano the r i n t e rp r e t a t i on wou ld be t ha t F i t t s ' l aw gove rns no t on ly t he execu t i on , bu t a l so


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t he p l ann ing o f a c t i ons , so t ha t m ove me n t execu t ion can be ad ju s t ed t o t he r equ i r eme n t so f t he goa l . Cons ide r , f o r example , an eve ry day li fe s i tua t i on w he re t he s am e behav io rar i ses , e .g . d r iv ing a car th ro ugh ga tes o f d i f fe ren t wid ths . A s the ga tes ge t nar ro we r , thed r ive r, even i f he is awa re t ha t t he ca r f it s t he ga t e w id th , w i ll spon t an eous ly s l ow dow nand nea r ly s t op t he ca r be fo r e ge t t ing t h roug h t he ga te . I n o the r t e rms , t he d r i ve r tends t oad jus t t he ve loc i t y o f h is c a r t o t he accu racy r equ i r emen t o f t he pa th : t he g r ea t e r t heaccu rac y need ed t o d r i ve t he ca r s a f e ly th roug h t he obs t ac l e s , the s l ow er t he ve loci t y . Th i sexam ple sugges t s tha t t he d r i ve r ' s behav io r , a s w e l l a s t ha t o f t he sub j ec t s pa s sing m en ta l l yth rough t he ga t e s, is de t e rmined by h is r ep r e sen t a ti on o f the ac t i on , r a t he r t han bycons t r a in t s a r is i ng du r ing execu t i on . Th i s j u s ti f ie s t he u se o f m o to r im age ry a s a t oo l f o ru n d e r s t a n d i n g t h e c o n t e n t o f m o t o r r e p r e s en t a ti o n s.

P H Y S I O L O G IC A L C O R R E L A T E S O F M E N T A L S I M U L A T IO N O F M O V E M E N TW h a t i s t h e s i tu a t io n o f m o t o r i m a g e s a m o n g t h e o t h e r m o t o r p h e n o m e n a , s u c h a s

m o to r i n t en t i on , p r epa ra t i on , o r execu t i on? Aga in , t he r e a r e s eve ra l pos s ib l e way s o fdea l i ng w i th t h is p rob l em . A f i rs t hypo thes i s s t a t e s tha t s imu la t ing a mov em en t i s t he s am eth ing a s a c tua l l y pe r fo rming i t, excep t tha t execu t i on is b locked . An a l t e rna ti ve hy po thes i ss t a t e s t ha t m en t a l s imu la t i on i s lim i t ed t o r ehea r s ing ea r l y s t ages o f ac t i on , we l l ahead o fexecut ion . Whereas the f i r s t hypothes i s genera tes spec i f ic predic t ions (e .g . one shouldexpec t f ind ing i n m o to r ima ge ry phys io log i ca l co r r e l a te s s imi l a r to t hose me asu red du r ingrea l a c ti on ) , t he s econd o ne i s mo s t l y a de f au l t hypo thes i s . I t wou ld on ly becom e p laus ib l ei f su f f i c i en t conv inc ing ev idence fo r t he f i r s t one cou ld no t be shown . I n t h i s and t hefo l l owing s ec ti ons , some o f t he se a rgumen t s w i ll be d i s cus sed unde r two m a in head ings ,t he phys io log i ca l changes occu r r i ng du r ing m en ta l s imu la t i on o f movem en t s , and t heef fec t s of menta l t ra in ing .Phys io log i ca l co r r e l a t e s have been r eco rded i n many expe r imen t s i nvo lv ing men ta lm o t o r i m a g e ry . E l e c t ro m y o g r a p h i c a c t iv i ty ( E M G ) w a s f r e q u e n t ly f o u n d t o i n c re a s e w i thr e s p ec t t o r e s t du r i n g m o t o r s i m u la t io n . J a c o b s o n [ 3 5 ] f o u n d m i c r o m o v e m e n t s a n dinc r ea sed EM G in t hose l imbs invo lved in imag ina l mo vem en t s , bu t no t i n thecon t r a l a t e r a l ones . These d is cha rges we re r e l a ted t o t he r equ i r emen t s o f the imag inedt a s k (e .g . r h y th m ) . S h a w [ 61 ] f o u n d E M G i n cr e as e to b e p r o p o r t i o n a l t o t h e a m o u n t o fimag ined e f fo r t . He nce t he com m on c l a im tha t t he k ine s the ti c image o f a mo t ion pa t t e rn i saccom pan ied by t he s ame i nne rva t i on pa t t e rn a s du r ing t he m o t ion i t s e lf ( s ee al so [28 ] and[30] ). We hn er e t a l . [ 65 ] l ooke d fo r a pos s ib l e re l a t ionsh ip be twe en t he amp l i t ude cou r se o ft h e m e n t a l ( i d e o m o t o r ) E M G a n d p r o c e s s u a l ta s k c h a ra c te r is ti c s. T h e y f o u n d f r e q u e n c ycha rac t e r i s t i c s i n t he men ta l t r a i n ing EMGs s imi l a r t o t hose i n t he ac t i ve t r a i n ing EMGs ,wh ich show s t ha t t he r e a r e t a sk - spec i fi c f r equenc i es in t he E M G o f t he r e laxed a rm du r ingmen ta l t r a in ing . T he f ac t t ha t E M G was found t o be qu i e scen t in s eve ra l expe r imen t s ( e. g.[ 66 ]) a l so i nvo lv ing m o to r im age ry does no t con t r ad i c t t he abov e i dea . I t m ay o n ly r e fl e ctbe t t e r i nh ib i ti on o f m ovem en t execu t i on unde r c e r t a in cond i t i ons . Con ve r se ly , t he f ac tt ha t muscu l a r a c t i v i t y i s some t imes on ly pa r t i a l l y b locked du r ing mo to r s imu la t i on ( a sshown by r e s idua l EMG) emphas i ze s t he de l i c a t e equ i l i b r i um be tween exc i t a t o ry andinh ib i t o ry i n f luences a t t he mo tone u ro n leve l and sugges ts t ha t mo to neu rons a r e c l o se t oth r e sho ld ( see be low) .T h e f a c t t h a t m e n t a l s i m u l a ti o n o f m o v e m e n t a c ti v a te s m o t o r o u t p u t w a s c o n f ir m e d b ya r ecen t s t udy o f sp ina l exci t ab i l it y du r ing m o to r image ry . B onne t e t a l . [5] instructed


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subjects either to press isometrically on a pedal, or to simulate mentally the same action.Two levels of strength (weak and strong) were used. The main result of this experimentwas that motoneuron excitability, as tested by the amplitude of spinal monosynapticreflexes, was increased during mental simulation. This increase was only slightly less thanthe reflex facilitation associated with the current performance of the same movement.Tendinous-reflex (T-reflex) amplitude was more increased than H-reflex amplitude. Inaddition, the change of reflexes in the leg imagined to be involved in the movement waslarger for a strong than for a weak simulated pressure. In accordance with some of theforegoing results, a weak EMG activity was found during mental imagery, which wasmodulated by the lateralization and intensity of the imagined movement.

The fact that the T-reflexes were more facilitated than H-reflexes is important toconsider. Both reflexes are conveyed via the same monosynaptic neuronal pathways butthe H-reflex, which is triggered by the electrical stimulation of the fibers, short-cutsneuromuscular spindles, while the T-reflex, which is triggered by a tendon tap, is aresponse to stretching these spindles. Insofar as the sensitivity of neuromuscular spindlesto muscular stretch is under the control of gamma motoneurons, an increase in excitabilityof the T-reflex, but not of the H-reflex, would result from a selective increase in gammamotoneuron activity. The possibility of a spindle activation during mental simulation of amovement is an interesting one. Spindle afferents are known to play a role not only duringmovement execution, but also for organizing the motor output during self-generatedactions [31, 531. For example, passively executed movements or vibrations of thecorresponding tendon strongly facilitate, via spindle activation, the initiation of voluntarymovements when such an initiation has became difficult, or even impossible, after either alasting immobilization or a cerebral lesion [7].

Other effecters normally not submitted to voluntary control, such as the autonomiceffecters, are also likely to be activated during motor imagery. This possibility was testedby instructing subjects to either actually perform or mentally simulate a leg exercise at twolevels of work. Heart rate, respiration rate and end-tidal PcoZ were measured in bothconditions. After a few seconds of actual exercise, respiration and heart rates began toincrease. The heart rate increased about 50% over the resting level value. In the mentalcondition where no work was produced, this increase was about 32% [l 11. The differencebetween the two situations might reflect the contribution of the reflex regulation ofrespiration and heart rates in response to peripheral factors. It remains that a largefraction of this immediate increase at the onset of exercise (both real and mental) would bedue to the effect of motor preparation, not to the metabolic changes. This point confirmsearlier experiments of Adams et al. [2] who showed that heart rate and cardiac outputalready increase notably within about 5 beats after exercise was started. Respirationchanges within one breathing cycle. These findings suggest that vegetative activationduring preparation to effort would be timed to begin when motor activity starts. Thiswould represent an optimal mechanism for anticipating the forthcoming metabolicchanges and shortening the intrinsic delay needed for heart and respiration to adapt toeffort (for a review, see [56]). Autonomic activation during imagined action thus pertainsto the same phenomenon of preparation to action and appears as a mandatoryconsequence of motor simulation.

The possibility that these autonomic changes were a consequence of muscular activitycan be ruled out. The spectroscopic analysis performed by Decety et al . [l 11, showing nochange in muscular metabolism during mental simulation, is against this possibility. In


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a d d i t i o n , G a n d e v i a e t a l . [ 23 ] obse rved g r aded ca rd iovascu l a r changes i n pa r a lyzedsub j ec t s a t t emp t ing mu scu l a r con t r ac t i ons . A s pa ra ly s i s was comp le t e , t he se changes cou ldno t be due t o r e s idua l muscu l a r a c t i v i t y .The i s sue o f phys io log i ca l co r r e l a te s o f m o to r s imu la t i on ha s a l so been i nves t i ga tedus ing b r a in ac t iv i t y ma pp ing t e chn iques . R o l and e t a l . [ 57 ] a sked no rm a l sub j ec t s toimag ine a r ap id a nd sk il led s equence o f d ig i t mov emen t s . They fo und a s i gn if i can t andl o c al iz e d r C B F c h a n g e m a i n l y in t h e s u p p l e m e n t a r y m o t o r a r e a ( S M A ) . D e c e t y e t a l . [15]s tud i ed no rm a l sub j ec t s imag in ing a g r aph i c m ovem en t (wr i ti ng ' one , two , t h r ee ', e t c .) .The sub j ec t s we re i n s truc t ed t o imag ine t he mov em en t a t t he ' f ir s t pe r son pe r spec t i ve ' andto t r y t o ' f e e l t he i r wr i t i ng hand ' . Reg ions co r r e spond ing t o t he p r e f ron t a l co r t ex , SMA,and a l so the cerebe l lum were ac t iva ted s igni f icant ly , as wel l as in the basa l gangl ia ( see a l so[581).A r ecen t con f i rm a t ion o f t he p ionee ring s t ud i es o f Ro l an d e t a l . [ 57 ] was p rov id ed by t heP E T d a t a o b t a in e d b y F o x e t a l . [ 19 ] and S t ephan e t a l . [ 62 ]. The y sho wed t ha t im ag inedf in g e r m o v e m e n t s a c t iv a t e t h e s u p p l em e n t a r y m o t o r a r e a ( S M A ) a n d p a r i et a l a r e a sb i l at e r al l y . Ac tua l m ovem en t a c t i va t es con t r a l a t e r a l s en so r imo to r co r t ex , a s we l l a s SM Aand pa r i e t a l a r ea s on bo th s i de s . S t ephan e t a l . a l so no t i c ed t ha t du r ing imag ina t i on t heS M A f o c i w e r e l o c a t e d m o r e a n t e r i o r l y t h a n d u r i n g e x e c u t i o n . C o n f i r m a t o r y d a t a w e r ea l s o o b t a i n e d b y t h e f u n c ti o n a l M a g n e t i c R e s o n a n c e I m a g i n g ( f M R I ) t e c h n iq u e . S a n e s e ta l . [ 60 ], i n s t udy ing fM R I ac t i va t i on du r ing bo th execu t ed and im ag ined f i nge rmovemen t s , f ound t ha t , whe rea s an t e r i o r mo to r a r ea s ( such a s SMA) a r e ac t i va t ed i nbo th cond i t i ons , M1 is a c t i va t ed on ly du r ing execu t ion . Ha l l e t t e t a l . [29] , a l so us ingf M R I , r e p o r t e d t h a t s o m e a c t i v a t i o n o c c u r r e d i n p r i m a r y m o t o r c o r t e x d u r i n g i m a g i n e dmo vem en t s . The l eve l o f a c t iva t i on was l e ss du r ing im ag ina t i on t han du r ing execu t i on .

F ina l l y t h is p ro b l em was r e inves t iga t ed r ecen t l y by De ce ty e t a l . [14] , us ing PE T. Three -d im ens io na l g raspa ble o bjec ts (cy l inders and spheres of d if fe ren t sizes, co lors ando r i en t a t i ons ) we re p r e sen t ed t o sub j ec t s who were i n s t ruc t ed t o ' imag ine t hemse lvesg ra sp ing t he ob j ec t s w i th t he i r r igh t han d ' . A s i gn i fi c an t rCB F inc r ea se was ob se rved i na r ea s conce rn ed w i th mo to r behav io r . A t t he co r ti c a l leve l, a r ea 6 in t he i n f e r io r pa r t o f thef ron t a l gy rus w as s t r ong ly ac t i va t ed o n b o th s ides . Ac t i v i t y a l so i nc r ea sed i n l e f t p r e f ron t a la r ea s , ex t end ing t o t he do r so l a t e r a l f r on t a l co r t ex ( a r ea s 9 and 46 ) , and i n t he pa r i e t a llobule (a rea 40) . F ina l ly , the an te r ior c ingula te cor tex (a reas 24 and 32) was b i la te ra l lyac t i va t ed . A t t he subco r t i c a l l evel , t he cauda t e nuc l eus was foun d t o b e s t r ong ly ac t i va t edon bo th s i de s and t he ce r ebe l l um was i nvo lved on ly on t he l e f t s i de . The absence o fl abe ll ing i n t he SM A p rope r i n th i s expe r imen t c an b e exp la ined by t he t ype o f m ove m en ts imu la t ed by t he sub j ec ts . In t he p r e sen t c a se , s imu la t ed m ovem en t s we re v i sua ll y gu idedreach an d g r a sp m ovem en t s d i r ec t ed a t ex te rna l ob j ec t s , whe rea s i n p r ev ious s tud i e s whe reS M A w a s f o u n d t o b e a c t i v a t e d , s i m u l a t e d m o v e m e n t s w e r e r a p i d s e q u e n t i a l a n dpurpose l e s s i n t e rna ll y gene ra t ed f i nge r mov emen t s .

Con sc ious ly r ep r e sen t i ng an ac t i on t hus i nvo lves a pa t t e rn o f co r ti c a l a c t i va t ion wh ichre semble s t ha t o f an i n t en t i ona ll y execu t ed ac t i on [20 , 52 ] . A t t he m om en t , m os t o f t heava i l ab l e ev idence shows t ha t t he p r im ary m o to r co r t ex is si len t when no ex ecu t i on occu r s .B u t , a s t h e a b o v e H a l l e t t e t a l . ' s expe r imen t [ 29 ] sugges ts , t he u se o f m ore s ensi ti vet echn iques migh t no t con f i rm th is conc lu s ion . O the r r ecen t obse rva t i ons u s ing t r ansc r an i a ls t imu la t i on o f p r imary m o to r co r t ex go i n t he same d i r ec ti on i n showing an i nc r ea se i nm oto r r e sponses du r ing men ta l s imu la t i on o f mo vem en t o r r e l a ted ac t i v i ti e s ( [25 , 29 ], s eebe low) . Ac t i va t i on o f mo to r co r t ex , howeve r , is no t t he so l e exp l ana t i on fo r t h is


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MOTOR IMAGERY 1425

enh anc em en t o f co r t i c a l re s ponsi venes s . The co r t i co f uga l vo l l ey p r od uced by co r t i c a ls t imu la t ion c ou ld be m or e e f fi cien t, s imply because of the increase in re f lex exc i t ab i l i ty a tthe sp ina l l eve l . S imi la r d i scuss ions have been ra i sed by exper iments in o ther types ofm en t a l i m age r y , s uch a s v i s ua l i m age r y [ 43 ] o r i n t e r na l s peech [ 32 ].

Som e o f t he re s u l ts p r e s en t ed i n t h is s ec t i on r a is e t he p r ob l em o f t he m e chan i s m and t hel o c us o f m o t o r i n h i b it io n d u r i n g m o t o r s i m u l at io n . W e k n o w t h a t d u r i n g p r e p a r a t i o ninhib i t ion occu rs a t the segm enta l sp ina l l eve l: hence the decrea se of sp ina l re fl exes du r ingt he p r ep a r a t o r y pe r i od and t he i r re i nc r eas e s ho r tl y be f o r e t he m o vem en t s ta r t s [ 6 ]. Thepa t t e r n o f s p i na l exc i tab i li ty du r i ng m o t o r s i m u l a t i on , wi t h a m ar ke d i nc r ea s e i n T- r e fl ex ,i s t hus c l o s e r t o t ha t o f r e a l a c t i on t han t ha t o f p r epa r a t i on . T he m echa n i s m s by wh i ch t hem ot o r co m m an d is a c ti ve ly i nh i b it ed s hou l d t hus d i f fe r f o r p r epa r a t i on and s i m u l a t ion . I nt he f o r m er ca se , t he m ov em en t wou l d be b l ocked by a m as s ive i nh i b it i on ac t i ng a t t hes p i na l leve l to p r o t ec t m o t one ur on s aga i ns t a p r em a t u r e t r i gge ri ng o f a c t i on [ 55 ]. I n t hel a t t e r c a s e , t he exc i t a t o r y m o t o r ou t pu t wou l d be coun t e r ba l anced by ano t he r , pa r a l l e l ,ou t pu t f r om o t he r ( no n - p r i m ar y ) m o t o r co r t ic a l a r ea s , wh i ch wo u l d on l y pa r t ia l l y po l a r i zet h e m o t o n e u r o n s .

T H E E F F E C T S O F M E N T A L T R A I N I N GI f one cons i de r s the s t r ong r e la t ions h i ps o f m o t o r i m age r y t o t he neu r a l s ubs t r a te , i t i s

l og ica l to expec t t ha t t he cen t r a l changes p r odu ced d u r i ng i m age r y will a f f ec t s ubs equen tm o t o r p e r f o r m a n c e . C o n v e rs e ly , th e o b s e r v ed c h a n g e s m i g h t r e p re s e n t a n e x p l a n a t i o n f o rt hos e e f f ec ts kno wn t o a r is e a s a r e s u lt o f m e n t a l t r a in i ng .T h e i n fl u e nc e o f m e n t a l t r a i n in g u s i n g m o t o r i m a g e r y o n m o t o r p e r f o r m a n c e h a s b e e nr ecen t l y r ea s se s s ed and con f i r m ed by s eve ra l expe r i m en t s [ 17 ] . I t ha s been s how n t h a tm e n t a l t r a i n i ng a f fec ts no t o n l y g l oba l m o t o r pe r f o r m ance ( e. g. m us cu l a r s t r eng th , [ 66 ]) ,bu t a l s o a s pec t s o f t he pe r f o r m ance no r m a l l y t hou gh t t o be m or e s pec if ic ou t com e s o ft r a in i ng , s uch a s r educ t i on o f va r iab i l it y and i nc r ea se i n t em po r a l cons i s tency . Th i s wass h o w n b y V o g t et al. [ 64 ]. Th ey exam i ned how s ub jec t s l e a rned t o r ep l ic a t e a pe r i od i cm ovem en t pa t t e r n d i s p l ayed v i s ua l l y . T r a i n i ng was e i t he r phys i ca l ( r ep l i c a t e t he v i s ua lp a t t e r n b y f l e x io n - e x te n s i o n o f t h e e l bo w ) , m e n t a l ( f o r m a m e n t a l im a g e o f t h e m o v e m e n tneeded t o r ep l i c a t e t he pa t t e r n ) , o r obs e r va t i ona l . Phys i ca l t e s t b l ocks we r e pe r f o r m ed( wi t hou t v i s ua l f e edback ) a f t e r t he t r a i n i ng pe r i od i n each m oda l i t y . Phys i ca l and non-phys i ca l t ypes o f p r ac t i c e we r e f ound t o exe r t s im i l a r e f fec ts on r ep l ic a t i on o f m ove m en tf o r m , s pa t i a l s cal ing , cons i s tency o f m ov em en t t em po , and cons i s tency o f r e la t ive t im i ng .H o w c a n t h e b e n e fi t o f m o t o r i m a g e s o n m o t o r e x e c u ti o n b e e x p la in e d ? A s n om ov em en t occu r s , t he f i r s t exp l ana t i on i s t ha t o f a pu r e l y cen tr a l e f fec t. I nc r ea s ed t ra f fi c i nneu ra l c i rcu i t s could b e respons ib le for im prov ing synap t ic e f f icacy in c r i t ica l par t s o f thes ys t em , s uch a s , f o r exam pl e , i n t he ce r ebe l l um o r b a s a l gang l ia . Th i s wo u l d r e s u l t i ni nc r ea s ing t he capac i t y o f t he s ys tem f o r tun i ng m o t on eur o na l popu l a t i ons o r s ha r pen i ngcoo r d i na t i o n be t ween agon i s t and an t agon i s t m us c l e g r oups . Yu e and Co l e [ 66] cam e tot he conc l us i on t ha t t he i nc r ea s e i n m us c l e s t r eng t h f o l l owi ng m en t a l t r a i n i ng d i d no tp r i m ar i l y r e s u l t f r om neu r a l changes a t t he execu t i on l eve l , bu t a t h i ghe r p l ann i ng andpr og r am m i ng l eve l s o f t he m o t o r s ys t em . The a l t e r ed p r og r am s i n t u r n wou l d ach i eves t r eng t h ga i ns by ac t i ng on t he s p i na l c i rcu i t ry .Th i s c en t r a l exp l ana t i on s eem s t o be s up por t ed b y t he e f f ect s o f m ag ne t i c t rans c r an i a ls t i m u l a t i on o f m o t o r co r t ex . The s i ze o f t he exc i t ab l e a r ea devo t ed t o f i nge r m ovem en t s


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1426 M. JEANNEROD

was fo und to i nc r ea se a s movem en t s we re r epea t ed ove r t ra in ing pe r iods ( a f ac t r epo r t ed i nan ima l expe r imen t s by Grunbaum and She r r i ng ton a s e a r ly a s 1903 , s ee [27 ] ) .In te res t ing ly , th i s increase was ob serve d for imagina l t ra in ing as wel l [29] . A re le vanto b s e r v a t i o n w a s m a d e b y G a n d e v i a a n d R o t h w e l l [2 5]: t h e y s h o w e d t h a t ' c o n c e n tr a t in g 'on o ne ha nd musc l e w i tho u t a c t i va t i ng i t i nc r ea sed t he e f f ect o f sub th re sho ld m agne t i cs t imu la t i on o f the co r t i c a l a r ea co r r e spond ing t o t ha t m usc l e ( and no t o f o the r musc l e s ) .Thus , t he r e i s a s e l ec ti ve enhance men t o f co r ti c a l r e spons ivenes s du r ing image ry o r r e l a t edprocesses .

The cen t r a l exp l ana t i on i s an i n t e r e s t i ng one , a s i t ha s imp l i ca t i ons fo r t he p roces s o fl ea rn ing i n gene ra l. T hus , t he r e w ou ld be an i nne r r egu l a t i on o f ex t e rna l pe r fo rm ancew i t h o u t r e c o u r se t o a c t u a l m o v e m e n t a n d t o t h e r e la t ed s e n s o r y f e e d b a c k a n d k n o w l e d g eo f r e su l ts . Th i s exp l ana t i on is compa t ib l e w i th a h i e ra r ch ica l m ode l o f a c ti on ge ne ra t i onwh ere an in te rna l repres enta t ion of the ac t ion re leases e f fe ren t s igna ls in lower leve lmechan i sm s , an d whe re a com par i son o f t he e f f er en t d i s cha rges w i th t he i n t e rna lr ep re sen t a t i on o f t he ac t i on i s pe r fo rme d ( see [40 ] and [44 ]) . A s imi la r m ode l w i ll bedeve loped i n t he nex t s ec t i on .I n t h e a b o v e h y p o t h e s is , t h e E M G a c t iv i ty o b s e rv e d d u r in g m e n t a l t r a in i n g w o u l d h a v eto be con s ide red m ore a s an e f f ect t han a c ause o f t he p roces se s t ak ing p l ace du r inglearn ing . The recent f ind ings on sp ina l re f lexes dur ing motor imagery repor ted in thep rev ious s ec t i on , howeve r , sugges t an a l t e rna t i ve - - a lbe i t complemen ta ry - - i n t e rp re t a t i on .A ton i c a f f e r en t d i s cha rge a r i si ng f rom the musc u l a r sp ind l e s du r ing im age ry (pos s ib ly duet o i n c r ea s e d g a m m a m o t o n e u r o n a c ti v it y ) c o u l d h a v e i m p l ic a t io n s f o r s u b s e q u e n t s h a p in go f m o t o r p e r f o r m a n c e a n d i m p r o v e m e n t i n l ea rn in g . N o t e t h a t t h is i n te r p r e t a ti o n i scom pa t ib l e w i th t he ' c en t r a l ' ex p l ana t i on above , a s it a l so i nvo lves t he ac t i v i t y o f a c en t r a lgene ra to r f o r t r igge r ing t he sp ind l e ac t i v it y . A s imi la r exp l ana t i on wo u ld ho ld fo r changesi n t h e m o t o r s y s t e m d u r i n g m o t o r p r e p a r a ti o n . M e U a h et al.'s [46] exper im ents in them o n k e y s h o w e d t h a t a s m a l l p r o p o r t i o n o f b ic e p s m o t o r u n i ts w e r e ac t iv e d u r in g t h ep repa ra t i on pe r iod p r eced ing t he i n s t ruc t i on t o m ake a f l ex ion o f the a rm. These un i ts ,wh ich ha d a l ow th r e sho ld and a l ow d i scha rge r a t e, s t op ped f i ri ng sho r t ly be fo re t hemovemen t began . Th i s p r epa ra to ry muscu l a r a c t i v i t y , i t was sugges t ed , p l ayed animp or t an t r o l e in a sub seque n t movem en t , i nc r ea s ing t he s ti ff nes s o f t he musc l e andreduc ing i t s t ime cons t an t i n r e spond ing t o t he phas i c command . I t cou ld a l so havep rov ided a f f e r en t i n fo rma t ion t o f ac i l i t a t e t he cen t r a l neu rons r e spons ib l e fo r gene ra t i ngthe phas i c comm and . D i sen t ang l ing t he 'c en t r a l ' and ' pe r iphe ra l ' sou rce s o f i n fo rm a t ionfo r men ta l p r ac t i c e wou ld r equ i r e s t ud i e s i n dea f f e r en t ed pa t i en t s . Th i s wou ld i nvo lvet e s t i ng t he ab i l i t y t o gene ra t e an imag ined movemen t i n a dea f f e r en t ed l imb ( i n pa t i en t ssu f fe r ing pe r iphe ra l neu ro pa thy , f o r example ) . I f v iv id image ry p e r s i st ed i n t he se pa t i en t sand i f ga in s in sub seque n t pe r fo rma nce w e re ob t a ined , t h i s wo u ld ru l e ou t a r o l e o f sp ind l ea f f e ren t s , even if E M G d i scha rges w e re reco rded .

A M O D E L F O R S E L F - G E N E R A T E D A C T IO N SThe f i na l s ec t ion wi ll dea l w i th an a t t em p t a t de s ign ing a s imp le mod e l o f gene ra t i on andcon t ro l o f goa l -d i rec t ed ac t i on co mp a t ib l e w i th t he occu r r ence and t he e f fec ts o f men ta limage ry . A t e s t ab l e hypo thes i s w i l l be p roposed , imp ly ing t ha t mo to r r ep re sen t a t i ons fo rac t i ng and fo r imag ing a r e one and t he s ame th ing , and t ha t t he two modes o f

r ep re sen t a t i ons a r e on ly d i s t ingu i shab l e by t he c i r cums tances i n wh ich t hey a r e gene ra t ed .


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MOTOR IMAGERY 1427

This hypothesis obviously rejects the notion of motor images as an epiphenomenonunrelated to the generation of actions (for a discussion of this point, see [39]).Motor representations are conceived here as 'internal models' of the goal of an action.The goal of an action, however, can be specified at several different levels which represent

different aspects of the same action [59]. It can be defined as the final result at which theaction is intended. I t can also be defined as a sequence of intermediate actions needed toachieve the final result; or as a program of coordinated joint movements; or as a set ofmotor commands, etc. (see [37]).The first assumption of the model is that these functional (and possibly, anatomical)levels of representation are organized hierarchically. This hierarchy can be conceived as asequence of different modules (Fig. 1): The complete internal model of the action to beperformed is stored at the higher level. Activation of the modules has to follow a preciseorder, such that the global internal model of the action activates an appropriate plan,which in turn activates motor programs, etc. This conception of action generation is inagreement with previous models. Hierarchical architectures similar to those of computerprograms were often used to conceptualize action plans where actions were represented assequences of steps involving tests and operations (see [48]). Norman and Shallice [49], forinstance, assumed that specification of the components of actions was carried out "bymeans of numerous memory schemas, some organized into hierarchical or sequentialpatterns, others in heterarchical or independent parallel (but cooperating) patterns" ([49],p. 5). In this conception, any given action sequence was represented by an organized set ofschemas, with one--the source schema--serving as the highest order control andactivating the other component-schemas for the individual movements of that action.When a given source-schema had been selected, component-schemas were controlled byhorizontal and vertical processing threads. Horizontal threads determined the order ofactivation of the component-schemas and thus specified the structure for the desiredaction sequence, al though vertical threads determined activation values for these schemas.Activation values involved attentional control, motivational factors, etc. The notion ofschema, which is a convenient one for accounting for the way representations are stored atintermediate and lower levels (see, e.g. [4]) is probably too restrictive for higherrepresentat ional levels. Intentions, by definition, do not always correspond to preexistingschemas and may represent newly built mental structures.The other important feature of the proposed model is that it involves controlmechanisms which are activated in parallel with the main stream o f informat ion (Fig. 1).These control mechanisms imply a storage of operations performed at each level in asmany motor memories as there are levels. These memories are used as a comparator forcontrolling the unfolding of the action. Reafferences picked up at the periphery anddocumenting the current state of the action are fed into the motor memories and can beused to signal its degree of completion. If the desired action has been completed, thereafferences and the content of the memories will coincide, and the latter will be erased. I fthe action is incomplete, residual activity will persist in the memories: this will reactivatethe corresponding module and generate corrections. Finally, if the desired action does nottake place, memories will remain fully activated, which in turn will maintain the wholesystem activated.A first attempt at testing the validity of this model can be made by describing thefunctioning of the lower levels. Hence, the program module sends commands to theexecution level (e.g. direction and extent of a movement). If an external perturbat ion arises


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1428 M. JEANNEROD

o f s ta te sin te rn a l o f c o n t e x t c o n s t r a i n t s fx t e r n a l [ p e r t u r b a t i o n s ]

~ ACTION

Fig. I. A functionalcha rt for self-generatedactions. The ma in flowof informationgoes hrou! 1 hecognitive steps of action (e.g. intending, planning, programm ing and executing). Ea ch o f theselevels can be influen ced by level-specificenvironm ental cues, which define the c ontext or theconstraints of a given action, or which arise from perturbations during execution.The h igher level(intending) is supposed to be less influenced by environmentalcues and m ore by internalizedcues.The lower row depicts level-specific hort-term mem orieswhich are activated by changes occurringat eac h of the cognitive steps, and deactivated by reafferencesarising from movem ent execution.Fo r d etails see text.d u r i n g e x e c u t i o n a n d t h e c o r r e c t e ff e c t i s n o t o b t a i n e d , t h e r e a f f e r e n c e s ( e ,g . v i su a l ,p r o p r i o c e p t i v e ) a r is i n g f r o m t h e p e r i p h e r y w il l n o t c o r r e s p o n d t o t h e r e p r e s e n t a t i o n o f t h ed e s i re d s t a t e s t o re d i n t h e m e m o r y , a n d t h e m e m o r y w i ll n o t b e c a nc e l le d . A n e r r o r s i gn a lw i ll b e t r a n s f e r r e d t o t h e p r o g r a m l e v e l w h i c h w i ll c h a n g e i t s s p e c i fi c a t io n u n t i l e x e c u t i o n isf i n a l ly c o r r e c t . T h i s c o m p a r i s o n p r o c e s s c a n b e v e r y f a s t (s e e e x a m p l e s b e l o w ) : t h i s i s ap r e r e q u i s it e f o r p r e v e n t i n g t h e e r r o r s i g n al f r o m p r o p a g a t i n g t o h i g h e r l e v el s. If , h o w e v e r ,t h e a l t e r a t i o n o f t h e p r o g r a m i s n o t s u f fi c ie n t t o o b t a i n t h e d e s i r e d e f f ec t , t h e p r o g r a m l e v e lw i l l r e m a i n a c t i v a t e d a n d t h e e r r o r s i g n a l w i l l p r o p a g a t e t o t h e p l a n , e t c .

T h i s m e c h a n i s m , c l as s ic a ll y k n o w n i n n e u r o p h y s i o l o g y a s th e ' c o r o l l a r y d i s c h a r g e ' o r' e f f e r e n c e c o p y ' m e c h a n i s m [3 4, 63 ], p o s t u l a t e s t h a t s i g n a l s g e n e r a t e d b y t h e m o t o r c e n t e r sp r o v i d e i n f o r m a t i o n a b o u t f u t u r e m o v e m e n t s b e f o r e th e y h a v e r e a c h e d t h e e ff e c to r . T h i sm o d e o f r e g u l a t io n ( a l s o t e rm e d " i n t e r n a l f e e d b a c k " b y M i le s a n d E v a r t s [ 47 ]) h a s a m a j o ra d v a n t a g e i n m i n i m i z i n g c o r r e c t io n d el a y s w h e n a p e r t u r b a t i o n o c c u r s d u r i n g e x e c u t i o n o fa m o v e m e n t . F o r e x a m p l e , w h e n t h e v is u a l t a r g e t o f a r e a c h i n g m o v e m e n t is s u d d e n l yd i s p la c e d d u r i n g t h e m o v e m e n t a c o r r e c t i o n c a n b e g e n e r a t e d w i t h i n 1 00 m s e c o r l e ss , s ot h a t t h e t a r g e t is c o r r e c t l y a c q u i r e d w i t h o n l y li t tl e i n c r e a s e in m o v e m e n t t im e . T h i sc o r r e c t i o n is a c h i e v e d t h r o u g h a k i n e m a t i c r e a r r a n g e m e n t d u r i n g t h e e a r l y p h a s e o f t h em o v e m e n t ( se e [5 1]). T h e r e a r e a l s o e x a m p l e s o f f a s t c o r r e c ti o n s i n o t h e r t y p e s o fm o v e m e n t s . A b b s a n d G r a c c o [1 ] h a v e d e s c r ib e d r a p i d c o m p e n s a t i o n ( w i th i n 5 0 m s e c o rl es s) o f p e r t u r b a t i o n s a p p l i e d t o a r t i c u l a t o r s d u r i n g s p e e c h ( s ee a l so [ 9] f o r a s i m i l a r r e s u l tw i t h m e c h a n i c a l p e r t u r b a t i o n s a p p l i e d t o fi n g er s d u r i n g r a p i d g r a sp s ). T h i s t y p e o fc o r r e c t i o n i m p l i e s a d y n a m i c c o m p a r i s o n b e t w e e n t h e i n t e r n a l l y g e n e r a t e d s i g n a l s w h i c h


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MOTOR IMAGERY 1429

r ep r e sen t t he de s i r ed movemen t , and i ncoming s i gna l s wh ich mon i to r t he cu r r en t s t a t e o fexecu t i on . S ince t he r ea f f e ren t s igna l s a r e de l ayed w i th r e spec t to t he c om m and s igna l, t hec o m p a r a t o r m u s t l o o k a h e a d i n t im e a n d p r o d u c e a n e s ti m a t e o f t h e m o v e m e n t v e lo c i tyc o r r e s p o n d i n g t o t h e c o m m a n d . T h e i m a g e o f th is e s t im a t e d v e l o c i ty i s u s e d a t t h e i n p u tl evel f o r com pu t ing t he ac tua l p os i t i on o f the e f f ec to r w i th r e spec t t o t he t a rge t . T hem o v e m e n t s t o p s w h e n t h e t w o s i g n a l s c o r r e s p o n d . S i m u l a t i o n e x p e r i m e n t s s h o w e d t h a taccu ra t e co r r ec t i ons can be gene ra t ed when t a rge t pos i t i on i s pe r t u rbed , w i thou t no t ab l einc r ea se i n movemen t du ra t i on , wh ich i s compa t ib l e w i th t he obse rved da t a [ 33 ] .

A p o s s i b l e a n a t o m i c a l s u b s t r a t e f o r t h e s e m e c h a n i s m s c a n b e f o u n d i n t h e m a n yco l l a t e r a l s a r i s i ng f rom the py ramida l t r a c t , and d i s t r i bu t i ng t o subco r t i c a l s t r uc tu r e sinvo lved i n mo to r con t ro l , such a s t he s t r i a t um, t he ven t r a l l a t e r a l nuc l eus and t heven t ro basa l co mp lex o f t he t ha l amus . A t t he b r a in s t em l evel , py r am ida l co ll a t e ra l scon t r i bu t e t o t he do r sa l co lum n nuc l e i ( see [47 ]) . These co l l a t e ra l s cou ld ca r ry i n fo rm a t ionc o n c e r n i n g t h e ' d e s i r e d ' m o v e m e n t , t o b e c o m p a r e d w i t h r e a f f e r e n t p r o p r i o c e p t i v ein fo rma t ion conce rn ing t he ac tua l l y pe r fo rmed movemen t . A s imi l a r o rgan i za t i on s eemsto ex i s t i n t he sp ina l co rd . The ce rv ica l p rop r io sp ina l n eu rons , wh ich r ece ive a cop y o f t hem o t o r c o m m a n d s a n d w h e r e v is u a l a n d p r o p r i o c e p t iv e r e a ff e re n t si gn a ls h a v e b e e n s h o w nto conve rge , co u ld p l ay t he ro l e o f a com para to r ( e.g . [ 3 ]) .

The nex t po in t t o b e d i scus sed is t he r e l evance o f th i s mode l t o mo to r images . I t isno t e wo r thy t ha t , i n cond i t i ons o f no rm a l execu t i on ( i. e. when t he execu t ed and t her ep re sen t ed ac t i ons co inc ide ), t he r e is u sua l l y no awarenes s o f t he con t en t o f t her ep re sen t a t i on a t a ny leve l and no im age i s expe r ienced . Th i s i s exp l a ined b y t he f ac t t ha tm o to r im age ry and execu t i on have d i ff e r en t t ime cons t an t s . Because image ry , un l ikeexecu t i on , imp l i e s sub j ec t ive awarenes s , i t take s l onge r t o appea r . I f image ry ac tua l l yoccu r r ed i n cond i t i ons o f no rm a l execu t i on , i t wo u ld be de l ayed w i th r e spec t t o t hec o r r e s p o n d i n g a c ti o n . T h i s e f f ec t w o u l d b e e v e n m o r e p r o n o u n c e d i n c a se s o f f a stmo vem en t s o r du r ing t he occu r r ence o f co r r ec t i ons ( s ee [8 ] and [38 ] ) . By con t r a s t ,awa ren es s i s a com m on expe r i ence in cond i t i ons wh e re t he ac t i on is de l ayed , i ncomple t e lye x e c u t e d o r b l o c k e d . A c c o r d i n g t o t h e a b o v e m o d e l , t h o s e a r e c o n d i ti o n s w h e r e t h e m o t o rme mo r i e s a r e no t o r i ncomple t e ly e r a sed , and w he re t he r ep r e sen t a t iona l l eve l s a r e kep tac t i va t ed : t h i s pe r s i s t i ng ac t i va t i on wou ld t hus be t he subs t r a t e f o r ( consc ious ) mo to rimages . In add i t ion , beca use ac t iva t ion pers i s t s a t a l l l eve ls of the sys tem w hen ac t ion i sno t pe r fo rm ed , t he r e is a pos s ib i l i ty t ha t t he con t en t o f t he m o to r im age r e fl e ct s a c t i v i ty o fany o f t he se leve ls . The ac tua l l y expe r ienced image wo u ld dep end on t he l eve l wh ich w ou ldbe p ro be d ( e .g . by t he expe r imen te r ) w i th t he p rov i so t ha t t h i s l eve l wo u ld be consc ious lyaccess ib le . The con t en t o f h ighe r l evels o f the sy s t em shou ld i n p r i nc ip l e be m ore acce s s ib l eto sub j ec t i ve expe r i ence t han t ha t o f the l ower ones .

Obse rva t i ons made i n expe r imen ta l l y pa r a ly sed sub j ec t s a r e i n t e r e s t i ng t o cons ide r i nth is con t ex t , a s t he s i t ua t i on c r ea t ed by pa r a ly s is bea r s som e ana logy w i th men ta ls imu la t i on : i n bo th ca se s , a c t i on i s r ep r e sen t ed , no t execu t ed , even t hough t he l eve l a twh ich mo to r ou tpu t i s b locked i s c l e a r l y d i f f e r en t i n t he two cond i t i ons . Sub j ec t s w i thexpe r imen ta l pa r a ly s i s o f one l imb expe r i ence s t r ong s ensa t i ons o f e f fo r t wh en t he ya t t em p t t o m ove t ha t l imb . To t a l l y cu r a r i zed sub j ec t s a r e ab l e t o f ee l and t o de sc r i be the i ri n t ended m ove me n t s [ 24] . Ga nde v i a [ 21 ] p rop osed an i n t e rp r e t a t i on fo r t he s ensa ti ons o fe f fo r t r epo r t ed by sub j ec t s w i th weakened musc l e s on t he i r a t t emp t s t o move . Hesugges t ed t ha t n eu ra l t r a ff ic in mo to r c o r t i co fuga l pa th s m igh t be r ead o f f by t he sub j ec t sand u sed a s t he r e l evan t s i gna l f o r t he obse rved i l l u s ions . I ndeed , comple t e pa r a ly s i s


12/14

1430 M. JEANNERODf o l l owing py ramida l l e s i ons a t t he co r t i c a l l eve l i s no t a ccompan ied by s ensa t i ons o fi nc r ea sed e f fo r t o r heav ines s , p r ec i s e ly because no t r a f f i c occu r s i n t he mo to r pa thwaysa f t e r such le s ions . Sensa t ions r eapp ea r du r ing pa r t i a l r e cove ry o f movem en t s , when ne u ra lt r a f f i c r e e s t ab l i she s . The s ame hypo thes i s wou ld accoun t f o r pe rmanence o f s ensa t i ons o fe f fo r t i n a ll c a se s o f d i s ta l pa r a ly si s , whe re co r t i co fuga l pa thw ays a r e no t a l t e r ed . I n a m orer ecen t pape r , t he s ame au th o r p rop osed t ha t t he d i s cha rge r e spons ib le f o r the se sensa t i onsdoes no t a r i s e d i r ec t l y f r om the co r t i co fuga l pa thway , because t r ansc r an i a l e l e c t r i c a ls t imu la t i on o f t h is pa th wa y does no t p ro duce a s ensa t i on o f e f fo rt . I n s t ead , t he sub j ec texpe r iences a pa s s ive mov em en t [ 22 ]. He t he r e fo r e sugges t ed t ha t t he r e l evan t d i s cha rgesfo r t he se s ensa t i ons a r i s e f r om s t ruc tu r e s such a s p r emoto r co r t ex o r ba sa l gang l i a .

I t i s i n t e re s t ing t o no t e i n t h is con t ex t t ha t ba sa l gang l ia have been foun d t o be ac t i va t eddu r ing m en ta l s imu la t ion o f m ovem en t [ 14] . I n add i t ion , m o to r image ry ha s been foun d t obe cons ide r ab ly s l owed i n pa t i en t s w i th Pa rk in son ' s d i s ea se , whe rea s i n t he s ame sub j ec t s ano rm al ima gery a b i l i ty for o ther ty pes of ima ges (e.g. v i sua l) i s p reserv ed [16].

T h e a b o v e s p e c u l a t io n b e a r s s o m e r e le v a n ce t o t h e r o le o f m o t o r i m a g e r y a n d m e n t a ls imu la t i on o f mov em en t i n mo to r l e a rn ing ( t he so - ca ll ed men ta l p r ac t i c e ). As a l r ead ys t ressed ear l ie r in th is paper , the pos tu la ted pers i s t ing ac t iva t ion of cent ra l loop s dur ingm oto r im age ry r ep re sen t s a pos s ib l e subs t r a t e f o r improv ing synap t i c e f fi c acy i n t hec i rcu i ts wh ich a r e r ehea r sed . Ano the r exp l ana t i on is r ep re sen t ed by t he pos s ib l e gam m aac t i va t i on du r ing m o to r image ry . Th i s exp l ana t ion de se rves ca r e fu l cons ide r a t i on , bec ausethe r e su l t i ng sp ind l e d i s cha rge wou ld be ve ry d i f f e r en t f r om tha t p roduced du r ing r ea lmo vem en t . D ur ing r ea l mo vem en t , d i s cha rges a r is e f r om sp ind le s in t he musc l e an t agon i s tt o t he movemen t , whe rea s , du r ing s imu la t ed movemen t d i s cha rges a r i s e f r om the agon i s tmusc le . This d i f fe rence i s a c r i t i ca l one in the present model : sp indle d i scharge dur ingm ove m en t ex ecu t i on i s one o f the r ea f f e r en t s i gna l s by wh ich com ple t i on i s s igna l led t o t hesys t em, whe rea s sp ind l e d i s cha rge du r ing mo vem en t s imu la t i on w ou ld i n f ac t r e i n fo r ce t hea c t iv i ty o f t h e m o t o r m e m o r y a n d c o n t r i b u t e t o t h e a c t i vi ty o f t h e c e n tr a l l o o p .

F ina l l y , t he mod e l a l so p r ed i c ts a s imi l a r exp l ana t i on fo r o the r t ypes o f l e a rn ing , such a sl ea rn ing o b t a ined by r epea t ing t he t o -be - l ea rned mov eme n t . Because a t t he ea r l y st ages o fl ea rn ing ac t i ons a r e pe r fo rmed ve ry s l owly , l a rge e r ro r s i gna l s a r i s e f r om the pe r i phe ry ,due t o i naccu ra t e pe r fo rmance . These cond i t i ons c r ea te a s i tua t i on w he re t he cen tr a l l oopsr ema in ac t i va t ed and can be r ehea r sed . Th i s sugges t s t ha t l e a rn ing by pe r fo rming wo u ldno t be subs t an t i a l ly d i f f e ren t f r om l ea rn ing by imag ing , a s t he neu rop hys io log i ca tsubs t r a t e wou ld be t he s ame i n bo th ca se s .

A c k n o w l e d g e m e n t s - - W o r k suppor t ed by t he H uma n Fron t i e r Sc ience Program and by t he P61e Rh6ne -A lpes de sSciences C ogni t ives. I thank P. D om iney for he lpful discussions.

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Jeannerod, M. (1995). Mental Imagery in the Motor Context. Neuropsychologia, 33(11), 1419-1432