Exp rin Res (26) DOI.7/s22-6-456-2 3 RESEARCH ARTICLE 4 Giovnni Mirell Pierpolo Pni 5 Mrtin Pre Stefno Ferrin 6 Inhiitory control of reching movements in humns 7 8 Received: August 25 / Accepted: Mrch 26 2 3 49 Ó Springer-Verlg 26 5 6 Astrct ehviorl flexiility provides very lrge 7 repertoire of ctions nd strtegies, however, it crries 8 cost: potentil interference etween different options. 9 The voluntry control of ehvior strts exctly with the 2 ility of deciding etween lterntives. Certinly inhi- 2 ition plys key role in this process. Here we exmined 22 the inhiitory control of reching rm movements with 23 the countermnding prdigm. Right-hnded humn 24 sujects were sked to perform speeded reching 25 movements towrd visul trget ppering either on 26 the sme or opposite side of the reching rm (no-stop 27 trils), ut to withhold the commnded movement 28 whenever n infrequent stop signl ws presented (stop 29 trils). As the dely etween go nd stop signls in- 3 cresed, sujects incresingly filed to inhiit the 3 movement. From this inhiitory function nd the rec- 32 tion times of movements in no-stop trils, we estimted 33 the otherwise unoservle durtion of the stopping 34 process, the stop signl rection time (SSRT). We found 35 tht the SSRT for reching movements ws, on verge, 36 26 ms nd tht it vried with the reching rm nd the 37 trget position even though the stop signl ws centrl 38 stimulus. In fct, sujects were lwys fster to withhold 39 reching movements towrd visul trgets ppering on 4 the sme side of the reching rm. This ehvior strictly 4 prllels the course of the rection times of no-stop 42 trils. These dt show tht the stop nd go processes 43 intercting in this countermnding tsk re independent, G. Mirell Æ P. Pni Æ M. Pre Æ S. Ferrin (&) Deprtment of Humn Physiology nd Phrmcology, University L Spienz, Pizzle Aldo Moro 5, 85 Rome, Itly E-mil: stefno.ferrin@unirom.it Tel.: +-6-49936 Fx: +-6-4969236 ut most likely influenced y common fctor when under the control of the sme hemisphere. In ddition, we show tht the point eyond which the response cnnot e inhiited, the so-clled point-of-no-return tht divides controlled nd llistic phses of movement processing, lies fter the inter-hemispheric trnsfer. Keywords Motor control Æ Countermnding Æ Humn Æ Reching Introduction Considerle efforts hs een directed to understnding the neurl system underlying the control of reching movements, nd significnt dvnces hve een mde with respect to the preprtion nd the execution of these movements (Georgopoulos 986, 996; Klsk et l. 997; Wise et l. 997; Cminiti et l. 998; Grzino et l. 22). Nevertheless, much less ttention hs een given to mechnisms suserving the inhiition of these movements. The ility of suppressing n impending ction is fundmentl property of executive control, nd the quntittive study of the inhiitory control of ction ws mde possile with the introduction of the countermnding prdigm (Logn 994). This prdigm is test of suject s ility to withhold commnded movement in response to n infrequent stop signl. In prctice, stopping ecomes incresingly more difficult s the dely etween go nd stop signls is lengthened, nd the inhiition function tht descries the proility of stopping cross the rnge of stop signl delys (SSD) cn e exploited to estimte the length of time needed to cncel the commnded movement, i.e., the stop signl rection time (SSRT). Thus fr, the countermnding prdigm hs een used for studying the inhiitory control of different motor cts: eye movements (Hnes nd Schll 996; Hnes nd Crpenter 999; Cel et l. 2; Logn nd Irwin 2; Asrress nd Crpenter 2; Colonius et l. 2; Kornylo et l. 23), eye-hed gze shifts (Corneil P. Pni Dottorto di Ricerc in Neurofisiologi, University L Spienz, Pizzle Aldo Moro 5, 85 Rome, Itly M. Pre Deprtment of Physiology, Queen s University, Kingston Cnd ON K7L 3N6 44 45 46 47 48 49 5 5 52 53 54 55 56 57 58 59 6 6 62 63 64 65 66 67 68 69 7 7 72 73 74 75 76 77 78 79 8 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6
8 nd Elsley 25), mnul responses (Logn 98; Logn 82 nd Cown 984), hnd squeezes (De Jong et l. 99), 83 nd simple rm movements (McGrry nd Frnks 997; 84 Kudo nd Ohtsuki 998). The results show tht the rce 85 model pplies eqully well to ll types of responses, 86 suggesting tht the inhiitory control of ction oeys the 87 sme principles cross effectors nd/or motor systems 88 (Logn nd Irwin 2). In the present report, we 89 dpted the countermnding prdigm to the study of 9 the inhiitory control of visully guided reching 9 movements. This new experimentl pproch is the first 92 step towrd our generl gol of elucidting the neurl 93 sis of this executive process, in prllel to efforts di- 94 rected to understnding the inhiitory control of scc- 95 dic eye movements (Hnes et l. 998; Stuphorn et l. 96 2; Ito et l. 23; Pré nd Hnes 23). 97 An interesting chrcteristic of rm movement is how 98 the performnce of the left nd right rm differs in 99 certin motor tsks. The oservtion tht right-hnded sujects produce left-rm movements with shorter rection times hs een presented s evidence tht there 2 is left rm dvntge in the preprtion of pointing 3 nd reching movements (Vely nd enoit-durocrd 4 999; Mieschke et l. 2; rthelemy nd oulinguez 5 22,; Neely et l. 25). This phenomenon hs een 6 ttriuted to speciliztion of the right hemisphere 7 for sptil processing nd visuosptil ttention 8 (Mesulm 98; Fisk nd Goodle 988; Vely et l. 9 2; rthelemy nd oulinguez 22). Furthermore, it hs een shown tht the rection times of movements mde y oth left nd right rms re shorter when the go 2 signl ppers in the visul field ipsilterl to the rm 3 used y the suject (Vely nd enoit-durocrd 999; 4 rthelemy nd oulinguez 22; Cvin-Prtesi et l. 5 24). This symmetry is thought to reflect the inter- 6 hemispheric trnsmission of informtion (shore 98; 7 Mrzi et l. 99; Mrzi 999). y studying right-hn- 8 ded sujects mking left- nd right-rm reching 9 movements to left nd right visul trgets in counter- 2 mnding rech tsk, we wished to determine whether 2 nd how the ility to withhold movement in response 22 to centrl stop signl is dissimilr. Given the rce 23 model ssumption of independence of go nd stop 24 processes, we expected stopping ility not to e influ- 25 enced y performnce symmetries relted to either the 26 reching rm or the trget position. 27 The countermnding rech prdigm cn lso shed 28 some light onto nother importnt issue, nmely, whe- 29 ther the cognitive process underlying the inhiition of 3 ction is lterlized. Aron et l. (23) hve shown tht 3 humn ptients with highly selective lesions to the right 32 inferior frontl gyrus (IFG) hve defective stopping 33 ehvior. The SSRTs of these ptients were significntly 34 longer thn controls nd they correlted with the 35 mount of dmge to the right IFG. This study strongly 36 suggests the right hemisphere s exerting preferentil 37 inhiitory control over ction. In contrst, Vn der 38 Schoot et l. (23) found tht stopping performnce 39 ws etter with stop signl presented in the right visul field, therey suggesting mjor involvement of the left hemisphere in inhiitory control. Given this cler discrepncy, possily ecuse of differences in experimentl procedures, we wished to exmine further this issue. In this report, we will show tht the SSRT ssocited with humn reching movements is close to tht found for hnd key-press (Logn nd Irwin 2), ut remrkly longer thn tht of sccdes (Hnes nd Crpenter 999; Cel et l. 2; Kornylo et l. 23). We will lso show tht humns re lwys fster oth to execute nd to withhold reching movements towrd visul trgets presented on the sme side of the reching rm, even if the stop signl ws lwys presented centrlly. In ddition, we will present evidence tht the inhiition of reching movement is not lterlized. Lstly, we will ddress how the inter-hemispheric trnsmission of motor signls reltes to the point of no return, the moment t which the go process leves its controlled phse, during which movement cn e inhiited, to enter its llistic phse, during which movements cnnot e inhiited. As in the clssic Poffenerger tsk (Poffenerger 92), we found shorter rection times when the trget is presented on the sme side of the responding rm (uncrossed comintion) thn when is presented on the opposite side (crossed comintion). This extr time, the so-clled crosseduncrossed-difference (CUD), is ttriuted to the interhemispheric trnsmission of signl through the corpus cllosum (Mrzi et l. 99; Mrzi 999). In greement with the findings of Cvin-Prtesi et l. (24), we will show tht the llistic stge occurs fter inter-hemispheric trnsmission. A rief report hs ppered previously (Mirell et l. 24). Mterils nd methods Sujects nd pprtus Ten right-hnded sujects etween the ges of 22 nd 36 yers (men 27.±4.2), with norml or corrected-tonorml visul cuity were tested. Sujects hndedness ws determined using the Edinurgh hndedness inventory (ryden 977) nd only prticipnts with homogeneous pttern of hnd preference were included. Ech suject completed two experimentl sessions on two different dys, one using the right rm nd the other using the left rm. The order in which the rm ws used in ech session ws counterlnced cross sujects. Sujects were seted in drkened, sound-ttenuted chmer with their eyes 4 cm from 2 PC monitor (CRT noninterlced, refresh rte 85 Hz, 8 6 resolution, 32 it color depth) where visul stimuli were presented. Stimuli consisted of red circles (2.434 cd/m 2 )of 2.5 dimeter ginst drk ckground of uniform luminnce (<. cd/m 2 ). The PC monitor ws coupled with touch screen (MicroTouchä, smpling rte 2 Hz) for touch positions monitoring. The presenttion 4 4 42 43 44 45 46 47 48 49 5 5 52 53 54 55 56 57 58 59 6 6 62 63 64 65 66 67 68 69 7 7 72 73 74 75 76 77 78 79 8 8 82 83 84 85 86 87 88 89 9 9 92 93 94 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6
95 of stimuli nd dt cquisition ws controlled y the 96 CORTEX rel-time control system (http://www.cor- 97 tex.slk.edu) running on PC. The temporl rrnge- 98 ments of stimulus presenttion were synchronized with 99 the exct presenttion time of the visul stimulus deter- 2 mined y the monitor updte rte. All sujects gve their 2 informed consent nd they were pid 5 for ech session. 22 The experimentl procedures were pproved y the locl 23 ethics ord nd performed in ccordnce with the ethicl 24 stndrds lid down in the 964 Declrtion of Helsinki. 25 ehviorl tsks 26 Sujects were first mde fmilir with the pprtus nd 27 the primry tsk of mking reching rm movements to 28 visul trgets. Once fmilirized, the sujects performed 29 lock of go-trils (go-only tsk), from which we 2 mesured the men nd stndrd devition of the indi- 2 vidul s RTs in the sence of stop signl. RT ws 22 determined s the time difference etween time of the 23 occurrence of the go signl nd movement onset. After 24 short rek, they performed the countermnding tsk. 25 This tsk consisted of rndom mix of 75% no-stop 26 trils (Fig. ) nd 25% stop trils (Fig. ). Ech 27 no-stop tril ws identicl to those in the go-only tsk 28 nd egn with the presenttion of red circle t the Fig. Schemtic of the tsk. Temporl sequence of the visul displys for no-stop nd stop trils. All trils egn with the presenttion of centrl stimulus. After vrile holding (5 8 ms), the centrl stimulus disppered nd, simultneously, trget ppered (go signl) t one of two loctions, either to the right or to the left of the centrl stimulus. In the no-stop trils sujects hd to perform speeded reching movement towrd the peripherl trget within mximum time (see Methods for further detils). On frction of interleved trils (25%) the centrl stimulus reppered (stop signl) fter vrile delys (SSDs), instructing the suject to inhiit movement initition. In these stop trils, if sujects countermnded the plnned movement keeping the rm on the centrl stimulus the tril ws scored s success. Otherwise if sujects executed the reching movement the tril ws scored s filure. Dotted circle indictes the size of the tolernce window for the touches (dimeter 5 ) Centrl Stimulus Trget Hnd position center of the disply. Sujects were required to touch it with the index finger of their reching hnd for vrile period of 5 8 ms (holding time). Then the centrl red circle disppered (go signl), nd simultneously trget ppered on the horizontl plne rndomly t one of two loctions, either.3 (8 cm) to the right or the left of the centrl stimulus. Sujects hd to perform speeded reching movement towrd the peripherl trget. The stop trils only differed from the no-stop tril y the reppernce of the centrl red circle, which cted s the stop signl instructing the sujects to inhiit their movements nd which ws presented fter vrile dely, the SSD. Stop trils in which sujects successfully cncel their movements were defined s stop-success trils, while stop trils in which they fil to cncel, leving the centrl position, were defined s stop-filure trils. Sujects were given feedck in the form of tone when their responses in either no-stop or stop trils were correct. Ech suject performed totl of, countermnding trils in five locks of 2 trils, with interleved resting periods when requested. These experimentl sessions included stop trils whose SSD vlues were previously determined with one or more locks of 2 countermnding trils (men 2.8±2.2). Five vlues of SSD were used nd they rnged from 2 to 8 units of refresh rte (or 23.6 nd 22.4 ms, respectively). The STOP TRIAL Centrl Stimulus Trget Stop-Signl Hnd position tolernce Holding Time NO-STOP TRIAL Go-signl Go-signl SSD Rection Time Stop-signl Success Success Filure Success Filure Success 29 22 22 222 223 224 225 226 227 228 229 23 23 232 233 234 235 236 237 238 239 24 24 242 243 244 245 Holding Time RectionTime 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6
246 ctul SSD vlues were djusted to the performnce of 247 ech suject in ech experimentl session so tht move- 248 ments were successfully inhiited in 85% of the stop 249 trils with the shortest SSD nd in 5% with the longest 25 SSD. Since ech suject performed two experimentl 25 sessions one using the right rm nd the other the left 252 rm on different dys, the SSD vlues were djusted 253 seprtely for ech rm. 254 efore performing the countermnding tsk, sujects 255 were instructed tht in some stop trils they would not 256 e le to withhold the movement nd tht they should 257 not e trouled y their performnce. We stressed tht 258 the importnce ws to respond to the visul trget s 259 quickly nd ccurtely s possile nd not let the stop 26 signl interfere with their performnce. At the end of 26 ech countermnding lock, sujects were informed 262 out the chnges in their men RTs with respect to the 263 men RT otined during the initil go-only session, 264 nd they were sked, whether necessry, to mintin the 265 sme speed level (see lso Ozyurt et l. 23). We lso 266 discourge sujects to dopt the strtegy of slowing 267 down to cncel more esily their movements y impos- 268 ing n upper RT limit, defined s the men RT of go 269 trils during the go-only tsk plus five SDs. The no-stop 27 trils with RTs higher thn the upper RT were thus 27 identified s errors during tsk performnce, ut kept 272 for the finl nlysis. Finlly, the peripherl trget ws 273 present only for 35 ms in stop trils to give dditionl 274 feedck to the sujects if they filed to cncel (Fig. ). 275 Dt nlysis 276 To quntify the inhiitory ility of ech suject, inhi- 277 ition functions were constructed y plotting the pro- 278 ility of stop-filures s function of SSD. To derive 279 relile prmeter estimtes for the inhiition function, 28 the dt were fit with cumultive Weiull function of 28 the form: W ðtþ ¼c ðc dþ e ðt=þ ; 283 where t is the time fter trget presenttion, the time t 284 which the inhiition function reches 64% of its full 285 growth, the slope, c the upper limit nd d ws the 286 lower limit of the function (Hnes et l. 998). Since, y 287 definition, the inhiition function could not ssume 288 negtive vlues or vlues igger thn one, the vlue of c 289 ws set to nd the vlue of d to. 29 Results 29 Rection times of reching movements 292 Rection times of reching movements mde in the 293 no-stop trils were ffected y the sptil reltionship 294 etween trget position nd the reching rm. Figure 2, 295 contrsts the cumultive distriutions of the RTs of reching movements mde y one representtive suject with either rm nd trget. RTs of reching movements mde with the left rm were significntly shorter when directed to trgets positioned in the ipsilterl hemifield (26±2.2 vs. 283±.8 ms; Kolmogorov Smirnov-test, p<.). The sme result ws otined with the right rm (257±.7 vs. 32±2. ms; Kolmogorov Smirnovtest, p<.). Figure 2c nd Tle summrizes the verge (±SE) RTs for reching movements mde y the ten righthnded sujects during the no-stop trils of the countermnding tsk to the left nd right trgets with their left nd right rms. An nlysis of vrince on RTs of no-stop trils, with reching rm nd trget position s fctors, ws performed cross sujects. This nlysis reveled significnt interction etween reching rm nd trget position (two-wy ANOVA with repeted mesures, df=9, F=32.5, p<.5), ut no significnt min effects (rm: df=9, F=2.72, p=.3; trget: df=9, F=5, p=.38). Post-hoc nlyses (Newmn Keulstest) showed tht sujects hd significntly shorter RTs for trgets presented on the sme side of their reching rm (right: p<.5; left: p<.5). In ddition, right-rm reches to trgets positioned in the left hemifield hd RTs significntly longer thn left-rm reches to trgets positioned in either the left (p<.) or right hemified (p<.5). ecuse of the interction etween reching rm nd trget position, ll our nlyses took into ccount these fctors seprtely. We lso estimted the CUD y sutrcting the men RT of ech suject in the uncrossed conditions (e.g., right rm to right trget) from tht otined in the crossed comintions (e.g., right rm to left trget). This mesure verged 24 ms (±4.2) nd ws positive in ll sujects, regrdless of the reching rm, mening tht uncrossed comintions lwys yielded shorter RTs. Our estimte of CUD exceeds the rnge (8. 5.5 ms) reported in studies of pointing movements (Vely nd enoit-durocrd 999; oulinguez et l. 2; rthelemy nd oulinguez, 22,; Vely et l. 22). The presumly greter cognitive lod of the countermnding tsk, due to the unpredictle presenttion of the stop signl, my explin this discrepncy. In this respect, it must e noted tht the men CUD otined in the lock of go-only trils (5±4 ms) ws within the rnge of previously reported vlues nd significntly smller thn tht otined in the no-stop trils of the countermnding tsk (pired t-test, df=9, p<.5). Contextul influences on rection times Motor responses often tend to hve longer RTs with the introduction of stop trils in the countermnding tsk (Lppin nd Eriksen 966; Ollmn 973; Logn 98; Rieger nd Guggel 999; Cvin-Prtesi et l. 24). We lso oserved this contextul effect in our experiments. In ech suject, the RTs of the reching movements produced during the no-stop trils of the 296 297 298 299 3 3 32 33 34 35 36 37 38 39 3 3 32 33 34 35 36 37 38 39 32 32 322 323 324 325 326 327 328 329 33 33 332 333 334 335 336 337 338 339 34 34 342 343 344 345 346 347 348 349 35 35 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6
Cumultive Proility Cumultive Proility c Suject ST Arm: LEFT Trget Right Trget Left 2 3 4 5 36 34 32 3 28 Suject ST Arm: RIGHT Trget Right Trget Left 2 3 4 5 countermnding tsk were significntly longer thn those otined in the initil lock of go-only trils, even though our experimentl design included low proility (5) of stop signl trils (Logn 994) s well s detiled instructions nd continuous feedck (Ozyurt et l. 23). Figure 3, contrsts the cumultive distriutions of the RTs of reching movements mde y one representtive suject in go-only nd no-stop trils. The RTs of oth left- nd right-rm movements produced in the nostop trils of the countermnding lock were significntly (Kolmogorov Smirnov-test, p s <.) longer thn those oserved in the lock of go-only trils (left: 27±.5 vs. 232±3.5 ms; right: 287±.7 vs. 24±4 ms). A similr increse in RTs for either reching rm ws found to e significnt (p s <.) in ll ten sujects. Furthermore, the mount of procrstintion for the right (6±4. ms) nd left rm (76±.3 ms) ws not significntly different (pired t-test, df=9, p=.32). To exclude the possile confounding effect of ftigue, we rn two of the sujects in series of consecutive locks of go-only trils (six for the first suject, 2 for the second). The men RTs of oth sujects in the lst lock ws significntly shorter thn tht otined in the initil lock (first suject: 28±2.7 vs. 296±2. ms, Kolmogorov Smirnov-test, p<.; second suject: 289±3.2 vs. 328±2.2 ms, p<.). Ftigue ws therefore not fctor. In ddition to the glol increse in RTs of motor responses in the countermnding tsk, the occurrence of stop trils hs lso een reported to hve locl effect on the RTs of responses produced in the immeditely following no-stop trils (Cel et l. 2; otvinick et l. 2; Jones et l. 22; rown nd rver 25). Figure 4, shows n nlysis of the influence of stop trils on the RTs of the left- nd right-rm reching movements mde y one suject in the four susequent no-stop trils. RTs in the first no-stop tril immeditely fter stop tril were significntly longer thn the following three no-stop trils oth for the right rm (one wy ANOVA, df=3, p<.; post-hoc Tuckey Krmer, p s <.5) nd for the left rm (one wy ANOVA, df=3, p<.; post-hoc Tuckey Krmer, p s <.5). The slowing effect of stop-success trils nd stop-filure trils did not differ (not shown). We oserved similr sequentil effect cross ll sujects (Tle 2). An nlysis of vrince with RT of the nostop tril sequence nd reching rm s fctors, reveled min effect of the reching rm in eight sujects oth Trget Left Arm Right Arm Left Trget Right Fig. 2 Rection times (RTs) of no-stop trils for reching movements in reltions to the rm used nd trget presenttion. Pnel nd show, for n exmple suject, the cumultive distriutions of the RTs of no-stop trils for right (solid trces) versus left (dotted trces) trgets using the left nd the right rm, respectively. Pnel c shows the men vlues (± SE) of RTs cross ll sujects (n=) for the no-stop trils in the countermnding lock for either rm nd trget position 2 2 4 5 6 Tle Men RTs (±SE) nd percent correct of no-stop trils in the countermnding locks cross the entire popultion Men RT Trget right Trget left (%) Corr Trget right Trget left Right Arm 3±9. 338±.5 94.5 93. Left Arm 32±8.7 3±9.7 92.9 94.8 Disptch: 29.3.6 Journl: 22 No. of pges: 6 352 353 354 355 356 357 358 359 36 36 362 363 364 365 366 367 368 369 37 37 372 373 374 375 376 377 378 379 38 38 382 383 384 385 386 387 388 389 39 39 392 393 394 395 396 397 398 399 4
Suject ST Arm: LEFT Suject ST Arm: RIGHT Cumultive Proility Go trils No Stop trils 2 3 4 5 Fig. 3 Rection times (RTs) for reching movements in the go-only tsk nd in the countermnding tsk. Pnel nd show the cumultive distriutions of the rection times (RTs) of lock of 35 3 25 Suject MX Arm: LEFT Men RT No Stop trils 2 3 4 Sequence of No Stop trils Fig. 4 After-effect of stop signl ppernce in the tril sequence. Pnel nd shows the effect of stop signl on the RTs of four consecutive no-stop trils for the left nd the right rm of one 4 fter correct nd wrong stop, with the RTs of six eing 42 shorter for left-rm reches. Most importntly, the min 43 effect of tril sequence on RT ws significnt in ll 44 sujects (p s <.). A significnt interction etween the 45 two fctors never occurred. 46 In order to estlish the durtion of the slowing effect 47 of the stop signl we performed post-hoc nlysis on Tle 2 Men RTs (± SE) of sequence of four no-stop trils fter correct/wrong stop signl Go trils No Stop trils 2 3 4 5 trils with (no stop trils; dotted trces) nd without stop trils (go trils; solid trces) intermixed for the left nd the right rm of one suject, respectively 35 3 25 Suject MX Arm: RIGHT Men RT No Stop trils 2 3 4 Sequence of No Stop trils suject, respectively. In ech pnel the dotted line represents the men RT of no stop trils. The lck dots represent the men RT (± SE) for ech ctegory of no stop tril the RTs of the correct no-stop tril sequence (Newmn Keuls-tests). Seprtely for ech suject-rm comintion, we counted the numer of times in which the RT of the first no-stop tril ws significntly longer (p s <.) thn tht of the three susequent trils (three comprisons were mde for ech suject-rm comintion). In the sme wy we estlished how mny times the second First no-stop Second no-stop Third no-stop Fourth no-stop RT right rm correct trils 35±.3 332±9.3 322± 35±8.8 RT left rm correct trils 337±8 32±.3 39±9.9 34±8.2 RT right rm wrong trils 342± 324± 38±9. 36±8.3 RT left rm wrong trils 328±9.3 32±8.8 38±9.8 32±8.6 48 49 4 4 42 43 44 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6
45 tril ws slower thn the third nd the fourth trils in the 46 sequence (two comprisons were mde for ech suject- 47 rm comintion). Finlly we counted the numer of 48 times in which the third tril ws slower thn the fourth. 49 Considering ll comintions together, the first no-stop 42 tril ws slower tht the other in the sequence 66.7% 42 (4/6; ten sujects, two rms, three comprisons) of the 422 cses. In contrst the second tril ws slower thn the 423 third nd the fourth trils in the sequence just in the 424 5% (6/4; ten sujects, two rms, two comprisons) of 425 the cses, while the third nd the fourth trils never 426 differed. Similr results hve een otined for wrong 427 no-stop tril sequence (not shown). Overll this result 428 indictes tht the effect of stop tril ws generlly 429 limited, when present, to the immeditely following no- 43 stop tril. 43 ehviorl estimte of reching movement cncelltion 432 One of the min gols of the present study ws to esti- 433 mte the length of time required to cncel commnded 434 reching movement, the SSRT. This vlue cnnot e 435 mesured directly ut cn e estimted from the P (Stop filure) c P (Stop filure) Suject ST 35.3 58.8 7 82.4 94. SSD (ms) LL L LR ehviorl performnce in the countermnding tsk using the rce model developed y Logn (Logn nd Cown 984; Logn 994). In the countermnding tsk, the inhiition of reching movements depended on the SSD. Sujects successfully cnceled their reching movements to trget when the stop signl ws presented fter short SSD, ut they incresingly filed with the lengthening of the SSD. Figure 5, plots the proility of not cnceling movement s function of SSD (the inhiition function) for one representtive suject reching with the left nd right rm. The three functions in ech pnel represent the inhiition functions for the crossed comintion (e.g., right rm to left trget), the uncrossed comintion (e.g., right rm to right trget), nd oth comintions pooled together. One cn see tht the inhiition control of this suject did not depend on the reching rm: the proility of not cnceling the reching movements ws lwys lowest t the shortest SSD, it incresed with the lengthening of the SSD, nd ws highest t the longest SSD. Nevertheless, the overll ility of this representtive suject to cncel reching movement in the uncrossed conditions ws lower thn in the crossed conditions. P (Stop filure) P (Stop filure) SSD SSD2 SSD3 SSD4 SSD5 LL L LR d Suject ST 23.5 47. 58.8 7 94. SSD (ms) SSD SSD2 SSD3 SSD4 SSD5 RR R RL RR R RL 436 437 438 439 44 44 442 443 444 445 446 447 448 449 45 45 452 453 454 455 456 457 458 459 Fig. 5 The inhiition functions (IF), represented y the est fit of the Weiull function, (see Results for further detils), for one experimentl suject re shown for the left () nd the right hnd (). In ech plot the solid grey line represents the IF for crossed conditions, the thin lck line the IF for uncrossed conditions nd the dotted line oth comintions pooled together. Pnels c nd d show the verge IF cross the entire popultion (n=) for the left nd the right hnd, respectively. Dt from individul sujects were comined y verging for ech single SSD the proility of generting movement even though stop signl ws presented. In ll instnces the proility of stop filure incresed s function of the SSD nd is consistently higher for crossed thn for uncrossed stimuli. L left rm, R right rm, LL left rm left trget, LR left rm right trget, RL right rm left trget, RR right rm right trget 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6
46 The popultion nlysis showed very similr results 46 (Fig. 5c, d). We performed two-wy-anova on the 462 proility of responding, with SSD nd trget positions 463 s fctors. The min effects were significnt for oth 464 rms. The proility of responding incresed with SSD 465 (left rm: df=4, F=9.6, p<.; right df=4, rm: 466 F=8.7, p<.), nd its vlue in the crossed condi- 467 tion ws higher thn in the uncrossed condition (left 468 rm: 4±.3 vs. 3±.4, df=, F=8.3, 469 p<.; right rm: 3±.3 vs. 6±.4, df=, 47 F=45.4, p<.). It is worthy to note tht there ws 47 no sttisticl difference in the SSDs cross rm used. For 472 instnce the men vlue (± SE) of the shortest SSD for 473 the right hnd cross ll sujects ws 83.5±9.3 while 474 tht for the left hnd ws 7±8.6 (p= df=9 475 pired t-test). The sme holds true for the longest SSD 476 (54.7±5 for the right hnd vs. 44±.9 for the 477 left hnd; p=.32 df=9 pired t-test). Furthermore, in 478 most cses, the SSDs of ech single suject in the two 479 experimentl sessions, showed smll if ny differences. 48 Strting from the RT distriutions otined for ech 48 suject during no-stop trils (e.g. Fig. 2, for suject 482 ST) nd inhiition functions (e.g. Fig. 5, for the sme 483 suject) of ech comintion of rm used nd trget 484 presenttion, we estimted the corresponding SSRT. We 485 used two estimtion methods. The first method of esti- 486 mting the SSRT ssumes tht it is rndom vrile. 487 This estimte relies on the mthemticl demonstrtion 488 tht the men SSRT is equl to the difference etween 489 the men RT during no-stop trils nd the men vlue of 49 the inhiition function (Logn nd Cown 984). We 49 computed the men of the inhiition function from the 492 est-fit of the Weiull functions, W(t), s follows: Men of inhiition function ¼ X ½ðW ðtþ W ðt ÞÞtŠ=½W ðt mx Þ W ðt minþš; 494 where t rnges from the minimum to the mximum stop 495 signl dely in -ms intervls, while W(t mx) nd 496 W(t min) represent the mximum nd the minimum 497 proilities of responding. Overll, the Weiull func- 498 tion fits hd men r 2 of.74 (±.2) nd the Chi- 499 squred test ws lwys nonsignificnt (p s >.9). We 5 computed the SSRT for ech sujects nd for ech 5 comintion of reching rm nd trget presenttion. 52 The verge (± SE) SSRT estimted with this pproch 53 cross ll of the sujects ws 26±2.7 ms. 54 The second method of clculting the SSRT mkes 55 the ssumption tht the SSRT is constnt. Although 56 this ssumption seems implusile, its violtion does not 57 significntly lter the result of the nlysis (Logn nd 58 Cown 984; nd 997). y this method, the SSRT is 59 estimted y integrting the distriution of RT in no- 5 stop trils, eginning t the onset of the go-signl, until 5 the integrl equls the oserved proportion of noncn- 52 celled trils t tht SSD. This point is tken s the 53 finishing line of the stop process, nmely the longest RT 54 possile efore ll reching movements ecome inhi- 55 ited y the stop process. The SSRT t ech SSD is then equl to the difference etween the finishing line nd the given SSD. The men (± SE) SSRT cross ll of the sujects with this pproch ws 28±2 ms. Since the SSRTs otined with the two methods were not significntly different (pired t-test, df=9, p=.3) in the following we will consider just the SSRT estimted with the first method ecuse it is the one tht mkes the most cceptle ssumption, nmely, tht the SSRT is rndom vrile. Figure 6 nd Tle 3 summrizes the men vlues of SSRT (± SE) otined cross sujects for ech reching rm nd trget position. As we proceeded previously for the nlysis of RTs of no-stop trils, we explored the chnges of SSRT in reltion to oth the reching rm nd the trget position with two-wy ANOVA with repeted mesures. There were no significnt min effects (rm: df=9, F=4, p=.52; trget: df=9, F=.78, p=), ut the interction etween the two fctors ws significnt ( df=9, F=.7, p<.). A post-hoc nlysis (Newmn Keuls-test) reveled tht the SSRT ws significntly shorter in the uncrossed condition, i.e., when the movement ws going to e mde to trget positioned on the sme side of the reching rm (right rm: p<.; left rm: p<.5). In ddition, the SSRT ssocited with the inhiition of right-rm reching movements to the left trget ws longer thn tht found for left-rm reches to the sme trget (p<.5). These findings closely resemle the results otined for the RT of reching movements. We therefore wondered whether SSRT nd RT were relted. Figure 6 shows tht this ws not the cse (r=6 df=38, p>.5). Since SSRTs nd RTs hve different vriility (see Tles, 3) we trnsformed ll scores in Z-vlues. However, even fter this procedure the two sets of vlues did not correlte (r=7, df=38, p>.5; not shown), mening tht there is no liner reltionship etween the SSRTs nd the RTs. We lso computed the CUDs of the SSRT. The verge (± SE) CUD ws 8±3.4 ms nd it ws positive for ll sujects, mening tht the uncrossed comintions lwys yielded shorter SSRTs. The CUDs ssocited with the SSRTs were significntly shorter thn those ssocited with the RTs (pired t-test, df=9, p<.5). Figure 6c shows, however, tht these two mesures were highly correlted (r=.97, df=8, p<.). The CUDs mesured from the RTs of stopfilure trils (26.6±. ms) were lso significntly longer thn the CUDs of the SSRTs (pired t-test, df=9, p<.5). These two sets of vlues were lso linerly relted (r=.73, df=8, p<.5). Figure 7 shows how vrile were the inhiition functions cross sujects within single experimentl condition ecuse of the different SSDs tht were chosen to ccount for ech suject s RTs. To compenste for tht suject-to-suject vriility, we normlized the proility t ech SSD tht sujects fil to cncel successfully their reching movements into Z score of the reltive finishing time (ZRFT; Logn nd Cown 984) with the following expression: 56 57 58 59 52 52 522 523 524 525 526 527 528 529 53 53 532 533 534 535 536 537 538 539 54 54 542 543 544 545 546 547 548 549 55 55 552 553 554 555 556 557 558 559 56 56 562 563 564 565 566 567 568 569 57 57 572 573 574 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6
24 Tle 3 Men SSRTs (± SE) cross ll sujects for ech response rm nd for ech position of the trget Trget right Trget left SS SS c CUD 22 2 8 3 25 2 Trget Left Arm Right Arm Left Trget Right r=6 Slope=.5 5 25 3 35 4 6 4 2 r=.97 Slope=.22 2 4 6 CUD SS Right rm 97±2.2 27±3.2 Left rm 2±.7 97±2.2 ZRFT ¼ðRT no - stop SSD SSRTÞ= SD no - stop : The slope of the resulting inhiition function could e tken s mesure of the inhiitory control of ny experimentl condition, nd ny differences in slopes etween sujects nd/or experimentl conditions s evidence of distinct inhiitory processes (Logn nd Cown 984). Figure 7 shows tht, once normlized, the inhiition functions otined in ll our sujects for the right-rm-right- trget condition, ecme closely ligned. Figure 7c shows the verge slope otined fter normliztion. Comprle lignment ws oserved in ll experimentl conditions (not shown). Tle 4 reports the verge vlues of the slopes of the inhiition functions cross ll sujects. An nlysis of vrince (two-wy ANOVA with repeted mesure) performed on the slopes of the normlized inhiition functions, with reching rm nd trget position s fctors, reveled tht the slopes did not differ significntly (rm: df=9, F=.32, p=8; trget: df=9, F=.5, p=.33; interction: df=9, F=.58, p=4). We conclude from this nlysis tht the process underlying the inhiition of reching movements ws the sme cross ll conditions nd sujects. Independence of GO nd STOP processes The rce model ssumes tht the ehviorl outcome in the countermnding prdigm depends on rce etween two independent processes: () go process initited y the go signl tht cn led to the execution of the reching movement; nd (2) stop process initited y the stop signl tht cn inhiit the commnded movement (Fig. 8). If the stop process is fster thn the go process, the response is inhiited. If not, the response is initited. Since the durtion of the go nd stop processes re oth rndom vriles the result of the rce is stochstic. The proportion of stop trils in which the commnded response is successfully inhiited (stop-success trils) nd in which it is not (stop-filure trils) is determined y the distriution of the finishing times of the go nd stop processes (Fig. 8). One of the centrl ssumptions of the rce model is tht the go nd stop processes re stochsticlly independent, nmely, tht the finish time of ech process is uncorrelted with the finish time of the other process. To test whether the go nd stop processes intercted, we considered how well the rce model predicted the RTs of the reching movements tht escped Fig. 6 Estimte of the stop signl rection time (SSRT) nd reltionship with the rection times (RTs) of no stop trils for reching movements. Pnel shows the men vlues of SSRTs (±SE) cross ll ten sujects, seprtely for ech response rm nd for ech position of the trget. Pnel shows the reltionship etween the RTs of no stop trils nd the corresponding SSRT mesured seprtely for ech response rm nd for ech position of the trget. Pnel c shows the reltionship etween the crossed uncrossed difference (CUD, see text for further detils) of the RTs of no stop trils nd tht of the corresponding SSRT 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6 576 577 578 579 58 58 582 583 584 585 586 587 588 589 59 59 592 593 594 595 596 597 598 599 6 6 62 63 64 65 66 67 68 69 6 6 62 63 64 65 66 67 68 69
Tle 4 Men slopes (± SE) cross ll sujects for ech response rm nd for ech position of the trget P (Stop filure) P (Stop filure) c P (Stop filure) E GM CH ET ST CS P SA OD MX 5 5 2 SSD (ms) 2 2 ZRFT r=.9 Slope=.37 E GM CH ET ST CS P SA OD MX inhiition (Logn nd Cown 984). Reching movements were produced in oth the no-stop trils nd the stop-filure trils, ut the ltter were initited ecuse the go process finished efore the stop process. Therefore, considering the distriution of the RTs of the no-stop trils, the responses tht escpe inhiition should e those corresponding to reching movements tht hd RTs shorter thn the SSD plus the estimted SSRT (Fig. 8, light region of RT distriution). Given the ove resoning, three predictions should e stisfied (Logn nd Cown 984; Logn 994). First, the men RT in stop-filure trils should never e longer thn the men RT in the no-stop trils. Second, the men RT in stop-filures trils should lengthen with incresing SSD. Third, the men RT in the stop-filure trils t ech SSD should e equl to those predicted from the rce model. Figure 9 shows tht these predictions were stisfied. Figure 9 shows the cumultive RT distriution of right-rm reches to the left trget mde y representtive suject. This exmple illustrtes tht the RTs in stop-filure trils (288±3.3 ms) re significntly shorter (Kolmogorov Smirnov-test; p<.5) thn the RTs in no-stop trils (36±3.3 ms). From the sme dtset, Fig. 9 shows tht the RTs in the stop-filure trils increses s function of SSD nd tht they re not significntly different from those predicted y the rce model (pired t-test; p s >.5). To check if t the popultion level the predictions were stisfied, we used two different pproches. First of ll, we considered how mny times ech single suject fulfilled ech of the three predictions. The RTs in stopfilure trils were significntly longer thn the RTs in the no-stop trils (Kolmogorov Smirnov-test; p s <.5) in 68/2 (ten sujects; two rms, two trgets; five SSDs) cses (or 84%). In ll occurrences the men RTs in stopfilures trils lengthen with incresing SSD (one wy ANOVA, df=4, p s <.5). Finlly, in the 33 out of 2 (ten sujects, two rms, two trgets, five SSD) cses (or 66.5%) the oserved men RTs in the stop-filure trils t ech SSD wre equl to those predicted. As second pproch we considered the popultion s whole y collpsing single sujects dt together. Figure 9c shows for the condition in which sujects mde right-rm reches to the left trget, tht the cumultive distriution of the RTs in stop-filure trils cross ll sujects (36±.75 ms) is significntly different (Kolmogorov Smirnov-test, p<.) nd shifted to the left with respect to the RT distriution otined in no-stop trils (338± ms). Furthermore, 2 2 ZRFT Fig. 7 Normliztion of inhiition functions. Pnel shows the inhiition functions of ech of the ten sujects otined when they were using the right rm nd the trget ws presented in the right hemi field. Pnel shows the sme inhiition functions fter the normliztion of ech stop signl delys (SSDs) of ech suject into Z vlues of the reltive finishing time (ZRFT). Pnel c shows the est liner regression fit considering ll dt coming from the entire popultion 2 2 4 5 6 Trget right Trget left Right rm.37±.2 3±.3 Left rm.39±.2.38±.4 Disptch: 29.3.6 Journl: 22 No. of pges: 6 62 62 622 623 624 625 626 627 628 629 63 63 632 633 634 635 636 637 638 639 64 64 642 643 644 645 646 647 648 649 65 65 652 653 654 655 656 657 658 659 66 66 662 663 664 665 666 667 668 669
Activtion Process Stop-filure tril 67 s shows in Fig. 9d, with the only exception of the 67 shortest SSD, RTs in the stop-filure trils increses 672 significntly long with the SSD (one wy ANOVA, 673 df=4, F=.5, p<.). It hs een lredy shown 674 tht t the shortest SSD this prediction cn e violted 675 proly ecuse of the very few stop-filure trils 676 (Logn nd Cown 984; Logn 994). Similr results 677 were otined in the other three experimentl condi- 678 tions. 679 A further evidence of the independence etween the 68 go nd stop processes comes from the comprison 68 etween the CUD vlues mesured for the RTs in the 682 no-stop trils of the countermnding tsk (when only 683 the go process is ctivted) nd the corresponding vlues 684 mesured for stop-filure trils. If stop-filure trils 685 represent instnces in which the responses escped 686 inhiition, thn the CUD in oth situtions should not 687 differ. In greement with this prediction, the CUD 688 mesured from the stop-filures trils ws not signifi- 689 cntly different from the CUD mesured from no-stop 69 trils (27±3.4 vs. 24±3.4 ms; pired t-test, df=9, 69 p=.53). # Events GO STOP SSRT SSD5 GO end strt SSRT Stop-success tril Discussion Using the countermnding prdigm, we showed tht the length of time needed for humn suject to cncel their reching movements is out 2 ms. This estimte, however, vries s function of the reching rm nd trget position: sujects lwys took less time to cncel their reching movements towrd visul trgets ppering on the sme side of their reching rm, the sme condition tht led to shorter rection times. This new form of sptil comptiility is interpreted s consequence of the existence of common fctor influencing oth the go nd the stop processes. GO 5% STOP SSRT REACTION TIME (RT) DISTRIUTION of NO-STOP TRIALS Stop-success P(Stop filure)=85% Stop-filure Go-signl Stop-signl End of Stop process Time from go signl Fig. 8 Logic underlying the rce model. The rce model represents the performnce in the countermnding tsk ssuming tht GO process (solid line) independently rce ginst STOP process (dotted line) towrd their respective threshold (roken horizontl line). The GO nd STOP processes re initited y the presenttion of the trget nd the stop signl, respectively. In no-stop trils, only the GO process is ctive, nd movement is generted when the GO process finishes. In stop trils, since the stop signl is lwys presented t vrile delys (SSDs) fter the go signl, the STOP process egins fter the GO process hs egun. In ll instnces in which the GO process finishes efore the STOP process, the STOP end strt reching movement is not cncelled (, left) nd vice vers (, right). The time it tkes to respond the stop signl is the stop signl rection time (SSRT). In the predictions of the outcome of the rce etween STOP nd GO process for the longest SSD (SSD5) re shown. Considering hypotheticl distriution of the RTs of the no-stop trils, the responses tht escpe inhiition should e those corresponding to reching movements tht hd RTs less thn the SSD5 plus the estimted SSRT. In our design, the longest SSD should e such tht the sujects could inhiit the movement just 5% of times (drk region of distriution) Inhiitory control of reching movements Our ehviorl estimte of reching movement cncelltion verged 26 ms, vlue close to tht reported for mnul key presses in severl experiments (see for review Logn 98; Logn nd Cown 984), ut much 692 693 694 695 696 697 698 699 7 7 72 73 74 75 76 77 78 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6
Suject MX 3 Suject MX 8 Cumultive Proility c Cumultive Proility Stop filure trils No Stop trils 2 3 4 5 Stop filure trils No Stop trils 2 3 4 5 6 7 Fig. 9 Independence of go nd stop processes. Pnel nd c show the cumultive distriutions of the rection times (RTs) of no-stop trils versus tht of stop filures trils cross ll stop signl delys (SSDs), for suject () nd for the entire popultion (c; n=), respectively in the condition in which they were using the right rm nd the trget ws presented in the left hemi field. The cumultive distriution of the RTs of stop-filure trils is significntly shifted to the left respect to tht of the no stop trils in oth cses. As fr s the popultion is concerned, the cumultive distriutions were otined y collpsing together the single RTs of no stop- or stop 79 longer thn cncelltion estimtes ( 5 ms) 7 otined for sccdic eye movements (Hnes nd 7 Crpenter 999; Cel et l. 2; Kornylo et l. 23). 72 Could the processes inhiiting the production of eye nd 73 rm movements e different? 74 Although our experimentl design ws comprle to 75 tht of Hnes nd Crpenter (999), we cnnot ddress 76 this issue directly ecuse eye movements were not 77 simultneously recorded. Logn nd Irwin (2) did, 78 however, study the inhiitory control of eye nd hnd 79 movements under identicl conditions. In their 72 experiment sujects were instructed to respond to visul 72 stimuli either with n eye movement or y pressing key. 722 They found significnt difference etween oculr nd 723 mnul SSRT estimtes, result they interpreted s 724 supporting the hypothesis tht seprte processes, gov- 725 erned y common principles, inhiit eye nd hnd d 29 27 25 23 35 33 3 29 58.8 7 82.4 94. 5.9 SSD (ms) Oserved Predicted movements. One importnt detil overlooked y this study is tht SSRT estimtes include the durtion of the llistic phse of movement processing (Logn nd Cown 984). This is ecuse SSRTs re computed from the rection times of the no-stop trils, which encompss oth controlled nd llistic processes. Differences etween SSRT estimtes could therefore not only e due to different inhiitory processes t ply ut possily to differences etween the llistic processing of oculr nd hnd movements. If we ssume tht the llistic processing of rm movements is longer thn tht of eye movements, then the controlled processing could e identicl nd possily under the control of the sme inhiitory process. This hypothesis is, however, mitigted y reports indicting tht the point of no return occurs very lte in the processing of mnul responses (Osmn et l. 99) nd tht such movements cn even e 5.6 3 9.3 9 7 7 3.5 7.3 2.7 SSD SSD2 SSD3 SSD4 SSD5 filure-tril of ll sujects. Pnel shows the ctul versus the predicted reching movement ltencies for stop-filure trils in the sme suject nd condition of. Verticl rs t ech dt point indicte one stndrd error of the men. The numers ove the dt points indicte the numer of stop filure trils t ech SSD. The men vlues of the RTs of stop-filure trils cross ll ten sujects re shows in d. These vlues were otined y collpsing together the single RTs of stop filure trils of ll sujects seprtely for ech SSDs. The numers ove the dt points indicte the men numer of stop filure trils t ech SSD 726 727 728 729 73 73 732 733 734 735 736 737 738 739 74 74 742 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6
743 inhiited fter muscle ctivtion (De Jong et l. 99) 744 nd initition (Kudo nd Ohtsuki 998). These oser- 745 vtions question whether controlled nd llistic pro- 746 cesses in the progrmming of skeletomotor movements 747 cn e distinguished. It is, however, possile tht 748 reching inhiition does not entil only the suppression 749 of centrlly generted motor commnds ut tht it cn 75 lso operte (during the llistic process of the centrl 75 progrmming) to suppress issued commnds t the level 752 of the periphery (De Jong et l. 99). These peripherl 753 inhiitory processes my thus prticipte, downstrem of 754 the motor pln, in the online dptive control of move- 755 ments tht is ecoming well documented (e.g., Desmur- 756 get nd Grfton 2). Determining whether the 757 reching SSRT we estimted is longer thn the oculr 758 SSRT ecuse of longer llistic phse of centrl motor 759 processing will necessitte further investigtions with 76 neurophysiologicl methods. 76 An lterntive to the hypotheses descried ove is 762 tht the differing oculr nd reching SSRTs otined 763 with very similr experimentl designs rise from dif- 764 ferences in the orgniztion of the respective motor 765 systems. In the cse of sccde inhiition, fovelly 766 presented stop signl could e sufficient to directly 767 suppress the motor commnds of sccde-relted neu- 768 rons in frontl eye field (Hnes et l. 998) nd superior 769 colliculus (Pre nd Hnes 23) vi the ctivtion of 77 locl neurons with fixtion-relted ctivity. Such locl 77 inhiitory network hs not een descried within the 772 neurl system involved in the production of reching 773 movements. It is thus plusile tht reching inhiition 774 simply requires longer time ecuse the stop process 775 initited in response to the fovel stop signl cn only 776 exert its suppression of centrl motor commnds 777 through rther indirect pthwy. 778 Neurl sis of inhiitory control 779 The neurl sustrtes underlying the countermnding of 78 reching movements hve not een explored. Neurl 78 ctivity involved in the inhiitory control of rm 782 movement hs only een studied using the Go/No-Go 783 prdigm. In this prdigm, neurons whose ctivity ws 784 relted to the decision to move or not to move hve een 785 found in oth premotor (Klsk nd Crmmond 995) 786 nd motor (Miller et l. 992; Port et l. 2) cortices of 787 primtes. However, there is n importnt difference 788 etween the countermnding nd the Go/No-Go tsk. 789 In the former, the signl for inhiiting the movement is 79 presented fter the go signl, while in the ltter the stop 79 signl precedes the go signl. Therefore in the counter- 792 mnding tsk it is n ongoing response tht hs to e 793 inhiited, in the Go/No-Go tsk it is potentil move- 794 ment. Likely the neurl ctivity in the two tsks could e 795 very different. It is not cler wht re the motor res 796 involved in regulting the initition of reching move- 797 ments. A good cndidte is the dorsl premotor re 798 (PMd) ecuse of the strong presence of set-relted ctivities (Wise 985; Johnson et l. 996) involved in the preprtion to mke the movement nd ecuse of the existence of direct projection to the spinl cord (Dum nd Strick 996). The oservtion tht reching movements ecome more impulsive nd uncontrolled fter lesion of the premotor cortex (Moll nd Kuypers 977) suggests tht this corticl re my exert role in reching inhiition. Another possiility is tht reching inhiition could involve the primry motor cortex, the min source of movement-relted ctivity to the spinl cord. It would e interesting to compre the dischrge properties of motor cortex neurons in the countermnding tsk with the known properties of neurons within sccde executive centers (Hnes et l. 998; Pre nd Hnes 23). Other motor res, including the supplementry motor res nd the cingulted res of the frontl loe could e lso involved on signling conflicts nd error situtions similrly to wht oserved for the countermnding sccde tsk (Stuphorn et l. 2; Ito et l. 23; Curtis et l. 25). Finlly, different prefrontl res could e involved t different levels s suggested y the numerous studies in humns (Liddle et l. 2; Rui et l. 2; Wtne et l. 22; Aron et l. 23; Hsegw et l. 24). A new form of sptil comptiility We found unexpectedly tht reching SSRT depended on which rm ws used nd within which visul hemifield the trget fell. As fr s reching movements is concerned, it is well known tht rection times re shorter when trget is presented on the sme (uncrossed) side of the reching rm thn when it is presented on the opposite (crossed) side (Mrzi et l. 99; Vely nd enoit-durocrd 999; Vely et l. 2; rthelemy nd oulinguez 22; Cvin-Prtesi et l. 24). This phenomenon most certinly results from the orgniztion of the visul nd motor pthwys, i.e., ech hemifield is represented in the contrlterl hemisphere nd ech rm is controlled y the contrlterl hemisphere. Processing would e speeded up when limited to single cererl hemisphere in the uncrossed sitution, nd the necessry inter-hemispheric trnsfer of informtion would slow down processing in the crossed sitution. In our study, every single suject rected significntly fster in the uncrossed sitution rther thn in the crossed one, nd no significnt difference ws oserved etween left nd right rm. We thus found no evidence for ny symmetry in the production of reching movements. ecuse we designed our experiment with stop signl tht ws presented t the fove nd did not possess ny sptil ttriute, we did not expect to find difference in SSRT etween the two different experimentl conditions (crossed versus uncrossed). Our prediction rose from the considertion tht the stop signl most likely reches oth hemispheres t the sme time nd, in principle, it should hve een eqully effective in eliciting the 799 8 8 82 83 84 85 86 87 88 89 8 8 82 83 84 85 86 87 88 89 82 82 822 823 824 825 826 827 828 829 83 83 832 833 834 835 836 837 838 839 84 84 842 843 844 845 846 847 848 849 85 85 852 853 854 2 2 4 5 6 Disptch: 29.3.6 Journl: 22 No. of pges: 6