Visual regulation of manual aiming

Visual regulation of manual aiming * Romeo

Chua and DlgbvI Ellmtt

Abstract

Recent dlscusslons k g , C;lrson et al 1992, m press, Elhott et dl 1991. Pileson et al 1986) on the nature of motement legulat~on hLw gwen we to the idea that USUCHIW~O~ control ot pomtmg mw_ proceed m J contmuous’ or pseudo-contmuous’ fdshlon The emergence of this concept has been stlmulated by fmdrngs whrch haw shown thLlt the presence of USUJ mkx-matIon contrlbutes to pertormnnw accurLwI (Carson et al 199 2, Elktt et nl 1991) despite the absence ot

h‘js contmuous’ terms of adjustments 7 To date, little conslderxlan been given to this ISSUE In d recent stud), Carson et 4 (in press) have ewmnt:d, m Ljddltlon to zero crossmgs, the presence ot slgnlflcant de1 ILjtIc)ns’ m the xceleratlon protlles (see L~lsoKm Donkelax and Franks 1991) ot Lmtmg movements Swwflcnnt dewtlons reflect unchxxterlstlc changes In xceler~tlon iihlch do not lewd to Li tr,jnsltIon m the sign ot 1979, 1981 CU-son et J in press iicceltx~tion biz g . see Cxlton Mewr et al lY88, bn DonlAw and Frxnhs lYY1) Gwen thLjt I contmuous I womotor control rnq qxrJtcl YIP graded ~~l;lt~~nc ot the gLjrn’ of muscle xtwty m the xtwz limb Kxson et ~1 tn press, Elliott 1992. Elliott et al 1991 I, psrh
Evperlment

1

SubJzcts wre 10 right-handed (Oldtleld 19711 undergr,~duate and gradu,lte students [-I mJs, 6 fern&, Loge 17-27 \txrs) All subjects L hLld normal or corrected-to-nornlal \ lwn _

The subject held Lomouse (Sumn~Sketch J-button mouse) wth the right hand ‘lnd mndt: Al mowments wth this mouse The mouse IS Lehloned such that cjt Its dnnerlor end I\ d clear plastic pointer the cents of which IS demwxted bl 3 circle 0 5 Inches III dwleter TNO wro Intersect dt the centre of this clrculdr rI1rw iormmg a crosshw wthrn the circle It IS the wordmates of the crosshair InterrcectIon on the tablet surface which IC transLlted by the gr,jphlcs tablet ‘jnd computer :IC the lwxtlon ot the mouse Subjects held the mouse wth

the tip ot their mdev finger plxed wthm the crosshdlr circle The location of the mouse (crosshaIr on the t&let, and thus the locatmn ot the mde\ finger. MS translated mto the coordmates of the cursor on the monitor This trLmslatlon was performed wth unity gain such that a I mm displacement of the mouse crosshaIr resulted m a 1 mm displacement of the cursor on the monitor The cursor dppexed s d blue solid circle 3 mm m diameter The output signal tram the graphics tablet itas sent to a computer (AMA325 386 computer). and wns sampled at a rate of 1% 85 Hz In order to constram mowment oL the mouse to J smgle dlmenslon hsn~s), ;1 wooden track ws secured to the tablet surtax along Its _ mldlme The track &wed for the mouse to be moved forwwdLund bxkwrd treely. whde lmmng IL~ttfrL~ldisplacement to approum~Mv _ 1 mm To reduce nwwment resistance due to frlctlon, pLjper maskmg tape MS uwd to cover the bottom wrtace ot the mouse Smce ;1ll mownwntc wre to be made wthout wton of the hmb, a sheet ot black pILlstIc MS mounted wer the tablet to chldd the hmb tram UC\\

A repeated measures (2 x 3) tactor~C~l desw ws rSmplowd The two mdependen t \ arldbles lf sre wwn cond&n ( Vwon-C&or VC, No Vwon-Cursor NC) and target size (5 mm, 10 mm, 20 mm) A smgle target distance ot 130 mm was used, thsreb> Jleldmg three mde\ of dlfflculty levels (3 7, 4 7. 5 7) The subject Itas seated tacmg the grLlphw tablet Llnd nwmtor, wth the mldlme ahgned approwwtely wth the momtor In this owntttic-n. the track on the tdblct surface ~3 to the right ot the wbject’l; mldlme, thereby allowng tar movements of the rght hmb to be nwde comfortably wthm IpsllaterC~l space The subject cst sufflclentlv close to the tablet such that the ensuing reachmg mwements did not necessitate Ml extennson of the arm Ambrent Illunwx~tlon wthm the black-walled experiment chmlber ms provided bv a table lamp wth Lo 3 wtt neon bulb The lamp leas txed a\iatv_ tram the monttor and subject Contrast and brightness lewl~ ot the momtor were hspt approvlmately the same for Jl sublets Each trial began wth ~1dlspk ot the mouse-curer Ljnd the home circle or Girting powon The- home circle consisted ot A 8 mm diameter red circle wth a central crrculx opening 4 mm tn diameter, and itas located centrally ,jt the bottom ot the momtor display This

ctdrtmg posltlon corresponded to rI central loctltlon ~pprourinatsl~ c 8 cm tram the bottom edge ot the tablet’s xt~w wx Once the home circle and cursor ;Ippeared on the monitor, the cubJect moved the cursor to the centre of the home circle dnd remamed motIonless once this had been ~ccomphshed Three seconds tollowng the apptaraxx of the home clrcle Lwl cursor. II arcuLr target ;Ippecrred cjt the upper rayon 01 the c11spla~ The centre ot the target cias alIgned \c Ith that ot the home ulrcle Cjnd was katted 130 mm tram the centre ot the home clrcltz The subyxt MS tree to mitlate mownlent Lit dnytmw dnd MS mstructed not to be concerned wth rextmn tme The subject 1~115Instructed to mow as qu&l~ ‘jnd JS xcurLMp I as possible, dnd to ensure that whenew- EIS~JI fsedtwh was aallable, the cursor should ‘It lea5t be m wntxt 111th the txget The subject MS also wtructed to remmm motlonlew dt the end ot the mwement until the teedbxh d~splq clppe;lred Tw 1 ISIO~I condltlons wre emplowd These wlon condltlons _ pert~m onh to viwn ot the cursor, LIs all mownwnts wxe nwle wthout CWOII of the Imb In the Vw-w-Cur-xx WC) condltlon, the cursor remwwd w~ble throughout the nwwment In the No VIWIICursor (NC) condition, the cursol dlsappexsd JS won A ~tc centre wlr nlowd outside 01 the smII cerltrL~l openmg ot the home circle The cursor subsequentI\ _ reappeared at the end ot the trml At the end ot exh trial In both c’ondltms, Lofeedbach d~~pLw _ WIS pro\ ded which showd the home c-m+, txget, the curer clt Its Inltlal posItIon ,lt the tme of the target s dppexmce, and the cursor at its locmon ,tt the tme the mwement ended Dependmg upon the mture of the movement. either one, ‘1 hght-colored cursol, or tw. a hght md &Irk-solored cursor MS shown The du4 cursor dlspLw occurred whenewr the ewcuted mowwnt contLwwd a dlccrete second,lnr mowment \ihlch MS hkel, mltrated lrlte mmrdmg to temporJ cr~tmons Thu\. subjects receI\ ed feedback which mtormed them that Lln unLwxptLlbIe delay occurred betwen the end of their mltlLll nwwtlent md ,m cursor a-we;Itteipted correction ’ The powion of the &irk-colored sponded to the powon of the mouse/ mdts\ finger ,\t the time the subject ttxmmL~ted the mownwnt completely The hght-colored cursor

mdlcated where the mowment was mltlL~lly ternwwted prw to the attempted correctIon (see section on Datd Reduction tar J defmltlon of movement termmation) Each subyxt partwpdttsd In II~single e\permwntL~I Sewon At the start ot the session, c- demonstration ;t the t:lsk :lnd mstructwnc were prcwded The sublect then performed one bloch of 12 prxtlce tr& for each vwon condition Wrthm each practrce bloch 4 trrdll;, wre performed tor each target uze Follow me these two practice blochs, 2 sets of 6 blocks of trials ( 12 tr&,i blocck) wre pertormed Exh set contamed one block ot each one ot the SL\ wwn bv txget CIZC condition combmdtlons Block/ condition order wthm the first set MS randomized wth the constrxmt that three wncecutw blocks could not be ot the >drnrn WPIO~ condltlon The second set ot bloch5 were randomized m the c;~me tdshlon. HI c?ddltlon to the constwnt th‘lt, In combmatton wth the first set oi SI\ bloch\, no threr: concwutwe blocks ot trldls could be oi the wmts i won condltlorl The first set ot h blochs wre concldersd ‘1s pr,ictlce ‘It exh ot the umyue USK-MIb> txget size condltlons, Ljnd only the IN wt ot 6 bloch\ wre analyzed In addltlon, wrthm each block ot 12 tr&, the first 2 trials wrre considered AS prxtwx Ljnd wre Au not ~nc‘luded In the anal) ses

Fallowng the e\perlmentdl seeions, and for exh tri4, the rw dlsplxement d&j were Wered usmg J second order dual pdss Butterwxth Mter IION pass cutott trequencj oi 6 0 Hz) InstLlntLlneoul wloclt> MS cAculatsd b> dlfferentlLltmp dIsplxement dLltLl uwg (1 two pomt central finite dlftersnce ~Igorlthm InstLmtLineous xcelsrL~twn US cAculated by dWrentl;.ltmg WIOCI~Y dLjtLj ucmg the wme algorrthm Smce the mwemmnn ws constrwned to the >-dw, kmematlc data were anrtlyzed wth respect to this am onlv An mteractw program we developed to Identrtv certLim crltlcL~l points In the displacement. wlocltl, :jnd ,Icc~leratlon-profrl~s tar e;lch trial The bwnnmg ot cl mcxment IUS dellned ‘1s the first instance at which the InstLlntLjnrous wloclt\l _ ws gre,-rter th:m or equL~l to 1 0 mm/s The end of the nwwment IUS dstmed c~s the tu-st Instance A which mstantmec-us UZIOCI~ fell wthin + 1 0 mm IS, and remLuned wthm these thresholds for a period greater thLjn 10 samplmg frLjmes (72 0 ms) It d subsequent rexceleratlon hn Increase in wlocit~ in

either- the posltw or negatrw dlrectlon and ewesdmg threchold wlues) occurred prior to the end ot the 10 trame wndow followmg the temporarIIv _ defmed mwement end, then dependmg upon the twture ot this reacceler~tlon. mownlent end itas redefmed It the mcrtxw In v&-~lty ws equal to or eweeded ~1duration of 72 m5 i 10 frames). then Ci sexch tar d nts\t movement end WC done xcordmg to the same crlterwn nnd procedures L~bow Howwr, rf the dur;ltwn NL?S less than 10 frames and wtoclt~ I returned to wthm threshold wluex then movement end wc not redstmed end WC taken a~ the pre1 ~oush I detmed endpamt Mowment time ws therefore cAculated 3s the number ot trames betwen the defmed start and end ot the mowment, dl\ lded ID> the wllplmg trequency Constant error w\ c&ul;lted as the slened dlWrence behwen the y-coordmL~tes 01 the centre of the cursorLand the centre ot the txgrt trrotirfrcll tifms poIntI marked on the wloclt\ _ and xcelrr~tlon protIles aided m the ldentlllcatlon ot kmemdtlc IndIce\ ot mowmsnt modltlcatlonc Four types ot mod[tIc~tIons wre dAned and enumerated from the hInt:nwIc ddtd tar each trial These were slgmfrcant dei1atlonc In the xc&I~~tlon protlle prior to peek wloc~~ _ dnd tollw mg pal\ wloclt>. neg~~tlw to posltlw trxn$ltlon\ ot the xcelerzitlon priflle 111 the pewd hehwen peA velocity ‘jncf the end ot the mowment, ‘jnd rwerwl~ ot the (-in-ectlon of movement To Identlhl I slgnltrcdnt dewtlons In the xceter~tlon tlxe In the period behveen the start ot the mowment Jnd peak wloc~t~. a wwch ior a rewrsal point, other than thLlt ot pe,ik L~ccelelC~tlon MS flrlt pertormed It such a rewrwl pwnt ws present. then the wcond rewrscll pomt followng the stxt ot the mowment WC mxked For Lo srgmtlcdnt dwatlon to be present, tw- crlterlon\ had to he wtlsfwd Flrrt, the a-nplltude betwen this marhed rewrsat and the consequent one h,id to wt15hF_ the predstrned dmplltude crlterlon ot WC of the greatest absolute m~gmtuds m LwxlerL~tlon Second. the duration betwsn thll;l nwhed rrwrsal md the pomt ,it ifhlch the nwgnltudtt of xceler~tlon tell beIon thLjt value ‘it the nwhtxl rewlwl h&Id to equal or eweed the temporal crlterron of 72 m~llwcunds (ct \ dn DonkeLar and Franhs 1991) To Identlhl slgnlflc:lnt ds~~~t~on~ rn the period betwxn peak veloc10 and the end of the mowment, a vxrch WI\ again pertormed tar ,W~l ‘ct?ltvl Crltwl

t

anv rswrsal pomts other than that of peak negatw xceleratwn If suih a reversal pomt baas present, then the second reversal pomt followng the pomt of peak velocity was marked For d slgmtlcant dsvlatlon to be present, two crlterl&s had to be satisfied First, ths amphtude btstween this marked reclerwl md the consequent one had to satIs& the amplitude crlterlon Second, the duration between this marked revw-wl. Ljnd the nekt rwxsal point (or ths end ot the mowment) had to sa~sf$ the temporal crlterlon To Identlh _ zero crossmgs In the mxleratlon protlle ~ollowng peak wloclty, a search IUS made ior a negatlce to posItwe trannstton m acceleratwn For any such transItIons to be deimd a~ d zero crossmg tht: peak’ m the wlwtg profile associated wth this transition had to sat& two crlterlons First, the amplitude ot the \elwty peak. nwctsured trom the relatlce wlwty mmlmum to this peC~k. had to equal or exceed 5 0 rim/s Second. the durLjtlon betwxn the start ot this lreacc&rdtIon m wloc~~ dnd the point at which the velc~c~tymlue fsll belong that ~nlus Lit the start had to sat&y tht: temporal crlterlon RtxxsJs m the mowment wx rdsntlfled from a change m the sign ot wloc~ty For ths mownlent rewr~al or negatwe vel&ty to be slgmtlcant. ths absolute magmtuds of nqntw WIOCI~J hdd to rennlLjm aboce the \eloclty threshold (1 0 mm/s) for a mmmwm duration eyuL-l to the temporal crlterlon For each trial, performance dntLl wrc aiallable relatmg to mowment time, constant error, and wrlable w-or Kmematlc data wrt3 Lwulable relatmg to peak iebclhf of the motvment, time to peal\ wloolty as a proportlon ot the movement time, L-nd thr: displacement dt the time of peak celoc~ty expressed ds d proportlon ol target dlstdnce In addition, data pertaining to movement modlflcdtlons Included the number ot slgmflcant dewtlons In the acceleration profile prior to peak vslocih and followrng peLjk wlocltv, the number of negative to posltlvt: z&o crossings of the accsleratlon protlle followng peak wloc~ty. dnd movemrnt rwxsals

Mean values tar each ot the dependent Earlables were obtained tram wlls termed from a combmatton of the Independent tactors. wth each mean value calculated from 10 trials ’ Wht:newr possible, the dependent Larlables were analyzed separateIF ” using a 2 x 3 (VI-

R. Chua, D. Elliutt / Visual regulation

374

-

NC

-

vc

5

Target Size (mm) Fig. 1. Experiment

1: Movemen t time (ms) as a function of Vision condition and target size (mm).

sion condition X target size) repeated measures analysis of variance. For those variables for which parametric analyses were not possible due to skewness, non-parametric analyses were performed. Performance measures

Analysis of movement time revealed a main effect for Vision condition, F(1,9) = 10.96, p < 0.01, and an effect for target size approaching conventional levels of significance, F(2,18) = 3.50, p = 0.051, which reflected a tendency for movement times to decrease with increasing target size and hence, decreasing index of difficulty (e.g., Fitts 1954). Consistent with studies which have shown faster movement times in situations of degraded visual conditions (e.g., Carson et al. 1990; in press; Elliott et al. 1991; Elliott and Madalena 1987; Wallace and Newell 1983), subjects executed their movements more rapidly in the NC condition compared to the VC condition (see fig. 1). However, since target information was always available, this result was not a consequence of subjects attempting to acquire the target prior to the decay of some target representation (cf. Elliott

’ For the movem ent modification variables, the sums, rather than the means, were used for analysis. This was done merely for convenience and does not affect the outcome of the analyses.

1992. Ellwtt Lind h4ndalena 1987, Elliott et ‘11 1991) The mtlusnce ot wual condltlons on maement time contradicts the predlc t Ion of the dual s snt mode I that the duration 0t SUbJW oiements shou Id be mde Pen dent of I usual condltlons cf 1990

According to Mew- et ~1 (1988). movement endpoints should be dlstrlhuted &out the centre ot the target and that this distrlbut on should be Independent ot wsud conditions The fmdmgs tor const cmt error m the present eupwment concur wth these predlctlons kf Elliott and MdddIewj 1987. Elliott et ‘11 1991, PrLjblLjnc et aI 1979d. 1986) Speaf~callq. subjects did not elhlblt ~nv_ consistent tendencies to undershoot or owrshoot the centre ot the target region Mowow-, endpoint bias did not ~aq wth vision condition The variable error Llnalbsls Indicated that subjects \\ere more wnslstat In the VC condltlon compared to ths NC condition, F( 1,91 = 91 63. p < 0 001 This vwon nwn effect xcounted for a rel~t1\,el\ large proportion ot the w-wee Ik = 0 512) The etfect wac medki bv target size. indicated bi the mterxtlon ot w~on dnd target size. Fkl8) = 153, /7 < 0 05 Figure: 2 Illustrates thLlt wth 1 won ot the cursor, subjects movement endpoints became more variable with increasing target size* mdlcxmg that subjects nwde use ok the larger target region (e E, lxger effectlie target wdths) When vlslon of the cursor was not ;l\.alldble, however. this sttect was not apparent

Whtle the ktnemdttc dn~lvses rewaled no stentftcant effects for peak wloctlv _ (cl MacKenzte it al 1987, MtlnrSr Gd I~az 19W, tj matn ettecl hr \‘tston condttton WE tound for both the ttme to penh wAoctt\ ds a proportton of nwwment ttme (TPV/MT). F( I,91 = 72 04, /I < 0 tio1. t (2 = 0 hh51, dnd the dtstLlnce trawlled at the ttme ot pe& wloutt\ cl5 3 proportton of the txgef dtstdnce (PVDISP/DIST). F(l.W”= 39 60, p < 0 001 A3 deptcted tn ftg 3. cubJscts
rephcates MacKenzie et al (1987) The mmmwl mfluencs ot txget we In this study may haie been due to the lrmwd number ot target wdths empbed_ The longer deceleration phase characterlstlc of accurate 1 usually guided mowments has tradltwnally been presumed to allow tme for the usual detectlon and reduction of errors m the movement The modlflcatrons exammed durmg this movement phase were zero crossmgs (ZC). reversals (REVS), and slgmflcant deblatlons m the acceleratton proflle f~llowmg peak wloclty (PVSD), as described earlrer Slgmflcant devlatlons occurrmg prior to peak \elocltv _ (SDPV) wre also euammed Since all of these variables ewept POD had markedlv _ skewed drstrlbutlons. a Friedman analysis ot variance by ranks bias employed as necessaq A 3 x 3 repeated measures PVSD re;reJed a main eitect

analysis of karlance for Vwon condltjon,

performed on F( 1.9) = 7 17,

11~ < 0 025 Figure 5 illustrates the gredter number of deilatlons In the acceleratron proflles ot movements made with wlon of the cursor wallable (see also Carson et al in press) The mtlal analys of ZC using a Frledmdn anwa wedled a slgnlflcant eftsct amongst the SL\ b IsIon x target we condltlons, t ‘(5. II r = 10) = 31 89, p < 0 001 In order to determine A ths source of this effect. further tests were performed to examine speclflc questlons The test tar an etfect of target size talled to weld slgmflcance A _

second test recwiled ii significant etfect for \‘lwIn condltlon, 13 1, tound :2: = 10) = 10 00. p < 0 005 thus mdlcatmg thLlt the dlfkrence In the mltd SL\ condltron analyw MS llhely due to the mklusnce ot \ w-m Speclflcall\, subjects euhlblted a greater number ot secondaw xceleratlons m their mocemsnts \+hen 1 won MS avallnblc (tee table 1) Sample wkwtv protks are depicted m tlgs 6 ‘-nd 7 Inltlal SIX level Frledman andlvses pertormed c-n SDPV :jnd on REVS failed to establish slgnlflcanx Therefore. no turther tests were pertormed on these iarlables Thus, ot the four vxl;rbles reldtmc to

mownent modrtlcatlons, the number ot slgnrtlcant dewatlons tolkw mg peak ~IOCI~J and zero crossmgs wre kmnd to GUJ as Lofunction ok mmn condmon Prttsunmbl~. this reflects a grater number ok

R. Chua, D. Elliott / Visual regulation

Time

379

(milliseconds1

Fig. 6. Sample velocity (mm/s> and acceleration (mm/s/s) profiles for a movement in which no modifications were observed.

modifications in movements made under visual guidance, and these modifications, in turn, contributed to the better performance observed. This particular finding is in direct opposition with those of previous investigators (e.g., Carson et al. 1992; Elliott et al. 1991; Meyer et al. 1988) who have found that performance advantages associated with visual feedback were independent of the number of corrections in the movement trajectory. Currela tional analyses

To examine whether any relationships existed between selected performance and kinematic measures, a number of correlational anal-

380

R, Chum, D. Elliott / ?%wd regulation

Time

(milliseconds)

Fig. 7. Sample velocity (mm/s) and acceleration significant deviation and zero crossing

(mm/s/s) following

profiles for a movement in which a peak velocity were evident.

yses were performed. Correlations were performed between movement time (MT) and the total number of modifications (MODS), between the time from peak velocity to the end of the movement (TPV-END) and absolute error (AE), and the total number of modifications and absolute error (cf. Carson et al. in press; Elliott et al 1991). For each analysis, within-subject correlation coefficients were derived for each subject for each condition. These coefficients were then transformed into Fisher z-scores and analysed using a 2 Vision condition X 3 target size repeated measures analysis of variance. The overall positive relation between MT and MODS (grand mean z = 0.85, p < 0.001) indicated that increased movement durations were

associated wth more modltlcatlons to the trajsctory Mwx-wr, this relation was more pronounced when wslon of the cursor \bas wallablts ( - = 1 01) compxed to when It ws not (z = 0 68) FI I,91 = 5 72, PC< 0 05 The posItwe relation suggests that ,jdlustnwnts to the trqsctoo are time consuming ’ Thus, ths long&- moxmment durations obsened when cwon IS awlable may be du& ths greater number ot I lsual feedbxk-based modrtlcatlons There was an overall negatw rAtron bstwsn TP\ -END and AE (grand mean z = -0 19, 11 < (J 01). mdlcatmg that mowments wth longer dsc&ratlon phases wrc assoclatcd wth less error (5 g , Elliott et L~I 1991) This relation ws prrmarlI\ present m the VC condltlon (Z= - 0 39) and sssentlall\ absent In the NC condltlon (:z = -0 Ol), F(1.Y) = 10 89, p < 001 The extent ot the nsgatw reldtton ws grc3;lter tar the smLlllest target SIZC (5 mm z = - 0 38. 10 mm z = -0 17. 20 mm z = -0 03. F(2.18) = 4 31, 1, < 0 05 The relation betwxn MODS Ljnd AE wre tound to be mdependent ot both Wslon condition and txget CIZ~ There ws, hwwwr, an owrdll nsg&jtIw relation behwen thts tw> variables (grand mtxn L w = -0 20, /I < 0 01) Thus. It dppexs that betttx xcurxc _ was achwed bf hen modlflcatlons wre made to thus mcwment (cf Carson et al 1991. In press, Elhott et al 1991) Howew-, If modltlc~tlonc made wth visual feedbach nre assumed to bs more ,jccurLjte (Meyer et dl 1988L then w-w might c\pcct a stronger negatlw rxlatlon when ucmn IS avallablz, contrxy to ths present findings

Dtscrtssiou The prlmanl _ purpose of this stud\ _ was to examme the rntluence of w-yng usul condltmn and target SIZE on manual almmg pertorrndncc dnd armmg trqcctorlss Of mterect wrs the r&wonshIps betutsen the performance ‘wl kmematlc measurec, Ljnd hw thece Indices 1 arrsd 1%Ith I Isual condltlon The Ljwlabllrtp I ot wual mformatlon prowd to be d potent determmant of performance and tht: underlyng mowmrSnt kmsmatlcs Superw performance conwtenw itas xhwed when vlslon ws zallable Mortx-wr, It appw-s t h,jt *this ptxtormdncr: adI ‘Intags ws de-

rwd tram a modIf~cL~tlonal procesc subsumed by UW-MI Changes In the hmematIcs of the limb mowmmntt Llppeared to be retlectw ot the dltferences In almIng 1 mabht~~ The mlpxt oi th; usunl rAmpul~tlon on mowment trdIeCtorles ws prlmxrly wldent during the latter phclse ot the mowment The ~bwnce ot 3n mfluence ot wslon on the magmtude ot peak velocltv suggests thLlt I wal reguLltwn occurred prmw-111 durmg the dsceleratlw phase ot the mocvnwnt Karwn et ‘11 In press) Indeed the usud mL~nrpulatlon proved to be 3 potent determmdnt oi the relatlw time spent In deceler3rlng the mowment Addltwn~lly. the tmw betwen peak v&at\ and the end oi the movement also predicted termmJ LlccuracJ wh;n wlon of the cursor ws awL~ble It IC also during this phme In \ihlch cte see a v~uall~ _ reIdted dltterence m the number oi rno[ ement adlustment\ Speclf~oally. there wre more zero crossmgs :wd wmtlcdnt dewatlon~ In the acceler~~tron protIle ot mocements ewcuted In the presence ot wcual teedbxk Thee semndan xceler,~tlons ,lnd dewwons In the axeleratlon trace, retlsctmp Jdlustments to the trqectoq. apparently contributed to better axuracv aid wnslctE:IlC~

Trxlltlonnl clowd-loop models of \ w&motor control a~sunw that uw11 IC used to detect dnd reduce dls~rcpaxxs hetwxn the pocltlon l%cY, 1911) It corrects adJustot the hLlnd and txget (ESg , Lxle mats to the movement trdlecton a-e baed upon wcual teedhack (e g Crossm~n ‘jnd Goodew 19&‘1983. Howrth et aI 1971, Woodworth 18991, then these correcttons should not be ew.l~~t In mownwntr male wthout uwd tsedbd Elhott et A 1991) In our ddtLj, the presence ot adJustmats to the mownlent trcllecto17v doplte the c clbxnce ot 1 lwn k g q see L~lso Cawn et 111 In press. Ellrott et L~l 1991 Jednnerod 198-L hieyx et ‘-1 1988) oontllcts wth models \fhlch posit that correctwe submowments are medIated by Lo~~Iw~ comp,wcon ot the powIons ot the limb ,lnd txpet it: g , Keels M-IS, 1WlI The prewnce of thee adlul;tments despite the A-wnce of usud mfornwlon ut the hmb pocltlon suggest+ that the! mw be based upon other courcec of mtormatlon, fol ew-nple, kmesthetlc tsedbxh ’ Gwn thrit performance remam mtsrlor to that \i hen USIOII IS a al-

able. kmesthetlc

feedbach

appears

to be a relatwzlv

poor source ot

Information upon which to hse correctw adjustments; Furthermore. In the present task, kmesthetlc feedback IC dlswxwd from the USLLII mtormatlon and therctore would be expected to prwdc less direct Information regarding the po5ltwn of the hmb/cursor. compared to a situatt:on m which J traditIonal pomtmg tLjsk IS emplowd Although the pertormance drtfersncec betwen the EISU~ condotlons dre comparable to what has been demonstr,lted prewwsl\ (e p q Ellrott et al 1991), the kmemdtic tmdmgs contrast wth others k g , Carson et al 1992, Elhott et al 1991, Mqer et ‘11 19881 Prewws studies (Carson et al 1992, Elhott et al 1991, hlqer et al 198X) have L&d to sha\\ annv 111the number of wcondaw _ xc&x+ _ differences tlons The present study denwwrates a dl&rence In the number ot ztxo crowngs dnd slpnltlcant dwatlonb as ;1 function tlon (see also Cxson et al In press) The dIscrt:panc!

01 USKMI condobshwsn thew

studies dnd the present ma! partI\ be A function ot the tape of task _ employed For example. the requirement that mwementl;, must be termmated entlrel> by muscle actwit! (and friction) might result m a gre
Experiment

2

In this study. w wrr3 concerned with Meyer et al’s (1988) trsatment ot ho\{ correctw submowments are &pxed In movements composed of hw submowments. Meyer et dl W8SI assww that the secondq submovement tollow the primary submwement wthout delav This teature ot the mods1 IS mconwtent wth their dssumptton that secondq submovements arc prepared on the bws of wual feedback of the priman: submovement endpoint (set: blew- et al 1988 euperlment 2) If the preparation of the secondaw sub&wment IS mediated by visual teedback, important usual mtormatlon must be dewed from earlier stages ot the mwement If there IS to be no delaj _

1 trom ot the

teedto the mowment accuracy

(cf Bedubdton ,-ind Hay 1086, Carlton 1981) Absence tram the termm~~l portlons ot the trapxtor?, (assumed to encompass the prlman submocemsnt endpomt) would be eqected to have less mtluence on accuracy (cf GrIton 19Sl) ElperlmetIt 2, theretore, IUS designed to mwstlgate the effects ot selectlwl> ewludmg UW:II teedbach during different stages of the nwwmment trqxtow _ VISLI~ feedback WIS manipulated by either pro\ ldmrr cwon ot the cursor Mslon-Cursor), no \ won ot the cursor (No -‘isunCursor), ~MN-I ot the curer onl!_ on the first half of the trLqectorv i FHW, or 1 w>n of the curcor onlv_ on the last hdt ot the trqectoi (LH\ 1 In thrs study, our intent w-5 to haw sublects prepare to mc~w to ot I Cal

feedbxk

tLlrgets m the wme

mc~nner, regxdlee

ot whether

complete

01

only

pxtl~~l teedbach MS nlallnble It would not t-e too surprlsmg to find that dltterent stratepw are employed tor Full Vlclon :jnd No Vwon conditions (e g _ Elliott and Allard 1985, Zel,tzmk et al 1983) Ot greater concern \tas that dltterent strategies wre not adopted kor the pLlrt131 teedback conditions Theretore, in ;rttempts to ,nwd thrc potentlJ problem. the tw p:jrtlJ teedhA condltlons wxe combmsd wth sIther r’l Vwon-Cursor or No i won-Cursor wndltlon Thus, a block ot trAs consisted ot either the \‘won-Cursor condltlon corn-bmed it 1th the hw Partial-Vision condltlon\. or the Nu Yeron-Cursor wndrtron combmed wth the PLwtlL&Vlcmn condltmm It us ot interest to see whether the xwlts for the hw pCvtlLII tsedhtick conditions wwld differ dependrng on \i hether the! wre combmsd wth the \ won-Cursor or No VlsIon-Cursor condltlon Since bc?rlmg target vzt: had onlv a mrnmx~l mtluence on the dependent measures m expwment l-. this txtor ws ehmmated m experiment 2 Thus, Vwon condltron renamed the sole mdependent fxtor Subyxts m the present stud\ also recsrwd more otenslw plxtlce than subjects In expermwnt 1 Although practice effects wre not dealt wth dIrectI\ _ In this work it \+a\ Lw ICCW which could be e\ammed afterwrdq should It prow to be ot Interest Theretortl wblects m the present stud\ p;.lrtlclpatcld m an :jddltionA elpermwn-

tal sesslon

Identical

to expsrlment

1 This

session was considered

as

Prd ct1ce

Sub~scts were 10 right-handed (Oldfleld 1971 I undergraduate and graduate students (5 male, 5 female, age 17-28 Iwars) from McM;lster unlcersltk _ All hJd normal w corrected-to-normal wlon trrs apparatus

was ldentml

to those m elperlment

1

There wxe tour tvpes ot USUJ feedback mampulatlons t 1) Vrwn-Cursor WC), I; \b hlch vwon of the cursor ws awl&le throughout the mowment, (2) No Vwon-Curl;or (NC). m which the curcw ws extmgulshed lmmedratelv upon leawng the home circle, (3) First-Half Vision (FHV), In which *wslon ot the cursw was ac&ble only on the first half ot the trqectory
Du til

I

thctw~l

The datLl tram the twst session wre not analvzed For the Vwon ML\ block, -I \C trials were packed at r&lndom tram trials l-20. 21-40, 31-60, 61-W. Lwd (‘31~100 thus gwng 1~total ok 20 trials The wme process ws pertormud tar NC trials tram the No Vlslon ML\ blouh These trials wre anJ\zed Jong wth the trrals tar the P3rt~al-Vwon _

20 trk ior each ot the sn ~won conditions Thus, there wre condo t Ions The rem;imder of the ddtd reduction ‘ind hmemat1c ~n~l~stss Ljnd _ the dependent \xlahles wre Identical to epwment 1

In contrast to elperlment 1, an&w ot mowment time ws mdependent ot Usron condition Althkgh the mqorltv I ot trials MY2 1 wthm Lous1on combmatlon group wrtz either VC or NC. the mowment time ettect tound kx- these conditrons m e\perlment 1 MS absent hers Psrh;lps the presence ot the Partial-i’lslon trials wthm the tw \W-UI hh blochs negated the mowt~ent time dlttewnce tound pro IOUSI~ EuammaClon -of cc-nstcjnt error rwxled ,ln overall ten&xc\ to owrshoot (grand nwjn = 0 81. 11 < (1 (11b md
tIo[ls. F( 1,451 = 21 43. JI < 0 001 This IS presumiib \: a consequence ot the longer duration for bihlch usual rnlormatlon IC ~?vak~ble m the LHV condltwns and the proumvty of the cursor I I the target during this period No dItferences were found between thew condltlonc and the VC condltlon No constant error dltierences wrre tound between the VC and NC condltlons kt Elliott et al 1991 Pr;lblanc et al 197%) There WIS d difference behwen the NC condltwn ilnd the hw FHV condltlons F( 1.45) = 22 97, 11 < 0 001 Slmllar to eupwment 1. the ~wllablllty ot \ ISU~ mkwmatlon w’;l found to mfluence subrects’ conslstenq, F(5,45) = 9 67, 11 < 0 001 Post-hoc compxrsons &e&d that sublects wrre less wrlable ishen they were certam that home i usual mformatlon wuld be a~allablt: durmg the ensumg mwement compared to the sltudtton In which such certamb MS not possible. F! 1,-U) = 20 27, 17 < 0 001 Figure 9 ~llustrdtes these dIfferewes betwen the Vlswn-cursor Combmatlon condltlons and the No Vlswn-Cursor combmntron condltlons Further compxwns show4 that USIOII of the cursor throughout the mowment \\a5 of benetlt compnred to wslon only on the first halt. F( 1,351 = 16 67. /I < 0 001 and sImllxlv, the sank bentlilt \#a\ ewdent toI USIOII durmg the ldst halt. F( 1.45) = I5 93 11 < 0 001 There wre no d1tkrenws hetwen the VC condltlon ,ind the hw LHV crlndltlons Although pertormance m the FHV comhtrons were Inkrlor to the \‘C and LHV condltlons, J compdrwn of the hw FHV condrtlons ‘jgamst

the NC condltlon reveLlIed that some benefit was gamed from the earlv visual Information that was aia~lable, F( 1,35) = 4 71, p < 0 OS A tmai comparison to examine an) possible dlfterences bebxen the PartI&Vision condltlons m the Vlslon MLK and those In the No Vlslon ML\ revealed no slgnlflcant differences Thus, performance varI&lllb m the Partial-Vision conditions were Independent of the type ot vlslon combination group (Vision vs No Vlslon) m which these partral conditions jcere embedded

bv_ UsIon perimen t 1 peak veloc~tv was unaffected condition (e g , Carson et dl m press, WEll~ott et dl 1991) vlslon condltron was found to influence both the time to peak ~eloaty as a proportion of movement time (TPV/MT), F(5,45) = 3 62. p < 0 01, and the proportion of the target distance trakelled at peak ieloclh (PVDISP/DIST), F&IS) = 125. 12 < 0 005 An ercammatlon of the5e effects revealed that for both variables, the primary Influence was exerted bv the type of vision combmatlon group (TPV/MT F( 1.45) = 14 53, p 2 0 001. PVDISP/DIST F(1.45) = 31 12, p < 0 001) Subjects spent a greater proportion of the movement In the deceleration phase when they were certain that the cursor would aluay5 be wslble for at least d portion of the trajectow _ (see fig 10) Moreover, as a

pexed (LHV condltlon) or disappeared IFH\ condltlon) ljt the mldpornt ot the traJecto0 Analvses ot movement modlflccltlons failed to reveal a slgnltlcant _ effect for an\: of the modlflcatlon variables The absents of an effect tor Vwon condition 1s consistent wth the results of prwous mwctlgators (Elliott et ‘11 1991, Mewr et al 1988) but IS In direct contrast I wth those of e\perlment 1 The sltuatlon arises, therefore, m c\ hch slrnilx pertormance advantages xe found tar movements made under msud guldnncc. wth ths notable eweptlon that, unhke expwmsnt 1 these advantages mhlch are prewm&l\ _ due to the presence ot v~su~~I mtormatlon are no longer related to the number of mowment modltlcations y

CorrelatIonal analwes Identical to experiment 1 wre pertormcd on _ the same group ot dependent iarlables Smlll;lr to experiment 1, there ws an overall posltw relation between MT dnd MODS (grand mean - = 0 76, p < 0 001) An mtluencs of vlslon condrtlon, F&U) = 2 03. ;I < 0 05. was due to a slglnlflcant palrwse difference between the tjso extreme means (Tukey I%D. y < 0 09 A negatw relation was tound between TPV-END and AE (grand mean z = -0 19, 11 < 0 01) Post-hoc analqsls (Tukey HSD, /7 < 0 05) ot dn ettect tar wlon condltron, F(,C,W = 3 83. 1; < (101, revenled onlv a dlfterence hehwen the eweme means There was also a small ne&e relation behveen MODS and AE (grand mean z = -0 14. 11 < 0 011 A slgnlflcant difference between the two eweme rnedns (Tukev HSD. /I < 0 09 resulted 111 dn effect for vlslon condltlon, F(5.4:i) =3 12. /I 005 Purtrul- blot1 corzdrtiom To examme whether the manner In iihlch the Partial-Vwon condotlons were grouped (I e . m combmntlon wth VC trial5 or wth NC tr&, or blocked on their ow) affected pertormance and the manner In which subjects prepared for upcommg movements, analyses wre performed which Included the data from the blocked PartlAVlslon condltlons The analyws utrhzed 3 factorial arrangement ot tbfo Par-

tl&Vwon condltlons (FHV and LHW and three Mwtype condltrons t Ptwlal-Vlwn conditions m combmatlon wth VC t&s (VW. III combmatlon with NC trials (NX). and blocked on their own (BLK)). to emplovI a 2 x 3 (PartI&i’won x MI\-tvpe) repeated measures dnalysls I ot variance Ptv toi~?ZL7~lCL’mt’c7 wrtzs

Analvsis ot mowment time rewaled dri eftect tar Partral-Vision conditlci F( 1,9) = ,r 74. 11< (105. with WbJKtS takmg longer to complete therr nwwments m the LHV conditions (FHV = 593 ms, LHV = 6-U) rn4 Presumable. the longer mwemmnt times retllect the subjects’ cllttempts to utllrzi the visual rntormatlon whwh becomes awl~ble once the mldpomt ot the trayzctoq IS redched nnd to mAe tiny necessarv JdJustments to the mowtent Subjects mcwment xcurax I ‘jnd boas. reflected In constant error, WC Ljftected b\ both Partial-Vision condltlon FI l-9) = 32 13, 11c 0 001. and ML\-~PS. F(2.18’1 = 3 S3. y < 0 05 As might be elpectsd. wbJects wre n&e accurate \‘Ihen usual Information ws ava~lahle on the last half ot the trajectory ( -0 28 mm) In contrast, bihen usual mlorm~~tron was restrwted to the mitral half ot the trqectoq. subjects overshot the target centrrs 12 54 mml Figure 12 Illustrates the overall overshootmg that occurred In the FHV conditions The reason tar this remamn unclwr When the Pdrtlal-YeIon trr;lls wre either blocked or combmed with VC tr&, SubJects wrre nwre cwzurate (VI~KIII ML\ =

0 84, Blocked = 0 49) than the sltudflon In whrch these trials wrre combmed wth NC trials (No Vlslon ML\ = 2 (It>) A srgnlfwant per~tse dlfferenct: was found between the Blocked and No Vlslon MLX condltlons (Tukev HSD, p < 0 115) It seems subjects tended tc- be more accurate &n certam that some usual Information wuld be wallable during the ensuing movement (e g , see Elhott and Mar-d 1985, Zelazmk

et al

Partial-Vlslon F(1,9)

1983)

condltlon

= 26 70, p < 0 001

was the sole Influence Once

dgam.

wth

on karlable error, the longer durations for

which visual mtormdtlon IS available durmg the LHV condltlons, subjects Mzre able to mamtam greater consistency (FHV = 7 10 mm, LHV = 5 11) compared to the FHV conditions

Krrttvnn ttc rntwslri

4s Analvsls ot peak veloc~~ agam revealed no slgnrtlcant effects. whk the analyses for TPV)MT and PVDISP/ DIST revealed only an Interaction ot Partial-Vision and MLK-type condltlons for both varlahles (TPV/MT FQ.18) = 5 36, p < 0 025. PVDISP/DIST F(2.18) = 10 487, p = 0 001 As IS ewdsnt m fig 13, the mteractwn for TPV/MT isas reflectw of subjects spending a greater proportlon ot the movement m deceleration m tht: LHV condrtlon compared to the FHV condwon when these condltrons were

-

FH\

Frg 1-l

blocked When these condltlons wre combmed wth either VC CM-NC trials, there were no wch dWmmcss In pxallel. subJects travelled
The prImam _ purpose of: this studb MS to elannlme the &tects ot selectwel~ ewludmg wual teedbxk -on amwq pertormance and dqnamlcs It correctw adjustments xrf bed on vestal teedhack trom the precedmq mltlal wbmowment. then, gwen the mtrmslc delws_ tn wsuomotor p&xssmg, this teedbxk must he derlwd from the errrller stage\ of the InltlA suhmwement dnd not It5 endpomt (Cxson et A m press, ct Meyer et al 1988) This mlplles. wth respect to the

R Chr

D

Ellrc~tt

I ~s11drc ~lrl~rtlu~l

present study, that Important VISUA mformatlon can be gmed tram the first half of the mowmment trqecton’ (ci Car lton 981) In the present study, It ws assumed that tt1e III ml halt ot the trqectow wuld encompass thLjI portion ot the prmwrv submowment prior to- Its endpoint, and the Idtter half of the t&ectow wuld therefore contLjm the xtual endpoint Followng this Ime of reawnmg. It was therefore hvpothesrzed that the usual feedbxk tram the mltlal half of the trqectory wuld be Important, gown that It encompassed that portion ot the prmlaw _ submowment tram \j hlch usual mtormatlon WIS required for the prepL~rL~tlon ot the cecondarv _ submocement (ct Mewr et al 1988) Clearly. this mltlal crittempt ot the present stud\ suffers from the rlmple mL~mpulatlon ot USMI Lsed upon particular segments of the trqecton It IS ‘In mlprecw manlpuIatlon wth uhlch to ewmme the present ICSUSS ol mtetect The L-pproprIatcS method of testmg the abow hlpothesls wuld be to mL?nlpuL--te the ~wlab~l~tv of \ ISMI fesdbxh &out the xtudl endpoint ot the prmlanl _ subr&ement. rather th;ln some portion of the trajectonr An Ljddltwwl problem IS that movements c;w contain sew-lltl &wrectlons not Jurt one This nmpl~f~es the problem dt hand since noi! one must consider ho\{ later corrections are prepared Theretore the ISSUS concernme the role ot usmn m the prepxatlon of correctwe subnwwments &Its turther mwcltlg‘ltlon The present tmdmgs mdlcate that rZrt“jter Ljd\antages wx-t3 xcrued tram irsual mformdtlon on the latt& halI ot the mowment These results xe cupportw ot the wwk ot Cxlton t 19511, m th< U~KXI ot the Idtter stages of the trajecton _ stxm~ more crltlc:jl for pertornxlnce There IS Come EdIdsnce to surest that eariF \ rsual mfornwtron wl; ben&xl when compared to :%o VISIOII condltron (ct Cxlton lcllyl, Beaubaton ;Ind HEY It ws mltralh _ expected that this _ 19MA howwr. early \ ISUJ mtormatlon wuld have ~1 grsdter contrlbutlon One powble problem IS th,-it In srtuattons 111\\hlch WUA mlormatwn IS ewluded on the mrtlal portion of the trqecton. subjects rndi Lldopt a strate&T m \fhlch they slou their mwxtwnts once wlon becomes aiallable (e g q SW also Carlton 1981) The) ma> Jso spend a greclttx Ltnwunt ot time trLwrsmg this ILItter portion ot the tralector?,, thereby allowng sufflclent time to utlhzs the usual mtormdtlon N hlch beconw ZW~III&I~ This ma\ theretore compensate for the lxk ot wsual mtormatron during the -mltldl portion ot the trLqectoo A second powblllt\ IS that the umd~rnenc~on~~ht~~ of the drmmg task and rhe

possrblvI lesser Importance ot the early c ~wal mformatlon (compared to when the cursor IS close to the target and errors are easier to detect) ma\_ haw reduced the mpact of wud teedback from the mltlal halt ot the traJt:ctoq In e\perlment 1, a longer decelerJtwe phase ws assocwed wth better terminal accuracy \i hen \ won was wallable Moreover, the wsu;llty-bnsed pertormance adwltage ws attributed to Li gwlter number ofwual feedbxk-based adJustments to the mowmmnt In the present study. the time betwen peak velwtv _ dnd the end ot the movement hgam predicted &otute error Howwr, c~ dlBr:rence ot greater importzmce IS that the present pertormance advantage cdn no tonw- be attributed to rt prevalence ot I wall\-bdsed wrrectw A_ lustLments The ,~bsencc: ot Cl uwd mfluence on the number oi modIf[catrons In the trajectory IS In direct opposltlon to expermwnt 1 (see also Carson et al In press), but IY corwstent wth other wwk (e p ‘1 Carson et al 1992, Ettlott et dl 1991. Meyer et A 19Stz) There xc’ a number of powblhtres \ihv_ the wwaltv mediated performance advantage ws no longer retlected In the kmematlc measures First. there wre Logredter number of errors (target mlssed comptetel~ 1 commItted In the \ won-Curwr trILlIs compxed to slperlment I Those subjects who commItted Lopredter number ot errors nuy haw pad msufWent xtentlon to accuraq demands It mat’ be thL-t h,td these sublects complred wth xcurxv _ demLlnds. a greater number of correc‘tIons would hLw been wdsnt In the Vlsron-Cursor that these ddJustments xe retr& The assumption IS, of course yulred dnd lead to better termmal L~ccur~c~ (e g q we elperlment 1) Houewr, despite the larger number ot err&s committed, the Impxt ot wslon upon almIng consistency 5tltl remnms wdent A second Aernatw I\ that the more extenswe prxtlce recewed bl the subJeW (ct ekperlment I) mav haw resulted m a mode of wual-motor control to which the current ‘kmemL~tlc mdlces wxs mcensltw On d r&ted note, 3 third posslbltlty IC that USUJI regulat[on ot the Lw-mng mowments occurred In a contmuous. Lagwll as Lodwrete fdshlon (e g q see Cxson et al 1992, m pre55, Elliott et a1 19911 Although the use c-11 siyiiflcant dewatlons in the acceter~tIon trace ws originAv proposed to mdeu a contmuow or pseudo-contmuous mode ot ~womotor control, the present wdence suggests that this measure \+as not suttlcwtlv sensltwe to the nwdItlcL~tw-wl process thnt ma) hLwz occurred N hen ~ISUA teedhxk N ~~ JL L~~lAle

General discussion models of v~s~~m~cmr control (e g , CLwlton 1981, Keele 1968) hdw generallv emCrossman dnd Goodebe 1!%3/198?. phaslzed the importance of wslon m medldtmp the modlflc&onal processes inherent In xcurdt(: hmb movements The precise nature of this regulatory process remains J topic ot debate The prswnt wrk sought to euamme whether wu:~l regulation proceeds In an mtermlttent, discrete, or m c? contmuous, tashlon In addltlon, d recent model ot limb control, the Stochastic Optlmlzed Dun1 Suhmowment Model. proposed b\ Mewet al (1988L ws exammed Ot prmaw mtemt _ ws the I& of visual feedbach wthm the general trmw&rk ot the model md speclfrc& rn the preparatmn of secondary, correcttw, submovements It has been demorwtrdted that wclon can be 3 potent mflluencs on performL~nce in goal-directed limb mxennlent~ md on the kmerndtws underlyng these sctmns k g . see also Ellmtt et al 1991) This poses EI problem tar the dual submovement model (hlevsr _ et aI NW In Iihlch the role ot ~slon 1s nmm~zed or gwen less thLln sufflclsnt consider+ tlon In the present wrk, w hdw show that the duration of subyxts niow~ients, perform;ince consistency. number ot mownlent correctlons. dnd the symmeto ot the kmem~tlc protllss were ~11 found to LIQ L~s‘1 function of USU~IIcondltlon These tmdmgs die mconslstent wth the predlctlons of the dual submowment model (hlewr et 4 lY8S) ’ The ISSUCot how corrects submowments xe pre&ed dnd the role ~hlch wlon ploys m the process ,w,~lts further mwstlg~~tlon The rwture of ~~~suL~l-motorregulation h:ls recewd recent attention (e g , Carson et al 1992, in press, Elliott 1992, Elliott et ii1 1991, P6llsson et al 19S6) The purpose ot emplowng the present kmetnatlc m&xes MS to Llttetnpt to mde\ dllferent forms of trqxrory tmdlb cattons dnd Inter the nature ot the underlwng mode ot control The use ot slgnlflcant dwlatlons’, m addition t6 zero crossmgs’. promded d nwre sensltw mtmsrre of these adjustments kf Gwson et al 1992, Elliott et al 1991, Meyer et al 19881 Slgnltlcant deilatlons m the xceleratlon trace tollowng peak celoclty presumably reflect LldJustnwits of the braking impulse wthout Closed-loop

It appears that wuomotor regulation ot alming mowmmstt GUI occur rn both discrete and contmucwc tashlon hlloreow-. the mode of control, and the number and type ot trayxtory adJustments. may w-y from trial to trial, dependmg on the pxtlcular details ot the swatIon at hand It certainly would not be ‘t surprise for wsuom~~tor reguldtlon to ekhlblt thrs t-‘le~b~hty Atter all, one inherent feature of motor control processes should be the &~llty to adapt to iaryng demands The control of hmb mowments should he sensltwe to a number of factors. these mw_ mclude, among others, the demands of the tal;k, the goal ot the performer. the task spec~t~c eupewnce of the pertormer, and enwonmental condltlons (e g . SW Martemuk et ‘11 1987)

References

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