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Bimanual reaching across the hemispace: Which hand is yoked to which?

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Bimanual reaching across the hemispace: Which hand is yoked to which?
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  Bimanual reaching across the hemispace: Which hand is yoked to which? Gavin Buckingham a,b, ⇑ , Gordon Binsted c , David P. Carey a a Vision Research Laboratories, School of Psychology, University of Aberdeen, Aberdeen, UK  b Centre for Brain and Mind, University of Western Ontario, London, ON, Canada c Coordinating Perception and Action Lab (CPAL), University of British Columbia, Okanagan, BC, Canada a r t i c l e i n f o  Article history: Accepted 15 September 2010Available online xxxx Keywords: Bimanual coordinationManual asymmetriesHemispatial asymmetriesMotor controlAttentional asymmetries a b s t r a c t When both hands perform concurrent goal-directed reaches, they become yoked to one another. Toinvestigate the direction of this coupling (i.e., which hand is yoked to which), the temporal dynamicsof bimanual reaches were compared with equivalent-amplitude unimanual reaches. These reaches wereto target pairs located on either the left or right sides of space; meaning that in the bimanual condition,onehand’s contralateral (moredifficult) reach accompaniedbytheother hand’s ipsilateral (easier) reach.By comparing which hand’s difficult reach was improved more by thepresence of theother hand’s easieripsilateral reach, we were able to demonstrate asymmetries in the coupling. When the cost of bimanualreaching was controlled for the contralateral reaching left hand’s performance was improved, suggestingthattheleft handisyokedtotherightduring motoroutput. Incontrast, theright handshowedthegreat-est improvements for contralateral reaching in terms of reaction time, pointing toward a dominant roleforthelefthandintheprocessespriortomovementonset.Theresultsmaypointtowardamechanismforintegrating the unitary system of attention with bimanual coordination.   2010 Elsevier Inc. All rights reserved. 1. Introduction Bimanualcoordinationisarelativelycommonfeatureofgoal-di-rected interaction outside of the laboratory in humans. Both handstendtoplaycomplementary,yetdistinct,rolesinmanydailyactiv-ities,suchastyingshoelaces,buttoningshirts,andevenwritingonasheet of paper. Despite the apparent independence of the hands inthese tasks, many experimental studies have indicated that thereare severe temporal constraints upon the hands during discreteand rhythmic bimanual coordination (Corcos, 1984; Kelso, 1984; Kelso, Putnam, & Goodman, 1983; Kelso, Southard, & Goodman,1979; Marteniuk, Mackenzie, & Baba, 1984, Swinnen, 2002). Here, weinvestigatehowthisdichotomyisresolvedwithinthemotorsys-tembydeterminingifonehandisyokedtotheotherduringsimplevisually-guidedbimanualreaches.When individuals move both hands together, they tend to tem-porallysynchronisethemovementofonehandwiththeother.Thissynchronisation effect is even strong enough to induce uninten-tionalshiftsfromanti-phasecoordination(i.e.,movingoutoftime)to in-phase coordination (moving in time – Kelso, 1984; see Swin- nen (2002) for review). Recent work by De Poel, Peper, and Beek(2007)indicatesthatthistemporalsynchronisationisnotsymmet-rical. Rather than synchronising to a ‘middle ground’ (the equiva-lent of both limbs making simultaneous phase shifts of 90  ), thenon-dominantlimbwasmorestronglyinfluencedbythedominantlimb than vice versa (see also Byblow, Carson, & Goodman, 1994;De Poel, Peper, & Beek, 2006). Interestingly, this asymmetry ap-pears tobemodulatedbythedirectionof attentionduringthetaskitself (De Poel, Peper, & Beek, 2008), mirroring suggestions fromearlier research that attention may be laterally-focussed duringbimanual coordination as an expression of cerebral lateralization(Peters, 1981).Similar effects of temporal synchronisation have been demon-strated during discrete bimanual tasks. When reaching with bothhands, the usual pattern of temporal asymmetries evident inone-handed reaches with either limb (the small left hand reac-tion time advantage and the larger right hand movement dura-tion advantage) are ameliorated, and the hands start and finishtheir reaches at the same time (Kelso et al., 1979). This temporalcoupling even takes place when the hands must make move-ments of different amplitudes from one another – with bimanualreaches taking substantially longer to terminate than unaccom-panied movements of equivalent amplitude (Kelso et al., 1983).Functionally, this means that the hand with the shorter reachis slowed down by the hand with the more difficult, longerreach. It is clear that, at least conceptually, the timing mecha-nisms of both rhythmic and discrete bimanual coordination aresimilar. However, despite valiant efforts to computationally mod-el discrete movements as truncated rhythmic movements (e.g.,Ronsse, Sternad, & Lefèvre, 2009), the differences in the locusof neural activation between these coordination modes (Schall,Sternad, Osu, & Kawato, 2004) suggests that such a generaliza-tion may be inadequate. 0278-2626/$ - see front matter    2010 Elsevier Inc. All rights reserved.doi:10.1016/j.bandc.2010.09.002 ⇑ Corresponding author at: University of Western Ontario, London, ON, Canada. E-mail address:  gbucking@uwo.ca (G. Buckingham).Brain and Cognition xxx (2010) xxx–xxx Contents lists available at ScienceDirect Brain and Cognition journal homepage: www.elsevier.com/locate/b&c Please cite this article in press as: Buckingham, G., et al. Bimanual reaching across the hemispace: Which hand is yoked to which?.  Brain and Cognition (2010), doi:10.1016/j.bandc.2010.09.002  With the current study, we aimed to determine whether evi-dence for a leading role for the dominant hand could be demon-strated during a discrete bimanual reaching task. In contrast tothe work examining asymmetries during rhythmic bimanualmovements, only very limited evidence exists of either hand ‘giv-ing more ground’ than the other during discrete bimanual tasks.Marteniuk et al. (1984) elicited small asymmetries in the directionof the coupling, by comparing bimanual reaches with equivalentunimanual reaches while ‘handicapping’ one hand with a heavierstylus than the other. As the handicapped right hand slowed theleft hand more than the converse condition, it could be suggestedthat the left hand was ‘bound’ to the right hand. However Marte-nuik’sstudyfailedtoprovidecompellingevidenceofasymmetricalyoking in all the subjects, perhaps due to variability of many uni-manual performance measures. An obvious way to improve thechances of demonstrating asymmetries in coupling would be tofurther increase the magnitude of the baseline (i.e., unimanual)asymmetries – increasing the difference between uncoupled uni-manual hand differences with those in a coupled bimanual reach.Hemispatial asymmetries may allow just such a reduction in theperformance of one hand relative to the other.When a hand performs a reach into its own space, the move-ment is completed in a shorter duration and higher peak velocitiesthan an equivalent amplitude reach across the body midline. Themechanisms underlying this ipsilateral/contralateral reachingasymmetry are still under some debate. Some researchers havesuggests that the contralateral reaching deficits are merely anexpression of the increased attentional demands of common stim-ulus–response compatibility effects (see Procter and Reeve (1990)for review). However, most researchers tend to view the hemi-space asymmetry in terms of inter- vs. intra-hemisphere process-ing. Therefore, for an ipsilateral reach, the hemisphere thatprocesses the visual target is also responsible for the eventual mo-toric output. Therefore the transfer from one hemisphere to theother that must occur prior to a contralateral reach somehowslows the overall performance (Velay & Benoit-Dubrocard, 1999).Conversely, studies by Carey, Hargreaves, and Goodale (1996)and Carey and Otto-de Haart (2001) have found compelling evi-dence that the within/betweenhemisphere processing theory can-not fully account for the deficits seen when reaching across thebody midline. Carey and colleagues instead proposed that the dif-ficulty in reaching across the body midline was due mostly to bio-mechanical factors, such as the larger centre of mass around thetwo-joint system necessary for contralateral reaching (ipsilateralreachinggenerallyrequireslittlemorethanextensionof theelbow joint). It must be noted however that for the purposes of the cur-rent experiments, the underlying causes of the hemispatial reach-ing asymmetries are not relevant. The crucial point here is thatmovements made into the contralateral hemispace are slowed rel-ative to ipsilateral equivalents. Thus, in order for bimanual reachesto left or right space to be synchronised in time, one hand must beslowed down or sped up, relative to the performance that can beattained when reaching in a unimanual circumstance.Withthecurrentstudy,weaimedtodeterminewhichhandwasyoked to which by combining the contralateral reaching disadvan-tage (Carey et al., 1996) with the temporal yoking usually seen be-tween the hands during bimanual reaches of different amplitudes(Kelso et al., 1983). Participants reached with both hands at thesametimetowardtargetpairsthatweredifferentamplitudesawayfromthestartlocationintherightorlefthemispace.Therefore,onehand performed an easy, ipsilateral reach while its counterpartperformed a more difficult, contralateral reach to contact the tar-gets–imposingakinematic(andperhapsmorenaturalistic)deficitupon one hand, analogous to the differentially weighted styli de-scribed by Marteniuk et al. (1984). This paradigm contrasts themajority of bimanual reaching tasks, where both hands tend toreachonlyintotheirrespectiveipsilateralsidesofspace(e.g.,Kelsoet al., 1979; Marteniuk et al., 1984; however, see Experiment 3from Kelso et al. (1983) for an exception).To determine which hand was yoked to which, the changes inperformancebetweenbimanualandequivalentunimanualreachesto either hemispace were examined. It was hoped that this taskwould give an indication of which hand’s performance was moremalleable to the temporal coupling exhibited during bimanualreaching. One possibility would be that both hands tend toward acentral‘compromise’duringbimanualreaching,indicatingthatthecouplingwassymmetrical,asimpliedbytheworkof Kelsoandcol-leagues(1979),Kelsoandcolleagues(1983).Alternatively,therighthand’sperformancemaybemoreaffectedbythelefthand,presum-ablybyreducingitslevelofperformancetoaccommodatethe,com-paratively less able, non-dominant limb. However, based on therhythmic bimanual studies of  Peters (1981) and De Poel et al.(2006),DePoeletal.(2008),wepredictedthattherighthandwouldbe ‘in-charge’ of the left. This behavioural profile would lead to in-creasedchangesinthelefthand’sperformancebetweenunimanualandbimanualreachingtoequivalenttargets.Thesealterationsmayeven take the form of an ipsilateral reaching right hand using theyoking to improve the performance of a contralateral reaching lefthand,relativetoanequivalentunimanualreach.Unfortunately, such a direct comparison may not be feasible. Alargebodyofevidencehasdemonstratedthatbimanualreachingin-cursasubstantialcostintermsofreactiontimeandmovementdura-tion compared to unimanual equivalents (see Ohtsuki (1994) forreview). The cause of this bimanual cost is not only largely unde-fined, but also refractory to the purpose of the current work. Wethereforedecidedtopartialoutthisbimanualcostfromtheinferen-tialanalyseswherebimanualandunimanualreachesarecompared,in order to get a clearer picture of any interactions between hand,spaceandcoupling.Tonormalisetheunimanualreachingscorestothose of the bimanual conditions in all measures, the overall uni-manualmeanforaparticularmeasurewassubtractedfromtheover-all bimanual mean. The value yielded from this calculation – the‘bimanual cost’ (reported for each measure in the results section)was then added to, or subtracted from, the unimanual scores asappropriate.Thisadjustmenttotheunimanualvaluesforanypartic-ular measure served to make comparisons between bimanual andunimanualreachingmorereadilyinterpretable. 2. Methods  2.1. Participants Eighteen postgraduate students and staff members of the Uni-versity of Aberdeen School of Psychology (eight male) took partin this study (mean age=27.5years,  SD  =6.4). All participantswere right-handed (mean score=26.4/30,  SD  =4.9, as measuredby a modified version of the Waterloo Handedness Questionnaire;Steenhuis & Bryden, 1989), with 13 of the sample showing righteye sighting dominance. Participants were naïve to the hypothesisand gave written informed consent prior to testing. All procedureswere approved by the Ethics Committee of the School of Psychol-ogy at the University of Aberdeen.  2.2. Apparatus and data reduction Ahorizontallightemittingdiode(LED)gridandcustomPCsoft-ware were used to deliver the central fixation and target stimulus.Infraredreflectivemarkerswereattachedtotheparticipant’sindexfingers. The position of these markers were monitored with athree-camera ProReflex motion analysis system (Qualisys Inc.),recording at 240Hz. The camera positions were calibrated prior 2  G. Buckingham et al./Brain and Cognition xxx (2010) xxx–xxx Please cite this article in press as: Buckingham, G., et al. Bimanual reaching across the hemispace: Which hand is yoked to which?.  Brain and Cognition (2010), doi:10.1016/j.bandc.2010.09.002  to each testing session. The 3-dimensional position data fromeach2s trial were low-passfilteredwitha 10Hzdual-pass Butterworthfilter. These data were then differentiated to yield measures of velocityin eachplane andcombinedto givea measureof resultantvelocity (using 3 Wave software written in Labview, NationalInstruments Inc.). Fromthe resulting velocity profile,  reaction time ,  peak velocity  and movement end time (i.e., the total time taken tocompletethereach)foreachhandwasdetermined(movementon-set/offset threshold=50mm/s). From these measures, the finaldependant variable,  movement duration  (movement end time –reaction time), was calculated.  2.3. Procedure Participants sat on a height adjustable chair in front the LEDboard, with both index fingers on pre-defined ‘home’ pointsmarked by Velcro pads 9cm apart. Following a verbal pre-cue(‘‘Ready . . . ”), a fixation light appeared for a random duration be-tween600and1000ms. At the sametime as fixationoffset, thevi-sual targets appeared indicating that participants must reachtoward the targets as quickly and as accurately as possible. Partic-ipants performed bimanual and unimanual reaches in six blockedconditions: ‘unimanual right hand right space’; ‘unimanual righthand left space’; ‘unimanual left hand right space’; ‘unimanual lefthand left space’; ‘bimanual right space’; and ‘bimanual left space’.While the testing order of these blocks were counterbalanced be-tween subjects, side of space was blocked so that targets appearedinonehemispaceatatime(e.g.,allrightspaceblocksbeforeallleftspace blocks for half of the participants). Furthermore, the poten-tial path of each hand being marked with semi-translucent mask-ing tape to ensurethat participants reachedto the targets withtheappropriatehandinthebimanualconditions.Thismeasurewasta-ken to avoid the left hand reaching toward the right target, andvice versa – an event that occurred occasionally during pilot test-ing. It must be noted, however, that the masking tape providedonly a guideline so that hand would contact it’s own target – therewere no constraints on the reaching movement itself, which wereperformed with a normal 3-D trajectory (i.e., participants did notslide their hands along the table surface).The targets appeared in one of three locations for each hand,requiring a movements of either 13cm (short), 27cm (medium)or 41cm (long) along a path (marked with tape) at 45   angle fromthe target board’s near edge, towardtheleft or right sides of space.The amplitude of the reach was manipulated in order to supple-ment the hemispace variable and provide an even stronger indica-tion of which hand is yoked to which – a contralateral reachinghand yoked to an shorter-amplitude ipsilateral hand movementis likely to further increase the ‘enhancement potential’ of thebimanual reach over its unimanual equivalent. In every bimanualreach combination, one hand reached to the medium distance tar-get in conjunction with either a shorter, identical, or a longer dis-tance reach of the other hand. Fig. 1 demonstrates these reachconditions schematically.The twobimanual blocks comprised20medium/short, 20 med-ium/medium and 20 medium/long trials for each side of space,yielding 120 bimanual trials in total. The four unimanual blockscomprised of 10 medium, five short and five long reaches for eachhand in each space, yielding a further 80 trials. These reach condi-tions were chosen to enhance the unpredictability of target loca-tion, increasing the spatial demands of the task to accommodatethe fact that targets were confined to one side of space at a time.  2.4. Analysis All trials where the participant’s reaction time failed to fallwithin the range of 100–500ms were removed from the dataset(<10% of any participant’s trials in any condition). To normalisethe unimanual reaching scores to that of the bimanual conditionsin any one measure, the overall unimanual mean for a particularmeasure was subtracted from the overall bimanual mean. The va-lueyieldedfromthiscalculation–the‘bimanualcost’ (reportedforeach measure in the results section) was then added to, or sub-tracted from, the unimanual scores as appropriate. These adjustedunimanual values were used in the comparisons with the valuesfrom the bimanual conditions.Only the velocity profiles of hands reaching to the medium tar-getswereanalysedinboththebimanualandunimanualconditions(10trials per hand per condition), inorder to examine the factor of ‘reach context’ in the statistical analysis. This ‘context’ factor con-tained four levels:  alone  (i.e., unimanual),  with short   (i.e., 14cmshorter),  with medium  (i.e., a reach of identical amplitude), or  withlong   (i.e., 14cm longer). The final factorial design yielded a re-peatedmeasures4(reachcontext:alone,withshort,withmedium,with long)  2 (hand: left, right)  2 (space: left, right) ANOVA.This ANOVA was performed on all the measures of interest (reac-tion time, peak velocity and movement duration) separately, usingGreenhouse–Geisser corrections to account for inhomogeneity of the covariance where necessary. Post-hoc Bonferroni-corrected  t  -testswereusedtoexaminesignificantinteractions,comparingper-formance for the  alone  reaches into contralateral space with thevarious bimanual combinations ( with short, with same, with long  ). 3. Results and discussion  3.1. Reaction time The unimanual (i.e.,  alone ) reaction time was increased by22ms to account for the ‘bimanual cost’ in this measure. The maineffects of both hand ( F  (1,17)=15.23,  p  <.005) and context( F  (2.0, 33.6)=11.17,  p  <.001)weremoderatedbyahand  context Fig. 1.  The schematics of the reach conditions for the left hand into right space. G. Buckingham et al./Brain and Cognition xxx (2010) xxx–xxx  3 Please cite this article in press as: Buckingham, G., et al. Bimanual reaching across the hemispace: Which hand is yoked to which?.  Brain and Cognition (2010), doi:10.1016/j.bandc.2010.09.002  interaction ( F  (3,51)=5.45,  p  <.005). There was also a significanthand  spaceinteractionobserved( F  (1,17)=6.59,  p  <.05),inaddi-tion to a hand  space  context interaction ( F  (1.6,27.4)=3.97,  p  <.05). Post-hocanalysis of the crucial hand  context interactionshowedthat,althoughjustfailingtomakethecriteriaforstatisticalsignificance due to Bonferroni corrections, the right hand wasclearly improved by the presence of its counterpart performing ashorter reach when reaching into contralateral space, showing a15msimprovementfromthe alone  conditiontothe with short   con-dition( t  (17)=2.85,  p  <.01–seeFig.2[upperpanel]).Interestingly,this marginal improvement was not apparent for reaches yoked toa hand reaching the same distance (  p  =.26) or a longer distance(  p  =.99). Presumably, the lack of improvements in reaction timefor these latter reaches was due to the counter-effect of increasedreaction time when reaching to more distant targets (Munro,Plumb, Wilson, Williams, & Mon-Williams, 2007). The left handshowed no improvement at any distance – yoking the hand toshorter (  p  =.37), same distance (  p  =.44), and longer (  p  =.09)reaches all elicited roughly equivalent reaction time to unimanualreaches into contralateral space. Thus, in terms of reaction time, itappears that the right hand was improved by the left hand whenmoving into the left (contralateral) side of space.  3.2. Peak velocity The unimanual (i.e.,  alone ) peak velocity was reduced by180mm/s to account for the ‘bimanual cost’ in this measure. Themain effects of both hand ( F  (1,17)=8.43,  p  <.05) and context( F  (1.8,30.8)=8.49,  p  <.001) were accompanied by a significanthand  context interaction ( F  (3,51)=3.19,  p  <.05). Furthermore,there was a significant hand  space interaction ( F  (1,17)=40.07,  p  <.001) and a significant hand  space  context interaction( F  (1.4,23.3)=4.09,  p  <.05). Post-hoc analysis did not indicate thateitherhandwassignificantlyimprovedbythepresenceofitscoun-terpartwhenreachingintocontralateralspace(all  p value’s>0.03).Therewas,however,astrongtrendforthelefthand’speakvelocityto be increased when yoked to the shorter-reaching right hand (a121mm/s improvement from the  alone  condition to the  with short  condition ( t  (17)=2.32,  p  =.03 – see Fig. 2 [middle panel]).  3.3. Movement duration The unimanual (i.e.,  alone ) movement duration was increasedby 17ms to account for the ‘bimanual cost’ in this measure. Inaddition to the main effects of context ( F  (3,51)=7.70,  p  <.001)and hand ( F  (1,17)=7.56,  p  <.05), there was a significant hand  -space interaction ( F  (1,17)=7.26,  p  <.05). Post-hoc analysisshowed that the left hand was significantly affected by the pres-enceof anotherhandwhenmovingintocontralateral space, show-ing a 23ms improvement when moving with a shorter reachinghand ( t  (17)=2.99,  p  <.0083 – see Fig. 2 [lower panel]). Thisimprovement was also clear when the hand was combined withanother hand reaching the same distance (20ms improvement t  (17)=3.79,  p  <.005). When combined with a longer reachinghand, however, the 14ms improvement did not achieve signifi-cance following Bonferroni corrections (  p  =.04). As with the reac-tion time data, we suspect that this improvement is most robustwiththeshorterreachcombinationduetothecounter-effect ofin-creased movement durations over increased movement ampli-tudes (Fitts, 1954). The right hand showed no significantimprovement when yoked to shorter (  p  =.25), identical (  p  =.27),or longer reaches (  p  =.69). Thus, the left hand’s performance incontralateral space was improved by the presence of the righthand.These experiments were designed to investigate which handwas coupled to which, by comparing temporally-yoked bimanualreaches with equivalent unimanual reaches across the hemispace.It was predicted that the ‘subordinate’ hand would be able to uti-lise this temporal yoking between the hands to improve its perfor-mance for the difficult, contralateral, reaches. As some evidenceexistsforarighthandleadroleindefiningbimanualreachingtasks(Marteniuk et al., 1984), it was predicted that the left hand wouldbe yoked to the right. To test this hypothesis we utilised wellestablished differences in movement kinematics in left and rightspace. Reaction time is generally taken as a measure of perceptualprocessing and motor preparation. However, evidence for hemi-spatial asymmetries is mixed, with some studies finding a robustadvantage for ipsilateral reaching (e.g., Ishihara, Imanaka, & Mori,2002), while others fail to demonstrate any such difference (e.g.,Carey et al., 1996). The measures of peak velocity and movementduration are taken to reflect the ‘quality’ of movement output in Fig. 2.  Meanmedianreactiontime,peakvelocity, andmovementtimeasafunctionof hand and condition. Only the  alone  (adjusted to remove the effect of thebimanual reaching cost) and  with short   reach contexts into contralateral space areplotted, reflecting the most robust findings for all the measures. Error barsrepresent standard error of the mean.4  G. Buckingham et al./Brain and Cognition xxx (2010) xxx–xxx Please cite this article in press as: Buckingham, G., et al. Bimanual reaching across the hemispace: Which hand is yoked to which?.  Brain and Cognition (2010), doi:10.1016/j.bandc.2010.09.002  aiming tasks where speed is a requirement. It is in these outputmeasures where the largest ipsilateral advantages are shown, rela-tive to contralateral performance in unimanual reaching (e.g., Bar-the´le´my&Boulinguez,2002;Carey&Otto-deHaart,2001).Noneof the ipsilateral advantages in these measures are known to beasymmetrical with regard to hand – the right hand ipsilateraladvantage in the various measures is equal to the left hand ipsilat-eral advantage.To further increase the potential effect of this yoking, reacheswere combined into various amplitude pairs: a medium distancereach in conjunction with either a longer, shorter or identicalamplitude counterpart. This manipulation created the situationwhereahandreachingintocontralateralspacewascombinedwithnot only an ipsilateral reach, but a much shorter ipsilateral reach –the most favourable conditions for the contralateral reachinghand’s performance to improve.Once the ‘bimanual cost’ (a factor unrelated to our examinationof hand and hemispace) was accounted for, substantial enhance-ments in performance were detected for contralateral reachinghands when yoked to an ipsilateral shorter-reaching counterpart.Interestingly however, the asymmetry was not consistent acrossall the measures of coordination. In line with the hypothesis, theleft hand showedthegreater performanceenhancement incontra-lateral space while yoked to a shorter-reaching counterpart interms of peak velocity and movement duration. Surprisingly, how-ever,therighthandshowedthegreaterperformanceenhancementin terms of reaction time. 1 These findings support the right hand’shypothesised ‘in-charge’ role when performing bimanual reaches.However, the contrasting direction of the asymmetry in the reactiontime measure implies that the right hand only maintains this ‘in-charge’ status during movement output itself, while the left hand‘rules’ during movement preparation. Therighthand’scontralateralreactiontimeimprovementunderbimanual conditions was, initially, an unexpected finding. How-ever, while the direction of this asymmetry would appear to beat odds with the hypothesis of the left hand being yoked to theright hand, the domain-specific asymmetries are not incompatiblewith the unimanual reaching literature. Unimanual reaching stud-ies have demonstrated a small but consistent left hand advantagein term of reaction times, usually attributed to the privileged in-tra-hemisphere access to the right hemisphere which generallyshows an advantage for visuo-spatial tasks (Barthe´le´my & Boulin-guez, 2002). There are even suggestions that this reaction timeadvantage is especially prominent when the spatial demands of the task are high (e.g., with many possible target locations – Car-son, 1989; or spatially complicated targets – Carson, Goodman, &Elliott,1992).Thehigherlevelsof‘improvement’fromtheuniman-ual to the bimanual conditions for the right hand, indicating theleft hand’s pre-motor dominance may be a further behaviouralmarker of the right-hemisphere’s spatial processing dominance.We suggest that the left hand’s relative improvement of the righthand’smovementpreparationtimeisrelatedtothislefthandreac-tiontime advantage, and that botharelikely tobe relatedto a spa-tial processing advantage for the contralateral (right) cerebralhemisphere.The opposite pattern of improvements was seem in the kine-matic variables describing the motor output (peak velocity andmovement duration). The right hand’s improvement of the lefthand during motor output can be interpreted in terms of asymme-triesintheallocationof attention, that appeartoplayafundamen-tal role in how we coordinate our hands (De Poel et al., 2006; DePoeletal.,2008;Peters,1981;Riek,Tresilian,Mon-Williams,Copp-ard, & Carson, 2003). Given the attentional bias framework thatmuch of the recent rhythmic (De Poel et al., 2008), and discrete(Buckingham & Carey, 2009; Buckingham, Main, & Carey, in press)bimanual tasks have been described in terms of, it is possible thatthese asymmetries are another behavioural marker of a bias inattentiontowardtherighthandduringtheoutputstagesofbiman-ual reaching. Of course, as we did not manipulate attention in thecurrent work(cf. Buckingham&Carey, 2009), we cannot make anystrong claims regarding attentional asymmetries from this task.However, an attentional bias toward the right hand does providea plausible explanation for the asymmetry in yoking. Combiningan asymmetrical temporal yoking between the hands with anattentional bias would mean that the demands on the motor andattentional systems during bimanual coordination were reduced.Thus, not only would the potential degrees of freedom for the mo-torcommandsbehalvedbythetemporalyoking,inaclassicmotorsynergy explanation of the coupling (e.g., Kelso et al., 1984), thecomputational demands on the visuomotor system would be sim-ilarly reduced. This hypothesis may also only be applicable to bal-listic movements, where temporal constraints make saccadicviewing of all the targets an ineffective strategy for the coordina-tion of two simultaneous reaches (Hesse, Nakagawa, & Deubel,2010). This (rarely discussed) difference between ballistic andfeedback-stylemovementsmayaccountfortheinconsistenciesbe-tweenourfindings,andotherswherethemovementsuncoupleto-ward the end of a bimanual reach (for example, the high accuracydemands, longer reach latencies, and temporal uncoupling seen inthe eye movement study of  Riek et al. (2003)).Of course, attention may not be a deciding factor in the asym-metrical yoking at all. Marteniuk et al. (1984) have suggested thatneural overflow between each hand’s respective motor cortex isresponsible for the temporal coupling in bimanual coordination,and it is asymmetries in this asymmetrical ‘leakage’ that accountfor non-symmetrical bimanual coordination. This account has re-ceived some support from recent neuroimaging work, showingasymmetries in motoric overflow in a rhythmic coordination taskwithrecentneuroimagingwork(Aramaki,Honda,Okada,&Sadato,2006) suggestingthat thelefthemisphereisabletoexert influenceover the right hemisphere. With the current dataset however, it isdifficult to speculate or distinguish between the attentional andneural factors that may influence performance in this task, beyondnoting that an attentional bias makes for a simple and intuitivecounterpart to a between-hand yoking mechanism.To conclude, the experiments reported in this paper have pro-vided some evidence that the left hand may be ‘yoked’ to the righthandatthelevelofmotoroutput.Thisyokingmayberelatedto, oreven a consequence of a rightward bias in attention, in order to al-low the unitary attentional system to be integrated into control of the two hands.  Acknowledgments This project was supported by a 6th Century studentshipawarded to G. Buckingham by the College of Life Sciences & Med-icineat theUniversityof Aberdeen, andbyaninternationalincom-ing short visit grant, awarded to D.P. Carey and G. Binsted by theRoyal Society. References Aramaki, Y., Honda, M., Okada, T., & Sadato, N. (2006). Neural correlates of thespontaneousphasetransitionduringbimanualcoordination. Cerebral Cortex, 16  ,1338–1348.Barthe´le´my, S., & Boulinguez, P. (2002). Manual asymmetries in the directionalcoding of reaching: Further evidence for hemispatial effects and right 1 It should be noted that analyses of variance on the data performed with the non-adjusted unimanual values produced the same main effects and interactions as thereported ANOVA’s (i.e., with adjusted unimanual values). This adjustment scoremerely made the interactions easier to interpret within the context of our hypothesis,giving the comparisons appropriate meaning. G. Buckingham et al./Brain and Cognition xxx (2010) xxx–xxx  5 Please cite this article in press as: Buckingham, G., et al. Bimanual reaching across the hemispace: Which hand is yoked to which?.  Brain and Cognition (2010), doi:10.1016/j.bandc.2010.09.002
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