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Hemispheric asymmetry and interhemispheric transferin reaching programming
\ PERGAMON
Neuropsychologia 26 "0888# 784Ð892
Hemispheric asymmetry and interhemispheric transfer in reaching programming Jean!Luc Velay \ Simone Benoit!Dubrocard Lab[ Neurobiologie Integrative et Adaptative\ UMR CNRS 5451\ Universite de Provence\ 02286 Marseille\ France Received 07 December 0886^ accepted 8 November 0887
Abstract The purpose of this study was to explore the intrahemispheric processes and the interhemispheric transfer that occur during the programming of a pointing movement[ Twenty _ve subjects participated in this experiment] 01 were right!handed "Rhr#\ 01 left! handed "Lhr#\ and 0 was left!handed with a posterior callosal lesion[ The task consisted in producing an open loop pointing response toward a visual target appearing brie~y on the right or the left of a central _xation point[ Reaction times "RTs# were shorter for the Rhrs when reaching with the left hand than with the right hand[ No such hand!related di}erence was observed in the Lhrs[ The left hand advantage indicates that one process was faster in the right hemisphere of Rhrs[ This faster process appears not to be visual but motor or visuomotor[ For either hand\ responses were faster when the target appeared in the visual _eld homolateral to the pointing hand "uncrossed condition# than when it appeared contralaterally to the hand "crossed condition#[ The crossed vs uncrossed di}erence did not vary between Rhrs and Lhrs or between the hands[ The transfer time between the hemispheres was symmetrical whatever its direction[ The partially callosotomized left!handed subject was two!fold slower than the control Lhrs[ His uncrossed responses were faster than the crossed ones\ but his interhemispheric transfer time was very asymmetrical] it was normal from right to left hemisphere but was highly increased in the opposite direction[ An attempt at modelling the RT data is proposed and the possibility of di}erent callosal locations for the interhemispheric transfer is discussed[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] Reaching^ Right!handers^ Left!handers^ Corpus callosum^ Lesion^ Reaction time
0[ Introduction In primates\ hand reaching movements may be con! sidered the archetype of the visually triggered spatial activities[ To be e.ciently performed\ they require that a series of visuospatial and sensorimotor processes be achieved from the localization of the visual target to the motor commands that are sent to the arm muscles ð16\ 23Ł[ In most studies devoted to the neural mechanisms underlying reaching movement programming\ the pre! ferred hand was used because it was thought to be the more skilled and thus the less variable[ For some time\ however\ there has been interest in the possible hemi! spheric lateralization of this task[ The performances of the two hands were often discordant[ Certain studies evidenced a superiority of the right hand\ either in accu! racy or in speed or both\ indicating left hemisphere domi! nance in the preparation and control of pointing movements in right!handers "Rhrs# ð10\ 11\ 13\ 35\ 43Ł[ Corresponding author[ Fax] ¦22!93!80!17!75!58^ e!mail] velayÝ newsup[univ!mrs[fr
The right hand system may be superior in processing the sensory feedback ð13Ł\ or the left hemisphere may be more appropriate to specify the force parameters of the movement ð36Ł[ Nevertheless\ according to the classical hemispheric dichotomy\ the right hemisphere seems more involved in spatial activities "see ð20Ł for a review#[ Both motor and spatial aspects are important in programming a reaching movement\ and some component left hand: right hemisphere advantage should be observed in this task[ In fact\ other studies showed that the left hand could be faster and:or give rise to a greater accuracy ð09\ 04\ 06\ 10\ 17\ 18Ł[ This left!hand advantage would re~ect the right hemisphere superiority for the spatial processes prior to the movement[ Accordingly\ patients having right or left hemisphere lesions are of interest[ Fisk and Goodale ð12Ł observed a lengthening of the reaction time "RT# only with right!damaged patients\ and they pro! posed that the right hemisphere is more involved in deter! mining the spatial position of the target whereas the left hemisphere is more involved in controlling movement execution[ Conversely\ Haaland and Harrington ð29Ł
9917!2821:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved PII] S9917!2821"87#99038!2
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reported that RTs were longer in left!damaged patients but not in right!damaged ones relative to control[ The di}erent experimental procedures might explain the dis! crepant results[ In all\ the results argue for large par! ticipation of the right hemisphere during reaching movement programming[ Yet it remains unknown whether spatial localization of the target or planning of the movement itself\ or both\ are favored[ In addition to the hand used\ another factor may deter! mine the contribution of each hemisphere\ namely the visual _eld in which the stimulus appears[ Because of the crossed visual pathways\ each visual hemi_eld projects to the opposite hemisphere[ As a consequence\ a better performance when the target is for example in the left visual hemi_eld can be accounted for by special visual competences of the right hemisphere[ If the target appears on the same side as the hand "uncrossed situation#\ the visual input and the motor output occur in the same hemisphere[ Under this intrahemispheric condition no communication with the other hemisphere is theoretically required[ On the contrary\ the crossed situation\ in which the visual input is given to one hemisphere and the motor response is mediated by the other\ requires an inter! hemispheric transfer of information\ and therefore the movement programming would take more time[ This well known experimental paradigm\ with simple and distal motor responses\ showed that the uncrossed responses were faster than the crossed ones "see ð4\ 00\ 28Ł for a review#[ In addition\ the interhemispheric transfer time is greatly increased in patients having a total or partial section of the corpus callosum "CC#\ which is assumed to mediate the interhemispheric integration of visuomotor responses ð49Ł[ Thus\ in such motor responses\ some information is transferred between the hemispheres via the CC[ It is still debated whether this information is visual\ visuomotor\ or motor\ and whether the transfer is the same in both directions ð15\ 28Ł[ The question of interhemispheric transfer is justi_ed only if the motor response is actually mediated by the hemisphere controlateral to the reaching hand ð1Ł[ Ana! tomical data and split!brain studies have shown that hand and _nger movements are controlled mainly by the con! tralateral hemisphere\ but arm movements may be con! trolled by either hemisphere ð01\ 14Ł[ The experiments devoted to interhemispheric transfer have therefore used only very distal responses[ However\ the spatial com! ponent of these so!called key!pressing responses was poor with respect to that required in programming a spatially oriented movement[ In fact\ as underlined by Jeannerod ð23Ł\ the investigators were more interested in movements in response to targets appearing in the visual _eld than in movements directed at these targets[ Contrary to the key!pressing responses\ the arm reaching movement involves principally the shoulder and the elbow[ Since this movement might be mediated by either the con! tralateral or the ipsilateral hemisphere\ an inter!
hemispheric transfer would not be necessary in the crossed pointing[ The majority of the few studies on interhemispheric transfer in directed movements have shown that the crossed responses in Rhrs were initiated more slowly than the uncrossed ones ð04\ 11\ 12\ 40Ł[ No such work has been done with left!handers "Lhrs#\ nevertheless they are thought to have a less pronounced hemispheric asymmetry ð03Ł and a larger corpus callosum "CC# than Rhrs ð42Ł[ This last point is still debated however\ and anatomical data suggest that the regional size of the CC is unrelated to its sensory!motor functions ð0Ł[ Furthermore\ the consequences of callosal lesions on the programming of directed movement are not estab! lished[ The purpose of this study was to investigate the respec! tive contribution of each hemisphere and the inter! hemispheric communication during the programming of a visuomanual pointing movement[ We measured the RT with the two hands\ in uncrossed and crossed situations\ and both in Rhrs\ Lhrs\ and in a subject with a partial lesion in the posterior part of the CC[ We assumed that the right hemisphere in Rhrs may have a special role in programming this type of motor response\ and that this would probably induce a left!hand advantage[ We expected this hand asymmetry to be weaker or absent in the Lhrs[ Finally\ if the directed movement programming requires interhemispheric communication through the callosal pathways\ we expected a larger {crossed! uncrossed di}erence| "CUD# in the patient[ Part of the results have been presented at the Second Meeting of European Neurosciences ð6Ł[
1[ Methods 1[0[ Subjects The study was approved by the Ethics committee of the University[ Subjects were 14 men "11Ð24 years of age#[ They were all volunteers and had all given their verbal consent[ They all had normal or corrected to nor! mal visual acuity in both eyes[ Among the 13 normal subjects\ some were members of the laboratory sta}\ and the others were students at the university[ The latter were paid for participating[ Twelve of them were right!handed and 01 left!handed according to the Edinburgh Hand! edness Inventory[ The patient "17 years old# was left! handed "−79# without a familial history of left hand! edness[ He su}ered from a disconnection syndrome after a trauma that occurred in 0881 "3 years before the experi! ment#[ At that time\ he presented hemorrhagic bilateral fronto!parietal contusions[ At the date of the experiment\ the neurological evaluation was normal\ but the sagittal section MRI showed a region of hypodensity including the middle and posterior part of the trunk of the corpus callosum\ sparing the splenium ð05Ł[ The most obvious
J[!L[ Velay\ S[ Benoit!Dubrocard : Neuropsycholo`ia 26 "0888# 784Ð892
symptoms were a left tactile and visual hemi_eld anomia and a left hand agraphia[ Further details regarding this patient can be found in Ceccaldi et al[ ð05Ł[ 1[1[ Task The task consisted in producing an open loop pointing response[ The subjects were required to point with their right or left hand as rapidly and accurately as possible toward a visual target appearing brie~y on the right or the left of a central _xation point "FP#[ Since we were interested in the processes during movement program! ming\ the subjects were required to be very fast to force them to program all the movement parameters before its onset\ that is during the RT\ and to minimize the corrections during its execution[ The RT in some pointing experiments were long "see ð11\ 29Ł#\ and this might have prevented the emergence of a right hemisphere superi! ority[ For the same reasons\ we used a visual open!loop movement\ because if the subjects could see their hands during the movement execution "visual closed!loop#\ they could correct some defaults of programming\ particularly in presence of a structured visual context ð41Ł[ Finally\ corrections are still possible during the ongoing move! ment\ even under open!loop conditions if the target remains present because the visually de_ned target pos! ition can be compared with the proprioceptively de_ned arm position ð31\ 33Ł[ To avoid these corrections\ the target was lit!o} before movement onset[ Before the experiment\ 53 trials "21 with each hand# were run to make the subjects familiar with the pointing task requirements[ Speed and accuracy were equally emphasized[ During this training session\ the subjects were informed about their speed and accuracy[ 1[2[ Procedure Each subject was seated with his head in a chin and forehead restraint\ facing a vertical panel situated 36 cm from his eyes\ and whose bottom was _tted with a small starting platform[ He gazed at the FP\ which was a green light!emitting diode "4 mm in diameter# at the center of the panel\ exactly straight!ahead at eye level[ By con! tacting the starting platform with his hand\ the subject triggered the visual target onset[ The visual target was one among four red LEDs symmetrically arranged 5 and 01> to the right and left of the FP[ It was switched on very brie~y "49 ms#\ and the subject was instructed to keep _xating the FP\ which remained lit until the end of the trial[ Fifty!ms exposure is well below the time necess! ary for stimulus foveation and\ after some training\ all subjects could refrain from making saccadic eye move! ments during the target presentation[ The subjects were then required to point toward where the red target had just appeared[ The small plate used as the starting point of the movement was 34 cm beneath the grid!center[ It
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was in the midsagittal plane and at the same location for right! and left!hand responses[ The pointing thus consisted of a vertical movement\ from bottom to top\ involving mainly shoulder and elbow[ Its amplitude was 34[2 and 35[0 cm for the 5 and 01> targets respectively[ The panel consisted of a grid!patterned printed circuit "21×21 cm#\ and as soon as the subject|s _nger contacted it\ a computer recorded the pointing coordinates and emitted a tone signalling that the subject could begin the next trial[ He then placed his hand again on the starting platform\ triggering the target for the following trial\ and so on up to the end of the series[ The pseudo!random order in which the targets were illuminated could not be predicted[ The whole series of reaches was run in total darkness\ the subjects were unable to see their hands\ and they were given no information about the accuracy of their movements[ Experiments were run in two blocks\ one with each hand[ Half of the Rhrs and the Lhrs began with the right hands and half with their left hands[ One set of 21 measurements "eight for each target# was taken with each hand[ Spatial accuracy was determined for each subject and each target by calculating the centroid of the eight poin! ting shots[ Then\ two variables were computed] "0# the mean radial error "in mm#\ i[e[ the distance between the centroid and the target\ and "1# the pointing surface "in mm1#\ i[e[ the dispersion of the eight pointings with respect to the centroid[ We measured the RTs "in ms#\ i[e[ the time separating the target appearance and the movement onset\ and the movement time "MTs\ in ms#\ between movement onset and _nger contact with the grid[ 2[ Results The various combinations of the experimental factors "1 hands×1 _elds×1 target excentricities# resulted in eight experimental situations[ Spatial and temporal data on the patient and the right! and left!handed groups were analyzed separately in a series of ANOVAs[ The comparisons of the patient|s data were done by Student|s t tests[ 2[0[ Spatial accuracy The mean radial error was very close in Rhrs "23[2 mm\ S[D[ 04#\ in Lhrs "27[4 mm\ S[D[ 19#\ and in the patient "25[0 mm#[ The mean pointing surfaces were not statistically di}erent between the Rhrs "061 mm1\ S[D[ 70#\ the Lhrs "042 mm1\ S[D[ 56#\ and the patient "013 mm1#[ For both radial error and pointing surface\ none of the main factors or interactions reached the signi_cance[ 2[1[ Movement time Table 0 shows the MTs[ In intact subjects\ only the main e}ect of excentricity appeared to be signi_cant
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Table 0 Mean movement times
Table 1 Mean reaction times LVF
RVF
LVF
Lh
Rh
Lh
Rh
Rhrs] mean S[D[
199[9 24
100[0 32
119[7 49
084[3 36
Lhrs] mean S[D[
191[0 44
125[2 53
120[8 60
Patient] mean S[D[
496[9 18
408[4 14
419[9 17
RVF
Lh
Rh
Lh
Rh
Rhrs] mean S[D[
194[1 14
117[9 18
101[1 28
119[4 20
119[5 46
Lhrs] mean S[D[
113[7 29
125[5 18
122[8 23
113[6 22
357[9 06
Patient] mean S[D[
351[6 12
360[0 25
459[4 15
351[8 34
LVF] left visual _eld^ RVF] right visual _eld^ Lh] left hand^ Rh] right hand[
"F"0\11# 05[6\ P ³ 9[990#] the MTs were shorter for the 5> targets "101[7 ms# than for the 01> targets "105[6 ms#[ In addition\ the group by hand interaction was signi_cant "F"0\11# 4[1\ P ³ 9[94#] the Rhrs tended to be faster with their right hand "192[1 vs 109[3 ms\ F"0\00# 1[9\ ns# whereas the Lhrs tended to be faster with their left hand "106[9 vs 117[4 ms\ F"0\00# 2[2\ P ³ 9[09#[ The hand by visual _eld interaction was also signi_cant "F"0\11# 57[8\ P ³ 9[990#] In both groups\ although the distances to the targets were identical\ ipsilateral reaches were completed more quickly than contralateral reaches "194 vs 114 ms#[ The di}erence was similar in Rhrs "07 and in Lhrs "12 ms\ F"0\00# 27[6\ P ³ 9[990# ms\ F"0\00# 21[7\ P ³ 9[990#[ The patient was much slower than the Lhrs at reaching "F"0\00# 07[4\ P ³ 9[994#[ He was faster with his right than with his left hand "383 vs 403 ms\ T"51# 2[56\ P ³ 9[990# but only in the non!crossed situation[ Similarly to the intact subjects\ he was faster in ipsilateral than in contralateral reaches "377 vs 419 ms respectively\ T"51# 3[98\ P ³ 9[990#[ However\ the di}erence was greater for his right hand "41 ms\ t"51# 4[36\ P ³ 9[990# than for his left dominant hand "02 ms\ T"51# 0[10\ n[s[#[ 2[2[ Reaction time Table 1 presents overall RT results[ In intact subjects\ the ANOVA revealed no main e}ect of group\ visual _eld\ and target excentricity[ A main e}ect of response hand appeared "F"0\11# 5[3\ P ³ 9[91#^ latencies for reaches with the left hand "108[9 ms# were shorter than with the right "116[4 ms#[ However\ the hand by group interaction was signi_cant "F"0\11# 3[3\ P ³ 9[94#\ and this left hand advantage was present only in Rhrs "198 vs 113 ms\ F"0\00# 09[8\ P ³ 9[90# and not in the Lhrs\ who were equally fast with both hands "118[2 vs 129[6\ F"0\00# ³ 0#[ In Rhrs this hand di}erence did not depend on the side where the target appeared] it was
LVF] left visual _eld^ RVF] right visual _eld^ Lh] left hand^ Rh] right hand[
signi_cant in the crossed "04[7 ms\ F"0\00# 7[52\ P ³ 9[91# and in the uncrossed situation "04[2 ms\ F"0\00# 7[66\ P ³ 9[91#[ The hand by visual _eld interaction was signi_cant "F"0\11# 39[2\ P ³ 9[990#] both in Rhrs and Lhrs\ for either hand\ responses were faster when the target appeared in the visual _eld homolateral "107[7 ms# than contralateral "116[6 ms# to the pointing hand[ The mean CUD was 7[8 ms[ Because it seemed greater in Lhrs "CUD 09[4 ms\ F"0\00# 099[9\ P ³ 9[990# than in Rhrs "CUD 6[1 ms\ F"0\00# 6[8\ P ³ 9[91#\ an ANOVA was done directly on the CUDs[ It showed that the CUDs did not di}er either between groups or between hands[ The patient\ who was left!handed\ responded much slower than the Lhrs "F"0\00# 60[5\ P ³ 9[9994#[ He was globally faster with his right than with his left hand "T"51# 2[56[ P ³ 9[990#[ But\ as shown in Table 1 and Fig[ 0\ this di}erence was due only to the crossed situation in which he was much slower\ whereas like the intact Lhrs he was equally fast with both hands in the uncrossed situation[ In other words\ his mean CUD was 42 ms "T"51# 3[46\ P ³ 9[990# but it was 7 ms with his right hand "T"29# 9[47\ ns#\ close to that of the control group\ and it reached 87 ms with his left hand "T"29# 09[14\ P ³ 9[990#[ 3[ Discussion Since we were concerned with movement program! ming\ RT was the dependent variable of primary interest[ Nevertheless we also considered the spatial performance and the MT in order to check for any trade!o} between the RT and these variables[ For the two groups of intact subjects and the patient\ the pointing accuracy did not vary in function of the experimental situations\ and par! ticularly it did not di}er with the hand used\ contrary to studies showing a greater precision for the left ð17Ł or for the right hand ð10Ł[ In both groups\ MT was shorter with
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Fig[ 0[ Rh] right hand^ Lh] left hand^ RVF] right visual _eld^ LVF] left visual _eld[ Schematic representation of the experimental data[ The half ellipses symbolize the cerebral hemispheres[ Three processes were distinguished from the visual input to the motor output] the visual process "V#\ the visuomotor transformation "VM#\ and the motor process per se "M#[ The grey area between the hemispheres denotes the CC[ The dashed zone in the patient CC represents the lesioned area[ The intrahemispheric and interhemispheric pathways are drawn in solid and dashed lines\ respectively[ The RTs in each situation are on the side of each line[ The values in the CC under or above the dashed lines are the measured CUDs and represent the transfer times[ Note the 05 ms extra time in the LH of Rhrs[ To be coherent with the measured RT\ the transfer in both directions must occur before this longer step[
the dominant hand[ In other words\ the dominant hand was able to execute the required movement with the same accuracy but more quickly than the other hand[ Such an advantage for the Rh in Rhrs has often been shown\ and it re~ects an LH superiority in de_ning the temporal parameters of the motor commands\ or in using the feed! backs\ or both ð10Ł[ The MT to reach the more excentric targets was 3 ms longer\ likely because the distance from the starting platform was slightly greater for these targets[ In the Lhrs\ Rhrs\ and the patient\ the movements cross! ing the midline were slower than those that did not\ although the mean distances from the starting platform to the targets were identical[ A similar result was obtained in intact Rhrs ð04\ 06\ 11Ł and in patients with a callosal agenesis ð22Ł[ This di}erence in MT was greater in acal! losal than in intact subjects ð22Ł\ suggesting that it might also somehow result from interhemispheric di}erences and not only from the diverse biomechanical constraints imposed by the two directions of movements[ This was also true in our patient but only when he used his right hand[ We have no de_nite explanation for this asym! metric e}ect\ which might be related to his partial callosal lesion[ The main part of the discussion will focus on the RTs\ _rst in the intact subjects and then in the patient[ The data are represented graphically in Fig[ 0[ The intra and interhemispheric di}erences were calculated by com!
bination and subtraction of the four hand and visual! _eld situations ð21Ł[ 3[0[ Handedness Rhrs and Lhrs had not previously been compared in reaching movements[ Their RTs were not di}erent\ sug! gesting that handedness did not in~uence the time required to program a visuomanual reaching movement[ This result is congruent with those of key!pressing responses in SRT in which Rhrs and Lhrs did not exhibit di}erent RT ð28\ 32Ł[ 3[1[ Visual hemi_eld asymmetry Visual detection was reported to be faster in the LVF in a key!pressing task ð3Ł\ suggesting that the right hemi! sphere would be better at detecting the onset of a light and therefore might be expected to mediate faster responses[ However\ Marzi and colleagues ð28Ł observed that this _eld e}ect was present only in the crossed situation and was thus not attributable to some hemispheric asymmetry in visual detection but to the interhemispheric integration of visuomotor responses[ Such a _eld e}ect was not observed in the experiments involving visuomanual reaching in response to the appearance of lateral visual stimuli ð09\ 04\ 11\ 17Ł[ Nevertheless\ Guiard and col!
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leagues ð17Ł hypothesized that the left!hand advantage was due to a right hemisphere superiority in solving the problems of spatial localization[ This hypothesis was tested further by increasing the complexity of the spatial processing required to establish the position of the target ð06Ł[ Increasing the spatial complexity did not increase the magnitude of the left!hand advantage\ and it was concluded that it did not re~ect a superiority of the right hemisphere in visuospatial localization but more likely in the movement programming itself[ This conclusion is supported by the _nding that neither hemisphere was dominant for visual localization in either Rhrs or Lhrs[ 3[2[ Hand asymmetry The shorter RT "−05 ms# exhibited by the Rhrs with their left hand in our experiment agrees with previous results ð09\ 04\ 06\ 10\ 18\ 35Ł[ Note that in simpler responses like key!pressing\ Rhrs respond faster with their right than with their left hand ð38Ł or at the same speed with both hands ð7\ 28Ł\ but a left hand advantage has not been evidenced[ The left!hand advantage seems to be typical of reaching movements\ and it has been regarded as a right hemisphere superiority in pro! gramming this type of movement[ This discrepancy between the SRT _nger responses and the CRT reaching movements may result from two causes] _rst\ the muscles concerned are not the same and there might be a right hemisphere advantage for the responses involving proxi! mal muscles ð08Ł[ Secondly\ contrary to key!pressing responses\ pointing movements involve an important spa! tial component\ and this would favor the left hand:right hemisphere ð04\ 06\ 15Ł[ In other words\ some time is lost in the left hemisphere of the Rhrs with respect to their right one[ In the pro! gramming of a visually triggered motor response\ one of the three processes usually considered\ namely visual "V#\ visuomotor "VM#\ and motor "M#\ is longer in the left hemisphere of the Rhrs "Fig[ 0#[ The longer step does not seem to involve visual processings\ since RTs were not di}erent in LVF and RVF[ It is unlikely to result from slower motor processing in the left hemisphere than in the right because such a general motor advantage for the right hemisphere:left hand was not evidenced in simpler motor responses ð28Ł\ but see ð08Ł and in more complex movements known to depend on the left hemisphere ð25Ł[ More probably\ the slower step in the left hemisphere belongs to the visuomotor processes\ linking the position of the target in space to the motor command[ In pointing movements\ these processes are particularly important because a trajectory has to be de_ned to drive the hand from its initial position to where the target appeared\ and the right hemisphere would be more e.cient for achieving these spatial operations[ Studies of patients with hemi! spheric lesions support this idea ð12\ 29Ł[ Nevertheless we could not conclusively identify the locus of the longer
step and hence the 05 ms extra!time have been placed between motor and visuomotor processes in Fig[ 0[ Inter! estingly\ this hand asymmetry was not present in the Lhrs\ who seem to have a less pronounced hemispheric lateralization than the Rhrs in general ð03Ł\ and for spatial abilities in particular ð27Ł[ 3[3[ Interhemispheric transfer In the intact subjects\ the RTs were faster in uncrossed than in crossed situations[ The mean CUD was 7 ms\ a value higher than in the classical SRT responses "about 3 ms^ see ð28Ł#[ In principle\ the pointing task must be compared to CRT tasks since there is choice between several responses[ Two cases are possible in CRT tasks] usually\ no CUD is observed^ in some instances\ CUD is higher than in SRT tasks "29Ð49 ms#[ This discrepancy has not been explained[ The short anatomical CUD might appear only in the rapid re~ex!like visuomotor responses and would not be seen in CRT because it is too short with respect to the long decision processes required in CRT tasks ð2Ł[ From this point of view\ the reaching response shares several features with the SRT task] "0# the motor aspect of the response is very simple\ and the link between stimulus and response is straightforward^ "1# the response does not require complex cognitive pro! cessings and the RT are usually short "199Ð149 ms#^ "2# contrary to most CRT tasks\ a positive CUD\ in the same range as the one reported here "around 09 ms#\ is generally reported ð04\ 10\ 11\ 40Ł[ In any case\ as observed in key!pressing movements\ interhemispheric transfer in directed movements was as fast for Lhrs as Rhrs ð28Ł[ Finally\ that RT did not di}er signi_cantly between the two transfer directions in the two groups suggests that the interhemispheric transfer was not asymmetric in the reaching response\ unlike for key!pressing responses ð8\ 02\ 15\ 28Ł[ Note that for the Rhrs\ when the target was in the RVF\ the crossed pathway was faster than the uncrossed one "−8 ms# "Fig[ 0#[ This apparent paradox is easily accounted for if one admits that the transfer from the LH to the RH occurs before the 05 ms extra! time[ For similar reasons\ the transfer from RH to LH must also occur before the longer step^ otherwise the LVF!Rh condition would have resulted in a 101 ms RT instead of 117 ms[ We placed both transfers at the same level of the CC in Rhrs but they might be separated\ provided the transfer occurs before the left hemisphere long step[ The representation in Fig[ 0 is simple and _ts correctly with the data although\ by simply subtracting the RTs in the four hand and visual _eld situations\ one cannot unambiguously di}erentiate interhemispheric transfer di}erences and intrahemispheric asymmetry in this kind of protocol ð21Ł[ We have applied the same modelling to other data for reaching in Rhrs ð04\ 10Ł[ Overall RT di}ered\ but the modelling _ts correctly with
J[!L[ Velay\ S[ Benoit!Dubrocard : Neuropsycholo`ia 26 "0888# 784Ð892
the data and both the transfer times and the extra!time in LH were close to ours[ In the Lhrs\ the location of the transfers were de_ned in function of the patient|s results[ 3[4[ Patient The patient|s performance accuracy was not a}ected by his callosal lesion but he was much slower than the intact Lhrs[ Note that he was also slower in the situations that a priori do not require an interhemispheric transfer of information "uncrossed situations#[ This whole!_eld impairment was observed in cases of callosal agenesis and callosal lesion ð5\ 22\ 38Ł[ It may result from the suppression of some non!speci_c facilitatory ð26Ł or inhibitory ð15Ł in~uence of the CC on intrahemispheric processings[ The mean CUD of the patient was 42 ms\ a value greater than in the intact subjects[ This further supports the hypothesis that the CUD in reaching task does re~ect the callosal interhemispheric transfer[ In SRT\ patients with a congenital absence of the CC often showed a CUD of about 04 ms ð19\ 24\ 39\ 30\ 34Ł^ in callosotomized patients\ the CUD was on average greater than 49 ms ð1\ 07\ 19\ 28\ 38\ 49Ł[ When the CC is lesioned\ the transfer would likely occur through subcortical path! ways ð1\ 37\ 00Ł[ Surprisingly\ in our patient\ the CUD was very di}erent in the two transfer directions] his trans! fer time was similar to the controls in the R to L direction but it was multiplied by 01 in the L to R direction "Fig[ 0#[ Although similar observations have been made in patients with a total commissurotomy and in whom the CUD was particularly elevated in the R to L direction "ð38Ł^ and see ð28Ł#\ such a clear!cut asymmetry in transfer had not been described[ This marked dichotomy pre! sumably results from the incomplete lesion of the pat! ient|s CC] his L to R callosal transfer of information seemed disrupted whereas his R to L transfer seemed normal[ Two explanations are possible] the transfers in both directions occur side to side in the same postero! median zone of the CC^ the _bers conveying the L to R information are in the lesioned area and the _bers conveying information in the reverse direction are at the anterior boundary of the lesioned area "Fig[ 0#[ In this case\ the same type of signals\ probably visuomotor!inte! grated information\ would be exchanged in both direc! tions through the posterior part of the CC[ Alternatively\ the R to L transfer might occur through a distant and intact\ anterior or posterior\ part of the CC[ In this case\ the information exchanged would di}er in both directions ð15Ł[ Visuomotor signals would be transferred from the LH to the RH through the posterior part of the CC "damaged in this patient#^ from the RH to the LH the signals would be either visual\ through the splenium "spared in this patient#\ or motor through the anterior CC[ The splenium itself does not seem to be essential for interhemispheric communication since Di Stefano et al[ ð19Ł showed that a callosotomized patient whose splenium
890
was spared had a transfer time similar to that of two acallosal subjects[ The role of the anterior part of the CC is discussed] it has been reported that it is not involved in interhemispheric transfer since lesions in it do not seem to modify the CUD in any direction ð49Ł\ but other results argue directly for its participation in SRT ð21Ł[ However\ most of the data on partial or total callosal lesions concern Rhrs\ and observations on Lhrs are lack! ing[ In addition\ the data were obtained with simple vis! uomotor responses\ and the consequences of the same lesions might di}er with reaching movements[ We adopted the more parsimonious hypothesis\ which did not presuppose that transfer di}ered in the two direc! tions and in Rhrs and Lhrs "Fig[ 0#[ Studies with Lhrs with lesions of the anterior part of the CC or Rhrs pre! senting posterior or anterior lesions would be useful to precise the callosal locus of the transfer in each direction and group[ 4[ Conclusion The results con_rm previous data suggesting that\ in Rhrs\ the right hemisphere might have a special role in the programming of directed movements[ We show that this hemispheric asymmetry does not exist in Lhrs[ On the other hand\ although the motor response involved mainly proximal muscles\ a crossed motor pathway was used instead of a direct ipsilateral command ð08\ 22Ł[ In fact\ in terms of the joints and muscles involved\ the movement is indeed proximal but in terms of its spatial representation\ it may be considered very distal since the relevant spatial trajectory concerns the index tip\ i[e[ the most distal part of the arm[ In consequence\ some infor! mation has to be transferred between the two hemispheres in the crossed situations\ both in Rhrs and Lhrs[ That the CUD increased with the callosal lesion con_rms both that the CUD re~ects the interhemispheric transfer and that the fastest transfer normally occurs through the CC[ There are several hypotheses to explain the very asym! metric interhemispheric transfer time of the Lhr patient^ one is that the information transferred might di}er in the two directions\ but this needs to be con_rmed[ Acknowledgements This study was funded by the CNRS[ The authors thank J[C[ Gilhodes for his valuable comments on the manuscript\ J[L[ Demaria for his helpful technical assist! ance and Gary Burkhart for revising the English[
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uncrossed pathway responding in callosal agenesis] reply to Kinsbourne and Fischer[ Neuropsychologia 0863^01]176Ð89[ Roy EA\ Elliot D[ Manual asymmetries in visually directed aimings[ Canadian Journal of Psychology 0875^39]098Ð10[ Roy EA\ Elliot D[ Manual asymmetries in aimed movements[ Quarterly Journal of Experimental Psychology 0878^30A]490Ð05[ Sergent J[ Subcortical coordination of hemisphere activity in com! missurotomized patients[ Brain 0875^098]246Ð58[ Sergent J\ Myers JJ[ Manual\ blowing\ and verbal simple reactions to lateralized ~ashes of light in commissurotomized patients[ Per! ception and Psychophysics 0874^26]460Ð7[ Tassinari G\ Aglioti S\ Pallini R\ Berlucchi G\ Rossi GF[ Inter! hemispheric integration of simple visuomotor responses in patients
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with partial callosal defects[ Behavioral and Brain Research 0883^53]030Ð8[ Van der Staak C[ Intra! and interhemispheric visual!motor control of human arm movements[ Neuropsychologia 0864^02]328Ð 37[ Velay JL\ Beaubaton D[ In~uence of visual context on pointing movement accuracy[ Cahiers de Psychologie Cognitive 0875^5]336Ð 45[ Witelson SF[ Hand and sex di}erences in the isthmus and genu of the human corpus callosum] a post!mortem morphological study[ Brain 0878^001]688Ð724[ Woodworth RS[ The accuracy of voluntary movement[ Psycho! logical Review "Series of Monograph Supplements# 0788^2]0Ð003[
Neuropsychologia 26 "0888# 784Ð892
Hemispheric asymmetry and interhemispheric transfer in reaching programming Jean!Luc Velay \ Simone Benoit!Dubrocard Lab[ Neurobiologie Integrative et Adaptative\ UMR CNRS 5451\ Universite de Provence\ 02286 Marseille\ France Received 07 December 0886^ accepted 8 November 0887
Abstract The purpose of this study was to explore the intrahemispheric processes and the interhemispheric transfer that occur during the programming of a pointing movement[ Twenty _ve subjects participated in this experiment] 01 were right!handed "Rhr#\ 01 left! handed "Lhr#\ and 0 was left!handed with a posterior callosal lesion[ The task consisted in producing an open loop pointing response toward a visual target appearing brie~y on the right or the left of a central _xation point[ Reaction times "RTs# were shorter for the Rhrs when reaching with the left hand than with the right hand[ No such hand!related di}erence was observed in the Lhrs[ The left hand advantage indicates that one process was faster in the right hemisphere of Rhrs[ This faster process appears not to be visual but motor or visuomotor[ For either hand\ responses were faster when the target appeared in the visual _eld homolateral to the pointing hand "uncrossed condition# than when it appeared contralaterally to the hand "crossed condition#[ The crossed vs uncrossed di}erence did not vary between Rhrs and Lhrs or between the hands[ The transfer time between the hemispheres was symmetrical whatever its direction[ The partially callosotomized left!handed subject was two!fold slower than the control Lhrs[ His uncrossed responses were faster than the crossed ones\ but his interhemispheric transfer time was very asymmetrical] it was normal from right to left hemisphere but was highly increased in the opposite direction[ An attempt at modelling the RT data is proposed and the possibility of di}erent callosal locations for the interhemispheric transfer is discussed[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] Reaching^ Right!handers^ Left!handers^ Corpus callosum^ Lesion^ Reaction time
0[ Introduction In primates\ hand reaching movements may be con! sidered the archetype of the visually triggered spatial activities[ To be e.ciently performed\ they require that a series of visuospatial and sensorimotor processes be achieved from the localization of the visual target to the motor commands that are sent to the arm muscles ð16\ 23Ł[ In most studies devoted to the neural mechanisms underlying reaching movement programming\ the pre! ferred hand was used because it was thought to be the more skilled and thus the less variable[ For some time\ however\ there has been interest in the possible hemi! spheric lateralization of this task[ The performances of the two hands were often discordant[ Certain studies evidenced a superiority of the right hand\ either in accu! racy or in speed or both\ indicating left hemisphere domi! nance in the preparation and control of pointing movements in right!handers "Rhrs# ð10\ 11\ 13\ 35\ 43Ł[ Corresponding author[ Fax] ¦22!93!80!17!75!58^ e!mail] velayÝ newsup[univ!mrs[fr
The right hand system may be superior in processing the sensory feedback ð13Ł\ or the left hemisphere may be more appropriate to specify the force parameters of the movement ð36Ł[ Nevertheless\ according to the classical hemispheric dichotomy\ the right hemisphere seems more involved in spatial activities "see ð20Ł for a review#[ Both motor and spatial aspects are important in programming a reaching movement\ and some component left hand: right hemisphere advantage should be observed in this task[ In fact\ other studies showed that the left hand could be faster and:or give rise to a greater accuracy ð09\ 04\ 06\ 10\ 17\ 18Ł[ This left!hand advantage would re~ect the right hemisphere superiority for the spatial processes prior to the movement[ Accordingly\ patients having right or left hemisphere lesions are of interest[ Fisk and Goodale ð12Ł observed a lengthening of the reaction time "RT# only with right!damaged patients\ and they pro! posed that the right hemisphere is more involved in deter! mining the spatial position of the target whereas the left hemisphere is more involved in controlling movement execution[ Conversely\ Haaland and Harrington ð29Ł
9917!2821:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved PII] S9917!2821"87#99038!2
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reported that RTs were longer in left!damaged patients but not in right!damaged ones relative to control[ The di}erent experimental procedures might explain the dis! crepant results[ In all\ the results argue for large par! ticipation of the right hemisphere during reaching movement programming[ Yet it remains unknown whether spatial localization of the target or planning of the movement itself\ or both\ are favored[ In addition to the hand used\ another factor may deter! mine the contribution of each hemisphere\ namely the visual _eld in which the stimulus appears[ Because of the crossed visual pathways\ each visual hemi_eld projects to the opposite hemisphere[ As a consequence\ a better performance when the target is for example in the left visual hemi_eld can be accounted for by special visual competences of the right hemisphere[ If the target appears on the same side as the hand "uncrossed situation#\ the visual input and the motor output occur in the same hemisphere[ Under this intrahemispheric condition no communication with the other hemisphere is theoretically required[ On the contrary\ the crossed situation\ in which the visual input is given to one hemisphere and the motor response is mediated by the other\ requires an inter! hemispheric transfer of information\ and therefore the movement programming would take more time[ This well known experimental paradigm\ with simple and distal motor responses\ showed that the uncrossed responses were faster than the crossed ones "see ð4\ 00\ 28Ł for a review#[ In addition\ the interhemispheric transfer time is greatly increased in patients having a total or partial section of the corpus callosum "CC#\ which is assumed to mediate the interhemispheric integration of visuomotor responses ð49Ł[ Thus\ in such motor responses\ some information is transferred between the hemispheres via the CC[ It is still debated whether this information is visual\ visuomotor\ or motor\ and whether the transfer is the same in both directions ð15\ 28Ł[ The question of interhemispheric transfer is justi_ed only if the motor response is actually mediated by the hemisphere controlateral to the reaching hand ð1Ł[ Ana! tomical data and split!brain studies have shown that hand and _nger movements are controlled mainly by the con! tralateral hemisphere\ but arm movements may be con! trolled by either hemisphere ð01\ 14Ł[ The experiments devoted to interhemispheric transfer have therefore used only very distal responses[ However\ the spatial com! ponent of these so!called key!pressing responses was poor with respect to that required in programming a spatially oriented movement[ In fact\ as underlined by Jeannerod ð23Ł\ the investigators were more interested in movements in response to targets appearing in the visual _eld than in movements directed at these targets[ Contrary to the key!pressing responses\ the arm reaching movement involves principally the shoulder and the elbow[ Since this movement might be mediated by either the con! tralateral or the ipsilateral hemisphere\ an inter!
hemispheric transfer would not be necessary in the crossed pointing[ The majority of the few studies on interhemispheric transfer in directed movements have shown that the crossed responses in Rhrs were initiated more slowly than the uncrossed ones ð04\ 11\ 12\ 40Ł[ No such work has been done with left!handers "Lhrs#\ nevertheless they are thought to have a less pronounced hemispheric asymmetry ð03Ł and a larger corpus callosum "CC# than Rhrs ð42Ł[ This last point is still debated however\ and anatomical data suggest that the regional size of the CC is unrelated to its sensory!motor functions ð0Ł[ Furthermore\ the consequences of callosal lesions on the programming of directed movement are not estab! lished[ The purpose of this study was to investigate the respec! tive contribution of each hemisphere and the inter! hemispheric communication during the programming of a visuomanual pointing movement[ We measured the RT with the two hands\ in uncrossed and crossed situations\ and both in Rhrs\ Lhrs\ and in a subject with a partial lesion in the posterior part of the CC[ We assumed that the right hemisphere in Rhrs may have a special role in programming this type of motor response\ and that this would probably induce a left!hand advantage[ We expected this hand asymmetry to be weaker or absent in the Lhrs[ Finally\ if the directed movement programming requires interhemispheric communication through the callosal pathways\ we expected a larger {crossed! uncrossed di}erence| "CUD# in the patient[ Part of the results have been presented at the Second Meeting of European Neurosciences ð6Ł[
1[ Methods 1[0[ Subjects The study was approved by the Ethics committee of the University[ Subjects were 14 men "11Ð24 years of age#[ They were all volunteers and had all given their verbal consent[ They all had normal or corrected to nor! mal visual acuity in both eyes[ Among the 13 normal subjects\ some were members of the laboratory sta}\ and the others were students at the university[ The latter were paid for participating[ Twelve of them were right!handed and 01 left!handed according to the Edinburgh Hand! edness Inventory[ The patient "17 years old# was left! handed "−79# without a familial history of left hand! edness[ He su}ered from a disconnection syndrome after a trauma that occurred in 0881 "3 years before the experi! ment#[ At that time\ he presented hemorrhagic bilateral fronto!parietal contusions[ At the date of the experiment\ the neurological evaluation was normal\ but the sagittal section MRI showed a region of hypodensity including the middle and posterior part of the trunk of the corpus callosum\ sparing the splenium ð05Ł[ The most obvious
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symptoms were a left tactile and visual hemi_eld anomia and a left hand agraphia[ Further details regarding this patient can be found in Ceccaldi et al[ ð05Ł[ 1[1[ Task The task consisted in producing an open loop pointing response[ The subjects were required to point with their right or left hand as rapidly and accurately as possible toward a visual target appearing brie~y on the right or the left of a central _xation point "FP#[ Since we were interested in the processes during movement program! ming\ the subjects were required to be very fast to force them to program all the movement parameters before its onset\ that is during the RT\ and to minimize the corrections during its execution[ The RT in some pointing experiments were long "see ð11\ 29Ł#\ and this might have prevented the emergence of a right hemisphere superi! ority[ For the same reasons\ we used a visual open!loop movement\ because if the subjects could see their hands during the movement execution "visual closed!loop#\ they could correct some defaults of programming\ particularly in presence of a structured visual context ð41Ł[ Finally\ corrections are still possible during the ongoing move! ment\ even under open!loop conditions if the target remains present because the visually de_ned target pos! ition can be compared with the proprioceptively de_ned arm position ð31\ 33Ł[ To avoid these corrections\ the target was lit!o} before movement onset[ Before the experiment\ 53 trials "21 with each hand# were run to make the subjects familiar with the pointing task requirements[ Speed and accuracy were equally emphasized[ During this training session\ the subjects were informed about their speed and accuracy[ 1[2[ Procedure Each subject was seated with his head in a chin and forehead restraint\ facing a vertical panel situated 36 cm from his eyes\ and whose bottom was _tted with a small starting platform[ He gazed at the FP\ which was a green light!emitting diode "4 mm in diameter# at the center of the panel\ exactly straight!ahead at eye level[ By con! tacting the starting platform with his hand\ the subject triggered the visual target onset[ The visual target was one among four red LEDs symmetrically arranged 5 and 01> to the right and left of the FP[ It was switched on very brie~y "49 ms#\ and the subject was instructed to keep _xating the FP\ which remained lit until the end of the trial[ Fifty!ms exposure is well below the time necess! ary for stimulus foveation and\ after some training\ all subjects could refrain from making saccadic eye move! ments during the target presentation[ The subjects were then required to point toward where the red target had just appeared[ The small plate used as the starting point of the movement was 34 cm beneath the grid!center[ It
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was in the midsagittal plane and at the same location for right! and left!hand responses[ The pointing thus consisted of a vertical movement\ from bottom to top\ involving mainly shoulder and elbow[ Its amplitude was 34[2 and 35[0 cm for the 5 and 01> targets respectively[ The panel consisted of a grid!patterned printed circuit "21×21 cm#\ and as soon as the subject|s _nger contacted it\ a computer recorded the pointing coordinates and emitted a tone signalling that the subject could begin the next trial[ He then placed his hand again on the starting platform\ triggering the target for the following trial\ and so on up to the end of the series[ The pseudo!random order in which the targets were illuminated could not be predicted[ The whole series of reaches was run in total darkness\ the subjects were unable to see their hands\ and they were given no information about the accuracy of their movements[ Experiments were run in two blocks\ one with each hand[ Half of the Rhrs and the Lhrs began with the right hands and half with their left hands[ One set of 21 measurements "eight for each target# was taken with each hand[ Spatial accuracy was determined for each subject and each target by calculating the centroid of the eight poin! ting shots[ Then\ two variables were computed] "0# the mean radial error "in mm#\ i[e[ the distance between the centroid and the target\ and "1# the pointing surface "in mm1#\ i[e[ the dispersion of the eight pointings with respect to the centroid[ We measured the RTs "in ms#\ i[e[ the time separating the target appearance and the movement onset\ and the movement time "MTs\ in ms#\ between movement onset and _nger contact with the grid[ 2[ Results The various combinations of the experimental factors "1 hands×1 _elds×1 target excentricities# resulted in eight experimental situations[ Spatial and temporal data on the patient and the right! and left!handed groups were analyzed separately in a series of ANOVAs[ The comparisons of the patient|s data were done by Student|s t tests[ 2[0[ Spatial accuracy The mean radial error was very close in Rhrs "23[2 mm\ S[D[ 04#\ in Lhrs "27[4 mm\ S[D[ 19#\ and in the patient "25[0 mm#[ The mean pointing surfaces were not statistically di}erent between the Rhrs "061 mm1\ S[D[ 70#\ the Lhrs "042 mm1\ S[D[ 56#\ and the patient "013 mm1#[ For both radial error and pointing surface\ none of the main factors or interactions reached the signi_cance[ 2[1[ Movement time Table 0 shows the MTs[ In intact subjects\ only the main e}ect of excentricity appeared to be signi_cant
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Table 0 Mean movement times
Table 1 Mean reaction times LVF
RVF
LVF
Lh
Rh
Lh
Rh
Rhrs] mean S[D[
199[9 24
100[0 32
119[7 49
084[3 36
Lhrs] mean S[D[
191[0 44
125[2 53
120[8 60
Patient] mean S[D[
496[9 18
408[4 14
419[9 17
RVF
Lh
Rh
Lh
Rh
Rhrs] mean S[D[
194[1 14
117[9 18
101[1 28
119[4 20
119[5 46
Lhrs] mean S[D[
113[7 29
125[5 18
122[8 23
113[6 22
357[9 06
Patient] mean S[D[
351[6 12
360[0 25
459[4 15
351[8 34
LVF] left visual _eld^ RVF] right visual _eld^ Lh] left hand^ Rh] right hand[
"F"0\11# 05[6\ P ³ 9[990#] the MTs were shorter for the 5> targets "101[7 ms# than for the 01> targets "105[6 ms#[ In addition\ the group by hand interaction was signi_cant "F"0\11# 4[1\ P ³ 9[94#] the Rhrs tended to be faster with their right hand "192[1 vs 109[3 ms\ F"0\00# 1[9\ ns# whereas the Lhrs tended to be faster with their left hand "106[9 vs 117[4 ms\ F"0\00# 2[2\ P ³ 9[09#[ The hand by visual _eld interaction was also signi_cant "F"0\11# 57[8\ P ³ 9[990#] In both groups\ although the distances to the targets were identical\ ipsilateral reaches were completed more quickly than contralateral reaches "194 vs 114 ms#[ The di}erence was similar in Rhrs "07 and in Lhrs "12 ms\ F"0\00# 27[6\ P ³ 9[990# ms\ F"0\00# 21[7\ P ³ 9[990#[ The patient was much slower than the Lhrs at reaching "F"0\00# 07[4\ P ³ 9[994#[ He was faster with his right than with his left hand "383 vs 403 ms\ T"51# 2[56\ P ³ 9[990# but only in the non!crossed situation[ Similarly to the intact subjects\ he was faster in ipsilateral than in contralateral reaches "377 vs 419 ms respectively\ T"51# 3[98\ P ³ 9[990#[ However\ the di}erence was greater for his right hand "41 ms\ t"51# 4[36\ P ³ 9[990# than for his left dominant hand "02 ms\ T"51# 0[10\ n[s[#[ 2[2[ Reaction time Table 1 presents overall RT results[ In intact subjects\ the ANOVA revealed no main e}ect of group\ visual _eld\ and target excentricity[ A main e}ect of response hand appeared "F"0\11# 5[3\ P ³ 9[91#^ latencies for reaches with the left hand "108[9 ms# were shorter than with the right "116[4 ms#[ However\ the hand by group interaction was signi_cant "F"0\11# 3[3\ P ³ 9[94#\ and this left hand advantage was present only in Rhrs "198 vs 113 ms\ F"0\00# 09[8\ P ³ 9[90# and not in the Lhrs\ who were equally fast with both hands "118[2 vs 129[6\ F"0\00# ³ 0#[ In Rhrs this hand di}erence did not depend on the side where the target appeared] it was
LVF] left visual _eld^ RVF] right visual _eld^ Lh] left hand^ Rh] right hand[
signi_cant in the crossed "04[7 ms\ F"0\00# 7[52\ P ³ 9[91# and in the uncrossed situation "04[2 ms\ F"0\00# 7[66\ P ³ 9[91#[ The hand by visual _eld interaction was signi_cant "F"0\11# 39[2\ P ³ 9[990#] both in Rhrs and Lhrs\ for either hand\ responses were faster when the target appeared in the visual _eld homolateral "107[7 ms# than contralateral "116[6 ms# to the pointing hand[ The mean CUD was 7[8 ms[ Because it seemed greater in Lhrs "CUD 09[4 ms\ F"0\00# 099[9\ P ³ 9[990# than in Rhrs "CUD 6[1 ms\ F"0\00# 6[8\ P ³ 9[91#\ an ANOVA was done directly on the CUDs[ It showed that the CUDs did not di}er either between groups or between hands[ The patient\ who was left!handed\ responded much slower than the Lhrs "F"0\00# 60[5\ P ³ 9[9994#[ He was globally faster with his right than with his left hand "T"51# 2[56[ P ³ 9[990#[ But\ as shown in Table 1 and Fig[ 0\ this di}erence was due only to the crossed situation in which he was much slower\ whereas like the intact Lhrs he was equally fast with both hands in the uncrossed situation[ In other words\ his mean CUD was 42 ms "T"51# 3[46\ P ³ 9[990# but it was 7 ms with his right hand "T"29# 9[47\ ns#\ close to that of the control group\ and it reached 87 ms with his left hand "T"29# 09[14\ P ³ 9[990#[ 3[ Discussion Since we were concerned with movement program! ming\ RT was the dependent variable of primary interest[ Nevertheless we also considered the spatial performance and the MT in order to check for any trade!o} between the RT and these variables[ For the two groups of intact subjects and the patient\ the pointing accuracy did not vary in function of the experimental situations\ and par! ticularly it did not di}er with the hand used\ contrary to studies showing a greater precision for the left ð17Ł or for the right hand ð10Ł[ In both groups\ MT was shorter with
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Fig[ 0[ Rh] right hand^ Lh] left hand^ RVF] right visual _eld^ LVF] left visual _eld[ Schematic representation of the experimental data[ The half ellipses symbolize the cerebral hemispheres[ Three processes were distinguished from the visual input to the motor output] the visual process "V#\ the visuomotor transformation "VM#\ and the motor process per se "M#[ The grey area between the hemispheres denotes the CC[ The dashed zone in the patient CC represents the lesioned area[ The intrahemispheric and interhemispheric pathways are drawn in solid and dashed lines\ respectively[ The RTs in each situation are on the side of each line[ The values in the CC under or above the dashed lines are the measured CUDs and represent the transfer times[ Note the 05 ms extra time in the LH of Rhrs[ To be coherent with the measured RT\ the transfer in both directions must occur before this longer step[
the dominant hand[ In other words\ the dominant hand was able to execute the required movement with the same accuracy but more quickly than the other hand[ Such an advantage for the Rh in Rhrs has often been shown\ and it re~ects an LH superiority in de_ning the temporal parameters of the motor commands\ or in using the feed! backs\ or both ð10Ł[ The MT to reach the more excentric targets was 3 ms longer\ likely because the distance from the starting platform was slightly greater for these targets[ In the Lhrs\ Rhrs\ and the patient\ the movements cross! ing the midline were slower than those that did not\ although the mean distances from the starting platform to the targets were identical[ A similar result was obtained in intact Rhrs ð04\ 06\ 11Ł and in patients with a callosal agenesis ð22Ł[ This di}erence in MT was greater in acal! losal than in intact subjects ð22Ł\ suggesting that it might also somehow result from interhemispheric di}erences and not only from the diverse biomechanical constraints imposed by the two directions of movements[ This was also true in our patient but only when he used his right hand[ We have no de_nite explanation for this asym! metric e}ect\ which might be related to his partial callosal lesion[ The main part of the discussion will focus on the RTs\ _rst in the intact subjects and then in the patient[ The data are represented graphically in Fig[ 0[ The intra and interhemispheric di}erences were calculated by com!
bination and subtraction of the four hand and visual! _eld situations ð21Ł[ 3[0[ Handedness Rhrs and Lhrs had not previously been compared in reaching movements[ Their RTs were not di}erent\ sug! gesting that handedness did not in~uence the time required to program a visuomanual reaching movement[ This result is congruent with those of key!pressing responses in SRT in which Rhrs and Lhrs did not exhibit di}erent RT ð28\ 32Ł[ 3[1[ Visual hemi_eld asymmetry Visual detection was reported to be faster in the LVF in a key!pressing task ð3Ł\ suggesting that the right hemi! sphere would be better at detecting the onset of a light and therefore might be expected to mediate faster responses[ However\ Marzi and colleagues ð28Ł observed that this _eld e}ect was present only in the crossed situation and was thus not attributable to some hemispheric asymmetry in visual detection but to the interhemispheric integration of visuomotor responses[ Such a _eld e}ect was not observed in the experiments involving visuomanual reaching in response to the appearance of lateral visual stimuli ð09\ 04\ 11\ 17Ł[ Nevertheless\ Guiard and col!
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leagues ð17Ł hypothesized that the left!hand advantage was due to a right hemisphere superiority in solving the problems of spatial localization[ This hypothesis was tested further by increasing the complexity of the spatial processing required to establish the position of the target ð06Ł[ Increasing the spatial complexity did not increase the magnitude of the left!hand advantage\ and it was concluded that it did not re~ect a superiority of the right hemisphere in visuospatial localization but more likely in the movement programming itself[ This conclusion is supported by the _nding that neither hemisphere was dominant for visual localization in either Rhrs or Lhrs[ 3[2[ Hand asymmetry The shorter RT "−05 ms# exhibited by the Rhrs with their left hand in our experiment agrees with previous results ð09\ 04\ 06\ 10\ 18\ 35Ł[ Note that in simpler responses like key!pressing\ Rhrs respond faster with their right than with their left hand ð38Ł or at the same speed with both hands ð7\ 28Ł\ but a left hand advantage has not been evidenced[ The left!hand advantage seems to be typical of reaching movements\ and it has been regarded as a right hemisphere superiority in pro! gramming this type of movement[ This discrepancy between the SRT _nger responses and the CRT reaching movements may result from two causes] _rst\ the muscles concerned are not the same and there might be a right hemisphere advantage for the responses involving proxi! mal muscles ð08Ł[ Secondly\ contrary to key!pressing responses\ pointing movements involve an important spa! tial component\ and this would favor the left hand:right hemisphere ð04\ 06\ 15Ł[ In other words\ some time is lost in the left hemisphere of the Rhrs with respect to their right one[ In the pro! gramming of a visually triggered motor response\ one of the three processes usually considered\ namely visual "V#\ visuomotor "VM#\ and motor "M#\ is longer in the left hemisphere of the Rhrs "Fig[ 0#[ The longer step does not seem to involve visual processings\ since RTs were not di}erent in LVF and RVF[ It is unlikely to result from slower motor processing in the left hemisphere than in the right because such a general motor advantage for the right hemisphere:left hand was not evidenced in simpler motor responses ð28Ł\ but see ð08Ł and in more complex movements known to depend on the left hemisphere ð25Ł[ More probably\ the slower step in the left hemisphere belongs to the visuomotor processes\ linking the position of the target in space to the motor command[ In pointing movements\ these processes are particularly important because a trajectory has to be de_ned to drive the hand from its initial position to where the target appeared\ and the right hemisphere would be more e.cient for achieving these spatial operations[ Studies of patients with hemi! spheric lesions support this idea ð12\ 29Ł[ Nevertheless we could not conclusively identify the locus of the longer
step and hence the 05 ms extra!time have been placed between motor and visuomotor processes in Fig[ 0[ Inter! estingly\ this hand asymmetry was not present in the Lhrs\ who seem to have a less pronounced hemispheric lateralization than the Rhrs in general ð03Ł\ and for spatial abilities in particular ð27Ł[ 3[3[ Interhemispheric transfer In the intact subjects\ the RTs were faster in uncrossed than in crossed situations[ The mean CUD was 7 ms\ a value higher than in the classical SRT responses "about 3 ms^ see ð28Ł#[ In principle\ the pointing task must be compared to CRT tasks since there is choice between several responses[ Two cases are possible in CRT tasks] usually\ no CUD is observed^ in some instances\ CUD is higher than in SRT tasks "29Ð49 ms#[ This discrepancy has not been explained[ The short anatomical CUD might appear only in the rapid re~ex!like visuomotor responses and would not be seen in CRT because it is too short with respect to the long decision processes required in CRT tasks ð2Ł[ From this point of view\ the reaching response shares several features with the SRT task] "0# the motor aspect of the response is very simple\ and the link between stimulus and response is straightforward^ "1# the response does not require complex cognitive pro! cessings and the RT are usually short "199Ð149 ms#^ "2# contrary to most CRT tasks\ a positive CUD\ in the same range as the one reported here "around 09 ms#\ is generally reported ð04\ 10\ 11\ 40Ł[ In any case\ as observed in key!pressing movements\ interhemispheric transfer in directed movements was as fast for Lhrs as Rhrs ð28Ł[ Finally\ that RT did not di}er signi_cantly between the two transfer directions in the two groups suggests that the interhemispheric transfer was not asymmetric in the reaching response\ unlike for key!pressing responses ð8\ 02\ 15\ 28Ł[ Note that for the Rhrs\ when the target was in the RVF\ the crossed pathway was faster than the uncrossed one "−8 ms# "Fig[ 0#[ This apparent paradox is easily accounted for if one admits that the transfer from the LH to the RH occurs before the 05 ms extra! time[ For similar reasons\ the transfer from RH to LH must also occur before the longer step^ otherwise the LVF!Rh condition would have resulted in a 101 ms RT instead of 117 ms[ We placed both transfers at the same level of the CC in Rhrs but they might be separated\ provided the transfer occurs before the left hemisphere long step[ The representation in Fig[ 0 is simple and _ts correctly with the data although\ by simply subtracting the RTs in the four hand and visual _eld situations\ one cannot unambiguously di}erentiate interhemispheric transfer di}erences and intrahemispheric asymmetry in this kind of protocol ð21Ł[ We have applied the same modelling to other data for reaching in Rhrs ð04\ 10Ł[ Overall RT di}ered\ but the modelling _ts correctly with
J[!L[ Velay\ S[ Benoit!Dubrocard : Neuropsycholo`ia 26 "0888# 784Ð892
the data and both the transfer times and the extra!time in LH were close to ours[ In the Lhrs\ the location of the transfers were de_ned in function of the patient|s results[ 3[4[ Patient The patient|s performance accuracy was not a}ected by his callosal lesion but he was much slower than the intact Lhrs[ Note that he was also slower in the situations that a priori do not require an interhemispheric transfer of information "uncrossed situations#[ This whole!_eld impairment was observed in cases of callosal agenesis and callosal lesion ð5\ 22\ 38Ł[ It may result from the suppression of some non!speci_c facilitatory ð26Ł or inhibitory ð15Ł in~uence of the CC on intrahemispheric processings[ The mean CUD of the patient was 42 ms\ a value greater than in the intact subjects[ This further supports the hypothesis that the CUD in reaching task does re~ect the callosal interhemispheric transfer[ In SRT\ patients with a congenital absence of the CC often showed a CUD of about 04 ms ð19\ 24\ 39\ 30\ 34Ł^ in callosotomized patients\ the CUD was on average greater than 49 ms ð1\ 07\ 19\ 28\ 38\ 49Ł[ When the CC is lesioned\ the transfer would likely occur through subcortical path! ways ð1\ 37\ 00Ł[ Surprisingly\ in our patient\ the CUD was very di}erent in the two transfer directions] his trans! fer time was similar to the controls in the R to L direction but it was multiplied by 01 in the L to R direction "Fig[ 0#[ Although similar observations have been made in patients with a total commissurotomy and in whom the CUD was particularly elevated in the R to L direction "ð38Ł^ and see ð28Ł#\ such a clear!cut asymmetry in transfer had not been described[ This marked dichotomy pre! sumably results from the incomplete lesion of the pat! ient|s CC] his L to R callosal transfer of information seemed disrupted whereas his R to L transfer seemed normal[ Two explanations are possible] the transfers in both directions occur side to side in the same postero! median zone of the CC^ the _bers conveying the L to R information are in the lesioned area and the _bers conveying information in the reverse direction are at the anterior boundary of the lesioned area "Fig[ 0#[ In this case\ the same type of signals\ probably visuomotor!inte! grated information\ would be exchanged in both direc! tions through the posterior part of the CC[ Alternatively\ the R to L transfer might occur through a distant and intact\ anterior or posterior\ part of the CC[ In this case\ the information exchanged would di}er in both directions ð15Ł[ Visuomotor signals would be transferred from the LH to the RH through the posterior part of the CC "damaged in this patient#^ from the RH to the LH the signals would be either visual\ through the splenium "spared in this patient#\ or motor through the anterior CC[ The splenium itself does not seem to be essential for interhemispheric communication since Di Stefano et al[ ð19Ł showed that a callosotomized patient whose splenium
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was spared had a transfer time similar to that of two acallosal subjects[ The role of the anterior part of the CC is discussed] it has been reported that it is not involved in interhemispheric transfer since lesions in it do not seem to modify the CUD in any direction ð49Ł\ but other results argue directly for its participation in SRT ð21Ł[ However\ most of the data on partial or total callosal lesions concern Rhrs\ and observations on Lhrs are lack! ing[ In addition\ the data were obtained with simple vis! uomotor responses\ and the consequences of the same lesions might di}er with reaching movements[ We adopted the more parsimonious hypothesis\ which did not presuppose that transfer di}ered in the two direc! tions and in Rhrs and Lhrs "Fig[ 0#[ Studies with Lhrs with lesions of the anterior part of the CC or Rhrs pre! senting posterior or anterior lesions would be useful to precise the callosal locus of the transfer in each direction and group[ 4[ Conclusion The results con_rm previous data suggesting that\ in Rhrs\ the right hemisphere might have a special role in the programming of directed movements[ We show that this hemispheric asymmetry does not exist in Lhrs[ On the other hand\ although the motor response involved mainly proximal muscles\ a crossed motor pathway was used instead of a direct ipsilateral command ð08\ 22Ł[ In fact\ in terms of the joints and muscles involved\ the movement is indeed proximal but in terms of its spatial representation\ it may be considered very distal since the relevant spatial trajectory concerns the index tip\ i[e[ the most distal part of the arm[ In consequence\ some infor! mation has to be transferred between the two hemispheres in the crossed situations\ both in Rhrs and Lhrs[ That the CUD increased with the callosal lesion con_rms both that the CUD re~ects the interhemispheric transfer and that the fastest transfer normally occurs through the CC[ There are several hypotheses to explain the very asym! metric interhemispheric transfer time of the Lhr patient^ one is that the information transferred might di}er in the two directions\ but this needs to be con_rmed[ Acknowledgements This study was funded by the CNRS[ The authors thank J[C[ Gilhodes for his valuable comments on the manuscript\ J[L[ Demaria for his helpful technical assist! ance and Gary Burkhart for revising the English[
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