Human Movement Science 31 (2012) 791–800

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Human Movement Science

journal homepage: www.elsevier .com/locate/humov

Adaptation of hand movements to double-step targetsand to distorted visual feedback: Evidence for sharedmechanisms

Gerd Schmitz a,⇑, Otmar Bock b, Valentina Grigorova c, Steliana Borisova c

a Institute of Sport Science, Leibniz University Hannover, Am Moritzwinkel 6, 30167 Hannover, Germanyb Institute of Physiology and Anatomy, German Sport University, Am Sportpark Muengersdorf 6, 50933 Koeln, Germanyc Institute of Neurobiology, Bulgarian Academy of Sciences, Acad. G. Bonchev St. 23, 1113 Sofia, Bulgaria

a r t i c l e i n f o a b s t r a c t

Article history:Available online 10 December 2011

PsycINFO classification:2330

Keywords:TransferSaccadesSensorimotor adaptationMotor learningAttention

0167-9457/$ - see front matter � 2011 Elsevier B.doi:10.1016/j.humov.2011.08.003

⇑ Corresponding author. Tel.: +49 511 762 2191E-mail addresses: gerd.schmitz@sportwiss.uni-h

(V. Grigorova), stelaborisova@abv.bg (S. Borisova).

Visuomotor adaptation of hand movements has been studied withtwo paradigms: double-step targets and distorted visual feedback.Here we investigate whether both procedures are based on a com-mon adaptive mechanism. Subjects adapted either to double-steptargets or to distorted feedback, each requiring a change ofresponse angle by �15�. The magnitude of adaptation was largerwith rotated feedback but magnitude of aftereffects was compara-ble, suggesting that the difference was due to strategic effectsrather than visuomotor recalibration. Most importantly, subjectswho adapted to double-step targets and were then exposed torotated feedback performed as well as subjects who had fullyadapted to rotated feedback, i.e., there was nearly 100% transferfrom double-steps to rotations; likewise, the transfer from rota-tions to double-steps was almost 100%. From this we conclude thatboth types of adaptation share a common mechanism forrecalibration.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

In an ever-changing environment, the central nervous system needs to constantly recalibrate therules by which it transforms sensory inputs to motor outputs. To evaluate the underlying principles,investigators often induce sensorimotor adaptation by distorting visual feedback about hand

V. All rights reserved.

; fax: +49 511 762 2196.annover.de (G. Schmitz), bock@dshs-koeln.de (O. Bock), valgri@bio.bas.bg

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movement direction (Bock, Abeele, & Eversheim, 2003; Cunningham, 1989; Krakauer, Ghez, & Ghilard-i, 2005; Prablanc, Tzavaras, & Jeannerod, 1975; Wang & Sainburg, 2005) or amplitude (Bock, 1992):goal-directed reaches initially miss their target because of this manipulation, but they gradually adaptwith practice until the distortion is compensated. Studies have shown that adaptation to rotated feed-back compensates for distortions throughout the full ±180� range (Bock et al., 2003; Cunningham,1989), benefits from a preceding adaptation to smaller discordances (Abeele & Bock, 2001; Wigmore,Tong, & Flanagan, 2002; Woolley, Tresilian, Carson, & Riek, 2007), transfers poorly to untrained targetdirections (Krakauer, Pine, Ghilardi, & Ghez, 2000; Roby-Brami & Burnod, 1995), but transfers well toan untrained effector (Anguera, Russell, Noll, & Seidler, 2007; Harris, 1963; Sainburg & Wang, 2002).Imaging and lesion studies suggest that adaptation to rotated feedback is located in a distributed brainnetwork involving the cerebellum, frontal and parietal cortex, and the basal ganglia (Girgenrath, Bock,& Seitz, 2008; Imamizu, Kuroda, Miyauchi, Yoshioka, & Kawato, 2003; Seidler, Noll, & Chintalapati,2006; Shadmehr & Holcomb, 1997; Werner, Bock, Gizewski, Schoch, & Timmann, 2009; Werner, Bock,& Timmann, 2009).

In another approach to the study of sensorimotor adaptation, visual feedback is not manipulated;instead, the target is displaced at response onset such as to require a change of response direction oramplitude. This double-step paradigm was mainly used with saccadic eye movements (Deubel, 1987;McLaughlin, 1967), but it has also been applied with success to adapt hand movement direction (Bock,Schmitz, & Grigorova, 2008) and amplitude (Magescas & Prablanc, 2006). Just like adaptation to ro-tated feedback, double-step adaptation benefits from a preceding adaptation to smaller steps(Schmitz, Bock, Grigorova, & Ilieva, 2010), is direction-specific (Deubel, 1987) and can transfer be-tween effectors (Bock et al., 2008). However, it compensates for directional distortions only up toabout 15� (Schmitz et al., 2010), which is a substantial difference to the ±180� compensation of rotatedfeedback and thus suggests the existence of distinct adaptive mechanisms. In fact, it has been arguedthat double-step adaptation resides at the output stage of the sensorimotor system (Magescas &Prablanc, 2006), while adaptation to distorted feedback is located at the input stage (Simani, McGuire,& Sabes, 2007). The existence of distinct mechanisms is in accordance with imaging and lesion studies,which found double-step adaptation to be located in the cerebellum and the superior colliculus (Des-murget et al., 1998, 2000; Takeichi, Kaneko, & Fuchs, 2007; Xu-Wilson, Chen-Harris, Zee, & Shadmehr,2009), i.e., in regions that partly differ from those implicated in the adaptation to distorted feedback(see above). However, the evidence derived from imaging and clinical data is not conclusive, since itwas based on eye movements in case of double-step adaptation, but on hand movements in case ofadaptation to rotated feedback; this leaves open whether the observed topographic differences reflectthe adaptation type, or rather the effector type.

The question whether adaptation is based on one common, on multiple overlapping or on multipleindependent mechanisms, is of general interest for our understanding of the brain’s functional plas-ticity. We therefore decided to shed more light on this issue by evaluating whether adaptation to dou-ble-step targets will transfer to distorted feedback, and vice versa: a substantial transfer would argueagainst the existence of completely independent mechanisms.

We also studied whether effector types share common or distinct adaptive mechanisms. It has beenshown before that eye and hand movements can adapt concurrently to double-step targets (Bekker-ing, Adams, & Pratt, 1995) and that double step adaptation of the eyes can be facilitated by a precedingdouble step adaptation of the hand (Bock et al., 2008; Grigorova, Bock, Borisova, Ilieva, & Schmitz,2010). In the present study we investigated whether double-step adaptation of the eyes is facilitatedby a preceding hand adaptation to rotated feedback.

2. Methods

2.1. Subjects

Seven females and thirteen males (24.8 ± 2.1 years) with no prior experience in sensorimotor re-search participated in this study. They exhibited no overt sensory or motor deficits except for cor-rected vision. All signed their informed consent to this study, which was pre-approved by the

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Ethics Committee of the German Sport University. Eleven subjects were assigned to group DS (adap-tation to double steps) and nine to group ROT (adaptation to rotated feedback).

2.2. Procedures

The experimental apparatus was the same as in our previous studies (Bock et al., 2008; Schmitzet al., 2010). Subjects sat at a distance of 40 cm from a computer monitor, with their head stabilizedby a chinrest and a headband. A visual target was presented for 750–1550 ms in the center of thescreen and then jumped in one of eight randomly selected directions (0, 45, 90, . . .,315�– where 0�is rightwards), onto an imaginary circle of 11 cm radius about the center. In single-step trials, the tar-get remained stationary for 650–760 ms before returning to the center; in double-step trials, itjumped after 200 ms by �15� along the circle and returned to the center after a constant interval of640 ms.1

Subjects were instructed to follow the targets quickly and accurately with their hand and theireyes. To point at the target they moved a stylus on a horizontal digitizing tablet (CalComp DrawingBoard III, resolution 0.25 mm). Visual feedback was provided by an on-screen cursor which – through-out most phases of the experiment – moved along with the stylus from the center to the target andback in the same fashion as a ‘‘mouse pointer’’ moves along with a computer ‘‘mouse’’: left-rightand fore-aft stylus motion induced left-right and up-down cursor motion of the same amplitude. Sinceall our subjects were accustomed to the use of a computer ‘‘mouse’’, they found it easy and intuitive tocontrol the cursor with the stylus, but we nevertheless decided to give them 40 practice trials beforestarting the actual experiment. During some experimental phases, we rotated the stylus-cursor rela-tionship by +15� about the screen center, and subjects therefore had to rotate stylus direction by �15�in order to reach the target with the cursor.

The experiment was subdivided into episodes of 20 pointing trials, which were separated by shortrest breaks. Group DS started with a baseline phase of four episodes with single-step targets and non-rotated cursor feedback. Next came the adaptation phase of 20 episodes with double-step targets andnon-rotated feedback, and then the transfer phase of five episodes with single-step targets and rotatedfeedback. It was followed by the refresh phase of two episodes as in the adaptation phase, and finallyby the aftereffect phase of two episodes with single-step targets and non-rotated feedback. Procedureswere similar for group ROT, except that episodes with double steps and with rotated feedback wereinterchanged.

2.3. Data registration and analysis

Hand position was sampled at 75 Hz. Sampling was lowered to 50 Hz in double-step trials to syn-chronize the digitizing tablet and the eye camera. The data were smoothed twice by three-point cen-tral averaging. Then, we determined the angle between the actual direction of hand movement and areference direction that would aim the cursor straight at the target during the baseline phase. Accord-ing to this definition, the optimal angle would be 0� during the baseline phase and �15� by the end ofthe adaptation phase. We calculated this angle at two instants during each response, once 100 ms aftermovement onset (initial angle) and once when the hand reached its most eccentric position and slo-wed down or stopped (final angle). Reaction time was defined as the interval between target appear-ance and movement onset and movement duration as the interval between movement onset andmovement end. Hand movements with reaction times >400 ms (3% of trials) were discarded, as thistime is sufficient to start reprogramming of hand movements towards the second target step (vanSonderen, Denier van der Gon, & Gielen, 1988).

Movements of the left eye were recorded during baseline and double-step trials with an infrared-light sensitive oculometer (ISCAN RK-426PC, resolution 1�, sampling rate 50 Hz). We determined thesaccade angle as the actual direction of eye movement and a reference direction that would aim theeyes straight at the target during the baseline phase. Note that these directions are measured in

1 We have shown before that adaptation to steps triggered after 200 ms and to those triggered by saccade onset is equivalent(Grigorova et al., 2010).

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screen-centered coordinates in contrast to the usual gaze angle that is measured in head-centeredcoordinates. We further determined, saccadic reaction time. Eye movements were discarded whenreaction times exceeded 270 ms, again to avoid reprogramming (Becker & Jürgens, 1979).

For subsequent statistical testing, we calculated the medians of response angles and those of reac-tion times for each subject and episode. Data were submitted to repeated-measures analyses of var-iance (ANOVAs) with the between-factor group and the within-factor episode, or with the two within-factors effector and episode. Where necessary, the degrees of freedom were adjusted according toGreenhouse and Geisser. Post hoc comparisons were made with Fishers LSD-tests.

3. Results

3.1. Hand movements

During the baseline phase, hand movements towards the targets had a reaction time of281 ± 19 ms, an initial angle of 4.15 ± 4.38�, a movement time of 454 ± 67 ms and a final angle of0.64 ± 1.28�. None of those variables differed significantly between groups (initial angle: F(1,18) = 2.32, p > .05, reaction time: F(1, 18) = 1.48, p > .05, final angle: F(1, 18) = 2.09, p > .05, duration:F(1, 18) < .01, p > .05). The hand either stopped (66% of all trials) or substantially reduced its tangentialvelocity at its most eccentric position, and then returned to the center.

Fig. 1 a shows the baseline-adjusted initial angles of hand movements in the subsequent phases.Both groups gradually changed their responses in an adaptive sense, i.e., hand angles became morenegative, but group ROT performed better than group DS. The benefit of group ROT was producedby a quick initial change and was constant throughout the adaptation phase. Thus, both groupsadapted in parallel. An ANOVA of the adaptation phase accordingly yielded a significant effect of epi-sode, F(19, 342) = 5.21, p < .001) and group (F(1, 18) = 12.98, p < .01), and no significant interaction(F(19,342) = 1.52, p > .05).

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5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33episode

initi

al h

and

angl

e [d

eg]

DS ROT

{----------------------------------------------------------------------------------------------------}{------------------------}{--------}{---------} adaptation transfer refresh

phase phase phase aftereffect

phase

Fig. 1. Baseline-adjusted initial hand angles during the adaptation, transfer, refresh and aftereffect phase of both subjectgroups. Symbols represent the across-subject means of the median hand angle in each episode and error bars the pertinentstandard deviations. Group DS adapted in 20 episodes to double-step target displacements, then in five episodes to rotatedfeedback, again in two episodes to double-step target displacements, and finally de-adapted in two episodes with single-steptargets and without rotation of feedback. The opposite regimen was used for group ROT.

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Adaptive performance of group ROT worsened throughout the transfer phase, dropping to the end-adaptation level of group DS; at the same time, group DS improved to the end-adaptation level ofgroup ROT. Both groups reverted to their own end-adaptation levels at the onset of the refresh phase.Thus, group differences in these two phases did not depend on the preceding adaptation regime, butrather on the momentary distortion type. This observation is confirmed by an ANOVA of the transferand refresh phase, which yielded neither a significant effect of Group (F(1, 18) = 2.94, p > .05) nor Epi-sode (F(6, 108) = 0.97, p > .05), but a highly significant interaction (F(6, 108) = 10.86, p < .001). Post-Hoc decomposition confirmed that all episodes of the transfer and refresh phase differed significantlybetween groups (at least p < .05).

Considering the characteristics of each distortion type, we calculated the magnitude of transfer foreach subject k from one group as

Trk½%� ¼A� Tk

A� R� 100 ð1Þ

where A is the mean of the initial hand angles of the first five adaptation episodes from the othergroup, R is the mean initial angle of both refresh episodes from the other group and Tk is the meaninitial angle of the five transfer episodes for subject k. Thus, Trk = 0% indicates that k performed duringthe transfer phase like novices from the other group, and Trk = 100% that k performed like fullyadapted subjects from the other group. The magnitude of transfer across all our subjects was94.1 ± 65.1%, and differed significantly from the value of 0% (F(1, 18) = 40.01, p < .001), but did not dif-fer between groups (F(1, 18) = 0.15, p > .05). In other words, we found full transfer in both groups.

An ANOVA of the aftereffect phase yielded no significance for Group (F(1, 18) = 0.02, p > .05) or Epi-sode � Group (F(1, 18) = 0.32, p > .05), but a significant effect of Episode (F(1, 18) = 10.97, p < .01)which confirms successful de-adaptation from the first to the second episode.

Baseline reaction times were 281 ± 19 ms, and increased by 10 ± 17 ms during the adaptation phase.Baseline-adjusted values did not differ between groups during the adaptation phase (F(1,18) = 1.79,p > .05), the refresh phase (F(1, 18) = 1.64, p > .05) nor the aftereffect phase (F(1, 18) = 0.74, p > .05),but were 10 ms shorter in ROT than in DS during the transfer phase (F(1, 18) = 9.67, p < .01).

In all phases of the experiment subjects corrected their movements towards the targets as reflectedby the difference between initial and final hand angle. During exposure to double-step targets subjectscompensated the sensorimotor discordance completely: In the adaptation and refresh phase of groupsDS baseline-adjusted final angles (�15.97 ± 1.84� and �16.65 ± 1.54�) were similar to final angles inthe transfer phase of group ROT (�16.18 ± 0.98�). Movements were less corrected during adaptationto rotated feedback: In the adaptation and refresh phase of group ROT baseline-adjusted final angles(�13.76 ± 0.73� and �13.54 ± 2.20�) were similar to those in the transfer phase of group DS(�13.55 ± 2.99�). Differences between groups were significant in the adaptation phase (F(1,18) = 13.43, p < .01), transfer phase (F(1, 18) = 7,59, p < .05) and refresh phase (F(1, 18) = 12.80,p < .01). Furthermore, final angles differed between groups in the aftereffect phase (F(1, 18) = 6,20,p < .05): subjects from group DS (�0.94 ± 0.86�) corrected their movements more than subjects fromgroup ROT (�2.78 ± 2.06�).

Baseline-adjusted movement durations differed as well between both adaptation paradigms. Theywere enhanced during adaptation to double-step targets (adaptation and refresh phase of group DS:74 ± 68 ms and 18 ± 55 ms; transfer phase of group ROT: 68 ± 78 ms) and reduced during adaptationto rotated feedback (adaptation and refresh phase of group ROT:�41 ± 43 ms and�43 ± 57 ms; transferphase of group DS:�9 ± 47 ms). Those changes differed significantly between groups in the adaptationphase (F(1, 18) = 20.44, p < .001), transfer phase (F(1, 18) = 6.53, p < .05) and refresh phase (F(1,18) = 5.94, p < .05), but they did not differ in the aftereffect phase (�32 ± 60 ms, F(1, 18) = 2.61, p > .05).

3.2. Eye movements

Eye angles during the baseline phase were 0.25 ± 2.34�. They did not differ significantly betweengroups (F(1, 18) = 0.01, p > .05) or episodes (F(1, 18) = 4.35, p > .05). Fig. 2 illustrates that eye adapta-tion of group DS proceeded in a similar fashion as hand adaptation; indeed, ANOVA of the adaptationphase revealed no significant effects for effector (F(1, 18) = 0.99, p > .05) or Episode � Effector (F(19,

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angl

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eg]

DS ROT

{----------------------------------------------------------------------------------------------------}{------------------------}{--------}{---------} adaptation transfer refresh

phase phase phase aftereffect

phase

Fig. 2. Baseline-adjusted eye angles. Symbols represent the across-subject means of the median eye angle in each episode anderror bars the pertinent standard deviations. Group DS adapted to target displacements of �15� in 20 episodes of the adaptationphase and two episodes of the refresh phase, and de-adapted in two episodes without target displacement during the aftereffectphase. Group ROT adapted to target displacements of �15� in five episodes of the transfer phase.

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152) = 0.95, p > .05), only for episode (F(19, 152) = 6.04, p < .001). Eye angles during the refreshphase of group DS were consistently less negative than the pertinent initial angles of the hand (effec-tor: F(1, 8) = 7.99, p < .05; episode: F(1, 8) = 2.15, p > .05, Episode � Effector: F(1, 8) = 0.11, p > .05) andthe same holds for the aftereffect phase (effector: F(1, 8) = 10.82, p < .05, episode: F(1, 8) = 6.44, p < .05,Episode � Effector: F(1, 8) = 4.81, p > .05).

We did not register corresponding eye data from group ROT since with rotated hand feedback, theeyes look at the target rather than at the cursor, and their angle therefore is near zero throughout theadaptation phase (Grigorova & Bock, 2006). Eye angles during the transfer phase of group ROT werenot reliably different from that during the first five adaptation episodes of group DS (Group: F(1,18) = 1.18, p > .05, Episode: F(4, 72) = 8.26, p < .001, Episode � Effector: F(4, 72) = 1.12, p > .05), i.e.,we have no evidence that adaptation of hand movements to rotated feedback transferred to dou-ble-step saccades.

Baseline reaction times of the eyes (232 ± 14 ms) were significantly lower than those of the hand(F(1, 8) = 18.17, p < .01). They decreased in the first adaptation episode by 73 ± 92 ms (t(8) = �2.39,p < .05), and then gradually increased again (Episode: F(19, 152) = 2.55, p < .001), thus becoming com-parable to baseline values at the end of the adaptation phase (t(8) = �0.37, p > .05).

4. Discussion

4.1. Hand adaptation

This study compared the adaptation of hand movements to double-step targets and to rotated vi-sual feedback, each requiring a change of initial hand angles by �15�. In accordance with previouswork (Bock et al., 2003, 2008; Roby-Brami & Burnod, 1995), subjects adapted gradually in both para-digms. However, responses were consistently more efficient throughout the adaptation phase withrotated feedback. This advantage of rotated feedback didn’t persist into the aftereffect phase, whichsuggests in accordance with established reasoning that adaptive recalibration of the sensorimotor

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system was similar (Jeannerod, 1988) and differences were based on strategic adjustments. Strategicadjustments do not reflect ‘‘true adaptation’’ (Weiner, Hallett, & Funkenstein, 1983) and involve sev-eral cognitive processes as anticipation, cue or cognitive learning or implicit or explicit movementadjustments (Bedford, 1993; Bock, 2005; McNay & Willingham, 1998; Pisella et al., 2004; Redding& Wallace, 1993; Redding & Wallace, 1996; Roby-Brami & Burnod, 1995). A common explicit strategyis to countervail a sensorimotor discordance by pointing to a virtual target next to the real target(Welch, 1978). This kind of strategy can immediately offset goal-directed movements, persist through-out the adaptation phase and be additive to recalibration (Mazzoni & Krakauer, 2006; Suelzenbrueck &Heuer, 2009). The group differences of the present study have similar characteristics. Therefore, weconclude that they rather reflect differences in strategic adjustments than in recalibration.

The strategic advantage switched groups during the transfer phase and switched back at the onsetof the refresh phase, i.e., it was specific for the distortion type (rotation of feedback) rather than forsubjects from group ROT. As a possible explanation of this finding: Taylor and Ivry (2011) reportedthat strategic adjustments of initial hand angles are related to the error signal between final handand target angle. In accordance to these results final angles differed systematically between distortiontypes in the present study. During adaptation to double-step targets subjects corrected their move-ments and (over)compensated the distortion, whereas during adaptation to rotated feedback a finalerror remained. According to Taylors and Ivrys hypothesis the remaining error should trigger a stra-tegic adjustment of initial hand angles during adaptation to rotated feedback, but not during adapta-tion to double steps. This is exactly what we found in our study.

Nevertheless, the difference of initial angles between both groups is larger than the difference offinal angles. Therefore, further mechanisms might be involved: discordant feedback might focus sub-jects’ attention on the cursor, while the double-step paradigm might focus it on the target; it wasshown before that subjects perform better throughout the adaptation phase when they attend the cur-sor rather than the target (Grigorova, Petkova, & Bock, 2006). Another explanation might be a distri-bution of attention in the double-step paradigm, when the hand adapts concurrently with the eyes(‘‘saccadic adaptation’’). A distribution of attention can reduce adaptive performance and does not af-fect feedback-control (Taylor & Thoroughman, 2007).

The most important finding for the purposes of our study concerns the transfer phase. Adaptationtransferred to almost exactly 100% from double-step targets to rotated feedback and vice versa, i.e.,what has been learned in one paradigm was applied unconditionally in the other. Moreover, whensubjects switched from double-step targets to rotated feedback, they even were able to capitalize fullyon the strategic benefit of the latter distortion. Our findings are therefore compatible with the viewthat both discordances activate a common mechanism for adaptive recalibration, but differently stra-tegic processes.

The hypothesized existence of a common mechanism seems to be at odds with the finding thatadaptation to double-step targets doesn’t exceed 15� even if step magnitude is much larger (Schmitzet al., 2010), while adaptation to visual rotation shows no such constraint (Cunningham, 1989; Bocket al., 2003). This discrepancy between paradigms cannot be explained by strategies, since it persistsduring the aftereffect phase. Instead, it could reflect differences of perceptual processes located up-stream from the adaptive mechanism: it might be easy for our visual system to determine that thefeedback cursor moves 60� past the target, but difficult to establish that the target jumped by 60�along a circle. As a consequence, the common adaptive mechanism may not receive adequate errorinformation with large double-steps, and thus can’t compensate them efficiently. This topic mightbe investigated in a further study.

4.2. Saccadic adaptation

In group DS, eye angles changed with a similar time-course as hand angles during the adaptationphase, but showed much less adaptive change than hand angles during the aftereffect phase: afteref-fects were only 9% of the discordance for the eyes, but amounted to 39% for the hand. This is in con-trast to previous studies on eye adaptation with the hand held still: there, aftereffects of the eyes wereas high as 24–36% (Alahyane et al., 2007; Bock et al., 2008; Xu-Wilson et al., 2009). One possible inter-pretation for this discrepancy is that adaptive recalibration of saccades - as gauged by aftereffects

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(Bock, 2005; McNay & Willingham, 1998; Pisella et al., 2004; Redding & Wallace, 1996) – is strongwhen the eyes move alone, but is marginal when eyes and hand move together, possibly becausethe latter situation can be mastered more efficiently by yoking oculomotor commands to arm motorcommands rather than by recalibrating both motor systems (Cotti, Vercher, & Guillaume, 2011). Thisinterpretation is consistent with the finding that eye and arm movements are naturally coupled (Gran-kek et al., 2009; Jackson, Newport, Mort, & Husain, 2005; Neggers & Bekkering, 2002), but it is difficultto reconcile with the present observation that during the transfer phase of group ROT, eye and handdirections were dissociated by 5.9�. We therefore favor the alternative interpretation, that adaptiverecalibration of saccades in group DS actually was as strong as in studies where the eyes moved alone,but was washed out during the transfer phase of DS: during that phase, subjects were exposed to dis-torted hand feedback and single-step targets, such that any previously established saccadic recalibra-tion should be detrimental for performance, and therefore should be revoked. This view is supportedby our findings from group ROT: subjects from this group were exposed to rotated hand feedback andsingle-step targets, which induced no adaptive recalibration of saccades since the subjects movedtheir eyes like novices during the subsequent transfer phase.

The dissociation of eye and hand movements in group DS and the lack of transfer from rotated-feedback adaptation of the hand to double-step adaptation of the eyes in group ROT argue for distinctadaptive mechanisms for both effectors. Nevertheless, both mechanisms can interact in dependence ofthe context (Bekkering et al., 1995; Bock et al., 2008; Cotti et al., 2007; Grigorova et al., 2010; Mages-cas & Prablanc, 2006).

5. Conclusion

In sum, our data suggest that there is no fundamental difference between hand adaptation to dou-ble-step targets and hand adaptation to rotated feedback. Dissociable effects of hand and eye adapta-tion argue for distinct adaptive mechanisms for these effectors and might explain topographicdifferences of adaptation paradigms in the human brain.

Acknowledgments

This work was supported by DFG and the Bulgarian Academy of Science (Exchange Grant BUL113/148/0-1). Thanks are due to Dipl.-Ing. L. Geisen and Dipl.-Ing. P. Grozdev for software development.

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