Cerebral Cortex Advance Access originally published online on October 10, 2007
Cerebral Cortex 2008 18(6):1395-1406; doi:10.1093/cercor/bhm173
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Memory Formation in the Motor Cortex Ipsilateral to a Training Hand
1 Human Cortical Physiology Section and Stroke Neurorehabilitation Clinic, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20817, USA, 2 Laboratoire de Neurophysiologie, Université catholique de Louvain, B-1200 Brussels, Belgium, 3 Sezione di Neurofisiologia Clinica, Dipartimento di Scienze Neurologiche e del Comportamento, Universita’ di Siena, I-53100 Siena, Italy, 4 Cortical Physiology Research Group, Department of Neurology, University Medical Center Hamburg-Eppendorf, 20246 Hamburg, Germany
Address correspondence to Dr Leonardo G. Cohen, Human Cortical Physiology Section, Stroke Neurorehabilitation Clinic, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20817, USA. Email: cohenl{at}ninds.nih.gov
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Abstract |
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Cortical reorganization within the primary motor cortex (M1) contralateral to a practicing hand has been extensively investigated. The extent to which the ipsilateral M1 participates in these plastic changes is not known. Here, we evaluated the influence of unilateral hand practice on the organization of the M1 ipsilateral and contralateral to the practicing hand in healthy human subjects. Index finger movements elicited by single-pulse transcranial magnetic stimulation (TMS) delivered to each M1 were evaluated before and after practice of unilateral voluntary index finger abduction motions. Practice increased the proportion and acceleration of TMS-evoked movements in the trained direction and the amplitude of motor-evoked potentials (MEPs) in the abduction agonist first dorsal interosseous (FDI) muscle in the practicing hand and decreased the proportion and acceleration of TMS-evoked abduction movements and MEP amplitudes in the abduction agonist FDI in the opposite resting hand. Our findings indicate that unilateral hand practice specifically weakened the representation of the practiced movement in the ipsilateral M1 to an extent proportional to the strengthening effect in the contralateral M1, a result that varied with the practicing hand's position. These results suggest a more prominent involvement of interacting bilateral motor networks in motor memory formation and probably acquisition of unimanual motor skills than previously thought.
Key Words: inhibition • ipsilateral M1 • motor learning • plasticity • TMS • training
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Introduction |
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Skill acquisition is accomplished through repetition, which in turn leads to performance improvements (Shadmehr and Mussa-Ivaldi 1994
; Nudo, Milliken, et al. 1996; Bays and Wolpert 2007). Encoding of motor memories in the primary motor cortex (M1) may represent one of the steps preliminary to the acquisition of a motor skill (Donchin et al. 2002; Krakauer and Shadmehr 2006). For example, practice of finger movements in a particular direction leaves a short-lasting memory trace in the contralateral M1, as evidenced by an increased probability, at the end of the training period, that transcranial magnetic stimulation (TMS) applied over M1 will elicit movements in the practiced direction (Classen et al. 1998). This cortical reorganization associated with practice (Nudo, Wise, et al. 1996; Classen et al. 1998) is thought to engage long-term potentiation-like mechanisms (Butefisch et al. 2000) that could support learning and memory processes (Rioult-Pedotti et al. 1998; Sawaki et al. 2002).
In addition to changes in the M1 contralateral to a practicing hand, it has been reported that the M1 ipsilateral to a hand performing a novel motor task is active to various degrees (Kawashima et al. 1994; Chen et al. 1997) and that this activity decreases progressively as the task becomes overlearned (Daselaar et al. 2003; Rossini et al. 2003; Bischoff-Grethe et al. 2004). This activation has been interpreted, depending on the context, as a possible epiphenomenon of task complexity (Chen et al. 1997; Hummel et al. 2003; Rossini et al. 2003; Verstynen et al. 2005), as erroneous localization of a predominantly dorsal premotor activation (Hanakawa et al. 2005), or as an active contribution to the learning effort (Grafton et al. 2002).
Besides, previous studies evaluated changes in corticomotor excitability of the ipsilateral M1 during performance of a unilateral motor task by means of TMS and reported both inhibitory (Leocani et al. 2000; Duque, Mazzocchio, et al. 2005; Koch et al. 2006) and facilitatory (Muellbacher et al. 2000; Hortobagyi et al. 2003) effects. A possible explanation for these conflicting results during task performance is that the relative net effect of performing a unilateral hand movement on corticomotor excitability of the other hand varies from inhibitory to facilitatory depending on the behavioral set (Liepert et al. 2001; Sohn et al. 2003; Perez et al. 2007; Perez et al. forthcoming). It remains to be determined whether changes in the organization of M1 ipsilateral to a moving hand, so far identified during unilateral movements, persist after the end of a training period. In other words, it would be useful to determine if unilateral hand training, in addition to encoding a memory trace in the contralateral M1 (Classen et al. 1998), elicits reorganizational changes in the ipsilateral M1 and if so the characteristics or specificity of this form of use-dependent plasticity. Understanding these characteristics could potentially strengthen the design of emerging neurorehabilitative interventions in the clinical arena (Whitall et al. 2000; Wolf et al. 2006).
In summary, the functional details of reorganizational changes in the ipsilateral M1 after motor training are incompletely understood. To address this issue, we utilized an experimental protocol that permits the evaluation of movement kinematics and motor-evoked responses elicited by single-pulse TMS applied over the human M1 (Classen et al. 1998). Our main findings were that unilateral motor practice encodes reciprocal memory traces in both M1s, characterized by strengthening of the representation of the practiced movement in the M1 contralateral to the practicing hand and by reduction of the mirror movement representation in the opposite M1, an effect that varied in magnitude with the practicing hand's position.
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Materials and Methods |
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Subjects
Fifteen healthy volunteers (9 women and 6 men; 33.3 ± 2.2 years) participated in Session1, 9 of whom were also involved in Sessions 2 and/or 3 (n = 8 in each session) provided that 1) TMS applied over M1 elicited isolated and consistent contralateral abductions of the nondominant index finger and 2) they were able to perform brisk voluntary abduction movements of the dominant index finger. All participants gave written informed consent, and the Institutional Review Board of the National Institute of Neurological Disorders and Stroke approved the study protocol. Two individuals (no. 9 and no. 11 in Tables 1–3) were left-handed according to the Edinburgh handedness inventory (Oldfield 1971).
Subjects sat comfortably with both forearms supported on an armrest adapted for each individual. The practice of the dominant index finger was performed with the hand positioned palm down (Session 1) or turned at 90° thumb up (Sessions 2 and 3), and the nondominant nonpracticing hand was always placed palm down at rest (Fig. 1a). The 3 sessions were performed on separate days. The index fingers were kept completely unrestrained, and the 3 last fingers of each hand were attached to the armrest with tape.
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Motor Practice
Preceding practice, the optimal scalp positions for induction of contralateral index finger movements with the lowest stimulus intensity were determined and marked on the scalp to ensure identical placement of the coil throughout the experiment. This technique has proven reliable in the past to ensure stable positioning of the coil location in experimental designs requiring several TMS testing blocks (Gilio et al. 2003; Duque, Vandermeeren, et al. 2005; Pal et al. 2005). Applied in this way, TMS induced abduction movements in the nondominant index finger of all subjects (inclusion criteria, see above). In Session 1, we tested the hypothesis that TMS-evoked abduction movements in the nondominant index finger could be altered by practice of the dominant index finger consisting of repetition of index finger abductions (mirroring the direction of TMS-evoked abduction movements in the nondominant hand), as shown in Figure 1a, Session 1. In 2 additional experiments performed on 2 separate days in a randomized order (4 subjects performed Session 3 before Session 2, whereas the others performed Session 2 before Session 3), the practicing hand was turned at 90° thumb up to investigate the effect of the same abduction movements performed in a direction different from that in Session 1 (Fig. 1a, Session 2) or of flexion movements performed toward the midline (mirroring the direction of TMS-evoked abduction movements in the nondominant hand as in Session 1, Fig. 1a, Session 3). The practiced movements were paced at 1 Hz, and each practice session lasted for 30 min, a duration sufficient to encode a memory trace that reflects the kinematic details of the trained motions in the cortical representation of the practicing finger (Classen et al. 1998). Accuracy and consistency of practiced movements were monitored online by the investigators, and, if necessary, instructions were repeated to the subject. Additional measurements were obtained all along experimental sessions to calculate offline the strength (length of the acceleration vector) and accuracy (dispersion and angular deviation from the instructed practice direction) of the training movements (see Table 1 for individual data on practice kinematics).
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Transcranial Magnetic Stimulation
The effect of practice was investigated with the subjects at rest by applying TMS to both M1s before and after practice with the 2 hands palm down (Fig. 1b). This hand position was used for testing pre- and posttraining TMS-evoked index finger movement directions and motor responses in all 3 sessions, even when practice was performed with the hand turned at 90° thumb up in Sessions 2 and 3. TMS was applied by means of a figure-of-8 magnetic coil (diameter of wings 70 mm) connected to a Magstim 200 magnetic stimulator (Magstim, Whitland, Dyfed, UK). The coil was placed tangentially on the scalp with the handle pointing backward and laterally at a 45° angle away from the midline, approximately perpendicular to the central sulcus (Werhahn et al. 1994). The hot spot was defined for both M1s as the optimal position for eliciting an isolated and consistent contralateral index finger movement. The resting motor threshold (rMT) was defined, at the hot spot, as the minimal TMS intensity needed to elicit motor-evoked potentials (MEPs) larger than 50 µV peak-to-peak in the relaxed first dorsal interosseous (FDI) in 5 out of 10 consecutive trials. The TMS intensity was then adjusted to elicit mild index finger movements in all trials in each of the 2 hands. Same intensities (% output) were used during pre- and postdeterminations in all 3 sessions (see Table 2). Sixty TMS pulses were delivered to each M1 at 0.2 Hz before and after each practice session to determine TMS-evoked finger movement directions and MEP amplitudes in both hands according to a well-established protocol (Classen et al. 1998).
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Electromyography and Kinematic Recording Procedures
Electromyography (EMG) activity and MEPs were recorded bilaterally from surface electrodes placed over the FDI (MEPFDI), acting as abduction (and to a lesser extent flexion) agonist, and from the first palmar interosseous (FPI, MEPFPI), acting as abduction antagonist (illustrated in Fig. 1a,b). During practice, there was no EMG activity or movements detectable in the nonpracticing hand as reflected by EMG and accelerometric monitoring. Movements of each index finger were recorded by means of 3-dimensional accelerometers (Kistler Instrument, Amherst, NY) mounted on the distal phalanx of the index fingers. The direction of TMS-evoked and voluntary index finger movements was calculated from the direction of vectors constructed from the first peak acceleration in the 2 orthogonal axes of the principal movement plane (abduction–adduction and flexion–extension) (Classen et al. 1998). The acceleration along the horizontal movement axis (abduction–adduction) was also quantified separately. Arbitrarily, abduction was always assigned positive values, whereas the opposite direction (adduction) was assigned negative values. EMG and acceleration signals were digitized at 5 kHz using a data collection–analysis program written in LabView (National Instruments, Austin, TX). Please note that what is described as M1 plasticity in the present study refers to plastic changes in the corticospinal system.
Measurements
The effect of the 3 separate practice sessions was examined by evaluating TMS-evoked movement directions and MEP amplitudes before and after motor practice in both the practicing and nonpracticing hands positioned palm down at rest regardless of the position held during the practice period (Fig. 1a). That is, the nonpracticing hand remained at rest in all 3 sessions.
Practice Zones
Practice zones (PZs) were defined in all sessions as ±45° quadrants centered on the index finger practiced movement (PZM; abduction, flexion) and on the index finger practiced direction (PZD, inward, upward, see Fig. 1b). A different measure, PZALL, defined as PZM + PZD reflected the combined effect of muscle activity and direction during the practice period. In Session 1, PZM (abduction) and PZD (inward) coincided, expressed as PZ = PZALL in Figure 1b. In Sessions 2 and 3, because of the different practice hand position (Fig. 1a), PZM and PZD diverged (Fig. 1b). In Session 2, subjects practiced index finger abductions in an upward direction (Fig. 1a). Therefore, PZs (defined with the hand palm down) corresponded to a PZM in abduction and a PZD upward (Fig. 1b). In Session 3, subjects practiced index finger flexions in an inward direction (Fig. 1a). Therefore, PZs were defined with PZM in flexion and PZD inward (note again the divergence of PZM and PZD, Fig. 1b). The rationale for defining separately the 2 PZs was to capture the effects of muscle activity, PZM, and direction of movement, PZD, during the practice period. We evaluated the changes in the proportion of TMS-evoked movements falling within the PZs as a function of practice. For the nonpracticing hand, we defined a mirror practice zone (PMZ), which was centered on abduction in all sessions and was therefore the mirror image of PZ (in Session 1, PZM and PZD were the same, Fig. 1b), PZM (in Session 2), or of PZD (in Session 3, see above and Fig. 1b). We also defined a parallel practice zone (PPZ), which was the parallel image of PZ, PZM, or PZD (in Sessions 1, 2, and 3, respectively) and other nonspecific intermediate locations (PIZ). We then calculated changes in the proportion of TMS-evoked movements that fell within the PMZ, PPZ, and in PIZ in the nonpracticing hand after each of the 3 practice sessions, once both hands were at complete rest (see Fig. 1b). These 3 PZs in the nonpracticing hand were defined to distinguish how practice in one hand altered the cortical representation of the mirror or parallel movements in the nonpracticing hand (Fig. 1b).
MEP Amplitudes
MEPFDI and MEPFPI amplitudes were measured in each hand and compared before and after practice.
Data Analysis
All data are expressed as mean ± standard error. Distribution normality was tested by the Kolmogorov–Smirnov tests for each variable. First, in order to test whether training kinematics were comparable across the 3 different practice sessions, the length of the acceleration vector, the dispersion, and the angular deviation of the trained movements were analyzed by means of one-way repeated measure analysis of variance (ANOVARM) with SESSION(Session1/Session2/Session3) as factor. Second, for each session and for each hand, we tested the effect of practice on the rMT, MEP amplitudes, and motor kinematics of TMS-evoked movements (horizontal acceleration and proportion of TMS-evoked movements in PZALL, PZM, PZD, PMZ, PPZ, and PIZ) by means of one-way ANOVARM with PRACTICE(pre/post) as factor. In addition, we used one-way ANOVARM with factor SESSION(Session1/Session2/Session3) to study the relative influence of the 3 different practice sessions on MEP amplitudes and TMS-evoked movements in the nonpracticing hand (post/pre). Paired t-tests corrected for multiple comparisons were used for post hoc analysis when appropriate. Finally, we determined correlations by means of the Pearson and Spearman tests when adequate.
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Results |
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Motor Training Kinematics
Motor training kinematics including the length of the acceleration vector of the practiced movements and the dispersion and angular deviation of the training motions from the instructed practice direction revealed no differences across the 3 training sessions (see Table 1 for group and single subjects performance), despite the different movement directions and muscles involved across sessions.
rMT before and after Practice
rMTs were comparable in the practicing and nonpracticing hands at baseline in all 3 sessions (Table 2, ANOVARM, not significant [ns]). Training did not result in significant changes in rMT in neither hand in any of the 3 sessions (Table 2, ANOVARM, ns). Before practice, the proportion of TMS-evoked movements in the practicing hand falling in the PZ in Session 1, PZM in Session 2, and PZD in Session 3 was comparable: 0.43 ± 0.09, 0.46 ± 0.12, and 0.36 ± 0.11 for Sessions 1, 2, and 3, respectively. Similarly, the proportion of TMS-evoked movements in the PMZ for the nonpracticing hand was also comparable in the 3 sessions: 0.73 ± 0.05, 0.59 ± 0.10, and 0.61 ± 0.08 for Sessions 1, 2, and 3, respectively, indicating that each practice session did not influence baseline determinations in the following sessions.
Effect of Different Forms of Right-Hand Motor Practice
Session 1
Training in this session involved index finger abduction movements performed inward with the hand palm down.
Effects on the practicing hand
ANOVARM revealed that practicing abduction index finger movements with the palm down (Fig. 1a, Session 1) induced a significant increase in the proportion of TMS-evoked movements falling in PZ (a ±45° quadrant centered on the mean practiced index finger abduction movements, F = 15.82, P = 0.002, Table 3 shows all individual subjects’ data), in the acceleration peak of TMS-evoked abduction movements (F = 5.34, P = 0.04) and on the amplitude of MEPs recorded from the agonist muscle (MEPFDI, F = 4.48, P = 0.05) but not from the antagonist muscle (MEPFPI, F = 1.48, P = 0.25, all individual subjects’ data are shown in Table 4). These findings are consistent with previous reports indicating that training strengthens the representation of the practiced finger movement in the M1 contralateral to the practicing hand (Classen et al. 1998). Figures 2 and 3a show results from a single subject and group data, respectively, whereas Tables 3 and 4 show all single subject data.
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Effects on the nonpracticing hand
More importantly, we found that the proportion of TMS-evoked movements decreased in the mirror PMZ (from 0.73 ± 0.05 to 0.36 ± 0.07, F = 29.56, P < 0.001) but increased in the parallel PPZ zone (from 0.06 ± 0.02 to 0.29 ± 0.07, F = 10.43, P = 0.006, Fig. 3a,b) and in the intermediate PIZ zone (from 0.21 ± 0.05 to 0.35 ± 0.08, F = 7.25, P = 0.017). It is noteworthy that the increase in the proportion of TMS-evoked movements in the PPZ of the nonpracticing hand was similar to that in the PIZ (paired t-test, t = 0.85, P = 0.41, see Fig. 3b), suggesting that practice did not specifically enhance representation of that particular (practiced) movement direction (toward the left) in the nonpracticing hand. In addition, practice induced an overall decrease in the horizontal acceleration (reduced abduction component) of TMS-evoked movements (F = 21.82, P < 0.001, see Fig. 3a) and the amplitude of MEPFDI (F = 9.33, P = 0.009, Figs 2 and 3, and Tables 3 and 4) in the nonpracticing hand. The decreased horizontal acceleration of TMS-evoked movements in the nonpracticing hand correlated with the increase in the proportion of movements evoked in PZ in the practicing hand (r = –0.06, P = 0.02). The amplitude of MEPs in the nonpracticing FPI, an index finger adductor muscle, was not influenced by training (F = 1.38, P = 0.26).
Session 2
Training in this session involved index finger abduction movements performed upward with the hand turned at 90° thumb up (Fig. 1a).
Effects on the practicing hand
As mentioned above, the length of the acceleration vector of the practiced finger abduction movements and the dispersion and angular deviation of the training movements from the instructed practice direction were comparable in Sessions 1 and 2, despite the different postures (Fig. 1a, Table 1).
Despite this homogeneity in motor training kinematics, practice in Session 2 did not demonstrate an increase in the proportion of TMS-evoked movements in PZALL in the practicing hand (from 0.88 ± 0.17 to 0.98 ± 0.04, F = 2.8, P = 0.14, n = 8, see circular histograms in Fig. 4a, left), possibly due to a ceiling effect, because at baseline most subjects already showed a maximum proportion of TMS-evoked movements in PZALL. Consistent with this hypothesis, all subjects who had lower baseline proportion of TMS-evoked movements in PZALL in the practicing hand showed a strong increase (ranging from 0.76 ± 0.17 to 0.96 ± 0.04, n = 4). We found no evidence for a preferential enhancement of PZM or PZD in the practicing hand performing this task.
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Effects on the nonpracticing hand
Following this form of practice, we found no quantifiable training-related changes in the direction of TMS-evoked movements in the nonpracticing hand (ANOVARM, ns, Fig. 4a,b, left).
Session 3
Training in this session involved index finger flexion movements performed inward with the hand at 90° thumb up (Fig. 1a).
Effects on the practicing hand
The length of the acceleration vector of the practiced finger movements and the dispersion and angular deviation of the training movements from the instructed practice direction were comparable in Sessions 1 and 3, despite the different motions involved (Table 1). However, similar to Session 2, practice did not show training-related increases in the proportion of TMS-evoked movements in PZALL (from 0.49 ± 0.36 to 0.68 ± 0.32; F = 2.1, P = 0.19, n = 8, Fig. 4a, right) or any convincing evidence for a preferential enhancement of PZM or PZD (ANOVARM, ns, see Fig. 4a).
Effects on the nonpracticing hand
Training flexion index finger movements into the same direction in space as in Session 1 (inward) led to a significant reduction in TMS-evoked movements in the mirror PMZ (ANOVARM, F = 5.46, P = 0.05, Fig. 4a,b, right), and the acceleration of TMS-evoked movements tended to decrease (F = 4.71, P = 0.067). Consistently, ANOVARM revealed a significant effect of the factor SESSION on practice-related changes (post/pre) in the proportion of TMS-evoked movements falling in PMZ (F = 11.84, P = 0.002, Fig. 5), on the acceleration peak of TMS-evoked abduction movements in the nonpracticing hand (F = 10.70, P = 0.009), and in MEPFDI amplitudes (F = 5.50, P = 0.025). The reduction of TMS-evoked movements in PMZ for the nonpracticing hand correlated with the increase in the proportion of TMS-evoked abduction movements in PZD for the practicing hand (r = –0.83; P = 0.005).
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Discussion |
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Motor practice results in cortical reorganization within the M1 contralateral to the practicing hand (Kleim et al. 1998
; Nudo 2003; Stefan et al. 2005). Understanding the degree to which the M1 ipsilateral to a practicing hand reorganizes and the characteristics of this reorganization following unimanual hand practice were the goals of this investigation. The Paradigm
Using our paradigm, it has been shown that repetitive practice of finger movements in a particular direction leaves a short-lasting memory trace in the contralateral M1, as evidenced by an increased probability, at the end of the training period that TMS applied over M1 will elicit movements in the practiced direction (Classen et al. 1998). The effects of this type of training on the organization of the ipsilateral M1 outlasting the training period are not known. In this investigation, we addressed this question by studying TMS-induced kinematic and EMG responses from muscles agonistic and antagonistic to the practiced movement in both hands, providing information on cortical organization of both finger motor representations (Stefan et al. 2005).
Effects of Motor Training on the Organization of the Ipsilateral M1
The main result of this study was that practice of unilateral index finger abduction movements inward decreased the proportion of TMS-evoked index finger abduction movements in the nonpracticing resting hand to an extent proportional to the increase in the practicing hand. Corticospinal excitability changes were consistent with these kinematic observations, showing a decreased excitability of the index finger abduction agonist FDI in the nonpracticing hand. Corticospinal excitability of the index finger adductor muscle (FPI, abduction antagonist) in the nonpracticing hand was not influenced by training, indicating that practicing right index finger abduction to the left did not result in a facilitation of the muscle controlling index finger movements in the same direction (toward the left) in the nonpracticing hand. These results indicate that unimanual hand practice, as implemented in Session 1, induced reciprocal plastic changes in mirror movement representations in both M1s. These changes were characterized by strengthening of the representation of the practiced movement in the contralateral M1 and a proportional weakening of the mirror movement representation in the ipsilateral M1.
In Sessions 2 and 3, we tested the effects of practicing the same index finger movement in another direction in space (abduction upward, Session 2) and another index finger movement (flexion) but in the same direction in space (inward, Session 3) relative to Session 1. The proportion of TMS-evoked index finger abduction movements in the nonpracticing hand decreased only when practice movements mirrored the direction of abduction movements in the nonpracticing hand (abduction inward in Session 1, flexion inward in Session 3) but not when practice movements were performed upward even if they were also abduction (Session 2). Corticospinal excitability changes were consistent with these kinematic observations, showing a decreased excitability of the index finger abduction agonist FDI in the nonpracticing hand only following practice movements that mirrored its action, that is inward (abduction in Session 1, flexion in Session 3) but not following practice of abduction movements toward a different direction in space as in Session 2. Therefore, despite comparable motor training kinematics, stable baseline primary endpoint measures of plasticity in the nonpracticing hand, and comparable training effects on plasticity in the M1 controlling the practicing hand, training in Sessions 2 and 3 elicited fundamentally different changes in the organization of the M1 controlling the nonpracticing hand. Our results on differential plastic changes in the organization of M1 controlling the nonpracticing hand as a function of the 3 practice sessions, cannot be explained then by differential effects of training on M1 plasticity controlling the practicing hand in Sessions 2 and 3.
This is to our knowledge the first report of use-dependent changes in the cortical organization of an M1 that outlasts a period of training of the ipsilateral hand. Of additional interest is the hint that this form of cortical reorganization may be influenced by the kinematic details of the movements practiced by the opposite hand (see Fig. 5). A full exploration of this hypothesis was, however, beyond the goals of our experiment.
Effects of Motor Training on the Organization of the Contralateral M1
We found that practicing abduction index finger movements with the palm down (Session 1) induced an increase in the proportion of TMS-evoked movements falling in the PZ (PZ = PZALL), in the acceleration peak of TMS-evoked abduction movements, and in the amplitude of MEPs recorded from the agonist (MEPFDI), but not from the antagonist (MEPFPI) muscles, consistent with previous reports (Classen et al. 1998; Butefisch et al. 2000; Stefan et al. 2005). This finding is also in tune with our present understanding of the contribution of the contralateral M1 to motor learning and skill acquisition (Shadmehr and Brashers-Krug 1997; Lu and Ashe 2005; Ashe et al. 2006; Matsuzaka et al. 2007). We did not find convincing evidence for specific or differential increases in the proportion of TMS-evoked movements in PZM, PZD, or in PZALL = (PZM + PZD) of the practicing hand in Sessions 2 and 3. Therefore, training as implemented in Sessions 1 and 3, which induced comparable plasticity in the organization of M1 controlling the nonpracticing hand, had substantially different effects on the organization of the M1 controlling the practicing hand. Possible reasons for the lack of changes in the proportion of TMS-evoked movements in PZM, PZD, or PZALL of the practicing hand in Sessions 2 and 3 may include 1) most TMS-evoked movements already fell in the PZs before practice, reducing the sensitivity of this measure to detect additional increases after training in the practicing hand (ceiling effect), 2) training as implemented in these 2 sessions did not influence the proportion of TMS-evoked movements in PZM or PZD of the practicing hand, and 3) the change in hand position from practice "thumb up" (Fig. 1a) to TMS testing "palm down" (Fig. 1b) in Sessions 2 and 3 may have affected the sensitivity of our kinematic and EMG measures. We accepted these limitations because the experiment was designed primarily to secure consistency of motor training kinematics across sessions, a goal that was successfully accomplished (see Table 1), and to assess changes in TMS-evoked responses in the nonpracticing hand, which remained always in the same hand position (palm down, Fig. 1a,b).
Mechanisms of Reorganizational Changes in the M1 Ipsilateral to a Training Hand
Transcallosal interactions between motor regions in the 2 hemispheres do exist (Ferbert et al. 1992; Mochizuki et al. 2004). They link supplementary, primary, and premotor areas between themselves (Gould et al. 1986; Rouiller et al. 1994; Liu et al. 2002; Marconi et al. 2003) and are predominantly (but not exclusively) inhibitory. These interhemispheric connections operate in the process of generation of voluntary movements (Duque, Hummel, et al. 2005; Koch et al. 2006; Duque et al. 2007) and contribute to motor learning as well as to intermanual transfer of acquired motor skills (Perez et al. 2007; Perez et al. forthcoming). Differential excitability changes in both M1s have been suggested before when noted that 1-Hz repetitive TMS applied to one M1, which decreases motor cortical excitability under the stimulating TMS coil, results in a relative increase in excitability in the opposite unstimulated M1 (Gilio et al. 2003; Schambra et al. 2003; Pal et al. 2005). These effects reported in the past suggested that excitability of homologous movement representations in the 2 hemispheres might be to some extent interdependent and that each of them could be affected by excitability or activity-dependent changes in the other (Gilio et al. 2003). Our results now indicate that reciprocal plastic changes in the organization of the 2 M1s occur after unimanual motor training and that some of these changes are kinematically specific, influenced predominantly by the direction of the practiced movement rather than by the muscles involved, a finding consistent with the known role of M1 in motor control and aspects of spatial processing (Shen and Alexander 1997a; Kakei et al. 1999, 2003; Duque, Mazzocchio, et al. 2005).
On the other hand, it is also possible that interhemispheric interactions targeting the M1 ipsilateral to the practicing hand originated from non-M1s like premotor areas, where direction of movements are strongly represented (Shen and Alexander 1997b; Kakei et al. 2001, 2003). The premotor cortex exerts inhibitory influences on the contralateral M1 (Mochizuki et al. 2004) and is a major contributor to hand selection processes during unimanual movements in health (Schluter et al. 1998; Hoshi and Tanji 2000; Rushworth et al. 2003; Cisek and Kalaska 2005; Koch et al. 2006) and disease (Johansen-Berg et al. 2002; Fridman et al. 2004). Note that, despite previous data pointing to the contrary (Classen et al. 1998), we cannot rule out completely that spinal mechanisms, in which symmetry around the midline axis dominates, could have partially contributed to our results. Finally, one methodological consideration in this study is that our subjects were instructed to watch the motor practicing hand to enhance the effects of physical training alone, as commonly done in neurorehabilitative treatments (Stefan et al. 2005; Celnik et al. 2006).
Possible Role of Reciprocal Bihemispheric Plasticity after Unimanual Training
What could be the role of this form of bihemispheric plasticity after unimanual practice? In the healthy central nervous system, it is possible that reciprocal changes in the organization of the motor cortices contralateral and ipsilateral to a practicing hand may facilitate accuracy in learning of skilled unimanual finger movements or of bimanual movements that require differential contributions of the 2 hands (Johansson et al. 2006; Koch et al. 2006) and contribute to optimize movement focality (Davare et al. 2007) or to a more accurate control of newly learned lateralized distal hand motor skills (Swinnen 2002; Serrien et al. 2006), possibly through surround inhibition mechanisms (Sohn and Hallett 2004). From a clinical point of view, our findings may provide some insight into the mechanisms underlying the beneficial effects of newly developed rehabilitative interventions that appear to improve motor function in the paretic hand after brain lesions like stroke. Constraint-induced movement therapy is a rehabilitative treatment that focuses on intensive training of the paretic hand and restraint of movement in the intact hand (Rossini et al. 2003; Wolf et al. 2006). Noninvasive cortical stimulation with TMS or transcranial direct current stimulation (tDCS) that upregulates cortical excitability in the ipsilesional M1 (Hummel et al. 2005; Khedr et al. 2005; Kim et al. 2006) or that downregulates excitability in the contralesional M1 (Mansur et al. 2005; Takeuchi et al. 2005; Hummel and Cohen 2006) also appears to exert beneficial effects. On the other hand, we do not yet know the extent to which this form of reciprocal M1 plasticity after unimanual hand training is task specific. Is it for example as operational when more proximal or less focal arm motor training is implemented (e.g., during bilateral arm training [Whitall et al. 2000]) as when training precise focal distal hand movements (present study)? More work is required to sort out these questions.
In summary, we found that unilateral hand motor practice encodes reciprocal memory traces in both motor cortices, characterized by strengthening of the representation of the practiced movement in the M1 contralateral to the practicing hand and by reduction of the mirror movement representation in the opposite M1. These results highlight the role of interacting bilateral motor networks in motor memory formation and skill acquisition, an issue of relevance in motor control and neurorehabilitation.
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Intramural Research Program of the National Institutes of Health, National Institute of Neurological Disorders and Stroke.
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J.D. is a postdoctoral researcher at the Belgian National Funds for Scientific Research. We thank Richard Ivry and Joseph Classen for their critical comments on an earlier version of this manuscript. Conflict of Interest: None declared.
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