Cognitive Brain Research 23 (2005) 374 – 386
www.elsevier.com/locate/cogbrainres

Research report

Programming effectors and coordination in bimanual in-phase mirror
finger movements
Marie-Pierre Deiber a,b,*, Vicente Ibañez c, Roberto Caldaraa,
Carine Andrey a, Claude-Alain Hauert aaFaculté de Psychologie et des Sciences de l’Education, Université de Genève, UniMail, Boulevard du Pont d’Arve 40, CH-1211 Geneva 4, SwitzerlandbInstitut National de la Santé et de la Recherche Médicale, CERMEP, Boulevard Pinel 59, 69003 Lyon, FrancecUnité de Neuroimagerie Psychiatrique, Chemin du Petit Bel-Air 2, CH-1225 Chêne-Bourg, Geneva, Switzerland
Accepted 15 November 2004
Available online 21 January 2005

Abstract
We investigated cerebral activation during programming of in-phase symmetric finger movements in a precued response task. Partial
precues provided advance information about either mirror effectors or in-phase coordination of bimanual movements, while full precue
specified both response parameters and neutral precue no movement information. Effects of precueing were assessed on reaction time (RT),contingent negative variation (CNV), and alpha and beta event-related desynchronization (ERD). Information on coordination mode induced
less efficient preparation than information on effectors, as revealed by longer RT, but paradoxically the CNV was found of larger amplitude
for in-phase than for mirror precue. Full and in-phase precues were associated to largest cerebral activation, as reflected by CNV amplitude as
well as beta ERD. It is suggested that with in-phase precueing, abstract programming of coordination and concrete preparation of possible
effectors overlap, engaging more cerebral resources than when symmetric effectors are pre-specified. Alpha ERD underwent regional
modulations dependent on the type of preparation, pointing out the role of the right parietal region in visuomotor transformation with fullmovement programming, and the preferential implication of the dominant hemisphere and medial brain regions in synchronization of both
hand movements. Beta ERD topographical distribution suggested an increased implication of bilateral and medial motor regions in
anticipation to the response signal with incomplete movement preparation.
D 2004 Elsevier B.V. All rights reserved.
Theme: Motor systems and sensorimotor integration
Topic: Cortex
Keywords: Movement preparation; Bimanual; In-phase coordination; Mirror movements; Contingent negative variation; Event-related desynchronization

1. Introduction
In actions involving both hands, coordination refers toisomorphic joint movements (e.g., drumming) and contrasts
with cooperation, where each hand has a specific anddifferentiated role and cooperates with the other to produce
an unique action (e.g., to peel a fruit) [52,54]. Bimanual* Corresponding author. Department of Psychology, University ofGeneva, Room 5166 40 Bd du Pont d’Arve, Geneva, Switzerland. Fax: +41
22 379 92 29.
E-mail address: Marie-Pierre.Deiber@pse.unige.ch (M.-P. Deiber).
0926-6410/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cogbrainres.2004.11.009

coordination has been extensively studied in various
behavioral studies devoted to identify the main constraints
limiting its efficiency [43]. It is now well established that, inthe dynamics of bimanual coordinative mode, two preferred
forms of behavior express, during which either homologous
or antagonist muscle groups are simultaneously concerned[23]: in the former case, the movements are designed as inphase and, in the latter case, as anti-phase. The in-phasepattern proves to be the most natural and stable mode ofcoordination [22], resulting in a strong symmetry tendency
in bimanual movements in humans. Little is known about
the physiological mechanisms for this tendency. It is

M.-P. Deiber et al. / Cognitive Brain Research 23 (2005) 374–386

generally admitted that the in-phase coordination mode is
economical for the motor system since, in this case, a similar
motor command can be addressed to both limbs [4]. More,sensory reafferences tend to be temporally intermingled,
providing a virtually unique perceptive signal of control forthe system [6]. However, the view of a simplified motor
command cannot account for the high efficiency of in-phase
coordination when functionally different effectors areengaged. Another principle may thus apply for which thedirection of movements in the external space (e.g., righthand and foot simultaneously up) constitutes a clear
constraint on coordination, affecting a conceptual rather
than a sensorimotor level. These observations suggest that,in the bimanual coordinative mode, at least two levels must
be distinguished, namely an babstractQ level responsible forthe in-phase/anti-phase aspect of the coordination to be
produced, irrespective of effectors, and a bconcreteQ level atwhich the effectors themselves are specified [43]. The
functional properties of bimanual coordination would result
from the complex interplay of these abstract and concretelevels [6].
There is growing evidence favoring the view thatbimanual coordination is controlled by a widely distributed
cerebral network rather than a single anatomical structure[42]. While brain imaging studies have identified the
cerebral areas involved [17,20,37,40,47,48], recent EEG
studies have provided insights into the functional patterns ofactivity underlying bimanual coupling [1,9,11]. Thesestudies have been mostly based on comparative assessments,
contrasting either bimanual with unimanual movements, orbimanual movements of different effectors and/or complexity. To our knowledge, there is no available data on theneural mechanisms that could subtend the abstract andconcrete aspects of bimanual coordination. We intended toaddress this question by considering the most preferred andstable pattern of bimanual coordination, namely in-phasemirror movements. Because motor execution intermingles
recruitment of definite effectors in specific temporal relationships, we focused on the preparation phase of bimanual
finger movements. By using the movement-precueing technique and manipulating the information provided in the
precue, we intended to disentangle the aspects relative to therecruitment of symmetric effectors from those relative to the
preparation of in-phase coordination.
In warned reaction time (RT) tasks, a sustained negativity
called contingent negative variation (CNV) develops after
the precue during the foreperiod interval [45]. The more
movement dimensions are specified in the precue, the more
RT decreases and CNV amplitude increases [5,21,25,26,49].
Hence, the CNV potential has been proposed to provide anindex of abstract programming at a central level where all
movement parameters can be specified in parallel [25,49,
55]. Additionally, a recent study has shown that effectorspecific activity also contributed to the CNV potential, incase one motor program controls unilateral movements withdifferent effectors [21]. Thus, conveying information on

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both abstract and concrete levels of motor programming, theCNV potential represents a valuable index to characterize
the preparation process preceding bimanual movements.
Based on the results of Jentzsch’s study [21], a particular
CNV pattern could be anticipated in association to theprecueing of in-phase coordination: while effectors are yet
undefined, knowledge of coordination mode (bimanual inphase movements) would induce concrete programming ofpotential effectors in addition to abstract motor preparation,
resulting in large CNV amplitude. Preparation and executionof movements are also accompanied by an event-related
desynchronization (ERD) in alpha and beta EEG frequency
bands in regard to the central region, which has been
associated to sensorimotor cortex activity [24,31,32,46].
While the extended CNV scalp negativity might be thoughtas the summation of post-synaptic activities and thus
represents a depolarization of cortical cells indicating an
increase in their readiness to discharge [14,39], thedesynchronization of cortical rhythms, more focally distributed, is interpreted to reflect increased cellular excitability in functionally related groups of neurons [34,38,41].
Hence, CNV and ERD represent different aspects of neuralactivity, and their coupled analysis is susceptible to givecomplementary insight into the programming of effectors
and coordination mode in preparation of bimanual movements. While a few studies dedicated to motor research haveexamined simultaneously cortical oscillatory activities and
CNV potential [3,15,27,33], none of them have yet
investigated how bimanual movements are prepared.

2. Materials and methods
2.1. Subjects
Twelve normal volunteers participated in the study,
including 4 men and 8 women, with a mean age of 23.4 F
6.5 years. All subjects were right handed as measured by the
Edinburgh Inventory [29]. The protocol was approved by
the Ethical Committee of the University of Geneva, and all
the subjects gave their written informed consent for the
study.
2.2. Experimental paradigm
Subjects sat comfortably in a dimly lit room, with botharms resting on an especially designed table including
adjustable armrests. Their hands were positioned palm down
on a 4-element keypad (designed by NeuroScan Inc.,
Herndon, Va., USA), so that the index finger and the little
finger of each hand could be moved to press one specifickey.
A delayed reaction time paradigm was used. Visual
stimuli were presented at a constant viewing distance of
1.20 m and consisted of four signals arranged in a 3 cm greysquare on a black background. A preparatory stimulus (S1)376

M.-P. Deiber et al. / Cognitive Brain Research 23 (2005) 374–386

characterized by two or four empty signals was presented
for 2000 ms, requiring the subject to prepare his movement.
An imperative stimulus (S2) characterized by four filled
signals was then presented for 2500 ms, prompting the
subject to execute the movement within the corresponding
response window. A varying time interval (1.5–3 s) elapsed
after S2 before a new S1 was presented (Fig. 1). Movements
consisted in brisk bimanual in-phase or anti-phase flexion–
extension of the index and/or little fingers in different
combinations, with maximum flexion leading to keypress. A
single flexion–extension cycle was required for each finger
concerned. Bimanual in-phase movements corresponded toa simultaneous flexion–extension cycle of right and left
fingers, while anti-phase movements presented a 1808 phaselag between the cycle of each finger. Four preparation
conditions were designed, differing by the information
carried out by S1 (Fig. 1): (1) Full: complete movement
information, requiring in-phase keypress with both indexes;(2) Mirror: partial information about the fingers to move,requiring keypress with both indexes; (3) In-phase: partial
information about the mode of coordination of left and rightmovements, requiring in-phase keypress; (4) None: no
movement information (meaningless signal combination).
The four conditions were mixed at random in series of 68
trials. Focusing on the analysis of in-phase mirror movements led to select unique patterns for full as well as forpartial precues, with the mere constraint of equalized
number of response configurations between the mirror andthe in-phase conditions being respected (n = 3). As such,our experimental design had two main advantages: first, thelimited number of possible configurations contributed tokeep the duration of the whole experiment within reasonable
limits; second, the absence of the anti-phase configuration

for the coordination precue avoided the bias of a potential
preselection of the first hand designed, which could
confound the preparation of anti-phase coordination into
the preparation of first side effectors. Subjects had an initialpractice run, until they got familiar with the patterns andtheir associated responses. Six series were then acquired
within a session, giving a total of 102 trials per condition.
Additionally, a baseline condition was designed with thesame temporal structure as the preparation conditions, butusing meaningless S1 and S2 signals: the subjects wererequired to watch the stimuli while staying motionless (Restcondition). Two series of the Rest condition were acquired(totaling 102 trials), at the beginning and at the end of thesession.
2.3. Data acquisition
Continuous EEG was recorded from 61 surface electrodes, mounted on a cap (Quick Cap, NeuroScan Inc.). Data
were sampled at 500 Hz, the upper cutoff was 100 Hz andthe lower cutoff was set to DC (DC amplifiers and software
by NeuroScan Inc., Herndon, Va., USA). Linked earlobes
served as reference. The electrooculogram (EOG) wasrecorded bipolarly from two electrodes placed above andbelow the right eye. Two bipolar EMG channels wererecorded from surface electrodes positioned over the rightand left forearm flexors (flexor digitorum communis),electrodes of each pair being located ~15 cm apart. The
high pass filter for EMG was set to 10 Hz. Visual stimuli aswell as keypresses were automatically documented withmarkers in the continuous EEG file. Reaction time (RT)corresponded to the time interval between S2 and the first
keypress. For each condition, mean RTs were calculated forFig. 1. Visual stimuli for each experimental condition and temporal sequence of events in a single trial. Empty signals for preparation (S1), filled signals for
execution (S2). Left panel signals for left hand, right panel signals for right hand. Circle for index, triangle for little finger; two vertical bars for in-phase
movements; vertical and horizontal bars for anti-phase movements, starting with the side of the vertical bar. There is a single informative S1 configuration foreach of the Full, Mirror, and In-phase conditions: Full: bboth indexes, in-phaseQ; Mirror: bboth indexesQ; In-phase: bin-phaseQ. S1 is uninformative in None.

M.-P. Deiber et al. / Cognitive Brain Research 23 (2005) 374–386

all response patterns, as well as for the in-phase mirror
response pattern alone. Moreover, the RT to each response
pattern was studied in the None condition, in absence ofadvance information.
2.4. Data analysis
EEG signals were corrected for ocular artifacts using athreshold rejection algorithm (Neuroscan Inc.). The total
analysis window was 4500 ms, starting 1500 ms before S1.
The 4500 ms EEG trials were digitally filtered off-line (lowpass filter 70 Hz, slope 24 dB/octave) and a baseline
correction over 1500 ms prior to S1 was computed. The
CNV potential, largely present over the scalp, is a composite
waveform made of sub-cortical as well as cortical radial andtangential contributions [3,28]. For this reason, no procedure of re-referencing or spatial filtering was used on thedata, which could have noticeably attenuated some CNV
components. Moreover, the use of high spatial filtering such
as Laplacian transformation on scalp potential as well as on
spectral power data can result in complex topographicalpatterns particularly hard to interpret, mainly due to thedifficulty in identifying tangential sources [2,19]. For
congruence purposes in parallel analysis of CNV and
ERD, the ERD calculation was also performed on thedataset in common reference. Topographic analysis of eachelectrophysiological index was focused on differential distribution patterns across conditions rather than on anatomical
localization of activity for each condition.
The EEG trials were automatically scanned for contamination by muscular or electrode artifacts, and theremaining trials were inspected visually to control forminor artifacts. A minimum of 60 artifact-free trials wereaveraged per subject and condition, after merging trials
from the 6 series (within-subject-averaged waveforms).
There was an equal number of trials averaged per condition
for each subject. To study electrophysiological indices of
cerebral activity, we opted for a region of interest (ROI)
approach, which focuses on a restricted number of electrodes on the basis of the regional distribution of the indicesunder study.
2.4.1. Contingent negative variation
As previously reported [9], the CNV in preparation forbimanual movements displayed an extended scalp distribution centered medially on the scalp surface (Fig. 4A).
Three ROIs of 3 electrodes each were selected in order toaccount for a possible lateralization of the CNV potential
with the mode of preparation: left hemisphere (F3, C3,
P3), medial region (Fz, Cz, Pz), and right hemisphere (F4,C4, P4). Since the onset latency of the CNV was variable
between subjects and electrodes, and in order to avoid
resulting measurement biases, the CNV amplitude wasassessed during its sustained phase, i.e., between 1000 ms
and 2000 ms after S1. The 1000 ms analysis period wasfurther subdivided into two segments displaying special

377

characteristics: the initial CNV [1000;1500 ms] reached apeak of amplitude in the frontocentral region, while theterminal CNV [1500;2000 ms] declined in amplitude andwas more posteriorly distributed. In each of these 2
segments, the mean amplitude was calculated for eachelectrode of interest, and a logarithmic (log) transformation
was performed in order to obtain a Gaussian distributionof the values. For statistical assessment of CNV amplitude,
a 3-way repeated measures ANOVA was performed onthese mean log amplitude values, with Condition (Full,
Mirror, In-phase, None), Time ([1000;1500 ms],
[1500;2000 ms]), and ROI (Left, Medial, Right) as
within-subject factors.
2.4.2. Event-related desynchronization/synchronization(ERD/ERS)
In a first step, the reactive frequency ranges were
determined on the basis of visual inspection of the grandaverage power spectra in the 2 s delay period [35]:differences between preparation and rest conditions weremost prominent in the alpha (8–12 Hz) and beta ranges (15–25 Hz). In consequence, further analysis of the spectral
power time course was focused on these frequency bands,
using the standard technique of ERD/ERS [34]. The same
trials selected for CNV analysis were used for ERD/ERS
calculations. The signal was digitally filtered in the 8–12 Hz
and 15–25 Hz bands, respectively, using narrow band-passfilters ( 48 dB/octave). The filtered signals were thensquared and averaged across trials. The spectral power
magnitude at each electrode ( P) was normalized byexpressing the change in signal power as a ratio to theaverage power during the baseline (B) using the following
formula: %(ERD/ERS) = ((BP)/B)*100. The 1500-ms
period preceding S1 served as baseline. Thus, positive
percentage values correspond to a decrease in power, or
desynchronization (ERD), negative percentage values to a
power increase, or synchronization (ERS), and null values
to an absence of power variation.
In both alpha and beta bands, an ERD was elicited within
0 and 500 ms after S1 and maintained throughout the delay
period (Figs. 3B and C). ERD amplitude was assessed
during its sustained phase, i.e., between 500 and 2000 ms
after S1.This time window was further subdivided within
three time segments of 500 ms each, with the two last
segments matching those for CNV analysis: [500; 1000 ms],
[1000; 1500 ms], and [1500; 2000 ms]. Alpha and beta
ERDs were more focally distributed over the central, mesial,and parietal regions than the CNV potential (Figs. 5A and
6A). To account for this distribution, six ROIs of 2
electrodes each were selected, overlying brain structuresthat have been shown to be involved in motor preparation ina paradigm similar to the present one [10]: sensorimotor
cortex, parietal cortex, and mesial motor regions. The choiceof the electrodes included in each ROI was also guided byrecent motor EEG studies of task-related power andcoherence values [1,11,16]. Electrodes known to overlie

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Fig. 2. Mean reaction time (RT) and standard deviation to all patterns
confounded and to the in-phase mirror pattern (n = 12). For each measure,the RT is significantly different between conditions.

approximately the primary sensorimotor cortex weregrouped into two central ROIs, one on each hemisphere:
Left Central (LC): C3, CP3; Right Central (RC): C4, CP4.
Likewise, electrodes overlying the parietal region weregrouped into two parietal ROIs, one on each hemisphere:
Left Parietal (LP): P3, PO3; Right Parietal (RP): P4, PO4.
Electrodes in the midline were grouped into two mesial
ROIs: Anterior mesial, overlying the supplementary motor
area (SMA): FCZ, CZ; and Posterior mesial, overlying the
precuneus (PCu): CPZ, PZ. The ERD/ERS values for eachelectrode of interest were averaged within the 6 ROIs. ERD/ERS statistical analysis was performed independently in thealpha and beta frequency bands using 3-way repeatedmeasures ANOVAs with Condition (Full, Mirror, In-phase,None), Time ([500; 1000 ms], [1000; 1500 ms], [1500; 2000ms]), and ROI (LC, RC, LP, RP, SMA, PCu) as withinsubject factors.

3. Results
3.1. Performance
The overall error rate was very low (1.7% of all
responses). Incorrect responses (wrong keypresses) werethe most common (0.83%), followed by response omissions(0.52%) and early responses (0.15%). Most of the errors(48%) occurred in the None condition, 29% in the In-phasecondition, 13% in the Mirror condition, and 10% in the Fullcondition.
Fig. 2 presents the mean RT to all patterns confounded
and to the in-phase mirror pattern, respectively, and resultsfrom statistical analysis are given in Table 1. In both cases,
the Full condition was associated with shortest RTs, and theNone condition with longest RTs. RTs were longer in the Inphase than in the Mirror condition. There was a significant
effect of Condition on RT when confounding all responses
as well as when considering the in-phase mirror response
only (Table 1). Contrast analysis revealed that in both cases,
every condition was significantly different from each other(Table 1). The pattern type has a significant effect on choice
RT, as measured in the None condition (one-way ANOVA,
F(4,44) = 31.61, P b 0.001). Contrast analysis showed thatthe in-phase mirror response pattern was associated tosignificantly shortest RT (721 F 96 ms), while the other two
in-phase response patterns involving asymmetrical fingers
were associated to significantly longest RTs (left index–rightlittle finger: 1170 F 264 ms; right index–left little finger:
1240 F 235 ms). Intermediate RTs were recorded for thetwo anti-phase mirror patterns (both indexes, starting right:
906 F 88 ms, starting left: 951 F 111 ms).
3.2. CNV
For the four preparation conditions, the potential elicitedby S1 consisted in a fast event-related response immediately
following S1 and a slow rising negativity reaching amaximum at around 1000 ms then decreasing before S2
presentation; at S2, a fast response was observed similar tothe one observed after S1 (Fig. 3A). The fast responses
evoked by S1 and S2, also present in the Rest condition,
merely reflect initial processing of the visual stimuli andwill not be considered here. The slow negative potential
developing during the delay period corresponds to thecontingent negative variation (CNV), it was absent in the
Rest condition since no motor preparation was required(Fig. 3A). The CNV was largely distributed over the scalp,with a peak in the central region (Fig. 4A). CNV scalp
distribution was similar across all conditions, but amplitude

Table 1
Reaction time (RT) per condition
Full

RT(1)

RT(2)

417 F 96417 F 96

Main effect for condition(1) F(3, 33) = 96.0**(2) F(3, 33) = 94.4**

MirrorIn-phase

673 F 97890 F 158

598 F 85641 F 88

None

985 F 117

721 F 96

RT in milliseconds F standard deviation.
(1) all patterns confounded; (2) in-phase mirror pattern.
F: one-way repeated measures ANOVA; *P b 0.01; **P b 0.001.

Contrast analysis

F(1)

F(2)

Full vs In-phase
Full vs Mirror
Full vs None
Mirror vs None
In-phase vs Mirror
In-phase vs None

75.3**
105.3**
195.2**
158.2**
30.2**
16.2*

83.1**
83.0**
145.5**
160.2**
12.6*
27.6**

M.-P. Deiber et al. / Cognitive Brain Research 23 (2005) 374–386

379( F(1,11) = 7.98, P b 0.02). The ROI effect resulted from alarger CNV amplitude in the medial leads (Fig. 4C; Contrast
analysis, Medial vs Left hemisphere, F(1,11) = 12.05, P b
0.01; Medial vs Right hemisphere, F(1,11) = 13.75,
P b 0.01).
3.3. Alpha ERD
A weak initial ERS starting before S1 onset was elicitedin all conditions including Rest, rapidly followed by a
sharply increasing ERD in the four preparation conditions
only. The ERD culminated between 500 and 1000 ms, thendecreased (Fig. 3B). It disclosed a centroparietal distribution
and appeared of larger amplitude in the Full and In-phaseconditions, reaching 40% of baseline level (Figs. 3B and
5A). The 3-way ANOVA (Condition, Time, ROI) revealed
no significant main effects, but only an interaction
Condition  ROI ( F(15,165) = 3.52, P b 0.001). Two
features were mainly responsible for this interaction (Fig.
5B): (1) in RP and to a smaller extent in RC, the ERD
tended to be larger in the Full as compared to the In-phasecondition, whereas in all other ROIs it was higher in the Inphase condition; (2) the ERD over SMA was low in theNone condition relative to the other ROIs and conditions.
3.4. Beta ERD

Fig. 3. Grand average electrophysiological signals on Cz electrode for eachcondition (n = 12). S1 corresponds to time 0 and S2 occurs at 2000 ms. (A)Electrical potentials, revealing the slow negative component (CNV); (B)Alpha ERD/ERS (8–12 Hz); (C) Beta ERD/ERS (15–25 Hz).

differences could be observed between conditions essentially in its initial portion. The 3-way ANOVA (Condition,
Time, ROI) revealed significant main effects of Condition( F(3,33) = 7.19, P b 0.001), Time ( F(1,11) = 15.16, P b0.01), and ROI ( F(2,22) = 8.44, P b 0.01), but nointeractions between factors. The Time effect was due tolarger CNV amplitude in the initial as compared to theterminal time period (Fig. 4B). The effect of preparation
mode on CNV amplitude was significant in both time
periods (Fig. 4B). Contrast analysis revealed that during theinitial time period, the CNV amplitude was significantly
larger in the Full condition than in the Mirror and Noneconditions, respectively (Full vs Mirror, F(1,11) = 12.34,
P b 0.01, Full vs None, F(1,11) = 15.21, P b 0.01). The
initial CNV was also larger in the In-phase than in the
Mirror and None conditions (In-phase vs Mirror, F(1,11) =7.96, P b 0.02, In-phase vs None, F(1,11) = 5.13, P b 0.05).
In the terminal time period, only the Full and the Noneconditions significantly differed in terms of CNV amplitude

The four preparation conditions disclosed a sharp beta
ERD increase within 500 ms after S1 onset, comparable tothe alpha ERD increase although of less amplitude (~30% of
baseline level). The beta ERD reached a peak in the 500–
1000 ms time window, followed by a decrease, followed bya second increase culminating after S2 onset (Fig. 3C). Beta
ERD was more centrally distributed as compared to alpha
ERD and appeared of higher amplitude in the Full and Inphase conditions (Figs. 3C and 6A). The 3-way ANOVA(Condition, Time, ROI) revealed significant main effects(Condition, F(3,33) = 4.03, P b 0.02, Time, F(2,22) = 5.09,
P b 0.02, ROI, F(5,55) = 3.73, P b 0.02) as well as aninteraction Condition  Time ( F(6,66) = 3.99, P b 0.01).
The Condition effect pointed out higher ERD values in the
Full and In-phase conditions (Fig. 6B), and contrast analysis
revealed significant differences between Full and Noneconditions ( F(1,11) = 5.03, P b 0.05) and between In-phaseand None conditions ( F(1,11) = 7.85, P b 0.02). The time
curve was U-shaped (Fig. 6B), and contrast analysis
established that the ERD amplitude was significantly
smaller in the second time window (1000–1500 ms) as
compared to the first (500–1000 ms; F(1,11) = 7.65, P b0.02) and last time windows (1500–2000 ms; F(1,11) =11.35, P b 0.01). The ROI effect was issued from thecontrast between large ERD amplitude in the central ROIs
(LC, RC, and SMA) and reduced ERD amplitude in theparietal leads (LP, RP, Pcu; Fig. 6C). The interaction
between Condition and Time was largely explained by the
ERD pattern in the last time window, which showed an

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Fig. 4. CNV results. (A) Grand average topographic potential maps in the delay period between 500 and 2000 ms after S1 (n = 12). Top views of the head, dots
indicate electrode positions. Blue colors for negative, red colors for positive potential values. (B) Mean CNV amplitude (Fstandard error) within each time
window and condition across all ROIs (n = 12). (C) Mean CNV amplitude (Fstandard error) for each ROI across time windows and conditions (n = 12).

increase in all conditions except in the Full condition(Fig. 6B).

4. Discussion
4.1. Experimental remarks
The present study investigated the functional organization associated to the preparation of in-phase mirror movements, analyzing the cerebral activity subtending the
programming of symmetric effectors and in-phase coordination, respectively. While our full precue designed only inphase movements of both indexes, our partial precues
signaled either the two indexes as mirror effectors orinformed on the in-phase synchronization of bimanual
movements. Such a design introduced two principal
limitations to be evaluated. The first one concerns the factthat the in-phase movement of both indexes occurred more
frequently within a run, and an advantage of this responseover the others could be discussed in terms of practice aswell as programming bias. In-phase mirror movements
correspond to a privileged mode of bimanual coordination,and they are largely documented as being more stable, andthus easier, than any other bimanual movements [22,23,42].
We confirmed this fact when analyzing the RT to the fivedifferent response patterns in the None condition andshowing that in-phase movements of both indexes were
the fastest. Therefore, an unavoidable bias was definitely
present in favor of in-phase mirror movements, primarily

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381

Fig. 5. Alpha ERD/ERS results. (A) Grand average topographic alpha ERD/ERS maps (n = 12) from 240 ms to 2000 ms relative to S1. Red colors for ERD,
blue colors for ERS. (B) Mean alpha ERD value (Fstandard error) for each ROI and condition across the three time windows (500–2000 ms; n = 12). LC: Left
Central; RC: Right Central; LP: Left Parietal; RP: Right Parietal; SMA: Supplementary Motor Area; PCu: Precuneus.

due to the inherent properties of bimanual coordination.
Mixing the different precue conditions within a run
contributed to limit a potential strategic bias to prepare forin-phase mirror responses. It is also noticeable that while
responses involving the little fingers were the slowest, andproportionally more abundant in the In-phase than in theNone condition, the RT difference between these two tasks
was in favor of the In-phase condition. Such an observation
supports the dominant influence of precueing over the typeof finger involved in the response. The second limitation
concerns the potential confounding factor relative to theprediction of upcoming response difficulty. Indeed, the inphase and mirror precues pre-specified partly different
responses that were associated to different levels ofdifficulty, as reflected in their RTs in the absence of advance
information (None condition): in-phase movements ofheterogeneous effectors present in the In-phase condition
were slower than anti-phase index movements present in the
Mirror condition. Again, those differences in difficulty level
were unavoidable since they refer to inherent properties ofbimanual movements, characterized by preferential modesof coordination. However, facing these potential design
confounds essentially deriving from bimanual coordination
constraints, it was essential to ensure that the task effects
observed on RT were effectively due to the nature ofadvance information, rather than to any other motor
parameter. To this purpose, we performed an independent
analysis on the responses to the in-phase mirror pattern,which was present in all conditions. This analysis provided
the same results as the analysis involving all responses,
confirming the effect of the preparation mode on RT. We
can thus affirm that in our paradigm, the RT to S2

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Fig. 6. Beta ERD/ERS results. (A) Grand average topographic beta ERD/ERS maps (n = 12) from 240 ms to 2000 ms relative to S1. Red colors for ERD,
blue colors for ERS. (B) Mean beta ERD value (Fstandard error) within each time window and condition across all ROIs (n = 12). (C) Mean beta ERD value(Fstandard error) for each ROI across time windows and conditions (n = 12). LC: Left Central; RC: Right Central; LP: Left Parietal; RP: Right Parietal; SMA:
Supplementary Motor Area; PCu: Precuneus.

imperative stimulus is indeed dominantly affected by themode of preparation, independent of motoric aspects linked
to bimanual coordination, or potential predictions relative tothe difficulty of the upcoming response.
4.2. Mode of preparation and response performance
In accordance with previous literature [5,10,13,49], theshortest RTs were observed when the bimanual movement
was fully specified, i.e., when both the effectors and themode of coordination were predetermined. In addition, ourRT data showed that specifying the effectors was more
efficient on motor performance than the specification of
movement coordination. In other words, as far as homolo-

gous effectors and in-phase coordination are concerned, thecontrol system for bimanual movement appeared to takegreater advantage of a cue specifying the effectors to be
moved than the coordinative mode. These RT results arecompatible with the classical model of motor programming,
which suggests that not all precues are equally beneficial
and that movement effectors are the first parameters to be
specified [36].
4.3. Mode of preparation and cerebral activity
CNV as well as alpha and beta ERD recorded during the
preparation of bimanual movements displayed partially
distinct reactivity to the type of advance information. WhileM.-P. Deiber et al. / Cognitive Brain Research 23 (2005) 374–386

both CNV and beta ERD results suggested that themagnitude of cerebral activation was globally influenced
by the preparation mode, beta ERD underwent additional
temporal modulations that were dependent on the type ofprecue. On the other hand, alpha ERD showed regional
modulations according to the preparation process. Aninteresting finding concerned the large activation associatedto in-phase programming as reflected by CNV and beta ERD
amplitudes, which, in contrast to full preparation, wasassociated to poor motor performance.
4.3.1. CNV
CNV amplitude was sensitive to the preparation type,essentially in its initial 500 ms portion. The CNV potential
has been classically subdivided into an early wave,
supposed to reflect principally orienting reaction to theprecue, and a late wave, which was thought to be associatedto motor preparation as well as other non-motoric processes
such as response signal expectancy [45,50]. However, since
every study uses a specific time interval between the precueand response signal, and differs in terms of task parametersand complexity, there is a tremendous disparity in theliterature about CNV measurements, and no consensus
exists on the exact duration of the underlying processes. A
recent study using a unimanual forewarned RT task has
shown that the initial CNV was accompanied by an earlylateralized alpha ERD over the sensorimotor cortex,
suggesting that task-specific motor processing alreadyoccurs in relation to the warning stimulus [3]. This result
is supported by our own findings of alpha and beta ERD
arising early after S1. More importantly, it is compatible
with our observation that the initial CNV is significantly
influenced by the precue as a tool for motor preparation. The
precueing effects observed on initial CNV remained onterminal CNV, but of less amplitude. Thus, the present datasuggest that the programming of effectors and coordination
for bimanual movements would take place relatively early
during the instructed foreperiod and that a trace of it would
remain until the occurrence of the response signal. The
antero-posterior topographic differences between the initial
and terminal CNV likely result from the interplay of distinct
functional processes during the preparatory period: frontal,
mesiofrontal, and central activity related to orienting
attention, selection of motor action, and task-specific motor
processing would contribute to the frontocentral distributionof the initial CNV, while central muscle programming aswell as posterior sensory-related anticipatory attention forS2 would participate to the centroparietal topography of theterminal CNV [9,18].
The CNV was of similar amplitude in the Full and Inphase conditions, a result that contrasts with the performance data, RT in the In-phase condition being much longer
than in the Full condition. Earlier reports have generally
demonstrated a positive correlation between CNV amplitude and informative content of the preparatory stimulus, inparallel with a negative correlation between CNV ampli-

383

tude and RT [5,13,15,26,49,51]. However, Jentzsch andcolleagues also found that CNV amplitude with partial
precue specifying side only was similar than with fullprecue specifying effectors and side, and they suggested
that a motor program could be activated at an abstract leveleven when the effectors were not known [21]. Theyreached the conclusion that a common motor program
could control for the movements of functionally close
effectors and that when side only is known, both abstractand effector preparation could occur. Such parallel process
would account for comparable or even larger CNV
amplitude in the partial condition specifying side comparedto the full precue condition, due to overlapping abstract andeffector-specific motor activity. Our own findings arecompatible with this view, suggesting that when only thein-phase mode of coordination is known for finger movements, abstract programming and concrete effector preparation are taking place, resulting in large CNV amplitude.
Additionally, the fact that CNV amplitude was larger in the
In-phase than in the Mirror condition further supports the
hypothesis that overlapping programming processes are
involved in the former condition vs single preparation ofsymmetric effectors in the latter condition. Concerning
CNV topographic distribution, our data revealed a peak of
amplitude over the midline, in accordance with a previous
study on bimanual finger sequence preparation [9]. Such
localization is compatible with the implication of bilateraland medial motor structures in bimanual programming[42], regions that are also implicated in CNV generation[3,28].
4.3.2. Alpha ERD
In spite of a tendency for higher alpha ERD in Full and
In-phase conditions, our data did not evidence main effects
of preparation mode on alpha activity. Accordingly, we had
previously reported the lack of sensitivity of alpha activity
to the precise nature of the task by showing that alphaoscillations did not differentiate bimanual in-phase andanti-phase sequences [11]. In confirmation of a recent study[15], the absence of global alpha reactivity to specific task
attribute contrasts with the reactivity of the CNV potential,
which would better reflect the relevance of task parameters
to action performance. However, the present results
evidenced differences in alpha scalp topography according
to advance information, suggesting that modulations ofalpha oscillations indeed occur during bimanual motor
preparation and that they reflect regionally specificprocesses. These differences concerned the Full and Inphase conditions on one hand and the None condition on
the other hand. Regions of interest located in the righthemisphere, more specifically parietal regions, showedcomparatively higher alpha activity in the Full than in
the In-phase condition, while left hemispheric and medial
regions presented the converse pattern. A right parietal
activation in the Full condition would be concordant withthe completion of the stimulus-response transformation

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M.-P. Deiber et al. / Cognitive Brain Research 23 (2005) 374–386

process in the presence of all movement parameters [7,51].
Programming in-phase coordination engaged predominantly left hemispheric and medial regions, an observationcompatible with the hypothesis that temporal coupling inbimanual movements may depend on a pace-keeping
mechanism preferentially controlled by the dominanthemisphere [12,53]. Enhanced activity observed for electrodes located over the midline structures additionallysuggests the participation of higher-level sensorimotor
areas (SMA, cingulate motor area, precuneus) in programming in-phase coordination. This observation is consistent
with dipole source analysis [21,25] and further supports the
hypothesis that medial activity could be related to abstract
motor programming. The None condition contrasted withthe others essentially for the reduced activation in electrodes overlying the SMA region, as reflected by low alpha
ERD. This observation in a condition where no specificmotor preparation of any sort could be achieved isconsistent with the commonly accepted role of SMA bothin preparation of movements [30] and bimanual coordination [43].

with the view that in bimanual coordination, a concrete levelthat concerns the effectors can be distinguished from an
abstract level relative to the coordinative mode, the former
being associated with faster response execution than thelatter. A remarkable observation concerns the fact that
cerebral activity associated to bimanual motor programming
could not be simply predicted by the behavioral results. Ourdata showed that programming in-phase coordinationnecessitated more cerebral resources than programming
symmetric effectors. In line with a previous motor preparation study contrasting partial programming of side withcomplete, side and effector preparation [21], these findings
support the hypothesis that intermingled brain processes
could be engaged in the preparation phase for bimanual
movements. In particular, large cerebral activity associatedto in-phase coordination programming could result from theoverlap of abstract motor programming, putatively locatedin the higher-level mesial motor regions, and effectorspecific activity originating in corresponding primary motor
areas.

4.3.3. Beta ERD
The beta oscillations underwent modulations in the 2-s
preparation period, which is compatible with their fast
rebound properties [8]. In the last period of the delay, within
500 ms before S2, the beta ERD tended to accentuate in allconditions except in Full. The resulting interaction between
time and condition factors could reveal a special sensitivityof beta oscillations to expectancy, since the system had to
get ready for processing additional information at S2 in the
Mirror, In-phase, and None conditions. The overall larger
beta ERD with in-phase and full precues paralleled the resulton CNV amplitude, suggesting more intense cerebral
activation during these information conditions. However,beta ERD appeared less sensitive than initial CNV, since it
differentiated only the Full and In-phase conditions from theNone condition. This is consistent with the view that beta
oscillations correspond to large-scale processes tuning
cortical excitability and that finer task-dependent modulations occur at cortical level, as revealed by modifications of
post-synaptic responses reflected in the CNV. Bilateralcentral and mesiofrontal distribution of beta ERD is
compatible with the activation of the motor-related neuronal
network [44].

Acknowledgment

4.4. Towards an understanding of bimanual motor
preparation
In-phase mirror movements represent a privileged modeof bimanual coordination, as confirmed by the present
performance results for various combinations of coordinative modes and hand effectors. Focusing on motor programming, we have shown that performance gained morewhen the precue specified mirror effectors rather than thein-phase coordinative mode. Such finding is compatible

This work was supported by the Swiss National Science
Foundation, grant no. 31-59516.99.

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