Clinical Neurophysiology 112 (2001) 1419±1435

www.elsevier.com/locate/clinph

Alpha band power changes in unimanual and bimanual sequential
movements, and during motor transitions
M.-P. Deiber a,b,*, R. Caldara a, V. IbanÄez c, C.-A. Hauert aaFaculteÂ de Psychologie et des Sciences de l'Education, UniversiteÂ de GeneÁve, UniMail, Boulevard du Pont d'Arve 40, CH-1211 Geneva 4, SwitzerlandbInstitut National de la SanteÂ et de la Recherche MeÂdicale, CERMEP, Boulevard Pinel 59, 69003 Lyon, FrancecUniteÂ de Neuroimagerie Psychiatrique, Chemin du Petit Bel-Air 2, CH-1225 CheÃne-Bourg, Geneva, Switzerland
Accepted 14 March 2001

Abstract
Objective: To investigate the cortical activation during execution of unimanual and bimanual synchronous and asynchronous ®nger
sequences, as well as during transitions between those sequences.
Methods: Task-related power (TRPow) analysis of multichannel surface EEG was used to examine the regional oscillatory brain activity
in the lower (7.8±9.8 Hz) and upper (10.8±11.8 Hz) alpha band. Unimanual to bimanual, bimanual to unimanual, and unimanual tounimanual transitions, prompted by visual cues, were studied in 10 right handed subjects.
Results: (1) Execution of unimanual and bimanual movements was accompanied by a bilateral activation over the central regions. (2) The
7.8±9.8 Hz TRPow decrease was more prominent for left and bimanual movements, suggesting sensitivity of the lower alpha band to task
dif®culty. (3) No difference in alpha oscillatory activity was found between bimanual synchronous and asynchronous sequences. (4)Transitions between motor sequences were invariably accompanied by a mesioparietal TRPow decrease in the lower alpha band. (5)This mesioparietal activation was contingent to the change of motor program, and could not be accounted for by the change of visualcue, or related attentional processes.
Conclusion: The 7.8±9.8 Hz mesioparietal activation most likely re¯ects a posterior parietal motor command initiating transition between
motor programs. q 2001 Elsevier Science Ireland Ltd. All rights reserved.
Keywords: Bimanual movement; Motor transitions; EEG; Alpha band; Parietal cortex

1. Introduction
Oscillatory brain activity in the alpha band (8±12 Hz) is
reduced or suppressed during preparation and execution of
movements, a phenomenon ®rst described as `blocking' of
the central mu rhythm (Gastaut et al., 1954; Chatrian et al.,1959), and which has been later referred to as `event-related
desynchronization' (ERD) (Pfurtscheller and Aranibar,
1977; Pfurtscheller and Berghold, 1989; Toro et al.,1994). Desynchronized alpha band activity is interpreted
to re¯ect increased cellular excitability in thalamocorticalsystems, whereas synchronized alpha band activity occurring immediately after movement execution, or over remotecortical regions, re¯ects a cortical idling state (Steriade,
1993; Toro et al., 1994; Pfurtscheller et al., 1996;
Pfurtscheller and Lopes da Silva, 1999). The alpha ERD
is often divided into two subcomponents, namely a lower* Corresponding author. Tel.: 141-22-705-9263; fax: 141-22-705-9229.
E-mail address: marie-pierre.deiber@pse.unige.ch (M.-P. Deiber).

frequency (8±10 Hz) and a upper frequency (10±12 Hz)
component. These two components differ in terms of amplitude and topography, the lower alpha component being
smaller and more posterior than the upper component(Toro et al., 1994; Andrew and Pfurtscheller, 1997; Manganotti et al., 1998; Babiloni et al., 1999; Pfurtscheller et al.,2000), but their functional signi®cance remains an object of
debate.
There are few EEG and MEG studies devoted to thecomparison of brain activity underlying unimanual andbimanual movements. Most of them have examined theactivity preceding movement and at initiation of movement,
less often during movement performance. Responses related
to bimanual movements have been generally reported oflarger amplitude than those related to unimanual movements (Kristeva et al., 1990; Lang et al., 1990; Kristeva etal., 1991; Urbano et al., 1998; Cui and Deecke, 1999a). In
addition, there is suggestive evidence that the temporal
aspect of coordination between the two hands in¯uences
the magnitude of movement-related potentials (Lang et1388-2457/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved.
PII: S 1388-245 7(01)00536-3

CLINPH 2000798

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M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

al., 1988; Cui and Deecke, 1999b). Concerning oscillatory
brain activity, execution of unimanual movements areaccompanied by a bilateral alpha ERD over the central
regions, with predominance over the contralateral hemisphere (Toro et al., 1994; Leocani et al., 1997; Gerloff etal., 1998a; Manganotti et al., 1998; Andres et al., 1999;
Babiloni et al., 1999). The alpha ERD associated to bimanual movements has been examined only recently, and is
described as bilaterally distributed with mesial posterior
extension (Andres et al., 1999). However, no systematiccomparison has been performed with unimanual movements.
In the present paper, we investigated changes in the alphaband oscillatory activity during execution of unimanual andbimanual sequences of ®nger movements. We used a taskrelated power approach, which relates to continuous task
performance rather than to a single event (Gerloff et al.,1998a). Our ®rst objective was to evaluate whether thetask-related power changes could differ between unimanualand bimanual sequences. We examined synchronous andasynchronous sequential movements of both hands, inorder to explore the relation between the alpha band oscillatory activity and the temporal mode of bimanual coordination. Our second objective concerned the transition
between unimanual and bimanual motor programs, andvice versa. Production of movement typically consists ofan initiation, followed by an execution phase, and thesephases have speci®c electrophysiological correlates(Deecke et al., 1976; Lang et al., 1988; Gerloff et al.,1998b). Additionally, motor behavior is characterized byconstant transitions between movements, which could
correspond to the (re)organization of cerebrally encoded
motor programs (Kelso, 1984; Buchanan and Kelso,1993). Our purpose was to further specify the pattern ofoscillatory activity associated with transition between
movements, as compared to initiation and execution of
movement, to evaluate whether motor transition appeared
as a simple shift between cerebral activity patterns, orwhether it could imply some speci®c activity, possibly
driven by speci®c brain regions.
Our motor tasks consisted in sequential tapping movements of the index and middle ®ngers, and were instructedby visual cues, which triggered the initiation of movement
as well as the change of motor program. The lower andupper frequency components of the alpha band wereanalyzed separately, in an attempt to investigate further
their functional signi®cance in relation to motor performance and transition between motor programs.

2. Methods
2.1. Subjects
Ten normal volunteers were studied, including 3 men and7 women, with a mean age of 24.4 ^ 3.2 years. All subjects

were right handed as measured by the Edinburgh Inventory(Old®eld, 1971). They all gave their written informed
consent for the study.
2.2. Experimental design
2.2.1. Main experiment
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 ®nger and the
middle ®nger of each hand could be moved to press one
speci®c key. A motor sequence is de®ned by a successive
press of the index ®nger (1) and the middle ®nger (2). There
were 4 different motor sequences: unimanual right: R1±R2(Right); unimanual left: L1, L2 (Left); bimanual synchronous: R1±L1, R2±L2 (Syn); bimanual asynchronous: R1,
L1, R2, L2 (Asyn).
Visual stimuli were presented at a constant viewing
distance of 1.20 m, and consisted of two juxtaposed 1.8
cm gray squares on a black background, ®lled or not by ablack cross to indicate the sequence type (Fig. 1). A trial was
designed as follows: a ®rst stimulus (stimulus A) waspresented on the screen for a duration of 2.75 s, requiring
the repetitive execution of a motor sequence at a regular,
self-paced rate (execution A). A second stimulus (stimulusB) was then presented for 2.75 s, prompting the change toanother motor sequence to be performed repetitively at a
rate similar to the ®rst sequence (execution B). A third
stimulus (stop signal), presented for 0.7 s, required immediate cessation of the movements (Fig. 1). Five motor conditions of 40 trials each were tested, de®ned by the repetitive
execution of a ®rst motor sequence (execution A) and atransition to a second motor sequence, to be also executed
repetitively (execution B): 1. Right-Bimanual, 2. LeftBimanual, 3. Syn-Unimanual, 4. Asyn-Unimanual, 5.
Unimanual-Unimanual. These 5 motor conditions corresponded to 3 types of transitions: unimanual to bimanual(U±B), bimanual to unimanual (B±U), and unimanual tounimanual (U±U). Conditions 1 and 2 belonged to U±Btransitions; in each of these conditions, there was an equal
number of transitions to Syn and Asyn sequences, occurringin random order. Conditions 3 and 4 belonged to B±U transitions; in each of these conditions, there was an equal
number of transitions to Right and Left sequences, occurring in random order. In condition 5 (U±U transitions), trials
starting with right (50%) and left (50%) hand were randomized, and transitions were dictated by the nature of execution A (i.e. if execution A is Right, then execution B is Left,and vice versa). In order to reduce anticipation of stimulus
B, 15% of trials were added at random in every condition, in
which duration of stimulus A or B was either shortened orincreased; these trials were not included in the analysis. A
rest condition was also studied in which the subject had toremain still while looking at the stimuli on the screen, allM.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

1421

Fig. 1. Experimental paradigm. Transition: U±B, unimanual to bimanual; B±U, bimanual to unimanual; U±U, unimanual to unimanual. Condition: 1, Rightbimanual; 2, Left-bimanual; 3, Syn-unimanual; 4, Asyn-unimanual; 5, Unimanual-unimanual. Unimanual movements are indicated by one square ®lled with ablack cross, the side of which codes movement side. Bimanual movements are indicated by two ®lled squares; facing squares code for synchronous movements, shifted squares for asynchronous movements. The motor sequence corresponding to each of the visual patterns is indicated in italics. The rest condition
is not illustrated. Control: control experiment composed of the `Right constant' and `Right mixed' conditions. (*): no change of motor program required. For
conditions 1±4, and both control conditions, stimulus B has a 50% chance to be of one type or another. For condition 5, it is stimulus A which has a 50% chance
to be of one type or another, stimulus B being determined by stimulus A. Bottom: temporal axis corresponding to one trial; numbers in italics correspond to the
11 overlapping epochs of 1024 ms duration used for TRPow calculation. The gray circle represents the stop signal.

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M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

patterns being presented in random order. A session
included two replications of each condition, and was subdivided into two blocks: in the ®rst block, the order of presentation of the conditions was pseudo-randomized between
subjects, and in the second block, this order was reversed(e.g. 3, 5, 1, rest, 4, 2; 2, 4, rest, 1, 5, 3).
2.2.2. Control experiment
A control experiment was designed on 8 new right handed
normal volunteers (3 men, 5 women, mean age: 30.4 ^ 9.4
years) with two objectives: ®rst, to evidence a potential
effect of change of visual stimulus, independently of the
motor response; second, to test the effect of change ofmotor program as opposed to no change. In the two control
conditions, the `Right constant' and the `Right mixed'
conditions, stimulus A was always the right pattern andstimulus B was either the right or the bimanual synchronous
pattern, both presented in the same proportion (Fig. 1).
Temporal parameters were the same as in the main experiment. The ®rst objective was tested by the `Right constant'condition, in which subjects were required to keep doing theright hand sequence, independently of stimulus B pattern.
Thus, the effect of change of visual stimulus could be
isolated, the motor response remaining constant. In order
to avoid complete ignorance of stimulus B, since no change
in motor program was required, the subjects had to countsilently left pattern stimuli occurring in 10% of the cases;
trials corresponding to these patterns were rejected fromanalysis. The second objective was tested by the `Right
mixed' condition, in which the subjects were to follow the
instruction given by stimulus B, i.e. either keep doing theright hand sequence (Right pattern) or change to the bimanual synchronous sequence (Syn pattern). Thus, the effect ofchange of motor program could be tested directly incomparison with the absence of change. Each condition
was performed twice in a row, starting with the `Right
constant' condition, so that the subjects were not yetaware of the motor signi®cance of the Syn pattern presented
at stimulus B, thus avoiding mental association between
pattern and movement.
2.3. Data acquisition
Continuous EEG was recorded from 47 surface electrodes, mounted on a cap (Easy-Cap, FMS, MuÈnchen,
Germany). Data were sampled at 1000 Hz, the lower cutoff
was 0.05 Hz and the upper cutoff 100 Hz (DC ampli®ers andsoftware by NeuroScan Inc., Herndon, Va., USA). Linked
earlobes served as reference. A potential limitation of such areference is that it may induce greater alpha power suppression at most distant electrode sites. Computation of a reference-free derivation could theoretically overcome thislimitation, but on the other hand such derivation tends tounderestimate distributed cortical sources with largely
radial generators, which can be a disadvantage for detecting
activation of mesial and lateral premotor areas. In any case,the use of a subtractive approach to determine task-related
power changes (see below for detailed description) minimized signals common to activation and rest conditions.
Two bipolar EMG channels were recorded from surface
electrodes positioned over the right and left forearm ¯exors(¯exor digitorum communis), each pair of electrodes being
located ~15 cm apart. The high pass ®lter for EMG was set
to 10 Hz. Each visual stimulus as well as each key press
were automatically documented with a marker in the continuous EEG ®le. Temporal occurrence of each key press could
thus be retrieved off-line, for calculation of reaction time tostimulus A (RT), reaction time to stimulus B, designed astransition time (TT), and time elapsed between each
keypress (tapping interval).
2.4. Data analysis: task-related power (Gerloff et al.,1998a)
Since EEG analysis was performed on ongoing motor
performance, a task-related power (TRPow) approach waschosen (Gerloff et al., 1998a), rather than the classical ERD
methodology which usually relates to phasic changes of EEG
signals associated to single, discrete events. In contrast withclassical ERD, TRPow is computed by discrete Fourier transform of ®xed time periods, thus reducing temporal resolution
but allowing high frequency resolution. EEG signals werecorrected for ocular artifacts using a threshold rejection algorithm (Neuroscan Inc.), and digitally ®ltered off-line (1±40Hz, slope 24 dB/octave). For each condition, the trials weredivided into segments of identical duration on which thespectral power of the signal could be calculated: on every
occurrence of stimulus A, the continuous EEG recording was
divided into 11 overlapping epochs of 1024 ms (Fig. 1). Afterremoval of slow drifts by linear trend correction, and baseline
correction using the entire epoch window, the single epochs
were inspected automatically for artifacts, and doublechecked manually. Because whole trials in which artifacts
occurred were excluded, there was an equal number of replications for each epoch. A minimum of 30 artifact-free trials
was obtained per subject for each of the two transition types
within a motor condition, after merging trials from the two
acquisition blocks. Each single epoch was hamming windowed to control for spectral leakage.
For spectral power analysis, a discrete Fourier transformwas computed for each 1024 ms epoch, allowing afrequency resolution of 0.98 Hz. Spectral power was calculated on each epoch for all frequency bins between 1 and 30
Hz, and the power samples per epoch were averaged acrossall trials within each condition. In order to reduce the effects
of inter-subject and inter-electrode variation in absolute
spectral power values, as well as to eliminate power oscillations due to visual stimulation, task-related power at anelectrode x (TRPow x) was obtained by subtracting rest
(Pow x rest) from corresponding activation conditions (Powx activation), according to Eq. (1)TRPowx  Powx activation 2 Powx rest1

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

Therefore, task-related power decreases (activation) are
expressed as negative values while task-related powerincreases are expressed as positive values. A potential
disadvantage of using a rest condition temporally independent from the motor conditions is that differences in arousal
and attention could occur. However, this is mitigated by thefact that these factors ¯uctuate randomly within and acrosstrials during rest, thus limiting consistent effects on TRPow
calculation. For analysis of the lower and upper alphacomponents, the non-overlapping frequency ranges of 7.8±
9.8 and 10.8±11.8 Hz were, respectively, selected. Powerchanges corresponding to each frequency range wereobtained by averaging the power value of the respective
frequency bins.
2.5. Statistical analysis
Subtraction of power values as shown in Eq. (1) may be
used to demonstrate task-related changes of the mean values.
However, the variance of spectral power values is more variable at higher power level, and therefore a transformation of
the power values is necessary prior to further statisticalevaluation. Since logarithmic (log) transformation stabilizes
the variance of spectral power estimates (Halliday et al.,1995), we used this procedure to give approximately constantresidual variance at all levels of the response. Thus, for statistical analysis, Eq. (1) becomes Eq. (2)




2
logTRPowx  log Powx activation 2 log Powx rest
Electrodes of interest were de®ned similarly to the region-ofinterest approach that has been used in neuroimaging techniques such as PET and fMRI. The choice of electrodes was
also guided by recent motor EEG studies of task-related
power and coherence values (Gerloff et al., 1998a; Andres
et al., 1999). Electrodes known to overlie approximately the
primary sensorimotor cortex were grouped into two laterocentral regions-of-interest (ROI), one on each hemisphere:
ROI 1: C3, CP3 (left hemisphere); ROI 2: C4, CP4 (righthemisphere). Electrodes of interest in the midline weregrouped into two mesial ROIs: ROI 3: FCz, Cz (anterior
midline); ROI 4: CPz, Pz (posterior midline, or mesioparietal). ROIs 3 and 4 are likely to overlie approximately the
supplementary motor area (SMA) and the precuneus, respectively (Steinmetz et al., 1989). Position of the 4 ROIs, eachconsisting of two electrodes of interest, is detailed in Fig. 5.
Two separate statistical analysis were performed, the ®rst
one designed to compare the different types of motor
sequences, and the second one aimed at evaluating the
TRPow changes accompanying the transition between the
different motor sequences. In the ®rst analysis, the steadystate execution of the motor sequences was examined within
the time period corresponding to execution A: the mean
logTRPow values for epoch 2 and 4 were computed foreach condition and entered into a two-way repeatedmeasures analysis of variance (ANOVA), with SEQUENCE(Right, Left, Syn, Asyn) and REGION (ROI 1±4) as within-1423

subject factors. In the second analysis, the question was totest whether the transition between motor sequences could
be related to a speci®c pattern of TRPow, that is, distinct
from stable execution as well as initiation of a motor
sequence. Thus, our working hypothesis could be modeledby 4 consecutive motor states, i.e. initiation, execution A,
transition, and execution B. To ®t with this model, and to
reduce the number of statistical variables, a unique time
window was selected to represent each of the motor states,with the constraint that they should not be overlapping. RT
and TT ranges were respectively used to assign the initiationand the transition phase to the appropriate epoch (seeSection 3), and the epochs contiguous to the transition
epoch were selected to represent execution A and B. As aresult, the analyzed time data set was composed of epochs 1,
4, 6, and 8, representing each of the 4 motor states respectively. For each of the 3 transition types (U±B, B±U, and U±U), a two-way repeated measures ANOVA was performedon the logTRPow values, with EPOCH (1, 4, 6, 8) and
REGION (ROI 1±4) as within-subject factors.
For the control experiment, the statistical analysis was
restricted to the time window corresponding to transition(epoch 6; see Section 3). A two-way repeated measures
ANOVA (CHANGE, REGION) was performed on eachcondition: in condition `Right-constant', the effect of changeof visual stimulus was tested, whereas in condition `Right
mixed', the effect of change of motor program was tested.
Statistical analysis was performed separately on the lower(7.8±9.8 Hz) and on the upper alpha band (10.8±11.8 Hz).
Statistical threshold was set at P , 0:01.
3. Results
3.1. Motor performance
Performance is presented in Table 1 for each type of
motor sequence. Tapping interval refers to the mean interval
between the elements of the sequence (intervals R1±R2 forright sequence, L1±L2 for left sequence, R1/L1±R2/L2 forbimanual synchronous sequence), calculated over the ®rst
and second repetitions of the sequence. For bimanual asynchronous sequence, the mean of the 3 within-sequence intervals (R1±L1, L1±R2, R2±L2) is calculated. Since tapping
interval was not different in execution A and B in any of the
motor sequences, the mean tapping value is presented. The
mean tapping interval is of 431 ms, corresponding to amovement rate of 2.3 Hz. It is remarkably constant withineach type of sequence (on average, within-subject variability 10.6%, inter-subject variability 27.7%), and is not significantly different between the types of sequence (one-way
ANOVA, F  3:5).
3.1.1. Conditions 1±4
RT is of 403 ms on average, and is not signi®cantly different between conditions (F  2:2). TT, of 582 ms on average,

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M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

Table 1
Motor performance a
Motor sequenceMotorcondition

RT

CV

ErrorsMotor transition

TT

CV

ErrorsTapping interval

CV

ErrorsRight

1

410 ^ 11728.6

0448 ^ 57
417 ^ 92

12.6
22.1

8
4

443 ^ 121

449 ^ 65376 ^ 96

14.4
25.6

7
4

27.2
10.5

5

5
3

424 ^ 1204410 ^ 123

30.1

5

28.2
9.8
27.3
9.6

6

Asyn

0
2
0
1
3
0
6
9
7
12

28.2
12.4

Syn

12.9
15.2
14.8
13.6
23.7
18.0
18.1
10.0
16.1
10.6

1

5
2

544 ^ 70 b
637 ^ 97 c
478 ^ 71561 ^ 76 d
633 ^ 150 e
470 ^ 84565 ^ 102569 ^ 57
563 ^ 91584 ^ 62431 ^ 122Left

Syn±right
Asyn±right
Left±right
Syn±left
Asyn±left
Right±left
Right±syn
Left±synRight±asyn
Left±asyn

424 ^ 116

19aMotor conditions 1±5 as described in text and Fig. 1. Syn, synchronous; Asyn, asynchronous; RT, reaction time; TT, transition time; CV, coef®cient ofvariation. Values of RT, TT and tapping interval are mean values over 10 subjects ^ standard deviation, in milliseconds. CV values are mean values over 10
subjects, expressed in % (CV  (standard deviation/mean)*100). CV values in italics correspond to mean of within-subject CV (i.e. within-subject variability).
Errors represent the total number of keypress errors over 10 subjects.
b
Repeated-measures ANOVA, contrast analysis: Syn-right vs. Asyn-right, P , 0:01.
c
Asyn-right vs. Left-right, P , 0:001.
dSyn-left vs. Right-left, P , 0:001.
eAsyn-left vs. Right-left, P , 0:001.

is signi®cantly longer than RT within each condition (Right:
F  22:8, P , 0:001; Left: F  17, P , 0:001; Syn:
F  46:2, P , 0:001; Asyn: F  15:3, P , 0:001). This isan expected observation, for the necessity of changing the
motor program at stimulus B without prior knowledge on the
new sequence required. TT does not vary according to thepreceding motor program, except for the right sequence: ittakes longer to switch to a right sequence when the preceding
sequence is bimanual asynchronous than when it is bimanualsynchronous (F  10:5, P , 0:01).
3.1.2. Condition 5
In this condition, in which stimulus A can be either theright or the left pattern, RT is of 448.5 ms on average. RT
values are not signi®cantly different from equivalent values
in conditions 1 and 2, in which stimulus A is ®xed (Right:
F  4:9; Left: F  2:5). TT is of 474 ms on average, anddoes not signi®cantly differ from RT (Right: F  0:9; Left:F  0:9). On the other hand, TT values in condition 5 aresigni®cantly shorter than equivalent values in conditions 1
and 2, in which there is an uncertainty on stimulus B (Right:
F  18:7, P , 0:001; Left: F  16:7, P , 0:001).
There was a very low percentage of keypress errors
during tapping, with a maximal value of 0.4% for bimanual
asynchronous sequence. For the initial keypress, the maximum of errors is of 1.25% for conditions 1±4 (corresponding to condition 4), and of 2% for condition 5. For thetransition keypress, a maximum of 6% errors is attained
when the preceding sequence is bimanual asynchronous.
3.2. Time course of TRPow changes
Fig. 2 illustrates the mean TRPow changes during the 11
time epochs, for the 3 transition types. In the lower alphaband, the TRPow values reach a plateau during epochs 2±4,
after some variability is observed at initiation of the motor
sequences (epoch 1). When the ®rst motor program is unimanual (U±U and U±B transitions), the TRPow values start
decreasing during epoch 5; they reach a minimum at epoch
6 for U±U transitions, but continue to decrease during thebimanual motor program for U±B transitions. When the ®rst
motor program is bimanual (B±U transitions), the TRPow
decrease is of large amplitude, reaching a maximum at
epoch 6; TRPow values start increasing right after epoch6, corresponding to the transition to the unimanual motor
program. It is noteworthy that, before and after transition,
the TRPow decrease in the lower alpha band is of smaller
amplitude in unimanual than in bimanual sequences. In theupper alpha band, the TRPow changes over time are lesspronounced than in the lower alpha band.
Steady state execution of the motor sequences was examined during execution A (mean power value over epochs 2
and 4), after it was veri®ed that the TRPow values withineach motor sequence were not different during execution A
and B (i.e. before and after transition). For the time course
model of transition between motor programs (see Section2), epoch 1 (0±1024 ms) corresponded to motor sequence
initiation, since the RT range across all 5 conditions is of
262±716 ms. Epoch 6, starting 190 ms before stimulus B
and extending up to 834 ms after it, was identi®ed as corresponding to transition, since the time needed to realize thechange of motor program fell between 371 and 770 ms (TTrange). Epochs 4 and 8, contiguous to epoch 6, wereselected to represent execution A and B, respectively.
3.3. Steady state execution of sequences
Figs. 3 and 4 show, in the lower and upper alpha band

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

respectively, the topographic TRPow maps during the 3following phases of movement: initiation of motor sequence(row A); steady state execution of motor sequence (row B);
and transition between motor sequences (row C). MeanlogTRPow values during steady state execution of each
motor sequence are presented in Fig. 5. The results of thetwo-way repeated measures ANOVA (SEQUENCE,REGION) are shown in Table 2.
3.3.1. Lower alpha band (7.8±9.8 Hz)
Fig. 3B shows that the execution of unimanual andbimanual sequences are associated with bilateral TRPow
decrease in the central region (i.e. ROI 1 and 2). For bimanual sequences, the TRPow decrease extends to the mesioparietal region (i.e. ROI 4). The ®rst-order statistical effectof SEQUENCE (Right, Left, Syn, Asyn) is signi®cant

1425(Table 2), and Fig. 5A shows that the logTRPow decrease
for all ROIs is less pronounced in the right than in the othersequences (contrast analysis, Right vs. Left: F  15:7,
P , 0:01, Right vs. Syn: F  16:5, P , 0:01, Right vs.
Asyn: F  8:7, P , 0:01). On the other hand, logTRPow
values are not signi®cantly different between left, synchronous and asynchronous sequences. The factor REGION
discriminates the data in the following way: the logTRPow
decrease is larger in ROI 1 and 2 (laterocentral regions) thanin ROI 3 and 4 (mesial regions). There is no signi®cantinteraction between SEQUENCE and REGION.
3.3.2. Upper alpha band (10.8±11.8 Hz)
Fig. 4B shows that the TRPow decrease is focused overthe sensorimotor region bilaterally (i.e. ROI 1 and 2) with
reduced mesial extension compared to the lower alpha band,
especially concerning bimanual sequences. Statisticalanalysis on logTRPow values does not evidence any significant difference between sequences, but between regions(Fig. 5B, Table 2): as in the lower alpha band, the logTRPow decrease is larger in laterocentral ROI 1 and 2 than inmesial ROI 3 and 4.
3.4. Transition between motor sequences
Time course of mean logTRPow values over epochs 1, 4,
6 and 8 for each transition is shown in Fig. 6. The results ofthe two-way repeated measures ANOVA (EPOCH,REGION) for each transition type are shown in Table 3.

Fig. 2. Time course of task-related power over the 11 epochs of 1024 ms
duration. Corresponding scale in milliseconds is given at bottom, witharrows indicating occurrence of stimulus A (0 ms), stimulus B (2750 ms)
and stop signal (5500 ms). Each data point represents the average of the 8electrodes of interest (C3, CP3, C4, CP4, FCz, Cz, CPz, Pz), over conditions 1 and 2 for U±B transitions, conditions 3 and 4 for B±U transitions,and condition 5 for U±U transitions (n  10). (A) Lower alpha band (7.8±
9.8 Hz); (B) Upper alpha band (10.8±11.8 Hz).

3.4.1. Lower alpha band (7.8±9.8 Hz)
Transition from one sequence to another, whatever theunimanual or bimanual nature of the ®rst and second
sequences, is associated with a mesioparietal TRPow
decrease extending to the right hemisphere (ROI 4 and
ROI 2) (Fig. 3C). The mesio-posterior distribution of
TRPow decrease at transition between motor sequences isdistinct from the bilateral laterocentral distribution of
TRPow decrease during stable execution of unimanualand bimanual sequences (Fig. 3B). For all 3 transition
types, an interaction between EPOCH and REGION isobserved, suggesting that the time effect is different according to the region (Table 3). Fig. 6A reveals that the logTRPow decrease is maximum in epoch 6 for all transition
types; the interactive effect between EPOCH and REGION
is explained by the sharper change of logTRPow values for
ROI 4 and ROI 2 across time. A sharper slope is observed inthese regions between phases of unimanual sequence execution and phases of transition, i.e. between epochs 4 and 6 for
U±B and U±U transitions, and between epochs 6 and 8 for
B±U transitions.
3.4.2. Upper alpha band (10.8±11.8 Hz)
The transition epoch is comparable to epochs of steadystate execution for all types of transitions, although withslightly more pronounced mesiocentral TRPow decrease1426

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

Fig. 3. Grand average of topographic task-related power maps (n  10) in the lower alpha band (7.8±9.8 Hz). The dots indicate electrode positions. TRPow
decreases (area of `activation', negative TRPow values) are coded in blue/green and TRPow increases (positive TRPow values) are coded in red/orange. (A)Initiation of motor sequences, corresponding to epoch 1. Right-1 and Left-1 correspond to condition 5, initiation of right and left hand sequences, respectively.
(B) Steady state execution of motor sequences, corresponding to average of epochs 2 and 4. Maps corresponding to steady state execution of unimanual
sequences for Right-1 and Left-1 are similar to the maps illustrated in row B for Right and Left, respectively, and are not illustrated for simpli®cation. (C)Transition between motor sequences, corresponding to epoch 6. TRPow maps have been averaged over conditions 1 and 2 for U±B transitions, and conditions 3
and 4 for B±U transitions. Note the bilateral distribution of TRPow decrease at initiation and stable execution of the sequences, contrasting with themesioparietal distribution at transition between motor sequences. When stimulus B is predictable, transition between unimanual sequences (row C, U±U)
is also distinct from initiation (row A, Right-1 and Left-1).

Fig. 4. Grand average of topographic task-related power maps (n  10) in the upper alpha band (10.8±11.8 Hz). Same conventions as in Fig. 3. Note that thebilateral distribution of TRPow decrease is predominant during the 3 phases of movement (A±C).

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

1427

3.5. Transition epoch
In the lower alpha band, the passage from one motor
sequence to another is characterized by a mesioparietal
TRPow decrease similar among all transitions, as illustrated
in Fig. 7. A two-way ANOVA (TRANSITION, REGION)
does not show any signi®cant effect of the transition typeupon the logTRPow values in epoch 6 (F  4), only asigni®cant effect of REGION (F  12:7, P , 0:001), withmaximal logTRPow decrease in ROI 2.
3.6. Control experimentFig. 5. Grand average of the logTRPow values (n  10) during steady state
execution of each condition, illustrated for each ROI. (A) Lower alpha band
(7.8±9.8 Hz); (B) Upper alpha band (10.8±11.8 Hz). Electrode placement
and position of the ROIs are indicated on the view of the top of the head.
ROI 1: left central (C3, CP3); ROI 2: right central (C4, CP4); ROI 3:
mesiofrontal (FCz, Cz); ROI 4: mesioparietal (CPz, Pz). Note the enhanced
TRPow decrease for left and bimanual sequences, particularly in the lower
alpha band.

(Fig. 4C). In contrast with the lower alpha band, an interaction between EPOCH and REGION is observed only in
U±B transitions, and refers merely to a sharper effect oftransition in the right laterocentral region (ROI 2; Fig.
6B). In B±U transitions, there are independent effects of
EPOCH and REGION, whereas in U±U transitions, only
REGION has a signi®cant effect (Table 3).
Table 2
Statistical results in steady state sequence execution a

SEQUENCE(Right, Left, Syn, Asyn)REGION (ROI 1±4)
SEQUENCE £ REGIONa

Lower alphaUpper alpha

d.f.

F

P

F

P

3

6.5

*

3.6

NS

3
9

21.8
4.2

**
NS

18.4
4.4

**
NS

Repeated-measures ANOVA on task-related power, log-transformed
data: *P , 0:01; **P , 0:001; NS, non-signi®cant.

Fig. 8 shows the topographic TRPow maps corresponding
to stimulus change and to motor program change, in thelower and upper alpha frequency bands. The map of stimulus change is obtained from the `Right constant' condition
by subtracting the map at epoch 6 when the stimulus
remains constant from the map at epoch 6 when the stimulus
is changing. This difference map does not show any consistent residual TRPow, suggesting that the change of stimulus
has no effect on the TRPow. This observation is con®rmed
by a two-way ANOVA performed on the log TRPow values
in the `Right constant' condition, with STIMULUS
CHANGE (change, no change) and REGION (ROI 1±4)
as within-subject factors: no signi®cant effect is evidenced,
in any of the frequency bands.
The map of motor program change is obtained from the`Right mixed' condition by subtracting the map at epoch 6
when the motor program is not changing from the same map
when the motor program is changing. This difference map
reveals mainly in the lower alpha band a TRPow decrease
centered in the mesioposterior region, similar to the one
observed at transition between motor programs (Figs. 3Cand 7). In the lower alpha band, a two-way ANOVA
performed on the log TRPow values in the `Right mixed'
condition con®rms a signi®cant interaction between
MOTOR CHANGE (change, no change) and REGION(ROI 1±4), which suggests that the effect of change inmotor program is different according to the region (Table4). This effect is signi®cantly more pronounced for ROI 4
compared to ROI 1 and ROI 3, but not compared to ROI 2(contrast analysis, change motor program vs. no change, ROI
1 vs. ROI 4, F  39:9, P , 0:001; ROI 3 vs. ROI 4,
F  13:7, P , 0:01; ROI 2 vs. ROI 4, F  1:74). In theupper alpha band, there is no signi®cant effect of MOTORCHANGE, nor interaction between MOTOR CHANGE and
REGION, but only an effect of REGION (Table 4).
4. Discussion
4.1. Motor performance
Keypress rate was remarkably constant in all sequences,
despite the self-paced mode of tapping. This stability
excludes movement rate as a confounding variable in the1428

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

Fig. 6. Time course of logTRPow values over epochs 1, 4, 6, 8 for each type of transition, illustrated for each ROI (n  10). (A) Lower alpha band (7.8±9.8 Hz);
(B) Upper alpha band (10.8±11.8 Hz). The logTRPow are averaged over conditions 1 and 2 for U±B transitions, and over conditions 3 and 4 for B±U
transitions. Changes of logTRPow over time are prominent in the lower alpha band. Note that in every transition type, logTRPow values are minimal during
epoch 6.

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

1429

Fig. 7. Grand average of topographic task-related power maps (n  10) in the lower alpha band (7.8±9.8 Hz) for each transition during epoch 6. Same
conventions as in Fig. 3. Note the reproducibility of the mesioparietal TRPow decrease for each transition type.

comparison between sequences. The ®nding that reaction
time was not different between unimanual and bimanual
sequences is consistent with recent observation (Foltys etal., 2001). In U±B and B±U transitions, the transition time
between motor sequences exceeded the simple reaction time,
which could result from at least 3 factors: (1) the longer time
Table 3
Statistical results within each transition type a

U±B transitions
EPOCH (1, 4, 6, 8)
REGION (ROI 1±4)
EPOCH £ REGION
B±U transitions
EPOCH (1, 4, 6, 8)
REGION (ROI 1±4)
EPOCH £ REGIONU±U transitions
EPOCH (1, 4, 6, 8)
REGION (ROI 1±4)
EPOCH REGIONa

Lower alphaUpper alpha

d.f.

F

P

F

P

3
3
9

16.9
57.2
5.4

**
**
**

20.9
51.2
4.2

**
**
**

3
3
9

13.9
67.3
7.2

**
**
**

5.2
57.0
2.6

*
**
NS

3
3
9

3.5
25.5
2.5

NS
**
*

1.9
28.4
1.5

NS
**
NS

Same conventions as in Table 2.

Fig. 8. Grand average of topographic task-related power maps (n  10) in
the lower and upper alpha band during epoch 6 for the control experiment.
Maps corresponding to `Stimulus change' are obtained from the `Right
constant' condition, by subtracting trials without change of stimulus B
from trials with change. Maps corresponding to `Motor change' areobtained from the `Right mixed' condition, by subtracting trials withoutchange of motor program from trials with change. Note that the mesioparietal TRPow decrease is associated exclusively to the change of motor
program, and is prominent in the lower alpha band.

1430

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

presumably needed to program a new sequence when the
motor system is already engaged in performance; (2) the
inhibition of the ®rst sequence; and (3) the unpredictabilityof the forthcoming transition signal, which prevented anticipatory behavior. The in¯uence of the third factor appears
particularly strong, since transition time was signi®cantly
shorter in the U±U transitions in which the transition signal
was predictable. The transition time was similar among U±B
and B±U transitions, and remained independent on the motor
sequence preceding the change. Only transitions from asynchronous bimanual movements tended to take more time,
especially those to right sequences. This observation can be
explained by a natural tendency to complete the asynchronous sequences before the change, completion which takes
theoretically twice the time of the other two-element
sequences. Overall, the data suggest that the performanceof unimanual and bimanual sequential movements wascomparable, as were the transitions between those movements. The present observations are compatible with the
behavioral literature (Rosenbaum, 1991).
4.2. TRPow during steady state execution of ®nger
sequences
In both lower and upper alpha bands, TRPow decreases
were prominent over the central, and secondarily, mesial
regions. Differences of TRPow amplitude between central
and mesial regions tended to be larger in the upper alphaband. During execution of unimanual movements, a bilateral TRPow decrease was observed, in accordance withprevious topographic ERD studies (Toro et al., 1994;
Leocani et al., 1997; Crone et al., 1998; Gerloff et al.,1998a; Andres et al., 1999; Babiloni et al., 1999). In EEG
and MEG studies, more data is available in the interval
preceding movement than during movement performance,
but yet bilateral response to unimanual movements is
described (Toro et al., 1993; Salmelin et al., 1995; Urbano
et al., 1998). In contrast, in functional imaging studies of®nger movements, there is a general consensus for sensorimotor cortex (SMC) activation lateralized contralaterally(Deiber et al., 1996; Catalan et al., 1998; JaÈncke et al.,
1998; Toyokura et al., 1999). Some descriptions of ipsilateral SMC activation have been made in relation to movements of the non-preferred hand (Kawashima et al., 1993;
Table 4
Statistical results in epoch 6 of the `Right mixed' condition a

MOTOR CHANGE (yes, no)
REGION (ROI 1±4)
MOTOR CHANGE £REGIONa

Lower alphaUpper alpha

d.f.

F

P

F

P

1
3
3

11.1
4.1
7.0

NS
NS
*

9.8
7.6
4.1

NS
*
NS

Same conventions as in Table 2.

Kim et al., 1993), as well as to the complexity of hand
movements (Rao et al., 1993; Shibasaki et al., 1993; Kawashima et al., 1998). However, the absence of ipsilateral activation in most studies using metabolic measures remains
dif®cult to conciliate with data on oscillatory brain activity.
The neurophysiological mechanisms underlying alpha reactivity remain largely unknown, and a common idea is thatalpha suppression could re¯ect activation of a distributed
cortical network system capable of supporting motor function. Within this function, and because of the connections
existing between neurons of a same functional system, activation of a subset of the network could result in widespreadalpha suppression (Crone et al., 1998; Pfurtscheller et al.,2000), in contrast with the circumscribed foci of activation
detected with neuroimaging techniques. A contralateral
predominance of ERD has generally been described inunimanual movements, although statistical testing was notalways performed (Leocani et al., 1997; Crone et al., 1998;
Gerloff et al., 1998a; Manganotti et al., 1998; Babiloni et al.,1999). Our own data did not evidence statistically signi®cant larger TRPow decrease over the hemisphere contralateral to movements, although a trend was observed in both
lower and upper alpha bands.
During execution of bimanual ®nger sequences, the activation re¯ected by the TRPow decrease reached a maximumof amplitude in the 4 scalp regions examined. In the 7.8±9.8
Hz range, the distribution of the TRPow decrease extended
to the mesial region of the scalp, in accordance with earlier
observation (Andres et al., 1999). However, the amplitudeof the TRPow decrease for bimanual movements remained
higher in the central region. On the other hand, we did notcon®rm neuroimaging ®ndings suggesting a predominant
activation of the left hemisphere in bimanual movements
performed by right handers (JaÈncke et al., 1998; Viviani etal., 1998). Absence of lateralized hemispheric control for
simple bimanual movements has also been reported recently
in a study of reaction time involving transcranial magnetic
stimulation (Foltys et al., 2001).
Comparison between ®nger sequences revealed that in
the lower alpha band, the type of sequence has a signi®canteffect on the TRPow decrease, but that this effect is independent on scalp location: for right sequences, the TRPow
decrease was smaller than for left and bimanual sequences.
In contrast, Toro and co-workers (Toro et al., 1994) did notobserve any difference of the ERD response to movementsof the right and left hand, but they tested single digit movements as opposed to two-digit sequential movements in thepresent study. A possible explanation for the largest TRPow
decrease observed in left and bimanual sequences would bethat for right handed subjects, these sequences are moredemanding than sequences performed with the right hand.
The relative proportion of keypress errors between thesequences tends to support this hypothesis, since righthand sequences constantly counted the smallest number of
errors, at initiation and during execution of movements.
Considering right and left hand movements, this notion of

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

task dif®culty, i.e. requirement of a greater effort to performwith the non-preferred hand, has already been proposed to
explain a tendency for less asymmetric hemispheric
responses to left than right hand movements (Lang et al.,
1990; Kawashima et al., 1993; Kim et al., 1993; Urbano etal., 1998; Stephan et al., 1999). Interpretation of the greater
sensitivity of the lower alpha band to the nature of the motor
sequence can only be speculative. Andrew and Pfurtscheller(1997) have proposed that, although both lower and upperalpha components are probably generated in the underlying
neocortical circuitry, the lower alpha component centeredaround 9 Hz is related to a global generation-mechanism(corticocortical interactions), in contrast with the 12 Hz
component related to local circuitry (intracortical interactions). Lower alpha desynchronization could re¯ect generaltask demands, and be non-task speci®c (Klimesch et al.,
1998; Pfurtscheller et al., 2000). In this context, one could
speculate that an enhanced global mobilization related totask dif®culty could be in¯uencing the extensive connections linking the motor system, resulting in a desynchronization preferentially centered in the lower alpha frequency
band. Our observation on the absence of regional speci®city
in the differences between unimanual and bimanual ®nger
sequences is concordant with most ®ndings from ERD,
movement-related activity, and functional imaging studies.
Concerning the SMC, there is a consensus about the similarity of activation in each hemisphere during bimanualsynchronous movements, activation which is not signi®cantly larger than in the hemisphere contralateral to unimanual movements (Urbano et al., 1998; Andres et al., 1999;
Toyokura et al., 1999; JaÈncke et al., 2000). On the other
hand, mesiofrontal activation is reported either of thesame order (Andres et al., 1999; Toyokura et al., 1999) orlarger in bimanual synchronous than in unimanual movements (Lang et al., 1990; Kristeva et al., 1991; Urbano et al.,
1998; Cui and Deecke, 1999a; JaÈncke et al., 2000).
However, a larger mesiofrontal response in bimanual movements could result from the overlap of the SMA activity ineach hemisphere (Urbano et al., 1998), and thus would notnecessarily indicate that the SMA contributes preferentiallyto bimanual movements.
Overall, the two following ®ndings: (1) that the TRPow
decreases are not only enhanced during bimanual movements, but also during left hand movements; and (2) that
this enhancement does not show any signi®cant regional
speci®city, support the hypothesis that the notion of energy
requirement to perform a task could be more in¯uent on themagnitude of spectral power decrease than the notion ofbimanuality, especially in the lower alpha band. In otherwords, our results in the 7.8±9.8 Hz frequency range suggestthat the TRPow decreases can be larger for bimanual thanfor unimanual movements, but that this effect is consecutive
to enhanced task dif®culty rather than to the bimanual natureof the movements.
Our study addressed another important issue, the comparison between bimanual synchronous and asynchronous

1431

sequences of movements. The synchronous and asynchronous functional modes refer to the phase relationshipbetween the movements of each hand, and can also be
designed as in-phase and anti-phase patterns. These patterns
have been shown to represent the most stable temporal
couplings, with maximal stability for the in-phase mode ofcoordination (Kelso, 1984; Tuller and Kelso, 1989). Few
studies have examined the cerebral correlates of the twomodes of bimanual coordination. Negative cortical DC
shifts have been shown of larger amplitude over the mesiofrontal region during execution of alternate compared tosimultaneous index ®nger movements, but no difference
was found between simultaneous ¯exion-¯exion (inphase) and ¯exion-extension (anti-phase) movements(Lang et al., 1988). In a more recent study, the Bereitschaftpotential has been reported of larger amplitude for spatiallycongruent (i.e. anti-phase) than for anatomically congruent(i.e. in-phase) bimanual index ®nger movements (Cui and
Deecke, 1999b). Similarly, fMRI data suggest that SMC andmesial wall activation is higher during anti-phase ascompared to in-phase ®nger or ®st movements (Stephan etal., 1999; Toyokura et al., 1999). Our own data did notreveal any difference in the alpha oscillatory activity
between the synchronous and asynchronous modes ofcoupling for sequential movements. This negative ®nding
could again be interpreted in relation to the large distributed
cortical network system that may subtend alpha suppression
in relation to motor function (see above), which would not
be sensitive to certain speci®c parameters of movements.
An alternative hypothesis is that the timing commands
generating the different modes of bimanual coupling are
not re¯ected in the alpha frequency band. Lastly, there isa possibility that the sequential nature of our tasks is responsible for speci®c activity patterns that would mask potential
differences related to the mode of temporal coupling
between the two hands.
4.3. TRPow during transition between ®nger sequences
A main purpose of the present study was to analyze thepattern of cerebral oscillatory activity associated withmotor transition, by contrast with motor initiation andexecution. A major ®nding is that the transition between
motor sequences is accompanied by a TRPow decrease
localized over the mesioparietal region in the 7.8±9.8 Hz
frequency band. This topography is distinct from the bilateral distribution present at initiation or stable execution of
motor sequences. We will refer to this phenomenon as
mesioparietal activation, since it is postulated that a
TRPow decrease re¯ects the interruption of synchronizedactivity in functionally related groups of cortical neurons,
which can be seen as a correlate of increased cellular excitability (Pfurtscheller and Lopes da Silva, 1999). In the
10.8±11.8 Hz frequency band, there is a tendency for amesial TRPow decrease at transition, but the distribution
remains essentially bilateral and does not focus over the1432

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435

4.3.1. Visual stimulation
In our paradigm, motor sequences were instructed via
visual signals. Stimulus A instructed the ®rst motor program
until occurrence of the transition signal, stimulus B, which
instructed the second motor program. Thus, the change fromstimulus A to stimulus B was an obliged experimental
feature, and the question can be raised whether the mesioparietal effect at transition could simply result from thischange. It must ®rst be noticed that calculation of TRPow
is based on subtraction of a passive viewing task temporally
matching the motor tasks, which should ensure the cancellation of effects related to the presentation of visual stimuli.
To further test the effect of change of visual stimulus, the`Right constant' condition was designed in which subjects
performed the same motor sequence during the whole trial,
although the visual signal changed at stimulus B in half ofthe cases. The absence of remaining activation whensubtracting trials with no change at stimulus B from trials
with change at stimulus B con®rms that the change of stimulus cannot by itself be responsible for the mesioparietal
activation.
Considering the unpredictability of the transition signal,
there is a possibility that the mesioparietal activation could
relate to the absence of knowledge about the forthcomingtransition. However, we have an argument that invalidates
this hypothesis: the mesioparietal activation was present attransition between unimanual sequences, in which the forthcoming transition stimulus was always predictable. On theother hand, in these U±U transitions the mesioparietal activation was absent at initiation of motor program (stimulusA), and yet the left or right hand side of the ®rst motor
sequence could not be predicted (Figs. 3A and 4A). Consequently, the mesioparietal activation appears independent
on the predictability of the visual signal.

movement but not affected by attention (Nashmi et al.,1994). In relation to the visual modality, a decrease of theparieto-occipital 10 Hz activity has been reported when
visual stimuli are selectively attended (Foxe et al., 1998).
This observation is consistent with the presence of a 10±12
Hz ERD in the occipital region in the period preceding avisual stimulus providing knowledge of results (Bastiaansen
et al., 1999, 2001). Together, these ®ndings suggest that thealpha desynchronization in the parieto-occipital region
could re¯ect active engagement of the visual areas in the
anticipatory period of a relevant visual stimulus.
In our tasks, attention to the transition signal merely
includes a sustained attention component, present when
two patterns are attended with equal probability, and aselective attention component, occurring in case attentional
priority has to be given to one pattern in favor of the other(Coull, 1998). Combined examination of the results frommain and control experiments provide some clues on thepotential role of these attentional processes in the mesioparietal activation reported in our study. In the main experiment, sustained attention is necessarily higher in the motor
tasks as compared to the rest condition, and thus could playsome role in the TRPow changes observed. In the `Right
mixed condition', the two possible patterns for stimulus B
are attended equally. Yet, the mesioparietal activation
survives the subtraction between two types of trials involving the same level of sustained attention, suggesting thatthe activation is independent on the sustained attention
component. On the other hand, the tasks in the main experiment are not supposed to require selective attention, sinceboth possible patterns of stimulus B must be equallyattended: the presence of the mesioparietal activation inthese tasks thus suggests that it is independent on selective
attention. In summary, although a de®nite assessment ofattentional effects remains beyond the scope of the paper,
the present observations support the hypothesis that themesioparietal activation evidenced at transition between
motor programs does not primarily depend on sustained or
selective attentional processes to the instruction stimulus.

4.3.2. Visual attention
The motor responses required in our paradigm wereconditioned by visual stimuli, and thus they were intrinsically dependent on visual attention. We attempt to clarify
this issue in the interpretation of our results, aware of theexistence of data suggesting that the alpha ERD is sensitive
to attention. In a word recognition task, Dujardin and coworkers have observed a widespread desynchronization in
the alpha band with high attentional load, reaching its maximum just before motor response (Dujardin et al., 1993). In
an oddball paradigm with targets and non-targets, slower
alpha frequencies (6.4±10.4 Hz) have been shown to desynchronize in relation to attentional demands such as alertness
and expectancy (Klimesch et al., 1998). However, contradictory ®ndings have been reported, showing that oscillations in the 7±13 Hz frequency range are disrupted by

4.3.3. Motor transitions
The preceding arguments suggest that the mesioparietal
activation is not due to the sensory input instructing thechange, nor to attentional processes related to it. This activation occurs at transition between motor programs, andthus it is likely to be associated with the motor output aspectof the change. Since bimanual execution of movements was
associated with marked increase in mesial activation, an
initial question would be whether the mesioparietal activation could re¯ect a simple transitional state from unimanual
to bimanual motor performance. This hypothesis can bediscarded for two reasons: (1) transitions from bimanual
to unimanual movements also elicited mesioparietal activation, although unimanual movements are characterized by areduced mesial activation compared to bimanual movements; (2) the mesioparietal activation was also elicited atmesioposterior region, as observed in the lower alpha band.
The different hypothesis about the nature of the 7.8±9.8 Hz
mesioparietal activation are discussed in the following
sections.

M.-P. Deiber et al. / Clinical Neurophysiology 112 (2001) 1419±1435transition between unimanual movements. A decisive test toensure that the mesioparietal activation is due to the motor
aspect of the change is to show that it remains when comparing a situation involving a motor change with a situation of
constant motor production. The `Right mixed' condition
tested this hypothesis in contrasting trials requiring a changeof motor sequence with trials in which the initial sequence
had to be pursued. The results clearly showed that themesioparietal activation was present when a transition
occurred as compared to a situation without transition, indicating that it is contemporary to the change of motor
program.
The mesioposterior distribution of the TRPow decrease attransition is compatible with the involvement of the posterior parietal cortex in the change between motor programs.
Our data further suggest that this implication is speci®callyrelated to the transition from one motor plan to another, andnot to motor initiation from a rest state, since in the latter
situation no parietal activity comparable to the one observed
at motor transition was elicited. Recently, a PET study
examining the brain activity related to the change between
in-phase and anti-phase bimanual movements has evidenced
a unique focus of activation in the posterior border of the left
angular gyrus, and has concluded to a key role of the posterior parietal cortex in commanding the change between two
bimanual motor programs (de Jong et al., 1999). The anatomical location found by these authors appears distinct fromthe mesioparietal activation evidenced in the present work,
but procedural differences between the two studies have to
be underlined: ®rst, the group of de Jong et al. (1999)
focused on transitions between bimanual motor programs,
which were not studied here, and second, the anatomical
generators responsible for the lower alpha parietal oscillatory activity cannot be de®nitely resolved by our method.
Nevertheless, both studies demonstrate a role of parietal
cortex in the change between motor programs, and ourown data further suggest that mesioparietal activation
occurs independently on the unimanual or bimanual natureof these programs.
Several animal studies have demonstrated that the posterior parietal cortex is concerned with the intention about the
movement to make (Andersen et al., 1997). In particular, theactivity of the lateral intraparietal area has been shown to bealtered by changes in the monkey's intentions with regard toeye movements, independently of overt behavior (Bracewell et al., 1996). Intention-related signals have been
described in the posterior parietal cortex, that are independent of attentional processes (Snyder et al., 1997). Our data
do suggest that, when the motor system is engaged in task
performance, the posterior parietal cortex might subserve acommand function to change the motor program on external
instruction. The respective contribution of intention andaction in this command function remains beyond the experimental reach. The ®nding of a mesioparietal activation
centered in the lower alpha band, discussed as re¯ecting
corticocortical connections (Andrew and Pfurtscheller,

14331997), is consistent with the representation of the parietal
cortex as a key structure for sensorimotor transformations,through the integration of cognitive functions such as intention, attention, and selection of targets (Andersen et al.,1997). Our data further suggest that the intervention of theparietal cortex in changing the motor program, as re¯ectedby the mesioparietal TRPow decrease in the lower alphaband, concerns more speci®cally the processes related to
motor production rather than visual processing.
Acknowledgements
M.-P. Deiber was supported by the MHV grant no. 3255030.98 from the Swiss National Science Foundation, inthe context of the Plurifaculty Program of Cognitive
Neuroscience, University of Geneva. V. IbanÄez was
supported by the Swiss National Science Foundation,
grant no. 31-59516.99.
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