Exp Brain Res (2004) 159: 389–399
DOI 10.1007/s00221-004-2101-0

RESEARCH NOTES

Roberto Caldara . Marie-Pierre Deiber .
Carine Andrey . Christoph M. Michel . Gregor Thut .
Claude-Alain Hauert

Actual and mental motor preparation and execution:a spatiotemporal ERP study
Received: 11 February 2004 / Accepted: 17 August 2004 / Published online: 12 October 2004
# Springer-Verlag 2004

Abstract Studies evaluating the role of the executive
motor system in motor imagery came to a generalagreement in favour of the activation of the primary
motor area (M1) during imagery, although in reduced
proportion as compared to motor execution. It is still
unclear whether this difference occurs within the preparation period or the execution period of the movement, or
both. In the present study, EEG was used to investigate
separately the preparation and the execution periods ofovert and covert movements in adults. We designed aparadigm that randomly mixed actual and kinaesthetic
imagined trials of an externally paced sequence of finger
key presses. Sixty channel event-related potentials wererecorded to capture the cerebral activations underlying the
preparation for motor execution and motor imagery, as
well as cerebral activations implied in motor execution andmotor imagery. Classical waveform analysis was combined with data-driven spatiotemporal segmentation analysis. In addition, a LAURA source localization algorithm
was applied to functionally define brain related motor
areas. Our results showed first that the difference between
actual and mental motor acts takes place at the late stage ofthe preparation period and consists of a quantitative
This study was supported by a grant (1114–56777.99) from the
Swiss National Science Foundation and by the Programme Commun
de Recherche en Génie Biomédical 1999–2002
R. Caldara . M.-P. Deiber . C. Andrey . C.-A. Hauert
Faculty of Psychology and Educational Sciences, University ofGeneva,Geneva, Switzerland
C. M. Michel . G. Thut
Functional Brain Mapping, Department of Neurology,
University Hospital of Geneva,
Geneva, Switzerland
C.-A. Hauert (*)
FPSE,
40 Bd. du Pont d’Arve,
UNIMail, 1211 Geneva 4, Switzerlande-mail: claude-alain.hauert@pse.unige.ch
Tel.: +41-22-3799261
Fax: +41-22-3799229

modulation of the activity of common structures in M1.
Second, they showed that primary motor structures are
involved to the same extent in the actual or imagined
execution of a motor act. These findings reinforce andrefine the functional equivalence hypothesis between
actual and imagined motor acts.
Keywords Motor preparation . Motor execution .
Imagined or actual response . Negative contingent
variation . EEG segmentation

Introduction
The traditional information processing view of motor
behaviour (Theios 1975) distinguishes between preparatory and executive processes in the sequence of operations
leading to a motor response. Studies on monkeys related
motor preparation to the primary motor area (M1) (Ashe etal. 1993; Georgopoulos et al. 1989; Tanji and Evarts
1976). In human subjects, several studies (EEG or MEG:
Deecke 1987; Deecke et al. 1969; event-related desynchronization in the EEG alpha band: Pfurtscheller 1989;
implanted subdural recordings: Neshige et al. 1988)
showed that M1 exhibits a contralateral maximal activity
in the late preparatory period of a self-paced motor
response. This activity follows an important, bilateral
activity of the supplementary motor area (SMA). Deiber etal. (1996), in a positron emission tomography study,
compared different conditions of motor preparation using
a pre-cued reaction time paradigm. They identified a
network of cerebral structures involved in the motor
preparatory processes including the contralateral frontal
and parietal cortices, ipsilateral cerebellum, thalamus andcontralateral basal ganglia. Of particular interest, the sameneuronal network also turned out to be characteristic of
motor execution (Catalan et al. 1998). All these findings
are from overt movement studies.
One of the most challenging topics, in the motor field of
the cognitive neuroscience domain, is the investigation of
the mental imagination of a motor act, namely the mental

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representation of a self performing motor act without overt
movement. Several studies tried to evaluate the role of theexecutive motor system, in particular M1, in motor
imagery (MIm). The earliest positron emission tomography work on this issue failed to show an activation of thisarea during MIm (Decéty et al. 1988; Roland et al. 1980).
However, these studies suffered from a clear procedural
limitation due to the fact that the term ‘motor imagery’
was used as equivalent to visual imagination of a motor
act. As noted by Jeannerod (1994), MIm is not equivalent
to visual imagination, this latter process implying a visual
mental image and not a kinaesthetic internal representationof the action. Sensu stricto, MIm should definitely beunderstood as the “(...) conscious mental rehearsal of a
motor act without performing any overt movement [andimplying] that the subject feels himself executing a given
action” (Schnitzler et al. 1997, p. 201).
In this framework, recent functional neuroimaging
studies showed an activation of M1 during MIm by
instructing subjects to practice kinaesthetic mental imagery (EEG: Beisteiner et al. 1995; Lang et al. 1996; Naito
and Matsumura 1994; Pfurtscheller and Neuper 1997;
MEG: Lang et al. 1996; Schnitzler et al. 1997; fMRI:
Lotze et al. 1999; Porro et al. 1996, 2000; Roth et al. 1996;
PET: Lang et al. 1994). In some of these studies (Lotze etal. 1999; Porro et al. 1996, 2000), this activation was
weaker compared to the motor execution (MEx) condition.
To date, there is a general agreement (resulting in the‘functional equivalence hypothesis’, Jeannerod 2001) infavour of the activation of M1 during MIm, although in
reduced proportion as compared to MEx.
However, it is still unclear whether the difference ofactivation intensity in M1 between actual or imagined
motor acts occurs within the preparation period or the
execution period, or both. The aim of the present eventrelated potential (ERP) study is to examine this issue.
Indeed, the ERP technique provides a direct measure ofcognitive processes as they occur and thus represents a
powerful technique with which to track the time course of
the functional processes involved. A first set of ERP data
has been provided on this question by Cunnington et al.
(1996), who studied motor cerebral potentials associatedwith the preparation of an actual or imagined externallypaced motor sequence. The level of cerebral activity(maximal peak amplitude of the late potential preceding
the response—contingent negative variation, CNV) appeared higher in the preparation of an actual motor
response than in the preparation for imagining movement.
The authors concluded that the two types of motor
responses involve similar preparatory processes, most
likely related to the SMA. More recently, Jankelowitz andColebatch (2002) provided comparable results and interpretation. Altogether, these data suggest the involvementof similar processes between the preparation for motor
execution and for motor imagination. However, bothparadigms used in these ERP studies systematicallyinvolved several repetitions of the same type of response
within a time period (fixed-blocks design), which does notfavour a careful and vivid mental motor realization. In thesame way, no attempt was made in order to control thesubject’s engagement in this covert activity. In addition,these studies focused only on the preparation period, and,hence, they cannot provide a direct measure of the cerebral
activity underlying the execution processes of covert andovert movements, and its relationship with the preparation
period. As a consequence, the question of the origin of theweaker activity observed for M1 in the MIm compared to
the MEx condition cannot be elucidated by these results.
To avoid these methodological limitations, the temporal
resolution of the EEG was used here to investigate
separately the preparation and the execution periods ofovert or covert finger movements in a paradigm randomly
mixing actual and kinaesthetic imagined trials. During theMIm condition, some intermittent verification trials wererandomly introduced in order to control that imagined
finger movement sequences were correctly realized. The
task consisted of the production of a simple externallypaced sequence of key presses involving three fingers ofthe dominant hand. Before each sequence, a visual signal
instructed the subjects on the way, actual or imagined, theymust produce their response. It then became possible tocompare, in the same experimental design, the cerebral
activations implied in the preparation to motor execution(P-MEx) and the preparation to motor imagery (P-MIm),as well as to compare the cerebral activations implied in
motor execution (MEx) and motor imagery (MIm).

Methods
Subjects
Ten healthy, right-handed subjects (five men, aged 21–43
years, laterality index >0.6 according to Bryden’s questionnaire, 1977), students or research assistants, were
tested. They did not present any history of neurological or
psychiatric disease and took no medication at the time of
the recording. All subjects gave their written informed
consent and the protocol was approved by the ethics
committee of the University of Geneva (Faculty ofPsychology and Educational Sciences).
Stimuli and procedure
Subjects were seated in front of a 17” screen located at a
distance of 1.20 m. The task consisted of a sequence offinger key presses. The subject’s right forearm was placedon an inclined plane (the side of the hand on the upside) to
avoid tactile stimulations that could occur during the
motor imagery task from the contact of the fingers with the
response keys placed at the front.
For the MIm condition, subjects were instructed toimagine fingers’ movements in a kinaesthetic way, namely
to try to feel the sensations that are usually felt in themuscle-tendon complexes when actually executing the
movement. Just before the experimental session, theparticipants were extensively trained to make a kinaes-

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thetic image of the activity of their fingers in an imagined
thumb-fingers opposition repeated sequence.
1.

Preparation to execution (P-MEx)/Preparation to
imagination (P-MIm): A preparation period of 1 s wasdetermined by the interval between the onset of avisual preparatory stimulus (PS) and the onset of thefirst auditory stimulus (S1), which acts as a go signal
for the execution/imagination period (Fig. 1). The PSof 900 ms specifying the mode of realization of the
movement (circle = execution; triangle = imagination)
were presented at random. The figures were displayedin white on a black background with an approximate
size of 1° of visual angle.
2. Motor execution (MEx)/Motor imagination (MIm):
The task consisted of the execution/imagination of a
fixed sequence of finger presses on response keys. The
movements (right hand forefinger—middle finger—
third finger) were externally paced by an auditory
signal (500 Hz, 80 dB, 500 ms in duration) with aninterstimulus interval (ISI) that randomly varied
between 1.6 and 2 s; the sequence was repeatedthree times in a row during a trial (see Fig. 1). A
central cross was present during the entire experiment;
subjects were instructed to continuously fixate thiscross. To maximize attention and compliance with theimagery task, we elaborated verification trials inwhich subjects, after having heard a sound of a higher
frequency (1,000 Hz, 80 dB, 500 ms in duration) that
could appear at any point of the finger sequence, had
to start to actually execute the movement with thefinger following the one they just used for the imagery
task. Reaction times in the motor execution period
were recorded through a response box with three keys
(Neuroscan Inc., Herndon, VA, USA).
Some training trials (3 MEx trials, 1 MIm trial, and 2
MIm verification trials presented in random order) werefirst administered to the subjects. This training session
helped them to become familiar with the task requirements

Fig. 1 Experimental paradigm (P-MEx preparation to execution, PMim preparation to imagination, PS preparatory signal, Sn auditory
stimuli, ISI interstimuli interval, randomly varying between 1,600

and to learn the association between the preparatory
stimulus and the corresponding mode of realization of the
movement.
The experiment comprised 80 MEx and 80 MIm trials,
as well as 20 MIm verification trials that were not includedin the analysis. The events were equally and randomly
distributed within five blocks (16 MEx, 16 MIm and 4
MIm verification trials; total number of events = 36 perblock). The order of the blocks was counterbalanced
between subjects to control for order effects.
EEG recordings and analysis
EEG was continuously recorded over experimental blocks
from 60 Ag/AgCl electrodes mounted on an elastic cap(Easy-Cap, FMS, Munich, Germany) according to therevision of the 10/20 system (American Electroencephalographic Society 1991). The data were digitized at asampling rate of 1,000 Hz and the band-pass filtering was
fixed to 0.15–70 Hz; the impedance of all electrodes was
kept under 5 kΩ. Linked ear-lobe electrodes served as
reference. A bipolar EOG monitored vertical eye movements. The Neuroscan software (Neuroscan Inc., Herndon,
VA, USA) was used for the recording and analysis of theEEG data. EEG signals were corrected for ocular artefacts
using an algorithm implemented in the software. They
were baseline corrected and band-pass filtered (1–30 Hz,
24 dB/oct). Sweeps with an amplitude exceeding ±40 μV
in any of the scalp channels were eliminated (on average,
approximately 65% of the trials, corresponding to 52
events per condition, were kept for each subject). For theremaining valid trials, and for each subject, the recorded
electrophysiological signal was separately averaged acrossall the electrodes and experimental conditions, resulting inindividual ERPs for each condition. Then, ERP signals
were averaged across all the subjects in their respective
conditions, resulting in the grand mean average percondition. For each grand-mean average separately, theand 2,000 ms). For the MEx and MIm periods, the analyses wererestricted to S4 (forefinger response, see text)

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average reference signal was calculated and obtained offline by the sum of the activity in all recorded channels
divided by the number of channels. Finally, grand-mean
ERPs were rescaled against their respective averagereference signal.
Due to the experimental design, separate analyses wereconducted for the preparation and the execution period.
Preparation period
Data were subjected to two independent analysis procedures consisting of: (1) ERP waveform analysis and (2)analysis of ERP map topography.
First, EEG data were analysed by a waveform analysisof the contingent negative variation (CNV), traditionallyassociated with the processes preceding motor execution
in delayed reaction time paradigm (Walter et al. 1964).
Separate ERPs time-locked to the PS onset were obtainedfor P-MEx and P-MIm over the 1-s preparation period; thebaseline correction was set to the 200-ms interval before
PS onset. The waveform analysis of the CNV wasconducted in both conditions with a repeated measures
ANOVA on the mean amplitude of the last 500 ms onelectrodes C3, CZ, C4 (where maximum CNV signals arerecorded, cf. Bonnet et al. 1998; Jankelowitz andColebatch 2002).
Second, ERP map topographies were subjected tospatiotemporal analysis (Lehmann and Skrandeis 1980;
Pascual-Marqui et al. 1995), aiming to objectively definestable surface ERP topographies (segments of stable maps)
and the time intervals where they rapidly change from one
stable configuration into another (segment borders). This
procedure is based on findings showing that surface ERP
map topographies are not randomly distributed over time,
but are rather composed of a sequence of dominant stablescalp topographies (segments) (Lehmann 1987; Michel etal. 1999a), each presumably reflecting different functional
stages of information processing at the brain level, the socalled functional microstates (Brandeis and Lehmann
1986; Lehmann and Skrandeis 1980; Michel et al. 1992;
Pascual-Marqui et al. 1995). The map strength over time isrepresented by the global field power (GFP), which is thespatial standard deviation of the average reference maps’potential distribution, which is mathematically expressedas:vﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃuPunu ðxi  x Þt
GFP ¼ i¼1nwhere x̄ represents the average reference, which is

n
P

xi n

i¼1

where n = number of electrodes, x i = voltage at electrodei.
The GFP values are sequentially calculated for eachtime point over time, and usually periods of map stability

are represented by high values of the GFP. Spatiotemporalsegment maps were defined in the grand-mean ERP map
series using a clustering procedure that statisticallydetermines, with a cross-validation criterion, the optimal
number of segments and their respective time ofoccurrence (Pascual-Marqui et al. 1995). This technique
has been successfully applied to the study of cognitive(Caldara et al. 2003, 2004; Khateb et al. 2000; Michel etal. 1999b; Pegna et al. 1997), sensory (Ducommun et al.
2002) and motor processes (Thut et al. 1999, 2000). This
analysis was carried out for P-MEx and P-MIm separately(Cartool software, D. Brunet, HUG, Geneva, Switzerland).
Third, to search for specific processes, i.e. processes
present or dominant in one condition when compared tothe other, segments of both conditions P-MEx and P-MIm
were compared by means of a fitting procedure applied toindividual data. This procedure consisted of calculating foreach subject the spatial correlation coefficients between agiven segment map and the successive ERP maps of eachcondition in the corresponding time intervals (e.g.
Ducommun et al. 2002; for a review, see Michel et al.
2001). This analysis was conducted in order to assess how
well a given segment map explains a given condition. The
goodness of fit, i.e. the variance explained by this segment
in a defined condition, was expressed by the percentage ofglobal explained variance index (GeV). This parameter
equals the sum of the explained variances over the time
windows of interest, weighted by the strength of the map
at each moment in time. In order to identify which maps, ifany, distinguish the two conditions, a repeated measures
ANOVA with factors Conditions and Maps was calculatedon the GeV data. A significant statistical difference would
indicate that one condition is significantly better explainedby one given map than another; as a consequence, thismap is specific to this condition.
Execution period
Stimulus-locked waveform analyses are not appropriate toinvestigate MEx and MIm conditions. Indeed, any
possible difference of amplitude between motor execution
and motor imagery could be due to a “latency jitter” inmotor-related components (Picton et al. 2000) caused by
the fact that people are responding at different times.
When response is overt, this effect can be neutralized byusing a response-locked waveform analysis, analysis that
permits the identification of the best motor activations
related to the execution of the movement. Then, for this
precise stage of motor execution, a time window and its
related electrical scalp topography can be objectively
defined by means of the spatiotemporal segmentation
analysis. The map topography occurring in this particulartime period in motor execution, encapsulating the motor
potential, will be referred to as the motor map throughout
the article. Finally, by using the fitting procedure, it
became possible to assess to what extent this motor map isrepresented in MEx and MIm conditions respectively.

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Concretely, the analysis for the execution period
consisted first of a segmentation analysis based on the
response-locked signal of the MEx condition. Given its
high temporal stability (see “Behavioural results”), movement-related potentials were averaged on the fourth
response (S4 in Fig. 1). The period considered was −200
to 400 ms (0 ms = response) with a baseline correction
period set −700 to −500 ms before the response. Second,
we carried out a fitting of the identified maps in thestimulus-locked signal of MEx and MIm (ERPs averaged
separately for MEx and MIm on the S4 auditory stimulus
from 0 to 400 ms; baseline period: −200 to 0 ms); this wasfollowed by a two-way repeated measures ANOVA on the
GeV values, with Conditions and Maps as factors.
In the same way, the motor map was also fitted
backwards into the two types of preparation periods inorder to assess the respective involvement of primary
motor processes in preparation for actual and imagined
movements. A two-tailed paired t-test contrasting conditions was then conducted on the GeV values resulting
from this fitting.

Fig. 2 Average reference ERP waveforms recorded over electrodes
C3, CZ and C4 for the P-MEx and P-Mim on the top. Note that
CNV was larger for the hemisphere ipsilateral to the finger

Finally, the ERPs map fitting procedure also provides
information about when in time a given segment map isbest represented (time point of Best explained Variance—
BeV). Indeed, this index may be considered as a peakvalue in terms of the electrical scalp topography distribution and was used to reveal differences in timing, i.e.
latencies. These values were compared between conditions
using t-tests for the identified motor map.
Source localization
A three-dimensional distributed linear inverse solution
called LAURA was used to estimate the brain activity
underlying the segment map topography. The LAURA
algorithm calculates the local autoregressive average withcoefficients that depend on the distances between solution
points. This incorporates the known biophysical laws
regarding the spatial attenuation of the source strength(Grave de Peralta et al. 2001, 2004; Michel et al. 2001).
The lead field applied to this model was calculated on arealistic head model with 4,024 solution points, equally

movement. On the bottom are reported for illustrative purposes
the average reference ERP waveforms for the MEx and MIm
conditions. Positive values are up

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distributed in the grey matter of the average brain provided
by the Montreal Neurological Institute (MNI, Montreal,
Canada). Several simulation and application studies
showed that this localization procedure reveals meaningful
estimates of the 3D distribution of the intracerebral
sources (e.g. Itier and Taylor 2004; Michel et al. 2001;
Murray et al. 2004; Ortigue et al. 2004; Schnider 2003).

Execution/Imagination period, an early (0–250 ms) and alater poststimulus window (250–500 ms) was defined.
Separate repeated measures ANOVAs on mean values of
EMG signals, with the Window and the Condition as
factors, were used to assess significant differences in the
preparation and the Execution/Imagination periods.

Results
EMG
Preparation period
In order to verify that the participants correctly adapted tothe tasks, EMG activity of the right flexor digitorum
communis was recorded for all the conditions on the rightflexor digitorum communis using bipolar surface derivation. EMG was transformed in absolute value andanalysed. For the preparation period the last 500 ms was
taken into account, and an early (500–750 ms) and a later
pre-stimulus window (750–1,000 ms) was defined. For the

The waveform analysis of the CNV shows maximal valuesof the signal on electrodes C3, CZ and C4 (see Fig. 2, toppart). The Conditions (2) × Electrodes (3) repeatedmeasures ANOVA performed on the data during the late
CNV (500–1,000 ms) indicated that only the factor

Fig. 3 Global field power (GFP), segmentation maps of P-MEx
and P-Mim, and corresponding scalp topographies. The maps are
viewed from the top, with the nose up and the left ear left. The GFP

represents the spatial standard deviation of the average reference
maps’ potential distribution. The star indicates a significant
difference for map 5 between the two experimental conditions

Waveform analysis

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conditions is significant, the mean amplitude of the signal
being greater for P-MEx than for P-MIm (F(1,9)=13.51, p=.005).
Segmentation analysis
The segmentation analysis returned five different stablemap configurations in each condition respectively (PMEx: Fig. 3, maps 1–5; P-MIm: Fig. 3, maps 6–10). The
values of the GeV of each map were then calculated. The
repeated measures ANOVA computed on GeV with thedesign Conditions (2) × Maps (10) indicated a significant
effect compared to the factor Maps (F(9)=3.69; p =.001)
and an interaction effect (F(1,9)=12.91; p <.001, see Fig.
4). The GeV value associated with map 5 is significantly
different across the conditions as confirmed by post hoc
contrasts (p =.016). Maps 1, 2, 3 and 4, identified in PMEx segmentation, appear as representative in the P-MIm
condition. Reciprocally, maps 6, 7, 8, 9 and 10, resulting
from P-MIm segmentation, are also representative of PMEx. The differences between P-MEx and P-MIm
revealed by the segmentation analysis take place during
the late phase of the preparation period (map 5), in
accordance with the CNV results.
Motor execution/Motor imagery period
Behavioural results
The analyses were processed on the responses to the fourth
auditory stimulus which presented the highest temporalstability (mean reaction time: 233±8 ms, i.e. a coefficientof variation of 3.4%).

the literature (Deecke et al. 1976), map segment 2 can beconsidered as reflecting the presence of the motor
potential. Additional objective arguments in favour of
this interpretation are: (a) the temporal location starting
around the response onset and ending 80 ms after, (b) its
temporal correspondence with a burst of EMG activity and(c) its spatial location on the central region of the left
hemisphere as located with the algorithm LAURA (seeFig. 5); the LAURA inverse solution identified the
maximum activity within the Broadman area 4 on the
pre-central gyrus with the Talairach coordinates x =−41, y=−10, z =44.
Stimulus-locked ERP
The maps identified by the segmentation analysis in the
response-locked ERP in MEx were fitted in the MEx andMIm conditions respectively. The results are reported inTable 1.
A Conditions (2) × Maps (4) repeated measures
ANOVA on GeV values obtained with the fitting procedure of the four response-locked MEx maps in thestimulus-locked signal revealed only a significant effectfor the factor Maps (F(3)=7.825; p =.001). Furthermore,
the time points of the BeV of map 2 in MEx (mean = 200ms) and in MIm (mean = 222 ms) did not differ (t -test, t=.30, p =0.76). Likewise, ERP waveforms in the central
part of the scalp were not different between these
conditions (see Fig. 2, bottom part). To sum up, theTable 1 Globally explained variance indexes (%) and theirstandard errors (±) resulting from the fitting procedure of the fourmaps identified in the Response-locked ERP MEx condition, within
the MEx and MIm conditions. Only a significant effect for the maps
as a factor was observed
Maps

Response-locked ERP in MEx
The segmentation analysis revealed four stable map
configurations (Fig. 5). On the basis of previous data in
Fig. 4 Mean and standard errorof GeV for each map in the PMEx and P-MIm conditions(significant differences between
conditions: * p <.05)

MEx
MIm

1

2

3

4

6.90 (2.03)
8.04 (1.96)

12.96 (2.51)
13.30 (1.98)

6.00 (2.44)
1.80 (0.83)

14.80 (2.66)
19.88 (2.41)

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Fig. 5 From the top: GFP, mean EMG of the right flexor digitorum
communis, the four segmentation maps identified in the responselocked MEx and their respective topographies. The maps are viewed
from the top, with the nose up and the left ear left. LAURA inverse

solution of the identified motor component (map 2); maximum
corresponds to Broadman area 4 on the pre-central gyrus with theTalairach coordinates x =−41, y =−10, z =44

motor map was equally present in both conditions andoccurred in a similar time period.

EMG activity

Motor map in preparatory period
Lastly, the motor map was fitted into the preparatory
period (between 500 and 1,000 ms) for P-MEx and PMIm. The t -test conducted on the GeV data resulting fromthis fitting revealed a significant effect (t(9)=2.308, p=.023) according to condition: GeV was higher in the
preparation of effective as compared to imagined execution (22% and 17%, respectively).

EMG activity for all the conditions is reported in Fig. 6.
Separate repeated measures ANOVAs on mean values of
EMG signals, with Window and Condition as factors,
were used to assess significant differences in the preparation and the Execution/Imagination periods. Low residual muscular activities were observed during P-MEm and
P-MIm, and no significant differences were found between
these conditions. In the Execution/Imagination period therepeated measures ANOVA revealed only a significant
effect (F(1,9)=53.792, p =.00004) for the Condition factor,
the EMG activity was greater in the MEx (m=2.24 μv)
compared to the MIm (m=0.39 μv) condition, and post hoc
t-tests significantly identified this effect in both time
windows ( p <.001). Altogether, these results show that theparticipants correctly adapt to task demands.

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Fig. 6 EMG activity for the P-MEx and P-MIm on the left; MEx and MIm, on the right. Note, as expected, the presence of an EMG burst
uniquely in the MEx condition

Discussion
Two main findings were established in the present ERP
study: (1) motor imagery and motor execution differ in thelate stages of the preparatory period, and (2) motor
imagery and motor execution are characterized by thepresence of motor maps to the same extent in theexecution periods.
Preparation
Direct evidence for cerebral activity preceding a motor
response comes from single-cell studies in monkeys(Alexander and Crutcher 1990) and intracranial recordings
in humans (Ikeda et al. 1992; Neshige et al. 1988). This
activity, generally considered to reflect processes of
movement preparation, is described in these studies asbilaterally symmetrical. The waveform analysis on ourdata showed a difference of signal amplitude between PMEx and P-MIm on the CNV, the activation being more
important during P-MEx, which is in line with previous
results (Bonnet et al. 1998; Cunnington et al. 1996;
Jankelowitz and Colebatch 2002). The difference between
P-MEx and P-MIm is marked during the last 500 ms prior
to the go signal. This finding suggests a recruitment ofsimilar neural processes in the P-MIm and P-MEx
conditions except in the final stage period related to the
execution of the movement.
Segmentation analysis refined this result and also
demonstrated the presence of differences between the
two types of preparation. One map differed significantly
between P-MEx and P-MIm, and was more representativeof P-MEx. This observation confirms that a functional
difference is present in the process involved when asubject prepares to execute or prepares to imagine a motor
act, and that this difference is temporally located at the endof the preparation period. fMRI studies showing reduced
activation in M1 during MIm (Lotze et al. 1999; Porro etal. 1996, 2000; Roth et al. 1996) lead to the assumptionthat the difference, located in our data at the preparationlevel, concerns M1. This assumption is supported by the
results of the fitting procedure of the motor map into the
preparatory period: the motor map was significantly betteraccounted for in the P-MEx than in the P-MIm condition.
This result suggests that the functional difference between
the preparatory periods of each condition is restricted toprimary motor processes. Interestingly, as expected, weak
EMG activity was present to the same extent in bothconditions. This observation demonstrates that subjects
adapted to the task and indicates that the observed
differences cannot be related to a real execution of amovement.
Concluding on this point, our data suggest that theorigin of the functional difference between an actual or amental motor act would initially be situated in the latestages of the preparation level and mainly consists ofquantitative modulations of the activity of primary motor
structures. This conclusion is in line with Cunnington etal.’s results (1996).
Execution
A map corresponding to the motor component of the
response was identified in the response-locked signal of
the MEx condition. In agreement with recent fMRI data
(Lotze et al. 1999; Porro et al. 1996, 2000; Roth et al.
1996), the LAURA inverse solution identified theactivations of this scalp topography in the Talairach
coordinates corresponding to the left M1 area.
As showed by the segmentation analysis, the motor map
was explained to the same extent in the motor imagery andexecution conditions, indicating a comparable activationof this brain structure on both conditions. It is worth
noting that this similar pattern of activity occurs despite an
expected difference in the EMG activity between theconditions (an EMG burst was observed for the MEx398

condition), demonstrating independence between the
EMG signal and the activity in M1. The presence of acomparable amount of activity in M1 in MEx and MIm
conditions is not in line with fMRI studies that showed aweaker activation of M1 during MIm (Lotze et al. 1999;
Porro et al. 1996, 2000). The reasons for such a
discrepancy remain unclear. Some procedural aspects
differentiate these studies from the present one. First,
they did not separate the preparation and execution
periods. Second, the task requirements and the procedure
adopted in our experiment favoured the involvement ofkinaesthetic mental imagery. This point was not specified
by Lotze et al. (1999). Now, it is well known that, in theabsence of explicit instructions, the preferred mode ofmental imagery in adult subjects is visual (Hall and Martin
1995). However, Porro et al. (1996, 2000) explicitly
required from their subjects, during motor imagery, thatthey “imagine (...) feeling the sensations associated with
finger-tapping, while keeping the hand still” (Porro et al.
1996, p. 7,689). The only noticeable difference with theprocedure used in our study is the introduction ofverification trials in the present experiment. It cannot be
excluded that the random introduction of these trials,
amongst the experimental trials, could have influenced thesubjects to adopt a strategy of imagery favouring a highlevel of activation in primary motor structures. As aconsequence, the absence of differences between theamount of variance explained by the motor map in motor
imagery and motor execution might be explained by theprocedural choices adopted in the present study.
Conclusion
The realization of actual and mental motor acts isdifferentiated by a modulation of the activity within thesame neuronal network, which takes place at the latestages of the preparation period. This network includes
primary motor areas in the execution, actual or imagined,of a movement. These findings provide supplementary
evidence in favour of the functional equivalence hypothesis and suggest that common structures, including M1,
are necessary in the actual or kinaesthetically imagined
execution of a motor act.

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