Knowing when to respond and the efficiency of the cortical motor command: a Laplacian ERP study.

Abstract

The objective was to test whether motor preparation can modulate the efficiency of the cortical motor command. The electroencephalogram (EEG) was recorded from electrodes located over the primary sensorimotor cortices during the performance of a between-hand choice reaction time task in which foreperiod duration (the interval between the warning and the imperative signals, 800 vs. 2800 ms) was varied across blocks of trials. In order to increase the spatial resolution of the EEG traces, surface Laplacian was estimated. The amplitude of the negative wave developing over the hemisphere contralateral to the response was smaller for the short foreperiod associated with the best performance level. These results indicate that the activation of the primary sensorimotor cortex involved in the response is less pronounced for the short foreperiod, suggesting that temporal advance information increases the efficiency of the cortical motor command.

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Research Report
Knowing when to respond and the efficiency of the cortical
motor command: A Laplacian ERP study
Christophe Tandonneta,⁎, Borís Burlea, Franck Vidala,b, Thierry Hasbroucqa,b
aCentre National de la Recherche Scientifique and Université de Provence, Laboratoire de Neurobiologie de la Cognition, Marseille, France
bInstitut de Médecine Navale du Service de Santé des Armées, Toulon, France
A R T I C L E I N F O A B S T R A C T
Article history:
Accepted 16 June 2006
Available online 25 July 2006
The objective was to test whether motor preparation can modulate the efficiency of the
cortical motor command. The electroencephalogram (EEG) was recorded from electrodes
located over the primary sensorimotor cortices during the performance of a between-hand
choice reaction time task in which foreperiod duration (the interval between the warning
and the imperative signals, 800 vs. 2800ms) was varied across blocks of trials. In order to
increase the spatial resolution of the EEG traces, surface Laplacian was estimated. The
amplitude of the negative wave developing over the hemisphere contralateral to the
response was smaller for the short foreperiod associated with the best performance level.
These results indicate that the activation of the primary sensorimotor cortex involved in the
response is less pronounced for the short foreperiod, suggesting that temporal advance
information increases the efficiency of the cortical motor command.
© 2006 Elsevier B.V. All rights reserved.
Keywords:
Time preparation
Foreperiod duration
Motor cortex
Event-related potential
Surface Laplacian
1.Introduction
Inordertoinvestigatehowanticipatoryprocessescanimprove
behavioral performance, one can use advance information
about either the characteristics of the forthcoming movement
(event preparation) or the timing of the upcoming stimulus
(time preparation). Variations of the foreperiod duration (the
interval between the warning and the imperative signals)
allow to study time preparation. Previous behavioral study
showed, when the foreperiod is varied across blocks of trials,
that the force generated to execute the response is smaller
after short than after long foreperiods (Mattes and Ulrich,
1997). This result suggests that less force is required when
temporally optimally prepared. A direct prediction of this
result is to suppose that the motor structures generating a
weaker movement are themselves less activated. The present
study was aimed at testing this prediction.
A popularmeasure derived from the electroencephalogram
(EEG) to assessthe activation of corticalmotorstructuresis the
‘lateralized readiness potential’ (LRP), a computation of the
differencebetweencorticalactivitiesmeasuredbyanelectrode
contralateral to the response minus an electrode ipsilateral to
theresponse(Grattonet al., 1988). Thismeasure hasbeen used
to investigate event preparation (see, e.g., Osman et al., 1995;
Leuthold et al., 1996; Müller-Gethmann et al., 2000) or time
preparation (see Müller-Gethmann et al., 2003). It must be
noted that the LRP computation, by construction, does not
allow to specify the respective contributions of each distinct
motor cortex (Gratton, 1998; Eimer, 1999; Vidal et al., 2003).
Moreover, a drawback of the LRP is that it is computed from
monopolar recordings, whose spatial resolution is roughly in
the 6- to 10-cm range. This drawback can be circumvented by
estimating the surface Laplacian (Nunez, 2000), which
increases the spatial resolution of the EEG to 2–3cm. Acting
B R A I N R E S E A R C H 1 1 0 9 ( 2 0 0 6 ) 1 5 8 – 1 6 3
⁎ Corresponding author. LNC, Faculté St Charles, case C, 3 place Victor Hugo, 13331 Marseille cedex 3, France. Fax: +33 488 57 68 72.
E-mail address: christophe.tandonnet@up.univ-mrs.fr (C. Tandonnet).
0006-8993/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.brainres.2006.06.052
available at www.sciencedirect.com
www.elsevier.com/locate/brainres

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as a high-pass spatial filter, surface Laplacian estimation
removes the blurring effect of the diffusion of the currents
through the highly resistive skull (Nunez, 1981); this transfor-
mation is a good approximation of what would be the
corticogram (Gevins et al., 1995). Moreover, by allowing to
separatelyexaminethetimecourseofactivitiescorresponding
to different foci, surface Laplacian estimation secondarily
enhances the temporal resolution of EEG (Law et al., 1993).
Recent data demonstrate that the activity of the sensorimotor
cortices contralateral and ipsilateral to the response can be
directly derived from EEG recordings by estimating surface
Laplacian (Taniguchi et al., 2001; Vidal et al., 2003; Tandonnet
et al., 2003, 2005a). In the context of a between-hand choice
reaction time (RT) task, these studies showed a negative wave
over the sensorimotor cortex contralateral to theresponseand
a positive wave over the sensorimotor cortex ipsilateral to the
response about 100ms prior to the response. The negativity
overthecontralateralcortexissimilarto theinitialslopeof the
motor potential described by Deecke et al. (1969; see Taniguchi
et al., 2001). As the distribution of the premotor potentials is
comparable in humans and in monkeys (Arezzo and Vaughan,
1975), intracerebral recordings in monkeys, showing that the
initialslopeof themotorpotentialarisesfromlayerVof area4,
indicate that the initial slope of this potential corresponds to
the emission of the motor command from the primary motor
area contralateral to the involved hand (Arezzo and Vaughan,
1980). Hence, the negative wave obtained after surface
Laplacian estimation over the sensorimotor cortex contral-
ateraltotheresponselikelyreflectstheemissionofthecortical
motor command (Vidal et al., 2003). Thanks to stimulations
studies(Hasbroucqetal.,2000;Burleetal.,2002a),thepositivity
overtheipsilateralcortexcanbeinterpretedasaninhibitionof
the cortex involved in the incorrect response; such an
inhibitory mechanism could protect against error commission
(for a review, see Burle et al., 2004).
The objective of the present study was to test whether
time preparation can modulate the efficiency of the cortical
motor command. To this end, we varied the foreperiod
duration in a choice RT task and we tested the amplitude
and the latency of EEG components observed after surface
Laplacian estimation over the primary sensorimotor cortices,
compared to the LRP. We investigated the negative wave that
develops prior to the response over the cortex involved in the
response. In the context of the present between-hand choice
RT task, we expected a cortical inhibition of the cortex
involved in the alternative (incorrect) response and we
therefore also analyzed the positive wave that develops over
this cortex.
2.Results
The overall error rate (hand error and late response) was
0.84%. The error rate was arcsine transformed before being
submitted to a two-tailed two-paired Student's t test. There
was no difference in error rate between the short foreperiod
condition (0.90%) and the long foreperiod condition (0.79%; t(9)
<1). Erroneous trials were discarded from further analyses.
After electrical artifact rejection, some trials presented EMG
activity before the EMG burst in the muscle involved in the
response. As these trials cannot, strictly speaking, be con-
sidered as ‘correct’ (Smid et al., 1990; Hasbroucq et al., 1999;
Burle et al., 2002b), they were also discarded from further
analyses. The overall rejection rate was 34% for the short
foreperiod condition and 30% for the long foreperiod condi-
tion. There was no relation between trial rejection and
foreperiod duration (t(9)=1.59, p=0.15).
Reaction time was submitted to analyses of variance
involving twofactors: hand (left/right) andforeperiod duration
(800ms/2800 ms). Reaction time was shorter for the short
(297 ms) than for the long foreperiod (308 ms; F(1,9)=6.44,
p<0.05).Reactiontimewasfasterfortheright (296ms)thanfor
the left hand (309 ms; F(1,9)=10.25, p<0.025). There was no
interaction between these two factors (F(1,9)=2.15, p=0.18).
Confirming a classic result, the RT performance was better for
the short (800 ms) than for the long (2800 ms) foreperiod for
variations acrossblocksof trials(BertelsonandTisseyre,1969).
The two grand-averaged surface Laplacians over the
primary motor cortices, contralateral and ipsilateral to the
response, and the grand-averaged lateralized readiness
potential are presented in Fig. 1. Statistical analyses were
conducted in order to test whether time preparation mod-
ulatesthe amplitudeand the latency of these components. For
the Laplacians, ANOVAs were performed with the same
design as the RT, involving two factors: hand (left/right) and
foreperiod duration (800 ms/2800 ms), although for the LRP,
Student's t tests were performed.
In order to estimate the amplitude of the Laplacian
components, we used a procedure based on calculation of
Fig. 1 – Amplitudes of the grand-averaged surface Laplacians
(μV/cm2) as a function of time (ms) over the primary motor
cortices contralateral (top) and ipsilateral (bottom) to the
response for the short (thin) and for the long (thick)
foreperiods. Inset: grand-averaged lateralized readiness
potential (μV). Negativity is up. Traces are time-locked to
EMG onset. Baseline was taken from −150 to −100ms before
EMG onset.
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areas. Because a comparison between the maximal amplitude
and a baseline involves to arbitrary define a time window,
which is problematic when data are time-locked to the
response, we made a comparison independent of the baseline.
To determine the time windows corresponding to the minimal
and the maximal activities, we used linear regression mea-
sures (Taniguchi et al., 2001; Vidal et al., 2003). These slope
analyses were conducted for each component and for each
foreperiod condition on the grand-averaged traces, starting
from −200ms and +100ms time-locked to EMG onset. For each
trace, we measured the slope values during 20ms consecutive
epochs by computing a linear regression for each epoch; the
slope value was defined as different from zero if it exceeded a
criterion (5%) relative to the maximal slope of all epochs of
each trace (Tandonnet et al., 2003). The analysis was
performed backward and forward starting from the EMG
onset. Backward, the inferior boundary of the last time period
for which the slope value was superior to the criterion defined
the minimal value. Forward, the superior boundary of the last
time period for which the slope value was superior to the
criterion defined the maximal value. For the contralateral
negativity, the minimal value was −60 ms and −20 ms time-
locked to EMG onset for the right and the left responses,
respectively, in the short foreperiod condition and −100 ms
and −60 ms time-locked to EMG onset for the right and the left
responses, respectively, in the long foreperiod condition; the
maximal value was +40 ms time-locked to EMG onset for both
foreperiod conditions and both responses. For the ipsilateral
positivity, the minimal value was −40 ms and −60 ms time-
locked to EMG onset for the right and the left responses,
respectively, in the short foreperiod condition and −100 ms
and −60 ms time-locked to EMG onset for the right and the left
responses, respectively, in the long foreperiod condition; the
maximal value was +120 ms time-locked to EMG onset for
both foreperiod conditions and both responses.
To test whether preparatory processes modulate the
cortical motor command, we estimated the amplitudes of
the contralateral negativity and the ipsilateral positivity in
both foreperiod conditions on the individual averages. The
surface during a 40-ms epoch centered on the maximal value
obtained on the grand-averages was compared to the 40-ms
epoch preceding the minimal value. The differences between
these surface measures were submitted to an ANOVA. The
surface of the contralateral negativity was smaller for the
short (6.67μV cm−2ms) than for the long foreperiod (11.63μV
cm−2ms; F(1,9)=8.62, p<0.025). The surface of the contral-
ateral negativity was not different for right and left responses
(F(1,9)<1). There was no interaction between these two factors
(F(1,9)<1). To further check that the 40-ms epoch preceding
the minimal value on the grand-averages did not reflect any
changes in activity, we measured the slope during this period
and we tested if the slopes were different from zero. One
sample two-tailed Student's t tests revealed no reliable
significant changes in activity for the contralateral negativity
(all Fs<1; except for the left response in the short foreperiod
condition, F(1,9)=2.01, p=0.19). The surface of the ipsilateral
positivity was not different for the short (21.79μV cm−2ms)
and for the long foreperiod (21.16μV cm−2ms; F(1,9)<1 ) nor for
right and left responses (F(1,9)<1); there was no interaction
between these two factors (F(1,9)<1).
As previous LRP studies analyzed the LRP amplitude at
EMG onset, we used this measure to compare the LRP with
the Laplacians. In order to compute the difference in
amplitude between conditions, we used the same baseline
for the LRP and the Laplacians. There was no foreperiod
effect on the amplitude of the LRP (t(9)<1). On the other
hand, with the same method of measurement, the ampli-
tude of the contralateral negativity was lower for the short
foreperiod (−0.136μV/cm2) than for the long foreperiod
(−0.225μV/cm2; F(1,9)=8.87, p=0.015). There was no effect
of hand nor interaction between foreperiod and hand
(Fs<1). For the ipsilateral positivity, the numerical differ-
ence between short (0.188μV/cm2) and long foreperiod
(0.138μV/cm2) was far from significance (F(1,9)=1.40,
p=0.267). There was no effect of hand (F(1,9)=1.01,
p=0.341) nor interaction between foreperiod and hand
(F(1,9)=1.15, p=0.311).
Visual inspection of the grand-averaged traces revealed
that the amplitudes of the LRPs for the short and long
foreperiods are very similar. (Fig. 1, inset). Although it is
never possible to demonstrate the similarity of two waves, it
could be useful to attempt to quantify the agreement
between the two experimental conditions. In this end, we
performed a statistical analysis based on correlation coeffi-
cients (Bravais–Pearson's r). As additional coefficients could
be useful to deliver information of similarity (see Schröger,
1998), we also used another coefficient (the city-block
distance; Schröger et al., 1993). For each subject and for
each foreperiod condition, we computed the mean of the
time points of the LRP during the interval from −100 to +100
relative to EMG onset. We then examined the similarity of
the two sets of values by computing the correlation
coefficient (r=0.9342) and the city-block distance (sum of
the absolute value of the difference of each pair of values,
each value corresponding to one experimental condition of
one subject; C=10.83). As the sample size is small (10
subjects), we used a randomization test in order to estimate
the distribution of the sets of values (e.g., Edgington, 1980).
We (1) pooled the values for both conditions; (2) draw 50,000
of random samples from this pool; (3) computed the
coefficients for each sample; (4) computed the cumulative
distribution function of the values of the coefficients; and (5)
determined the critical value for a given significance level α
(Schröger, 1998). The probability for a correlation of 0.9342
was lower than 0.025 and those for a city-block distance was
lower than 0.0001, indicating high similarity of the LRP of the
two foreperiod conditions at an individual level.
Inorderto estimate theonsetof theLaplaciancomponents,
we used the slope analysis in specific time windows described
earlier in this section and we applied it on the individual
averages. For the contralateral negativity, the onset was
earlier for the short (−45ms) than for the long foreperiod
(−63ms; F(1,9)=6.39, p=0.032). The onset was not different for
right and left responses (F(1,9)=1.25, p=0.293). There was no
interaction between these two factors (F(1,9)<1). For the
ipsilateral positivity, the onset was not statistically different
for the short (−54 ms) than for the long foreperiod (−44 ms;
F(1,8)=1.50, p=0.252). There was no difference between the
right and the left responses (F(1,9)<1) nor interaction between
these two factors (F(1,9)=1.82, p=0.210).
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3.Discussion
After surface Laplacian estimation, EEG measures show that
the time between the onset of the contralateral negativity
onset and the EMG onset is shorter for the foreperiod that
leads to the best performance, which replicates previous
findings (Tandonnet et al., 2003). The novel contribution of the
present study lies in the finding that the contralateral
negativity is smaller for the short foreperiod, thereby
strengthening the view that time preparation affects the
implementation of the cortical motor command. An inter-
pretation compatible with the effect obtained on the latency of
the contralateral negativity was that the motor command is
less variable from trial-to-trial (Tandonnet et al., 2003).
However, a sharper distribution should also result in a higher
amplitude of the contralateral negativity, a prediction invali-
dated by the present results. Another possibility is that the
contralateral negativity is in average closer in time to the EMG
onset, or in other words, that the distribution of the trial-to-
trial contralateral negativities is shifted. Indeed, the results on
both the latency and amplitude suggest that time preparation
does affect the threshold rather than the slope of the motor
command from the primary motor cortex contralateral to the
involved hand. Note that the effect on amplitude was not
observed on the LRP time-locked to the EMG onset: The
amplitudes for the short and long foreperiods are extremely
similar, and, if any, the effect would be higher amplitude for
the short foreperiod during the time preceding and following
the EMG onset (see Fig. 1, inset). In other words, the effect, if
any, would be in the opposite direction. Hence, based only on
the LRP results, we would have concluded that foreperiod
duration does not affect motor processes. The present results
thus exemplify that inferences based on the LRP computation
can be misleading. A first reason of the discrepancy between
the LRP and surface Laplacian estimates is that the LRP is
based on monopolar EEG, whose spatial resolution is lower
than surface Laplacian estimation (see Section 1). Hence,
compared to LRP, surface Laplacian estimation allows a more
precise view of the activity of the primary sensorimotor
cortices and therefore reveals effects invisible on LRP. A
second reason is that preparation decreases the activation of
the sensorimotor cortex involved in the correct response and
increases the inhibition of the sensorimotor cortex involved in
the alternative (incorrect) response, as suggested by recent
EMG data (Tandonnet et al., 2005b). In such an eventuality, the
visual difference observed over the sensorimotor cortex
involved in the incorrect response, although non-statistically
significant in the present data, could mask the effect observed
over the contralateral cortex by some kind of counterbalan-
cing effect.
The results of the present study suggest that activation of
the primary motor cortex involved in the response is less
pronounced when the foreperiod is short and can be
interpreted in the framework of Näätänen's (1971) model. In
this model, the speed of response execution depends on the
distance between a level of motor activation, the ‘motor
readiness’ and a response threshold, the ‘motor action limit’.
In Näätänen's model, the effect of foreperiod duration on RT is
attributed to variations of motor readiness that in turn results
in variations of the distance to the motor action limit: The
more prepared the response, the closer to the motor limit.
Mattes and Ulrich (1997) extended this model and further
assumed that the central motor command depends on the
distance to the motor action limit: The larger the distance, the
greater the activation needed to reach the motor action limit.
This increase of activation should involve a greater number –
or a higher discharge frequency – of recruited motoneurons.
Hence, the motor commandwould recruitmore motoneurons,
or increase more the discharge frequency of the recruited
motoneurons, for long than for short foreperiods. According to
this notion, provided that the foreperiod is manipulated
across blocks of trials, sensorimotor cortex activation during
the RT interval should be higher for long than for short
foreperiods because, in this later case, a small increase in
excitation is sufficient to trigger the response. Assuming that
motor command can be reflected by the changes in EEG
activity observed in this experiment, the present results
provide physiological evidence in favor of the notion that
time preparation involved a decrease of sensorimotor cortical
activation that can be viewed as an increase of the efficiency
of the cortical motor command.
4.Experimental procedure
Ten right-handed subjects (6 men; 4 women), aged 23–27 years
(M=25), with normal or corrected-to-normal visual acuity
volunteered for the experiment. Experiment was conducted
with the understanding and the written consent of each
participant. The subjects were seated in an armchair, each
hand gripping a vertical tube on which a response button was
fixed. The two response buttons were to be operated with the
left and right thumbs, respectively. The subjects faced a
horizontal display of three light-emitting diodes (LEDs), 1.5 m
distant at eye level. The LEDs were fixed 1 cm apart. The
central LED (blue) served as a fixation point and the two outer
LEDs (bicolor green/red) displayed the response signals. The
two outer LEDs were displayed simultaneously to avoid
lateralization in event-related potentials. The visual angle
formed by the two outer LEDs was 1°.
Each trial began with the fixation LED lighting in blue. After
a constant foreperiod of either 800 ms or 2800 ms, a response
signal was given by switching on simultaneously both outer
bicolor (green/red) LEDs. One of these LEDs lit up in green and
the other in red. The response was a pressure of the thumb
corresponding to the side indicated by the response signal.
The time allowed for the response was 600 ms, after which
time the response signal and the fixation LED extinguished.
The inter-trial interval was 2600 ms.
The subjects were to produce as fast as possible a pressure
with either the left thumb in response to one response signal
or the right thumb in response to the other response signal.
Half the subjects had to produce the response ipsilateral to
the green stimulus and the other half had to produce the
response ipsilateral to the red stimulus. The color (green/red)
to which the subjects had to produce the response and the
duration of foreperiod for the first block were counter-
balanced across subjects. The instructions emphasized both
speed and accuracy.
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The experiment comprised 10 blocks of trials. Each block
consisted of 64 trials preceded by four warm-up trials. The two
conditions of foreperiod duration (800 ms and 2800 ms) were
alternated every other block. In each block, the first order
sequential effects for trial-to-trial transitions were counter-
balanced (Possamaï and Reynard, 1974).
The EMG activity of both flexor pollicis brevis was recorded
with paired Ag–AgCl electrodes, 6 mm in diameter, fixed about
2 cm apart on the skin of the thenar eminences. The signal
was amplified (Grass P511, gain 5000), filtered (bandpass:
10–300 Hz, 6 dB/octave; selective ‘notch’ 50 Hz filter) and
digitized online (A/D rate 1 kHz). In each trial, the EMG activity
was recorded during the 1600-ms or the 3600-ms extending
200 ms before the onset of the warning signal to the end of the
time allowed for the response. The EMG traces were visually
inspected and the EMG onsets were hand-scored by an
experimenter (see Hasbroucq et al., 1999).
The EEG was recorded from 12 Ag/AgCl scalp electrodes.
The reference and ground were, respectively, on the right and
left mastoids. Impedances were kept below 5 kΩ (at 30 Hz). The
brain structures underneath the electrodes were identified on
the basis of reports by Homan et al. (1987) and Steinmetz et al.
(1989). In order to estimate the time course of the surface
Laplacian by the source derivation method (Hjorth, 1975), as
modified by MacKay (1983), we used an electrode configura-
tion that partly differs from the 10 to 20 electrode system
(Jasper, 1958). Each ‘nodal’ electrode, at which surface
Laplacian was computed, was at the center of 3 surrounding
electrodes that formed the vertices of an equilateral triangle.
The nodal electrodes were placed over FCz, FC-C1, FC-C2, C3
and C4. The inter-electrodes distance was 1/20th of the inion–
nasion plus tragus–tragus distance (3.6 cm in average). The
electro-oculogram (EOG) was recorded bipolarly between
electrodes situated above the right eye at its outer canthus.
EEG and EOG signals were amplified (Grass P511, gain 20,000
for EEG and 5000 for EOG), filtered (bandpass: 0.01–100 Hz,
6 dB/octave; selective ‘notch’ 50 Hz filter) and digitized online
(A/D rate 250 Hz). Ocular artifacts were subtracted by
Semlitsch et al.'s (1986) method as implemented in Neuros-
can©. Monopolar recordings were visually inspected and trials
presenting large ocular artifacts or local artifacts (at one site
only) were rejected. The EEG was epoched off-line into periods
of 1200 ms, starting 700 ms prior to the EMG onset and ending
500 ms after the EMG onset. The monopolar recordings were
averaged time-locked to EMG onset; surface Laplacians were
estimated on the basis of monopolar recordings.
Acknowledgments
The authors are indebted to Dr. Steven A. Hackley and to two
anonymous reviewers for helpful comments.
R E F E R E N C E S
Arezzo, J., Vaughan Jr., H.G., 1975. Cortical potentials associated
with voluntary movements in the monkey. Brain Res. 88,
99–104.
Arezzo, J., Vaughan Jr., H.G., 1980. Intracortical sources and
surface topography of the motor potential and
somatosensory evoked potential in the monkey. Prog. Brain
Res. 54, 77–83.
Bertelson, P., Tisseyre, F., 1969. The time-course of preparation:
confirmatory results with visuals and auditory warning
signals. Acta Psychol. (Amst.) 30, 145–154.
Burle, B., Bonnet, M., Vidal, F., Possamaï, C.A., Hasbroucq, T.,
2002a. A transcranial magnetic stimulation study of
information processing in the motor cortex: relationship
between the silent period and the reaction time delay.
Psychophysiology 39, 207–217.
Burle,B., Possamaï, C.A., Vidal, F.,Bonnet, M.,Hasbroucq, T.,2002b.
Executive control in the Simon effect: an electromyographic
and distributional analysis. Psychol. Res. 66, 324–336.
Burle, B., Vidal, F., Tandonnet, C., Hasbroucq, T., 2004.
Physiological evidences for response inhibition in choice
reaction time task. Brain Cogn. 56, 141–152.
Deecke, L., Scheid, P., Kornhuber, H.H., 1969. Distribution of
readiness potential, pre-motion positivity, and motor potential
of the human cerebral cortex preceding voluntary finger
movements. Exp. Brain Res. 7, 158–168.
Edgington, E.S., 1980. Randomization Tests. Dekker, New York.
Eimer, M., 1999. Facilitatory and inhibitory effects of masked
prime stimuli on motor activation and behavioural
performance. Acta Psychol. (Amst.) 101, 293–313.
Gevins, A., Leong, H., Smith, M.E., Le, J., Du, R., 1995. Mapping
cognitive brain function with modern high-resolution
electroencephalography. Trends Neurosci. 18, 429–436.
Gratton, G., 1998. The contralateral organization ofvisual memory:
a theoretical concept and a research tool. Psychophysiology 35,
638–647.
Gratton, G., Coles, M.G., Sirevaag, E.J., Eriksen, C.W., Donchin, E.,
1988. Pre- and poststimulus activation of response channels: a
psychophysiological analysis. J. Exp. Psychol. Hum. Percept.
Perform. 14, 331–344.
Hasbroucq, T., Kaneko, H.,Akamatsu, M., Possamaï, C.A., 1999. The
time-course of preparatory spinal and cortico-spinal
inhibition: an H-reflex and transcranial magnetic stimulation
study in man. Exp. Brain Res. 124, 33–41.
Hasbroucq, T., Akamatsu, M., Burle, B., Bonnet, M., Possamaï, C.A.,
2000. Changes in spinal excitability during choice reaction
time: the H reflex as a probe of information transmission.
Psychophysiology 37, 385–393.
Hjorth, B., 1975. An on-line transformation of EEG scalp potentials
into orthogonal source derivations. Electroencephalogr. Clin.
Neurophysiol. 39, 526–530.
Homan, R.W., Herman, J., Purdy, P., 1987. Cerebral location of
international 10–20 system electrode placement.
Electroencephalogr. Clin. Neurophysiol. 66, 376–382.
Jasper, H.H., 1958. The ten twenty electrode system of the
international federation. Electroencephalogr. Clin.
Neurophysiol. 10, 371–375.
Law, S.K., Rohrbaugh, J.W., Adams, C.M., Eckardt, M.J., 1993.
Improving spatial and temporal resolution in evoked EEG
responses using surface Laplacians. Electroencephalogr. Clin.
Neurophysiol. 88, 309–322.
Leuthold, H., Sommer, W., Ulrich, R., 1996. Partial advance
information and response preparation: inferences from the
lateralized readiness potential. J. Exp. Psychol. Gen. 125,
307–323.
MacKay, D.M., 1983. On-line source density computation with a
minimum of electrodes. Electroencephalogr. Clin.
Neurophysiol. 56, 696–698.
Mattes, S., Ulrich, R., 1997. Response force is sensitive to the
temporal uncertainty of response stimuli. Percept.
Psychophys. 59, 1089–1097.
Müller-Gethmann, H., Rinkenauer, G., Stahl, J., Ulrich, R., 2000.
Preparation of response force and movement direction: onset
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