The Role of Parietal Cortex in Awareness
of Self-generated Movements: a
Transcranial Magnetic Stimulation Study
Penny A. MacDonald and Tomáš Paus1
Awareness of self-generated movements arises from comparing
motor plans, and the accompanying (hypothetical) efference copy,
with the visual and proprioceptive consequences of movement. Here
we used repetitive transcranial magnetic stimulation (rTMS) to
investigate the role of a posterior region in the superior parietal
lobule (SPL) in this process. Nine healthy volunteers performed a
finger extension actively and passively while wearing a CyberGlove;
the glove recorded these (actual) finger movements and used this
information in real time to move a virtual hand displayed on a
computer screen. To assess the participant’s awareness of movement onset, we introduced a delay between the onset of the actual
and virtual movement (60–270 ms, 30 ms increments); the task was
to judge whether the virtual hand movements were delayed relative
to the actual hand movements. Low-frequency rTMS (15 min, 0.6 Hz)
was applied either over the left SPL or the left temporal cortex
(control site) to decrease excitability of these regions and, in turn,
test their role in the awareness of self-generated movement.
Following the SPL stimulation, participants’ assessments of asynchrony were impaired for active but not passive movements. No
significant changes were observed after rTMS applied over the
control site. We suggest that these findings are consistent with the
role of the SPL in evaluating the temporal congruency of peripheral
(visual) and central (efference copy) signals associated with selfgenerated movements. As such, this region may contribute to the
sense of ‘agency’ and its disturbances in disorders such as apraxia
and schizophrenia.
To determine whether or not an action is self-generated requires
comparison of the motor plan, and the hypothetical efference
copy (Sperry, 1950; von Holst and Mittelstadt, 1950), to the
sensory consequences resulting from the production of the
action (Teuber, 1964; Soechting and Flanders, 2001). Both visual
(i.e. seeing one’s hand move) and proprioceptive (i.e. feeling
one’s hand move) inputs are relevant for this decision. This
arrangement allows for a comparison between the actual versus
the predicted sensory consequences of an action. Several lines of
evidence suggest that posterior parietal cortex is involved in this
comparison and, in turn, might give rise to the awareness of
self-determined actions. The present study tested this hypothesis
by applying transcranial magnetic stimulation (TMS) over one of
the parietal subregions and, subsequently, assessing consequences
of such stimulation on the awareness of self-generated movements. This experiment was inspired by previous observations
made by Sirigu et al. (Sirigu et al., 1999) that patients with
lesions to the left parietal lobe are impaired in this respect.
Before describing in detail the Sirigu study, we will brief ly
review current evidence implicating the posterior parietal
cortex, and the related fronto-parietal circuits, in the integration
of visual and somatosensory inputs with motor outputs.
Neuroimaging studies carried out in healthy human participants consistently revealed significant increases in cerebral
blood f low in the posterior parietal cortex during the performance of many types of movements (Grafton et al., 1992, 1996;
Kawashima et al., 1996; Rizzolatti et al., 1996; Faillenot et al.,
1997; Binkofski et al., 1998; Gordon et al., 1998; Ramnani et al.,
2001). Fink et al. (Fink et al., 1999) investigated specifically
the role of the parietal cortex in monitoring self-generated movements; they examined the effect of providing false visual feedback (i.e. presenting the participant’s mirror-imaged fingers to
represent the opposite hand) during in-phase and out-of-phase
bimanual finger opening and closing. Significant activations in
posterior parietal cortex occurred for both in-phase and out-ofphase movements, with increased activation during the false
visual feedback. This pattern of activation suggests that posterior
parietal cortex is involved in the comparison of sensory and
motor information during self-generated movements.
Single cell recordings in non-human primates provide further
evidence that posterior parietal cortex participates in the
production and evaluation of self-generated movements. Studies
have shown that many neurons in the medial intraparietal sulcus
(MIP) respond preferentially to movement (Johnson et al., 1996)
but also respond to visual and somatosensory stimuli, especially
when these stimuli are within reaching distance of the monkey
(Colby and Duhamel, 1991, 1996; Colby and Goldberg, 1999).
Furthermore, experiments with macaques have revealed that
visual receptive fields of the bimodal neurons enlarge with tool
use, extending to represent the new distance in extrapersonal
space attainable by reach with the tool (Iriki et al., 1996). Finally,
some neurons in MIP change their activity simply in relation to
the intention to perform reaching movements (i.e. motor plan),
prior to the presentation of the target of the movement and prior
to the enactment of the movement itself. The latter observation
suggests that, in addition to sensory and tactile/proprioceptive
information, MIP receives an efference copy generated in
relation to the preparation and/or execution of movement
(Anderson et al., 1997; Colby and Goldberg, 1999).
Studies of neural connectivity, using injections of retrograde
tracers in macaque monkeys, have further revealed that MIP,
along with adjacent posterior parietal areas (e.g. V6A, PEc, PEip)
project predominantly to dorsal premotor regions [i.e. PMd
or dorso-rostral area F2 (Cavada and Goldman-Rakic, 1989;
Rizzolatti et al., 1997, 1998; Wise et al., 1997; Matelli et al.,
1998)] and, to a lesser extent, to the primary motor cortex (Wise
et al., 1997). In human participants, Chouinard et al. (Chouinard
et al., 2002) also demonstrated connectivity between premotor
and parietal regions. Combining positron emission tomography
(PET) with TMS, they showed that TMS of dorsal premotor areas
(i.e. PMd/F2) resulted in a distal blood-f low response in the
posterior superior parietal cortex.
A final line of research investigating the role that posterior
parietal cortex might play in the determination of self-initiated
actions arises from the study of patients with lesions of the
© Oxford University Press 2003. All rights reserved.
Cerebral Cortex Sep 2003;13:962–967; 1047–3211/03/$4.00
Introduction
McMaster University Medical School, Hamilton, Canada and
1
Montreal Neurological Institute, McGill University, Montreal,
Canada
posterior parietal lobe. Lesions in the parietal lobe, particularly
in the lef t hemisphere, result in various impairments in the
performance and sequencing of actions, namely apraxia
(Critchley, 1953; Heilman and Rothi, 1982; Poizner et al., 1995;
Sirigu et al., 1995; Haaland et al., 2000). As mentioned previously and particularly relevant to the aim of the current study
are findings by Sirigu et al. (Sirigu et al., 1999) suggesting
that patients with apraxia are impaired relative to control
participants in differentiating self-generated movements from
experimenter-generated actions. They tested three such patients
with lesions to the left parietal lobe; they were asked to perform
movements with their right and left hands while they were
prevented from directly observing these actions. Visual feedback
was provided via a video monitor and consisted of (a) the
participant’s own gloved hand performing the instructed action,
(b) the gloved hand of the experimenter performing the
instructed action, or (c) the gloved hand of the experimenter
performing an action different from that performed by the
participant. The participant’s task was to indicate whether the
movement presented on the video monitor was his/her own.
When visual, proprioceptive, and motor information were
identical (i.e. when a participant actually saw his or her own
hand performing the instructed movement) patients with
apraxia and control participants were highly accurate. Conversely, when visual information was clearly discrepant from
proprioceptive and motor information (i.e. when participants
saw the hand of the experimenter performing a different action)
both groups performed comparably and correctly. However,
when greater, although imperfect, overlap existed between
visual, proprioceptive, and motor information and finer discriminations were required (i.e. when the experimenter’s hand was
presented performing the instructed action) all participants
performed more poorly, with the performance of the patients
being significantly impaired relative to that of controls. This
pattern occurred for both right and left hand gestures, and is
consistent with a view that (the left) posterior parietal cortex is
necessary for distinguishing actions generated by self from those
generated by others, particularly when only subtle differences
exist between movements.
The aim of the current study was to test experimentally in
healthy participants whether the left posterior parietal cortex is
crucially involved in the determination of agency of movements.
This study focused on the left parietal cortex in light of the
higher prevalence of apraxia following lesions to the left vs right
hemisphere, respectively (Freund, 1992). Further, the study
design did not permit inclusion of the right hemisphere in the
same groups of participants (see below). The approach was
similar to that employed by Sirigu et al. (Sirigu et al., 1999).
Healthy participants performed finger movements, either extension of the index or middle finger, while wearing a CyberGlove
that recorded, in real time, their actual hand movements and
used this information to move a virtual hand displayed on a
computer screen. Although the movements of the virtual hand
matched those of the actual hand exactly, the former were
delayed by various inter vals relative to the latter. The participants’ task was to identify the trials on which they detected a
delay between the actual onset of the action and the onset of the
virtual hand movement.
Participants performed one block of these asynchronydetection trials before and another block after low-frequency
stimulation of the left posterior parietal cortex with repetitive
transcranial magnetic stimulation (rTMS). Low-frequency (1 Hz)
rTMS has been shown to decrease excitability of the motor
system (Pascual-Leone et al., 1995; Chen et al., 1997;
Gerschlager et al., 2001; van der Werf and Paus, 2002; Chouinard
et al., 2003), and has also been used to investigate brain–
behaviour relationships [e.g. (Kosslyn et al., 1999; Hilgetag et
al., 2001)]. Given the proposed role of posterior parietal cortex
in the integration of proprioceptive, visual, and motor information for the evaluation of movements, we expected that
asynchrony detection would be impaired following rTMS. This
finding would support the necessary function of parietal cortex
in the determination of self-generated movements.
Methods
Participants
Twelve McGill University students (nine females and three males)
volunteered to participate in this experiment. Three female participants
did not return for the second session of the study and were therefore
excluded from analyses. A ll participants were right-handed and had
normal or corrected to normal vision. They ranged from 22 to 27 years of
age, with a mean age of 24 years. The study was approved by the Research
Ethics Board of the Montreal Neurological Institute and Hospital, and
written informed consent was obtained from all participants.
Overall Design
We obtained T1-weighted magnetic-resonance (MR) images from all
participants prior to their participation in this experiment. The experiment comprised two sessions on separate days: parietal cortex was
stimulated in one session while the temporal cortex (a control site) was
stimulated in the other session (Fig. 1). During the first session, we
determined the motor threshold for each participant as well as the
localization of parietal and temporal rTMS stimulation sites. Otherwise,
both sessions proceeded as follows: (a) asynchrony-detection trials, (b)
rTMS, and (c) asynchrony-detection trials. The rTMS was applied to the
experimental region on 1 day and to the control region on the other in
counterbalanced order across participants.
Transcranial Magnetic Stimulation
Determination of Motor Threshold
The threshold was defined as the minimal stimulation intensity required
to elicit reliably a twitch in the first dorsal interosseus muscle of the
participant’s right hand. Single TMS pulses were applied using a Magstim
200 magnetic stimulator (Magstim, Whitland, Dyfed, UK) connected
through the BiStim module, and a figure-of-eight coil to the scalp in the
region above the primary motor cortex. The stimulation intensity was
varied until a level was reached at which (a) observable muscle twitches
were reliably elicited for at least 50% of stimulations, and (b) lesser levels
of stimulation failed to consistently elicit these obser vable muscle
contractions. This intensity, the TMS motor threshold, was used to set the
intensity of stimulation during the rTMS phases of the experiment.
Repetitive TMS
TMS was applied using the same set up described above. The centre of the
figure-of-eight coil was positioned over the experimental (i.e. left parietal
cortex) region during one session, and over the control (i.e. left temporal
cortex) region on another, occurring in counterbalanced order across
Figure 1. Experimental design using one possible counterbalancing order.
Cerebral Cortex Sep 2003, V 13 N 9 963
Figure 2. Target sites. PAR, superior parietal cortex (x = –36, y = –64, z = 54); TEMP,temporal cortex (x = –66, y = –16, z = 8).
participants. Four participants received stimulation of the parietal lobe in
the first session and five participants received stimulation of the temporal
lobe during the first session. Figure 1 presents an example counterbalancing order and serves as an illustration of the study design. The short
axis of the figure-eight coil was perpendicular to the interhemispherics
fissure, with the coil current f lowing in the latero-medial direction; the
coil was held in place by a mechanical holder. Participants were asked to
remain immobile and were placed in a chin rest to reduce the probability
of movement during stimulation.
The intensity of TMS stimulation was 90% of motor threshold (50–70%
of the maximum stimulator output). A total of 550 TMS pulses were
administered at a rate of 0.6 Hz. The rTMS phase lasted ∼15 min.
Localization of Parietal and Temporal rTMS Stimulation Sites
Frameless stereotaxy was used to target the stimulation sites (see Fig. 2).
First, the MRI of the participant’s brain was transformed into standardized
stereotaxic space with an automatic feature-detection algorithm (Collins
et al., 1994). Next, the exact locations of the experimental region in the
left posterior parietal cortex and the control region of secondary auditory
cortex in the temporal lobe were identified in standardized stereotaxic
space (parietal cortex: x = –36, y = –64, z = 54; temporal cortex: x = –66,
y = –16, z = 8); the parietal coordinates are based on the results of our
recent TMS/PET study of fronto-parietal connectivity (Chouinard et al.,
2003). These MRI locations were then transformed to the participant’s
native brain coordinate space with an inverse version of the native-tostereotaxic transformation matrix (Paus, 1999). Following the determination on the MRI of the experimental and control regions using these
participant-specific coordinates, localization of the scalp positions
overlying the site was next achieved using a three-dimensional infrared
optical-tracking system (Polaris System, Northern Digital Inc., and
Brainsight software, Rogue Research Inc.). Using an optically tracked
pointer, positioned perpendicularly to the head, we determined and
marked with a grease pen the scalp positions overlying the cortical
regions of interest. In both sessions, the centre of the coil was placed
against these locations on the scalp. The participant was asked not to
wash off these marks so that there was no need to perform the registration on the second day of the experiment.
Asynchrony-detection Task
To perform the asynchrony-detection task, participants wore a CyberGlove (Immersion 3D Interaction Inc.; w w w.immersion.com/products/
3d/interaction/cyberglove.html) that recorded, in real time, the movements of their wrists and fingers and used this information to move a
virtual hand on a computer screen. In this way, movements of the virtual
hand corresponded exactly to the movements of the participant’s actual
hand. The participants saw only the virtual hand displayed on the computer screen, while their actual hand, fitted with the data glove, was
obscured from view inside a black box positioned directly in front of
them. The visual feedback presented on the computer screen was
asynchronous with the actual onset of movement. The onset of the virtual
hand was delayed at various intervals ranging from 60 to 270 ms, in 30 ms
increments, relative to that of the actual hand. The participant’s task was
to indicate on each trial whether they detected an asynchrony between
the onset of movements of the virtual hand and those of their actual hand.
During each session (i.e. Parietal vs Temporal), all participants
964 Awareness of Movement and Parietal Cortex • MacDonald and Paus
performed 128 asynchrony-detection trials both pre-and post-rTMS. Participants performed the task actively for one block of trials (i.e. 64 trials)
and passively for another. The order of tasks (i.e. Active vs Passive) preand post-rTMS was counterbalanced across participants (See Fig. 1). In a
given subject, the same order of tasks for the pre- and post-rTMS blocks
was used in the Parietal and Temporal sessions. The inclusion of the
passive task allowed us to evaluate possible impairment in the processing
of movement-related proprioceptive inputs. For each task, there were
eight trials at each of the delay durations, from 60 ms to 270 ms in 30 ms
increments. The trials at these various delays were presented in randomized order. For trials that were performed actively, the participant
extended either the index or the middle finger of his or her right hand,
depending on the oral instruction given by the experimenter at the
beginning of the trial. For the passive trials, the participant’s index or
middle finger of his or her right hand was extended by the experimenter.
This was accomplished with padded finger rings placed on the ends of
the participant’s index and middle fingers, connected with clear nylon
wire via a pulley system to analogous finger rings that were placed on the
experimenter’s fingers. As the experimenter f lexed her index or middle
finger, the participant’s corresponding finger was passively extended.
Each trial proceeded as follows. The virtual hand appeared on the
screen. In the active block of trials, the experimenter gave the oral
instruction to extend the index or the middle finger. The participant
actively performed the movement. In the passive block of trials, the
participant’s index or middle finger was extended passively without prior
notice as to the finger that would be extended; no prior notice was given
to minimize the participant’s active involvement in preparing to move the
finger. The virtual hand displayed the corresponding action on the
computer monitor at various intervals of delay relative to the participant’s
actual finger movement. The participant responded ‘yes’ or ‘no’ to
indicate whether or not a delay was detected. A blank screen then
appeared for 1 s prior to the onset of the next trial. Each trial lasted on
average 5 s. In each block of trials, there were an equal number of index
and middle finger extensions, occurring in random order.
Results
Baseline Performance
A t-test was conducted on the proportion of delay-detected trials
per task prior to rTMS, irrespective of cortical region stimulated.
The proportion of delay-detected trials was calculated as the
number of trials per task on which the participant detected a
delay between the actual finger movement and the virtual finger
movement, divided by the total number of trials for that task.
The analysis revealed no difference in terms of accuracy of
delay-detection between the active and passive tasks, t = 1.261,
P > 0.200.
Effects of rTMS
Our hypothesis was that rTMS applied over the parietal site
would impair the detection of movement delay for active trials
only. Therefore, we used an a priori single degree-of-freedom
contrast of the parietal-active condition relative to all other
Figure 3. Difference scores (post-rTMS minus pre-rTMS) of the proportion of delaydetected trials per task, for each stimulation site.
Table 1
Proportion of delay-detected trials (SEM) pre-rTMS and post-rTMS, per task, for each stimulation
site
Region of rTMS
Parietal
Temporal
Active
Passive
Pre-rTMS
Post-rTMS
Pre-rTMS
Post-rTMS
0.47 (0.04)
0.39 (0.03)
0.37 (0.04)
0.38 (0.05)
0.42 (0.04)
0.37 (0.04)
0.41 (0.04)
0.39 (0.05)
Figure 4. Single-participant difference scores (post-rTMS minus pre-rTMS) of the
proportion of delay-detected trials per task, for each stimulation site.
Finally, the analogous contrast of the active-temporal with the
passive-temporal condition was not significant, F < 1. As
predicted, performance of the asynchrony-detection task was
unaffected in either the active or the passive condition by
stimulation of the temporal cortex.
Discussion
Region × Task conditions (i.e. parietal-passive, temporal-active,
and temporal-passive). This contrast was highly significant,
F(1,8) = 19.823, P < 0.002, ref lecting a decrease in the proportion of delay-detected trials in the active condition after rTMS
stimulation of the parietal region (Table 1). Student t-tests
confirm that a significant difference in performance of the
detection of asynchrony arose only in the parietal-active
conditions when comparing pre-rTMS and post-rTMS scores,
t = 4.966, P < 0.001, whereas for all other conditions (i.e.
parietal-passive, temporal-active, and temporal-passive), t < 1.
Post-rTMS of posterior parietal cortex, participants appreciated
the delay between their actual hand movements and those of the
virtual hand significantly less frequently than pre-rTMS. The
difference scores (post-rTMS minus pre-rTMS) of the proportion
of delay-detected trials per task, for each region of TMS
stimulation are presented in Figure 3. Figure 4 presents the
difference scores (post-rTMS minus pre-rTMS) of the proportion
of delay detected trials per participant for each task and each
region of TMS stimulation.
A second a priori single degree-of-freedom contrast examining the difference scores of the proportion of delay-detected
trials in the parietal-active condition with the parietal-passive
condition approached significance, F(1, 8) = 4.638, P = 0.063;
the post minus pre-rTMS difference was larger in the
parietal-active, as compared with the parietal-passive, condition.
Again, the latter finding ref lects the decrease in accuracy of
asynchrony detection in the active condition following rTMS
stimulation of the parietal cortex compared to equal performance in the passive condition pre- and post-parietal rTMS.
There are two principal findings in the current experiment.
First, participants performed comparably at baseline in the
active and the passive tasks. This equivalence confirms the
appropriateness of the passive task as a control measure. Second,
rTMS of a region in left posterior parietal cortex impaired
detection of asynchrony between actual and virtual hand
movements when these movements were self-generated. Given
that rTMS of this region did not alter performance of asynchrony
detection when the hand movements were passively performed,
a task with equivalent cognitive demands, we can conclude that
this change in performance was not due simply to a general
impairment in the processing of visual information associated
with the virtual-hand movement on the computer screen.
Further, the unaltered performance in the passive task suggests
that stimulation of the superior parietal lobule (SPL) did not
produce a gross impairment in the processing of movementrelated proprioceptive input. By including a control session, in
which rTMS was applied to a region in temporal cortex without
effect on performance, we can also refute the possibility that the
main finding in the current study was due to a general effect of
rTMS on active movements. Finally, this control session enabled
us to appreciate how performance in the active task changed
simply with practice and fatigue due to repetition of the task
after rTMS. We can therefore dismiss the possibility that our
finding was simply a function of task repetition.
The result of main interest in the current experiment is in line
with the finding of Sirigu et al. (Sirigu et al., 1999). They found
that patients with lesions to the left posterior parietal lobe were
more impaired than controls in distinguishing whether
movements presented via a video monitor were self-generated
versus experimenter-generated. The current study differed from
Cerebral Cortex Sep 2003, V 13 N 9 965
that of Sirigu et al. in a number of important ways, however.
Because the disruption of parietal cortex was focal, controlled
and temporary, this study was not subject to the confounding
effects of relatively large cortical lesions, and to the possible
functional reorganization of the affected neural circuits. Second,
baseline measures of performance for each participant could be
sought before the experimental manipulation (i.e. rTMS). Finally,
in the current experiment, we isolated temporal information
from kinematic information. The CyberGlove recorded hand
position, amplitude, direction, and rotation of movements from
distal and proximal inter-phalangeal joints, from metacarpal
phalangeal joints, as well as from the wrist, in x, y, z, pitch, roll
and yaw coordinates. In this way, the movements of the virtual
hand replicated the kinematics of the actual hand movements
with great fidelity, and any minor imprecisions in the representation were consistent across trials. Consequently, participants
could only rely on the temporal relation between the actual and
the virtual hand in evaluating movements. This is in contrast to
the Sirigu et al. study in which the experimenter-generated
movements varied in terms of kinematic, as well as temporal
information relative to the self-generated movements.
In the current experiment, temporal information about
movement provided the basis for consistently performing the
behavioural tasks. Theoretically, this temporal information about
hand movements could be derived in different ways. Participants
could compare the position of their actual hand at any point in
time with the position of the virtual hand. In this way, asynchrony would be determined by noting discrepancies between
proprioceptive (hand) and visual (screen) feedback. A lternatively, participants could use the initiation of a motor
programme as a reference point for comparing visual feedback
of the virtual hand. That is, asynchrony judgements could be
achieved by comparing efference information with the visual
feedback in our experiment.
A number of studies have addressed the question of awareness
of movement initiation. First, Libet et al. (Libet et al., 1983)
asked participants to estimate the onset of their actions.
Participants were instructed to note the time on a clock when
they initiated a movement. This subjective assessment of movement initiation was then correlated with electromyographic
(EMG) information about the actual onset of movement. They
found that participants’ estimates preceded the beginning of the
actual movement, on average, by 86 ms. Supporting this finding,
McCloskey et al. (McCloskey et al., 1983), asked participants
to judge the order of occurrence of an auditor y tone and a
voluntar y hand movement. Again using EMG as an objective
measure of movement onset, they found that participants only
deemed the tone and the movement to be synchronous when the
tone in fact preceded the movement. Finally, Haggard and Magno
(Haggard and Magno, 1999) replicated the finding of Libet et al.
that judgements of movement onset preceded actual movement.
In addition, they demonstrated that applying rTMS to premotor
cortex increased the delay between a ‘go’ signal and the
participants’ awareness that a movement had been initiated.
Despite delaying the appreciation of movement onset, the
initiation of the actual movement was not delayed. This
interesting dissociation between performance and awareness of
movement was further maintained by the finding that rTMS of
primary motor cortex delayed the onset of movement but not
the subjective appreciation of the beginning of movement,
which again preceded actual movement, relative to a control
condition.
Together, these findings are inconsistent with a view that
ascribes decisions about self-initiation of movement to sensory
966 Awareness of Movement and Parietal Cortex • MacDonald and Paus
(proprioceptive) feedback. This view would require awareness
of action onset to follow actual enactment of a movement.
Rather, the studies reviewed here suggest that determinations of
onset of self-generated actions are anticipator y, favouring an
account whereby decisions about the timing of actions result
from reference to efference information. That is, the efference
copy acts in some manner as a time-stamp for the initiation of
movement. The current findings are also interpreted in this light.
Following rTMS of the left SPL in human participants, virtual
hand movements that were delayed relative to actual, selfgenerated hand movements were judged as synchronous
significantly more often than before rTMS of this region.
Stimulation of SPL with rTMS caused a delay in the awareness of
self-generated movements. Participants became more likely to
judge the movements of the virtual hand, which were in fact
delayed by various inter vals ranging from 60 to 270 ms, as
occurring simultaneously with the initiation of the actual
movement. This finding is convergent with the results obtained
by Haggard and Magno (Haggard and Magno, 1999) following
rTMS of premotor cortex. In fact, we speculate that the stimulated parietal site is part of the same fronto-parietal network
investigated by Haggard and Magno, the function of which is to
plan, predict, and evaluate movements of self. This view is
plausible given the sensori-motor integration functions that have
been ascribed to MIP (Rushworth et al., 1997a, 1999; Wise et al.,
1997; Colby and Goldberg, 1999) in addition to the reciprocal
connectivity that exists between MIP and premotor regions
(Rizzolatti et al., 1997; Wise et al., 1997; Matelli et al., 1998;
Chouinard et al., 2003). Recall that TMS of a premotor region
(PMd/F2) resulted in significant activation of a distal region in
the posterior superior parietal cortex (Chouinard et al., 2003);
the coordinates for the parietal stimulation site in the current
study in fact were derived from that study. Ours is a study among
a number of others coupling premotor and parietal regions in the
production, assessment, and correction of movement (Critchley,
1953; Hyvarinen, 1982; Kalaska, 1991; Rizzolatti et al., 1997;
Rushworth et al., 1997a; Wolpert et al., 1998; Desmurget et al.,
1999; Iacoboni et al., 1999).
In summary, our findings suggest that a region of posterior
parietal cortex, which may be analogous with macaque MIP, is
crucially involved in the assessment of self-generated movements. That is, decreased cortical excitability of this region
impairs awareness of self-initiated movements. We contend that
this impairment arises due to interference with efference information about a self-generated movement along a fronto-parietal
pathway involving premotor regions and MIP. Future studies will
further explore this pathway as well as investigate any hemispheric asymmetry that might exist.
Notes
This study was supported by the Canadian Institutes of Health Research
and the Canadian Foundation for Innovation. We thank Drs Ysbrand van
der Werf, Marie-Helene Grosbras and Kate Watkins for their help in
determining motor thresholds and positioning the stimulating coil and
Jason Lerch for programming the CyberGlove software. We also thank
Drs Kate Watkins and Marie-Helene Grosbras for useful comments on the
manuscript.
Address correspondence to Tomáš Paus, Montreal Neurological
Institute, 3801 University, Montreal, Quebec, H3A 2B4, Canada. Email:
[email protected].
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