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Lateral Somatotopic Organization During Imagined and Prepared Movements
Pascale Michelon,
1
Jean M. Vettel,
2
and Jeffrey M. Zacks
1
1
Psychology Department, Washington University, St. Louis, Missouri; and
2
Department of Cognitive and Linguistics Sciences,
Brown University, Providence, Rhode Island
Submitted 11 May 2005; accepted in final form 1 October 2005
Michelon, Pascale, Jean M. Vettel, and Jeffrey M. Zacks. Lateral
somatotopic organization during imagined and prepared movements.
J Neurophysiol 95: 811– 822, 2006. First published October 5, 2005;
doi:10.1152/jn.00488.2005. Motor imagery is a complex cognitive
operation that requires memory retrieval, spatial attention, and possi-
bly computations that are analogs of the physical movements being
imagined. Likewise, motor preparation may or may not involve
computations that are analogs of actual movements. To test whether
motor imagery or motor preparation activate representations that are
specific to the body part whose movement is imagined or prepared,
participants performed, imagined, and prepared hand movements
while undergoing functional MRI scanning. Actual hand movements
activated components of the motor system including primary motor
and somatosensory cortex, the supplementary motor area, the thala-
mus, and the cerebellum. All of these areas showed strong lateral
organization, such that moving a given hand activated the contralat-
eral cortex and ipsilateral cerebellum most strongly. During motor
imagery and motor preparation, activity throughout the motor system
was much reduced relative to overt movement. However, significant
lateral organization was observed during both motor imagery and
motor preparation in primary motor cortex, the supplementary motor
area, and the thalamus. These results support the view that the
subjective experience of imagined movement is accompanied by
computations that are analogs of the physical movement that is
imagined. They also suggest that in this regard motor imagery and
motor preparation are similar.
I N T R O D U C T I O N
Motor imagery is the ability to imagine performing a move-
ment without executing it. Motor imagery and motor execution
overlap in their computational features and in their neural
substrates (for a review, see Jeannerod 1995). However, the
term “motor imagery” encompasses a range of computational
processes, which can be arranged hierarchically (Jeannerod
1994). High-level processes include memory for targets of
movement and attention to spatial locations. Low-level pro-
cesses may include mapping the effector-specific sequence of
commands necessary to make a desired movement. This re-
quires solving two difficult inverse problems: The inverse
kinematics problem is the mapping of a desired movement path
to the sequence of joint angles that will produce it. The inverse
dynamics problem is the mapping from those joint angles to a
sequence of muscle torques. We will refer to the performance
of these computations without overt movement as motor sim-
ulation. The subjective experience of motor imagery may
require one to perform a motor simulation. However, another
possibility is that it only involves high-level processes. The
question of whether motor imagery involves motor simulation
is important because motor simulation processes have a differ-
ent computational form than high-level memory and attention
processes: Unlike those processes, motor simulation stands in
an analog relationship to the imagined action. The primary goal
of the present study was to test whether neural correlates of
motor simulation could be identified during motor imagery. To
what extent motor imagery can be differentiated from motor
preparation, the readiness to perform an action, was also
assessed.
Just as the computations supporting motor imagery can be
arranged hierarchically, so to can the brain regions supporting
motor function (Dum and Strick 2002). Critical components
implementing motor functions include the primary motor cor-
tex (M1), the supplementary motor area (SMA), the premotor
cortex (PM), the cingulate motor zones (CZ), and the cerebel-
lum. During actual movements, many of these areas show
somatotopic organization. Two types of somatotopy can be
distinguished: lateral organization occurs when one-half of the
body is represented in a brain area predominantly in the
corresponding hemisphere (ipsilateral) or in the opposite hemi-
sphere (contralateral). Homuncular organization occurs when
nearby body parts within one-half of the body (e.g., the right
hand and right elbow) are represented in nearby brain loca-
tions. Lateral and homuncular organization have been observed
in M1, first during direct cortical stimulation in awake patients
(Penfield and Rasmussen 1950) and recently in neuroimaging
studies of motor activity (e.g., Alkadhi et al. 2002; Grafton et
al. 1991; Overduin and Servos 2004). Both types of somato-
topic organization in SMA and PM have been revealed by
work with nonhuman primates (Godschalk et al. 1995; Kurata
1989; Mitz and Wise 1987). A few recent studies using either
electrical stimulation in epileptic patients (Fried et al. 1991;
Yazawa et al. 1998) or neuroimaging during actual movements
(Colebatch et al. 1991; Fox et al. 1985; Maccotta et al. 2001;
Mayer et al. 2001) have provided evidence for both lateral and
homuncular organization in SMA.
The role of these regions in motor imagery has been studied
in a number of positron emission tomography (PET) and
functional MRI (fMRI) studies. Most of the initial PET studies
reported that M1 is not activated by motor imagery, and some
more recent fMRI studies are consistent with this finding
(Binkofski et al. 2000; Decety et al. 1988, 1994; Gerardin et al.
2000; Stephan et al. 1995). However, a growing number of
studies using fMRI have reported activity in M1 while subjects
imagined making a movement with their left or right hand
(Dechent et al. 2004; Leonardo et al. 1995; Lotze et al. 1999;
Luft et al. 1998; Porro et al. 1996; Roth et al. 1996). Activity
Address for reprint r equests and other correspondence: P. Michelon,
Dept. of Psychology, Washington Univ., St. Louis, MO 63130 (E-mail:
pmichelo@artsci.wustl.edu).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
J Neurophysiol 95: 811– 822, 2006.
First published October 5, 2005; doi:10.1152/jn.00488.2005.
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in SMA and PM has been consistently reported during motor
imagery. Results for the CZ, thalamus, and cerebellum have
been mixed (for reviews, see Decety et al. 2002; Picard and
Strick 2001).
In summary, several of the regions activated during actual
movement are activated during motor imagery. However, it is
not known what computations these areas are performing
during motor imagery. Brain activity during a motor imagery
task could result from high-level processes such as memory
and attention or from motor simulation. Somatotopic organi-
zation provides one means to distinguish between the two.
High-level representations of plans for actions or targets of
movement do not have to be somatotopically organized. How-
ever, the processes required by motor simulation should be
organized in such a way, if they have the same format as the
computations supporting actual movement. Thus if a motor
imagery task leads to motor simulation, activity during that
task should be somatotopically organized.
The evidence regarding somatotopic organization during
motor imagery is surprisingly sparse. Indirect evidence comes
from electroencephalographic studies, which have measured
the surface electrical activity during imagined left and right
hand movements, and found evidence for lateral organization
(Galdo-Alvarez and Carrillo-de-la-Pea¨na 2004; Pfurtscheller
and Neuper 1997; Pfurtscheller et al. 1999). Until recently,
there was little direct evidence for either lateral or homuncular
somatotopic organization during motor imagery. Two neuro-
imaging studies that reported tests for contralateral organiza-
tion did not find it (Lotze et al. 1999; Roth et al. 1996). Both
studies also failed to detect lateral somatotopic organization in
SMA during actual movements, suggesting that this null result
has been caused by lack of power or to particulars of the task
design. Another neuroimaging study reported significant ho-
muncular organization in M1 during motor imagery, but this
brief report included no direct tests of such organization
(Stippich et al. 2002). A recent study that showed clear ho-
muncular and lateral organization in M1 and PMC during
actual movements found no evidence of homuncular organiza-
tion in these areas during motor imagery and only weak
evidence of lateral organization in the hand area or PMC
(Hanakawa et al. 2005). Finally, two studies using reasoning
tasks likely to involve motor simulation did report lateral
organization in the intraparietal sulcus and superior parietal
lobe, but not in motor cortex (Johnson et al. 2002; Wolbers et
al. 2003). In fact, only one study has reported clear evidence of
homuncular organization of evoked brain activity during motor
imagery (Ehrsson et al. 2003). In this experiment, participants
performed and imagined bilateral hand, toe, or tongue move-
ments, and the resulting activity was directly compared across
tasks, revealing a clear superior/medial to inferior/lateral pro-
gression in M1 during both execution and imagery. The results
also provided some evidence for homuncular organization in
SMA and PM during imagery, but the data were less clear.
Thus more data are clearly needed regarding both lateral and
homuncular somatotopic organization during motor imagery.
The absence of evidence is surprising, given that such data are
crucial to understanding the processes underlying motor imag-
ery.
Closely related to motor imagery is motor preparation.
Motor p reparation can be defined as read iness to perform an
action. It may differ from mot or imagery in a t least two
substantive ways. First, it is possible t hat the typical sub-
jective e xperience of motor imagery d oes not require that
one perform a motor simulation, only to retriev e a stored
high-level representation of an action, whereas motor prep-
aration requires mo tor simulation to be ready to perform the
action on command. Second, the opposite could be the case:
Retrieval o f a high-le vel action pl an may be suffi cient to
prepare a mov ement, wherea s motor simulation may be
required to create the subjective experience of motor imag-
ery.
Existing evidence suggests that motor imagery and motor
preparation do not in fact differ substantially. The neural
substrates of motor preparation have been extensively studied
in monkeys using paradigms in which the animal is given
preparatory information and has to withhold the behavior
during a delay before receiving a go cue. Brain activity during
the delay period has been observed mainly in the prefrontal
cortex, PM, SMA, M1, and the parietal cortex (Alexander and
Crutcher 1990; Romo et al. 1992; Wise and Mauritz 1985).
fMRI studies of motor preparation in humans in which similar
delayed response paradigms were used also suggest that M1,
SMA, PM, the cingulate cortex, and the cerebellum are in-
volved in motor preparation (Cui et al. 2000; Lee et al. 1999;
Ramnani and Miall 2003; Richter et al. 1997; Toni et al. 2002;
Watanabe et al. 2002; Zang et al. 2003).
There is little neuroimaging evidence for or against soma-
totopic organization during motor preparation. Most of the
studies mentioned above involved preparing an action with
only one limb, which did not permit one to assess the degree to
which activity related to motor preparation is somatotopically
organized. One study asked participants to prepare a unimanual
movement of the left or right hand, but did not test for lateral
organization (Lee et al. 1999). However, ample indirect evi-
dence comes from electrophysiological studies in which the
“Bereitschaftspotential” or readiness potential (BP) was ob-
served. The BP is a slow negative wave that develops before
the onset of the movement (Deecke et al. 1969). After a
symmetrical onset (reaching a maximum amplitude above the
SMA), BP amplitude becomes larger over the hemisphere
contralateral to the movement side (above the precentral re-
gion) (Barrett et al. 1986; Cui and Deecke 1999; McAdam and
Seales 1969). Cortical recording studies with patients also
suggest that SMA may show homuncular organization during
motor preparation (Ikeda et al. 1992; Yazawa et al. 1998,
2000). These electrophysiological results suggest that motor
preparation involves simulation.
The study reported here was designed to answer two ques-
tions about the role of simulation processes in motor imagery
and motor preparation. First, which components of the motor
system, if any, show lateral somatotopic organization during
motor imagery? Answering this question is crucial to distin-
guish between motor simulation processes and higher-level
processes that may underlie the motor activity observed during
motor imagery. Whereas Ehrsson et al. (2003) examined ho-
muncular somatotopic organization, these experiments focused
on lateral organization. Second, does the distribution of lateral
somatotopic organization during motor preparation differ from
that during motor imagery?
Lateral somatotopic organization was measured during
movement, motor imagery, and motor preparation. Participants
were asked to execute, imagine, or prepare lateral rotational
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hand movements with either their left or their right hand, while
brain activity was measured with fMRI. For the preparation
task, the movements were countermanded on most trials (no-go
trials), but occasionally the movement was executed immedi-
ately after the preparation interval (go trials).
Three features of the experimental design were optimized to
maximize power to detect lateral somatotopic organization.
First, a large number of MRI measurements were collected for
each participant. Second, components of the motor system
were anatomically identified in each participant. Third, the task
designs were constructed to dissociate brain activity caused by
motor tasks from that caused by processing of instructional
cues. Specifically, the motor preparation task was designed to
distinguish between activity related to motor preparation per se
and activity related to processing the preparation cue. Such a
distinction could not be drawn in most of the previous fMRI
studies of motor preparation (Cui et al. 2000; Lee et al. 1999;
Richter et al. 1997; Watanabe et al. 2002; Zang et al. 2003),
with a few exceptions (Ramnani and Miall 2003; Toni et al.
1999, 2002). To separate motor preparation from cue process-
ing, the motor preparation task included both go trials and
no-go trials and baseline trials on which no movement was
prepared.
M E T H O D S
Participants
Twelve participants (4 females; mean age, 23.6 yr) were recruited
from the Washington University community. All participants had
normal or corrected-to-normal vision, were native English speakers,
were right-handed as measured by the Edinburgh Handedness Inven-
tory (Raczkowski et al. 1974), and reported no history of significant
neurological problems. Participants were paid and provided informed
consent in accordance with guidelines set by the Washington Univer-
sity Humans Studies Committee.
Imaging procedures
Imaging was conducted on a Siemens 3 T Vision System (Erlangen,
Germany). Noise cancellation headphones and ear plugs were used to
dampen scanner noise. Visual stimuli were generated on an Apple
Power Macintosh G3 computer using PsyScope (Cohen et al. 1993)
and were projected onto a screen positioned at the head of the magnet
bore by a LCD projector. Participants viewed the stimuli by way of a
mirror mounted on the scanner’s head coil. Padding around the head
and a piece of tape positioned across the forehead were used to
minimize head movement.
Structural imaging included a high resolution (1 ! 1 ! 1.25 mm)
sagittal T1-weighted MP-RAGE (TR " 2,100 ms, TE " 3.9 ms, flip
angle " 7°, TI " 1,000 ms) and a T2-weighted fast turbo-spin echo
(TSE) scan. Functional data were acquired using a T2-weighted
asymmetric spin-echo echoplanar sequence sensitive to blood oxy-
genation level– dependent (BOLD) contrast (TR " 2,048 ms, TE "
25 ms, 4.0 ! 4.0 mm in-plane resolution). Whole brain coverage was
achieved with 32 contiguous 4-mm slices. Slice tilts and offsets were
prescribed in relation to the AC-PC plane on the basis of fast
automatic atlas registration of a low resolution (2-mm cubic voxel)
MP-RAGE scan. The complete imaging session lasted #2 h.
Stimuli and tasks
Parti cipants performed three tasks during the f unctional runs. In
the perform task, they were instructed to perform a r otational
movem ent of the left or right h and at 1 Hz for #4 s (4.15 s). In the
imagi ne task, they were asked to imagine performi ng such a
movem ent but to ref rain from making any movement. In both
tasks , on each trial, an L, R, or X cue was presented . When the cue
was L or R, participants were instructed to begin moving or
imagi ning moving immediately and to continue for the duration of
the cue (4.15 s). If the cue was an X, they were to rest (base line
trial s). At the end of the trial, the L, R, or X was replaced after
4.15 s by a red octagon, which was their cue to stop. The octagon
remai ned on the screen for 2 s. The total trial duration (6.14 s)
corre sponded to th ree scanner acquisition frames. In both tasks,
one-t hird of the trials were left hand trials, one-third were right
hand trials, an d on e-third were ba seline (rest) trials. In the prep are
task, the same L, R, and X cues were used. Participa nts were
instr ucted that when an L or R was presente d they should prepare
a movement of their left or right hand. They were asked to continue
to prepare throughout the 4.15-s duration of the cue. If the cue was
an X, they were instructed to rest. L or R cu es were replaced either
by a red octagon, signaling that they were not to move on that trial
(no-g o), or by a green circle, which cu ed t hem to perform the
movem ent (go). X cues were always followed by a red oc tagon.
The octagon or circle remained on screen for 2 s, for a total trial
durat ion of 6.14 s. In the pre pare t ask, one-half of the trials were
no-go trials (one-half left hand, one-half right hand), one-quarter
were go trial s (one-half left hand, one-half right hand), and
one-q uarter were baseline trials.
In the three tasks, all stimuli were shown centrally. They were
presented according to a rapid presentation randomized event-related
design (Burock et al. 1998), in which each trial type (left, right, or
baseline) appeared with equal probability on each trial.
Procedure
Participants practiced the tasks before beginning the scanning
session. First, they learned the unilateral hand rotational movement
and its pace (1 rotation/s) using a metronome. In a mock scanner
environment, they practiced the three tasks as many times as needed.
Second, they practiced the tasks on a computer and completed 36
trials of the perform task, 36 trials of the imagine task, and 48 trials
of the prepare task. Finally, they practiced the tasks during the
high-resolution structural scan and completed 24 trials each of the
perform and imagine tasks and 32 trials of the prepare task. This
extensive practice session was conducted to minimize body move-
ments and errors during the scans.
Once participants completed the initial training and were made
comfortable in the scanner, structural images were acquired, and the
slice prescription for the functional data acquisition was computed.
The final practice session was conducted during the longest structural
scan, the high-resolution MP-RAGE scan. Participants then com-
pleted nine BOLD runs, three of each task. Performance of the
perform, imagine, and prepare tasks was blocked by scanner run. The
order of the three tasks was counterbalanced across participants. Each
BOLD run for the perform and imagine tasks lasted 359 s (60 3-frame
trials plus 4 initial fixation frames) and each run for the prepare task
lasted 477 s (80 3-frame trials plus 4 initial fixation frames). The first
four frames during which a fixation cross was presented were later
dropped from the analysis to permit stabilization of the longitudinal
magnetic field.
Each scanning session was videotaped to monitor for inappropriate
movements during the tasks.
fMRI data analysis
Preprocessing included 1) compensation for slice-dependent time
shifts, 2) elimination of odd/even slice intensity differences caused by
interpolated acquisition, 3) realignment of all data acquired in each
subject within and across runs to compensate for rigid body motion
(Ojemann et al. 1997), and 4) intensity normalization to a whole brain
813SOMATOTOPY AND MOTOR IMAGERY
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mode value of 1,000. The functional data were transformed into the
stereotaxic atlas space of Talairach and Tournoux (1988) by comput-
ing a sequence of affine transforms (1st frame EPI to T2-weighted
TSE to MP-RAGE to atlas representative target), which were com-
bined by matrix multiplication. Reslicing the functional data in con-
formity with the atlas involved only one interpolation. For cross-
modal (e.g., functional to structural) image registration, a locally
developed algorithm was used.
Ten motor regions were identified in each brain hemisphere, based
on anatomic landmarks (Fig. 1). The regions were traced based on a
parcellation of the motor areas proposed earlier (Crespo-Facorro et al.
1999, 2000; Picard and Strick 1996, 2001). The borders for each of the
regions are listed in Table 1, and descriptive statistics for the regions’
locations are given in Table 2.
For each participant, the fMRI signal was averaged over each
anatomically defined region and submitted to statistical analyses
based on the general linear model. The model included effects
corresponding to left hand, right hand, and baseline trials. For the
perform and imagine tasks, for each effect, a predictor variable was
constructed by creating a variable whose value was 1 during the 4-s
actual or imagined periods, and zero otherwise, and convolving that
variable with a model hemodynamic response function (Boynton et al.
1996). To measure the overall activation in a given region, we
estimated the fMRI signal change for both left and right hand trials,
relative to rest trials. To measure lateral organization, we compared
the fMRI signal change in the hemisphere contralateral to the hand
that moved or was imagined to move to fMRI signal change in the
ipsilateral hemisphere. For each effect of interest, the magnitude for
each participant for each region was averaged across the two hemi-
spheres. Group-level activity was characterized with random-effects
t-test.
For the prepare task, separate predictor variables were created to
model the hemodynamic response to brain activity during the 4-s
preparation intervals and brain activity during the 2-s response inter-
vals. By combining separate predictors to model the preparation and
response intervals with catch trials on which no movement was
prepared, this approach provides an estimate of activity during the
preparation interval uncontaminated by the actual movements during
the response period of the go trials (Ollinger et al. 2001a,b). The
analyses reported here focus on the preparation intervals, excluding
the go and no-go intervals, as the processes involved in countermand-
ing or executing a motor plan once it is formed were not the focus of
the study. t-tests were conducted to characterize the response of each
motor region during the preparation intervals. One set of t-tests
assessed overall changes in activity in each region, and a second set
assessed the degree of lateral organization.
R E S U L T S
For all fMRI analyses, we adopted an alpha level of 0.05,
using the Bonferroni procedure to correct for multiple compar-
isons across the 10 motor regions. As this is a quite conserva-
tive procedure, we will also discuss effects of interest that
approached but did not reach statistical significance after cor-
recting for multiple comparisons.
Task-related increases in activity
Activity caused by actual movement or motor imagery was
calculated by comparing activity during the left and right trials
in each task to baseline trials in the same task. During the
perform task, increases in BOLD activity were strongest in the
primary somatosensory cortex (S1), M1, SMA, the thalamus,
and the cerebellum (Fig. 2). For S1, 10 of the 12 participants
showed increases in BOLD activity during task performance;
for the other four regions, 11 of 12 participants showed
increases. At the group level, all five of these regions showed
significant increases. PM, anterior rostral cingulate zone
(RCZa), and posterior rostral cingulate zone (RCZp) showed
trends toward changes in activity during the perform task,
which were significant at the single-region level but did not
survive correction for multiple comparisons. RCZp increased
in activity, whereas PM and RCZa decreased.
During the imagine task, robust increases were found only in
SMA (Fig. 2). All of the 12 participants showed increased
activity in SMA in this task, and activity in SMA was signif-
icant at the group level. M1 increased consistently in activity
(11 of 12 participants), but this trend was significant only at the
single-region level and failed to survive correction for multiple
comparisons. PM, caudal cingulate zone (CCZ), RCZa, RCZp,
and the thalamus all showed less consistent changes in activity,
which were significant at the single-region level but did not
survive corrections for multiple comparisons. In RCZp, the
trend was toward increasing during the imagine task (7 of 12
participants); in the other regions, the trend was toward de-
creasing (8 of 12 participants for CCZ, 9 of 12 for PM and
RCZa).
Activity caused by motor preparation was calculated by
comparing activity during left and right preparation intervals to
the baseline, including both go and no-go trials. Overall activ-
ity during the preparation period of the prepare task showed a
pattern that was similar to that for the imagine task, but weaker
and more variable across participants (Fig. 2.) In SMA, the
most consistently activated region, 9 of 12 participants showed
increases. This led to a group-level trend that was significant at
the single-region level but failed to survive correction for
multiple comparisons. S1, PM, and RCZa showed trends to-
ward decreases in activity that were significant at the single-
region level but did not survive correction for multiple com-
parisons (9 of 12 participants for S1 and RCZa, 10 of 12 for
PM).
FIG. 1. A representative example of the parcellation of the 10 motor regions
identified anatomically for each individual. Sagittal slice shows the cerebellum
(CER), thalamus (TH), caudal cingulate zone (CCZ), posterior rostral cingulate
zone (RCZp), and anterior rostral cingulate zone (RCZa). Three reference planes
are also depicted, which were defined based on tracing a line between the anterior
and posterior commissures (AC-PC line): plane A (vertical line drawn through
where the central sulcus intersects the medial wall and perpendicular to the AC-PC
line), VCA (vertical line drawn through the anterior commissure and perpendicular
to the AC-PC line), and plane B (vertical line drawn from the most anterior point
of the internal genu of the corpus callosum and perpendicular to the AC-PC line).
Axial slice shows the primary somatosensory cortex (S1), primary motor cortex
(M1), supplementary motor area (SMA), presupplementary motor area (pre-
SMA), and premotor cortex (PMC). For a precise anatomic description of each
region, see Table 1.
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Activity during motor imagery and motor preparation was
directly compared at the individual and group levels, using a
t-test of the same form as those for the analyses of the single
tasks. These provided weak evidence for differences between
motor preparation and motor imagery. In three regions, the
group-level analyses indicated trends that were significant at
the single-region level but failed to survive correction for
multiple comparisons. In M1, the increase observed during
motor imagery was smaller during motor preparation. In CCZ,
the decrease observed during motor imagery was smaller
during motor preparation. In the thalamus, the decrease ob-
served during motor imagery switched to a very small increase
in the motor preparation condition.
Lateral organization
Lateral organization in each task was calculated by compar-
ing trials on which the cued movement (e.g., right hand) was
contralateral to a given region (e.g., in the left hemisphere) to
trials on which the cued movement (e.g., right hand) was
ipsilateral to the given region (e.g., in the right hemisphere).
For the prepare task, only the preparation interval was ana-
lyzed. In the perform task, every region that showed significant
overall increases in activity also showed significant lateral
organization (Fig. 3). This reflected contralateral organization
in the cortex and ipsilateral organization in the cerebellum, as
expected. Comparing Figs. 2 and 3 indicates that lateral orga-
TABLE 1. Six borders used to trace each motor region and the plane in which the region was traced
Superior Inferior Posterior Anterior Lateral Medial Trace in Plane
S1 Dorsal surface Superior portion:
cingulate
sulcus; inferior
portion:
termination of
sulcus
Postcentral
sulcus
Central
sulcus
Lateral edge Superior to cingulate:
medial wall; inferior to
cingulate: fundus of
sulcus
Axial
M1 Dorsal surface Superior portion:
cingulate
sulcus; inferior
portion:
termination of
sulcus
Central
sulcus
Precentral
sulcus
Lateral edge Superior to cingulate:
medial wall; inferior to
cingulate: fundus of
sulcus
Axial
SMA Dorsal surface Cingulate sulcus M1 VCA* Superior
Frontal
Sulcus
Medial wall Coronal
Pre-SMA Dorsal surface Cingulate sulcus VCA* Plane B† Superior
Frontal
Sulcus
Medial wall Coronal
PM Dorsal surface Extension of the
cingulate
sulcus to
lateral edge
M1 Plane B† Lateral edge Superior frontal sulcus Coronal
CCZ Cingulate sulcus Corpus callosum Plane A‡ VCA* Lateral edge
of
cingulate
sulcus
Medial wall Coronal
RCZp Cingulate sulcus Corpus callosum VCA* Plane B† Lateral edge
of
cingulate
sulcus
Medial wall Coronal
RCZa Superior
cingulate
sulcus
Inferior cingulate
sulcus
Plane B† Anterior
edge of
cingulate
sulcus
Lateral edge
of
cingulate
sulcus
Medial wall Coronal
Thalamus Edges
determined
based on
contrast
properties of
the structure
in T1 image
Axial
Cerebellum Edges
determined
based on
contrast
properties of
the structure
in T1 image
Sagittal
*VCA: vertical line drawn through the Anterior Commissure and perpendicular to the AC-PC line. †Plane B: vertical line drawn through the most anterior
point of the internal genu of the Corpus Callosum and perpendicular to the AC-PC line. ‡Plane A: vertical line drawn through where the central sulcus intersects
the medial wall and perpendicular to the AC-PC line. S1, primary somatosensory cortex; MC, primary motor cortex; SMA, supplementary motor area; PM,
premotor cortex; CCZ, caudal cingulate zone; RCZp, posterior rostral cingulate zone; RCZa, anterior rostral cingulate zone.
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nization was generally more consistent across participants than
the overall level of activity. In S1, M1, SMA, and the thalamus,
all 12 participants showed greater activity in contralateral
cortex during movement, and in the cerebellum, all 12 partic-
ipants showed greater activity in the ipsilateral hemisphere. In
CCZ, 11 of 12 participants had greater activity in contralateral
cortex. Lateral organization was significant at the group level
in S1, M1, SMA, CCZ, the thalamus, and the cerebellum.
RCZp showed a trend toward contralateral organization (10 of
12 participants) that was significant at the single-region level
but failed to survive correction for multiple comparisons.
Lateral organization during motor imagery is shown in the
middle panel of Fig. 3. Activity was greater in contralateral
than ipsilateral M1 for all 12 participants, and greater in
contralateral SMA for 11 participants. Both effects were sta-
tistically significant at the group level. In S1 and the thalamus,
10 of 12 participants showed greater activity in the contralat-
eral hemisphere during imagined movement. This led to a
significant group-level effect in the thalamus, whereas this
trend in S1 was statistically significant only at the single-region
level.
Lateral organization during motor preparation showed a
pattern similar to that for motor imagery (Fig. 3). However,
unlike overall activity, lateral organization was stronger and
more consistent across participants during preparation than
during imagery. All 12 participants showed greater contralat-
eral activity during motor preparation in both M1 and SMA,
and 11 of 12 showed greater contralateral activity in S1 and the
thalamus. At the group level, lateral organization was statisti-
cally significant in all four regions. There was a trend toward
group-level ipsilateral organization in the cerebellum (10 of 12
participants), which was significant at the single-region level
but did not survive correcting for multiple comparisons.
Lateral organization during motor imagery and motor prep-
aration were directly compared at the individual and group
levels, using t-test of the same form as those for the analyses of
the single tasks. These provided no evidence for differences in
lateral organization between the two tasks.
Movement during the scanning sessions
The videotapes made during the scanning session were
viewed by a trained observer who coded for movement errors
during each trial. Hand movements while imagining or prepar-
ing a movement were extremely rare: one participant moved a
hand during one imagery trial and two participants moved a
hand during one trial each of the preparation task. Hand
movements during rest trials were minimal [mean " 0.73 $
FIG. 2. Overall changes in activity during motor performance, motor imagery, and motor preparation. Overall activation was calculated by comparing activity
during the left and right trials in each task to baseline trials in the same task. Each participant is represented by a single point for each region. Horizontal locations
of nearby points have been jittered for visibility. Asterisks in red indicate regions that differed significantly from 0, corrected for multiple comparisons across
regions.
TABLE 2. Location of the centers of mass of the motor region
Left Hemisphere Right Hemisphere
X Y Z X Y Z
S1 %33.9 (1.8) %31.4 (2.6) 54.3 (1.9) 35.0 (2.9) %29.3 (3.3) 55.2 (2.1)
M1 %31.4 (2.0) %18.1 (2.4) 54.1 (2.1) 31.5 (1.9) %16.8 (2.7) 55.7 (1.5)
SMA %13.1 (1.6) %10.8 (2.3) 62.6 (1.7) 13.5 (1.4) %10.4 (2.3) 63.2 (2.4)
Pre-SMA %13.4 (1.4) 7.8 (0.9) 58.4 (1.7) 13.6 (1.5) 7.8 (0.6) 60.0 (2.3)
Premotor %36.2 (1.9) 5.8 (0.9) 47.2 (2.6) 36.9 (2.2) 7.0 (1.3) 50.3 (2.1)
CCZ %6.7 (0.5) %23.6 (3.7) 37.0 (2.5) 7.8 (0.6) %22.1 (3.3) 37.6 (2.2)
RCZp %6.8 (0.8) 7.5 (0.9) 34.5 (1.9) 7.8 (0.7) 7.5 (1.0) 35.8 (2.3)
RCZa %7.9 (0.7) 28.2 (1.7) 9.6 (1.7) 9.0 (0.7) 28.1 (1.8) 10.0 (2.6)
Thalamus %8.1 (1.1) %19.9 (1.7) 7.0 (1.7) 9.8 (1.2) %18.8 (1.6) 6.8 (1.6)
Cerebellum %23.1 (1.6) %62.8 (1.1) %41.2 (1.6) 26.6 (2.0) %62.9 (0.8) %39.8 (1.5)
Locations are given as the mean x, y, and z coordinates in the atlas of Talairach and Tournoux (1988), with SD in parentheses. Note that several of these regions
are quite concave, in which case the center of mass is only an approximate description of the location of the region.
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0.99% (SD)] as were hand movements after the no-go cue
(mean " 1.67 $ 2.22%) or foot or leg movements in general
(mean " 0.96 $ 1.10%). A movement of the uncued hand was
occasionally observed during the actual movements task
(mean " 2.71 $ 3.20%) or during the go trials in the prepare
task (after the go cue; mean " 2.36 $ 2.30%).
Electromyographic experiments
Despite the fact that virtually no movement was observed
during the imagine task and during the preparation interval of
the prepare task, it is possible that small movements, not
detectable on the video image, could lead to detectable BOLD
signal in motor areas. To assess this possibility, two separate
EMG studies were conducted. A Grass polygraph (Model 7E)
was used. The EMG signal was sampled and integrated five
times per second (time constant of 0.2), and 60-Hz noise was
filtered out. The polygraph was calibrated so that each pen
deflection covered 50
!
V. The EMG was recorded from the
biceps muscles of both arms.
The goal of the first EMG study was to ensure that subjects
could perform the imagine task without actually moving.
Participants executed both the imagine and the perform tasks.
The goal of the second study was to test whether subjects could
perform the prepare task without actually moving during the
preparation period. Participants executed the prepare task,
which include a motor preparation component as well as a
performance component (go trials). For all three tasks, the task
design followed the one used during the fMRI study.
EMG experiment 1: imagine and perform tasks
Eight participants (6 females; mean age, 22.1 yr) were tested
who had not participated in the fMRI study. The perform and
imagine tasks used were the same as those used during the
fMRI study. First, after the movement was learned, participants
performed four blocks of 30 trials each (2 blocks of the
imagine task and 2 blocks of the perform task). This corre-
sponded to the practice session of the fMRI study. Second, they
performed two blocks of 39 trials each (1 of each task). This
corresponded to the second practice session of the fMRI study.
Finally, they performed two blocks of 60 trials each (1 of each
task). This corresponded to two BOLD runs of the fMRI study.
For each participant, the muscle response in each arm on
each trial of the perform and imagine tasks was estimated by
the difference between the minimum and maximum EMG
values during the trial. One set of analyses examined individual
participants’ performance. For each participant, within-subject
t-tests were conducted comparing muscle responses during
trials on which the participant imagined moving their hand to
rest trials, separately for the left and right hands. Across all
participants, no statistically significant differences were found
between imagined movement trials and rest trials [largest
t(52) " 1.6, P " 0.10]. In contrast, for every participant, the
difference between actual movements of each hand and rest
trials was statistically significant [smallest t(52) " 4.61, P &
0.0001].
A second set of analyses characterized performance of the
group as a whole with random effects t-test. The average
muscle response was computed for each participant, for each
trial type (left hand, right hand, and rest) in each task. The
results are presented in Table 3. There was no evidence that
EMG activity in the left or right bicep was greater during
imagined right movements than during rest [left: t(7) " %1.47,
P " 0.18; right: t(7) " 0.98, P " 0.36]. Similarly, activity in
the left bicep was not greater during imagined left movements
than during rest [t(7) " -.83, P " 0.43] and tended to be lower
in the right bicep during imagined left movements than during
rest [t(7) " %2.11, P " 0.07]. In contrast, EMG activity was
higher than rest in the left bicep for actual left movement and
FIG. 3. Lateral organization during motor performance, motor imagery, and motor preparation. Lateral organization was measured by comparing the signal
change in the hemisphere contralateral to the hand that executed the task to the signal change in the ipsilateral hemisphere. Format of the figure follows that of
Fig. 2, except that y-axis values describe degree of lateral organization rather than of overall activity.
TABLE 3. Mean evoked EMG activity (
!
V) in the left and right
biceps muscles while imagining a rotation of the left or hand or
while resting
Imagine
Left
Imagine
Right Rest
Left bicep 0.22 (0.23) 0.10 (0.14) 0.32 (0.52)
Right bicep 0.19 (0.16) 0.43 (0.50) 0.29 (0.26)
These values represent mean differences between maximum and minimum
values for each trial (SD across participants in parentheses). Data are from
EMG experiment 1.
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in the right bicep for actual right movement [left: t(7) " 7.19,
P & .001; right: t(7) " 9.97, P & 0.001]. Activity in the left
bicep during right movements and in the right bicep during left
movement did not differ significantly from activity during rest
[left: t(7) " 1.73, P " 0.13; right: t(7) " 1.71, P " 0.13]. In
summary, these data provided evidence that participants did
not covertly move an arm when they imagined moving it.
EMG experiment 2: prepare task
Eight participants who had not participated in the fMRI
study or in the first EMG study were tested (8 females; mean
age, 20.4 yr). The preparation task used in the fMRI study was
used. First, after the task was learned, participants performed
one block of 48 trials. This corresponded to the practice session
of the fMRI study. Second, they performed one block of 32
trials (2nd practice session of the fMRI study). Finally, they
performed two blocks of 80 trials each. This corresponded to
two BOLD runs of the fMRI study.
Recall that participants occasionally were cued to perform
the prepared movement (go trials). The EMG signal evoked by
the movement on those trials sometimes failed to return to
baseline before the next trial started. To avoid artifacts caused
by this carryover, all trials that followed a go trial were
excluded from the analyses. (In the neuroimaging analyses,
such carryover effects are controlled for by the trial counter-
balancing and statistical modeling of the carryover.) As before,
the muscle response in each arm on each trial was estimated by
computing the difference between the minimum and maximum
EMG values during the trial. To target the preparation period,
the interval examined began with the onset of the letter cue and
ended with the onset of the octagon or circle.
One set of analyses examined individual participants’ per-
formance. For each participant, within-subject t-tests were
conducted comparing muscle responses caused by prepared
movements to rest. Analyses were conducted separately for
each arm (e.g., comparing muscle responses in the right arm
during prepared right-handed movements to muscle responses
in the right arm during rest trials). Muscle responses during
preparing to move an arm were never significantly greater in
that arm than during rest trials. The only statistically significant
comparisons indicated lower EMG activity during preparation
than during rest [largest t(43) " 2.58, P & 0.02].
A second set of analyses characterized performance of the
group as a whole with random effects t-test. The average
muscle response was computed for each participant for each
trial type (left hand, right hand, and rest). The results are shown
in Table 4. Activity in the left or right bicep was not greater
during preparation of left-handed movements than during rest
[left: t(7) " %1.09, P " 0.31; right: t(7) " 0.86, P " 0.41].
Similarly, activity in the left bicep was not greater during the
preparation of right-handed movements than during rest
[t(7) " %1.23, P " 0.26] and was lower in the right bicep
during preparation of right-handed movement than during rest
[t(7) " %3.39, P & 0.02]. In summary, these data provided no
evidence that participants covertly moved an arm when they
prepared to move it.
D I S C U S S I O N
These data provide conclusive evidence for lateral organi-
zation in the motor system during both motor imagery and
motor preparation. Together with evidence for homuncular
organization during motor imagery (Ehrsson et al. 2003), they
indicate that when people imagine performing a movement or
prepare to perform that movement, they activate somatotopi-
cally mapped representations of the effectors involved. This
finding makes an important step beyond previous reports that
various components of the motor system show increased
BOLD activity during motor imagery by providing insight into
the nature of some of the computations underlying motor
imagery. In particular, this finding supports the view that both
motor imagery and motor preparation involve not just higher-
level memory retrieval and spatial attention processes, but also
motor simulation. One possibility is that somatotopically or-
ganized activity during motor imagery and motor preparation
reflects the computation of the inverse kinematics or inverse
dynamics of the imagined or prepared movement.
Lateral somatotopic organization
Across the motor system, lateral organization was strongest
during actual movement and somewhat weaker during imag-
ined and prepared movement. Replicating previous studies
(Colebatch et al. 1991; Fox et al. 1985; Grafton et al. 1991;
Mayer et al. 2001), statistically significant contralateral orga-
nization during actual movement was observed in S1, M1,
SMA, CCZ, and the thalamus, and significant ipsilateral orga-
nization was found in the cerebellum. Of special interest here,
significant lateral organization was observed during both motor
imagery and motor preparation in M1, SMA, and the thalamus,
and also in S1 during the motor preparation task.
The finding of lateral organization during motor preparation
is consistent with electrophysiological studies of neural activity
before the onset of a planned movement, known as BPs or
readiness potentials (Barrett et al. 1986; Cui and Deecke 1999;
McAdam and Seales 1969). Results from previous EEG re-
cordings from the skull surface suggest that BPs have two
components: an early one (BP1) that is not lateralized and
originates from SMA and a later one (BP2) that is contralateral
to the prepared movement and originates from M1 (Barrett et
al. 1986; Cui and Deecke 1999; McAdam and Seales 1969).
Our data suggest that the neural generators of both BP1 and
BP2 are lateralized. Given the low spatial resolution of elec-
trophysiological studies, left and right SMA cannot usually be
dissociated. This may explain why lateralization of potentials
originating in this area had not been observed earlier. In fact,
a recording study on patients with implanted electrodes suggest
that BPs are lateralized even in SMA (Ikeda et al. 1992).
TABLE 4. Mean evoked EMG activity (
!
V) in the left and right
biceps muscles while preparing a movement of the left or
right hand
Prepare
Left
Prepare
Right Rest
Left bicep 7.57 (5.12) 7.61 (5.04) 7.94 (5.69)
Right bicep 10.17 (5.75) 9.43 (5.57) 9.93 (5.48)
These values represent mean differences between maximum and minimum
values for each trial (SD across participants in parentheses). Data are from
EMG experiment 2.
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Overall magnitude of evoked responses
As was the case for lateral organization, the overall evoked
response was also stronger throughout the motor system during
actual movement than during motor imagery or motor prepa-
ration. Statistically significant increases during actual move-
ment were observed in M1, S1, SMA, the thalamus, and the
cerebellum. During motor imagery and motor preparation, the
overall level of activity throughout the motor system was
substantially reduced, remaining statistically significant only in
SMA during motor imagery. Overall changes activity in M1
during motor imagery did not differ significantly from baseline
activity, replicating previous results (Binkofski et al. 2000;
Decety et al. 1988, 1994; Gerardin et al. 2000; Stephan et al.
1995). There were no instances in which evoked responses to
motor imagery or motor preparation were larger in magnitude
than evoked responses to actual movements. This pattern is
consistent with previous studies of motor imagery (e.g., Porro
et al. 1996). Weaker activity in M1 during imagery than during
motor execution may come from inhibition originating in SMA
during motor imagery (Solodkin et al. 2004).
It is also possible that weakness of overall activity during
motor imagery and motor preparation was the result of the
repetitive movement used in this study. Indeed previous re-
search has shown that the amplitude of activity varies with task
complexity, in particular in SMA (Gordon et al. 1998; Shi-
basaki et al. 1993). Moreover, with repetition of the task,
participants may rely less on motor simulation than on memory
retrieval, which would reduce the lateralization pattern. It
would be interesting to replicate our results using more com-
plex motor tasks, such as finger-to-thumb opposition se-
quences. Regarding the relation between lateral organization
and overall activity in SMA, there are two possible outcomes.
First, it is possible that the greater the activity in SMA, the
greater the lateral organization. This would suggest that the
same neurons in SMA represent the limb to be moved and the
sequence of movement to be performed. Second, it is possible
that the lateral organization is independent of the overall level
of activity in SMA. This would suggest that different neurons
represents the limbs to be moved and the sequence of action to
be performed.
Importantly, an EMG study of performance of the three
tasks used found no evidence for micromovements during the
motor imagery task. This is consistent with previous reports of
similar tasks (Binkofski et al. 2000; Luft et al. 1998; Roth et al.
1996), although EMG activity has been shown during motor
imagery in other studies (Bonnet et al. 1997; Livesay and
Samaras 1998; Stephan et al. 1995). The present EMG data
indicate that the observed BOLD changes during motor imag-
ery in this paradigm cannot be attributed to actual movements.
The same conclusions apply to the motor preparation period of
the preparation task because no EMG activity was observed
during this interval.
Functional role of anterior cortical motor areas
Minimal overall changes in activity and minimal lateral
organization were observed in pre-SMA, PM, RCZa, and
RCZp. These findings support the view that these regions have
more in common with prefrontal cortex than lower levels of the
motor hierarchy. This interpretation is consistent with a num-
ber of previous findings. Although SMA has been associated
with motor execution per se in previous studies, pre-SMA has
not (Picard and Strick 1996). These results also are consistent
with the proposed dissociation between CCZ and the rostral
cingulate zones (RCZa and RCZp) (Picard and Strick 2001),
with RCZa implicated in conflict monitoring rather than motor
function per se (Braver et al. 2001). However, these results
provide no evidence for activity in RCZp during actual, imag-
ined, or prepared movement. One possibility is that the move-
ments studied here were too simple to evoke RCZp activity.
The relative absence of activity in PM during these tasks is
somewhat surprising, given its known role in motor planning
and execution (Dum and Strick 2002; Gerardin et al. 2000).
However, as with RCZp, it is possible that the movements
studied here were too simple to evoke responses in PM.
Another possibility is that previous studies, which used looser
anatomic criteria and/or spatial smoothing, conflated M1 ac-
tivity with activity in PM. Also of note, no evidence was found
for lateral organization in PM. This would seem to be at odds
with the finding of somatotopic organization in PM in nonhu-
man primates; however, microstimulation recordings indicate
that the hand area of monkey premotor cortex is fairly superior
and medial (Godschalk et al. 1995), which could place its
homolog in the human SMA.
Functional role of the cerebellum
Large portions of the cerebellum have been shown to play a
major role in motor control and coordination and have signif-
icant reciprocal connections with motor and somatosensory
cortex (Thach et al. 1992). These connections are relayed in
part through somatotopically mapped connections in the thal-
amus (Vitek et al. 1996). The cerebellum increased in activity
during motor performance but not during motor imagery or
motor preparation. Reductions in cerebellar activity during
motor imagery compared with motor execution had been ob-
served in previous studies (Decety et al. 1994; Lotze et al.
1999). Lotze et al. (1999) suggested that such reduced activity
may reflect inhibitory processes to block the execution of the
imagined movement. The neural generator(s) of such inhibi-
tory signals have not yet been identified. These signals may be
generated within the cerebellum itself (Lotze et al. 1999) or
may originate in the cortex. Potential candidate areas in the
cortex are the middle frontal gyrus and the inferior frontal
gyrus, which have been shown to be involved in the inhibition
of inappropriate responses (e.g., Brass et al. 2001; Watanabe et
al. 2002).
Interactions between excitation and inhibition
The thalamus was robustly activated during motor perfor-
mance, but showed essentially no overall change in activity
during motor imagery and motor preparation. (The largest
change in the thalamus during imagery or preparation was a
nonsignificant decrease.) However, statistically significant lat-
eral organization was observed in the thalamus during both
imagery and preparation. A similar pattern was observed in S1.
Regarding S1, one possibility is that nonspecific inhibitory
signals, which are not somatotopically organized, are inte-
grated with somatotopically organized excitatory signals in
regions projecting to S1, allowing motor simulation processes
819SOMATOTOPY AND MOTOR IMAGERY
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to proceed without the production of frank movements. Con-
sistent with this interpretation is the report of a patient with
bilateral parietal lesions (Schwoebel et al. 2002). When asked
to imagine performing movements, he frequently carried them
out, without awareness of doing so. Regarding the thalamus,
the processes involved may be different. The thalamus is
thought to select potential action plans via excitatory reciprocal
connections with the cortex and inhibitory connections coming
from basal ganglia (Mink 1996). The inhibitory output of the
basal ganglia is believed to act as a selective brake on motor
generator in the cortex to inhibit unwanted movements. Thus
decreased activity in the thalamus during motor imagery and
preparation compared with motor execution may reflect iden-
tical levels of inhibition during execution and imagery/prepa-
ration, but reduced excitatory activity coming from the cortex
(i.e., less amplification of correct movement plans).
Relationship between motor imagery and motor preparation
As noted in the Introduction, it is possible that motor
simulation could be selectively involved only in motor imag-
ery, or, alternatively, motor simulation could be selectively
involved only in motor preparation. The level of activity and
the pattern of lateralization elicited across motor regions during
motor preparation were quite similar to the level of activity and
the pattern of lateralization elicited by motor imagery. This
result suggests that motor simulation is involved in a similar
way during motor imagery and preparation. Such results sup-
port Jeannerod (1994)’s proposition that the difference be-
tween motor imagery and motor preparation is one of degree
rather than kind. Motor imagery is necessarily conscious,
whereas motor preparation may not give rise to a conscious
sensation. Jeannerod argued that when motor preparation is
prolonged, the intention to act becomes a motor image of the
action.
In addition to motor simulation, preparing a manual re-
sponse also may depend on attention to the planned response,
analogous to the role of visual attention in preparing to process
visual stimuli or move the eyes. Lesion and neuroimaging
studies suggest that such motor attention depends on areas in
the left parietal cortex (Rushworth et al. 1997, 2001). The
functional relationship between motor attention and motor
preparation is not yet clear. One possibility is that the parietal
attention system is necessary for performing a motor simula-
tion during motor preparation. It would be of interest to test
whether patients with compromised attentional systems show
reduced somatotopic mapping in frontal cortex during motor
preparation tasks.
There was weak evidence that the overall changes in activity
during motor imagery in M1 and CCZ were reduced in mag-
nitude during motor preparation. These trends, though sugges-
tive and consistent with previous results (Stephan et al. 1995),
should be interpreted with some caution. Difference such as
these could reflect specifics of the task design rather than
general processing differences between motor imagery and
motor preparation. For example, the presence of a variable go
or no-go cue after the preparation interval during the motor
preparation task may have encouraged participants to stop
motor preparation processes immediately, whereas the constant
stop cue during the motor imagery task may have exerted a
weaker stopping effect on motor imagery processes. Or, the
relative rarity of go trials may have reduced motivation to
prepare during the preparation interval. This leaves open the
question of whether activity during motor preparation reflects
reduced excitatory activity throughout the motor network, or
active inhibition at some stage. This question should be ad-
dressed in future studies focusing on inhibitory processes per se.
In conclusion, the results reported here support the hypoth-
esis that motor imagery is a mental analog of motor execution.
In the same way that visual mental imagery may preserve
specific visual properties of visual percepts, motor imagery
seems to include motor simulation processes closely related to
the form and timing of actual movements. In the domain of
visual imagery, studies of the degree to which mental imagery
preserves the spatiotopic mapping of visual experience have
been both contentious and informative (Klein et al. 2004;
Kosslyn 1994). It is our hope that these findings may produc-
tively constrain theories of motor imagery; in particular, we
believe these data render less tenable theories that posit that
motor imagery involves only the early stages of the motor
hierarchy.
The finding that motor imagery seems to engage motor
simulation has implications for neurorehabilitation, training,
and problem-solving. Measurement of somatotopically orga-
nized activity during motor performance and motor imagery
may provide a means to more accurately diagnose the nature of
processing deficits in patients with apraxia or other movement
disorders. The observation of motor simulation during mental
imagery provides a mechanism for the well-documented ben-
efits of mental practice of physical activities (e.g., Feltz and
Landers 1983). Finally, motor simulation during mental imag-
ery may interact with other forms of imagery to allow people
to simulate the consequences of actions they might take before
committing to the performance of a movement sequence.
A C K N O W L E D G M E N T S
We thank R. Larsen for the loan of the electrical recording equipment and
E. Akbudak, M. McAvoy, R. Buckner, and A. Snyder for technical assistance
with functional MRI data collection and analysis methods.
G R A N T S
This research was supported in part by the Mallinckrodt Institute of
Radiology.
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