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The Journal of Neuroscience, February 15, 2003, 23(4):1432
Sleep-Related Consolidation of a Visuomotor Skill: Brain
Mechanisms as Assessed by Functional Magnetic Resonance
Imaging
Pierre
Maquet1, 4,
Sophie
Schwartz2,
Richard
Passingham1, 3, and
Christopher
Frith1
1 Wellcome Department of Imaging Neuroscience, London
WC 1N 3BG, United Kingdom, 2 Institute of Cognitive
Neuroscience, University College London, London WC 1N 3AR, United
Kingdom, 3 Department of Experimental Psychology,
University of Oxford, Oxford OX1 3UD, United Kingdom, and
4 Cyclotron Research Centre, University of Liège,
4000 Liège, Belgium
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ABSTRACT |
Subjects were trained on a pursuit task in which the target
trajectory was predictable only on the horizontal axis. Half of them
were sleep deprived on the first post-training night
(n = 13). Three days later, functional magnetic
resonance imaging revealed task-related increases in brain responses to
the learned trajectory, as compared with a new trajectory. In the
sleeping group (n = 12) as compared with the
sleep-deprived group, subjects' performance was improved, and their
brain activity was greater in the superior temporal sulcus (STS).
Increased functional connectivity was observed between the STS and the
cerebellum and between the supplementary eye field and the frontal eye
field. These differences indicate sleep-related plastic changes during
motor skill learning in areas involved in smooth pursuit eye movements.
Key words:
functional neuroimaging; functional magnetic
resonance imaging; statistical parametric mapping; functional
connectivity; procedural memory; memory consolidation; sleep; sleep
deprivation; smooth pursuit eye movements
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Introduction |
Several lines of evidence indicate
that sleep is involved in memory trace consolidation. First, sleep
organization can be modified by recent learning both in animals
(Hennevin et al., 1995 ) and in humans (Maquet, 2001 ). Second, neurons
involved in recent waking experience are reactivated during
post-training sleep in rodent hippocampus (Pavlides and Winson, 1989 ;
Wilson and McNaughton, 1994 ; Kudrimoti et al., 1999 ; Nadasdy et al., 1999 ; Louie and Wilson, 2001 ) and in human cortex (Maquet et al., 2000 ). Third, sleep deprivation alters subsequent performance on the
learned task in animals (Hennevin et al., 1995 ; Smith, 1995 ) and in
humans (Maquet, 2001 ). Sleep deprivation studies suggest that sleep
occurring during the first hours after training sessions in animals
(Hennevin et al., 1995 ; Smith, 1995 ) or during the first post-training
night in man (Stickgold et al., 2000 ) plays a critical role in memory
trace consolidation, as measured by behavioral performance at a later date.
In several perceptual and motor skill learning tasks, performance
continues to improve hours after the training session has ended (Karni
and Sagi, 1993 ; Karni and Bertini, 1997 ; Karni et al., 1998 ). This
so-called "slow learning" is believed to lead to the consolidation
of the memory trace and to be sleep dependent (Maquet, 2001 ).
Accordingly, the learning of the pursuit rotor task, a visuomotor
procedural learning task, is known to be sensitive to sleep deprivation
on the first post-training night (Smith and MacNeill, 1994 ).
The effects of sleep on the cerebral correlates of skill learning has
not yet been characterized in humans. The aim of the present study was
to compare learning-dependent changes in regional brain activity after
sleep or sleep deprivation using a pursuit task (PT). We trained the
participants on a particular version of the PT (Frith, 1973 ) in which
the target trajectory was predictable on the horizontal but not on the
vertical axis (see Fig. 1A). Half of the subjects
were totally sleep deprived during the first post-training night (see
Fig. 1B). Three days later, during a functional
magnetic resonance imaging (fMRI) scanning session, they were exposed
to the previously learned trajectory and also to a new one in which the
predictable axis was vertical. This experimental design allowed for the
assessment of the effects of learning on brain activity, using
within-subject comparisons between learned and new conditions.
Our objective was to provide evidence that sleep deprivation disrupts
the slow processes that lead to memory consolidation. In contrast to
others (Drummond et al., 2000 ), we were not aiming to characterize the
immediate effect of sleep deprivation on human performance or
cognition. This is the reason why we adopted an experimental protocol
in which both sleeping and sleep-deprived subjects were retested after
at least two complete nights of sleep, i.e., in a state of arousal that
was similar across the two groups and between the training and retest
sessions (Stickgold et al., 2000 ).
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Materials and Methods |
Subjects. Normal subjects (13 females, 12 males; age
range: 19-24 years) were recruited by advertisement. They had no
history of medical, neurological, or psychiatric disease. None of them was on medication. The quality of their usual sleep was assessed by the
Pittsburgh Sleep Quality Index questionnaire (Buysse et al., 1989 ) to
check for the absence of obvious disturbances of sleep/wakefulness
cycles. The subjects were right-handed as indicated by the Edinburgh
Inventory (Oldfield, 1971 ). The subjects gave their written informed
consent to the study, which was approved by the Joint Ethics Committee
of National Hospitals and Institute of Neurology.
Experimental protocol. Subjects performed the PT
while lying in the scanner (see Fig. 1). A mirror box allowed them to
view the display (18 × 23°) generated by a PC (480 × 640 resolution; refresh rate 60 Hz) and projected by liquid crystal display
projector. Subjects were simultaneously shown the positions of a moving
target (red circle, 1°) and of a joystick (yellow dot, 0.5°;
refresh rate 25 Hz). By manipulating a custom-made joystick with their left hand, the subjects could move the position of the joystick on the
screen. The instruction was to maintain the joystick position as close
as possible to the moving target at all times. The left hand was chosen
to ensure that performance on the PT would not rely on preexisting
motor skills such as writing or drawing and to minimize interference
with normal daytime activity during the post-training period (because
the subjects were all right-handed). The subjects did not know that the
trajectory followed by the target (Fig.
1A) was manipulated in
a similar way as in Frith (1973) . The coordinates of the target were
described by a single sine wave (frequency: 0.423 Hz) along the
horizontal axis, and by the sum of four nonharmonic sine waves
(frequency: 0.267, 0.341, 0.413, and 0.673 Hz) on the vertical axis. As
a result, the trajectory followed by the target was easily predictable
along the horizontal axis but very difficult to predict along the
vertical axis. This trajectory was used to train the subjects and will
be referred to as the "learned trajectory."

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Figure 1.
Experimental design. A, The
bi-dimensional trajectory followed by the target during the training
session combined a regular movement on the horizontal axis and an
irregular movement on the vertical axis. B, Two
experimental groups were compared: half of the subjects were totally
sleep deprived during the first night after training on the PT and half
were allowed to sleep normally. All subjects continuously wore an
actimeter and were scanned while doing the PT on the third day after
training (see Materials and Methods). C,
Computation of the behavioral performance at the PT. Continuous
line indicates the joystick trajectory; dotted
line indicates the target trajectory. At each time point (40 msec), the distance between the target and the subject's trajectory
was computed. The subject was considered on target if this distance
(arrow) was smaller than half the SD of the
joystick-to-target distances observed for the subject during the
training session. The bottom display shows a typical
distribution of the joystick-to-target distances for one subject.
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The subjects were trained on the task in the scanner on the afternoon
of day 1, during a period of 5 min, between 2 and 6 P.M. (Fig.
1B). The subjects were not scanned during the
training session. They were trained in the MR scanner to ensure that
the task would be performed under the same conditions during the
training and retest sessions, i.e., with the same physical
characteristics for the presentation of visual inputs and same position
for motor performance. The training session was deliberately kept
short. The subjects developed only an imperfect skill on the task. In such a situation, learning and memory are more likely to depend on
sleep processes (Hennevin et al., 1995 ).
The subjects were only scanned on day 4, at the same time of day as
during the training session. During this scanning session, 30 18-sec-long blocks of PT were performed. Half of the blocks used the
learned trajectory. In the remaining blocks, the trajectory was rotated
by 90°, in such a way that the predictable axis became the vertical
one. Because the subjects had never been exposed to it, this trajectory
is referred to as the "new trajectory. The order of the learned and
new trajectories was randomized over subjects. Periods of fixation,
also 18 sec long, were interleaved between the PT blocks. The
coordinates of the target and the joystick were recorded every 40 msec,
during both the training and the scanning sessions (see below).
Functional MRI time-series were acquired at 2 Tesla using a Magnetom
VISION (Siemens, Erlangen, Germany) whole-body MRI
system, equipped with a head volume coil. Multislice T2*-weighted fMRI
images were obtained with a gradient echo-planar sequence using an
axial slice orientation (echo time = 40 msec; repetition time = 3.65 sec; 64 × 64 × 48 voxels; voxel size: 3 × 3 × 3 mm3). After the six initial scans were
discarded (to allow for magnetic saturation effects), each time-series
comprised 300 volume images. A structural T1-weighted sequence scan was
also obtained. The eye position was monitored on-line using an
eye-trajectory system (ASL, Model 504; Applied Science
Group, Bedford, MA).
The subjects were prospectively pseudorandomized into two groups (Fig.
1B). In the first group (sleeping group), the
subjects went home after the training session and slept as usual during the three post-training nights. In the second group (sleep-deprived group), the subjects stayed awake in the laboratory and were monitored during the first post-training night (until 7.00 A.M.). During this
night, the ambient light and the subjects' physical activity were
maintained as low as possible, and the subjects remained under the
constant supervision of the experimenters. They pursued their
usual activities on the following days and slept at home during the two
remaining nights. After a single night of total sleep deprivation,
individual performance on several tasks and subjective sleepiness are
completely restored after two nights of recovery sleep (Bonnet,
2000 ).
The physical activity of all the subjects was monitored continuously by
actimetry, from the end of the training session to the beginning of the
scanning session (sampling rate: 1/30 Hz) (Actiwatch, Cambridge
Technology). Subjects wore the actimeter on their right wrist
and were also asked to fill in a sleep log during the entire
experimental period.
All of the subjects were randomly assigned to each experimental group
and exposed to the same task characteristics during the training
session. Thus, no difference in the improvement in performance along
time was expected between the two groups unless the sleep
deprivation had a significant and deleterious effect on the acquisition
of this visuomotor skill.
Analysis of behavioral data. First, the subject's error was
computed as the Euclidean distance between the target and the joystick
location for each time point of the training session, and the SD was
computed (Fig. 1C). For the scanning session, the same
measures were computed at each time point during the PT blocks. The
time on target was used as the metric of subjects' performance. For each subject, it was computed as the number of time points (each 40 msec long) during which the distance was smaller than half the SD
computed during the training session. This method ensured that the same
metric was used to compute the performance in the training and scanning
session on an individual basis. Summing these points within each PT
block provided a measure of the time on target achieved during this block.
For the training session, the 5 min performance data were divided into
19 blocks of equal duration. These behavioral data were modeled by a
general linear model with repeated measures, using the repetition of
consecutive blocks as within-subject factor and the group (sleep vs
sleep deprived) as between-subject factor. For the scanning session,
the data were modeled by a general linear model with repeated measures,
using the repetition of consecutive blocks and the trajectory (learned
vs new) as within-subject factors and the group (sleep vs sleep
deprived) as between-subject factor. Post hoc t
tests were computed for differences between the groups or between trajectories.
The actimetry data were integrated over post-training periods of day
(D2, D3) and night (N1, N2, N3), defined by the time-to-bed and
wakeup times indicated in the individual sleep logs. The D4 data were
not considered in the analyses because they usually spanned only a few
hours (from wake up to the scanning session). Data were modeled by a
general linear model with repeated measures, using consecutive night
and day periods as within subject factor and the group (sleep versus
sleep deprived) as between subject factor. Post hoc
t tests checked for differences between the group for each
relevant time period.
Analysis of fMRI data. Functional volumes were analyzed
using Statistical Parametric Mapping http://www.fil.ion.ucl.ac.uk/spm/. They were corrected for head motion, spatially normalized to an echo
planar imaging template of 3 × 3 × 3 mm3 voxels conforming to the Montreal
Neurological Institute space, spatially smoothed with a Gaussian kernel
of 8 mm full-width at half-maximum (FWHM), and high-pass filtered
(1/140 Hz).
For each subject, changes in brain regional responses were estimated by
a general linear model in which the activity evoked in the PT blocks
with learned or new trajectory was modeled by boxcar waveforms
convolved with a canonical hemodynamic response function. Movement
parameters derived from realignment of the functional volumes were
included as covariates of no interest. The effects of interest were
then tested by linear contrasts, generating statistical parametric maps
[SPM(T)]. The images resulting from the comparison between learned
and new conditions were then further spatially smoothed (6 mm FWHM
Gaussian kernel) and entered in a second-level analysis, corresponding
to a random effects model, to account for intersubject variance in the
main effect of learning. Two analyses were performed. First, parameter
estimates for the learned and new conditions were compared in a
one-sample t test across all subjects to describe the main
effect of learning regardless of the group. Second, a two-sample
t test was used to evaluate the trajectory-by-group interaction.
On the basis of published work on motion perception, smooth eye
pursuit, eye-hand coordination, and motor learning, we expected that
changes in brain responses would occur in areas that participate in
performing the task: motion-related areas in the occipital and temporal
cortices, intraparietal sulcus, premotor cortex [including frontal eye
field (FEF)], supplementary motor area [including supplementary eye
field (SEF)], primary motor cortex, and cerebellum. Small-volume
correction of our fMRI results (Worsley, 1996 ) was computed on a 10 mm
sphere around the average coordinates published for the corresponding
relevant a priori location (Table 1, last column).
To examine whether sleep deprivation alters long-term functional
connectivity, analyses of psychophysiological interactions were
performed. These analyses searched for a modulation by the training
condition of correlations between the learning-related areas (see
below) [right dentate nucleus (DN); left supplementary motor area
(SMA)] and other distant areas (Friston et al., 1997 ). A new linear
model was constructed for each subject, using three regressors (plus
the covariates of no interest as in the initial model). One regressor
was the difference between the two main regressors of interest (learned
minus new). The second regressor was the activity in the reference
area. The third regressor represented the interaction of interest
between the first (psychological) and second (physiological)
regressors. Significant contrasts for this psychophysiological
regressor indicated a learning-related change in the regression
coefficients between any reported brain area and the reference region.
After smoothing (6 mm FWHM Gaussian kernel), these contrast images were
then entered into a second-level (random effects) analysis. A
two-sample t test was performed to assess the between-group
differences in learning-dependent changes in functional connectivity
(voxelwise threshold, p < 0.001 uncorrected; small-volume correction at p < 0.05).
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Results |
Behavioral data
Two subjects were discarded: one in the sleeping group because of
task-related movement artifacts in the fMRI time-series and another in
the sleep-deprived group because the subject's sleep was compromised
on nights 2 and 3 for professional reasons. The final number of
subjects in the sleeping and sleep-deprived groups was 11 and 12, respectively. At debriefing, none of them was aware of the different
spatial properties of the learned and new trajectories.
The behavioral results appear in Figure
2A. The statistical
analyses were run separately for the training and the scanning sessions. This is because the learning effect could be assessed within
the scanning session, by comparison of the subjects' performance on
the learned versus new trajectory. This analysis of behavioral data
shadows the analysis of fMRI data, essentially based on the within-session learning effect during the scanning session (see below).
For the training session, there was a significant effect of the
repetition of training blocks (F(18) = 1.659; p = 0.044), reflecting the improvement of
subjects' performance with time. There was also a significant effect
of the group (sleep vs sleep deprived)
(F(1) = 4.669; p = 0.041). Post hoc t tests confirmed that the
performance of the sleep-deprived subjects was lower than that of the
subjects in the sleeping group (p < 0.001). The repetition by group interaction was not significant
(F(18,1) = 0.518; p = 0.950), suggesting that the rate of learning during the training
session was not different between the two groups. For the post-training
session during fMRI, the effect of the trajectory (learned vs new) was
significant (F(1) = 18.603;
p < 0.001). Post hoc paired t
tests showed that the effect of trajectory (learned vs new) was
significant in both the sleep group (p < 0.001)
and the sleep-deprived group (p = 0.015). Most
importantly, the trajectory-by-group interaction was also significant
(F(1,21) = 4.862; p = 0.038), indicating that the sleeping subjects were significantly better on the trained than the new trajectory in comparison with the sleep-deprived group. The repetition (of the blocks) by group interaction showed a nonsignificant trend
(F(14,9) = 2.778; p = 0.055). No other interactions were significant. The group effect was
not significant (F(1) = 2.21;
p = 0.151), suggesting that the sleep-deprived subjects
were as good as the sleeping subjects (regardless of the status of the
trajectory).

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Figure 2.
Behavioral data. A, Time on target
(arbitrary units) during the training and scanning sessions, in the
sleeping and sleep-deprived group, for the learned (continuous
line) and new (dotted line) trajectories. Mean
time on targets is shown for successive 15 sec blocks; error bars
represent SEM. Units are the number of 40 msec intervals spent on the
target. B, Average movement activity measured by
actimetry in the sleep (white bars) and sleep deprived
(hatched bars) during the post-training period
(N1-N3). The activity was significantly
higher during the first post-training night (N1) in the
sleep-deprived group (*p < 0.01). No difference was noted
on the following days (D2, D3) and nights
(N2, N3). Error bars represent
SEM.
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The difference in performance during the training session is unlikely
to confound our results. First, because of the pseudorandomization of
the subjects and because the trajectories had the same features in all
subjects, differences in performance during the training session could
not be attributable to either a systematic population bias or a
variation in task difficulty. Second, the learning of the pursuit task
is a robust and replicable phenomenon (Eysenck and Frith, 1977 ).
Consequently, no ceiling effect is expected with the pursuit task, even
in the sleep-deprived subjects. Third and most importantly, the study
was designed in such a way that the learning effect could be assessed
by within-session effect. The critical contrast is the difference in
performance between the learned and the new trajectory during the
scanning session itself, regardless of the average value of
performance. In our case, performances during the scanning session were
matched between groups.
Actimetric data are shown on Figure 2B. The analysis
showed a significant overall variation of activity across days and
nights (F(4) = 125; p < 0.001) and a significant activity by group interaction (F(4,18) = 5.143; p = 0.001). Post hoc t tests comparing the two groups
showed a significant increase in activity during the first night in the
sleep-deprived subjects (p < 0.001), confirming
the efficacy of the experimental treatment. The activity during the second day tended to be lower in the sleep-deprived group, although the
difference was not significant (p = 0.071). No
other comparison approached significance.
Functional MRI data
The results are summarized in Table 1.
Main effect of learning
The responses to the learned trajectory were significantly larger
than to the new trajectory in three regions, regardless of the group:
the lateral nuclei of the cerebellum (hereafter referred to as DN), a
left medial frontal area, and the right cuneus (Fig.
3A). The latter did not
survive small-volume correction, using the coordinates of the nearest
motion-related area described in the literature (V3a; see references in
Table 1). It will not be discussed further.

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Figure 3.
fMRI data. A, Main effect of
learning. The first column shows the activation foci
(SEF/SMA on the top panel; DN on the bottom
panel), superimposed on the average normalized
structural MR image of the group. The second column
shows the peristimulus time course of the response in the corresponding
area (continuous line, responses to the learned
trajectory; dotted line, for the new trajectory). Error
bars represent SEM across subjects. B, Results of the
second-level analysis based on psychophysiological interactions. On the
top and bottom panels, brain areas are
connected with the SEF/SMA and DN, respectively, more tightly for
learned than new trajectories, and more so in sleeping subjects than in
the sleep-deprived group. The red arrowhead shows the
second area detected in the STS. Displays are thresholded at
p < 0.001 and coded according to the corresponding
color scale.
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The location of the DN activation was confirmed in reference to the
cerebellar atlas of Schmahmann et al. (2000) . The effect of learning on
DN was contralateral to the moving hand. Contralateral cerebellar
activations have already been reported in other learning situations,
for instance in eyeblink conditioning (Logan and Grafton, 1995 ;
Blaxton et al., 1996 ; Ramnani et al., 2000 ) and rhythm learning (Ramnani et al., 2000 ).
The left medial frontal area lies within the SMA (see references in
Table 1) at a level identified as the supplementary eye field (SEF)
(see references in Table 1). A medial prefrontal response ipsilateral
to the used hand is not unexpected. There are extensive
interhemispheric connections between homologous supplementary motor
areas (McGuire et al., 1991 ). Moreover, it could also be the case, as
for the left premotor cortex (Schluter et al., 1998 , 2001 ), that the
left SMA controls both hands and is dominant for action.
Trajectory-by-group interaction
The responses in the depth of the posterior superior temporal
sulcus (STS) to the learned trajectory were significantly larger in the
sleep group than in the sleep-deprived group (Fig.
4A). In other words,
the posterior STS responded more to the learned trajectory than to the
new one if the subjects were allowed to sleep on the first
post-training night.

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Figure 4.
Trajectory by group interaction. A,
Left panel, The superior temporal sulcus is
significantly more active in the learned condition in sleeping
subjects. The statistical results, displayed at p < 0.001, are superimposed on the average normalized structural MR
image of the group. Right panel, Peristimulus time
courses of STS response (continuous line, responses to
the learned trajectory; dotted line, for the new
trajectory; top row, sleep group; bottom
row, sleep deprivation group). Error bars represent SEM across
subjects. B, Lateral view of a glass brain in
the MNI space, showing the projections of the reported STS, as well as
MT/V5, biological motion, and related areas discussed in relation to
STS. Sources of the data displayed are indicated by first author and
year of publication.
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Psychophysiological interactions
A psychophysiological analysis using the DN as reference region
identified two areas in the STS area, a few millimeters away from the
area detected in the trajectory-by-group interaction. This result
indicates that these two areas are connected more tightly with the DN
in the context of the learned than the new trajectory, and more so in
sleeping subjects than in the sleep-deprived group. By applying a
small-volume correction, both were included in the same sphere around
the reference coordinates (Fig. 3B, bottom
panel), but only one peak survived the
p < 0.05 threshold.
The left SEF/SMA was more tightly correlated with the right premotor
cortex in the sleeping than in the sleep-deprived subjects in response
to the learned trajectory (Fig. 3B, top
panel). The area within the premotor cortex
corresponds to the frontal eye field (FEF) (see references in Table 1).
The activation lay in the depth of the precentral sulcus, in keeping
with the location of the pursuit area reported by Rosano et al.
(2002) .
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Discussion |
The present data reveal two important aspects of the cerebral
correlates of PT learning. First, they extend previous positron emission tomography (PET) results obtained on the standard version of
the task (pursuit rotor task using a circular trajectory). In our
particular case, because of intrinsic properties of the target path, an
optimal performance could only be achieved by developing implicitly
some model of the motion characteristics of the learned trajectory.
Furthermore, the pattern of brain responses observed here suggests that
in learning the task, the acquisition of appropriate ocular responses
is probably more critical than the development of new motor sequences
for the hand or to the improvement of eye-hand coordination. Indeed,
interactions between temporal cortex and the cerebellum as well as
between the FEF and the SEF are both implicated in conventional pursuit
eye movement pathways (Krauzlis and Stone, 1999 ).
Second, our results suggest that lack of sleep may hamper the
consolidation of recent memory traces, with detrimental effects on
later performance. In contrast, in the subjects allowed to sleep,
further processing of the memory traces is permitted during the first
post-training night. Consequently, their performance improves and
significant changes in patterns of regional brain activity are revealed
by functional neuroimaging.
Effect of learning
The performance on the learned trajectory was significantly better
than on the new trajectory in both the sleeping and the sleep-deprived
groups. The cerebral hemodynamic responses to the learned trajectory
were significantly larger than to the new trajectory in a medial
prefrontal area and the right DN.
The medial prefrontal area is probably the SEF. This would be
consistent with the psychophysiological interaction showing that this
area is functionally connected with the right FEF, a region involved in
controlling eye movements. In nonhuman primates, neuronal activity in
the SEF is related to smooth pursuit eye movements, especially when the
target motion is predictable (Heinen and Liu, 1997 ). Electrical
stimulation of this region modulates smooth pursuit eye movements (Tian
and Lynch, 1995 , 1996 ; Missal and Heinen, 2001 ). Alternatively, the
medial prefrontal region could correspond to the part of SMA that is
involved in hand action. In humans, an early PET study has shown that
SMA activity correlates with the time on target during a pursuit rotor
task (Grafton et al., 1994 ).
The increase in cerebellar signal is located in the DN. The DN has been
involved in tracking tasks (Brooks et al., 1973 ; Vercher and Gauthier,
1988 ) and in the control of visually guided movements (Mushiake and
Strick, 1993 ). Functional neuroimaging studies have described both
decreases and increases in cerebellar activity in response to learning
processes (Jenkins et al., 1994 ; Flament et al., 1996 ; Imamizu et al.,
2000 ). Recent evidence shows that the cerebellar hemispheres tend to
respond more with high movement errors, whereas a larger dentate
activation is observed when tracking performance is good (Miall et al.,
2001 ). The same observation is reported for other visually guided motor
tasks (Nezafat et al., 2001 ; Doyon et al., 2002 ).
Effect of sleep on experience-dependent brain activation
The posterior STS was found to be the only region differentially
more active for the learned trajectory in sleeping subjects than in the
context of sleep deprivation. The posterior STS (Fig. 4B) lies anterior to other motion-responsive areas,
especially the middle temporal area (MT/V5) (Watson et al., 1993 ;
Tootell et al., 1995 ; Dumoulin et al., 2000 ). Its functional role is
not yet characterized precisely. It responds to biological motion (Bonda et al., 1996 ; Puce et al., 1998 ; Grossman et al., 2000 ; Grezes
et al., 2001 ; Vaina et al., 2001 ) and to movement patterns of
interacting geometrical shapes (Castelli et al., 2000 ).
The observed posterior STS is also close to regions of the
temporal lobe that are active during smooth pursuit eye movements in
humans (Petit and Haxby, 1999 ; Schmid et al., 2001 ) (Fig.
4B). In nonhuman primates, stimulations (Komatsu and
Wurtz, 1989 ) and lesions (Newsome et al., 1985 ; Dursteler and Wurtz,
1988 ) of the superior temporal sulcus [area MT and medial superior
temporal (MST) area] affect saccades and pursuit eye movements.
The observed posterior superior temporal cortex is slightly anterior to
the areas reported for biological motion or smooth eye movements (Fig.
4B). It is even closer to the STS activation reported
during imitation of action (Iacoboni et al., 2001 ). Imitation would
require matching an observed action to an internal motor representation
and using it to organize future behavior (Rizzolatti et al., 2001 ).
Similarly, recent studies on motor preparation suggest that STS is
involved in extracting contextual and intentional cues during
goal-oriented behavior (Toni et al., 2001 ). We suggest that an internal
model of motion characteristics of the learned trajectory is built up
during the training session and consolidated during the post-training
night. At retest, to minimize the error, the information provided by
the current motion of the target has to be integrated with the stored
representation. The STS would be involved in this on-line integration,
which could not occur during the pursuit of the new trajectory. In
consequence, our results support the view that STS is not specialized
in the perception of social cues but is involved more generally in the
evaluation of complex motion patterns. Future research will have to
test these hypotheses.
Effect of sleep on experience-dependent changes in brain
functional connectivity
Psychophysiological interactions showed that the responses of the
DN were correlated with the activity in the posterior part of the STS
more tightly when the trajectory was learned than when it was new and
more so in sleeping subjects than in the context of sleep deprivation.
The STS area is the same as the one detected by the trajectory-by-group
interaction. This observation is consistent with a role of
temporo-ponto-cerebellar circuits in ocular following tasks. First,
projections from the superior temporal cortex to pontine nuclei are
identified in nonhuman primates and contribute to
cortico-ponto-cerebellar circuits (Ungerleider et al., 1984 ; Glickstein et al., 1985 ; Schmahmann and Pandya, 1991 ). These
projections are thought to be involved in smooth pursuit eye movements
in monkeys (Tusa and Ungerleider, 1988 ). Second, neurophysiological studies in primates show that ocular responses during trajectory tasks
are mediated by a pathway involving temporal areas, pontine nuclei, and
the cerebellum (Kawano et al., 1994 ; Takemura et al., 2001 ). Third,
theoretical models hypothesize that temporal cortices are involved in
building up an internal inverse model for eye movements during ocular
following responses (Wolpert et al., 1998 ). These proposals refer to
the STS in monkeys (areas MT and MST). The posterior STS area detected
here is more anterior than the human MT/V5 complex. We suggest that the
increased functional coupling between the DN and the STS might indicate
that STS provides the (ponto-)cerebellar circuits with information on
the eye trajectory appropriate for matching the learned trajectory.
These interactions occurred only in the subjects who slept during the
first post-training night.
Psychophysiological interactions also showed that the responses of SEF
were correlated with the activity in the FEF more tightly when the
trajectory was learned than when it was new and more so in sleeping
subjects than in the context of sleep deprivation. In nonhuman
primates, SEF and FEF are mutually connected (Huerta et al., 1987 ), and
neural responses in the FEF are related to smooth pursuit eye movements
(Gottlieb et al., 1994 ; Tanaka and Fukushima, 1998 ). Inactivation of
FEF impairs smooth eye movements (Shi et al., 1998 ),
whereas electrical stimulation of the FEF can generate pursuit eye
movements (MacAvoy et al., 1991 ; Gottlieb et al., 1994 ) and
modulate their gain (Tanaka and Fukushima, 1998 ; Tanaka and Lisberger,
2001 , 2002 ). We suggest that the increased functional connectivity
between SEF and FEF reflects a closer control of the eye movement
parameters such as their direction, speed, and gain. This is possible
because prediction of the target trajectory by the internal model
becomes more accurate.
Effect of sleep on learning
It should be noted that this experiment was not designed to
evaluate whether consolidation occurs exclusively during sleep. Even in the sleep-deprived subjects, the performance tended to be
better for the learned trajectory during the scanning session than
during the training session. This suggests that some consolidation does
take place during wakefulness. Indeed, it has already been reported
that consolidation of basic motor skills can occur within 5 hr of
wakefulness (Shadmehr and Holcomb, 1997 ).
Our results are consistent with the hypothesis of a favorable influence
of sleep processes on recent memory traces. The behavioral data
confirmed the observation by Smith and MacNeill (1994) . Total sleep
deprivation during the first post-training night disrupts subsequent
performance of the learned trajectory. The functional MRI data further
show that when sleep is allowed during the first post-training night,
regional responses are increased in critical regions usually activated
by performing learned motor sequences. Furthermore, the functional
connectivity is augmented between these regions and other areas known
to participate in the conventional smooth pursuit eye movement network.
These changes in connectivity might reflect better inverse modeling of
the ocular following response and better control over the oculomotor output.
 |
FOOTNOTES |
Received Aug. 13, 2002; revised Oct. 25, 2002; accepted Nov. 13, 2002.
This research was supported by the Wellcome Trust. P.M. is Senior
Research Assistant at the Fonds National de la Recherche Scientifique
(Belgique). P.M. was also supported by the Queen Elisabeth Medical
Foundation and the Royal Society. S.S. was supported by the Swiss
National Science Foundation (Grant 8210-061240). We thank Karl Friston
for his comments on a previous version of this manuscript.
Correspondence should be addressed to Pierre Maquet, Cyclotron Research
Centre B30, University of Liège-Sart Tilman, 4000 Liège,
Belgium. E-mail: pmaquet{at}ulg.ac.be.
 |
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