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Journal of Physiology (2001), 535.2, pp. 397-406
© Copyright 2001 The Physiological Society
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ABSTRACT |
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INTRODUCTION |
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A significant number of studies have addressed the functional anatomy of the cortical motor areas using non-invasive imaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Many studies have assessed the cerebral control of hand and arm muscles, but only a few papers have addressed the cerebral control of leg muscles (e.g. Fink et al. 1997). It would seem likely that the difference in the use of the upper and lower limbs is also reflected in their cerebral control. Knowledge of the cerebral control of leg movements is also essential for the prospective analysis and design of future therapeutic interventions in the rehabilitation of patients with lesions in the central nervous system.
In order to approach the question of the cerebral control of leg movements and to create a basis for future investigations, a number of basic issues in relation to the functional anatomy of the cerebral leg motor areas were addressed. The primary aim of the first part of the study (study 1) was to investigate whether a difference in the cerebral activation during co-contraction of antagonistic right-sided ankle muscles as compared to ankle plantar flexion and dorsiflexion may be disclosed with PET. This question was prompted by electrophysiological experiments in the monkey, which have suggested that different corticospinal cells may be involved in the control of stabilising co-contraction of antagonistic muscles as compared to extension-flexion movements (Humphrey & Reed, 1983; Fetz & Cheney, 1987). Non-invasive human experiments have given some indication that this may also be the case in man (Nielsen & Kagamihara, 1993; Nielsen et al. 1993).
It was further studied whether it is possible to dissociate the cortical representation of right-sided ankle plantar and dorsiflexors with PET and to what extent the cerebral activation is correlated to the force exerted by the subject.
In the second part of the study (study 2) we addressed the question of whether the cerebral activation differs when the subjects perform tonic as compared to dynamic right-sided ankle dorsi- and plantar flexion as well as isotonic compared to isometric contraction. Some of these issues have been addressed for upper limb cerebral motor areas (Dettmers et al. 1995), but given the different nature of the motor tasks in which the legs are involved, it is unknown whether the findings from these studies may be extrapolated to the cerebral leg motor areas.
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METHODS |
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Eight right-handed young healthy volunteers participated in each of the two studies after having given their written informed consent. The protocol was approved by Aarhus County Research Ethics Committee and conducted in accordance with the Declaration of Helsinki. All participants were informed of the calculated radiation dose of maximum 5.1 mSv. Female subjects were excluded if there was any possibility that they might be pregnant.
Experimental set-up
All investigated contractions involved muscles of the right ankle. The subjects were supine on the scanner bed with the right foot attached to a footplate, which was mounted on the scanner bed. The footplate was connected to a torque meter. The torque exerted on the footplate, and the voluntary rectified and integrated electromyographic activity (EMG) from the right soleus and tibialis anterior muscles were displayed on a monitor suspended in front of the subject. During all scans the subject gazed at the monitor. The subject's ears were plugged.
In study 1, three females and five males with a mean age ± S.D. of 27.1 ± 3.6 years (range 24-35 years) participated. Eight measurements were performed in the following situations, all on the right leg. (1) Rest condition with gaze fixed on the monitor displaying the EMG signal. (2) Tonic plantar flexion with steady torque and an EMG level in the soleus muscles at 5 % of maximum voluntary contraction (MVC) effort. (3) Tonic plantar flexion at 10 % MVC. (4) Tonic plantar flexion at 20 % MVC. (5) Tonic dorsiflexion with steady torque and an EMG level in the tibialis anterior muscles at 5 % MVC. (6) Tonic dorsiflexion at 10 % MVC. (7) Tonic dorsiflexion at 20 % MVC. (8) Tonic co-contraction of both the anterior tibialis and the soleus with an EMG level corresponding to 10 % of MVC for dorsi- and plantar flexion. The conditions were pseudorandomised and counterbalanced over the subjects.
In study 2, three females and five males with a mean age ± S.D. of 26.3 ± 2.9 years (range 22-32 years) participated. Seven measurements were performed in the following conditions, all on the right leg. (1) Rest condition with gaze fixed on the monitor displaying the EMG signal. (2) Tonic isometric plantar flexion at 10 % of MVC. (3) Tonic isometric dorsiflexion at 10 % of MVC. (4) Dynamic plantar flexion with an EMG level during the hold phase corresponding to 10 % of MVC. (5) Dynamic isotonic dorsiflexion with an EMG level during the hold phase corresponding to 10 % of MVC. (6) Dynamic isometric plantar flexion with an isometric torque during the hold phase corresponding to 10 % of MVC. (7) Dynamic isometric dorsiflexion with an isometric torque during the hold phase corresponding to 10 % of MVC. During the dynamic conditions, subjects were requested to make the EMG signal follow a prescribed ramp on the monitor. In this way the EMG amplitude and trajectory were matched between isometric and isotonic contraction. In all the dynamic tasks the ramp phase lasted 200 ms, the hold phase 0.5 s and the relaxation phase between contractions 2 s. The conditions were pseudorandomised and counterbalanced over the subjects.
EMG recordings
EMG signals were recorded by bipolar surface electrodes (non-polarisable Ag-AgCl, 2 cm between poles) placed over the following muscles in the right leg: tibialis anterior, soleus, medial and lateral gastrocnemius, biceps femoris and lateral vastus of the quadriceps. The background EMG activity was amplified (2000-20 000 times), band-pass filtered (5 Hz to 1 kHz), rectified, and sampled (2 kHz sampling frequency) on a concurrent computer workstation (Masscomp software, InfoWest Inc., Winnipeg, Canada). The integrated EMG signals were displayed as lines on the monitor in front of the subject. This made it easy for the subject to match the EMG levels and also to make a quantitative assessment of the level of EMG during the tasks. Care was taken that measurements were made at matched levels of background EMG activity in the different situations.
PET
Eight and seven tomograms of regional cerebral blood flow (rCBF) were acquired in studies 1 and 2, respectively, using the ECAT Exact HR47 PET camera (Siemes/CTI, Knoxville, TN, USA) operated in 3-D mode (Wienhard et al. 1994). Attenuation was measured by an initial 15 min Ga-68 transmission scan. A fast bolus of 500 MBq H215O dissolved in 5 ml saline was injected in the left antecubital vein and flushed with 10 ml saline within 6 s. Data were acquired in one 60 s frame with onset at bolus arrival to the brain (approximately 60 000 true coincident counts per second). Contractions began 10 s before injection and lasted throughout the scan.
MRI
For anatomical localisation each subject had a T1-weighted 3-D MR brain image consisting of 124 slices of 1.5 mm performed on a 1 Tesla GE Signa MR scanner (GE Medical Systems, Waukesha, WI, USA; 3D-FAST-SPGR-sequence without flow compensation, TE = 4 ms, TR = 24 ms).
Image analysis
PET volumes were reconstructed with measured attenuation and scatter correction and filtered to an isotropic full width half-maximum (FWHM) of 12 mm (Hann filter 0.15 cycles s-1). The individual 3-D MR brain volumes were aligned (Collins et al. 1994) in the Talairach coordinate system (Talairach & Tournoux, 1988) to the PET rCBF maps. The rCBF maps were aligned to compensate for any movement between scans (Woods et al. 1993).
Statistical analyses
After normalisation of the voxel count to an intracranial mean of 100 the t statistic was applied to the voxel-by-voxel subtracted rCBF maps assuming a Gaussian random field with zero mean and a unified standard deviation (S.D.) (Worsley et al. 1992). As the use of the unified S.D. increases the number of degrees of freedom (d.f.), the t values are in practice equal to a Z value (Worsley et al. 1992). The correlation analyses were performed where subjects were covariant. Due to technical problems with the image header-file in one subject correlation analyses could only be performed with seven subjects.
The assessment of the M1 peak activation location in study 1 was based on the average of the three EMG levels (5, 10 and 20 % of MVC). Significance was set at P < 0.05 corrected for multiple comparisons. The corresponding cut-off level of the t values depends on the cerebral areas searched according to the hypotheses. For a local search at the M1 of a sphere (2 cm diameter), statistical significance is reached for t > 3.08 (Worsley et al. 1992, 1996). For direct comparisons of the different conditions, a search was performed over the whole cortex, not just the precentral gyrus. This required an increased significance level of t > 4.3 for P < 0.05 corrected for multiple comparisons (Worsley et al. 1992).
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RESULTS |
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Study 1 - EMG recordings
Figure 1 shows EMG and torque recordings from a single subject during dorsiflexion (Fig. 1A), plantar flexion (Fig. 1B) and co-contraction (Fig. 1C). The torque level during dorsiflexion and plantar flexion corresponded to 10 % of the maximal voluntary dorsi- and plantar flexion effort, respectively. During co-contraction the subject was asked to maintain the torque at zero, while producing the same amount of tibialis anterior and soleus EMG activity as during dorsiflexion and plantar flexion. To facilitate this the rectified and integrated EMG activities from the two muscles were displayed as lines on the monitor in front of the subject. Only trials in which it was confirmed in the off-line analysis of the EMG signals that there was an equal amount of EMG activity during co-contraction as during dorsi- and plantar flexion were included in the subsequent analysis of brain activation. No activity was recorded in the knee muscles in any of the tasks and for all subjects.
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Figure 1. Sample EMG, torque and ankle joint position data from a single subject The upper traces in A-E show the torque exerted on the footplate, whereas that in F and G shows the ankle joint position. In A-C the lower traces show the soleus, tibialis anterior, biceps femoris and quadriceps EMG activity. In D-G the middle and lower traces show the soleus and tibialis anterior EMG activity. In A-C (study 1) the subject performed tonic isometric dorsiflexion, plantar flexion and co-contraction of ankle dorsi- and plantar flexors. The dotted lines in the upper traces in A and B show the baseline torque. During the co-contraction task the subject was requested to maintain the torque level at zero and produce the same level of EMG activity in the tibialis anterior and soleus muscles as during dorsiflexion and plantar flexion. The contraction level corresponded in all cases to 10 % of the maximal voluntary contraction level (MVC). In D and E (study 2) the subject performed isometric dynamic dorsi- (D) and plantar flexion (E). The subject was requested to make the torque signal follow a prescribed ramp in which the ramp phase lasted 300 ms and the hold phase 500 ms. The contractions were repeated every 2 s. The torque level during the hold phase corresponded to 10 % of MVC. In F and G (study 2) the subject performed isotonic dynamic dorsi- (F) and plantar flexion (G). The subject was requested to make the position signal follow the same ramp as for the isometric contraction. The load on the foot pedal was adjusted so that approximately the same amount of EMG activity was recorded from the tibialis anterior and soleus muscles as during the isometric dynamic contractions. | ||
Study 1 - brain activation
For all three contraction types (dorsiflexion, plantar flexion and co-contraction) the locations of the left M1 and supplementary motor area (SMA) peak activation areas are listed in Table 1 and displayed in Fig. 2. Isolated dorsi- and plantar flexion both elicited activation of the left M1 with a vector distance of 17 mm between the two peak activated areas. The area activated during dorsiflexion was larger than the area activated during plantar flexion, but there were no significant differences in the voxel-by-voxel analysis. Co-contraction activated a much larger volume: 14 ml (for t > 2.5) compared to 4.5 ml for dorsiflexion and 0.7 ml for plantar flexion. The co-contraction peak activation area was located inbetween but partially overlapping the areas activated by dorsi- and plantar flexion (Table 1 and Fig. 2).
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Figure 2. Transaxial and sagittal sections showing activation elicited by dorsiflexion, plantar flexion and co-contraction compared with rest (study 1) The rainbow scale shows the statistical t value (8 subjects) displayed on the average MR brain image of the subjects. 'z' indicates the Talairach co-ordinates (in mm) above a plane through the anterior-posterior commisure, 'x' indicates the Talairach co-ordinates (in mm) behind the anterior commisure. For co-ordinates of the M1 activation see Table 1. | ||
A statistical voxel-by-voxel comparison by subtraction of co-contraction from dorsi- and plantar flexion data showed significantly higher activation during co-contraction compared with dorsiflexion in the cerebellum (Talairach co-ordinates x, y, z: 11, -45, -26; t = 5.0) and compared with plantar flexion in both the cerebellum (x, y, z: 9, -45, -26; t = 4.8) and the anterior cingulate (x, y, z: 1, 10, 39; t = 5.3). In the voxel-by-voxel analysis no area showed significantly higher activation during dorsi- and/or plantar flexion compared with co-contraction. Assessment of the interaction between co-contraction and dorsi-/plantar flexion by a factorial analysis controlled for subject effects showed a significant difference in the right anterior cingulate (3, 13, 39; t = 4.3) and a sub-significant area in the left superior frontal gyrus (-23, 24, 39; t = 4.2).
The distances between the peak activation areas of the three contraction types were statistically assessed assuming that the extent of the activation was a point-spread function. The probability of a difference in location can be calculated by applying a two-tailed t test to the vector distance between the peaks using a weighted standard deviation with d.f. = n1 + n2 - 2, where n1 and n2 denote the number of subjects (image volumes) in the two analyses being compared. The weighted standard deviation for the two peaks was calculated based on the S.D. of the individual peak's point-spread functions using the peak t value (tpeak) of the activation (Sigmund & Worsley, 1995):
where FHWM denotes the full width at half-maximum of the isotropic filtered volume. The difference of 17 mm between the M1 peaks for dorsi- and plantar flexion was statistically significant (P < 10-6), as was also the case for the difference between the peaks for co-contraction and dorsiflexion (vector distance, 10 mm; P < 10-4), and co-contraction and plantar flexion (vector distance, 7 mm; P = 0.001).
A significant correlation was found in subject-corrected correlation analysis between cerebral activation and the level of EMG activity at three different levels of dorsal flexion (5, 10 and 20 % of MVC) and the baseline in the left cerebellum (Talairach co-ordinates x, y, z: -7, -68, -23; correlation coefficient = 0.74; t = 5.2) and in the left medial frontal M1 area (x, y, z: -5, -23, 62; correlation coefficient = 0.76; t = 4.8). The correlation analysis of the plantar flexion including the baseline condition showed significant activation of the left medial frontal M1 area (x, y, z: -8, -13, 60; correlation coefficient = 0.70; t = 4.4), the right cerebellum (x, y, z: 9, -57, -34; correlation coefficient = 0.69; t = 4.8), the left medial S1 (x, y, z: -11, -29, 51; correlation coefficient = 0.78; t = 5.3) and the left thalamus (x, y, z: -8, -15, 11; correlation coefficient = 0.76; t = 5.2).
Study 2 - EMG recordings
Figure 1D-G shows EMG recordings from a single subject during the dynamic tasks, which were investigated in study 2. The subject was asked to match the EMG activities recorded during the isometric (Fig. 1D and E) and isotonic tasks (Fig. 1F and G). The subject used for the illustration, as well as all other subjects, performed the isometric dynamic contractions without any visible EMG activity in antagonist muscles (i.e. m. soleus in the case of the dorsiflexion in Fig. 1D and m. tibialis anterior in the case of the plantar flexion in Fig. 1E). In contrast, some activity in the antagonist muscle could not be prevented in relation to the isotonic movements (Fig. 1F and G), when the subject had to bring the position of the foot pedal back to baseline after each movement. It was ensured that there was no activity in muscles acting at the knee in any of the tasks.
Study 2 - brain activation
For the subjects in study 2 tonic isometric dorsi- and plantar flexion only elicited activation of the left M1 and tended to activate the left SMA (Table 2). The difference in the location of the peak activation between tonic dorsi- and plantar flexion was 2 mm. Dynamic isotonic and isometric dorsi- and plantar flexion elicited more widespread activation, which involved, in addition to M1 and SMA on the left side, bilateral premotor and parietal sites (Table 2 and Fig. 3). The vector distance between the location of the M1 peak activation area for dorsi- and plantar flexion was 17 mm for dynamic isometric contractions and 2 mm for dynamic isotonic contractions.
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Figure 3. Transaxial sections showing activation by tonic isometric, dynamic isometric and dynamic isotonic dorsi- and plantar flexions over the right ankle (study 2) The rainbow scale illustrates t values > 3.0 (8 subjects) on the average MR brain image of the subjects. 'z' indicates the Talairach co-ordinates (in mm) above a plane through the anterior-posterior commisure. For co-ordinates and peak t values of activation see Table 2, and for statistical comparison between the contraction types see Table 3. | ||
A direct statistical comparison of the tonic isometric, dynamic isometric and dynamic isotonic contractions based on the averaged dorsi- and plantar flexion showed that there was higher activation in an area located in the left primary somatosensory cortex (S1) during the dynamic isotonic contraction (dorsi- and plantar flexion) than during the dynamic isometric contraction (Table 3; for visual inspection see Fig. 3). Compared with tonic isometric contractions, the dynamic tasks elicited significantly higher activation in the left premotor area, left SMA, left M1, right cerebellum, left precuneus and right fusiform gyrus (Table 3). Compared with the dynamic conditions, the tonic condition elicited significantly larger activation of the right anterior lateral and medial frontal structures (Table 3).
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DISCUSSION |
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Co-contraction versus dorsi-/plantar flexion
The main purpose of study 1 was to investigate whether cerebral activation differs between dorsi- and plantar flexion of the ankle and co-contraction of the antagonistic ankle muscles. Co-contraction clearly elicited a different activation pattern from isolated dorsi-/plantar flexion, not only in M1, but also in the cerebellum, the anterior cingulate and the left frontal cortex. As the EMG level in the soleus and tibialis anterior muscles during co-contraction was matched to that recorded during plantar flexion and dorsiflexion, respectively, a different level of activation of these muscles could not explain this observation. Care was taken that there was no visible EMG activity in any of the other muscles from which we recorded, but as it was not practically possible to record from all muscles, we cannot disregard the possibility that other muscles contributed to the different cerebral activation during co-contraction. Importantly, it would not be unlikely that some subjects contracted toe muscles or back muscles to a larger extent during co-contraction than during the other tasks, although they were specifically asked to avoid this. Another explanation could be that the more extensive activation during co-contraction is due to activation of inhibitory circuits to avoid undesired activation of other muscle groups. Nevertheless, the observation of a different M1 activation pattern during co-contraction compared with that for the isolated dorsi-/plantar flexions is consistent with previous studies. In the monkey it has been demonstrated that different corticospinal cells are active during co-contraction of antagonistic wrist muscles as compared to extension-flexion movements (Humphrey & Reed, 1983; Fetz & Cheney, 1987). In the cat, Capaday et al. (1998) have also documented a link between cortical areas controlling antagonist muscles. In humans, Nielsen et al. (1993) found that the short-latency facilitation of the soleus H-reflex was smaller during co-contraction of dorsi- and plantar flexors as compared to that during isolated plantar flexion and suggested that different populations of corticospinal cells were involved in the two tasks. This also receives some support from studies on the supraspinal control of spinal pathways during co-contraction as compared to plantar flexion/dorsiflexion (Nielsen & Kagamihara, 1992, 1993).
The larger activation in the cerebellum during co-contraction as compared to that during dorsi- and plantar flexion may be explained in different ways. The cerebellum is known to be involved in the control of agonist-antagonist activity and also to play a specific role in the co-contraction of antagonist muscles (Smith, 1981). Several of the subjects found it more difficult to perform the co-contraction task than contraction of the individual muscles. Thus, another possibility is that the increased cerebellar activation during co-contraction reflects the difficulty of the task. As none of the subjects showed any initial skill at performing the co-contraction task, but learned how to perform the task after some minutes of training before the experiments, it may be assumed that some (residual) learning took place in the course of each experiment. Since the cerebellum has been suggested to be involved in motor learning it would not be unlikely that the activation in the cerebellum during co-contraction reflects this (Raymond et al. 1996; Jueptner et al. 1997a). It is somewhat puzzling that the larger activation during co-contraction was seen in the left part of the cerebellum. This might suggest that the subjects also activated muscles on the left side of the body during co-contraction. However, since there was no activation in the right M1 during any of the tasks, we do not find this possibility likely.
Cerebral activation during dorsiflexion versus plantar flexion
Another aim of study 1 was to assess the possible difference between the cortical activation elicited by tonic dorsi- and plantar flexion. Both contractions resulted in increased activation in M1, but there was a significant difference in the location of the peak of the activation. Although study 2 was not designed to assess a difference between dorsi- and plantar flexion, we also found a significant difference in the location of the peak activation for the dynamic contractions, but not for the tonic isometric contractions in this group of subjects. The discrepancy between study 1 and study 2 in this respect may be explained by the different population of subjects, since the precise location of the central sulcus may vary by up to 20 mm (Talairach & Tournoux, 1988). A comparison of the co-ordinates of the central sulcus revealed that the average location of the central sulcus was located 6.5 mm more posterior in the subjects in study 2 as compared to the subjects in study 1. Furthermore, the variability of the location of the central sulcus was larger in study 2. In study 2 the mean displacement from the average location was 5.8 mm (S.D. = 3.3 mm) with a maximum difference of 21 mm between any two subjects. In study 1 the mean difference from the average location was 3.1 mm (S.D. = 2.3 mm) with a maximum difference between any two subjects of 11 mm. Some of the anatomical variability between subjects was compensated for by the spatial normalisation of the MR images, but this problem could not be entirely eliminated.
It seems of significance that the activated volume during dorsiflexion was larger than that during plantar flexion (Table 1). In cats and primates cortical stimulation more easily elicits activation of leg flexors than of leg extensors (Stewart & Preston, 1967). Transcranial magnetic stimulation in man has also revealed that ankle dorsiflexors are more easily activated than ankle plantar flexors (Brouwer & Ashby, 1992; Nielsen et al. 1993). This difference in the extent of cortical excitatory projections to flexors as opposed to extensors is also thought to explain the well-known pattern of flexor paresis and extensor tonus in patients with corticospinal lesions. Thus caution should be taken before accepting the present data along this line of thought. The torque levels during dorsi- and plantar flexion were matched relative to the maximal dorsi- and plantar flexion effort, but it might have been more correct to match the absolute torque levels during the two contractions.
Another concern is that the subjects may have contracted toe (and other) muscles to a larger extent during dorsiflexion than during plantar flexion. We did record from several muscles in order to minimise this problem and the subjects were instructed to perform the contractions without the use of the toe muscles. Nevertheless, we cannot exclude this possibility.
Correlation between EMG activity and cerebral activation
The subject-corrected correlation to the EMG level showed significant correlation in the left medial M1 area for both plantar and dorsiflexion as previously reported for hand muscles (Dettmers et al. 1995). Corticospinal cells increase their discharge in relation to increases in the static force level in the monkey (Cheney & Fetz, 1980). Human studies in which the muscular responses or the descending volleys evoked by transcranial magnetic stimulation have been investigated have also suggested that the level of corticospinal excitability increases with the static contraction level (Di Lazzaro et al. 1998). A correlation between EMG activity and cerebral activation was therefore to be expected, even though the maximal contraction level that we studied was only up to 20 % of the maximum possible contraction, compared to levels up to 60 % of the maximum contraction for finger movements in previous studies (Dettmers et al. 1995). A comparison of the significant results between dorsi- and plantar flexion suggests that plantar flexion may elicit a stronger sensory input with significant activation of both the left thalamus and the left medial S1 area.
In the cerebellum significant correlation to the EMG level was found on the left side for dorsiflexion, but on the right side for plantar flexion (although close to the midline). This latter finding was surprising and we have at present no explanation for it.
Comparison of tonic and dynamic contraction
The tonic isometric contraction showed a significantly higher activation of medial frontal gyrus/anterior cingulate gyrus compared with the two dynamic conditions (Table 3). Previous motor studies, which have found activation of the anterior cingulate, have assessed dynamic movements mainly of the upper extremities or studied saccades (Gaymard et al. 1998). The anterior cingulate has been implicated both in motor function (Paus et al. 1993; Picard & Strick, 1996) as well as in many different types of cognitive processing, including attention and pain (Mesulam, 1981). The function of the anterior cingulate in motor tasks has been related to many different aspects such as the imagination of movements (Stephan et al. 1995; Deiber et al. 1998), learning (Jueptner et al. 1997b; Rajah et al. 1998), and the planning, control and execution of movements (Remy et al. 1994; Stephan et al. 1995; Fink et al. 1997; Boecker et al. 1998). Primate studies have indicated that cortical microstimulation of the pre-SMA elicits slow tonic movements, whereas microstimulation of the SMA proper elicits fast, brisk and isolated movements (Luppino et al. 1991; Picard & Strick, 1996). These differences are possible explanations for the differential activation of the anterior cingulate in tonic and dynamic contractions.
During dynamic contraction as compared to tonic contraction, significantly more areas were activated including both bilateral premotor and parietal areas. A similar activation of both bilateral premotor, SMA and parietal areas during dynamic movement of the legs has been described before (Fink et al. 1997). The larger activation of the premotor and SMA areas is probably explained by the need for planning and initiating each of the individual contractions in the dynamic tasks. The prefrontal involvement in initiating voluntary movements of the foot has also been demonstrated by measurements of magnetic potentials over the vertex (Deecke et al. 1983). Furthermore, the dynamic tasks required a more intense surveillance of the monitor as the subjects were asked to make the EMG signal follow a slope drawn on the monitor. This probably also explains the activation of the superior parietal cortex, an area shown to be involved in task attention (Poranen & Hyvarinen, 1982; Johannsen et al. 1997). Increased activation of the superior parietal area and precuneus in relation to complicated hand movements has also been suggested to be related to shifts of the spatial direction of attention (Stephan et al. 1995).
Conclusion
The present study shows that the activation of both the M1 and non-primary motor areas depends on the type of movement/contraction. Precise definitions of the contractions are therefore important when comparing different mapping studies. Thus standardisation of the movement must involve the type of movement, level or force exerted and the frequency.
The results of this study support electrophysiological human and animal data which have suggested that slightly different populations of cortical neurones are involved in co-contraction of antagonistic ankle muscles as compared to isolated dorsi- and plantar flexion, although other explanations cannot entirely be ruled out. Dynamic contraction results in the activation of more and larger cortical areas than tonic contraction. This is probably related to the increased cortical demand for initiation and surveillance of the contractions. The larger activation in the primary somatosensory area during dynamic isotonic as compared to dynamic isometric contraction probably reflects the larger peripheral feedback in the former task.
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Acknowledgements
We are grateful for the help of electrical engineer Holger Kiilerich, Neurolab, Aarhus University Hospitals, for the construction of the footplate torque meter, and the technicians at the PET Centre, Aarhus University Hospital. This study was supported by grants from the Danish Medical Research Council, the Danish National Research Foundation, the Danish Sports Research Council, and the Danish Society of Multiple Sclerosis.
Corresponding author
P. Johannsen: PET Centre, Aarhus University Hospitals, Nørrebrogade 44, DK-8000 Aarhus C, Denmark.
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