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1 Unité de Physiologie et Biomécanique de la Locomotion, Département d'Éducation Physique et de Réadaptation, Université Catholique de Louvain, 1348 Louvain-la-Neuve, Belgium 2 Department of Physiology, Université de Montréal, Montreal, Quebec H3C 3J7, Canada
Submitted 7 April 2003; accepted in final form 31 July 2003
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ABSTRACT |
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INTRODUCTION |
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; Horak and Macpherson 1996; Massion 1992).
In humans, the sequence of postural adjustment preceding an upper limb voluntary movement is organized according to a consistent pattern (sequence of deactivation and activation in the leg muscles) and specific to the forthcoming voluntary movement and could therefore be considered as preprogrammed (Belenkiy et al. 1967; Bouisset and Zattara 1981; Brown and Frank 1987; Cordo and Nashner 1982; Crenna and Frigo 1991; de Wolf et al. 1998; Dietz et al. 2000; Horak and Macpherson 1996; Nashner and Forssberg 1986). Similar changes are also observed during discrete lower limb movements (Béraud and Gahéry 1995; Mouchnino et al. 1992; Rogers and Pai 1990) and during gait initiation (Brenière and Do 1986; Burleigh et al. 1994; Jian et al. 1993). In lower limb movements, the initial APA invariably involves a loading of the limb to be moved that displaces the center of gravity (CG) over the supporting limb.
Postural mechanisms during voluntary limb movements in cats have been studied by a number of laboratories and in a number of different tasks, including conditioned lift of the paw from a support surface (Birjukova et al. 1989; Di Fabio 1983; Dufossé et al. 1982; Gahéry et al. 1980), reaching to a target (Alstermark and Sasaki 1983; Alstermark and Wessberg 1985), and step initiation (Kolb and Fischer 1994). However, in most cases the analysis of the behavioral strategy has been restricted to an examination of the ground reaction forces (GRFs), primarily in the vertical plane, and an analysis of the center of vertical pressure (CVP) calculated from these data. Studies of electromyographic (EMG) activity in the different limbs have been restricted to a very few muscles (Alstermark and Sasaki 1983; Alstermark and Wessberg 1985; Di Fabio 1983; Dufossé et al. 1982; Ioffé et al. 1982). There is no detailed information for cats concerning the activation patterns of the postural muscles of the limbs during a voluntary movement nor, in particular, of the way in which these activation patterns relate to the sequence of kinematic and kinetic changes that define the postural strategy. Last, there is no detailed information of the manner in which the changes in postural activity are integrated with the changes that define the voluntary movement.
This latter aspect is particularly important with respect to the relationship between the APAs and the voluntary movements that follow. Although there are data from human studies to suggest that the APAs and the focal movement may be independently controlled (Benvenuti et al. 1997; de Wolf et al. 1998; Massion 1992), there is insufficient evidence to determine the nature of the relationship between the APAs and the focal movement during voluntary limb movements in the quadruped. Obtaining such information is critical to future experiments designed to determine the relative contribution of different neural structures to the control of these 2 aspects of motor control. The primary aims of the current study were thus threefold. First, we sought to detail the overall behavioral strategy adopted by the cat during a reaching task that required postural responses both in anticipation of the movement and during its execution. The primary goal here was to determine the relationship between the activity patterns of muscles in the moving limb and in the supporting limb to the kinetic requirements of the task. Second, we sought to determine the relationship between the APAs and the movement. Information on this aspect of the task provides important information as to whether posture and movement, in this model and in this task, are independently controlled or the result of a common control strategy. Finally, we intend to use the results from this study as a basis for determining the contribution of the pontomedullary reticular formation (PMRF), which has been strongly implicated in the control of posture (Luccarini et al. 1990; Mori 1987; 1989; Prentice and Drew 2001; Sakamoto et al. 1991), to the different components of this behavioral strategy.
Preliminary results were previously published in abstract form (Schepens and Drew 2000, 2001).
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METHODS |
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These experiments were carried out on 2 male cats, RS22 and RS23, weighing, respectively, 6.2 and 4.2 kg. Each animal was trained over a period of several months to walk on a treadmill and to step over obstacles attached to the moving treadmill belt and to make a discrete reaching movement with either the left or the right forelimb while standing unrestrained with each paw on a force platform. The task described here was essentially that introduced by Górska and Sybirska (1980) in which cats were trained to reach to a tube, placed medially and at shoulder level, and to retrieve a morsel of food. The tube was closed by an opaque shutter that was opened under computer control (see Fig. 1). During the task, the cats normally focused on the tube after the initial cue and made no overt orienting movements.
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After several months of training, the animals were prepared for surgery and implanted under general anesthesia (2–3% of isoflurane with oxygen) and in aseptic conditions. As previously described (Drew et al. 1986; Prentice and Drew 2001), pairs of Teflon-insulated, braided stainless steel wires were sewn through the muscle bellies of selected flexor and extensor muscles of all 4 limbs, as well as into nuchal and axial muscles. These wires were attached to a 51-pin connector fixed to the cranium of the cats. These cats were also instrumented for recording the activity of neurons in the PMRF but details on these procedures will be provided only when pertinent to the current experiments. The left forelimb was shaved and small discs (about 7 mm) of a light-reflecting tape were sewn to the skin over the scapula spine, the head of the humerus, the estimated joint center of the elbow, the distal head of the ulna, the metacarpophalangeal (MCP) joint, and the outside tip of the third digit (see Fig. 1). All surgical and experimental procedures in these experiments were carried out using the principles outlined by the Council of the American Physiological Society and the Canadian Medical Research Council and were approved by the Institutional body.
Protocol
After a recuperation period of about 1 wk, experiments were carried out 3–5 times weekly. Sessions lasted for 2 to 4 h a dayand continued for 3–5 mo in each cat. Data for this manuscript were obtained during the recording of neuronal activity during reaching movements (Schepens and Drew, in preparation). Trials in which the cat clearly moved before the Go signal (end of tone and simultaneous opening of shutter) were manually aborted and were not recorded. Reaches were made in blocks of 5–10 trials with each forelimb. In each experimental session, the initial trials were made as the cat reached to a standard target position and it is these data that are used in this study. For cat RS22, the target was positioned at a distance of 27 cm from the center of the front force platforms and at a height of 22 cm. For cat RS23, the target was 22 cm from the center of the front force platforms and at a height of 16 cm. These standard positions were determined during training as those with which the cat adopted the most natural-looking posture during the task without straining forward or crouching backward.
Ground reaction forces were recorded by mean of 4 strain-gauge force platforms (AMTI, 6 x 6 cm, model ORS6-5-1) supplying 3 force signals (identified by subscripts for vertical, V; anteroposterior, AP; and mediolateral, ML) and 3 moment signals. Throughout the study we will show the forces generated by each limb against the ground (FV, FAP, FML) using the coordinate system illustrated in Fig. 1 in which each vector points in the direction of positive force for each axis (see, e.g., Macpherson 1988). The platforms were calibrated before every experimental session and the calibration was verified several times during the experiment. The signals from these platforms were amplified at a gain of 2 K and low-pass filtered at 500 Hz. Electromyographic signals were amplified by a factor of 500 to 10,000 to provide a final signal of ±1–2 V and were band-pass filtered between 100 Hz and 3 kHz. EMG signals and GRFs were digitized at 1 kHz and stored on a computer. Data were recorded from about 2.5 s before the opening of the shutter to about 3.5 s after. Video recordings of all experiments were made with a Panasonic video camera (Model 5100, 60 fields/s, shutter speed 1/1,000 s) that was positioned so that the optical axis was perpendicular to the plane of the reaching movement. Video recordings were made only of the left forelimb because the camera was positioned on the left side of the animal. A digital time code (SMPTE), recorded onto the videotape and written into the header of the digitized data file at the beginning of each trial, ensured that kinematic, kinetic, and EMG data were accurately synchronized.
Data analysis
Trials were included for analysis in this manuscript only if the cat was stable before the Go signal and if the variation in FV in the reaching limb in the 1 s before the Go signal varied by <10% body weight. Any offset on the force channels was removed by using the force calibration file. The forces and the rectified EMG signals were filtered at 25 Hz with a dual-pass, second-order, digital Butterworth filter. We then displayed each trial, one at a time, with a time window of –500 to +1,000 ms, with respect to the Go signal, to manually measure the onset of selected events (EMG activity, force modifications). Several events were measured on all trials (see Fig. 3). These included: the first increase in the FV of the reaching forelimb (
FLV) occurring between the Go signal and the lift of the paw; the maximum of the FV (maxFLV) of the reaching forelimb observed between FLV and lift; the moment when FV of the reaching forelimb fell to zero (defined as lift onset); and the onset of the activity of the cleidobrachialis (ClB) muscle of the reaching limb. This latter muscle was chosen because its onset was always sharp and because it allowed comparison of biomechanical and neuronal events with those obtained during gait modifications, which were always synchronized to the onset of ClB activity (Drew 1993; Prentice and Drew 2001). We also measured the onset and offset of other selected muscles, when we were confident that we could determine a clear increase or decrease (see Fig. 3). In many cases, events were measured independently by both investigators. In such cases, differences between measures were frequently equal to the resolution of the display (3–5 ms).
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Force traces were additionally analyzed using the equations of Gahéry et al. (1980) (see also Dufossé et al. 1982) to determine an index of diagonality (ID) and an index of torsion (IT) (see Fig. 8). In brief the ID is a differential measure that is calculated as the ratio of change in vertical force in the diagonal limbs during the postural response as a function of total force change (Fig. 8A). A value of 1.0 indicates a completely diagonal pattern of activity of the type that would be expected from a rigid body. A value of 0.0 in contrast indicates that the changes are nondiagonal in nature. This differential equation is used because it gives a good indication of how the balance of weight support between the fore- and hindlimbs is changed during the response. In contrast, the measure of torsion (Fig. 8B) is derived from the total vertical forces and indicates the extent to which body weight is supported by one or the other pair of diagonal limbs. The value can vary between +1 and –1.
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Selected video recordings were digitized with a video grabber card (Miro DC 30+) and recorded to disc. By using custom-written software we could display each frame (resolution 640 x 480) and automatically detect the centroid of the (x, y) coordinates of the reflecting points attached to the skin of the cat. We used these (x, y) coordinates to reconstruct the trajectory of the limb and to calculate joint angles at the shoulder, elbow, wrist, and MCP joint (see Fig. 2). We did not undertake a detailed analysis of the kinematics of the reach: data are presented only to show that the basic form of the reach in our experiments was similar to those that have been described in detail by other authors (Alstermark et al. 1993a; Boczek-Funcke et al. 1998, 1999, 2000; Martin et al. 1995).
Because video recordings were available only for the left limb, the majority of the data presented are taken for reaches with the left limb. Some data are shown to emphasize that reaches with the left and right limbs were identical in all essential aspects.
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RESULTS |
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The results presented in this study were selected from experiments carried out over a period of several months for each of the 2 cats. From this large number of experiments we selected representative sessions, distributed throughout the experimental period, in which the cat made substantial numbers of consecutive reaches. The data presented in this manuscript are based on a total of 282 reaches with the left or right limb in cat RS22 and on 232 left and right reaches in cat RS23.
Kinematics
The reaching movements performed by both cats were characterized by a smooth displacement of the left paw from the support surface of the force platform to the tube containing the food (Fig. 2). The reaching movement consisted of an initial forward and upward displacement of the scapula together with a retraction of the humerus (anatomical extension of the scapulo-humeral joint), a flexion of the elbow, and a ventroflexion of the wrist and the MCP. This was followed by a protraction of the humerus (anatomical flexion of the scapulo-humeral joint), an extension of the elbow, and a dorsiflexion of the wrist and the MCP. The shape of the MCP trajectory in the sagittal plane (Fig. 2A) was almost sigmoid, composed of an initial lift phase followed by a second phase where the paw was directed forward into the tube. Although we did not make an extensive quantitative analysis of the reaching movement, inspection of the video recordings made during each experiment showed that both cats adopted a very similar kinematic reaching strategy. A quantitative analysis of 12 trials, selected from 2 experimental sessions in cat RS23, confirmed that the angular values presented for the data in Fig. 2B were representative (see Table 1). For these 12 trials the average time from the Go signal to the onset of lift was 295 ± 76 ms and the time from the Go signal to attaining the target (interrupting the photodiode) was 501 ± 99 ms; the average movement time was thus 206 ms.
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Forces and electromyographic activity: an overview
The reaching movements of the limb were preceded, and accompanied, by a characteristic sequence of activity in both the reaching limb and in the 3 supporting limbs. Figure 3 shows selected force and EMG data from a single trial during both a left (Fig. 3A) and a right (Fig. 3B) reach in cat RS22. During the left reach, the reaching limb (lFL) was initially loaded before being unloaded before the reach. In a complementary fashion, the right, supporting, forelimb (rFL) was unloaded before being loaded. There was an increase in FV in the right forelimb throughout the time that the reaching limb was off the ground. Substantial changes in FV were also observed in the left hindlimb (loaded) and the right hindlimb (unloaded).
Associated with these changes in FV, there were characteristic changes in EMG activity. Initially, there was a sharp increase in the activity of the left elbow extensor, the lateral head of the triceps brachii, lTriL, during the loading of the reaching forelimb. This was followed by cessation of activity in this muscle and concomitant activation of the lClB, which is one of the prime protractors and flexors of the forelimb. There was a second period of activation in lTriL slightly later in the reach. Similarly, in the right forelimb, there was an initial decrease in the level of activity of the right (r) TriL followed by a substantial and sustained increase in the level of activity. There was also an increase in the level of activity in the rClB, although this increase was relatively small compared with that observed in the same muscle during a right reach (Fig. 3B). There was also a sustained increase in the level of activity in the left vastus lateralis, lVL and a sustained decrease in the level of activity of the rVL indicating that the changes in force in the hindlimbs were actively produced. Changes in FV and in EMG activity were reciprocal during a right reach (Fig. 3B). These changes in FV and in EMG activity were very reproducible from trial to trial. Figure 3C, for example, illustrates the force and EMG traces for 7 trials taken from the same experimental session as illustrated in Fig. 3A. Inspection of these superimposed traces shows that there was little variation in either the form of the traces, their relative amplitude, or of the timing of the onset of the periods of activity in different force traces or in different muscles.
The traces in Fig. 3 emphasize the fact that there were clear changes in the level of EMG activity and the forces produced by the cat in all 4 limbs preceding the onset of the reach (onset of activity in lClB). We refer to both the changes in EMG activity that precede the reach movement, as well as the changes in force that result from the EMG activity, as APAs. The detailed nature of the relationship between these APAs and the movement will be addressed later.
Reaction and movement times
A quantitative indication of the overall reproducibility of these reaching movements can be obtained by examining the variability of some of the key components of the general behavioral strategy outlined with respect to the preceding figures. Figure 4 (see also Table 2) shows the time of onset of the first detectable increase in vertical force activity (lFLV) during a left reach for cat RS22 together with the time of lift and the time of onset of one of the prime movers, lClB, for all of those trials that fulfilled the criteria listed in METHODS (open bars). The figure also indicates the same data set after removal of statistical outliers (filled bars: see figure legend).
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For cat RS22, the initial postural change in the lFL (lFLV) followed the Go signal by a median value of 127 ms. Inspection of the data shows that most values were grouped tightly around the median value with very little scatter; 113/127 values fell within the inner fences of the boxplot (see Fig. 4 legend). Lift of the forelimb from the force platform followed the onset of the shutter by a median value of 364 ms (i.e., 237 ms after the onset of the initial force change). The median onset of activity in one of the prime movers of the forelimb (lClB) occurred 276 ms after the onset of the Go signal and thus followed the initial change in force activity by 149 ms but preceded lift by 88 ms. Trials made with the right forelimb were not statistically different (P = 0.30 for FLV) from those made with the left limb.
In the other cat used in this study, RS23, the movements were made slightly faster and with even less variability than those in cat RS22. Subjectively, this cat was more motivated to perform the task than the other cat. The other components of the movement were also made faster and were less variable (Table 2). Movements made by the right limb were only slightly different from those made with the left limb (P = 0.02 for FLV).
Changes in force distribution in all three planes
In addition to the large changes in FV, there were also more modest, but consistent, changes in the force in both the antero-posterior and the mediolateral planes. These are illustrated in Figs. 5, 6, 7. During the control period, before the reaching movement, there was always a net positive FAP observed at the forelimbs, indicating that the forelimbs were pushing forward. Anteroposterior forces in the hindlimbs were more variable but were generally negative (see Fig. 6), indicating that the hindlimbs of the cat were pushing backward. Together, these forces presumably aid in ensuring the overall stability of the body in the anteroposterior plane before the reaching movement (Macpherson 1988). Shortly after the Go signal, and before the onset of a reaching movement of the lFL, there was a marked increase in the magnitude of the negative FAP of the lHL, indicating that the left hindlimb was pushing backward and thus tending to push the body forward. Reciprocal changes were observed in the rHL, at least during the initial part of the movement. In the rFL, the FAP became more positive, indicating that this limb was pushing forward and thus tending to push the body backward; this would serve to counteract the forward momentum of the body caused by the increase in the FAP of the lHL.
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The changes in FML were very consistent. Values were positive in the 2 left limbs and negative in the 2 right limbs before the reaching movement, indicating that the cat was pushing outward with all 4 limbs. Again, this concerted force from all 4 limbs would tend to stabilize the body position. Before the reach, there was an increase in the magnitude of the FML in the lFL and a concomitant reduction of the magnitude of the FML in the rFL (see Fig. 6). These initial changes in FML probably reflect the fact that the cat is producing an extensor thrust in the left forelimb that is tending to push the body over the supporting limb (see DISCUSSION), although in the absence of any measures of CG we cannot make any definitive statements on this aspect of the behavioral strategy. Changes in FML in the hindlimbs were smaller but tended to follow the same pattern as that observed in the forelimb homolateral to the respective hindlimb (see Fig. 6).
Changes in the anteroposterior and mediolateral planes were similar in cat RS23 (not illustrated).
The resultant effect of these force changes on the overall posture of the cat during a single trial is illustrated in Fig. 7 both in vectorial format for each limb (Fig. 7, A and B) as well as with respect to the change in the center of vertical pressure (CVP) as calculated from the changes in FV in all 4 limbs (Fig. 7C). In the initial, control period (before the Go signal), as well as in the period before the first detectable change in lFLV, the CVP is stable; FV is distributed symmetrically between the left and right limbs and about 60% of the body weight is supported by the forelimbs. Subsequently, as the reaching limb is loaded and the supporting limb is unloaded, the CVP is transferred to the left of the midline and there is a slight shift forward. After this short period the situation is reversed as the reaching limb is unloaded and the weight is transferred to the right side. There is now a clear transfer of the CVP to the right-hand side where it remains for the rest of the trial. The major change after the lift is the transfer of the CVP forward as the cat moves its body forward to facilitate entry of the limb into the tube.
Diagonal versus nondiagonal postural changes and interlimb coordination
As illustrated in Fig. 8A there was little change in ID in the initial period after the Go signal as the reaching forelimb was loaded (maxlFLV). This suggests that the change in CVP observed at this time (Fig. 7) is primarily the result of changes in the distribution of weight in the forelimbs (Gahéry et al. 1980). However, as the reaching limb was unweighted (lClB onset) there was an increase in ID, which reached a maximum value of 0.55 ± 0.13 (SD: n = 113) in RS22 and a value of 0.43 ± 0.10 (n = 125) in RS23 at the time of lift. These values remained relatively constant for a period of 1 s after lift onset. They indicate that a diagonal change in weight distribution occurred involving both the forelimbs and the hindlimbs. It should be noted that the IT (Fig. 8B) was high in both cats, indicating that most of the total weight of the cat was supported on the diagonal limb pair: in the case of a left reach by the right forelimb and the left hindlimb.
The relationship between the changes in activity in the forelimb and in the hindlimbs was further analyzed by examining the change in magnitude of the FV in different pairs of limbs during 2 different periods: 1) from the Go signal to the time of onset of the reaching movement (lift: Fig. 9, A, C, and E) and 2) from the time of onset of the reaching movement (lift) for a period of 1,500 ms (Fig. 9, B, D, and F). As shown in Fig. 9A for an individual left reach trial, there was an excellent relationship between the changes in magnitude of each of the forelimbs and in each of the hindlimbs. In brief, the increase in FV in the rFL was compensated by a decrease in FV in the lFL and similarly for the 2 hindlimbs. In contrast, there was very little relationship between the changes in activity between any of the diagonal limb pairs or between the homolateral limbs. To determine how robust these results were, we performed this same analysis for all trials that were included in the database and then averaged the resulting coefficients of determination. Figure 9, C and E synthesize this analysis for cats RS22 and RS23, respectively, and confirm the results illustrated in Fig. 9A. Examination of only the changes that occurred during a 1,500 ms period after the lift signal (Fig. 9, B, D, and F) shows a similar finding.
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Two components are involved in the behavioral strategy
Although the movements made by each cat were very similar from one to the other there was, nevertheless, some variation in the movements, particularly with respect to the time of movement onset (Fig. 4 and Table 2). Moreover, as emphasized by Fig. 10, for 3 traces from each of the 2 cats used in this study, the time of the movement onset with respect to the Go signal varied much more extensively than did the time of the initial change in postural activity (lFLV). In the 6 examples illustrated, the initial changes in postural activity, represented by lFLV, as well as by the increased activity in lTriL, all occurred within 122 ms of the Go signal. In contrast, the onset of the movement, represented by the onset of activity in the lClB varied from 186 to 543 ms. Consequently, the time between lFLV and the onset of the movement also varied extensively, from a minimum of 114 ms to a maximum of 460 ms. This suggests that the initial changes in postural activity that precede the movement, and which are visible in both the EMG recordings and the force traces, are temporally decoupled from the onset of lift and thus of the reaching movement itself.
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These and other related relationships are illustrated in Fig. 11 in a quantitative manner for reaches with the left forelimb in cat RS22. The suggestion that the APA and the movement are temporally decoupled is supported by the finding (Fig. 11A) that there is no relationship between the first adjustment of the posture (lFLV) and the onset of the movement (lift). Rather, as illustrated in Fig. 10 for the isolated examples, the onset of the APA occurred at an almost constant latency after the Go signal, whereas there was a large variation in the onset of lift. That the onset of the APA is tightly linked to the Go signal is further suggested by the data in Fig. 11B, which plots the temporal difference between the time of lift and the onset of the APA (referred to as lead time; Vicario et al. 1983) as a function of the onset of lift. This shows a very high coefficient of determination (R2), confirming that the onset of the APA is tightly linked to the Go signal. Surprisingly, the postural changes preceding the movement, visible in both the left and the right forelimbs, appear to be only weakly coupled (Fig. 11C). This is, possibly, a false impression resulting from the very small range of latency values for these 2 measures. A stronger correlation is observed in Fig. 11D, which plots the relationship between the time of maximum force in the right, supporting limb and the onset of lift in the reaching limb.
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The relationships between some of the key changes in force during this behavioral activity and the onset or offset of activity in some of the principal muscles of the forelimb are shown in the middle column of this figure (Fig. 11, E–H). Figure 11E shows that there was a strong relationship between the onset of activity in the ClB and the moment of lift, confirming our suggestions that the ClB is one of the prime flexor muscles implicated in this movement. Similarly, there is a very good relationship between the end of the first period of activity in the lTriL and the onset of activity in the lClB, indicating a strong reciprocal relationship between the activity in the extensor and flexor muscles of the reaching limb. Similar relationships were observed for the other extensor muscles of the reaching limb (palmaris longus, PaL: ventroflexor of the wrist and digits; and the supraspinatus, SSp: extensor of the humerus; not illustrated). The onset of the initial change in force (lFLV) was not strongly coupled to the change in activity in the lTriL (Fig. 11G). As mentioned above with respect to Fig. 11C, this might reflect the very small range of latency measurements available for these 2 variables. Alternatively, it might reflect that the initial changes in postural support are the result of the activation of other muscles (e.g., unrecorded scapular muscles). The reciprocal relationship between activity in the left and right limbs was also observed when the onset of increased activity in the rTriL was correlated to the onset of lClB (Fig. 11H). This latter graph indicates that there was a strong relationship between the onset of activity in the muscles responsible for the postural responses in the supporting limb and the onset of the muscles responsible for the onset of the lift.
Figure 11, I–L illustrate the relationship between the time of peak force development (maxlFLV) to the time of the initial postural change (lFLV) and to the moment of lift and lClB onset. Inspection of these plots shows that there is a relatively strong relationship between maxlFLV and lift (Fig. 11I) but only a weak relationship between this event and lFLV (Fig. 11J). Note, however, that the time between the moment of peak force and lift onset is not constant but depends on the overall reaction time. This is illustrated by Fig. 11K, which shows a strong linear relationship between the time difference between these 2 events (lift-maxlFLV) and lift. Finally, as expected on the basis of the relationship illustrated in Fig. 11E, there was an equally strong relationship between maxlFLV and the onset of activity in lClB (Fig. 11L) as there was with lift (Fig. 11I).
Similar relationships were observed during right reach in this cat as well as for both the left and right reaches in the other cat (not illustrated). The only noticeable difference was a weaker relationship between maxlFLV and lift (R2 = 0.40) in cat RS23.
A detailed view of the overall pattern of EMG activity during this complex behavior is shown in Fig. 12. Muscles throughout the moving limb, as well as in the supporting limbs and in the axial musculature, were activated at different times during the reaching movement. The latency of activation of these muscles, at least in the 2 forelimbs, appears to fall into 2 groups: one that occurs relatively early in the behavior, at about the same time as the APAs in the forelimbs, and one that occurs later, just before and during the reaching movement.
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In the first, APA, group we include the early augmented responses observed in the extensor muscles of the reaching limb (the left limb in the illustrated example), including the TriL, the SSp, and the PaL. The earliest responses in these muscles in cat RS23 occurred at latencies of 60–70 ms after the Go signal. In addition, there was early activation, at similar latencies, of many of the shoulder muscles including both heads of the deltoids that we recorded [the acromiodeltoideus (AcD): retractor and outward rotator of the humerus; and the spinodeltoideus (SpD): retractor of the humerus] and in 2 of the heads of the trapezius muscle [the acromiotrapezius (AcT) and the cleidotrapezius (ClT)]. Early activation of EMG activity was also observed in the levator scapulae ventralis (LvS: extensor of the scapula). The early period of activation in the AcD and the SpD was normally less intense than the later period of activity observed in the same muscles (Fig. 12A). The reciprocal decrease in activity of the extensors in the supporting limb (rTriL, rAct) as well as in the rTrM also occurred at this same time. Although not illustrated in this figure, it should be noted that the responses were reciprocal for left and right reaches (see Fig. 3). Responses at similarly short latencies were also observed in most of the muscles of the hindlimbs as well as in the neck extensor (dorsiflexor), the biventer cervicus (BvC), and the complexus (Com). In the BvC there was frequently a decrease in the level of activity that preceded any of the changes in activity of the limb muscles (average latency of 52 ms in cat RS23).
In the second group, we include those muscles whose activity was clearly related to the movement itself. This included the flexor muscles acting around the elbow [the brachialis (Br) and the ClB] as well as the shoulder retractor, teres major, TrM, and the spinotrapezius (SpT, elevator of the scapula). The second burst of activity in the left limb extensors (SSp, TriL, and PaL) was also activated at this period of the reaching movement as was the second, larger period of activity in the shoulder muscles, AcD, SpD, AcT, and ClT. There was also activation of some of the muscles of the left hindlimb at this period (at least in one of the cats) as well as the only back muscle that we recorded, the longissimus dorsi (LoD, recorded at the L5 level). In addition to these muscles whose activity appeared to be activated at these 2 periods of the behavior, there were also some muscles [e.g., the latissimus dorsi (LtD: retractor of the humerus)] and the rhomboid muscles (RhB: retractor of the scapula) whose latency of onset was intermediate between those we just identified in the preceding paragraph.
To determine whether these muscles could truly be classified into 2 separate groups, one related to the APA and the other to the movement, we performed a similar analysis to that used to determine whether lFLV was better related to the Go signal or to the movement initiated by that stimulus (see Fig. 11) (i.e., we plotted lead time against ClB onset). The results of this analysis are illustrated in Fig. 13 for selected muscles from cat RS22 and synthesized in Fig. 14 for all of the muscles that we recorded in both cats. As expected on the basis of the latency measurements, the initial changes in activity of the extensors of the left, reaching limb, as well as those of the muscles acting around the shoulder and the extensors of the right, supporting limb were all better related to the Go signal than to the movement (filled circles in Fig. 13A; Fig. 14, A and D). Coefficients of determination (R2) for the plots using lead time, for all of these muscles, in both cats, were >0.50 (filled bars in Fig. 14, A and D). In contrast, the coefficients for the onset of activity in the lClB were <0.5 and in most cases <0.25. The coefficients of determination for the muscles tentatively classified in the second group [e.g., Br, TrM, and so forth were very high when plotted directly against the onset of activity in ClB (open circles in Fig. 13B and open bars in Fig. 14, A and D)] and very low when plotted as lead time (filled circles in Fig. 13B and filled bars in Fig. 14, A and D). Using an arbitrary threshold of 0.5, we can thus objectively identify 2 groups of forelimb muscles: one group (open bars >0.5 in Fig. 14) that are well correlated with the movement and another group (closed bars >0.5 in Fig. 14) that are best related to the Go signal and thus, by extension, to the APA (see DISCUSSION). Based on these criteria, only 3 of the forelimb muscles that we recorded (lRhb and lLtD in RS23 and rBr in RS22) could not be classified into one group or the other based on their initial period of activity.
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It was noteworthy that, although the onset of the activity of many of the forelimb extensor muscles was related to the Go stimulus, the end of this period of activity was better related to the movement (second series of bars for the limb extensors in Fig. 14). This suggests that, although the onset of the lift may be temporally decoupled from the onset of the APA, it is tightly linked to the end of APA. Moreover, it is also interesting to note that the second period of activity in the limb extensor muscles was very well related to the onset of the movement (third series of bars in Fig. 14). This suggests that the activity patterns observed in some individual muscles might be the result of 2, convergent, command signals, one for the APA and one for the movement.
Most of the muscles that we recorded in the hindlimbs were also activated at short latency and were better related to the Go stimulus than to the movement. As such, it is probable that the hindlimb muscles also contribute to the production of the APAs that preceded movement. However, it was also noticeable that this distinction was not as clear in cat RS22 and that some of the muscles in this cat even appeared to be better related to the movement than to the APAs. The reasons for this distinction are not clear. However, further inspection of the data revealed that this result was probably influenced by slower reaching movements in which the postural responses were indistinct and the first clear activation of the hindlimb muscles occurred coincidentally with the movement. Whether this simply reflects an inability to detect postural responses during these slow movements or whether it suggests different strategies with respect to hindlimb activation during slow and fast movements remains to be determined.
Finally, for the axial muscles, all of the muscles acting around the neck that we recorded showed a strong relationship to the Go signal and were activated before the loading of the reaching forelimb. In contrast, the only muscle recorded from the trunk, the longissimus dorsi, showed a better relationship to movement, in both cats.
We also considered the possibility that the groups of muscles that we identified may be the artificial result of the events that we used to make our analyses. In particular, it is possible that one should consider the movement to begin when the limb begins to unload (normally corresponding to maxlFLV) and not the moment of lift. Similarly, it is possible that, despite the close relationship between lift and lClB (Fig. 11E), our results might be different if we performed the analyses using lift rather than lClB as our synchronizing event. We thus repeated the preceding analyses using maxlFLV and lift as our markers for movement initiation rather than lClB. Figure 15 summarizes the essential points of this analysis. First, regressing the onset of activity in all of the muscles against lift rather than lClB made no difference to the results. As indicated in Fig. 15A and by Fig. 15C, the relationships obtained by using lift were almost identical to those obtained using lClB as our indicator. Second, the relationships obtained regressing maxlFLV instead of lClB resulted in uniformly weaker determinants of correlation. For example, Fig. 15B shows the results for the same 4 muscles illustrated in Fig. 15A. Comparison of the regressions for each muscle shows that better regressions were obtained using lift (and lClB; Fig. 13) as the indicator of movement rather than maxlFLV. These data are summarized in Fig. 15D, where it can be seen that R2, for the movement-related muscles (open circles) was (with 2 exceptions) always greater when the regression was made with lift than with maxlFLV. In other words, nearly all of the movement-related muscles were better correlated with the onset of lift than with the time of peak force. The relationships obtained using lead time as the value to be regressed also showed similar results (Fig. 15E; i.e., the correlations were better with lift than with maxlFLV). This is to be expected given that there was not a perfect relationship between maxlFLV and lift (Fig. 11I).
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DISCUSSION |
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), we propose that independent, or distributed, neural signals may be responsible, at least in part, for the APAs that precede the movement and the movement itself. Second, our recordings of EMG activity suggest that most, if not all, muscles are activated as one of 2 groups, either as part of the APA or as part of the movement itself. Third, we present evidence that the postural patterns in the hindlimbs may be activated independently of those in the forelimbs. Kinematics of the reaching movement
In these studies, we used a simple reaching paradigm that is readily comparable to that used in several other studies to investigate the kinematics of reach (Alstermark et al. 1993a,b; Boczek-Funcke et al. 1998, 1999, 2000; Martin et al. 1995). In general terms, there was little difference in the limb trajectory and joint angle changes observed in our study and those of others. As in most studies, the general form of the limb trajectory was sigmoid. We did not observe any clear demarcation into 2 phases as described by Martin et al. (1995), although the movement could be divided into a lifting phase and a thrusting phase on the basis of the joint angles. It is possible that the discordance in the form of the trajectory might reflect differences in the distance of the target from the cat. In our study the target was >20 cm from the cats' paw, whereas in the study of Martin et al. (1995) it was only 10 cm away. This may also explain why we observed a clear extension of the elbow before attaining the target, whereas Martin et al. (1995) described the thrusting part of the movement to be determined primarily by movement of the shoulder. Maximum joint angles at the elbow and wrist were similar to those reported by Martin et al. (1995).
Anticipatory postural adjustments that precede the movement
Most movements involve a complex series of postural changes that serve to maintain equilibrium and to prepare the body for the movement as well as to adjust the posture of different body segments before and during the movement itself (Gahéry 1987; Massion 1992). The majority of the anticipatory postural adjustments that we examined fall into the first category and we refer to them in our summary figure (Fig. 16) as preparatory anticipatory postural adjustments (pAPAs).
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As in the majority of experiments in humans that have examined these types of preparatory postural adjustments in response to both arm and leg movements (see INTRODUCTION), we observed clear changes in EMG activity and in the forces that preceded the onset of the movement. In the cat, these APAs were observed in all 4 limbs but were most obvious in the reaching forelimb in which there was an increase in FV that preceded the lift (force reduced to zero) by >200 ms (Table 2). Such a loading in the reaching forelimb was clearly described in the dog by Ioffé et al. (1982) and in the cat by Birjukova et al. (1989) but does not seem to have been observed in some of the other studies in which conditioned movements of the forelimbs have been studied (Di Fabio 1983; Dufossé et al. 1982). The fact that a distinct loading of the reaching limb was observed in nearly every movement made by each of our cats might reflect the fact that our animals had to make a substantial modification of their posture to reach to the tube in front of their body. A similar change in CVP was also observed by Kolb and Fischer (1994) in their study of step initiation and is also visible in the single trace illustrated by Alstermark and Sasaki (1983) during a reaching movement.
A major function of this loading of the limb is almost certainly to displace the CG within the triangle formed by the 3 supporting limbs, as shown clearly by the study of Ioffé et al. (1982). This is a coordinated response in which the increase in FV in the reaching limb during the APA occurs together with a decrease in FV in the support limb, as well as almost simultaneous changes in the hindlimbs (see Fig. 3). These responses are analogous to those that are adopted by humans when they are required to move a leg either as part of a discrete movement (Béraud and Gahéry 1995; Mouchnino et al. 1992; Rogers and Pai 1990) or during step initiation (Burleigh et al. 1994; Jian et al. 1993; Nissan and Whittle 1990). As in the reaching cats, these voluntary movements of the lower limbs involve a sequential strategy (Massion 1992) in which the initial action is an initial increase in FV under the moving limb that tends to move the CG in the opposite direction in preparation for the movement. In both conditions, it has been shown that the movement of the CVP to the moving limb displaces the CG over the support limb. Such changes in CG are not restricted to tasks in which a supporting limb is moved but are also seen in any task in which the movement requires a displacement of the body (e.g., pulling on a handle) (Lee et al. 1990).
Although EMG activity from one or two muscles was recorded in a few studies of voluntary movement in the cat (Alstermark and Sasaki 1983; Birjukova et al. 1989; Di Fabio 1983; Dufossé et al. 1982), the data in this manuscript provide the first detailed information on the muscle activation patterns that underlie the postural strategy. The EMG recordings clearly show that the changes in all of the forces in both the forelimbs and hindlimbs are active, in that the initial changes in force are preceded by changes in EMG activity. This was best detailed for the changes observed in the reaching forelimb in which we observed short latency activation of all of the forelimb extensor muscles that we recorded as well as many of the muscles acting around the shoulder (Fig. 12). In muscles such as the SSp, which showed a well-defined onset of activity, the earliest latency responses that we observed in cat RS23 [which responded faster than cat RS22 (Fig. 4 and Table 2)] were in the range of 60–70 ms. Similar latencies were observed in all of the muscles related to the APA. However, because the onset of activity in some of the muscles was not always well defined, we cannot make any definitive statements as to whether all of these limb extensor and shoulder muscles are activated simultaneously. Nevertheless, the results do indicate that the changes in the forces preceding movement are the result of a coordinated and distributed activation of muscles throughout the limb. Moreover, the appropriate postural preparation involves not only muscles in the moving forelimb but also activation of muscles in the supporting forelimb as well as in the 2 hindlimbs.
Movement initiation
As mentioned in a preceding paragraph, movement of a supporting limb involves a sequential motor act in which the CG is moved over the supporting limbs before the movement may begin. In such a complex act, one may question where the postural activity ends and where movement begins. This is a difficult question to answer and a number of different definitions are provided, generally centering on the first observed forward movement of the limb (see, e.g., Béraud and Gahéry 1995; Burleigh et al. 1994; Mouchnino et al. 1992). In our study we decided to define the onset of movement as the time either when the limb is lifted from the ground and the FV in the reaching forelimb falls to zero (lift) or as the time of lClB onset. As shown in Fig. 11E, the 2 events themselves are strongly related. The choice of lClB as a synchronizing muscle was made both because of the clear onset of activity in this muscle, which allowed for precision of measurement, and to allow future comparison of these data with those previously described during locomotion in which force recordings are not available and ClB was invariably used as the synchronizing muscle. We emphasize, however, that the onset of EMG activity in this muscle was also synchronous with the other principal flexors of the elbow, such as the Br, and the shoulder, such as TrM and SpT. Thus as for the APA, the reaching movement is the result of a coordinated and distributed activation of muscles acting throughout the limb.
Inspection of Fig. 10, however, clearly shows that in most of the trials the onset of ClB, and of the other muscles, occurred after the reaching limb began to unload (i.e., after maxlFLV). This is most evident in those trials in which movement was delayed (e.g., bottom traces of Fig. 10). As such, although the activity in the ClB, and the other muscles active at this time, clearly contributes to the reaching movement, it cannot contribute to the initial unloading of the limb. One may thus question whether the onset of movement should be defined as the time of maxlFLV and not lift. As stated above, this decision is debatable. In addition to the reasons given above for using lift and ClB as the indicator of movement, there are also several other factors that influenced us in this decision. For instance, comparison of the coefficient of determination calculated using either maxlFLV or lift as the synchronizing event revealed that all of the movement-related muscles had higher values of R2 when activity was plotted against lift than when plotted against maxlFLV (Fig. 15). In addition, apart from those muscles clearly related to the postural activity and starting before the onset of lFLV, only 2 of the reaching FL muscles (LtD and Rhb) discharged before maxlFLV and even in these 2 cases, the relationship was better with lift than with maxlFLV. Of course, it is possible that we did not record appropriate EMGs related to the limb unloading. However, given the large number of muscles that we did record, we suggest that the onset of the unloading might not be the result of an active flexion of the limb that removes it from the support surface but rather of the extensor thrust in the moving limb that displaces the CG over the support limb together with the simultaneous decrease in the extensor force in the supporting limb (Birjukova et al. 1989; Ioffé et al. 1982). Thus it is the initial displacement of the CG that unloads the limb and the flexor muscles only become active later to lift the limb from the support surface. In this viewpoint, maxlFLV appears as a transition point between the end of the postural response and the onset of the movement. This is supported by the finding that, whereas the onset of the postural response is decoupled from the onset of lift (and from maxlFLV), there is a significant relationship between lift and maxlFLV (Fig. 11I). In this respect our results support those investigators (Béraud and Gahéry 1995; Cordo and Nashner 1982; Mouchnino et al. 1992; Stapley et al. 1998) who argue that APAs have as a function not only the displacement of the CG to ensure stability but are also involved in facilitating and coordinating the focal movement.
Integration between posture and movement
A major controversy with respect to the integration of posture and movement is the extent to which the APAs and the focal movement are controlled as part of a single central command and the extent to which they are controlled independently. In some respect, this controversy arises because of the diverse nature of the tasks that have been used to study the question and because of the complex nature of the postural responses themselves. As we discuss in a later section, the postural responses that accompany a movement may have a different mode of control from those that clearly precede the movement. Massion in his 1992 review has come down strongly on the side of independent control of the APAs preceding movement and of the movement itself, primarily on the basis of the differences that are observed in reaction time tasks compared with self-paced tasks. The time constraints imposed by a reaction time task impose a reorganization of the APAs that are not observed during a self-paced task. In particular, the timing of the onset of the APA with respect to the movement is modified as is, in some cases, the pattern (Cordo and Nashner 1982; de Wolf et al. 1998; Horak et al. 1984). Other evidence also supports the viewpoint that the APAs and the focal movement may be independently controlled. Brown and Frank (1987), for example, showed that in conditions in which postural set was modified during a task in which subjects pulled on a lever, there was a change in the relative latency of the movement with respect to the latency of the APA. A similar lack of correlation was also observed between postural muscles and prime movers in the studies of Crenna et al. (1987), of Cordo and Nashner (1982), and in naive subjects in the study of Mouchnino et al. (1992). In contrast, trained dancers in the latter study (Mouchnino et al. 1992) showed a good correlation between the onset of activity of postural muscles and prime movers and a similar result was obtained by Horak et al. (1984). Our results make an important contribution to this argument concerning the organization of the control signals involved in the control of posture and movement by showing clear temporal decoupling of the APAs that precede movement and the focal movement itself, in a cued, reaching task.
In this study we used a simple analysis to show the relationship between different biomechanical measures and the stimulus or the movement. This analysis demonstrates in a simple and clear manner whether an event is better related to the movement or to the stimulus that initiates the movement (Chapman et al. 1986; Vicario et al. 1983). Here we have clearly shown that the onset of the APA was significantly related to the onset of the stimulus (Fig. 11B), whereas the onset of the movement was temporally decoupled from the onset of the APA (Figs. 10 and 11A). These results differ from 2 studies in humans that also specifically examined the relationship of the APA to the trigger stimulus. Neither Cordo and Nashner (1982) nor Horak et al. (1984) found any relationship between the onset of the APA and the stimulus during a rapid arm movement task. Although this may reflect either a species difference or the fact that our task required movement of a supporting limb, we consider that the difference in results more likely reflects differences in the instructions preceding the stimulus. In the 2 human studies (Cordo and Nashner 1982; Horak et al. 1984), highly trained subjects performed reaction time tasks in which subjects were expected to respond as quickly as possible to the stimulus, without any prior information. In contrast, our task was an instruction-delay task in which the animals received information as to which limb to move before the Go stimulus. Although the onset time of the stimulus was random with respect to the onset of the instruction, it seems quite likely that a preconfigured postural response was triggered by the stimulus at a relatively constant, short latency. Because this task was not a reaction time task (the cats were rewarded for any movements within 3 s of the Go stimulus) the variable time of movement onset reflects the speed of the overall sequence of activity and, probably, the motivation of the animal.
The decoupling of the APA preceding movement and the movement itself was also reflected in the pattern of activity of the EMG signals recorded from these cats. As for the kinetic measures, we obtained clear evidence that the large number of muscles that we recorded from the forelimbs were also clearly differentiated into 2 groups, one related to the APA and the other to the movement (Figs. 12, 13, 14). This further supports the idea that the strategy adopted by the cat is controlled by 2 distinct descending signals, one signal triggered by the stimulus and the other, later signal providing the detailed information about the movement to be made. Unfortunately, the current data set does not allow us to determine to what extent the postural responses are tailored to the details of the movement to be made (see, e.g., Benvenuti et al. 1997; Bouisset et al. 2000). Given the debate we raised in the previous section concerning what constitutes the onset of movement, we would emphasize that our conclusions hold regardless of whether one considers maxlFLV or lift to represent movement initiation. As illustrated in Fig. 11I, maxlFLV was significantly related to lift, but not to lFLV. Similarly, all the regression analysis for those muscles identified as being movement related were similarly classified when using maxlFLV as the synchronization point, although the correlations of determination were not as great as those obtained when using lift or ClB onset as an indicator of movement onset. The conclusions that there are two, temporally decoupled, signals are, therefore valid even given any doubts as to what constitutes movement initiation.
Although the results clearly show that the onset of the APA and the onset of the prime movement are temporally decoupled, the question of whether these 2 components of the overall strategy are planned independently or together remains problematic. As mentioned in the preceding paragraphs, the strategy adopted by the cat involves a sequential activity in which the CG is transferred to the other side of the body before the reaching movement. Because of the inherent biomechanical constraints of the movement, the initiation of the reach is highly dependent on the time taken for the CG to be transferred. The temporal decoupling between the onset of the APA and of the reach may thus, in part, be a reflection of the time taken for this weight transfer; this is reflected in the data illustrated in Fig. 10. The question then arises as to what extent the strategy is planned as a whole and to what extent, independently. It is possible, for example, that the movement is planned as a whole and that the temporal decoupling documented in the RESULTS reflects whether the cat intends to make a slow or a fast movement. However, it is equally possible that this temporal decoupling is indicative of parallel, or independent processes in which each of the components is planned separately. This is an issue that cannot easily be resolved. Nevertheless, the temporal decoupling observed strongly suggests independence in the control of the postural component and the movement component during execution of the strategy. As represented in the summary figure (Fig. 16; see also concluding remarks in DISCUSSION), we suggest that from a neurophysiological viewpoint there must exist separate signals that regulate the muscular activity involved in the postural component and that involved in producing the prime movement.
Moreover, there must also be a close interaction between these 2 signals. Indeed, the clear relationship between the postural component and the movement is illustrated in Fig. 11F, which documents the strong relationship between the end of the initial period of activity in the TriL of the reaching limb and the onset of activity in the ClB. Similar relationships were also observed for the other muscles whose onset was related to the stimulus and that discharged in a discrete burst (Fig. 12). This supports the suggestion that the duration of the postural response is adjusted to the requirements of the movement and is in agreement with the suggestion of Cordo and Nashner (1982) that movement is actively delayed until an appropriate postural support is prepared (see also Béraud and Gahéry 1995). This finding also implies that certain muscles receive convergent inputs supplying information provided by 2 different command signals, one related to the APA and the other related to the movement.
Anticipatory postural adjustments that accompany the movement
Associated with the movement of the limb as it was transported to the location of the food and then back to the force platform there were also clear modifications in extensor muscle activity and in the forces produced in the 3 supporting limbs. These changes in posture have as a function the support of body weight and the adjustment of equilibrium throughout the time that the reaching limb is not in contact with the support surface. As Massion (1992) has strongly emphasized, these responses are also anticipatory in nature in that they are controlled in a feedforward manner to anticipate the changes produced by the movement. Similar arguments have also been made for the postural responses that accompany modifications of locomotor gait (Lavoie et al. 1995; McFadyen and Winter 1991). Following the terminology of Gahéry (1987), we refer to these postural responses in Fig. 16 as accompanying anticipatory postural responses (aAPAs) to distinguish them from the pAPAs that precede movement initiation.
The anticipatory nature of these responses is shown in particular by the relationship between the onset of activity in the TriL of the supporting limb and the onset of activity of the ClB in the reaching limb (Fig. 11H). As previously shown by Alstermark and Wessberg (1985) there is a strong linear relationship between these 2 events. Moreover, the intercept of this graph (Fig. 11H) clearly shows that activity in the TriL of the supporting limb is activated almost simultaneously with the onset of activity in the ClB. In this case, it seems that the movement and the accompanying postural response are initiated by a single motor command (see Massion 1992). This suggestion is supported by our recent studies on the organization of the corticoreticulospinal system (Kably and Drew 1998a; Matsuyama and Drew 1997; Rho et al. 1997), which shows that neurons in the motor cortex that are implicated in the control of movement send collateral branches to the PMRF (Kably and Drew 1998b) where they could initiate postural responses adapted in time and magnitude to the focal movement (Drew et al. 2003; Prentice and Drew 2001).
Postural support pattern
Two basic stratagems for postural support have been described in quadrupeds. In response to an unexpected perturbation, cats adopt a support pattern in which weight is supported by the diagonal limb pair that includes the limb contralateral to the perturbed limb (Brookhart et al. 1965; Coulmance et al. 1979; Dufossé et al. 1982; Gahéry et al. 1980). In contrast, a conditioned forelimb movement leads to a nondiagonal pattern in which weight is supported primarily by the limb contralateral to the moving limb (Birjukova et al. 1989; Di Fabio 1983; Dufossé et al. 1982; Gahéry et al. 1980; Ioffé et al. 1988). Using the same formula to quantify the index of diagonality that we used in this study (Fig. 8), Birjukova et al. (1989) proposed that a value of <0.4 would indicate a primarily nondiagonal support pattern, whereas one of >0.7 would be indicative of a diagonal pattern. Di Fabio (1983) in his study suggested that a value >0.4 indicates a diagonal postural support pattern. Overall, the support patterns observed in the conditioned forelimb movements of Birjukova et al. (1989) (ID range = 0.21–0.29), Di Fabio (1983) (ID = 0.24), Dufossé et al. (1982) (ID = 0.31), and of Gahéry et al. (1980) (ID for cats <0.35) all fulfill the definitions for a nondiagonal pattern of support. Similarly, ID indexes obtained during perturbations by Dufossé et al. (1982) (ID = 0.82) fulfill the more restrictive definition and those obtained by Gahéry et al. (1980) (ID for cats 0.5–0.7) fulfill the less-restrictive definitions of Di Fabio (1983) for a diagonal support pattern.
The average values of 0.43 and 0.55 for ID that we obtained in the present study fall close to the definition of a nondiagonal pattern but are higher than those obtained in the studies listed in the previous paragraph. This finding of an ID pattern that falls outside of the 2 defined positions suggests that the postural pattern adopted by the cat might be less rigidly programmed than suggested by a clear demarcation into 2 separate patterns. Why this more diagonal pattern should be adopted in our task compared with those observed in other studies is not clear. However, it is possible that the postural pattern adopted by the cat is context dependent and is adapted to the type of voluntary movement that is being made. This suggestion is in agreement with the results of Kolb and Fischer (1994), who also found high values of ID during a task in which cats moved a forelimb from one support to another, further forward. Indeed, it is possible that the major goal of the cats in this task is not to produce a diagonal support pattern, per se, but rather that the diagonal support pattern reflects the requirement of the cat to push itself forward to assist the forelimb in reaching to the target. Finally, it has frequently been suggested that the innate postural pattern is the diagonal one, whereas the nondiagonal pattern accompanied learned or conditioned movements of the limb (Birjukova et al. 1989; Dufossé et al. 1982; Ioffé et al. 1982). Although our study does not directly address this issue, we should emphasize that these cats had already undergone several months of training before the start of the unit recording studies during which the present data were collected.
Independent control of forelimb and hindlimb muscles
Related to the issue of the coordination between the forelimbs and hindlimbs is the question as to whether the overall postural support pattern is programmed as a the result of a single command. Although not detailed here, the data illustrated in Fig. 3 clearly show that the initial changes in FV in the hindlimbs occurred at about the same time as in the forelimbs. This would suggest that the postural responses were produced as part of a single command. However, our analyses of the changes in magnitude of the FV in the forelimbs and hindlimbs during the reach (Fig. 9) revealed a strong correlation in the activity between the 2 forelimbs and between the 2 hindlimbs but only weak correlations between the other pairs of limbs (homolateral and diagonal). Moreover, our analysis of the latencies of activation of the hindlimb muscles equally suggested that there may be occasions in which hindlimb muscles are differentially activated. Together, these findings raise the possibility that the postural responses in the forelimbs and the hindlimbs may be controlled independently. It is interesting that Brookhart et al. (1965) also found the strongest coupling between transverse limbs during spontaneous distribution of weight in standing dogs. In this respect, it may be pertinent that our recent study on the discharge characteristics of reticulospinal neurons during voluntary gait modifications (Prentice and Drew 2001) showed that the most commonly observed pattern of modified discharge was one in which neurons discharged during the steps of the left and right forelimbs over the obstacles but not during the passage of the hindlimbs. Possibly such cells would play a particular role in coordinating activity in the 2 forelimbs and might be responsible for the tight link between the changes in the forces in the 2 forelimbs. Other cells were recorded that discharged to modifications of both hindlimbs or to modifications of all four limbs; such cells might be involved in coordinating hindlimb activity and in coordinating activity between the forelimbs and the hindlimbs.
Concluding remarks
The data presented here speak to the issue of the nature of the control of posture and movement in a reaching task in the cat. The results clearly show that the pAPAs that precede the focal movement are strongly time-locked to the stimulus. On the other hand, the postural adjustments that accompany the movement (aAPAs) are tightly time-locked to the movement itself. This organization is very similar to that proposed by Massion (1992)(see Fig. 16). We suggest that, at least at the level of the execution of the movement, independent, or distributed, control signals are at the origin of the APAs that precede movement and of the movement itself. We further suggest that the accompanying postural adjustments originate with the signal for movement. However, whether this simple strategy holds for similar types of movements in humans is not clear. Certainly, the arguments presented in the preceding sections strongly suggest that the pAPAs preceding the movement are organized similarly in both species and in diverse tasks. The situation for the aAPAs might not be as simple, given that the biomechanical constraints involved in maintaining equilibrium and postural support in a biped are more complex. Tasks such as raising a limb in humans involve not only adjustments of the CG but also complex kinematic changes throughout the body that accompany the movement. Recent experiments in microgravity (Mouchnino et al. 1996) suggest that these 2 types of responses might be independently controlled and modeling studies (Alexandrov et al. 2001) also suggest that responses of this type might be controlled independently. Nevertheless, the strategy used by the cat provides an ideal situation to test some of the neural control mechanisms implicated in different aspects of the integration of posture and movement. Neuronal signals related to the production of the APAs should similarly be related to the stimulus and should precede the earliest changes in EMG activity observed in the first group of muscles illustrated in Figs. 12, 13, 14. In contrast, neural signals related to the movement and/or to the anticipatory postural responses that accompany them, should be temporally independent of the stimulus and should be correlated to, and precede, the onset of activity in the second group of muscles. Our preliminary results from recordings in the PMRF (Schepens and Drew 2000, 2001; Schepens and Drew, unpublished observations) suggest that this structure might contribute to both of these functions.
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DISCLOSURES |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Address for reprint requests and other correspondence: T. Drew, Dept. of Physiology, University of Montréal, P.O. Box 6128, Station Centre-ville, Montréal, Québec H3C 3J7, Canada (E-mail: Trevor.Drew{at}umontreal.ca).
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ABSTRACT
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) positioned on the entrance of the tube at midheight indicated when the paw entered the tube. Markers placed on the skin of the animal allowed us to make kinematic measurements of the movement (see METHODS and 
