The role of action in postural preparation for loading and unloading in standing subjects

Transcription

1 Exp Brain Res (2001) 138: DOI /s RESEARCH ARTICLE Alexander S. Aruin Takako Shiratori Mark L. Latash The role of action in postural preparation for loading and unloading in standing subjects Received: 20 June 2000 / Accepted: 26 January 2001 / Published online: 25 April 2001 Springer-Verlag 2001 Abstract The main purpose of the present study has been to find an answer to the question: Can the subject generate anticipatory postural adjustments (APAs) when a predictable postural perturbation occurs in the absence of a voluntary action? Answering this question would allow us to distinguish between two competing hypotheses on the relation between APAs and voluntary movements. One hypothesis considers both APAσ and voluntary focal movements different peripheral patterns associated with a single control process, while the alternative hypothesis considers them outcomes of two parallel control processes. Healthy subjects performed series of loading and unloading trials that included: (1) catching a falling load onto another load held in extended hands; (2) catching a falling load onto a tray attached to the trunk; (3) allowing a falling load to hit another load out of the extended hands, causing an unloading; and (4) releasing a load held in extended hands by a voluntary shoulder movement. In series 1, precautions were taken to avoid possible small hand movements prior to the impact of the falling load. Available visual information on the trajectory of the falling load was manipulated. In all conditions, except when the subject s eyes were closed, APAs were seen with patterns that were adequate for counteracting expected perturbations. Quantitative electromyographic indices of APAs depended on the availability of visual information and particular methods of introducing postural perturbations despite the fact that the magnitude of the perturbation was always the same. Our findings support a hypothesis that control processes resulting in APAs can be different from control processes associated with focal voluntary movements. A.S. Aruin Department of Physical Therapy, University of Illinois, Chicago, Ill., USA T. Shiratori M.L. Latash ( ) Rec Hall-267, Department of Kinesiology, Pennsylvania State University, University Park, PA 16802, USA mll11@psu.edu Tel.: , Fax: Keywords Posture Anticipatory postural adjustment Loading Unloading Electromyogram Human Introduction Anticipatory postural adjustments (APAs) are seen as changes in the background activity of postural muscles that occur prior to a predictable postural perturbation (Belen kii et al. 1967; Cordo and Nashner 1982; Massion 1992). Such changes have been described for a variety of experimental manipulations, including postural tasks limited to an extremity (Dufossé et al. 1985; Lacquaniti and Maioli 1989) or involving the whole body (Belen kii et al. 1967; Bouisset and Zattara 1987; Aruin and Latash 1995). Two opinions have been expressed with respect to the relation between control processes underlying APAs and those underlying the generation of a focal voluntary movement. According to one hypothesis, these are parallel, independent control processes ( the parallel processes hypothesis ; Massion 1992). This view is corroborated, in particular, by observations of selective impairment of APAs in neurological patients (Dick et al. 1986; Viallet et al. 1987). An alternative hypothesis views APAs and focal movement as two peripheral patterns of a single multijoint control process (Aruin and Latash 1995; the single control process hypothesis ; De Wolf et al. 1998). This opinion has been supported by observations of a coupling between the magnitudes of voluntary action and of APAs, even when the magnitude of the perturbation remained constant (Aruin and Latash 1995). The relation between APAs and voluntary action by the subject has been a controversial issue. When a subject is asked to catch different loads released from different heights by an experimenter, there are APAs prior to the load impact in the arm muscles (Lacquaniti and Maioli 1989; Shiratori and Latash 2001) and in the trunk and leg muscles (Lavender et al. 1993; Shiratori and Latash 2001). However, a number of other studies using

2 loading and unloading procedures (Paulignan et al. 1989; Johansson and Westling 1989; Bennis et al. 1996) have reported no APAs when the perturbation is delivered by the experimenter, even if the forthcoming perturbation is temporally predictable (Struppler et al. 1993; Aruin and Latash 1995). One difficulty in generalizing findings in experiments with loading and unloading perturbations is the difference in tasks, possibly leading to different predictability of the forthcoming perturbations. For example, it is hard to compare the application of tapping perturbations to the wrist that are synchronized with a metronome (Struppler et al. 1993) with catching a free-falling object (Lacquaniti and Maioli 1989) or with an experimentertriggered forearm unloading when a warning tone is provided at a fixed interval (Dufossé et al. 1985). We have recently emphasized the importance of temporal predictability of a perturbation for the generation of APAs (Shiratori and Latash 2001). To deal with this problem, we designed a protocol within which loading and unloading perturbations could be triggered by an experimenter and delivered with the same degree of temporal predictability. This was done by releasing a load from a specific height that hit another load that was held in the subject s hands. The task was either to catch the falling load onto the load held by the hands leading to a loading perturbation or to allow the falling load to hit the hand-held load out of the hands, thus causing an unloading perturbation. In control series, we tried to make sure that the subjects did not perform a voluntary movement temporally coupled with the expected moment of impact of the falling load. If the subjects were unable to show appropriate APA patterns in the absence of a voluntary action, the single control process hypothesis would be supported. If, alternatively, the subject were able to generate APAs despite all the precautions, the parallel process hypothesis would receive support. Materials and methods Subjects Eight healthy men, between the ages of 27 and 52 years, without known neurological or muscle disorders took part in the study. All but two subjects were right-handed. Not all subjects participated in all the experimental series because of fatigue constraints. Seven subjects took part in all the series that comprised the main part of the experiment. The subjects gave informed consent approved by the Office for Regulatory Compliance of the Pennsylvania State University. Apparatus Subjects stood on a force platform (AMTI OR-6) which measured reaction force along the direction of gravity (F z ) and moment of force in the sagittal plane (M x ). Two loads were used in this experiment (Fig. 1A). A 2-kg small load made of metal (5-cm diameter 7-cm height) was suspended in the air with a cord via a pulley attached to the ceiling. The experimenter could release the cord, and the small load fell 0.5 m onto the other load held between the hands of the subject extended in front of the body. The load held by the subject was rectangular, made of sand and soft packaging material (21-cm length 14-cm width 8.8-cm height; 459 Fig. 1A, B Experimental setup. A small load was suspended in the air 0.5 m above a large load held by a subject (A) or above the tray apparatus worn by the subject (B) 2.5-kg weight), with a receptacle (9-cm diameter, 5 cm deep) carved in the middle. This load will be called the large load. In an attempt to minimize the commonly seen upward hand motion prior to the small load impact, in some series, a flat metal plank (1 cm wide) was placed horizontally over the wrists, lightly touching them, when the subject stood with the hands extended in front of the body. The plank was fixed to the external supporting frame placed approximately 0.5 m away from the right and left sides of the subject. In another series, the subjects wore a specially constructed tray apparatus made of plywood (see Fig. 1B), onto which the small load fell. This tray was attached to the body with belts at the trunk and shoulders. To increase comfort, a cushion was placed between the body and the tray. The total mass of the tray was approximately 5 kg. A unidirectional accelerometer (Sensotec, JTFM) was attached to different sites on the right hand or to the bottom of the tray depending on the experimental series. The accelerometer was aligned with the direction of the load impact on the hand, the tray, or the instructed movement. The accelerometer signals were used for data alignment. The surface electromyogram (EMG) was recorded in the following leg, trunk, and arm muscles on the right side of the body using disposable, self-adhesive electrodes: tibialis anterior (TA), soleus (SOL), rectus femoris (RF), biceps femoris (BF), rectus abdominis (RA), erector spinae (ES), flexor carpi ulnaris (WF), extensor carpi radialis (WE), biceps brachii (BIC), long head of triceps (TRI), anterior deltoid (AD), and posterior deltoid (PD). The electrodes were placed over the muscle bellies; the distance between the two electrodes of a pair was 3 cm. Signals from the electrodes were amplified ( 1,000) and bandpass-filtered ( Hz) prior to sampling. A Mac-IIci computer with customized software based on the Labview package was used to control the experiment and collect the data. All the signals were sampled at 500 Hz with a 12-bit resolution. The data were analyzed off-line with the customized software based on the Labview and Matlab package. Procedure In all the trials within the main experimental series, the experimenter released the small load suspended in the air 0.5 m above

3 460 either the large load held by the subject or the tray worn by the subject. The small load was released unpredictably within 3 s after a computer-generated tone. Each series included six trials. The experimental series were presented in a pseudorandom order. Seven subjects participated in the following series, which comprised the main part of the experiment. In loading series, the subjects were instructed to catch the small load onto the receptacle in the large load held between the hands ( hand-catch, see Fig. 1A) with their ring and little fingers of both hands underneath the large load. In unloading series, the subjects were instructed to allow the small load to hit the large load out of the hand, leading to unloading. The subjects were asked to hold the large load by pressing lightly with the palm and all the fingers on its sides. These experimenter-triggered unloading series will be addressed as hit-drop. Available visual information about the trajectory of the falling small load was manipulated in the hand-catch and hit-drop series as follows: (1) unobstructed vision of the small load along its whole trajectory (full vision); or (2) only the first 0.1 m of the small load trajectory could be seen (partial vision). For the fullvision condition, the subjects were instructed to watch the load drop from 0.5 m without moving the head. For the partial-vision condition, a cloth was placed 0.1 m below the suspended small load, which hid the rest of the trajectory of the metal load. The cloth was draped around the wrists so that the instant of load impact was not visible. A series with no vision (both eyes closed) was included for the hit-drop condition only. There were several additional series in which only some of the subjects took part. We restricted the total number of series performed by each subject to ten based on our earlier experience suggesting that mental and physical fatigue starts to affect a subject s performance after about trials: 1. A small amplitude hand movement toward the falling load was often observed prior to the load impact, and the subjects could not suppress this movement, even after an explicit instruction (cf. Lacquaniti and Maioli 1989). Two series were run in an attempt to minimize possible effects of this movement on APAs. First, the metal plank was placed above and across the wrists during the hand-catch task (five subjects). It touched both wrists lightly and prevented them from moving upward. Second, in a pilot study, four subjects caught the small load onto the tray attached to the trunk ( tray-catch, Fig. 1B). The location of the tray was adjusted so that the small load hit it in the same location as during the hand-catch series. In the experiments with the tray, the subject s hands were hanging freely down the sides of the body, with the palms facing toward the body. 2. The subjects were instructed to adopt the same body position as in the hit-drop series (Fig. 1A) and produce a fast, low-amplitude bilateral horizontal shoulder abduction movement, in a selfpaced manner within 3 s after a computer-generated tone, leading to release of the large load ( self-triggered unloading, eight subjects). Thus, there were four loading series: (1) hand-catch during full vision, (2) hand-catch during partial vision, (3) hand-catch with the plank, and (4) tray-catch. Besides, there were four unloading series: (1) hit-drop during full vision, (2) hit-drop during partial vision, and (3) hit-drop during no vision, and (4) self-triggered unloading. Data processing All signals were filtered with a 100-Hz low-pass, second-order Butterworth filter, and EMG signals were rectified. Individual trials were aligned by the first visible rise of the accelerometer signal indicating the moment of load impact for externally triggered loading (hand- or tray-catch series) or unloading series (hit-drop series). For the self-triggered unloading series, the first visible rise in the accelerometer signal indicating hand movement was used for data alignment. This alignment time will be referred to as time zero (T 0 ). In some cases, a whole-body motion or arm motion toward the load made the accelerometer signal unclear. Such trials were discarded from analysis (never more than 2 trials within a series; across all the subjects and all the series, 20 trials were discarded). Aligned trials within each series were averaged for each subject. To quantify the anticipatory changes in the muscle activity prior to the load impact or self-triggered movement, EMG signals were integrated from 100 to 0 ms ( EMG 100 ) with respect to T 0. This was further corrected for the background activity defined as the integral from 500 to 450 ms ( EMG 50 ) with respect to T 0 in the following manner: EMG= EMG EMG 50 To compare indices of EMG activity among different conditions and across subjects, normalization of EMG across subjects was necessary. Normalization was done separately for loading and unloading conditions. For each subject, the maximal absolute value of a given EMG index for a given muscle across either loading or unloading series was taken to be unity, and all other values of this particular index for this particular muscle were normalized with respect to the maximal value. Note that this method limited the range of changes of EMG indices from 1 to 1. Negative values corresponded to suppression of the background activity during APAs. Displacement of the center of pressure (CP) in the anteriorposterior direction ( CP y ) was calculated. Anticipatory displacement of the CP for all experimental series was quantified as the changes in CP displacement from 100 to 0 ms with respect to T 0. Statistical procedures included repeated-measures two-way and one-way ANOVAs with factors Perturbation (loading and unloading), Vision (full and partial during hand-catch series; full, partial, and none during hit-drop series), Load release (self and experimenter), and Plank (with and without). Results The main finding of the study is the qualitative demonstration that APAs can be generated in anticipation of an externally triggered loading or unloading postural perturbation in the absence of a voluntary focal action by the subject. APAs in the background muscle activity were observed as early as 200 ms prior to load impact, accompanied by shifts in the CP. They were seen in all the subjects, during both loading (catching into the hand-held large load or onto the tray) and unloading (hit-drop) tasks, under all manipulations, except when the subjects performed the task with eyes closed. General EMG patterns for loading and unloading perturbations For all the catching tasks (full vision, partial vision, with plank, and tray-catch, see the Materials and methods section), all the subjects showed an increase in the background EMG for ES and/or BF prior to the load impact onto the hand-held large load or onto the tray accompanied by an anterior CP shift. All hand-catch task modifications showed an increase in the background EMG activity for all the arm muscles prior to the load impact. The tray-catch series did not involve arms and was not accompanied by anticipatory changes in the arm muscle activity.

4 461 Fig. 2 EMG traces averaged across six trials by a representative subject who performed the hand-catch task under the full-vision condition. The vertical lines at time zero represent the moment of the falling-load impact onto the stationary load held by the subject. An early increase in the background level of EMG in erector spinae (ES), biceps femoris (BF), and all the arm muscles can be seen prior to the load impact. EMG scales are in bytes. Note that the bottom traces in each plot are inverted for easier comparison, and their scales are on the right y-axes. (RA Rectus abdominis, RF rectus femoris, TA tibialis anterior, SOL soleus, AD anterior deltoid, PD posterior deltoid, BIC biceps brachii, TRI long head of triceps, WF wrist flexor, WE wrist extensor) During unloading tasks such as hit-drop and selftriggered unloading all subjects showed suppression of the background EMG activity in ES and/or BF, accompanied by a posterior CP shift for all full-vision and partial-vision series, but not during the no-vision series. Figure 2 shows typical EMG traces for the trunk and leg (Fig. 2, left panels) and arm (Fig. 2, right panels) muscles on the right side of the body averaged across 6 trials by a representative subject for the hand-catch series during full vision. The subject caught the small load dropped from 0.5 m into the receptacle in the large load held in extended hands in front of the body. The vertical lines in Fig. 2 represent the instant of load impact on the large load. In this particular subject, ES, BF, SOL, and all the arm muscles showed an increase in the background muscle activity, which started as early as 150 ms prior to the load impact for the postural muscles and up to 200 ms prior to the load impact for the arm muscles. Figure 3 shows typical EMG traces for the leg and trunk muscles averaged over 6 trials by a representative subject who performed the hit-drop series with full vision. The small load was dropped from 0.5 m, hitting the large load held by the subject out of the hands. Note the suppression of the EMG in ES and BF prior to the load impact causing the unloading. Effects of vision and direction of perturbation The magnitude of APA activity was quantified as the integral of rectified EMG over 100 ms immediately prior to the load impact, corrected for the background activity ( EMG; see Materials and methods). Two-way ANOVAs with factors Perturbation (load-catch and hit-drop) and Vision (full and partial) have shown significant main effects of Perturbation for all the muscles except RA and RF (F 1,6 >19, P<0.01). In particular, APAs in leg and trunk muscles TA, SOL, BF, and ES, were significantly

5 462 Fig. 4 Normalized EMG indices averaged across the subjects for the hit-drop series under the three visual conditions for the dorsal leg and trunk muscles (with standard error bars). The magnitude of suppression of the background activity prior to the unloading was smaller under partial vision or no vision (closed eyes) conditions. This was significant for ES (asterisk) and close to significance for SOL Fig. 3 EMG traces for the leg and trunk muscles averaged over six trials by a representative subject performing the hit-drop series with full vision. Note the early EMG suppression in ES and BF prior to the load impact (time zero). Abbreviations are the same as in Fig. 2 larger for the loading series (catch) as compared to the unloading series (drop). Note that the initial conditions in the two series were virtually identical and differed only in the position of the fingers that either allowed or prevented load drop. No significant main effects of Vision were seen in the leg and trunk muscles and no significant interactions. Further analysis of the effects of Vision on APAs during loading and unloading perturbations was done taking into account that load-catch and load-drop series were performed under different sets of visual conditions. Two visual conditions were used in the load-catch series. When data only for these series were analyzed, WE was the only muscle to show significantly larger EMG under partial vision (F 1,6 =6.85, P<0.05). Three different visual conditions (full, partial, none) were used in the hit-drop series (six subjects completed the three conditions). As expected, no APAs in the muscle activity and no early CP y were observed for the no-vision condition. For the partial and full visual conditions, a decrease in the background EMG activity in one or more of the dorsal leg and/or trunk muscles (SOL, BF, and ES; cf. Fig. 3) was observed prior to the load impact causing the unloading. Figure 4 shows mean normalized EMG for the hitdrop series under the three visual conditions for the dorsal leg and trunk muscles. The magnitude of suppression of the background activity prior to the unloading was smaller when the subjects could see only the upper fragment of the trajectory of the falling small load (partial vision) or stood with their eyes closed. This was significant for ES and close to significance for SOL (F 2,5 =6.2, P<0.05 for ES; F 2,5 =3.26, P=0.08 for SOL). Further comparisons among visual conditions for ES EMG showed a significant difference between the full-vision and no-vision conditions (t= 3.52, P<0.01), but not between the full and partial visual conditions. When normalized EMG indices for full-, partial-, and no-vision conditions were compared with zero, there were significantly negative values under full vision for SOL, BF, and ES (one-group t-tests: t<5.06, P<0.01), a close-to-significant difference under partial vision for ES and BF (t= 2.21, P=0.078 for ES; t= 2.453, P=0.058 for BF), and no significant differences for the no-vision condition. The hand-catch series with full vision (loading) was accompanied by an anticipatory anterior shift of the center of pressure ( CP y ; ±0.001 m), whereas the hit-drop series (unloading) during full vision showed a posterior CP y ( ± m) prior to the load impact. Both shifts were significantly different from zero (t>5.5, P<0.001) and were also different from each other (F 1,7 =61, P<0.0001). No significant differences among CP y indices were observed across modifications of the unloading task or modifications of the loading task.

6 463 Fig. 6 Normalized EMG indices averaged across subjects for the self-triggered (Self-Init) unloading series and the hit-drop series (with standard errors bars). There is a trend for the anticipatory postural adjustments (APAs) in most postural muscles to be smaller for the hit-drop series (SOL is an exception). This difference was significant for BF (asterisk) Effects of minimizing arm movement prior to loading (plank and tray-catch) Fig. 5 EMG traces for leg and trunk muscles averaged over six trials for a representative subject who performed the self-triggered unloading task. An early suppression of the ES and BF activity prior to the shoulder movement that released the load (time zero) can be seen similar to patterns observed for the hit-drop series (cf. Fig. 3). Abbreviations are the same as in Fig. 2 Self- versus experimenter-triggered unloading Figure 5 shows typical EMG traces for leg and trunk muscles averaged over 6 trials for a representative subject who performed self-triggered unloading. Suppression of the ES and BF activity prior to the shoulder movement that released the load can be seen similar to patterns observed for the hit-drop series (cf. Fig. 3). Figure 6 shows normalized EMG indices averaged across subjects for the self-triggered unloading series and the hit-drop series. There is a trend for APAs in most postural muscles to be smaller for the hit-drop series, except for SOL. However, this difference was only significant for BF (F 1,5 =7.09, P<0.05; full data sets for these comparisons were available for six subjects). A plank was placed above and across the wrists to minimize an upward arm movement that could be seen prior to the load impact onto the hand-held large load. Twoway ANOVAs of EMG indices with factors Plank (with and without) and Perturbation (catch and drop) showed significant main effects of Perturbation for SOL, BF, and ES (F 1,4 >12, P<0.05). Larger APAs were observed during the catch series than during the drop series. There were no significant main effects of the Plank factor and no significant interactions. To eliminate the associated arm movement completely, we also included a pilot series in which the subjects wore a tray, which caught the load at the shoulder height without use of the arms. Figure 7 shows typical EMG traces for the tray-catch series averaged across 6 trials for a representative subject (same subject as in Fig. 2). Note an increase in the background activity of both BF and ES prior to the load impact onto the tray similar to that seen during the hand-catch series for this particular subject. The magnitude of the APA activity was different between the hand-catch and tray-catch series. The APA activity was smaller in the tray-catch series for ES and BF in this subject (cf. Figs. 2 and 7). When EMG indices were compared between hand-catch and tray-catch series, significantly smaller APAs were observed in TA, SOL, and ES for the tray-catch condition (F 1,3 >14.7, P<0.05). However, the original postural tasks were different between the two conditions: During handcatch, the 2.5-kg load was held in the outstretched hands, while, during tray-catch, the 5-kg tray was attached to the trunk. This was reflected in the difference in the baseline EMG activity between the two conditions. Significantly higher background activity was seen in RA and ES (F 1,3 >12.5, P<0.05) during hand-catch as compared to tray-catch series, while the difference for BF was close to significant (F 1,3 =8.43, P=0.062)

7 464 Fig. 7 EMG traces for the tray-catch series averaged across six trials by a representative subject (same subject as in Fig. 2) in the trunk and leg muscles. Note an increase in the background activity of both BF and ES prior to the load impact onto the tray similar to patterns seen during the hand-catch series for this particular subject (cf. Fig. 2). Abbreviations are the same as in Fig. 2 Discussion The main purpose of the present study has been to find an answer to the question: Can the subject generate APAs when a predictable postural perturbation occurs in the absence of a voluntary action? Previous studies have supplied controversial evidence. Dufossé et al. (1985) have reported that a major action by the subject is required to bring about APAs in experiments with arm unloading, while a finger motion triggering the same perturbation does not lead to visible APAs. Aruin and Latash (1995) have shown that a small finger motion is able to bring about APAs in experiments with unloading in standing subjects, while no APAs are seen when the subjects watch the experimenter triggering the same unload- ing. Similarly, Struppler et al. (1993) have reported that no APAs are seen in proximal arm muscles when a perturbation is delivered to a tendon at the wrist level by an experimenter, even when the perturbations are synchronized with a metronome. On the other hand, Lacquaniti and Maioli (1989) have observed APAs in arm muscles, while Lavender and Marras (1995) describe APAs in trunk postural muscles prior to load catching. In our previous study (Shiratori and Latash 2001), we observed APAs in both arm and leg and trunk muscles when the subjects were catching loads released unexpectedly from different heights by an experimenter. This observation seems to support the idea that APAs, in principle, can be generated without an explicit action by the subject (cf. Massion 1992) if the timing of the perturbation is predictable with sufficient accuracy. However, small movements of the subject s hands toward the falling object were observed in all the subjects, and the subjects were unable to suppress this motion even when their attention was brought to it. So, a possibility remained that a subliminal action was still present that could play the role of a trigger. Within the present study, we took all possible precautions to eliminate the possibility that subjects used such a motion in a similar paradigm involving catching loads by the subjects. Our manipulations involved adding a horizontal plank that limited the upward motion by the hands, using a tray attached to the subject s body so that hands were not involved in catching at all, limiting visual information on the final segment of the load trajectory prior to the impact, and even inverting the direction of the perturbation (the hit-drop series). The last manipulation involved a falling load hitting another load out of the subject s hands (an unloading). It was designed to make the postural perturbation rather unusual and require APA patterns qualitatively different from those seen in load-catching tasks. Despite all our attempts to eliminate possible triggering actions and deprive the subjects of crucial visual information, all the subjects were still able to generate patterns of APAs with qualitative features adequate for counteracting the effects of expected perturbations. In particular, APA patterns seen in the hit-drop series were qualitatively similar to those seen in series when the subjects released the load by a quick, low-amplitude shoulder adduction movement (cf. Aruin and Latash 1995). These patterns were significantly different from those seen in load-catching trials and led to early displacements of the CP in the opposite direction. Hence, we conclude that APAs can be generated in the absence of a voluntary action by the subject. Relations of APA magnitude to the context of the task The affirmative answer to the main question that the study addressed allows one to expect optimal APA patterns in all conditions when a subject expects a standard perturbation. However, quantitative features of APAs in

8 the present study showed dependencies on factors that did not affect the emergence of the APAs qualitatively. In particular, smaller APAs were observed in the hitdrop series than in the self-triggered series, while the expected magnitude of the unloading was kept constant. Similarly, smaller APAs were seen in trials when a standard load dropped from a standard height to a tray attached to the subject s body at shoulder level (traycatch) as compared to APAs seen prior to catching the same load in the subject s hands. These observations resemble earlier findings by Aruin and Latash (1995), who reported smaller APAs prior to a standard unloading when the unloading was triggered by a small finger motion as compared to trials when the same unloading was triggered by a shoulder motion. Some of these differences might be due to different levels of the background activation of postural muscles during quiet standing, as was true for some of the muscles when the tray-catch and hand-catch series were compared. However, in other series and for other muscles during the tray-catch versus hand-catch comparisons, there were no differences in the background muscle activation levels. So, apparently the control structures generated different APA patterns in anticipation of a standard perturbation depending on the context of the task, even when the context seemed to have no apparent mechanical effects on postural stability. Role of visual information in the generation of APAs In a study by Lacquaniti and Maioli (1989), APAs associated with load catching were seen when the subjects watched the upcoming object, but they were absent when the eyes of the subjects were closed but instead they received an auditory cue on the release of the object. It is known that some minimal information on object kinematics is needed for successful object-catching during juggling (for a review, see Beek and van Santvoord 1996). In our study, when the subjects could see the object during the first 10 cm of its trajectory after the object was unexpectedly released by an experimenter, i.e., during less than 50 ms, they were able to generate APAs timed appropriately with respect to the moment of load impact. Note that APAs in conditions of partial vision were smaller than during similar tasks with full vision of the object. This is another example of a dependence of APA magnitude on the task context. APA and focal movement: two control processes or two patterns of a single control process? Traditionally, anticipatory postural adjustments have been viewed as a separate component of a control process associated with a voluntary movement that is expected to lead to a postural perturbation (Bernstein 1947; Belen kii et al. 1967; Massion 1992). In a way, the cen- 465 tral controller has been viewed as having two pockets, one for selecting a program for a focal voluntary movement and the other for selecting an appropriate postural adjustment. This view has been corroborated, in particular, by observations of selective impairment of APAs in certain groups of neurological patients (Dick et al. 1986; Viallet et al. 1987; Bouisset and Zattara 1990). On the other hand, the close coupling of parameters of voluntary action and quantitative measures of APAs (Aruin and Latash 1995, 1996) have been used to support an alternative view, namely that a focal action and an associated APA represent two peripheral patterns of a single control process. The lack of reliable observations of APAs when predictable perturbations were not accompanied by a voluntary action by the subjects also corroborated this view. Our present observations speak clearly in favor of the former position. Despite the fact that quantitative features of APAs depended on the task context, including the presence or absence of a voluntary action, APAs could be generated when all possible precautions were taken to eliminate associated voluntary movements. Therefore, we conclude that APAs can represent consequences of a separate control process. Acknowledgements This study was supported in part by NIH grants HD and NS References Aruin AS, Latash ML (1995) The role of motor action in anticipatory postural adjustments studied with self-induced and externally triggered perturbation. Exp Brain Res 106: Aruin AS, Latash ML (1996) Anticipatory postural adjustments during self-initiated perturbations of different magnitude triggered by a standard motor action. Electroencephalogr Clin Neurophysiol 101: Beek PJ, Santvoord AAM van (1996) Dexterity in cascade juggling. In: Latash ML, Turvey MT (eds) Dexterity and its development. 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