فعالیت مغز در طول مشاهده و تصویرسازی حرکتی کارهای تعادل مختلف: مطالعه fMRI

After immobilization, patients show impaired postural control and increased risk of falling. Therefore, loss of balance control should already be counteracted during immobilization. Previously, studies have demonstrated that both motor imagery (MI) and action observation (AO) can improve motor performance. The current study elaborated how the brain is activated during imagination and observation of different postural tasks to provide recommendations about the conception of non-physical balance training. For this purpose, participants were tested in a within-subject design in an fMRI-scanner in three different conditions: (a) AO + MI, (b) AO, and (c) MI. In (a) participants were instructed to imagine themselves as the person pictured in the video whereas in (b) they were instructed simply to watch the video. In (c) subjects closed their eyes and kinesthetically imagined the task displayed in the video. Two tasks were evaluated in each condition: (i) static standing balance and (ii) dynamic standing balance (medio-lateral perturbation). In all conditions the start of a new trial was indicated every 2 sec by a sound. During AO + MI of the dynamic task, participants activated motor centers including the putamen, cerebellum, supplementary motor area, premotor cortices (PMv/d) and primary motor cortex (M1). MI showed a similar pattern but no activity in M1 and PMv/d. In the SMA and cerebellum, activity was generally higher in the dynamic than in the static condition. AO did not significantly activate any of these brain areas. Our results showed that (I) mainly AO + MI, but also MI, activate brain regions important for balance control; (II) participants display higher levels of brain activation in the more demanding balance task; (III) there is a significant difference between AO + MI and AO. Consequently, best training effects should be expected when participants apply MI during AO (AO + MI) of challenging postural tasks.

After several days of involuntary immobility patients show impaired postural control and increased risk of falling (Visschedijk, Achterberg, van Balen, & Hertogh, 2010). It is therefore important to take steps to counteract loss of postural control during the period of immobility. Motor imagery (MI) of balance tasks has been shown to improve static postural control in elderly people (Hamel & Lajoie, 2005). Similarly, action observation (AO) was shown to improve performance in a sitting-to-standing-to-sitting task and in walking (Tia et al., 2010). These findings provide evidence that both MI and AO can improve postural control, but the neural sites responsible for this improvement have not so far been identified. It is commonly agreed that the positive effects of MI and AO on physical task performance are probably explained by activation of overlapping brain areas during motor execution and MI as well as during motor execution and AO (Grezes et al., 2003, Jeannerod, 1995, Jeannerod, 2001 and Olsson et al., 2008). Jeannerod postulated the well accepted hypothesis that “the motor system is part of a simulation network that is activated under a variety of conditions in relation to action, either self-intended or observed from other individuals” (Jeannerod, 2001). This simulation network may differently be activated by different covert actions such as MI or AO although Jeannerod assumed a core network that pertains to all stimulation states (Jeannerod, 2001). Previous studies investigating actual execution of postural tasks with neurophysiological (Beck et al., 2007, Schubert et al., 2008, Taube et al., 2007 and Taube et al., 2006) and imaging methods (Ouchi et al., 1999, Taubert et al., 2010, Taubert et al., 2011a and Taubert et al., 2011b) concluded that primary motor cortex (M1), visual cortex, the anterior and posterior cerebellar lobes, the basal ganglia (especially the putamen) and the brainstem are all involved in balance control in humans. Studies have also shown that physical execution of more demanding postural tasks was associated with higher activity in the supraspinal centers associated with postural control such as the cerebellum, the putamen, the brainstem and various neocortical structures (Ouchi et al., 1999). However, brain activity during MI and AO of balance tasks is rarely known. Jahn et al., (2004) used functional magnetic resonance imaging (fMRI) to demonstrate that activity of the thalamus, basal ganglia (left putamen), left frontal gyrus and spinocerebellum (cerebellar vermis) was increased when participants imagined they were standing rather than lying down. Furthermore, the pattern of activity during imagined standing was different from the pattern of activity obtained during imagined walking and running, in which a six times larger activity of the cerebellum could be detected. The authors therefore concluded that control of an undisturbed upright stance involves low intensity cerebellar activity and sensorimotor control via the thalamus and basal ganglia (Jahn et al., 2004). However, so far no previous study has investigated brain activity during MI or AO of balance tasks which require participants to counteract external perturbation. Therefore, the first aim of the current study was to compare brain activity during a dynamic balance task (medio-lateral perturbation) with activity in a less demanding static balance task (maintaining an upright stance). It is well known from non-postural tasks that MI (Gerardin et al., 2000, Grezes and Decety, 2001, Hallett et al., 1994, Jeannerod, 2001, Kimberley et al., 2006, Lotze et al., 1999, Sirigu et al., 1995 and Stephan et al., 1995) and AO (Gallese et al., 1996, Grezes and Decety, 2001 and Neuper et al., 2005) activate brain regions that are also active during actual task execution. Ouchi et al., (1999) have further demonstrated that execution of more challenging standing tasks increased brain activity; we therefore hypothesized that activity in motor centers would be higher in the more demanding dynamic task than during static standing. The second main aim of the current study was to explore differences in brain activity according to the way participants mentally involved in the balance task. In a recent review article, Vogt, Rienzo, Collet, Collins, and Guillot (2013) have pointed out that MI and AO have been largely studied in isolation from each other but that combining both seems very promising. This statement was based on studies using electroencephalography (Berends, Wolkorte, Ijzerman, & van Putten, 2013) and fMRI (Macuga and Frey, 2012, Nedelko et al., 2012, Villiger et al., 2013 and Vogt et al., 2013) to demonstrate higher brain activity during AO + MI compared with AO and MI, respectively, in non-postural tasks. In order to clarify whether this phenomenon can also be applied to balance tasks, differences in neural activation between a) ‘motor imagery’ (MI), b) ‘actively’ (AO + MI) and c) ‘passively’ (AO alone) observed balance tasks were investigated by instructing participants either to a) imagine the balance task (MI), b) imagine themselves as the person displayed in the video (AO + MI) or c) simply to watch the video (AO). In analogy to observations in voluntary hand movements (Berends et al., 2013, Macuga and Frey, 2012 and Nedelko et al., 2012) we expected the activity to be greater during AO + MI than during AO or MI in both the static and dynamic balance task. In summary, the overall goal of this study was to identify differences in the pattern of neural activity evoked by MI, AO and AO + MI of differently demanding balance tasks that can be used to develop recommendations for the non-physical training of immobilized patients.

In summary, there is ample evidence that the SMA, premotor cortex, M1, basal ganglia (putamen), and cerebellum play a significant role in physically executed balance control (see section above). Now, the current study showed for the first time that these regions can also be activated by AO + MI of a dynamic balance task; MI produced comparable activity in the SMA, putamen and the cerebellum but non-significant activation of M1 and PMv/d. In contrast, AO did not activate any of these motor areas. Furthermore, for AO + MI and MI, activity was generally greater in the dynamic perturbation task compared to the static standing task. Based on these results it may be argued that best training effects should be expected when subjects apply MI during AO (AO + MI) of challenging balance tasks. This might be especially relevant for temporarily immobilized patients that want to reduce their risk of falling in the recovery phase after immobilization. However, future research in immobilized subjects has to verify that AO + MI indeed lead to faster regains in skill level.