, 2013 and Vogt et al., 2013). The current results further strengthen and extend this picture to balance tasks. The highest and most widespread levels of activity in motor related areas (M1, PMv & PMd, SMA, cerebellum, putamen) occurred during AO + MI, followed by MI, then AO. Conjunction analysis revealed largely overlapping patterns of activity in motor centers (SMA, cerebellum and putamen) when comparing the AO + MI and MI in the Gemcitabine concentration dynamic task. Interestingly, brain activity in the cerebellum, the precuneus, the posterior cingulate/cuneus and the primary motor cortex during AO + MI was not simply the sum of activity

of AO and MI; it was significantly higher than the sum of these two conditions. This suggests that MI during AO (AO + MI) evokes a supra-summative brain activity that cannot be obtained by simply adding activities from MI and AO. It is therefore assumed that AO + MI should be the most effective form of non-physical

balance training. Surprisingly, AO did not result in any see more significant activity of motor centers at all. This is in contrast to previous studies investigating brain activity during the observation of goal-directed movements of the upper extremity. In these studies activity in the premotor cortex, the primary motor cortex, the SMA, and the cerebellum was reported (Grafton et al., 1996, Grezes et al., 2003, Hari et al., Casein kinase 1 1998 and Jeannerod, 2001). Consequently, it might be speculated that the brain is differently

activated during observation of balance tasks than during observation of goal-directed movements of the upper extremity. This seems plausible as it was previously shown in a well-controlled study that corticospinal excitability was enhanced when observing transitive (i.e., goal-directed movements such as grasping a cup) but not when observing intransitive (i.e., movements not associated with a particular object or goal) hand gestures (Enticott, Kennedy, Bradshaw, Rinehart, & Fitzgerald, 2010). Thus, (the presented) balance tasks might in this sense be classified as intransitive movements consequently eliciting little brain activation when solely observing them without further mental effort. In any case, our results underline the importance of combining AO with MI (AO + MI) with respect to non-physical balance exercises as AO alone seems not appropriate to efficiently activate the relevant motor centers. One limitation of the current study is that the conditions (AO + MI, MI, and AO) were not randomized. It might therefore be argued that carryover effects or fatigue could influence the different conditions in a different way. However, considerable carryover effects are unlikely as the activity was always larger in the first condition than in the following ones. Fatigue is also unlikely as participants had sufficient rest between conditions in which they could relax.