544 Knoblich 2009), and to work towards joint goals (Vesper et al. 2010). Adopting an allocentric reference frame (in which objects can more easily be interpreted in relation to a co-actor’s body) may also facilitate imitation (Wohlsch- laeger et al. 2003) and other forms of joint learning (Csibra and Gergely 2009). It seems that participants were not explicitly aware of any change in behaviour or performance, suggesting that that switching from an ego- to an allocentric reference frame may be rather effortless. Although this may seem surprising, previous studies have reported similar ﬁndings, and it has been argued that different perspectives can be rapidly and effortlessly computed (Samson et al. in press). In fact, taking an allocentric perspective in some situations may happen more naturally and spontaneously than taking an egocentric view (Tversky and Hard 2009). It could be argued that both in the single and in the joint- attention condition, the hand pictures were mentally transformed by using purely visual strategies, merely comparing the visual shapes of stimuli (cf. Corradi- Dell’Acqua and Tessari 2010). This would imply that participants perceptually compared the shapes of the two hands rather than engaging in motor imagery or mapping the hands onto a body axis. However, a direct and con- tinuous comparison of the shapes of the two hands was not possible in the present experiments because they were never displayed simultaneously. Therefore, it is rather unlikely that participants engaged in purely visual rotation (Grabherr et al. 2007). In agreement with this, earlier studies suggest that participants use motor imagery as a default strategy when mentally transforming body parts and only use visual strategies when instructed to do so (Tomasino and Rumiati 2004). A further question concerns the role of action. In our task, we operationalized joint attention by having a second person attending to stimuli in order to act. This captures natural settings in which joint attention takes place, because we normally attend to objects with the intention to act on them (Humphreys and Riddoch 2004). Both partic- ipants attended to the stimuli with the same intention, which ensured that participants in joint-attention trials would direct their gaze to the screen in order to perceive the stimuli. A limitation of the present study is therefore that the role of action and the role of attention cannot be disentangled. It remains to be tested whether the joint- attention effect on mental rotation generalizes to settings where the other merely attends without acting. Note, however, that in order to minimize any potential effects of the other’s action, we made sure that the other’s actions could not be seen. Furthermore, both participants had the same stimulus–response mapping; thus, interference could not be caused by incompatibility of the two responses (Sebanz et al. 2005). 123 Exp Brain Res (2011) 211:531–545 There are several open questions that will be interesting for future research. We have suggested that joint attention from different spatial perspectives may lead people to take the other’s perspective into account and to process stimuli within an allocentric reference frame. Consequently, it would be insightful to test whether brain structures that are related to processing body parts from an allocentric versus egocentric perspective (Saxe et al. 2006) are selectively activated by joint attention from different spatial perspec- tives. Additionally, besides the processing of body parts, joint attention from opposite perspectives might also affect processing of other kinds of objects. Is a co-attendant’s frame of reference, for instance, also beneﬁcial when processing letters or words seen from a third-person per- spective? Given that motor imagery within an egocentric reference frame seems to be involved at least to some extent in transformations of abstract objects (Wexler et al. 1998), it is conceivable that priming an allocentric refer- ence frame through joint attention modulates transforma- tions of objects other than body parts. To summarize, we suggest that jointly attending to the same stimuli from different visual perspectives leads peo- ple to switch from a default egocentric reference frame to an allocentric reference frame. As other people’s perspec- tive and body orientation can more easily be taken into account within an allocentric reference frame (Amorim et al. 2006), the switch to an allocentric reference frame induced by joint attention may provide a mechanism for creating perceptual common ground in joint action and communication (Clark and Krych 2004; Richardson and Dale 2005; Richardson et al. 2007). Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which per- mits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. References Amorim MA, Isableu B, Jarraya M (2006) Embodied spatial transformations: ‘‘Body analogy’’ for the mental rotation of objects. JEP 135:327–347 Baron-Cohen S (1991) Precursors to a theory of mind: understanding attention in others. In: Whiten A (ed) Natural theories of mind: evolution, development and simulation of everyday mindread- ing. 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