Exp Brain Res (2011) 211:531–545 consists in the size of the overall effect of attention. This is likely due to the fact that experiments 1 and 2 contained more trials overall where initial hands were shown from the other’s perspective. In experiments 1 and 2, participants saw the initial hand equally often from a ﬁrst-person per- spective and from a third-person perspective (50% each). In Experiment 3, the initial hand picture was displayed from a ﬁrst-person perspective on 75% of the trials and from a third-person perspective only on 25% of the trials. However, the absence or presence of the overall effect of joint attention does not affect the interpretation of the results of experiment 3. General discussion The present experiments aimed at bringing together two aspects of joint attention that were addressed separately in previous research. Whereas research on gaze following has mainly focused on bottom-up, perceptual inﬂuences of joint attention, approaches on shared attention and shared intentionality have focused on the awareness of what is shared. The question we addressed here reaches into both domains and concerns the impact of sharing attention from different perspectives on object processing. Based on ear- lier ﬁndings (Tversky and Hard 2009), it can be hypothe- sized that joint attention triggers a switch from an egocentric to an allocentric reference frame. To recall, in an egocentric reference frame, objects are represented relative to the perceiver, whereas in an allocentric refer- ence frame, objects are represented relative to the envi- ronment (Klatzky 1998; Soechting and Flanders 1992; Volcic and Kappers 2008). In three experiments where participants judged the handedness of rotated hand pictures while engaging in joint or single attention, we found ﬂatter slopes of the rotation– performance curves when both participants attended to the same stimuli. This indicates that during joint attention participants suspended their egocentric frame of reference and adopted an allocentric frame of reference. Experiment 2 investigated whether social context modulates this joint- attention effect. Participants in this experiment took the other’s perspective into account in both cooperative and competitive settings. Finally, in experiment 3 the effect of joint attention on mental transformation was only observed following trials that primed an allocentric perspective. Taken together, the results provide evidence that sharing attention affects the processing of jointly attended objects. More precisely, the present results point towards a switch from an egocentric to an allocentric reference frame when people attend to objects jointly from different perspectives. This switch cannot be explained by the mere presence of another person (single-attention condition), suggesting that 543 joint attention, by highlighting the perspective of the co- actor, plays a crucial role in triggering an allocentric per- spective. It seems that participants computed the observed actor’s epistemic relation towards the object (Barresi and Moore 1996) only when the other was actually looking at the object. This implies that only when the other’s relation to the object and the difference to participants’ own rela- tion were highlighted through joint attention did they give up their egocentric reference frame to adopt an allocentric reference frame. We suggest that taking an allocentric perspective implies a change in the processes that people use to men- tally manipulate objects. In single attention, where an egocentric perspective was held, the mental transformation task was likely solved through motor imagery, whereby participants imagined moving their own hand to match the position of the rotated hand (de Lange et al. 2006; Kosslyn et al. 2001; Parsons 1987a, b, 1994, Parsons et al. 1995; Wexler et al. 1998). In contrast, in joint attention, the allocentric reference frame enabled participants to map a rotated hand onto the other’s body axis (Amorim et al. 2006; Lakoff and Johnson 1999; Tversky 2005). The ﬂat- tened slope in the joint-attention condition suggests that this process was beneﬁcial in larger rotation angles; when the rotated hand was in line with the other’s body, it could easily be mapped onto the other’s body axis. Therefore, the more stimuli were rotated, the faster participants were in joint-attention as compared to single-attention trials. Adopting an allocentric reference frame when jointly attending from opposite perspectives, thus, facilitated object processing especially when objects were turned towards the other. In contrast, slower responses to small angles of rotation in the joint-attention condition indicate that mapping the hand picture onto a body axis interfered with the default process of motor imagery occurring when the hand looked as if it belonged to one’s own body. However, other than the beneﬁt for larger angles of rota- tion, the slow-down in smaller angles was not present in all experiments. Especially when the instruction stressed speed (experiment 2), the cost was reduced for trials where the objects were not rotated towards the other. Hence, the costs of an allocentric reference frame seem less reliable than the beneﬁts. Taking an allocentric reference frame provides co-actors with a processing beneﬁt for objects that are depicted from the other’s perspective (thus are more easily processed from the other’s perspective). This processing beneﬁt may support the efﬁciency and ﬂuency of joint actions from different spatial orientations (Sebanz et al. 2006). In joint action contexts, co-actors often hold different views. Adopting an allocentric reference frame may help to inte- grate the consequences of one’s own and others’ actions, to predict each other’s impending actions (Sebanz and 123