536 Exclusion of 180 data All ﬁndings held when data at the 180 level were excluded from the analysis. RT increased signiﬁcantly with increasing angle of rotation [t(21) = 8.4, p \ .001], while slopes were ﬂattened in the joint-attention condition [t(21) = 2.6, p \ .05]. Intercepts differed signiﬁcantly [t(21) = 3.2, p \ .01]. Additional analysis including 3rd PP trials A 2 9 2 ANOVA with the factors perspective of ﬁrst-hand picture and attention showed a signiﬁcant main effect of the factor perspective of ﬁrst-hand picture [RTs: F(1, 21) = 43.0, p \ .001; errors: F(1, 21) = 23.3, p \ .001] on slopes. This was due to the fact that the rotation curve was nearly ﬂat in trials in which the ﬁrst-hand picture was shown from a third-person perspective [RTs and errors: ts(21) \ 1; see Fig. 4]. However, as can be seen in Fig. 4, RTs on 0 trials were faster than RTs on other trials (0 contrasted with all other degrees: [F(1, 21) = 15.8, p \ .01]). When 0 was excluded from the analysis, slopes of the rotation curves were still not different from zero [ts(21) \ 1]. Importantly, there was a signiﬁcant two-way interaction of attention and perspective of ﬁrst hand in RTs [F(1, 21) = 8.1, p \ .01]. This was due to the fact that attention affected only 1st PP trials, but not 3rd PP trials [t(21) \ 1]. There was no general difference in RTs between joint and single-attention trials [ts(21) \ 1]. Error rates were signiﬁcantly higher when the initial hand picture was seen from a third-person view [t(21) = 3.1, p \ .01] as compared to a ﬁrst-person view. Discussion The results of experiment 1 showed increasing RTs and error rates with increasing hand rotation. Most importantly, the results conﬁrmed our prediction that jointly attending to stimuli from different perspectives modulates the process- ing of these stimuli. The rotation curve was ﬂattened when two people jointly attended to the same stimuli, as per- formance in ‘easy’ trials (small angles of rotation) was slowed down compared to the single-attention condition, while responses were faster in ‘difﬁcult’ trials (larger angles of rotation). Thus, the other’s attention had a dif- ferential effect on the levels of rotation: the more the stimulus was turned towards the other person, the more participants beneﬁtted from joint attention. The same pat- tern of results was found when data of the 180 rotation condition was excluded. Participants did not seem to stra- tegically ﬂip the stimulus when it was rotated by 180 . The results suggest that when attending jointly from different points of view, participants may have suspended 123 Exp Brain Res (2011) 211:531–545 their egocentric reference frame and adopted an allocentric reference frame. This implies a transformation process whereby the rotated hand is processed by making use of the other’s body axis (Tversky 2005). Mapping the depicted hand onto the other’s body axis is beneﬁcial in high rota- tion angles where the hand is seen upside down and easily ﬁts the other’s body orientation. This explains why RTs for higher angles of rotation were faster in the joint-attention condition than in the single-attention condition, where participants likely used motor imagery from an egocentric perspective. Joint attention thus may provide a beneﬁt for stimuli rotated towards the other person. We also found that participants were slower for smaller rotation angles in the joint-attention condition compared to the single-attention condition. This may indicate interfer- ence between the egocentric reference frame and the allocentric reference frame primed through joint attention. When a hand is not rotated or only slightly rotated, motor imagery, which may constitute the default (Parsons 1994), is easily accomplished because the hand looks as if it belonged to one’s own body. In joint-attention trials, however, mapping hand stimuli onto the other’s body axis might interfere with motor imagery at these small rotation angles, leading to an increase in RTs. This interpretation is consistent with the claim that body parts can be spatially transformed by means of two different transformation processes, namely by motor imagery and by mapping stimuli onto a body axis (Amorim et al. 2006). Taken together, the present results are in line with the interpre- tation of a switch from an ego- to an allocentric reference frame in joint-attention trials. The results provide evidence that joint attention, but not the mere presence of another person, triggered a switch from an egocentric to an allocentric perspective. A co-actor’s attention to the same location may highlight the co-actor’s perspective and thereby change the reference frame that is used for spatial processing. This extends earlier ﬁndings showing that differences in perspective affect stimulus processing and verbal descriptions of visual scenes (Samson et al. in press; Tversky and Hard 2009). Results of the debrieﬁng session provided no indication that participants were aware of any change in behaviour or performance. This speaks against deliberate use of per- spective-taking strategies and suggests that people can rather effortlessly switch from an egocentric reference frame to an allocentric reference frame. Nonetheless, the task context may modulate the extent to which the other’s perspective is taken into account. If the task context calls for ‘fading out’ the other, it could be possible that the inﬂuence of the other’s perspective declines, or, vice versa, that it increases when the context demands focusing on the other. This was tested in experiment 2.