532 judged the amount of dots on virtual walls from either their own perspective or from the perspective of an avatar present in the scene. When participants judged how many dots they saw themselves, the avatar’s perspective inter- fered with their own, demonstrated by slower responses when the avatar saw a different amount of dots. The pro- cess underlying this effect was suggested to be a rapid, efﬁcient computation of the avatar’s perspective. When confronted with someone else having a different perspec- tive, participants had difﬁculties maintaining their purely egocentric view of the scene. Findings by Tversky and Hard (2009) suggest that another’s perspective also affects judgments about the spatial conﬁguration of objects. When participants were asked to describe spatial relations between objects in a picture, they showed a tendency to report the scene from the perspective of the person in the picture, especially when the question about the objects referred to object use (Tversky and Hard 2009). The authors claimed that although an egocentric perspective constitutes the default frame of reference, spatial perspective-taking occurs and ‘‘in some situations, taking the other’s perspective appears to be more natural and spontaneous than taking one’s own’’ (pp. 129). However, this study relied on verbal descrip- tions, and it is unclear whether such modulations of per- spective would manifest themselves in tasks that do not involve language use. The aim of the present study was to investigate whether joint attention from different perspectives modulates the reference frame that people adopt to process objects. Spatial characteristics of an object are usually encoded with respect to a reference frame. Reference frames can be egocentric, where objects are encoded relative to the per- ceiver, or allocentric, where objects are encoded relative to the environment rather than the perceiver (Klatzky 1998; Soechting and Flanders 1992; Volcic and Kappers 2008). We employed a rotation task that required gradual mental transformations of hands. This allowed us to measure dif- ferential effects of different perspectives, other than in previous studies where binary responses were collected (‘left’ versus ‘right’ in Tversky and Hard 2009; ‘yes’ versus ‘no’ when amount of dots was either the same or different in Samson et al. in press). We predicted that jointly looking at the same stimuli from different spatial perspectives would lead people to adopt an allocentric reference frame, where objects can be encoded relative to the environment or, respectively, to another person’s body orientation. This should be reﬂected in differential effects on mental rotation, depending on the degree of rotation. A further question that remains unanswered by earlier studies is whether the mere presence of another individual is sufﬁcient to make people consider another’s perspective or whether sharing attention plays a critical role. In order to 123 Exp Brain Res (2011) 211:531–545 address this question, we manipulated whether attention was shared or not while keeping the physical presence of the other person constant. Participants were sitting opposite each other while attending to objects on a ﬂat screen placed in between them. Attending alone or together, they performed a rotation task in which two pictures of hands were presented in succession, the second picture being rotated (handedness task). Using different angles of rotation, it is possible to get a parametric estimate of how participants perform mental transforma- tions when attending to the same stimuli alone or together. When handedness is judged by mentally transforming hand pictures, reaction times (RTs) are typically found to increase with the difference in orientation between the hand picture and participants’ own hand (Parsons 1987a, b, 1994; Parsons et al. 1995). Furthermore, RTs depend on the awkwardness of the depicted hand posture, suggesting that participants use motor imagery whereby they imagine the movement of their own hand to match the orientation depicted by the hand picture (de Lange et al. 2006; Kosslyn et al. 2001). Performing rotations of body parts based on motor imagery involves an egocentric reference frame. However, mental transformation processes of body parts can also be performed within an allocentric reference frame. This allows for body parts to be processed in relation to others’ bodies. It has been suggested that such transformations do not involve motor imagery of the depicted body parts but are accomplished by mentally mapping the body parts onto a body axis (head–feet, left–right; see Lakoff and Johnson 1999; Amorim et al. 2006). If joint attention leads participants to adopt an allocen- tric rather than an egocentric reference frame, this should be reﬂected in differential effects on mental rotation, depending on the degree of rotation. In particular, the rotation curve in the joint-attention condition should be ﬂattened. Participants should become faster for large rotation angles if largely rotated hand pictures are pro- cessed within an allocentric reference frame, where the hands can be mapped onto the other’s body axis. Alternatively, another person’s attention may increase the saliency of stimuli overall or it may increase partici- pants’ motivation. This should be reﬂected in a general effect, e.g., in an overall improvement of performance in the joint-attention condition. Slopes of the rotation–per- formance curves should not be affected and slopes should remain parallel. These predictions were tested in experiment 1. In two further experiments, we investigated whether the joint- attention effect is modulated by social context (cooperation versus competition, experiment 2) and by the degree to which the preceding trial primed the other’s perspective (experiment 3).