Introduction 1999 2003 2006 1997 2003 2007 2005 2004 2003 2005 2005 2006 2005 2007 2003 1998 2004 1986 2004 2004 2007 2005 1954 1964 1891 1 Fig. 1 dashed squares right  Method 2004 2003 2000 1996 1991a b 1954 1964 2001 In sum, in the present study consisting of a sequential motor task involving two actors we tested whether lifting height—a parameter that potentially could reveal an object’s weight—would be picked up by an observing co-actor and determine her subsequent handling of that object and whether she would be inclined to adapt her speed to the observed speed, or, alternatively, whether the interpersonal transfer of kinematic parameters would be incidental in nature, i.e., unrelated to the task constraints with which the individuals of the dyad were confronted. Method Participants Fourteen right-handed adult dyads participated in the experiment. Their ages ranged from 18 to 30 years; mean age was 22 ± 2 years. The 28 participants, 22 females and 6 males, formed 8 female dyads and 6 dyads of mixed gender. They received either course credits or payment (6 euros per hour) for their participation. Each participant signed an informed consent form. The study was approved by the local ethics committee and performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki. Materials X Y Z Four cylinders of 25 cm height were used, two with a diameter of 2.5 cm and two with a diameter of 6.5 cm. Two cylinders had a lightweight of 230 g and two cylinders had a weight of 835 g. To avoid any potential confounding between the effects of cylinder and target-area size all cylinders were given a 4-mm thick, circular base of 8 cm in diameter. This ensured that the thin and thick cylinders had the same size support base and could be repositioned in a comparable stable way which was a factor that also could contribute to the accuracy constraints since the participants were asked to place the cylinders on small and large circular target areas that were clearly marked on the table surface. Procedure and task n 1 In their control trials the participants of dyads 1–9 (arbitrary numbered) performed the fetch-part of the experimental task, i.e., at the start of their control trials they were confronted with a cylinder that was placed in the middle of the table and their task was to pick it up and place it in the area which was closest to their own starting position. Dyads 10–14 performed in their control trials the put-part of the experimental task, i.e., at the start of each trial they found the cylinder on the area closest to themselves and they were instructed to put the cylinder in the target area at the centre of the table. The data analyses showed that there were no systematic differences between the two groups of dyads as far as the targeted research questions concerned so the comparisons with the control trial data will not be addressed further in the remainder of this paper. The experiment was conducted by two experimenters who sat behind the participants. Each experimenter had a clear view on a separate computer screen which displayed information that specified the experimental conditions of every trial. In-between trials the participants were asked to close their eyes. After both participants had closed their eyes, the experimenters put the appropriate target pads on the table as well as the appropriate cylinder in the starting area of the putting-actor. Then, the two experimenters simultaneously tapped the participants on their lower arm, which indicated that they could open their eyes to start performing the trial. At the same time one of the experimenters started the movement recording. 1 Data analysis 2 Fig. 2 IRED light grey dark grey top-left graph top-right graph bottom-left graph bottom-right graph  2 2 2 2 Z t Results Effects of cylinder properties 3 1 F P F P F P F P Fig. 3 Means and standard errors of the lifting height (in cm) as a function of role (putting vs. fetching), object diameter (small vs. large) and object weight (light vs. heavy) for repetitions 1, 2 and 3 separately  Table 1 Results of repeated measures Anova on the lifting height according to a 2 role (putting vs. fetching) × 2 object mass (230 vs. 835 g) × 2 object sizes (2.5 vs. 6.5 cm in diameter) × 3 repetitions factorial design Factor df F P Role 1,13 <1 NS Object mass 1,13 86.5 <0.001 Object size 1,13 7.1 <0.05 Repetition 2,26 58.0 <0.001 Role × object mass 1,13 <1 NS Role × object size 1,13 1.3 NS Object mass × object size 1,13 14.4 <0.01 Role × object mass × object size 1,13 <1 NS Role × repetition 2,26 <1 NS Object mass × repetition 2,26 123.8 <0.001 Role × object mass × repetition 2,26 <1 NS Object size × repetition 2,26 3.67 <0.05 Role × object size × repetition 2,26 <1 NS Object mass × object size × repetition 2,26 3.78 <0.05 Role × object mass × object size × repetition 2,26 <1 NS t 4 4 N t i i t P t P i i F P Fig. 4 Left-hand graph i i i i Right-hand graph i i  4 i i N t i i t P t P i i F P 5 P R 2 Fig. 5 dashed line  Effects of target-area size variations 6 2 F Fig. 6 S L p f p f  Table 2 Results of repeated measures ANOVA on the mean wrist speed (in cm/s) according to a 2 role (putting vs. fetching) × 2 putting-size (small vs. large target area) × 2 fetching-size (small vs. large target area) factorial design Factor df F P Role 1,13 18.60 <0.01 Putting-size 1,13 75.07 <0.001 Fetching-size 1,13 33.90 <0.001 Role × putting-size 1,13 20.21 <0.001 Role × fetching-size 1,13 22.03 <0.001 Putting-size × fetching-size 1,13 <1 NS Role × putting-size × fetching-size 1,13 <1 NS 7 7 N t P t P Fig. 7 Left-hand graph i i Right-hand graph i i  7 i i N t P t P P R 2 R 2 Combined effects of size–weight illusion and speed-accuracy trade-off M Discussion 1998 2004 2004 More detailed analyses of incidental interpersonal transfer effects indicated that the situation was more complex than that. When focusing on the effects of between-trial cylinder-weight changes, the actor who was confronted with such changes first showed a systematically larger surprise-effect than the actor who was asked to transport the cylinder after the first actor had done so. A key finding here was that through the realized trajectory height during object transportation, the object mass was picked up by movement observation and integrated into the observer’s movement plan. Our prediction was therefore confirmed, i.e., the movements of an actor are likely to affect a subsequent, complementary movement generated by an observer. 2006 2003 1999 2003 8 Fig. 8 dashed line  Our interpretation of the current results does rely, of course, on the validity of both the weight-change effects and the co-variation of the actors’ movement speeds. It could be argued that the fetcher’s movement speed covaried with that of the putter due to a non-specific, visually primed increase in attention rather than the direct consequence of perceiving the putter’s object-transportation movements. However, attentional mechanisms are an unlikely explanation for our observations since the only visual cue that could have increased the attention in the fetcher was the putter’s movement speed. Whether the object-weight related adaptation in the fetcher to the observed movement of the putter was due to the inference of the weight of the object or to the observed kinematics per se, also remains a matter of discussion. If it was the former, then motor resonance (i.e., mirror-neuron system) may not be responsible for a direct transfer of the kinematics parameters. The effects of the object and the accuracy constraints differ in one fundamental aspect. The expected information transfer for the object is valid for the programming of the movement, while any hypothesized transfer of speed, by simple copying the observed movement to reduce dimensionality violates Fitts’ law. This implies that our hypothesis about the transfer of speed was either not valid or that the effect is intrinsically short lived because the duration of the effect is overruled by intrapersonal constraints. In other words, while the information about weight change is very relevant for movement programming, the examined speed changes were not. 2004 2005 2003 2005 2005 2004 1997 2003