GRIFFITHS AND TIPPER surface on which the objects were placed, one grey, the other dark green. Each participant’s obstacle was 20 cm from their hand’s resting position, and the target was 40 cm away. A retro reﬂective marker was placed on each par- ticipant’s wrist, and their movements were tracked using a Qualisys ProReﬂex motion capture system (Qualisys AB, Gothenburg, Sweden). Participants alternated between reaching and watching the other participant reach. Participants started with their eyes closed. The experimenter arranged the blocks for the acting participant. They were then instructed to open their eyes. One participant reached out and lifted the block, while the other, as instructed, observed the action. After the action was completed, the experimenter gave the instruction to close their eyes, the blocks of the ﬁrst participant were then removed, and the other participant’s blocks were put in place. Participants were then instructed to open their eyes again. Obstacles were present on half of the trials. Counterbalancing the current trial (N), N – 1 trial (the other participant’s reach), and N – 2 trial (the participant’s previous reach) gave eight poss- ible trial orders. Each trial order occurred (random- ized) 10 times for each participant. Figure 2 shows the key trial conditions. Panel A shows a trial sequence where the previous two reaches to the current trial did not involve obstacle avoidance— that is, a baseline of no obstacles (No) in any trial in the sequence (No–No–No). Panel B shows a sequence where Participant 1’s (currently acting participant) previous reach was over an obstacle (Ob) whereas Participant 2’s was not (No), result- ing in the Ob–No–No condition. The analysis of this reach sequence would reveal within-person priming effects—that is, the effect of the partici- pant’s own previous actions on their current action (as described in Grifﬁths & Tipper, 2009; Jax & Rosenbaum, 2007). Panel C shows a sequence where Participant 2’s previous reach, but not Participant 1’s own reach, was over an obstacle (i.e., No–Ob–No condition). The analysis of these reaches would reveal the effect of observing another person’s reach action. Results A number of trials were removed due to collisions or recording failure, and where participants had failed to open their eyes on the previous observation trial (1.18%). Only those trials where the current reach was without an obstacle were analysed. The trials where an obstacle was present were not analysed because there was little variation in height, as par- ticipants cleared the obstacle with as much room as possible, to avoid toppling the block (see also Grifﬁths & Tipper, 2009). The results are shown in Table 1. As predicted, there was no effect of observing the other partici- pant reach over an obstacle outside peripersonal space: N – 1, F(1, 15) = 0.00, p = .993. The par- ticipants were affected by their own previous reaches over obstacles: N – 2, F(1, 15) = 4.80, p = .045. There was no interaction, F(1, 15) = 0.25, p = .627. Further planned contrast t tests (two-tailed) between the baseline No–No–No and an obstacle at N – 2 (Ob–No–No) revealed a signiﬁcant difference, t(15) = 3.02, p = .004. No signiﬁcant difference was found between No–No– No and those trials where an obstacle appeared at N – 1 (No–Ob–No), t(15) = 0.33, p = .371. For complete data, Table 1 also shows reach height for trials where obstacles were present in both N – 1 and N – 2 (Ob–Ob–No), where higher reaches were also observed. However, because this condition reveals effects of own and other person’s prior reaches, it does not help our speciﬁc investigation of action observation and hence is not discussed further. EXPERIMENT 2: SHARED- WORKSPACE CONDITION Method Participants A total of 16 right-handed students (14 female), with a mean age of 21.3 years, participated in this study in return for course credits. All participants had normal or corrected-to-normal vision. 1244 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2012, 65 (7)