Figure 5. THR experiment.

(a) Experiment: Nine subjects performed reach movements to one of six hand targets holding a set of nine tools (eight tools and no-tool) presented randomly to them. The hand position data from a representative subject while holding different tools (colour code shown in c) is superimposed. (b) Each subject worked with one of two tool sets. The subjects were given visual feedback of their hand position, tool, start point and target before beginning each trial. The visual feedback of the tool and hand position were switched off once they started their movement. (c) Body representation change (BRC) model prediction: the BRC model assumes the tools lead to a proportional decrease in the perceived length of the upper arm and forearm. The colour code represents the tool orientation angles as shown in the left panel of c such that for example, the hand positions after reach with tools of 0 degree (θ_t=0) are averaged and represented by light green, those with tools of θ_t=45 are averaged and represented by orange and so on. The ‘no-tool’ condition is shown in red. The right panel of c shows the predicted tool-held hand position with respect to the target (black circle) averaged over the same targets as in the experiment. The model predicted that no-tool trials to reach the target (see red disk in black circle). It predicted that the tools to lead to both an overshoot (d) of the target, as well as a deviation (r) along the length of the tool, such that (d) the target miss angle θ_m is roughly 180° flipped with respect to the tool angle θ_t. The example hand position without a tool and the hand and tool position for a tool of θ_t=135 degrees is shown (not to scale). (e) The values of r and d were predicted to increase linearly with the tool length (orange–yellow trace). On the other hand, the VPC model predicts a deformation in the visual perception leading to only a target offset proportional to the length of the tool (green–yellow trace in d).