Spatial maps of the arm and tool: Accuracy, precision, and the effect of tool use

There is evidence that the sensorimotor system builds fine-grained spatial maps of the limbs based on somatosensory signals. Can a hand-held tool be mapped in space with a comparable spatial resolution? Do spatial maps change following tool use? In order to address these questions, we used a spatial mapping task on healthy participants to measure the accuracy and precision of spatial estimates pertaining to several locations on their arm and on a hand-held tool. To study spatial accuracy, we first fitted linear regressions with real location as predictor and estimated location as dependent variables. The slopes-representing estimation accuracy-were compared between arm and tool, and before to after tool use. We further investigated changes induced by tool use in terms of variable error associated with spatial estimates, representing their precision. We found that the spatial maps for the arm and tool were comparably accurate, suggesting that holding the tool provides enough information to the sensorimotor system to map it in space. While we did not observe changes in the accuracy of spatial maps following tool use, we did observe changes in their spatial precision. Although these effects were absent in a control experiment without tool use, the direct comparison between the two conditions did not yield significant differences, suggesting that the observed precision changes may be driven by non-specific factors. In all, our results suggest that tool users can build up a map of tool space that is comparable to body space.