Gaze-centered spatial coding of touch on a hand-held tool
Humans possess the remarkable ability to project tactile sensations outside their body and onto a hand-held tool that they are using. Despite nearly a century of research, the computations underlying this projection have not been adequately addressed. In the present study, we employed model-driven psychophysics to fill this gap. We hypothesized that tool-based sensory projection involves the remapping of touch from sensory feedback in the hand into an egocentric coordinate system. We first formalized the computational steps underlying tactile remapping. We designed a novel tool-sensing experiment that allowed us to rigorously test this model. In this task, participants contacted an object with a hand-held rod and then judged whether the object was above or below where they were currently looking. This comparison would only be possible if touch on the tool was projected outside the hand. Crucially, both object location and gaze position varied independently, allowing us to characterize the hand-to-space-to-gaze remapping process. Model-based curve fitting provided strong evidence that all participants in our task projected touch outside their body and into gazecentered coordinates. Crucially, the resolution of this projection was similar to what has been found for touch on the body. These findings provide the first step towards characterizing the computations underlying the spatial projection of touch on external objects, highlighting the incredible versatility of the sensorimotor system.