The hands' default location guides tactile spatial selectivity

Our skin is a two-dimensional sheet that can be folded into a multitude of configura-tions due to the mobility of our body parts. Parts of the human tactile system might account for this flexibility by being tuned to locations in the world rather than on the skin. Using adaptation, we scrutinized the spatial selectivity of two tactile perceptual mechanisms for which the visual equivalents have been reported to be selective in world coordinates: tactile motion and the duration of tactile events. Participants' hand posi-tion-uncrossed or crossed-as well as the stimulated hand varied independently across adaptation and test phases. This design distinguished among somatotopic selectivity for locations on the skin and spatiotopic selectivity for locations in the environment, but also tested spatial selectivity that fits neither of these classical reference frames and is based on the default position of the hands. For both features, adaptation consistently affected subsequent tactile perception at the adapted hand, reflecting skin -bound spatial selectivity. Yet, tactile motion and temporal adaptation also transferred across hands but only if the hands were crossed during the adaptation phase, that is, when one hand was placed at the other hand's typical location. Thus, selectivity for locations in the world was based on default rather than online sensory information about the location of the hands. These results challenge the prevalent dichotomy of somatotopic and spatiotopic selectivity and suggest that prior information about the hands' default position -right hand at the right side-is embedded deep in the tactile sensory system.SignificanceWhen we touch objects in the dark, we must take body posture into account to determine the objects' location. This could be achieved by mechanisms that are selective for tactile stimuli located at specific locations in the world irrespective of their location on the skin. Using adaptation, an established behavioral paradigm to probe the properties of neural mechanisms, we indeed found evidence for spatial selectivity beyond skin locations. Yet, this selectivity reflected the hands' typical not their actual location in space. Our result challenges the widely held idea that sensory events are coded exclusively in topographic reference frames and suggests that prior information about the location of the hands is embedded in the tactile sensory system.