A horizon for haptic perception.

The spatial limits of sensory acquisition (its sensory horizon) are a fundamental property of any sensorimotor system. In the present study, we sought to determine whether there is a sensory horizon for the human haptic modality. At first blush, it seems obvious that the haptic system is bounded by the space where the body can interact with the environment (e.g., the arm span). However, the human somatosensory system is exquisitely tuned to sensing with tools-blind-cane navigation being a classic example of this. The horizon of haptic perception therefore extends beyond body space, but to what extent is unknown. We first used neuromechanical modeling to determine the theoretical horizon, which we pinpointed as 6 m. We then used a psychophysical localization paradigm to behaviorally confirm that humans can haptically localize objects using a 6-m rod. This finding underscores the incredible flexibility of the brain's sensorimotor representations, as they can be adapted to sense an object many times longer than the user's own body. There are often spatial limits to where an active sensory system can sample information from the environment. Hand-held tools can extend human haptic perception beyond the body, but the limits of this extension are unknown. We used theoretical modeling and psychophysics to determine these spatial limits. We find that the ability to spatially localize objects through a tool extends at least 6 m beyond the user's body.