A somatosensory computation that unifies limbs and tools

Abstract

It is often claimed that tools are embodied by the user, but whether the brain actually repurposes its body-based computations to perform similar tasks with tools is not known. A fundamental body-based computation used by the somatosensory system is trilateration. Here, the location of touch on a limb is computed by integrating estimates of the distance between sensory input and its boundaries (e.g., elbow and wrist of the forearm). As evidence of this computational mechanism, tactile localization on a limb is most precise near its boundaries and lowest in the middle. If the brain repurposes trilateration to localize touch on a tool, we should observe this computational signature in behavior. In a large sample of participants, we indeed found that localizing touch on a tool produced the signature of trilateration, with highest precision close to the base and tip of the tool. A computational model of trilateration provided a good fit to the observed localization behavior. Importantly, model selection demonstrated that trilateration better explained each participant's behavior than an alternative model of localization. These results have important implications for how trilateration may be implemented by somatosensory neural populations. In sum, the present study suggests that tools are indeed embodied at a computational level, repurposing a fundamental spatial computation.

A horizon for haptic perception

“…Consistent with what we have observed in prior experiments (15,19), performance on the task was near ceiling. Localization in all participants was well fit with a linear model (means ± SE for R 2 : 0.91; range: 0.75-0.99).…”

“…It is indeed hard to imagine how a neural representation of body space could be so flexible as to incorporate a tool of the length used in the current study, extending its spatial representation out by 6 m. Instead, the present results are more in line with a functional account of embodiment, where sensorimotor computations are merely reused during tool use. Localizing touch on a rod may involve similar sensorimotor transformations (28), somatosensory computations (19,29), and neural processes (16) as localizing touch on the body. Given the high localization accuracy observed in the present study, we propose that this is also the case when localizing touch on rods of extreme length.…”

A horizon for haptic perception

“…Instead, the present results are more in line with a functional account of embodiment, where sensorimotor computations are merely re-used during tool use. Localizing touch on a rods may involve similar sensorimotor transformations (Yamamoto & Kitazawa, 2001), somatosensory computations (Miller et al, 2022;Miller et al, 2021), and neural processes (Miller et al, 2019) as localizing touch on the body. Given the high localization accuracy observed in the present study, we propose that this is also the case when localizing touch on rods of extreme length.…”