New preprint by Maij, Seegelke et al.: External location of touch is constructed post-hoc based on limb choice

post published on April 14, 2020

What we did and why

How do we localize touch on our body? There are several theories how. A prominent one says that we directly derive the 3D location of any touch, by integrating skin location of the touch with the posture of the touched limb. When we then want to say where on the body we were actually touched, we check which limb was at the 3D location at the time of touch.
We wondered whether this pans out (spoiler alert: no, it doesn't).
We asked participants to move their hands towards their body. During the movement, each hand received a tactile stimulus. The two stimuli were slightly separated in time. After the movement, we asked participants 1) to report which hand had received the stimulus and 2) to point to the location at which that stimulus had happened.
If participants use the 3D location of a touch to determine on which limb it occurred, then their pointing endpoints should be largely correct, even if they chose the wrong stimulus (and, with it, the wrong hand): any tactile location should be bound to its location in space.

What we found

When participants chose the incorrect hand, they did not point to where that incorret stimulus had been in space. Instead, they pointed to where the incorrectly chosen hand had been at the correct timepoint - that is, when the correct hand had received its stimulus!
The new preprint is a revision of a previous one. In the new preprint, we tested the effect over a range of four different time intervals between the two tactile stimuli, and we acquired data from a larger sample than in our original preprint. The new data were acquired in a new lab, by a new person, with different equipment (different brand motion tracker, sampling rate), and with scripts for experiment and analysis re-written from scratch. Still, the effect replicated and proved highly reliable across participants.

Why it‘s important

This study implicates that the presently popular idea of how touch is localized, and how we assign touch to body parts, is incorrect. Under this theory, participants should point to where the stimulus they chose was in space. Instead, we find that they construct a new location at which no stimulus ever occurred, by combining the correct stimulus time with the wrong hand. Therefore, stimulus location depends on which limb we think the stimulus occurred at - which is exactly vice versa from what previous theory suggested.