Relative finger position influences whether you can localize tactile stimuli

Abstract

To investigate whether the relative positions of the fingers influence tactile localization, participants were asked to localize tactile stimuli applied to their fingertips. We measured the location and rate of errors for three finger configurations: fingers stretched out and together so that they are touching each other, fingers stretched out and spread apart maximally and fingers stretched out with the two hands on top of each other so that the fingers are interwoven. When the fingers contact each other, it is likely that the error rate to the adjacent fingers will be higher than when the fingers are spread apart. In particular, we reasoned that localization would probably improve when the fingers are spread. We aimed at assessing whether such adjacency was measured in external coordinates (taking proprioception into account) or on the body (in skin coordinates). The results confirmed that the error rate was lower when the fingers were spread. However, there was no decrease in error rate to neighbouring fingertips in the fingers spread condition in comparison with the fingers together condition. In an additional experiment, we showed that the lower error rate when the fingers were spread was not related to the continuous tactile input from the neighbouring fingers when the fingers were together. The current results suggest that information from proprioception is taken into account in perceiving the location of a stimulus on one of the fingertips.

Fig. 1

The different finger configurations used in experiment 1: a fingers together, b fingers spread and c fingers interwoven. d The map of the two hands that was shown to our participants, with numbered dots indicating the possible stimulation sites

Fig. 2

Proportion of responses in experiment 1 averaged over all participants (with standard errors). a Symbols (and open bars) proportions of responses. Solid bars proportions normalized by the number of possibilities of this response occurring (for details, see text; values are given in Table 1). Inset the critical values of the normalized responses, enlarged for clarity. b Normalized proportions for the distance in numbers of fingers from the target on the stimulated hand (upper panel) or the other hand (lower panel). The positions on the other hand are calculated as if the same finger of that hand was stimulated (e.g. if the middle finger was stimulated, an error of distance 1 would be to either the index or the ring finger. An error of distance 1 on the other hand would be to the index or ring finger of the other hand)

Fig. 3

Results of experiment 2 in the same format as Fig. 2. The inset in a illustrates the two conditions. “Spread +” is the condition with additional tactile input