Is Tactile Shape Coding Invariant Against Small Location Shifts on the Skin?

A preliminary experiment was conducted to investigate the nature of location invariance of shape perception in touch. The small bar shape was presented on the fingertip by means of a dense pin-array stimulator. Participants were asked to report which of the two sequential pattern presentation groups contained the oddball pattern. In one group, the same pattern (e.g., one vertical bar) was presented sequentially six times at short intervals at a fixed location on the fingertip, and in the other group, the same five patterns and one different pattern (e.g., one oblique bar) were presented. The obtained performance of discriminating shapes was compared to that when the patterns were presented at random locations on the fingertip. We found that the discrimination performance dropped when the pattern had to be recognized at new locations. This result suggests that the tactile system cannot easily find the shape identity over different locations, even when the shape is very simple and position change is within a single finger.