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Abstract

In vision, the discovery of the phenomenon of saccadic suppression of displacement has made
important contributions to the understanding of Helmholtz’ stable world problem. Here we report a
similar phenomenon in the tactile modality. When scanning a single Braille dot with two fingers of the
same hand, subjects were asked to decide whether the dot was stationary or whether it jumped from one
location to another. The stimulus was created using refreshable Braille devices which have dots that
can be swiftly raised and recessed. In some conditions, the dot jumped from one location to another by
amounts of 2.5 and 5 mm. By monitoring the subject’s finger position we could ensure that the jumps,
if any, occurred when the dot was not touched by either finger. In some other conditions the dot did not
move. We found that in certain conditions, jumping dots were felt to be stationary. If the jump was
orthogonal to the finger movements, tactile suppression of displacement occurred effectively when the
jump was of 2.5 m but when the jump was 5 mm, subject easily detected it. If the jump was lateral, the
suppression effect occurred as well but less often when the artificial movement of the dot opposed the
movement of the finger. In such cases, the stimulus appeared sooner than when the brain could predict
it from finger movement, supporting a predictive rather than a postdictive differential processing
hypothesis.