Comparison of the human perception of hole size by the tongue and the fingers

Thirty blindfolded subjects assessed the size of holes (2.4-19.1 mm dia) that were presented to the tongue by using their fingers to select a matching hole from a comparator series of 38 holes (1.6-25.4 mm dia). Subjects consistently overestimated the size of holes less than 10 mm, but there was little disparity for holes greater 14 mm in diameter. The relationship between the size of hole presented to the tongue and that judged by the fingers to be of equal size was not linear across the range tested. The illusion, expressed as the ratio of apparent size to real size, could be described as an hyperbolic function of stimulus size.