Differences in the oral size illusions produced by cross-modality matching of peg and hole stimuli by the tongue and fingers in humans

Individuals overestimate the diameter of 1-mm-deep stimulus holes presented to the tongue when they use their fingers to select a hole of matching diameter. The aim here was to determine whether the oral size illusion evident for 1-mm-deep holes would also occur with 1-mm-high pegs of similar diameters. The illusion was studied in 24 individuals who were blindfolded during the trials. The two sets of test stimuli were (a) 5 cylindrical pegs (1 mm high) and (b) five circular holes (1 mm deep), each of 2.5, 5.0, 7.5, 10.0, 12.5 mm dia. The stimuli were held to the mouth with one hand, while the fingers of the free hand were used to select a matching object from a comparator series of either 1-mm-deep holes or 1-mm-high pegs ranging from 2.0 to 18 mm dia. Each stimulus was presented four times in a random order. The participants overestimated the diameter of the 1-mm-deep stimulus holes when these were matched with 1-mm-deep comparator holes. However, there was no illusion with four out of five 1-mm-high stimulus pegs when matched with 1-mm-high comparator pegs. In the ‘reverse’ experiment, there was no illusion with four out of five stimulus holes when these were matched with 1-mm-high comparator pegs. However, an illusion was evident for all the 1-mm-high stimulus pegs when these were matched with the 1-mm-deep comparator holes. Regardless of the nature of the stimulus (‘hole’ or ‘peg’), a mismatch between the stimulus and comparator was consistently seen only when the fingers probed comparator holes. The oral size illusion is not due to any intrinsic differences in the sensitivities of the tongue or fingers. Rather, the illusion is due to the inability of the fingertip to access small comparator holes.