The effect of stimulus form and dimensions on the oral size illusion in humans

Previous work has shown that individuals overestimate the diameter of holes presented to the tongue when they use their fingers to select matching holes from a range of comparators. This disparity is known as the oral size illusion. The hypothesis that this illusion arises from the greater pliability of the tongue surface compared with that of the fingertip was tested here. Two experiments were carried out on 64 participants. In experiment 1 (n=32), the stimuli were two sets of holes, 1 and 5 mm deep and 2.4, 5.6, 8.2, 12 mm dia. , which were presented to the tongue. The comparator was a series of holes 5 mm deep and 0.8-15 mm dia., which were explored manually. Participants significantly overestimated all diameters of the 1- and 5-mm deep stimulus holes (p<0.05). Apart from the 2.4-mm dia. holes, there were no significant differences between the illusions produced with the holes 1 mm and 5 mm deep (p0.7). In experiment 2 (n=32) individuals were tested with small cylinders ('pegs') 1 mm high and 2.6, 5.2. 7.6, 12.6 mm dia. and also with the four 5-mm deep stimulus holes used in experiment 1. The 'peg' stimuli presented to the tongue were matched against a similar set of 1-mm pegs, 2-18 mm dia., explored with the fingers. The comparator series for the 5-mm deep holes was the same as for experiment 1. Participants consistently overestimated the size of the 5-mm holes (p<0.001). However, there was no illusion with the peg stimuli (p=0.08), except for the 7.6-mm dia. stimulus (p<0.05). The results of experiment 1 did not support the experimental hypothesis that the oral size illusion was due to the greater pliability of the tongue surface. The result of experiment 2 suggests that the oral size illusion does not occur when peg stimuli are used instead of holes. The form and diameter of the stimulus is thus important in determining the magnitude of the oral size illusion.