The ability of haptics to modulate the Ebbinghaus illusion
Toshiko Mochizuki1*, Rie Kashiwabara1 & Keiko Omori2
(1: Japan Women’s University, Kanagawa 214-8565, Japan)
(2: Nihon University, Tokyo 156-8550, Japan) *motizuki@fc.jwu.ac.jp

Purpose: We investigated how the Ebbinghaus illusion (the phenomenon wherein the central circle looks larger if the circles around it are smaller in size and vice versa) could be modulated by haptics. Method: Ten university students (aged 21–22) observed the illusory figure under visual and bimodal conditions. Illusory figures composed of a central circle (5 cm in diameter) and five surrounding circles (3, 4, 5, 6, or 7 cm in diameter, size D) were presented one by one on a PC display. In the bimodal condition, the observers had to integrate the sizes of the visual and haptic central circles, which were congruent or incongruent in size. They simultaneously explored the haptic circle presented on the other side of the visual image by hand. Then, they decided the apparent size of the central circle among the comparison stimuli (4, 4.5, 5, 5.5, and 6 cm in diameter). Results: (1) In the visual condition, the Ebbinghaus illusion could be clearly found. The PSE (subjective equal size of the central circle) was larger than 5 cm when D = 3 cm, while it was smaller than 5 cm when D ≥ 4 cm. (2) In the bimodal condition, haptics modulate the amount of size illusion. Mean error score (The PSE �5 cm) of each central circle was significantly decreased by the haptic capture.