The influence of arm position on the Uznadze Illusion.

What is it about?

This study investigated how the position of our arms in space can influence our perception of object size. To this aim, we employed a well-known tactile illusion called the Uznadze Illusion. During this illusion, participants repeatedly held two spheres of different sizes (big or small) before touching two new test spheres. Depending on which hand was used to hold the small or big sphere, the size of the test spheres felt different. In the first experiment, participants evaluated the size of the test spheres while their arms were uncrossed or crossed. In the second experiment, participants matched the size of the test spheres while continuously changing their arm posture from uncrossed to crossed and vice versa. The illusion occurred regardless of arm posture, but the its strength was reduced when participants had their arms uncrossed. We discussed the results in relation to two factors that could influence haptic perception: low-level processing based on the stimuli conformation and high-level processing taking into account the arm posture.

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Photo by okeykat on Unsplash

Photo by okeykat on Unsplash

Why is it important?

This study shows that the position of the hands in external space does not affect the direction of the Uznadze haptic illusion, but it can influence its magnitude. Hence, the work provides further understanding regarding the role of the egocentric spatial location of body parts (i.e., arms) on the construction of haptic experiences of size through bimanual processing. Furthermore, the results presented support the use of the crossmodal method employed by Daneyko et al. (2020) for measuring the magnitude of the Uznadze illusion under different types of manipulation.