Measuring haptics with the eye: The case of the Uznadze illusion

In 1949 the Georgian psychologist Uznadze described a simultaneous haptic size contrast illusion that occurs after adapting, out of vision, one hand to a small sphere and the other to a large one. After the adaptation process (10-15 simultaneous exposures), participants are suddenly exposed to two spheres equal in size and weight, but the impression is that the sphere held by the hand adapted to the large sphere is much smaller than the other one. Here we experimentally sought for the mechanisms underpinning the illusion. One experiment tested the possibility of measuring the illusion’s magnitude with a matching method in which right-handed participants were asked to indicate the size of each hand-held sphere on a 12 step visual scale made of spheres painted matt black. Variables were: (a, within) size of the adapting spheres; (b, between) evaluation direction (right-left, left-right); (c, between) position of the small adapting sphere (right, left). Only variables a and b produced significant effects, confirming the possibility to use a visual scale to measure the illusion, however suggesting also a role of handedness in the evaluation process. Further experiments assessed the role of handedness and of simultaneous vs single-hand adaptation in the evaluation process.