Inside vs. outside: Haptic perception of object size

We have performed a psychophysical experiment to investigate differences in perceived object size when exploring the inside or outside of objects. The experiment consisted of five conditions, in which ten blindfolded subjects compared the size of circular disks and holes using either the index finger, two different probes, the finger-span method, or an infinitesimal virtual probe. The result showed significant negative biases for the conditions with the large probe and the finger-span method, meaning that an object felt on the inside should be larger than an object felt on the outside in order to be perceived as the same size. This indicates that subjects are unable to sufficiently correct for the diameter of the probe when exploring objects. At the same time, a general tendency was observed in all conditions that involved movement to feel the inside of objects as larger than the outside. This suggests that, in order to obtain a neutral estimate of object size in a virtual environment, one should use a virtual probe diameter of about 4% of the size of the object to be explored.