The precision of 'haptic' rod length perception is reduced by lack of visual precision

In studies on haptic rod length perception, participants conventionally report their length estimates by placing a visual landmark at a position equal to the rod’s perceived endpoint. We hypothesized that this visual aspect substantially increases the variability of the recorded length judgments. To examine this, we developed a virtual reality length judgment apparatus that provides better visual information. Participants performed a rod length perception task in both the conventional apparatus and the virtual reality apparatus. The variability of the length judgments was found to be higher in the conventional apparatus. We determined that between half and two-thirds of the variance in the conventional apparatus is haptic variance. Thus, vision accounts for between one-third and half of the variance that was previously thought to be haptic variance. Our finding implies that the virtual reality apparatus may be more suitable for studying subtle effects in haptic rod length perception.