Haptically straight lines.

In this research, we set out to investigate haptically perceived space. Large deviations with respect to physical space have already been shown to exist. Here, research on haptic space is continued by investigating straight lines constructed by touch. In four experiments, subjects were asked to produce straight lines between two reference markers that were in the horizontal plane at a fixed distance from each other. Each experiment corresponded to a different task: two different interpolation tasks, an intersection task, and a pointing task. Straight lines had an orientation that was approximately frontoparallel. Subjects used both hands; manipulation was unrestricted. Although we found considerable differences between observers, the overall pattern of results showed that haptically straight lines were generally curved away from the observer. However, in one of the interpolation tasks they corresponded to physically straight lines. In addition, the pointing task generally produced larger deviations than the other three tasks. Taken together, the results show that there is no unique definition of the straight line, a conclusion that questions the viability of the concept of haptic space.