The perception of length on curved and flat surfaces

In three experiments, observers judged the apparent extents of spatial inte rvals along the surface of a curved cylinder or a flat plane that was binoc ularly viewed in a natural, indoor environment. The observers' judgments of surface lengths were precise and reliable but were also inaccurate and sub ject to relatively large constant errors. These distortions differed among the observers, but they tended to perceive lengths oriented along the curve d dimension of the cylinder as being longer than physically equivalent leng ths in the noncurved dimension. This phenomenon did not occur when the obse rvers judged curved and noncurved paths on the flat surface. In addition, s ome observers' judgments of length were affected by changes in the distance to the cylinder, whereas others were affected by the cylinder's orientatio n in space. These results demonstrate that the perception of length on surf aces is highly dependent on the particular context in which the length occu rs.