Apparent-length expansion in whole contours and isolated parts: evidence for configuration-dependent processing

The visual system overestimates the length of structured contours relative to adjacent empty intervals of the same physical size. This study examined whether apparent-length expansion occurs across isolated contour parts as well as complete objects, and whether the expansion of the whole can be predicted from that of its components. Observers matched the perceived horizontal extent of complete pentagons, incomplete pentagons with one or two missing segments, isolated apices, pedestals, parallel line pairs, and rectangles using an adjustment method. All stimulus types produced consistent positive mean errors, indicating robust apparent-length expansion across a wide range of contour configurations. The magnitude of expansion depended on contour geometry rather than on physical size alone: removing contour segments did not reduce expansion and, in some cases, increased it. Isolated parts produced expansion effects comparable to or larger than those of the corresponding whole pentagons, but the sum of part effects did not predict the whole-object effect. These findings support a configuration-dependent account of apparent-length expansion and appear inconsistent with simple part-based additivity.