Scaled medial axis representation: evidence from position discrimination task

Previous experimental studies (e.g. Kovacs I, Julesz B. Nature (London) 1994; 370:644-646) have found enhanced contrast sensitivity at medial locations, supporting theoretical speculations that the visual system represents simple spatial regions by their medial axes. The core model (Burbeck CA, Pizer SM. Vis Res 1995;35:1917-1930) hypothesizes that the medial representation arises in a scale-specific way: the scale is determined by local object width, and it controls the resolution at which the medial locus and object width are encoded. Here we look for further evidence for a medial representation and test the idea that the resolution of the axis depends on object width. A new experimental paradigm was developed to infer sensitivity to position within individual figural regions, using circles as the figural regions. A probe dot was presented within a circle along a diameter at one location in one temporal interval and at a slightly different location on that diameter in a second temporal interval. The observer's task was to report the direction in which the probe dot had been displaced. Position discrimination thresholds were calculated and compared to two-dot separation discrimination thresholds. Data were obtained for two circle sizes. It was found that positional sensitivity was strongly enhanced near the center of the circle, and it was enhanced in a scale-dependent way. The results were tested against a scaled medial (core) model and against models assuming no medial representation. The core model was better able to account for the results.