Enhanced position sensitivity at scaled medial loci
Purpose: Kovacs and Julesz (Nature, 1994) found enhanced contrast sensitivity at medial locations, supporting theoretical speculations that the visual system represents regions by medial axes. Burbeck and Pizer's core model (Vis. Res., 1995) hypothesizes that the medial representation arises in a scale-specific way, with scale being determined by local object width, and that the spatial resolution of the medial axis is proportional to that scale. Here we look for further evidence for a medial representation and test the idea that the resolution of the axis depends on object width. Methods: A new experimental paradigm was developed to infer sensitivity to position within individual figural regions. A probe dot was presented within the object (a circle) along a radius; in a second temporal interval the object was presented again with the probe dot in a slightly different location along the same radius. 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 2-dot separation discrimination thresholds. Data were obtained for three circle sizes. Results: Positional sensitivity was enhanced near the center of the circle and it was enhanced in a size dependent way: the area of enhancement was larger for the larger objects, and the magnitude of the enhancement was greater for the larger objects. Conclusion: Results of this new position discrimination paradigm support the hypothesis that the visual system forms a medial representation and provide evidence for scaling of the spatial resolution of that representation.
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