An Illusory Contour Induces Texture Segmentation

It is known that neurons in V1 can signal a contour between two out-of-phase gratings [Grosof et al, 1993 Nature (London) 365 550 – 552]. We demonstrate that this type of illusory contour can segregate areas of surfaces without any luminance, contrast, or textural difference between the areas.

We have studied the conditions under which the illusory contour induces texture segmentation. The target was a circular contrast-inverted area (diameter 0.22 – 8.3 deg) in the centre of an isotropic narrow-band noise texture (centre spatial frequency 0.4 – 7 cycles deg⁻¹). Generally, segmentation was effortless in low-spatial-frequency textures but gradually disappeared with increasing spatial frequency although the contour remained visible. In a staircase experiment, the highest spatial frequency allowing segmentation was measured for each target size. The task was to tell whether the stimulus contained an object or just a contour. A negative power function relates the target diameter and the highest spatial frequency allowing segmentation. The visibility of the contour was independent of the target size. The illusory contour ‘captures’ the texture inside. However, the process is spatially limited. In a separate experiment, the subject used a cursor to point out how far from the contour the capture spreads. A negative relationship between the spatial spread and the spatial frequency of the texture was found. These findings are consistent with the idea that low-level mechanisms signalling illusory contours are involved in perceptual scene segmentation.