Are Angles Transformed into Blobs in Early Vision?

Abstract

When textures used in visual segmentation experiments are composed of angle elements, these angles seem to be transformed into ‘blobs’ at early stages of visual processing (‘blob hypothesis’; see Meinecke and Kehrer, 1994 Perception & Psychophysics56 326 – 334). These blobs can be imagined as ellipses in which the angles are embedded. Thus, ellipses/blobs are more or less elongated depending on the width of the underlying angle. We tested the blob hypothesis in a psychophysical experiment using two experimental conditions with angle textures. In condition A, subjects had to detect a target texture, in which the resulting blobs had a well-defined orientation, within a background texture, in which the blobs had a less-well-defined orientation. Condition B was identical, except that the target elements became the background elements and vice versa. If angles are really transformed into blobs in early vision, then condition A should yield a better performance than condition B, because it is much easier to detect ellipses among circles than circles among ellipses (Treisman and Gormican, 1988 Psychological Review95 15 – 48). Our results replicated this asymmetry and thus support the blob hypothesis. In a simulation study, in which Gabor filters were used as the input layer in a spatial filter model, we tried to gain a better understanding of the underlying mechanisms. The simulation indicated that the experimentally observed asymmetry might be due to particular features of the target textures, leading to higher Gabor target responses in condition A than in condition B, whereas background responses were very similar in both conditions.