The computation of shape orientation in search for Kanizsa figures.

Previous studies of visual search for illusory figures have provided equivocal results, with efficient search for Kanizsa squares (e.g., Davis and Driver, 1994 Nature 371 291-293) contrasting with inefficient search for Kanizsa triangles (e.g., Grabowecky and Treisman, 1989 Investigative Ophthalmology and Visual Science 30 457). Here, we investigated whether shape orientation can explain these differences. The results from three experiments replicated previous findings: Kanizsa squares in experiment 1 could be detected more efficiently than Kanizsa triangles in experiment 2. In addition, when controlling for stimulus complexity in experiment 3, we found search for Kanizsa diamonds intermediate in efficiency. Taken together, these results suggest an oblique effect in search for Kanizsa figures with cardinal shape orientations leading to more efficient performance than oblique shape orientations. Our findings indicate that both shape orientation and stimulus complexity affect search for illusory figures.