Investigative Ophthalmology & Visual Science,34:1288. 1993. [ARVO 1993; Sarasota, FL.]

Recent studies have shown that several scene-based properties can be determined rapidly and in parallel at preattentive levels, including surface convexity and concavity (Ramachandran, 1988), direction of illumination (Enns & Rensink, 1990), and three-dimensional orientation (Enns & Rensink, 1991). We show that in addition to these properties, preattentive vision is also sensitive to scene structure defined by shadows.

Evidence for this is based on a set of visual search experiments involving simple black-and-white figures that correspond to shadowed posts. Search is rapid when the target has an anomalous shadow (view figure from rightside up), but slows down when the distractors have anomalous shadows (view figure from upside down). This asymmetry in speed is destroyed when the areas corresponding to shadows are made lighter than the background or are surrounded by a thin white line; for these conditions, search is evidently based on image properties alone. It is also shown that the addition of white lines does not have a similar effect on preattentive processes based on shading, suggesting the operation of a separate mechanism for the processing of shadows.

(Figure viewed upside down)