Learning to search for visual targets defined by edges or by shading: Evidence for non-equivalence of line drawings and surface representations

A visual search task was used to test the idea that shaded images and their line-drawn analogues are treated identically from an early stage onwards i n human vision. Reaction times and error rates were measured to locate the presence or absence of a target in an array of a variable number of distrac tors. The target was a cube in one orientation and the distractors cubes in a different orientation. The stimuli were defined by lines alone, shading alone, or lines plus shading. Both the slopes and the intercepts of the sea rch functions (graphs of search time against number of displayed items) wer e higher for the line drawings than for the stimuli defined by shading. Ove r six experimental sessions, both the slopes and the intercepts fell for al l stimuli, but the relative differences between them were maintained. The d ata suggest that, at an equivalent stage of practice, line-drawn stimuli ar e processed more slowly than shaded stimuli in early vision.