Part-whole information assists in topological × topological but not in orientation × orientation conjunction searches

Visual search is a routine task used in everyday life and is an important field of research in cognitive psychology. In laboratory settings, it has been shown that search for a target defined by a unique conjunction of two colours is more efficient if one colour surrounds the other (a part-whole search) compared to when no such hierarchical structural relationship exists (a part-part search; Wolfe et al. in Perception & Psychophysics, 55, 537, 1994). A similar result has been shown to hold for size × size conjunction searches (Bilsky & Wolfe in Perception & Psychophysics, 57, 749, 1995). We show that this result also holds for topology × topology conjunction searches (where the stimuli are either hollow or filled), but not for orientation × orientation conjunction searches. We use the simultaneous-sequential paradigm to investigate a possible reason for the inefficiency of part-whole orientation search compared with the efficiency of part-whole searches of other features. We argue that two different attribute values from the same dimension can be processed independently, without interfering with each other for colour, size, and topology, but not for orientation. Because it is obviously more efficient to process a conjunction stimulus when both components of the conjunction can be processed without mutual interference, it follows that colour × colour, size × size, and topological × topology part-whole conjunction searches are likely to be more efficient than orientation × orientation part-whole conjunction searches.