Probing Distractor Inhibition in Visual Search: Inhibition of Return

The role of inhibition of return (IOR) in serial visual search was reinvestigated using R. Klein's (1988) paradigm of a search task followed by a probe-detection task. Probes were presented at either the location of a potentially inhibited search distractor or an empty location. No evidence of IOR was obtained when the search objects were removed after the search-task response. But when the search objects remained on, a pattern of effects similar to Klein's results emerged. However, when just the search-critical object parts were removed or when participants received immediate error feedback to prevent rechecking of the search objects, IOR effects were observed only when probes appeared equally likely at search array and empty locations. These results support the operation of object-based IOR in serial visual search, with IOR demonstrable only when rechecking is prevented (facilitating task switching) and monitoring for probes is not biased toward search objects. Many everyday visual inspection and search tasks involve the serial scrutiny of the environment until a target is detected. Such everyday tasks are similar to visual search experiments conducted in the psychological laboratory. In the laboratory, participants typically search for a target object among varying numbers of nontarget or distractor objects (the display size) and their reaction times (RTs) to detect the target are measured. Targets that are difficult to detect give rise to linearly increasing search RT/display size functions. Many theories of visual search (e.g., Cave & Wolfe, 1990; Treisman & Gelade, 1980; Wolfe, 1994) take such functions to be indicative of a spatially serial, item-by-item search process involving focal attention, where the shifts of focal attention may be entirely covert, that is, not accompanied by overt eye, head, and/or body movements. (In contrast, targets that are easy to detect give rise to fiat search RT/display size functions, consistent with the idea that the entire display, however many objects it contains, is searched in a spatially parallel manner.) One question of importance to all theories of visual search and inspection is how does the serial scanning mechanism keep track of where focal attention has already been so that it does not unnecessarily return to the same location or object? I