Abstract
When the roles of target and non-target stimuli are reversed, visual search asymmetry can result, with set-size effects so large for one target that capacity limitations are implied, but not for the other target (Treisman & Gormican, 1988). One explanation is that it is easier to find a deviant stimulus, such as a tilted line, amid prototypical non-targets (vertical lines) than vice versa: The deviant stimulus activates a unique channel whereas the prototypical stimulus does not. Another explanation is that it is easier to discern a stimulus with more of some quantifiable feature, such as a longer line, than one with less (a shorter line). The first explanation suggests demands are placed on attention whereas the second may not. Moreover, both explanations emphasize the features of simple stimuli.
Some researchers propose the local feature information of faces and their configuration help distinguish one face from another (e.g., Diamond & Carey, 1986). Thus search asymmetry explanations for simpler stimuli may also hold for the more complex stimuli of human faces. However, other researchers postulate faces are processed holistically so that neither the individual features nor their configuration is explicitly represented (Farah, et al., 1998).
We wanted to know (a) if demands are placed on attention or at an earlier level and (b) if similar explanations could account for processing simple and complex stimuli. To do so, we parsed processing into component parts and compared results to predictions of competing models (Davis, et al., 2003). Both simpler stimuli (tilted vs. vertical lines and longer vs. shorter lines) as well as more complex stimuli (human faces) were explored. Line length only affected early visual processing so that search asymmetry was eliminated when target discrimination was balanced across conditions. In contrast, tilted vs. vertical line search still resulted in a search asymmetry effect. Search results for faces also are revealing.
Partially supported by a grant from the National Science Foundation