Asymmetries in visual search: An introduction

In visual search tasks, observers look for a target stimulus among distract er stimuli. A visual search asymmetry is said to occur when a search for st imulus A among stimulus B produces different results from a search for B am ong A. Anne Treisman made search asymmetries into an important tool in the study of visual attention. She argued that it was easier to find a target t hat was defined by the presence of a preattentive basic feature than to fin d a target defined by the absence of that feature. Four of the eight papers in this symposium in Perception & Psychophysics deal with the use of searc h asymmetries to identify stimulus attributes that behave as basic features in this context. Another two papers deal with the long-standing question o f whether a novelty can be considered to be a basic feature. Asymmetries ca n also arise when one type of stimulus is easier to identify or classify th an another. Levin and Angelone's paper on visual search for faces of differ ent races is an examination of an asymmetry of this variety. Finally, Previ c and Naegele investigate an asymmetry based on the spatial location of the target. Taken as a whole, these papers illustrate the continuing value of the search asymmetry paradigm.