Comparative Visual Search : A Di erence that Makes a Di erence

In this article we present a new experimental paradigm: comparative visual search. Each half of a display contains simple geometrical objects of three di erent colors and forms. The two display-halves are identical except for one object mismatched in either color or form. The subject's task is to nd this mismatch. We illustrate the potential of this paradigm for investigating the underlying complex processes of perception and cognition by means of an eye-tracking study. Three possible search strategies are outlined, discussed, and reexamined on the basis of experimental results. Each strategy is characterized by the way it partitions the eld of objects into \chunks." These strategies are: (i) Stimulus-wise scanning with minimization of total scan path length (a \traveling salesman" strategy), (ii) scanning of the objects in xed-size areas (a \searchlight" strategy), and (iii) scanning of object sets based on variably sized clusters de ned by object density and heterogeneity (a \clustering" strategy). To elucidate the processes underlying comparative visual search, we introduce besides object density a new entropy-based measure for object heterogeneity. The e ects of local density and entropy on several basic and derived eye-movement variables clearly rule out the traveling salesman strategy, but are most compatible with the clustering strategy.