Search asymmetry in search for symmetry

Studies on visual search on vertical symmetry showed that both search for symmetry among asymmetry and search for asymmetry among symmetry are serial, but the latter was more efficient than the former (Olivers and van der Helm 1998). However, it is not entirely clear under what conditions a robust search asymmetry occurs. We conducted several experiments to elucidate characteristics and possible mechanism of this search asymmetry (more efficient search for asymmetry). In Experiment 1, we replicated the efficient search for asymmetry using vertically symmetrical solid shapes; reaction times were significantly faster and slopes were shallower in search for asymmetry among vertically symmetrical shapes than symmetry among asymmetrical shapes. In Experiment 2, we found that the search asymmetry was present with horizontally symmetrical shapes. In Experiment 3, the efficient search for asymmetry among symmetry was observed even when search items were dot patterns, which had failed to show search asymmetry in the previous study (Olivers and van der Helm 1998). We interpreted these findings in that the larger set size (3 to 9) than the previous study (1 to 4) resulted in the robust search asymmetry (i.e. more efficient search for asymmetry among symmetry). Subsequent experiments examined whether the search asymmetry observed in the present study is due to a faster processing for symmetric stimuli than asymmetric stimuli, or a grouping process for symmetric stimuli. Results have converged to suggest that the latter is more likely, that is, symmetric non-target items are perceptually grouped and therefore visual attention is guided to asymmetric target.