Abstract
The human visual system shows skilled performance in perceiving familiar objects. Visual search experiments have been employed to investigate this superiority of familiar objects. One of the most useful indexes is search asymmetry that familiar targets are less detectable than unfamiliar ones against the homogeneous background. Familiarity concerns faces. Faces are almost symmetrical, but strictly speaking, they are pseudo-symmetrical because of the slight differences between normal and mirror-reversed images of them. The present study focuses on this fluctuating asymmetry by examining if the human visual system draws a line between the normal and mirror-reversed images of our own faces in the visual search experiment. Before examining the facial asymmetry, we confirmed the validity of search asymmetry by comparing the search performance for our own faces and that for other faces. Observers made a slower response in the condition of their detecting the image of their own faces against the background of the images of others than in the reversed condition. This asymmetrical result reflected the specialty in perceiving our own faces. The same procedure was applied to examine the discrimination between the normal and mirror-reversed images of our own faces. The result showed the availability of the slight differences between the normal and mirror-reversed images of observers' own faces. This acute sensitivity thus yielded search asymmetry that better performance was obtained in the condition that the observers searched the mirror-reversed image among normal images compared with the reversed condition. This search asymmetry reflected the advantage of mirror-image over the normal image in familiarity. The human visual system holds a refined representation for our own faces as mirror-reversal. Such sensitivity may refer to the upper-bound of the visual ability for objects, because daily experiences of viewing the mirror-reversed face in the mirror have sharpened the sensitivity up to the highest level.