Fig. 1.

Face stimuli and experimental design of the psychophysics experiment. (A) Shows an example “morph line” between a pair of face prototypes (shown at the far left and second from the right), created using the photorealistic morphing system (Blanz and Vetter, 1999). The relative contribution of each face prototype changes smoothly along the line. For example, the fourth face from the left (the third morph) is a mixture of 70% of the face on the far left and 30% of the face on the far right. Four conditions, M3/6/9/12, which correspond to shape differences of 30%, 60%, 90%, and 120%, were examined in the present study. The difference between two prototype faces was defined as 100%; 120% difference was achieved by extrapolation along the morph line beyond face prototypes (Blanz and Vetter, 1999). (B) Design of the 2AFC face discrimination experiment. Subjects viewed a target face, followed by a mask, followed by two test faces, presented side-by-side, and had to indicate which of the test faces was identical to the previously presented target face. There were four levels of shape difference, M3/6/9/12, between the two test faces as explained in (A).