After visual adaptation to an object (e.g., a face) viewed from one side, the perceived viewing direction of the same object/face subsequently presented near its front view is biased in a direction opposite to that of the adapted viewpoint (
Figure 1). This aftereffect was termed viewpoint aftereffect. Observation of such a viewpoint aftereffect supports the existence of viewer-centered object representation in the human visual system (Fang & He,
2005; Ryu & Chaudhuri,
2006). Our previous study has shown that there was a very weak or a little transfer of the viewpoint aftereffect between objects from two different categories (e.g., no cross adaptation between a face and a paperclip), suggesting that viewpoint information is coded specifically for object categories. However, little is known about how sensitive the viewpoint aftereffect is to differences between objects within a category. Thus, the first question we addressed in this study is how the structural similarity between adapting and test faces modulates the viewpoint aftereffect. In the first and second experiments, we generated the adapting and the test stimuli through face morphing along identity dimension (Leopold, O'Toole, Vetter, & Blanz,
2001) and gender dimension (Webster, Kaping, Mizokami, & Duhamel,
2004), respectively. The difference in gender dimension is not just another case of identity difference. Gender forms a natural category and tends to be more salient than almost any other feature when looking at a face. Studying the neural coding of gender information is becoming an important topic in face perception (Ng, Ciaramitaro, Anstis, Boynton, & Fine,
2006). Viewpoint aftereffect transfer depends largely on whether view-selective face neurons in the human visual system are also tuned to face structures (e.g., identity and gender). Perrett, Hietanen, Oram, Benson, and Rolls (
1992) observed that most view-selective face neurons in monkeys' inferior temporal cortex are not sensitive to identity, but identity-sensitive neurons are often view selective. From this evidence, we would predict a strong but not complete transfer of the face viewpoint aftereffect when adapting and test faces are of different identities. Other vision researchers investigated this joint neural coding of face structure and view using face configural aftereffect (Leopold et al.,
2001; Webster et al.,
2004). They tested whether the face configural aftereffect can transfer across a substantial change in viewpoint, but there remains no consensus that face structure is view-specific coded (Anderson & Wilson,
2005; Jeffery, Rhodes, & Busey,
2006; Jiang, Blanz, & O'Toole,
2006).