Roboticists believe that people will have an unpleasant impression of a humanoid robot that has an almost, but not perfectly, realistic human appearance. This is called the uncanny valley, and is not limited to robots, but is also applicable to any type of human-like object, such as dolls, masks, facial caricatures, avatars in virtual reality, and characters in computer graphics movies. The present study investigated the uncanny valley by measuring observers' impressions of facial images whose degree of realism was manipulated by morphing between artificial and real human faces. Facial images yielded the most unpleasant impressions when they were highly realistic, supporting the hypothesis of the uncanny valley. However, the uncanny valley was confirmed only when morphed faces had abnormal features such as bizarre eyes. These results suggest that to have an almost perfectly realistic human appearance is a necessary but not a sufficient condition for the uncanny valley. The uncanny valley emerges only when there is also an abnormal feature.

Three experiments using computer-generated human figures showed that after a prolonged observation of eyes looking to the left (or right), eyes looking directly toward the viewer appeared directed to the right (or left). Observation of an arrow pointing left or right did not induce this aftereffect on the perceived eye direction. Happy faces produced the aftereffect more effectively than surprised faces, even though the image features of the eyes were identical for both the happy and the surprised faces. These results suggest that the eye direction aftereffect may reflect the adaptation of relatively higher-level mechanisms analyzing the other's eye direction.

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Natural human faces with abnormal visual features produce uncomfortable impressions, but artificial faces (e.g., robotic faces) do not necessarily do so. This is an example of the phenomenon called the uncanny valley. We hypothesized that this phenomenon indicates that natural and artificial faces are processed by different perceptual mechanisms, or they are processed differently by common mechanisms. We tested these hypotheses using a facial aftereffect where prolonged observation of adaptation faces with enlarged eyes induced a bias to underestimate the eye size of test faces. The results showed that adaptation to natural stimuli induced the aftereffect for both natural and artificial test stimuli. This suggests that the two types of faces engage common perceptual mechanisms. Adaptation to artificial stimuli also induced the aftereffect for natural test stimuli. However, artificial stimuli required a longer adaptation period (120 s) for the aftereffect to be induced compared to natural stimuli (60 s), suggesting that the processing of artificial faces by the human visual system may be inefficient. The uncanny valley may reflect that artificial faces are processed inefficiently by perceptual mechanisms that are common for processing natural and artificial faces.

After observing a face with the eyes looking to the left or right (adaptation stimulus), the perception of the eye direction of the subsequent face (test stimulus) is biased in the opposite direction of the adapted eye direction; this is called the eye direction aftereffect (EDAE). In the present study, the adaptation stimuli were rotated 90 degrees (clockwise or counterclockwise) or 180 degrees relative to the viewer. The EDAE was measured using upright test stimuli. For the 90 degrees rotation, prior observation of the 'leftward' and 'rightward' eye directions biased the perceived eye directions of the upright test stimuli to the right and left, respectively. These results suggest that the adaptation was induced utilizing an object-based (or face-based) reference frame. For the 180 degrees rotation, however, the results suggest that the adaptation was induced in a viewer-centered reference frame. The involvement of an object-based reference frame suggests that the EDAE reflected the adaptation of a relatively higher-level mechanism at least when the rotation angle from the upright position did not exceed 90 degrees .