Abstract
A person depicted in portrait paintings does not appear slanted even when observers move around. The gaze is also fixed to the observer. This constancy in angle of face/body orientation or gaze direction is called the Mona Lisa effect. Do observers realize the portrait was physically slanted when the effect occurs? What is the relationship between the effect for face/body and gaze? To answer these questions, we separately measured the perceived angle of face, gaze, and background while varying the physical slant of portrait itself. The stimulus was a computer generated face (19 × 12 deg) presented on a 3D LCD display. It was surrounded by a 24 × 24 deg black-contour frame filled with a noise texture. There were also no-frame and/or no-texture conditions. The slant was varied between ±30 deg. The observer was asked to judge the direction of gaze and the orientation of face or background in separate sessions. It was found that the perceived gaze almost always directed toward the observer regardless of slant angle or existence of frame or background. In contrast, the face orientation was judged facing the observer only in 40–50% of trials, and it was facing at the correct angle in 50–60% of trials. The background was perceived correctly in most trials. These results demonstrate special characteristics of eyes. The gaze is always directed to you even when the portrait is slanted and the background is perceived slanted. The face has intermediate characteristics: it is sometimes directed to you, but sometimes it appears slanted.