Changing the position of a light source illuminating a human face indu
ces an apparent shift of the perceived orientation of that face. The d
irection of this apparent shift is opposite to the shift of the light
source. We demonstrated the illumination-induced apparent orientation
shift (IAOS), quantified it in terms of the physical orientation shift
needed to compensate for it, and evaluated the results in the context
of possible mechanisms underlying orientation judgment. Results indic
ate that IAOS depends not only on the angle between the two light sour
ce positions, but also on the mean orientation of the face. Availabili
ty of cues coded in the visual texture of the face did not affect IAOS
. The most effective cue was the location of the visible outline of th
e face. IAOS seems to be due to a shift of this outline when shadowed
areas on the face merge with the black background. We conclude that an
important mechanism for orientation judgment is based on a comparison
of visible parts left and right of the profile line.