Mug Shots: Systematic Biases in the Perception of Facial Orientation within Pictorial Spaces

Pictures are 2-D projections of a 3-D world, so pictorial spaces behave differently than the 3-D visual spaces we inhabit. For instance, the angular orientation of a face pictured in half-profile view is systematically overestimated by the human observer – a 35° view is estimated to be approximately 45°. What is the cause for this perceptual orientation bias? We tested three different hypotheses. (1) The phenomenon is specific to pictorial projections due to the twofoldness of the medium and does not occur in 3-D space. (2) It can be explained with the depth compression expected when the vantage point of the observer is closer to the picture than the point of projection. (3) The visual system uses a shape prior that does not match the elliptical horizontal cross section of a typical head. Our results support the third hypothesis, and this effect can be mitigated through adding geometric information through structure-from-motion.