Where is the sun?

As illustrated by the well-known moon crater illusion, the human visual system uses a light-from-above assumption when it interprets shaded stimuli as 3-D shapes. Experimental results involving shaded stimuli also support this observation (Braun '90, '93; Kleffner & Ramachandran '91: Sun & Perona '93). While a light-from-above assumption is made for solving tasks involving ambiguous stimuli with shading consistent with both light-from-above and light-from-below, what assumption is made when lighting is from the side