The Development of the Light Source Bias in Shape-from-Shading

Objects that are lighter at the top usually appear convex due to an implicit assumption that light originates from above. This percept is stronger when the light source originates from the above-left, which may suggest a right hemisphere dominance in shape-from-shading. This leftward bias is consistently observed in Western adults who read from left-to-right. In right-to-left reading populations, however, the bias is more variable; at times presenting as a diminished leftward bias, or sometimes as a strong rightward bias. To understand the contribution of cultural factors, such as habitual scanning direction, to the development of the light source assumption, we tested children in Wales and Israel on a visual search task of shaded spheres at different stages in reading acquisition. We expected all pre-literate children to exhibit a leftward bias, detecting oddball circles with a shading pattern consistent with light coming from the above left, faster than other orientations. We predicted rightward shifts in Israeli children as reading fluency increased. This might indicate that the leftward bias is a developmental default that can be modified by cultural factors. We will discuss differences between Welsh and Israeli children and implications for the lateralisation hypothesis in shape-from-shading