Footprints sticking out of the sand. Part 1: Children's perception of naturalistic and embossed symbol stimuli

The shading information in images that depict surfaces of three-dimensional objects cannot be perceived correctly unless the direction of the illuminating light source is known, and, in the absence of this knowledge, adults interpret such images by assuming that light comes from above. In order to investigate if children make use of a similar assumption, we analysed data from 171 children between the ages of 4.6 and 10.8 years using 10 images (shown upright and upside-down) that could be perceived as either convex or concave. Each of five images depicted a naturalistic picture (e.g., a footprint), each of the other five depicted an embossed symbol (e.g., a square). On each of 20 trials, a child was presented with either an upright or upside-down image, and indicated whether the depicted shape appeared convex or concave. Our main findings are that (i) naturalistic stimuli are significantly more likely to be perceived as if light comes from above than symbol stimuli, and (ii) children's propensity to interpret stimuli as if light comes from above increases significantly with age, and at a similar rate for naturalistic and symbol stimuli. These results suggest that, irrespective of any innate competence, children's ability to interpret shading information is gradually refined throughout childhood.