Ambiguity and the 'mental eye' in pictorial relief

Photographs of scenes do not determine scenes in the sense that infinitely many different scenes could have given rise to any given photograph. Tn psy chophysical experiments, observers have (at least partially) to resolve the se ambiguities. The ambiguities also allow them to vary their response with in the space of 'veridical' responses. Such variations may well be called ' the beholder's share' since they do not depend causally on the available de pth cues. We determined the pictorial relief for four observers, four stimu li, and four different tasks. In all cases we addressed issues of reliabili ty (scatter on repeated trials) and consistency (how well the data can be e xplained via a smooth surface, any surface). All data were converted to dep th maps which allows us to compare the relief from the different operationa lisations. As expected, pictorial relief can differ greatly either between observers (same stimulus, same task) or between operationalisations (same o bserver, same stimulus). However, when we factor out the essential ambiguit y, these differences almost completely vanish and excellent agreement over tasks and observers pertains. Thus, observers often resolve the ambiguity i n idiosyncratic ways, but mutually agree - even over tasks - in so far as t heir responses are causally dependent on the depth cues. A change of task o ften induces a change in 'mental perspective: In such cases, the observers switch the 'beholder's share', which resolves the essential ambiguity throu gh a change in viewpoint of their 'mental eye'.