 There are also some well-known experimental effects suggesting that attention is allocated, not on the basis of straightforward spatial coordinates, but in ways that are constrained by the distribution of objects in space. The classic work in this area is John Duncan’s demonstration that attention shifts more readily between two locations that fall within the bounds of a single object than between equidistant locations that are separated by an object boundary (Duncan, 1984). Equally important here are findings from experiments using three-dimensional stimuli, and using virtual three-dimensional displays. These studies suggest that experimental participants are unable to differentially attend to depths in an empty space, but that these participants start to be able to shift their attention forwards and backward (to a place in front of or behind the point that their eyes are fixating) when the relevant locations are ones in which there are objects (see Yantis, 1998).