Bottom-up clues in target finding: why a Dalmatian may be mistaken for an elephant.

We provide informal psychophysical support for a strategy where bottom-up features guide attention toward a target, and the top-down path interprets hypothetical shapes at the target location--as opposed to a dominant top-down approach. In our survey, for which we used the familiar picture of a Dalmatian dog against a dappled background, (i) 75% of subjects initially found a bulging body which overlaps that of the dog, but final 'top-down' percepts were unexpected: nearly all subjects assigned an incorrect head and limbs to the body; (ii) after random rotation of texture elements overlapping computed features only 45% of subjects reported a bulging body, with a few adding limbs etc. The picture of the Dalmatian dog must therefore contain many bottom-up features--a top-down strategy may find 'incorrect' targets at correct target locations. Computational support for these claims is more easily constructed than one may expect. We could compute at least two bottom-up features, both useful in 3-D surface interpolation from 2-D scenes, which yielded significant values at the location of the Dalmatian dog: anisotropic texture compression and affine texture distortion cues. We therefore conclude that the role of top-down processing is overstated in a traditional example such as the Dalmatian dog picture.