Slant cue are combined early in visual processing: evidence from visual search

When looking for a target with a different slant than all the other objects, the time needed is independent of the number of other objects. Surface slant can be inferred from the two-dimensional images on the retinas using various cues. The information from different cues is subsequently combined to get a single estimate of slant. Is information from the individual cues or from the combined percept responsible for us so easily finding the target? To find out we compared combinations of two slant cues. The cues that we chose are retinal shape and binocular disparity. We compared search times for conditions with the same differences between the target and the other objects in each individual cue, but for each object the two cues either indicated the same slant or opposite slants. Search times were independent of the number of other items if the target clearly differed in perceived slant from the other items. Subjects systematically found the target faster when the cues indicated the same slant. We conclude that slant cues are combined locally throughout the visual field before the search process begins.