The visual search performances for metric and ordinal depth information show a pattern of dissociation

Neural evidence has shown that the processing of ordinal (categorical) spatial relation and metric (coordinate) spatial information involves different brain areas. Recent behavioral evidence suggests a difference in processing between metric depth (absolute distance) and ordinal depth (spatial relations in depth), but the mechanism underlying the difference is unclear. Here, we investigated the processing of the metric depth and ordinal depth by a visual search task. Items were presented at multiple depth planes defined by binocular disparity, with one item per depth plane. In the metric-search condition, the participants needed to search for a target that was presented on a particular depth plane, which was previously shown as the target depth, among one to three distractor depth planes. In the ordinal-search condition, they needed to search for a target that had a particular depth order, which was previously shown in numbers (‘1’ indicated the nearest depth plane, ‘2’ indicated the second nearest, and so on), among the distractor depth planes. The performances showed a pattern of dissociation. When searching for a metric depth, the overall reaction time (RT) increased as the depth separation between the target and the distractor became smaller. However, the depth separation showed no effect on the RTs for searching an ordinal depth. When searching for an ordinal depth, the RTs were faster when the target was presented at the nearest and the farthest depth planes than when it was presented at the middle planes. No such effect was found for searching a metric depth. Our finding indicates different underlying mechanisms for processing metric depth and ordinal depth information, which may involve distinctive activation in the dorsal and ventral pathways.