Abstract: PURPOSE: Depth, as an essential visual-spatial cue, is the primary guarantee to the normal life. Nevertheless, most research on visual working memory (VWM) were carried out in the absence of depth. Recently, several studies centering on VWM for depth initially found the difference between absolute depth (metric distance) and relative depth (ordinal relations among depth planes). Furtherly, the current study systematically investigated whether absolute depth and relative depth have the same capacity and whether they share the same characteristics during maintenance (i.e., consolidation process and the engagement of attention).
METHODS: In Experiment 1, we measured and compared the VWM capacity for absolute depth and that of relative depth adopting the change detection task. Participants were instructed to detect whether the probe and the memory item were placed in the same depth plane for the absolute depth group while to detect whether the numeral probe indicated the depth order correctly for the relative depth group. In Experiment 2, we examined the process of consolidation by adding masks at 50/ 200/ 500/ 900 ms after the offset of memory items. In Experiment 3, we tested whether attention is required for maintenance of the two kinds of depth by inserting a distraction task into the memory task.
RESULTS: 1) Overall, the memory performance for relative depth was greater than that for absolute depth. 2) Neither fine nor coarse absolute depth can survive perceptual interference from masks whereas relative depth can be consolidated well. 3) Attention distraction affected maintenance of coarse absolute depth rather than that of relative depth.
CONCLUSIONS: The results indicated that only relative depth is able to form the robust internal representation, suggesting that there exist two distinct stores for depth. These implied that absolute depth and relative depth might be processed separately through dorsal and ventral pathways, respectively, serving different goals (e.g., behavioral actions vs. visual memory).

Key words: Working Memory, Depth Perception, Depth Order, Metric vs. Ordinal