Working memory for depth indicates a serial-position effect 

One of the subsystems of Baddeley’s model on working memory is visuo-spatial sketchpad. It involves temporarily holding and processing visual information and spatial information. Although research on visual working memory is extensive, most studies employed visual stimuli presented at the fronto-parallel plane and few involve depth perception. To our knowledge, working memory for depth has not been investigated yet. Here, we explored depth working memory (DWM) by using a change detection task and an estimation task. The memory items were presented at various depth planes perpendicular to the line of sight, with one item per depth plane. The depth planes were separated by relative disparities ranging from -0.51◦ to 0.51◦ with a step of 0.17◦ using a Wheatstone stereoscope. Participants were required to make judgment on depth where the target (one of the memory items) located. We found that: 1) the change detection accuracy was much lower than that reported for visual working memory; 2) the memory performance (accuracy and estimation error) degraded with the number of memory items presented (set size); 3) the accuracy was higher for items presented at the nearest and farthest depth planes in relation to participants; 4) distance are overestimated for near depth planes and underestimated for far depth planes; 5) the performance was better when the probe was presented along with the other items originally in the memory array. These findings suggest that storage for depth information is more limited and imprecise than that for visual information, and the memory performance improves if references are provided. In addition, the advantage for memorizing the nearest and farthest depth suggests a serial-position effect, and indicates that DWM depends on the egocentric distance between an observer and the to-be-remembered object.