Working memory capacity and time-related decay are different for metric and ordinal depths

Depth, as a type of visuo-spatial information commonly used in daily life, hasn’t been well captured in the field of working memory (WM). Recent research focusing on WM for depth indicated the differences in performance between metric depth (absolute distance) and ordinal depth (spatial relations in depth). However, it remains unknown whether the two are different in terms of their memory capacity, temporal decay, and the processing stages they go through during memory formation. The current study is composed of three experiments that directly investigated these questions. In Experiment 1, we measured and compared the WM capacity for metric depth and that for ordinal depth. WM for metric depth was measured by detecting changes of depth planes after a delay with their depth order remaining unchanged (change detection paradigm), while WM for ordinal depth was measured by detecting changes of depth order denoted by numerals. In Experiment 2, we examined their temporal decay by testing how their memory performance changed as the retention time varied among 17/ 200/ 500/ 900/ 2000 ms. In Experiment 3, we explored whether the two could be stored in a fragile memory store with higher capacity, which was presumed to be protected by a retro-cue from interference with new visual inputs. Results showed that: 1) the memory capacity for ordinal depth was larger than that for metric depth; 2) fine metric depth gradually deteriorated over time whereas coarse metric depth and ordinal depth consistently showed no temporal loss; 3) both of them underwent a higher-capacity stage before entering into WM. The results suggest that there might exist two separate stores for depth, implying that metric and ordinal depth may be distinctively processed through dorsal and ventral pathways, respectively.