Surface assignment modulates object formation for visual short-term memory

Recent results have suggested that the operational units of visual short-term memory (VSTM) are whole objects, rather than features or the total amount of information to be remembered. Here, for the first time, the influence of surface assignment on object formation for VSTM was investigated. The observers had to memorize the features of four briefly presented (300 ms) two-part objects, followed by a mask and a cue indicating which object to report on. The experiments contrasted whether there were any apparent depth differences between the two parts of each object, and whether observers had to report on only one or both features of the post-cued target object. Depth differences induced with stereoscopic disparity, and with a pictorial depth cue (simple interposition of object features), interfered strongly with performance when both features of an object needed to be memorized, but aided performance when only a single feature needed to be remembered. Furthermore, there was considerable within-feature interference consistent with some previous findings, but contradicting others. The potential implications for conceptions of VSTM are discussed in the light of two hypothesized stages: an early feature-based stage, as well as a higher-level object-based stage where the depth manipulations exert their effects. The results argue for a strong modulatory influence of surface assignment on object formation for a VSTM task.