Searching for targets in visual working memory: investigating a dimensional feature bundle (DFB) model.

The human visual working memory (WM) system enables us to store a limited amount of task-relevant visual information temporally in mind. One actively debated issue in cognitive neuroscience centers around the question of how this WM information is maintained. The currently dominant views advocated by prominent WM models hold that the units of memory are configured either as independent feature representations, integrated bound objects, or a combination of both. Here, we approached this issue by measuring lateralized brain electrical activity during a retro-cue paradigm, in order to track people's ability to access WM representations as a function of the dimensional relation between WM items and task settings. Both factors were revealed to selectively influence WM access: whereas cross relative to intradimensional WM targets gave rise to enhanced contralateral delay activity (CDA) amplitudes, localization relative to identification task demands yielded speeded CDA and manual response times. As these dimension-based findings are not reconcilable with contemporary feature- and/or object-based accounts, an alternative view that is based on the hierarchical feature-bundle model is proposed. We argue that WM units may consist of three hierarchically structured levels of representations, with an intermediate dimensionally organized level that mediates between top-level object and lower-level feature representations.