The influence of working memory load on phase specific patterns of cortical activity.

This study investigated patterns of cortical activity associated with the temporally separated encoding, maintenance and retrieval phases of a working memory (WM) task. Eighteen healthy subjects completed a variable load version of the Sternberg Item Recognition Task. Brain regions showing activity on average across load were determined for each task phase. In addition, brain regions showing activity that increased linearly with load were determined for encoding and retrieval. Although previous fMRI studies have used event-related designs to isolate phase specific activity, this study differed in that design and analysis methods were optimized to ensure low multicollinearity between the conditions of interest: the duration of the intermediate phase (maintenance) was varied and load was selectively modeled for the encoding and retrieval phases. The brain areas showing activity on average across load for each phase combine to encompass regions identified in previous studies that have not attempted to separate phase specific activity. Encoding is associated with extensive load dependent activity, with the most robust activity in bilateral occipital and posterior parietal regions. Retrieval is associated with more selective load dependent activity, primarily in the anterior supplementary motor region and the right posterior cerebellum. The analysis strategies employed in this study could be used to further delineate the phases of WM that are most severely compromised in clinical populations with WM disturbances.