Individuals with mild traumatic brain injury (TBI) often have deficits in processing speed and working memory (WM) and there is a growing literature using functional imaging studies to document these deficits. However, divergent results from these studies revealed both hypoactivation and hyperactivation of neural resources after injury. We hypothesized that at least part of this variance can be explained by distinct demands between WM tasks. Notably, in this literature some WM tasks use discrete periods of encoding, maintenance, and retrieval, whereas others place continuous demands on WM. The purpose of this meta-analysis is to examine the differences in neural recruitment after mTBI to determine if divergent findings can be explained as a function of task demand and cognitive load. A comprehensive literature review revealed 14 studies using functional magnetic resonance imaging to examine brain activity of individuals with mTBI during working memory tasks. Three of the fourteen studies included reported hypoactivity, five reported hyperactivity, and the remaining six reported both hypoactivity and hyperactivity. Studies were grouped according to task type and submitted to GingerALE maximum likelihood meta-analyses to determine the most consistent brain activation patterns. The primary findings from this meta-analysis suggest that the discrepancy in activation patterns is at least partially attributable to the classification of WM task, with hyperactivation being observed in continuous tasks and hypoactivation being observed during discrete tasks. We anticipate that differential task load expressed in continuous and discrete WM tasks contributes to these differences. Implications for the interpretation of fMRI signals in clinical samples are discussed. (JINS, 2013, 19, 1–12)
