Aberrant modulation of brain activity underlies impaired working memory following traumatic brain injury

Impaired working memory capacity is a common and disabling consequence of traumatic brain injury (TBI) that is caused by aberrant neural processing. However, due to high heterogeneity in results across studies, it is challenging to conclude whether impaired working memory in this population is driven by neural hypo- or hyper-activation, and the extent to which deficits are perpetuated by specific working memory subprocesses. Using a combined functional magnetic resonance imaging and working memory paradigm, we tested the hypothesis that the pattern of neural activation subserving working memory following TBI would interact with both task demands and specific working memory subcomponents: encoding, maintenance, and retrieval. Behaviourally, we found that working memory deficits were confined to the high cognitive load trials. Our results confirmed our key prediction. Overall, TBI participants showed reduced brain activity while performing the working memory task. However, interrogation of the subcomponents of working memory revealed a more nuanced pattern of activation. When we simply averaged across all task trials, regardless of cognitive load or subcomponent, TBI participants showed reduced neural activation. When examined more closely, patterns of brain activity following TBI were found to interact with both task demands and working memory subcomponent. Participants with TBI demonstrated an inability to appropriately modulate brain activity between low and high demand conditions necessary during encoding and maintenance stages. Therefore, we demonstrate that conclusions about aberrant neural processing are dependent upon the level of analysis and the extent to which general cognitive domains can be parcellated into its constituent parts.