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Neural mechanisms of two different verbal working memory tasks: A VLSM study
Currently, a distributed bilateral network of frontal-parietal areas is regarded as the neural substrate of working
memory (WM), with the verbal WM network being more left-lateralized. This conclusion is based primarily on
functional magnetic resonance imaging (fMRI) data that provides correlational evidence for brain regions involved
in a task. However, fMRI cannot differentiate the areas that are fundamentally required for performing a
task. These data can only come from brain-injured individuals who fail the task after the loss of specific brain
areas. In addition to the lack of complimentary data, is the issue of the variety in the WM tasks used to assess
verbal WM. When different tasks are assumed to measure the same behavior, this may mask the contributions of
different brain regions. Here, we investigated the neural substrate of WM by using voxel-based lesion symptom
mapping (VLSM) in 49 individuals with stroke-induced left hemisphere brain injuries. These participants
completed two different verbal WM tasks: complex listening span and a word 2-back task. Behavioral results
indicated that the two tasks were only slightly related, while the VLSM analysis revealed different critical regions
associated with each task. Specifically, significant detriments in performance on the complex span task were
found with lesions in the inferior frontal gyrus, while for the 2-back task, significant deficits were seen after
injury to the superior and middle temporal gyri. Thus, the two tasks depend on the structural integrity of
different, non-overlapping frontal and temporal brain regions, suggesting distinct neural and cognitive mechanisms
triggered by the two tasks: rehearsal and cue-dependent selection in the complex span task, versus
updating/auditory recognition in the 2-back task. These findings call into question the common practice of using
these two tasks interchangeably in verbal WM research and undermine the legitimacy of aggregating data from
studies with different WM tasks. Thus, the present study points out the importance of lesion studies in complementing
functional neuroimaging findings and highlights the need to consider task demands in neuroimaging
and neuropsychological investigations of WM.