Functional asymmetries in human working memory
Fried, Peter Jacob
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Working memory is the cognitive ability to maintain and manipulate information in mind to guide behavior. This relies on the coordinated activity of a bilateral brain network, which has been modeled as a central executive in control of separate storage systems for verbal and spatial information. Evidence from human and nonhuman primate research demonstrates that the dorsolateral prefrontal cortex (dlPFC) is critical for manipulating information in working memory. However, whether the dlPFC is dissociable by the domain of information remains unsettled. Recent human studies using repetitive transcranial magnetic stimulation (rTMS) suggest the left and right dlPFC may play separable roles in manipulating verbal and spatial information. In the present study, this theory was investigated further with two experiments on healthy right-handed adults. Both experiments utilized the 3-back task of visual working memory with letters and locations serving as verbal and spatial stimuli, respectively. In Experiment 1, tasks were administered during functional neuroimaging in two formats: one using centrally-presented single letters as verbal stimuli, and dots in different locations as spatial stimuli; and another using single letters in different locations for both verbal and spatial tasks. At the whole-brain group-level, letter- and location-specific contrasts did not differ between formats, indicating verbal/spatial differences reflected discrete subsystems in working memory and not simply separate perceptual processing. Nevertheless, in the dlPFC, bilateral activity was observed across versions, suggesting its contributions to working memory are domain-independent. Experiment 2 tested whether this relationship was causal by assessing 3-back performance after applying low-frequency rTMS to the dlPFC. Following rTMS of the right dlPFC, accuracy improved on the letter task, but worsened on the location task, while the opposite was observed after left rTMS. These double-dissociations suggest left and right dlPFC operate as competing subsystems for manipulating verbal and spatial information, respectively. Thus, the observation of equivalent bilateral dlPFC activity during the letter and location tasks might reflect a left-lateralized system for verbally-encoded information and a right-lateralized system for nonverbal representations operating in parallel on all stimuli. Such a functional asymmetry would have implications for therapies aimed at ameliorating working memory impairments in disease and even normal aging.
Thesis (Ph.D.)--Boston University