Share or compete? Load‐dependent recruitment of prefrontal cortex during dual‐task performance

Low, Kathy A.; Leaver, Echo; Kramer, Arthur F.; Fabiani, Monica; Gratton, Gabriele · 2009 · Psychophysiology

DOI: 10.1111/j.1469-8986.2009.00854.x

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Summary

This study investigates how the dorsolateral prefrontal cortex (DLPFC) manages attention during dual-task performance, specifically testing whether distinct neural units within this region compete for resources or share them. Motivated by the Braver and Cohen model of executive control, which posits that attention is regulated through the selective activation and inhibition of different processing units, the authors sought to determine if such competition is measurable using the event-related optical signal (EROS). EROS was chosen for its high spatial and temporal resolution, allowing for the examination of spatial segregation within the DLPFC. The experiment involved 17 young adults who performed two interleaved tasks: an auditory oddball task (counting rare tones) and a visual Sternberg letter memory task (identifying probe letters in a set of two or four). Participants completed blocks with instructions to attend to only the auditory task, only the visual task, or both simultaneously. EROS data were recorded from the superior and middle frontal gyri, while electroencephalography (EEG) was recorded simultaneously to monitor P3 components. The design manipulated task priority (single vs. dual task instructions) and memory load (two vs. four letters) to assess graded changes in cortical activity. Behavioral results indicated that tone-counting accuracy decreased under dual-task conditions compared to single-task conditions, while Sternberg task performance remained stable, suggesting a prioritization of the visual task or greater vulnerability of the auditory task to interference. EROS analysis revealed differential recruitment of left and right DLPFC structures. The left middle frontal gyrus (MFG) was preferentially recruited by the visual memory task, whereas the right MFG showed spatially segregated competition between the two tasks. Specifically, activity in the right MFG was higher for target tones when they were attended (oddball condition) compared to when they were ignored (Sternberg condition), and this activity was modulated by memory load. The data demonstrated a reciprocal relationship: as the visual task load increased, tone-related activation in the right MFG decreased, consistent with a competition view of dual-task processing where tasks tap into common resource pools. These findings support the hypothesis that dual-task processing involves competition for neural resources within the DLPFC rather than the recruitment of entirely new brain regions or simple additive effects. The spatial segregation observed in the right MFG suggests that attention control may be implemented through the selective activation of specific, spatially organized units within the prefrontal cortex. This work provides neuroimaging evidence for resource allocation models of attention, showing that the brain dynamically adjusts cortical activation patterns based on task priority and load, with specific prefrontal sub-regions dedicated to managing different processing streams.

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• dual task performance