Abstract
The majority of neuroimaging studies focus on brain activity during performance of cognitive tasks; however, some studies focus on brain areas that activate in the absence of a task. Despite the surge of research comparing these contrasted areas of brain function, their interrelation is not well understood. We systematically manipulated cognitive load in a working memory task to examine concurrently the relation between activity elicited by the task versus activity during control conditions. We presented adults with six levels of task demand, and compared those with three conditions without a task. Using whole-brain analysis, we found positive linear relations between cortical activity and task difficulty in areas including middle frontal gyrus and dorsal cingulate; negative linear relations were found in medial frontal gyrus and posterior cingulate. These findings demonstrated balancing of activation patterns between two mental processes, which were both modulated by task difficulty. Frontal areas followed a graded pattern more closely than other regions. These data also showed that working memory has limited capacity in adults: an upper bound of seven items and a lower bound of four items. Overall, working memory and default-mode processes, when studied concurrently, reveal mutually competing activation patterns.
Highlights
Functional neuroimaging studies typically manipulate cognitive demand of tasks by changing executive load (e.g., n-back tasks; Owen et al 2005 for meta-analysis) or number of items on the display over a temporal delay (e.g., Sternberg tasks; Manoach et al 1997; Rypma et al 1999, 2002; Jha and McCarthy 2000)
We found mutually and inversely modulated linear relations between activity in areas associated with task difficulty and areas associated with control conditions as a function of the difficulty levels in this task
Areas associated with working memory increased as a function of difficulty and areas associated with default mode decreased as a function of difficulty
Summary
Functional neuroimaging studies typically manipulate cognitive demand of tasks by changing executive load (e.g., n-back tasks; Owen et al 2005 for meta-analysis) or number of items on the display over a temporal delay (e.g., Sternberg tasks; Manoach et al 1997; Rypma et al 1999, 2002; Jha and McCarthy 2000). In a coordinate-based meta-analysis of 24 n-back studies, Owen et al (2005) identified six cortical regions that were reliably activated by n-back tasks. By manipulating cognitive load, we expected to replicate this set of areas typically found with adults, and show how brain activity elicited by graded increases in cognitive load might affect activity found in the control conditions (i.e., no task)
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