Meta-analyses of functional imaging studies involving a wide range of tasks have identified a set of brain areas that typically have higher signal levels at rest than during cognitive tasks including the posterior cingulate cortex (PCC) and a medial frontal region including parts of medial frontal gyrus and ventral anterior cingulate cortex (MFG/vACC) (Shulman et al., 1997
; Mazoyer et al., 2001
). It has been suggested that these regions may form a default mode network that is active at rest and suspended during cognitive tasks (Raichle et al., 2001
). This is a reasonable hypothesis; however, there are other plausible explanations for these deactivations that have received less attention.
It is possible that decreased metabolic activity in these regions is associated with increased engagement of the regions. An inverse relationship between activation (as measured via functional imaging) and engagement appears to exist in other parts of the brain. For example, deactivation in the hippocampus (and adjacent regions) has been reported during a transverse patterning task (TPT) and during virtual navigation of a radial arm maze (RAM) (Astur and Constable, 2004
; Astur et al., 2005
). These task are believed to require engagement of the hippocampus, as the ability to perform TPT or to navigate RAMs is disrupted by hippocampal damage (Olton et al., 1979
; Reed and Squire, 1999
) (but see Bussey et al., 1998
). Thus, activity in the hippocampus during these tasks appears to be inversely related to engagement. One possible mechanism for such an inverse relationship is the coding of information in terms of neural synchrony rather than rate of neural firing. Along these lines, it is interesting to note that the theta rhythm (reflecting synchronized neural activity around the 4-7 Hz range) is associated with memory and cognitive function (Gevins et al., 1997
; Tesche and Karhu, 2000
) and has been related to decreased metabolism (Uecker et al., 1997
Thus, in some circumstance, deactivation of a brain area may be related to increased (rather than decreased) engagement. Fortunately, the degree of functional connectivity between brain areas can provide us with more direct information regarding their engagement with one another. This approach is used here to investigate the basis of deactivations in the default mode network.
Strong functional connectivity between nodes in the default mode network has been found at rest, supporting the theory that they function together during rest (Grecius et al., 2003
; Laufs et al., 2003
; De Luca et al., 2006
; Fransson, 2005
). The default mode theory would also predict that engagement between these regions should diminish during cognitive tasks, because of suppression of the circuit during task execution. However, connectivity between these regions has not been evaluated previously during cognitive tasks. In addition, the relationship between connectivity within the network and task performance has not been examined, although such connectivity-behavior analyses can provide unique insight into brain function (Hampson et al., 2006
). For example, if connectivity between regions in the default mode network during a cognitive task were negatively correlated with task performance, it would suggest that the circuit interferes with or distracts from cognitive processing.
This study tested the hypotheses that connectivity between the MFG/vACC and PCC differs during a working memory task and rest, and that connectivity between the two regions is related to task performance. Our hypotheses [on which our region-of-interest (ROI) analyses are based] focus on these two regions because they are frequently deactivated during functional imaging studies (Shulman et al., 1997
; Mazoyer et al., 2001
) and because their resting connectivity patterns in healthy adults have been described previously (Grecius et al., 2003
Interregional connectivity was assessed via two different methods. First, a seed region approach was used. This approach had the advantage that it provided connectivity maps for visualization, but the disadvantage that regions were not defined in the same manner, because the seed was treated in a special way. Therefore, a second analysis that defined the two regions in an identical manner was also performed (as suggested by an anonymous reviewer of this paper).