In the current study we investigated whether self reference would activate the DN as a whole, and if these regions would overlap with the DN as defined using functional connectivity measures in both the resting-state and the task conditions. We found a highly overlapping set of regions with more activity during the Self task relative to two externally-driven tasks and strong functional connectivity with the PCC that were very similar to models of the DN reported by others. We also identified some additional areas not typically considered to be part of this network, which may have been identified due to the comprehensive multivariate approach that we took to characterizing the DN across multiple analyses. However, this same pattern of activation and functional connectivity also was seen during the Other task. Therefore, there are three main findings from this study: 1) increased activity in the DN supports self reference, but is not specific just to judgments about the self, suggesting a broader role in processing multiple types of information that might form a social context of which the self is a part; 2) these activated regions are functionally inter-connected, suggesting that the DN as a whole, integrated network supports thought about ourselves and others that are close to us; and 3) the DN may encompass more regions than are currently thought to be part of this network. In particular, those regions that were identified in both the task and connectivity analyses (see ) may need to be considered for inclusion in the DN.
We based our experimental approach on the idea that if the DN, as an integrated whole, supports a particular cognitive process, then the best way to assess its function is to combine task-related DN deactivation, task-related DN activation, and functional connectivity, all of which have been used in isolation in previous studies. Our results supported the use of this approach, since our across-tasks activation analysis identified areas thought to be part of the DN, and that were functionally connected to the PCC. We found equivalent increases of DN activity for both the Self and Other conditions, and strong functional connectivity among DN regions in both conditions, consistent with other reports of similar activation for judgments of self and close others (Ames et al., 2008
; Ochsner et al., 2005
). This result indicates that engagement of the DN is not limited to self reference per se
, but plays a broader role. We have reported that the DN was engaged during theory of mind, as well as memory and self-relevant future thought (Spreng & Grady, 2010
), although the past and future self conditions also activated some DN regions more than did theory of mind. The results of this previous study, taken together with the current results, suggest that this network is involved in processing self-relevant information, but does not appear to be exclusive to it. Thus, although the results of this study are consistent with the idea that the DN facilitates the projection of the self across time and space (Buckner & Carroll, 2007
), our work would suggest that the DN also participates in processing information that may be relevant to the self but extends beyond the self to encompass information about other people. It is probably safe to say that the precise role of the DN remains elusive, although our results can perhaps rule out a narrow interpretation of its function. One reason that it has proven difficult to precisely define the cognitive function of the DN is that it is primarily active during internally-oriented cognition, which, because it is less constrained, can encompass numerous domains (Northoff & Bermpohl, 2004
) and by its very nature is likely to be fluid. A strength of our approach is that each analysis drew upon a different aspect of our data (activation, connectivity, task, rest), yet all our analyses identified similar brain regions. This converging approach provided evidence in line with previous studies showing that self reference is associated with activity in nodes of the DN, such as medial PFC and posterior cingulate (Fossati et al., 2003
; Gusnard et al., 2001
; Johnson et al., 2002
; Kelley et al., 2002
; Northoff & Bermpohl, 2004
; Uddin et al., 2007
), but went a step beyond these earlier data to show that the regions involved in processing personally relevant information are consistent with a functionally connected network, that network being the DN. In addition, we were able to show that increased activity in DN regions during the Self and Other tasks was not related to the slower response times in these tasks, but to the type of processing engaged during the tasks.
Despite the high degree of overlap between the areas that were active during the Self and Other tasks and the connectivity of the PCC, there were differences between the analyses. These differences could be due to several factors. For example, the DN is likely not a spatially rigid system, but may be modulated with brain state. That is, internally-driven task demands may call upon the DN in general, but may influence activity in some DN regions either upward or downward as the task is being performed (see also, Spreng & Grady, 2010
). This could explain why the right angular gyrus was functionally connected to the PCC, as part of the DN, but not activated for the Self and Other tasks. The tasks also may require the recruitment of additional processes, not mediated by the DN. The more extensive recruitment of left inferior prefrontal cortex seen during the Self and Other tasks, but not in the FC analyses (see , green areas), may be related to the engagement of cognitive control (Dove, Brett, Cusack, & Owen, 2006
; Seeley et al., 2007
) or autobiographical memory processes (Addis, McIntosh, Moscovitch, Crawley, & McAndrews, 2004
; Burianova & Grady, 2007
; Maguire & Frith, 2003
) during these tasks. This would be consistent with reports of predominantly left hemisphere activation during autobiographical memory tasks. In addition, the few differences noted between the two FC analyses may be due to fluctuations in network connectivity resulting from carrying out a task of any kind, vs. a less constrained cognitive state. Critically, the similarities across the three different analyses were more striking than the differences.
One strength of our study is that we used relatively simple tasks and tightly controlled the stimuli used in the tasks in an attempt to limit differences across conditions to task demands. We also used a sensitive multivariate analysis approach that allowed us to assess patterns of covariance across the task conditions as well as those related to activity in a specific brain region, resulting in a set of regions with a common covariance pattern regardless of the particular analysis used. With this converging approach we identified the regions currently thought to comprise the DN, as well as a few more. A number of these additional regions were found to be common across all three analyses, suggesting that the appearance of these areas was not due to a specific type of analysis. The regions found across all three analyses were several left hemisphere frontal regions (in the inferior, middle and superior frontal gyri), right thalamus, and caudate/putamen, lateral cerebellum, and anterior middle temporal gyrus bilaterally. Some of these regions have been reported in prior DN studies (e.g., Fransson, 2006
; Zuo et al., 2010
) but are typically not included as important nodes of the DN. We found all of them to be related to task as well as functionally connected during both tasks and rest, so the entire set of regions seen here may represent an upper bound of areas that potentially form the network. However, it is likely that the components of the DN that will be identified in any particular study depend to some extent on the method used to identify them, including the algorithm used (e.g., ICA or PLS), whether or not one uses a template, whether the experiment assesses task deactivation or connectivity, and choice of region to use as the seed. Thus, it still is not clear if the DN is composed of a core element that is always functionally connected and more variable subcomponents whose involvement in the network is transient and task-dependent, as suggested by some (Andrews-Hanna, Reidler, Sepulcre, Poulin, & Buckner, 2010
; Buckner et al., 2008
), or if the DN itself is more extensive than previously thought. Regardless, until more knowledge of the DN and its function accumulate, it may be useful to consider the network more inclusively. Applying a mask, using a template, or otherwise restricting assessment of the DN to an a priori
set of regions assumes that the structure of the network is completely known, and leaves no room for further exploration. Our results suggest that there is a need to cast a wider net when attempting to characterize and understand brain networks.
In conclusion, our study has provided evidence that the DN as an integrated network subserves internally-oriented cognition that includes, but is not strictly limited to, self reference. Our results also suggest that understanding the composition of the DN and its function will be well served by considering it more broadly and using a variety of analytic approaches.