Distant memories: Why the default mode network is important during states of internal focus

Since the early 2000’s work in cognitive neuroscience has highlighted a constellation of neural regions that are suppressed when engaging in external tasks as the hypothetical neural substrate for the mind-wandering state [1]. This neural system is anchored on the medial wall of the cerebral cortex in both anterior pre-frontal regions, and posterior cingulate cortex, as well as regions of lateral parietal cortex focused on the angular gyrus. This neural system is known as the default mode network and pioneering work by Malia Mason, Kalina Christoff and others demonstrated that this system is active when states of internal focus are high, either because task demands are low, or, in periods when experience-sampling probes indicate that are not especially concerned with the external environment [2-5]. Since then it has become apparent that the default mode network is not unique to the mind-wandering state – it can be active when tasks require participants to self-generate information that are unrelated to the external environment (for a review see [6]). We have recently shown that this does not need to be personally relevant information, the default mode network is active when considering the spatial layout of shapes if these are not visible when the decision must be made [7]. So the default mode network does not seem to be a system that is focused on particular mental content, as was initially believed, but instead may be important for information processing that does not arise from perception.

Although it is now well accepted that the default mode network plays an important role in internal experience, the structural basis that allows this system to support this function has only recently come to light. A paper led by Daniel Margulies at the Max Planck Institute in Leipzig, demonstrated that  the default mode networks role in internal experience may emerge from its’ location in the cortex. This study showed the regions in the default mode network are maximally distant from the systems concerned with perception and action both in in terms of the distance along the cortical surface, but also in terms of how close the neural activity in these regions tracked with signals from neural systems involved in seeing and acting [8]. Critically, this topological location allows the default mode network to support information processing that is unrelated to the external environment because it allows for functional separation between events taking place in the immediate environment and neural events within the heart of the cortical system. In the past I have argued that a process of attentional decoupling is an important enabling factor for states of internal focus [9, 10] and it seems that the topological location of the default mode network is such that it would maximise the possibility for neural activity to be discrepant with events in the external environment. In this way the work by Margulies and colleagues provides a structural account of how the mind has evolved the ability to escape the here and now.

References

  1. Raichle, M.E., et al., A default mode of brain function. Proceedings of the National Academy of Sciences of the United States of America, 2001. 98(2): p. 676-82.
  2. Mason, M.F., et al., Wandering minds: the default network and stimulus-independent thought. Science, 2007. 315(5810): p. 393-395.
  3. Christoff, K., et al., Experience sampling during fMRI reveals default network and executive system contributions to mind wandering. Proceedings of the National Academy of Sciences, 2009. 106(21): p. 8719-8724.
  4. Allen, M., et al., The balanced mind: the variability of task-unrelated thoughts predicts error monitoring. Frontiers in human neuroscience, 2013. 7.
  5. Stawarczyk, D., et al., Neural correlates of ongoing conscious experience: both task-unrelatedness and stimulus-independence are related to default network activity. PLoS One, 2011. 6(2): p. e16997-e16997.
  6. Andrews‐Hanna, J.R., J. Smallwood, and R.N. Spreng, The default network and self‐generated thought: component processes, dynamic control, and clinical relevance. Annals of the New York Academy of Sciences, 2014. 1316(1): p. 29-52.
  7. Konishi, M., et al., Shaped by the past: the default mode network supports cognition that is independent of immediate perceptual input. PloS one, 2015. 10(6): p. e0132209.
  8. Margulies, D.S., et al., Situating the default-mode network along a principal gradient of macroscale cortical organization. Proc Natl Acad Sci U S A, 2016. 113(44): p. 12574-12579.
  9. Smallwood, J., Distinguishing how from why the mind wanders: a process-occurence framework for self generated thought. Psychological bulletin, 2013.
  10. Smallwood, J., Searching for the Elements of Thought: Reply to Franklin, Mrazek, Broadway, and Schooler (2013). Psychological bulletin, 2013. 139(3): p. 542-547.

 

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