Dopamine helps the brain change gears: A review of "Dopamine-supports coupling of attention-related networks."

Before we begin, some background information...

It seems that networks and the idea of connectivity is on everybody's minds these days (pardon the pun).  It's a fairly safe bet to say that nearly everyone affiliated with neuroscience has more than a passing familiarity with the default mode and dorsal attention networks (i.e. see Raichle et al., 2001).  These networks are anticorrelated which is to say that when one is being more heavily accessed, the activity in the other is suppressed.  Broadly, the default mode is active at rest, during mind wandering and when thinking about the autobiographical past - it has been described as being related to internally directed attention.  The dorsal attention or task positive network on the other hand is engaged when attention needs to be directed outwards at a cognitively demanding task or situation. 

From Dang et al., 2012

What about the frontoparietal cognitive control network?  This network was discovered  much more recently.  Researchers had known for some time how important each of the constituent areas were for complex processing, task switching and manging multiple threads of attention, but it wasn't until Vincent et al., published their 2008 paper that these could be said to form a distinct network.  Vincent et al., noted that the frontoparietal control network was physically interposed between the dorsal attention and default mode networks and suggested that it might also functionally modulate the brain's access to these networks as well acting as a "master switch". 

This theory was tested and extended by Spreng et al., in a 2010 paper where they showed that during either internally or externally focused tasks connectivity with the default mode was modulated by the frontoparietal control network so that depending on the current goal, the frontoparietal control network could flexibly couple with either of the other networks. 

Jumping forward to the present study

Dang et al., (2012) suggested that as fluctuations in endogenous levels of dopamine is independently associated with activity in each of the three networks, perhaps it was also critical to allowing optimal switching between networks via the frontoparietal control network.  In essence, what they propose is a mediation effect, which means that while a relationship might exist between two things (i.e. the default mode network and the frontoparietal control network), this relationship might be explained by a third factor (which statisticians call a "lurking variable") - in this case, dopamine. 

Using PET (Positron Emission Tomography) to measure the uptake of a radioactively tagged chemical used by the body to produce dopamine, the researchers measured in each of their participants the density of dopamine produced in the substantia nigra and associated brain stem areas. 

They then used resting state fMRI data and seeds from previous analyses to extract the three networks in question.  By correlating each of the network nodes with each other, they came up with a measure of connectivity between each of the networks at rest.   This measure was found to be affected by endogenous levels of dopamine. 

But it's a bit more complicated than that, dopamine only significantly modulated the relationship between the default mode network and the frontoparietal control network at rest - meaning that the higher one's endogenous levels of dopamine, the better the frontoparietal control network was able to couple with the default mode.

Naturally, the next question is well, what about the dorsal attention/task positive network?  The logical extension of this work would be to assume that a parallel triumvirate exists between the dorsal attention network, dopamine and the frontoparietal control network that would be most prominent during task. 

It's a bit of a pity that the authors didn't report task data - but presumably this is in the pipeline.  Nevertheless, this paper makes an excellent theoretical push, and provides more evidence for the role of the frontoparietal control network. 

Dang, L. C., O’Neil, J. P., & Jagust, W. J. (2012). Dopamine supports coupling of attention-related networks. The Journal of neuroscience : the official journal of the Society for Neuroscience, 32(28), 9582–7. doi:10.1523/JNEUROSCI.0909-12.2012

Spreng, R. N., Stevens, W. D., Chamberlain, J. P., Gilmore, A. W., & Schacter, D. L. (2010). Default network activity, coupled with the frontoparietal control network, supports goal-directed cognition. NeuroImage, 53(1), 303–17. doi:10.1016/j.neuroimage.2010.06.016

 

Raichle, M. E., MacLeod, A. M., Snyder, A. Z., Powers, W. J., Gusnard, D. A., & Shulman, G. L. (2001). A default mode of brain function. Proceedings of the National Academy of Sciences U S A, 98, 676–682.