The world doesn't stop when night falls. From rabbits to night-club bouncers, there's a whole cast of nocturnal characters who come out to play. It's a similar story with the brain. When we disengage from the outside world, the brain doesn't go to sleep. Rather, there's a suite of neural regions, known collectively as the "default mode network", that spring to life. Over the last decade, this recognition of the brain's intrinsic functioning has led neuroscientists to perform numerous studies in which they scan the brain during rest, looking for areas that correlate with each other in the rise and fall of their activity.
We now have maps of "intrinsic connectivity networks" and findings showing how these patterns are affected by ageing and illness. But not everyone is happy. Many psychologists are concerned that this field reflects a "decognitivisation" of neuroscience, and that studies of a person's brain at rest are, almost by definition, poorly controlled (see "the Resting Brain" in this month's Psychologist magazine).
It is against this backdrop that Zarrar Shehzad and twelve co-authors have examined the reliability of resting-state networks across multiple brain scans, separated by up to 16 months. They say this is the first time the test-retest reliability of these networks has been explicitly tested. Their finding is that the functioning of a resting brain may well be unconstrained, but that some of the intrinsic networks that emerge are nonetheless stable over time.
The researchers scanned the brains of 26 participants three times. An initial scan was performed and then 5 to 16 months later, scans two and three were performed within 45 minutes of each other. Each scan lasted about five minutes during which time the participants were told to rest with their eyes open, whilst before them was a black background with the word "relax" written in white.
Shehzad's team focused on three regions of interest including those areas previously identified as the "default mode network" and its opposite number, the "task positive network". As in previous resting-state studies they found ample evidence of correlations in neural activity across various regions of the brain. What's more, these patterns of correlation tended to be moderately to robustly stable across the three scans. Stability of linkage between regions was greatest for those that were positively correlated (when one goes up, the other goes up), rather than negatively correlated (when ones goes up, the other goes down). The researchers also found that intrinsic correlations were particularly stable within the "default mode network", which extends from the prefrontal cortex along the midline to the parietal and medial temporal lobes.
Intrinsic correlations within the "task positive network", which is anti-correlated with the "default mode network" and which tends to be more active during extrinsic tasks, were less stable over time. The researchers said this probably reflects the fact that the task positive network is a superordinate system consisting of numerous sub-networks.
"Our findings support the audacious hypothesis that intrinsic connectivity networks, which are readily observed during resting state fMRI studies (as well as during task-based studies), reflect the fundamental self-organising properties of the brain," the researchers said.
Shehzad, Z., Kelly, A., Reiss, P., Gee, D., Gotimer, K., Uddin, L., Lee, S., Margulies, D., Roy, A., Biswal, B., Petkova, E., Castellanos, F., & Milham, M. (2009). The Resting Brain: Unconstrained yet Reliable Cerebral Cortex, 19 (10), 2209-2229 DOI: 10.1093/cercor/bhn256