Neuroscience has come a long way since the days of phrenology, when lumps on the outside of the skull were believed to denote enhanced size and strength of the particular brain region responsible for particular individual functions. Today's far more advanced neuroimaging technologies allow scientists to peer deep into the living brain, revealing not only its anatomical structures and the tracts connecting them but, in recent years, physiological descriptions of the brain at work.
Visualized this way, the brain appears to contain numerous "functional networks:" clusters of remote brain regions that are connected directly via white-matter tracts or indirectly through connections with mediating regions. These networks' tightly coupled brain regions not only are wired together, but fire together. Their pulses, purrs and pauses, so to speak, are closely coordinated in phase and frequency.
Well over a dozen functional networks, responsible for brain operations such as memory, language processing, vision and emotion, have been identified via a technique called resting-state functional magnetic resonance imaging. In a resting-state fMRI scan, the individual is asked to simply lie still, eyes closed, for several minutes and relax. These scans indicate that even at rest, the brain's functional networks continue to hum along -- albeit at lower volumes -- at distinguishable frequencies and phases, like so many different radio stations playing simultaneously on the same radio.
But whether the images obtained via resting-state fMRI truly reflect neuronal activity or are some kind of artifact has been controversial. Now, a new study led by neuroscientist Michael Greicius, MD, and just published in Science, has found genetic evidence that convincingly bolsters neuroimaging-based depictions of these brain-activity patterns.
Greicius and his colleagues analyzed gene-expression profiles -- molecular-biological proxy measurements of activity levels of each of the human genome's approximately 20,000 known genes -- of tissue samples from various parts of four different functional networks, excised from post-mortem human-brains.
From our news release on this study:
Using sophisticated statistical methods, they identified a set of 136 genes that showed a correlated pattern of gene expression in regions within each network... These 136 genes weren't specific to any single network, Greicius noted. Rather, "any one of these genes that was being expressed at a high, intermediate or low level in one region of any network, regardless of which network you'd picked, was also being expressed at corresponding levels in the other regions of that network," he said.
Evidence is accumulating that some neurodegenerative diseases propagate, in as yet poorly understood ways, along functional networks. Alzheimer's disease, for example, appears to spread from one brain region to the next within the brain's so-called default-mode network, which is activated when a person is asked to recall recent autobiographical events. The new study might make it easier to find out how this propagation comes about.
Could be that nerve cells that fire together not only wire together but tire together and, ultimately expire together.
Previously: New findings on exactly why our "idle" brains burn so much fuel, Having a copy of ApoE4 gene variant doubles Alzheimer's risk for women but not for men and A one-minute mind-reading machine? Brain scan results distinguish mental states
Photo by Crafty Dogma
