The brain's grey matter contains the cell bodies of the brain's neurons. Using MRI, we can ask two questions about the grey matter at a particular location in the brain – a question about structure, and a question about connectivity:
- HOW MUCH grey matter is there?
- HOW CONNECTED is it to other grey matter regions?
The first is called grey matter density; it is measured using high-resolution anatomical imaging. The second is called functional connectivity; it is measured by assessing synchronization of slow fluctuations in functional MRI signals, typically during rest.
While everyone's brain has the same rough anatomy and connectivity, we are not identical – people differ, in anatomy and in connectivity. How are such differences related? I would have thought that they were very closely linked, because grey matter that's "beefier" is likely "better connected" as well – the two being, so to speak, two sides of the same coin.
A recent study assessed grey matter density and functional connectivity in 333 people belonging to families whose genetic pedigrees have been established, to look for genetic control over anatomy and connectivity. That is, the authors did not look at differences between people in general; they used what's known about the genetics of these people to ask to what degree the differences in their brains were related to the differences in their genes. They did this for grey matter density, and for functional connectivity. Again, I would have thought that these would be very closely linked.
Surprisingly, however, the results suggest that "genes involved in connectivity are different from those involved in brain anatomy."
Now, this is a first study, and further research is necessary. But I would call this a happy surprise, because – like many scientific surprises – it suggests that we have more to learn.