For many years now, neuroscientists have been telling the subjects of experiments something like this: “Please lie in the MRI scanner and relax. When you see the task instructions come onto the screen in front of you, do your best.” The researcher would then use the brain’s activity during the “lie there and relax” period as a mere control condition; the object of scientific interest was always what “lights up” when a subject reads, makes financial decisions or performs some other task.
That has changed. It is now appreciated that the mind never rests. And that if we measure brain activation while a person lies in a scanner doing nothing, naturally occurring fluctuations will reveal networks that help elucidate the functional organization of the brain in fascinating new ways. Initial studies indicate that these “resting state” networks may help cast light on mental illness. And now, tantalizing new results suggest a significant link between these networks and intelligence.
Intelligence is a complex and historically controversial topic, in large part because it is difficult to define and to measure. Psychometricians have developed paper-and-pencil tests of general intelligence that tend to predict performance on a wide range of other tests and a number of life outcomes, like salary. Neuroscientists have used modern imaging methods to discover the neural correlates of intelligence as measured by these widely used tests. Many of these studies have examined the relations of IQ to brain anatomy, generally finding that greater grey matter volume or thickness across many brain regions correlates with higher IQ scores. Others have looked at functional measures taken while people perform tasks, generally finding that bilateral frontal and parietal regions are most often associated with performance on intelligence tests.
But now, for the first time, functional measures of the resting brain are providing new insights into network properties of the brain that are associated with IQ scores. In essence, they suggest that in smart people, distant areas of the brain communicate with each other more robustly than in less smart people.
In a recent paper, researchers at the Chinese Academy of Sciences, led by Ming Song, examined how resting brain networks differ between people who have superior versus average IQ scores. They used graph theory to quantify the network properties of the brain, such as how strong the communication is among distant brain regions. A graph is a mathematical representation that is composed of nodes (or brain regions) and connections between them (functional connectivity or temporal correlations), and can be used to characterize neural networks. Like prior researchers, they found that the posterior cingulate cortex is the hub of the human brain – it is the most widely and intensively connected region of the human brain at rest. Moreover, the strength of connectivity among distant brain regions was greater in people with superior than average IQ scores. Another 2009 study came to a similar conclusion, and noted that the strongest relations between resting connectivity and IQ were observed in the frontal and parietal brain regions, which have been most associated with performance on IQ tests.
Thus, remarkably, the strength of long-distance connections in the resting brain can be related to performance on IQ tests. We are often impressed when people make creative connections between ideas – perhaps long-range connectivity in the brain empowers such mental range.
These “at rest” findings fit well into what we know of how intelligence develops in children. Previous work discovered that in typical brain development there is a progression from local to distributed network connectivity. In children, there is strong local connection and weak distant connection. That changes with age: local connectivity decreases and long-distance connectivity increases. Intelligence by almost any measure increases with age until young adulthood. Interestingly, Earlier research also found that slower thinning of the neocortex (often interpreted as pruning of synapses) was associated with higher IQs in children; perhaps the slower pruning allowed for the establishment of long-lasting long-distance connections. Thus, the strength of long-distance connections in the brain may support the growth of intelligence and influence variation in adult intelligence.