Where in the Brain is Intelligence?

For years, Russian scientists harvested the brains of exceptionally smart people, trying to locate the source of their intelligence. After V.I. Lenin died in 1924, for example, the Russians invited the great German neuroanatomist Oskar Vogt to try to locate the “source of genius” in the leader of the Russian revolution. Vogt cut Lenin’s brain into more than 1,100 slices, but he found nothing exceptional except unusually large pyramidal cells.

The last brain that the Russians studied in this way was that of Andrei Sakharov, the nuclear physicist and human rights activist who died in 1989. From the dozens of brains they studied, the researchers made many observations about brain size, the density of neurons and the number of convolutions of the cortex, but their findings revealed next to nothing about human intelligence.

Today, scientists around the world continue to search for the physiological basis of human intelligence, but they also focus on genetic variation, which appears to determine about half of a brain’s cognitive ability on average, as measured by standard IQ tests. And by using modern scanning techniques, they are gaining much more detailed insights into the structure and function of the brain than the Russians could achieve through dissection.

The emerging consensus is that intelligence depends not just of the efficiency or power of various brain regions, but also on the strength of the connections that link them.

“These early attempts to find the physiological basis of intelligence were limited by a lack of modern technology,” said Richard J. Haier, a professor in the School of Medicine at the University of California, Irvine, who uses brain imaging to study higher cognitive processes. “With the advent of modern medical imaging it became possible to look for more subtle differences than you might find with gross anatomy.”

A model of human intelligence

By reviewing 37 neuroimaging studies, Haier and Rex E. Jung, a researcher at the Mind Research Network in Albuquerque, New Mexico, have developed a provocative theory that attributes human intelligence to a circuit that links the frontal lobes, which are involved in planning, organization and other highly developed human abilities, with the parietal region farther back in the brain, which integrates information from the eyes, ears and other senses. They believe this Parieto-Frontal Integration Theory, or P-FIT, best accounts for the evidence that intelligence depends on several brain regions tightly linked by axon tracks that form superhighways of information.

Prior to the P-FIT model, many neuroscientists considered the frontal lobes to be the seat of human intelligence. However, while damage to the frontal lobes may affect motivation, foresight, modulation of emotion and other higher functions, it seldom degrades IQ.

“The frontal lobes are important to intelligence according to almost every imaging study, but so are other areas,” said Haier. “That’s why we think intelligence involves a network that includes the frontal lobes, but is not exclusively based in the frontal lobes.”

The P-FIT model, according to Haier and Jung, also explains how the speed of transmissions between brain regions could make a crucial difference.

“Several studies have measured speed of neural processing, and they typically find a modest correlation with IQ tests,” Haier said. “The technology of measuring neural conduction velocity is a little primitive, but it is our belief that EEG studies that measure brain changes show that smarter people process information faster.”

Genes also contribute to intelligence, according to studies of identical twins, and paradoxically that contribution increases with age.

“In children, the contribution from genes is less than 50 percent,” Haier said, referring to identical twins. “In older people in their 60s, 70s and 80s, the number is more like 80 percent. As you grow older the heritability of intelligence increases. That means environmental influences become less significant over time, and genetic influences become more pronounced.”

The genetic contribution to intelligence undoubtedly involves many genes, each making a small contribution, but modern techniques for scanning the genome can compare billions DNA base pairs from many subjects very quickly. This will enable the DNA of very intelligent people to be compared with DNA taken from people of average intelligence, which could reveal genes that contribute to IQ.

Improving intelligence?

What will a better understanding of the physiology of intelligence bring us? Haier expects to see ways to measure intelligence more precisely.

“One of the things I can envision is a person getting an MRI scan from which predictions are made about their future academic achievement,” he said. “This would be more predictive than SATs.”

He also believes the biochemistry of intelligence will point to new ways of enhancing the brain’s performance.

“Some drugs used to treat Alzheimer’s are learning and memory drugs,” he said. “What happens if you can improve learning and memory in people who don’t have Alzheimer’s?”

Rex Jung predicts that a better understanding will create ways of enhancing intelligence through education and other techniques.

“Genes are not destiny,” he said. “Being able to look at genetic predispositions will provide incredible opportunities to work within the environment to increase intelligence and to work with individuals to maximize their intellectual potential.”

Fear of intelligence research

Understanding the physical basis of intelligence will require much more research, but both Haier and Jung believe such research is gaining momentum.

“But there’s a fear about this type of research, just as there is with research into the biological basis of violence and aggression,” Haier said. “There’s a fear of how that information would be used.”

In a recent paper in Brain on “Famous Russian Brains,” Alla A. Vein and Marion L.C. Maat-Schieman reviewed some of the findings made by Russian scientists who studied the brains of highly accomplished people. They found, for example, that the brain of Russian novelist Ivan Turgenev was an amazing 30 percent larger than average, while an exceedingly detailed study of the brain of Dmitrii Mendeleev, the creator of the periodic table, revealed highly developed frontal and parietal lobes on the left side.

Such findings may seem crude and even quaint by today’s standards, but Jung believes those scientists were on the right track, and he would like to take a look himself at some of the specimens they collected.

“You could image them and get detailed measurements,” he said. “I’d be very interested in cortical thickness in the frontal and parietal regions in the left hemisphere, which the Russians identified as having increased gyrification. You could do some very interesting experiments even today on those brains.”