neural connections
Neurons with more connections may store more memories
What makes some people smarter than others? It could come down to your individual brain cells - the bigger and faster your neurons, the higher your IQ. If confirmed, the finding could lead to new ways to enhance human intelligence.

Most intelligence research to date has identified brain regions involved in certain skills, or pinpointed hundreds of genes that each play a tiny role in determining IQ.

To go a step further, Natalia Goriounova at the Free University Amsterdam in the Netherlands and her colleagues studied 35 people who needed surgery for brain tumours or severe epilepsy. Each took an IQ test just before the operation. Then, while they were under the knife, small samples of healthy brain tissue were removed and kept alive for testing.

The samples all came from the temporal lobe. This brain area helps us make sense of what we see, recognise language and form memories, all of which factor into intelligence.

Bigger means better

Examining this tissue revealed that brain cells are significantly bigger in people with high IQ scores than those with lower scores. The bigger cells also have more dendrites - the projections that connect to other neurons - and the dendrites are longer, suggesting that these neurons may be capable of receiving and processing more information.

The connections between neurons are thought to be involved in storing memories, so it is likely that bigger cells have more "space" for memories, the team suggest.

"We've known there is some link between brain size and intelligence. The team confirm this and take it down to individual neurons," says Christof Koch at the Allen Institute for Brain Science in Seattle. "It's a beautiful study."

The team also tested neurons' ability to transmit electrical signals by putting current through them, gradually upping the frequency. Neurons from people with low IQs coped with low frequencies, but became fatigued and slower as the frequency rose. Cells from people with high IQs did not slow down, however.

Using computer models of brain-cell activity, the researchers predict that the neurons of people with low IQs send signals more slowly in general - a suggestion that chimes with observations that people with higher IQs tend to have faster reaction times.

The properties of brain cells explain about a quarter of the differences in IQ, says Koch. Genes, on the other hand, are thought to account for only around 3 to 7 per cent of the difference.

The study provides the first evidence that human intelligence could be shaped by the properties of individual brain cells - a finding that is likely to be controversial, says Koch. "Some people will say intelligence is so elusive and complex that the idea it can be tied to individual neurons is implausible," he says.

Extraordinary science

It remains unclear why some people have bigger brain cells than others, and whether this is a cause or a consequence of high IQ.

"We don't know if the differences are the results of experience, or if they are biologically determined," says Wendy Johnson at the University of Edinburgh, UK. To establish a clear link between brain-cell properties and intelligence, you would need to study thousands of tissue samples - not just those from 35 people, she says.

But given how difficult it is to study living brain tissue, this is the best one can hope for, says Koch. "If you want human tissue, the only other options are from aborted fetuses or dead brains," he says.

"What they did here is extraordinary neuroscience," says Richard Haier at the University of California, Irvine. "It's the beginning of being able to study intelligence neuron by neuron, and circuit by circuit." He wonders whether differences between cells in different regions of the brain might explain why some individuals excel in some aspects of intelligence but not others.

"This research could lead to neuroscience-based ways to enhance human intelligence - perhaps dramatically," says Haier. "We might be able to treat intellectual disabilities or prevent them from occurring."

Journal reference: bioRxiv, DOI: