Mild cognitive decline is a common consequence of ageing (Image: Frare Davis/Getty)

Dying cells may play only a small role in the brain decline that accompanies ageing. That is the suggestion from the first computer simulation of brain function that can solve intelligence tests almost as well as university undergraduates. The model promises to reveal how our brains and behaviour are affected by age, and might even offer a way of testing drugs that compensate for cognitive decline.

Psychologists have known for many years that our ability to think through novel problems – our “fluid intelligence” – gradually declines with age.

However, the reasons for this decline aren’t clear, because many features of the brain change as we age: neurons die; connections become sparser between regions of the brain and between individual brain cells; and our mental representation of different concepts becomes less distinct, among other changes. So far, psychologists have been unable to tease apart these possible explanations for cognitive decline.


Enter Chris Eliasmith, a theoretical neuroscientist at the University of Waterloo in Ontario, Canada, and his student Daniel Rasmussen. The pair used a computer to simulate the behaviour of about 35,000 individual brain cells wired together in a biologically realistic way.

What comes next?

Just like a real brain, their model encoded information as a pattern of electrical activity in particular sets of cells. The researchers set up the system to solve a widely used intelligence test known as Raven’s Progressive Matrices, which involves predicting what abstract symbol comes next in a sequence (see puzzle, right, in which you must select the correct option from the two rows of four symbols to fill the blank slot. Answer at the bottom of the article).

The model does this by searching for patterns in the input sequence and then using those to generate a prediction for the missing item in the sequence.

“We don’t tell it what patterns are there or what patterns it’s looking for,” says Eliasmith. Nevertheless, in repeated trials his model got the correct answer on an average of 18.4 of the 36 problems on the advanced version of the test.

By comparison, the average university undergraduate gets about 22 correct. Even when the model got one wrong, it made the same kinds of reasoning errors that humans do. “I think it’s the first instance of showing how a biophysically realistic model can be linked to fluid intelligence doing the exact same test that people do,” says Eliasmith.

Deleting cells

Having established that the model “thinks” similarly to humans, the researchers then mimicked the effect of brain cell death during ageing by randomly deleting up to 20 per cent of the brain cells. This made little difference, they found. People typically lose about 10 per cent of their brain cells as they age, and adults over 60 miss about eight more questions on Raven’s Matrices than when they were in their 20s.

In contrast, even models missing 20 per cent of their cells only missed one or two more questions, on average, than fully functioning models. This suggests that other changes in the ageing brain must be important in causing the decline in thinking power, says Eliasmith.

Despite its complexity, Eliasmith’s model is still much simpler than a real brain, and omits other changes that take place during ageing, says Philip Allen, a cognitive ageing researcher at the University of Akron, Ohio. Its great strength, however, is that it makes quantitative predictions about the factors that influence intelligence.

Eventually, researchers may be able to use the model to test other potential causes of cognitive decline – and even test whether drugs that affect brain function might help compensate for it, says Julia Spaniol, a psychologist at Ryerson University in Toronto, Canada.

Journal reference: Intelligence, DOI: 10.1016/j.intell.2013.10.003

Answer: The symbol that belongs in the blank slot is three arrows pointing upwards.