Does a ‘recycled’ brain region let us read?

The ability to read sets humans apart from other species, but we’re not completely sure how the brain developed the ability to let us do it.

We only began developing systems of reading and writing a few thousand years ago, and scientists suggest that’s too short a timeframe for our brains to have evolved new areas specifically devoted to recognising letters and words.

Some have thus hypothesised that parts of the brain originally used for other purposes (object recognition, for example) have been “recycled” or repurposed.

A new study led by neuroscientists at the Massachusetts Institute of Technology, US, published in the journal Nature Communications, adds weight to that idea by suggesting that a part of the brain in other primates also is capable of orthographic processing.

Rishi Rajalingham and colleagues were inspired, they say, by French cognitive psychologists who reported back in 2012 that baboons could learn to discriminate words from non-words.

Members of the team had previously found that the parts of the brain’s inferotemporal (IT) cortex that respond to objects and faces become highly specialised for recognising written words once people learn to read.

“These findings inspired us to ask if non-human primates could provide a unique opportunity to investigate the neuronal mechanisms underlying orthographic processing,” says co-author Stanislas Dehaene, from Collège de France, in Paris.

The researchers hypothesised that if parts of the primate brain are predisposed to process text, they might be able to find patterns reflecting that in the neural activity of nonhuman primates as they simply look at words.

To test that idea, they recorded neural activity from about 500 neural sites across the IT cortex of macaques as they looked at about 2000 strings of letters, some of which were English words and others nonsensical combinations.

“The efficiency of this methodology is that you don’t need to train animals to do anything,” Rajalingham says. “What you do is just record these patterns of neural activity as you flash an image in front of the animal.”

The researchers then fed that neural data into a simple computer model called a linear classifier, which learns to combine the inputs from each of the 500 neural sites to predict whether the string of letters that provoked that activity pattern was a word or not.

While the animal itself is not performing this task, the model acts as a “stand-in” that uses the neural data to generate a behaviour, Rajalingham says.

The model was able to generate accurate predictions for many orthographic tasks, including distinguishing words from non-words and determining if a particular letter is present in a string of words.

It was about 70% accurate at distinguishing words from non-words, the researchers say, which is similar to the rate reported in the 2012 study with baboons.

The researchers also recorded neural activity from a different brain area that also feeds into IT cortex: V4, which is part of the visual cortex. When they fed V4 activity patterns into the linear classifier model, the model poorly predicted (compared to IT) the human or baboon performance on the orthographic processing tasks.

The findings suggest, they say, that the IT cortex is particularly well-suited to be repurposed for skills that are needed for reading.