The intricate polyphony of the writing brain has been revealed in research showing how separate regions of the cortex cooperate to allow accurate spelling.

A team at Johns Hopkins University studied 33 people who were left with spelling impairments after suffering strokes, examining participants having trouble with both long-term and working memory.

People with long-term memory difficulties find it hard to remember how words are spelt, making educated guesses for irregular spellings such as “soss” rather than “sauce”. Those who have problems with working memory can remember how words are spelt, but find it difficult to put the letters together in the right order, for instance writing “liot” or “lin” instead of “lion”.

Researchers mapped the damage to each participant’s brain using MRI scans, and found left hemisphere impairments that divided into two groups: those with long-term memory problems had lesions towards the front of the brain or lower down at the back, while participants having difficulty with working memory showed lesions in the upper part of the cortex.

According to lead author Brenda Rapp, the research demonstrates that producing written words is a complex process that involves different processes in multiple subregions of the brain.

This flexibility may be part of the ‘evolutionary plan’ Brenda Rapp, lead author

“The components are sufficiently distinct that they can be separately damaged in the brain to produce very different patterns of performance,” Rapp said. “The system is not so highly distributed and interactive that regardless of which part of the system you disrupt, you would get the same pattern of performance.”

The study adds weight to those who argue that working memory is separated in the brain from long-term memory, rather than merely a subset of long-term memory that is in the focus of attention at any given moment.

This kind of architecture, where separate regions of the cortex cooperate to perform tasks that are highly practised and almost automatic, is exactly what you’d expect to find if the brain re-uses and recombines skills acquired at an earlier stage of evolution to perform new tasks, Rapp continued. Spelling is an ability that has been acquired by humans over a period that is extremely brief compared to evolutionary timescales, but is nevertheless deeply embedded. If such skills involve cooperation between such widely-spaced regions of the cortex, then “brains seem quite capable of repurposing areas intended for other or related functions by evolution. Presumably there are limits to how areas can be repurposed – understanding what these might be would be interesting, as it may help us understand which new skills may be problematic and why.”

The results of this study show that the brain is sufficiently flexible that it can adapt to “changing demands on thinking and behaviour”, Rapp added. “In fact, this flexibility may be part of the ‘evolutionary plan’.”

Researchers who are working in artificial intelligence may find “there is much to be gained by building thinking machines that are capable of ‘repurposing’ their circuitry in these ways”.