Simultaneous interpreters listen to one language and speak another at the same time. Scientists studied their brains with MRI. After interpreting training, some parts of the brain became thicker. But professional interpreters had less gray matter in some other parts.

Scientists think the brain first grows and later becomes more efficient. So a bigger brain does not always mean a smarter brain. What you can actually do in English is more important.

Learning languages is often said to "grow the brain," and simultaneous interpreters are a good group for testing this idea. In a study published in 2017, researchers scanned trainee interpreters before and after a master's program and compared them with multilingual people who did not train. Only the trainees showed thicker cortex in areas linked to hearing speech sounds, planning speech and controlling attention.

However, a 2014 study found that professional interpreters had less gray matter than other multilingual people in several language-related areas, and those with more interpreting hours tended to have less in some of them. One explanation is that the brain first expands and then becomes more efficient. Brain changes, therefore, do not simply mean better abilities.

Claims that language learning "grows the brain" are common in advertising, but the neuroscience of interpreters suggests a more complicated story. Simultaneous interpreting requires people to listen, translate and speak almost at once, placing extreme demands on language control and attention. In a longitudinal study published in 2017, trainee conference interpreters showed increases in cortical thickness after their master's program in regions linked to speech perception, speech planning and attentional control, whereas multilingual controls did not.

A 2014 cross-sectional study, however, found that professional interpreters had less gray matter than multilingual controls in several language-control regions, and in some areas the reduction correlated with cumulative interpreting hours. One influential model proposes that brain structure first expands when a demanding new skill is learned and later becomes more efficient, which would explain why both increases and decreases are reported.

Two further cautions apply. A Belgian study found changes in brain activity and connectivity after nine months of interpreting training, without any behavioral advantage on cognitive control tasks. An Italian study found that interpreting students already outperformed translation students in some cognitive measures before training began, raising the possibility of self-selection. Brain scans can show adaptation without showing superior ability.