MEANINGS of words can be hard to locate when they are on the tip of your tongue, let alone in the brain. Now, for the first time, patterns of brain activity have been matched with the meanings of specific words. The discovery is a step forward in our attempts to read thoughts from brain activity alone, and could help doctors identify awareness in people with brain damage.
Machines can already eavesdrop on our brains to distinguish which words we are listening to
, but Joao Correia at Maastricht University in the Netherlands wanted to get beyond the brain's representation of the words themselves and identify the activity that underlies their meaning. Somewhere in the brain, he hypothesised, written and spoken representations of words are integrated and meaning is processed. "We wanted to find the hub," he says.
To begin the hunt, Correia and his colleagues used an fMRI scanner to study the brain activity of eight bilingual volunteers as they listened to the names of four animals, bull, horse, shark and duck, spoken in English.
The team monitored patterns of neural activity in the left anterior temporal cortex - known to be involved in a range of semantic tasks - and trained an algorithm to identify which word a participant had heard based on the pattern of activity.
Since the team wanted to pinpoint activity related to meaning, they picked words that were as similar as possible - all four contain one syllable and belong to the concept of animals. They also chose words that would have been learned at roughly the same time of life and took a similar time for the brain to process.
They then tested whether the differences in brain activity were related to the sound of the word or its meaning by testing whether the algorithm could identify the correct animal while the participants listened to the Dutch version of the word.
The system was still able to identify which animal had been named, despite being trained with patterns generated for English words. For example, the word "horse" and its Dutch equivalent "paard" gave rise to the same brain pattern, suggesting that the activity represented the word's meaning - the concept of a horse. The team presented the work at the Society for the Neurobiology of Language Conference in San Sebasti?n, Spain, last month.
"This type of pattern recognition approach is a very exciting scientific tool for investigating how and where knowledge is represented in the brain," says Zoe Woodhead at University College London, who wasn't involved in the study. "Words that mean the same thing in different languages activate the same set of neurons encoding that concept, regardless of the fact that the two words look and sound completely different."
As resolutions in brain imaging improve, Correia predicts that a greater number of words will be predicted from brain activity alone. In principle, it might even be possible to identify whole sentences in real time, he says.
"The science fiction gadget that everyone wants is a mind-reading machine," says Matt Davis at the MRC Cognition and Brain Sciences Unit in Cambridge, UK. "This study is a useful contribution to that. It's helpful to know where to look."
However, the brain patterns that Correia identified were unique to each person. Brains are like faces - the eyes, nose and mouth are all in the same place, but the details can be different, says Davis. "The meanings might be stored in the same area, but the actual neurons would be idiosyncratic." To read someone's mind, a machine would first need to learn that individual's unique representation of each word. "You would have to scan a person as they thought their way through a dictionary," says Davis.
For now, knowing where to look for brain activity relating to meaning could help doctors identify awareness in people who have disorders of consciousness, such as locked-in syndrome. When a person hears a word that has two meanings, such as "bark", the brain processes the related sentence in a different way to an unambiguous one. "The brain needs to work a bit harder to understand a sentence with these words," Davis says.
This extra effort can be identified in a brain scan and is being used by Davis's team to work out whether a person with brain damage is processing meaning. The ability to pinpoint where different types of meaning exist in the brain will help to refine this technique, he says.
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