Incredibly detailed brain map shows activity of language-encoding neurons

By eavesdropping on brain activity, scientists have created the highest-resolution map yet of the neurons that encode the meanings of different words. The results suggest that, across individuals, the brain uses the same standard categories to categorize words – helping us convert sounds into meaning.

“This study is based on English words only. But it is a big step forward in understanding how the brain stores words in its language library,” said neurosurgeon Ziv Williams at the Massachusetts Institute of Technology in Cambridge. “By mapping the overlapping sets of brain cells that respond to different words, we can try to start building a thesaurus of the brain’s meaning.”

Semantic map

There is an area of ​​the brain called the auditory cortex that processes the sound of a word as it enters the ear. But it is the prefrontal cortex, an area of ​​higher-order brain activity, that processes the 'semantic meaning' of a word - the essence or gist of the word.

Some previous experiments have studied this process by analyzing images of blood flow in the brain, which is a measure of brain activity. Such an approach allows researchers to map the meaning of words to small regions of the brain.

Williams and his colleagues, however, found a unique opportunity to look at how individual neurons encode language in real time. His team recruited ten volunteers who were about to undergo epilepsy surgery, each of whom had electrodes implanted in their brains to pinpoint the source of their seizures. The electrodes allowed the team to record the activity of about 300 neurons in each volunteer’s prefrontal cortex.

As volunteers listened to short sentences containing about 450 words in total, the scientists recorded which neurons were firing and when. Williams found that about two or three separate neurons lit up for each word, even though the team recorded activity from only a tiny fraction of the prefrontal cortex’s billions of neurons. The researchers then looked at how similar the words were to the neural activity.

A multipotent neuron

There were words that the same groups of neurons responded to that belonged to similar categories, such as actions or words related to people. The team also found that words that the brain could associate with each other, such as 'duck' and 'egg', activated some of the same neurons. Words with similar meanings, such as 'mouse' and 'rat', triggered more similar patterns of neural activity than those triggered by 'mouse' and 'carrot'. Other groups of neurons responded to words related to more abstract concepts, such as the words 'above' and 'behind'.

Thus, the categories that the brain assigned to words were similar across participants, suggesting that the human brain is capable of synthesizing meaning groups in the same way. The prefrontal cortex neurons do not distinguish between words by sound, but only by their meaning. For example, when a person hears the word 'son' in a sentence, words related to family members will light up. But those neurons do not respond to the word 'Sun' in the same sentence, even though the words have almost identical pronunciations.

Mind reading

To some extent, researchers can determine what people are hearing by observing their neurons firing. While they can't reproduce the exact sentences, for example, they can tell that a sentence contains an animal, an action, and a type of food, in that order. Researchers can not only identify the neurons that correspond to words and their types, but also the order in which they are spoken. Having language at a natural rate will be crucial for future work on brain-computer interface devices that aim to restore speech to people with speech impairments.