As you read this, your neurons are firing – that brain activity can now be decoded to reveal the silent words in your head
TALKING to yourself used to be a strictly private pastime. That's no longer the case – researchers have eavesdropped on our internal monologue for the first time. The achievement is a step towards helping people who cannot physically speak communicate with the outside world.
"If you're reading text in a newspaper or a book, you hear a voice in your own head," says Brian Pasley at the University of California, Berkeley. "We're trying to decode the brain activity related to that voice to create a medical prosthesis that can allow someone who is paralysed or locked in to speak."
When you hear someone speak, sound waves activate sensory neurons in your inner ear. These neurons pass information to areas of the brain where different aspects of the sound are extracted and interpreted as words.
In a previous study, Pasley and his colleagues recorded brain activity in people who already had electrodes implanted in their brain to treat epilepsy, while they listened to speech. The team found that certain neurons in the brain's temporal lobe were only active in response to certain aspects of sound, such as a specific frequency. One set of neurons might only react to sound waves that had a frequency of 1000 hertz, for example, while another set only cares about those at 2000 hertz. Armed with this knowledge, the team built an algorithm that could decode the words heard based on neural activity aloneMovie Camera (PLoS Biology, doi.org/fzv269).
The team hypothesised that hearing speech and thinking to oneself might spark some of the same neural signatures in the brain. They supposed that an algorithm trained to identify speech heard out loud might also be able to identify words that are thought.