But he fought with more complex sentences.
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Once the safeguarding of mental privacy in place, the team began to test its inner speech therapy system with indicative words. The patients sat in front of the screen who displayed a short sentence and had to imagine saying it. The performance varied, reaching an accuracy of 86% with the most efficient patient and on a limited vocabulary of 50 words, but falling to 74% when the vocabulary was extended to 125,000 words.
But when the team went to tests if the prosthesis could decode an unstructured inner discourse, the limits of the BCI have become quite apparent.
The first unstructured inner speech test involved looking at arrows pointing up, right or left in a sequence on a screen. The task was to repeat this sequence after a short delay using a joystick. The wait was that the patients would repeat sequences like “Up, right, up” in their heads to memorize them – the goal was to see if the prosthesis would catch it. It did, but the performance was just above the level of chance.
Finally, Krasa and his colleagues tried to decode more complex sentences without explicit signals. They asked the participants to think on behalf of their favorite food or to remember their favorite quote from a film. “It didn’t work,” says Krasa. “What came out of the decoder was a kind of gibberish.”
In its current state, thinks Krasa, the neural prosthesis of inner speech is proof of concept. “We didn’t think it would be possible, but we did it and it’s exciting!” The error rates were too high, however, so that someone could use it regularly, ”says Krasa. He suggested that key limitation could be in the equipment – the number of electrodes located in the brain and the precision with which we can record the signal of neurons. Representations of inner speech can also be stronger in other brain regions than in the engine cortex.
The Krasa team is currently involved in two projects from the inner speech neural prosthesis. “The first is to ask the question [of] To what extent an inner speech is faster than BCI would be compared to an attempt at speech, ”explains Krasa.
Cell, 2025. Doi: 10.1016 / J.Cell .2025.06.015
