In the study, published in Frontiers in Artificial Intelligence today [16 October], the brain organoids received signals from a sensor detecting tactile patterns associated with different Braille letters. Each letter produced a distinct pattern of brain activity that a computer decoder could identify.
While signals from individual organoids were correctly decoded 61% of the time, combining three organoids increased accuracy to 83%. The team of three was also better at distinguishing the signals from artificial noise than the decoder working directly on the obscured signals, without the organoids' involvement.
Tianyi Liu, PhD student in the School of Engineering Mathematics and Technology at the University of Bristol and first author of the study, said: "We can take information from a real tactile sensor, turn it into a form that a living neural network can receive, and then recover meaningful information from the organoid response.
"That gives us a practical way to connect artificial sensors with biological neural systems, which are a very interesting possible route towards more energy-efficient computing."
From A to Braille
The biohybrid systems tested by the research team use brain organoids placed on multielectrode arrays and contained in a system that keeps the organoids alive and monitors their responses. These organoids are less than a millimeter across and lack several structures that a human brain has, but contain about 10,000 neurons, as well as supporting cells. The multielectrode arrays allow them to receive electrical signals.
First, the researchers applied a sensor that records tactile data to the 26 letters of the Braille alphabet. Then they converted this data to electrical stimulation patterns and presented the organoids with the different patterns to see if they would produce neural activity that could tell the scientists what stimulus they'd received.
The organoids' patterns of neural activity were classified using a simple machine learning algorithm, a support vector machine, that could sort the patterns and detect whether given signals corresponded to specific letters. A single organoid consistently produced a signal that could be accurately decoded as a letter about 61% of the time. However, three organoids working together produced a recognisable signal about 83% of the time.
Dr Benjamin Ward-Cherrier, Senior Lecturer in Bristol's School of Engineering Mathematics and Technology and the paper's senior author, added: "After processing by the organoids, the tactile data still contains enough information to classify Braille letters with good accuracy. Combining outputs from several organoids also showed an improvement in performance."
Three brains are better than one?
The researchers are unsure why three organoids performed better than just one. It could be because individual variation between the organoids means that they complement each other.
Tianyi Liu explained: "Different organoids may have slightly different network connections, developmental states, and ongoing activity, so the same input can produce similar but not identical responses.
"Combining those responses may give us complementary information that we would not get from a single organoid. At the same time, if every organoid behaves differently, it obviously makes it harder to build a predictable and standardised system."
To test robustness, the research team applied different noise filters to the Braille letter signals. They also used the same support vector machine to compare decoding from the organoid responses with direct decoding of the deliberately obscured signals. The machine decoding directly, without the organoids' involvement, turned out to be easier to fool than decoding from the organoid responses — it was less able to identify the letter signals among the artificial noise.
Closing the loop
The researchers stress that these experiments don't show if organoids can learn over time, they just show that the organoids received and responded to the Braille signals.
Tianyi Liu said: "We showed that tactile information could be encoded, processed through the organoids, and decoded, but we did not demonstrate learning or long-term changes in the organoids.
"We also tested one tactile task using a fixed encoding scheme and a relatively simple measure of neural activity. The next step is definitely to move beyond Braille. We are interested in things such as textures, edges, and different kinds of touch, and eventually in closed-loop tasks where the organoid receives feedback from what it does. That would allow us to start asking whether the organoid can actually adapt through experience."
Paper
'Encoding tactile stimuli for Braille recognition with organoids' by Tianyi Liu, Hemma Philamore and Benjamin Ward-Cherrier in Frontiers in Artificial Intelligence [open access]
