Lying in bed, unable to move a muscle, so-called locked-in patients have few ways to communicate with the outside world. But researchers have now found a way to use the widening and narrowing of the pupils to send a message, potentially helping these patients break the silence.
Doctors use the constriction of pupils under bright light to test whether a patient’s brain stem is intact. But our pupils also show the opposite response—dilation—based on our thoughts and emotions, says Wolfgang Einhäuser, a neurophysicist at Philipps University of Marburg in Germany. Einhäuser had been struggling to interpret changes in pupil size during decision-making when he began to wonder about a different application. He contacted Steven Laureys, a member of the Coma Science Group at the University Hospital of Liège in Belgium, to explore how the pupil could be used to communicate a choice.
Laureys works with locked-in patients, who have normal mental acuity but are paralyzed and unable to express thoughts to those around them. Many can control only the muscles that move their eyes; some, not even that. They can learn to communicate using EEG technology, in which electrodes on the scalp detect changes in brain activity. But applying the electrode cap is time-consuming, and the equipment is expensive, Einhäuser says. “If you imagine doing that every day, basically to communicate, that’s troublesome.”
To find a different technique, Einhäuser, Laureys, and colleagues reached back in time. “The pieces have been there since the early ’60s,” Einhäuser says. A 1964 study showed that our pupils dilate when we perform mental arithmetic, like attempting to multiply 27 and 15 with no pencil and paper, and that harder tasks led to more dramatic dilation. The team set up a camera and a laptop to explore this automatic response.
Healthy subjects were asked a yes-or-no question with one right answer, such as “Is your brother named Adam?” They watched a computer screen that displayed a math problem while announcing the response “Yes.” A few seconds later, it said “No” and showed a second problem. The subjects were asked to compute only the problem displayed with the correct answer and to ignore the other. A camera in front of their eyes measured precise changes in their pupils. It didn’t matter if they got the problem right, or if they completed it at all. Mental effort alone was enough to dilate their pupils and indicate which answer they chose, the team reports today in Current Biology. Einhäuser says that patients could use their pupils to respond to any question with two possible answers. The simple laptop and camera setup would cost less than $1000, he says.
Six healthy individuals and seven locked-in patients tried the new setup, answering a series of yes-or-no questions and computing the problem only for their chosen answer. The method had 84% to 99% success in reading the responses of healthy patients. Three of the seven paralyzed patients got better-than-chance results: The computer picked their intended answer between 67% and 84% of the time. Although far from perfect, these levels represent “very good accuracy,” according to Niels Birbaumer, a neurobiologist at the University of Tübingen in Germany who specializes in helping locked-in patients regain communication.
Einhäuser points out that with training, the effectiveness would improve dramatically: The patients could become more experienced at directing their mental effort, and the computer program would improve its accuracy through repeated trials.
The team also tried the system with patients who had more severe brain damage and whose level of mental ability was unclear. Only one of four patients was able to complete the task, and his results were no better than chance. But when he was instructed to try the math problem, his pupil response showed when he was computing. Einhäuser thinks, based on this result, that the system could also be a diagnostic tool for gauging brain function. Although this may only be a “piece in the big repertoire,” he believes that it could prove useful in remote hospitals with few resources.
Birbaumer is skeptical of this diagnostic use, saying that it will add little to existing techniques. He also notes that many patients already have communication systems based on moving their eyes side-to-side or up and down. But he acknowledges that for people who cannot control the muscles in their eyes, the pupil response could be a useful alternative.
The team now hopes to make the system faster and more user-friendly and to have patients try other binary choices, such as selecting letters to spell words. “For really complete locked-patients, even if they communicate one letter in a minute,” Einhäuser says, “that is still far above nothing.”