In Alexandre Dumas’s classic novel The Count of Monte Cristo, a character named Monsieur Noirtier de Villefort suffers a terrible stroke that leaves him paralyzed. Though he remains awake and aware, he is no longer able to move or speak, relying on his granddaughter Valentine to recite the alphabet and flip through a dictionary to find the letters and words he requires. With this rudimentary form of communication, the determined old man manages to save Valentine from being poisoned by her stepmother and thwart his son’s attempts to marry her off against her will.
Dumas’s portrayal of this catastrophic condition—where, as he puts it, “the soul is trapped in a body that no longer obeys its commands”—is one of the earliest descriptions of locked-in syndrome. This form of profound paralysis occurs when the brain stem is damaged, usually because of a stroke but also as the result of tumors, traumatic brain injury, snakebite, substance abuse, infection or neurodegenerative diseases like amyotrophic lateral sclerosis (ALS).
The condition is thought to be rare, though just how rare is hard to say. Many locked-in patients can communicate through purposeful eye movements and blinking, but others can become completely immobile, losing their ability even to move their eyeballs or eyelids, rendering the command “blink twice if you understand me” moot. As a result, patients can spend an average of 79 days imprisoned in a motionless body, conscious but unable to communicate, before they are properly diagnosed.
The advent of brain-machine interfaces has fostered hopes of restoring communication to people in this locked-in state, enabling them to reconnect with the outside world. These technologies typically use an implanted device to record the brain waves associated with speech and then use computer algorithms to translate the intended messages. The most exciting advances require no blinking, eye tracking or attempted vocalizations, but instead capture and convey the letters or words a person says silently in their head.
“I feel like this technology really has the potential to help the people who have lost the most, people who are really locked down and cannot communicate at all anymore,” says Sarah Wandelt, a graduate student in computation and neural systems at the California Institute of Technology in Pasadena. Recent studies by Wandelt and others have provided the first evidence that brain-machine interfaces can decode internal speech. These approaches, while promising, are often invasive, laborious and expensive, and experts agree they will require considerably more development before they can give locked-in patients a voice.
Engaging the brain—but where?
The first step of building a brain-machine interface is deciding which part of the brain to tap. Back when Dumas was young, many believed the contours of a