Ms. De Bruijne’s inability to move comes from a disconnect between her brain and muscles. Though she has lost the ability to move, her brain still generates an increase in electricity when she thinks about doing so. The brain-computer interface capitalizes on this.

Electrodes on her motor cortex, the region of her brain that controls voluntary movement, detect small electrical spikes when Ms. De Bruijne’s tries to move her right hand. Specifically, when she thinks about bringing her right thumb and ring finger together, wires transmit a signal to a typing software.

The software displays four rows of letters on a tablet highlighting one row at a time. When it gets to the row Ms. De Bruijne wants, she makes a “brain click” by thinking about the hand gesture. Then the program goes along the selected row, left to right. When the correct letter is highlighted, she makes another click. Letter by letter, she spells out her thoughts.

Some researchers have concerns about whether the system’s benefits are worth the risk of surgery.

“Because she can use an eye tracker, the brain-computer interface is not necessary” for Ms. De Bruijne to communicate, said Niels Birbaumer, a professor of medical psychology and neurobiology at the University of Tübingen in Germany. Dr. Birbaumer added that other noninvasive brain-computer interfaces had been shown to perform the same function as the communication system from Dr. Ramsey’s team.

There are always dangers with surgery, acknowledged John Donoghue, a professor of neuroscience at Brown University and the founding director of the Wyss Center for Bio and Neuroengineering. He added, however, that he thought the risks of this one were “not significantly greater” than those associated with more common procedures, such as deep-brain stimulation to treat Parkinson’s disease, or placement of pacemakers for heart arrhythmias.

Moreover, Dr. Donoghue said, Dr. Ramsey’s group used a safe, commercial device that the Food and Drug Administration has approved for treating Parkinson’s disease and essential tremor, a neurological disorder that causes involuntary and rhythmic shaking.