Scientists have developed a brain-computer interface that reads the brain's blood oxygen levels and enables communication by deciphering the thoughts of patients who are totally paralyzed and unable to talk.

In a trial of the system in four patients with complete locked-in syndrome - incapable of moving even their eyes to communicate - it helped them use their thought waves to respond yes or no to spoken questions.

People who are paralyzed except for up and down eye movements and blinking are classified as having locked-in syndrome. If all eye movements are lost, the condition is referred to as complete locked-in syndrome.

Researchers leading this trial said the brain-computer interface (BCI), which is non-invasive, could transform the lives of such patients, allowing them to express feelings and opinion to their loved ones and carers.

Counter to expectations, the researchers said, the patients reported being "happy" despite their condition.

"The striking results overturn my own theory that people with complete locked-in syndrome are not capable of communication," said Niels Birbaumer, a neuroscientist at Switzerland's Wyss Center for Bio and Neuroengineering, who co-led the study.

The trial, published in the journal PLOS Biology on Tuesday, involved four patients with amyotrophic lateral sclerosis (ALS) - a progressive motor neuron disease that destroys the part of the nervous system responsible for movement.

The researchers asked personal questions with known answers, such as: "Your husband's name is Joachim?", and open questions that needed yes or no answers, such as "Are you happy?".

The BCI technique used technologies called near-infrared spectroscopy and electroencephalography (EEG) to measure blood oxygenation and electrical activity in the brain.

"The machine records the blood flow... and calculates how (it) changes during "yes" and during "no", and the computer develops an idea, a pattern," Birbaumer told Reuters.

"And after a while, we know what the patient is thinking, when he thinks "yes", or when he thinks "no", and from that we calculate the answer."

The "known" questions elicited correct responses seven times out of 10, and the question "Are you happy?" resulted in a consistent yes response from the four people, repeated over weeks of questioning.

John Donoghue, director of the Wyss Center, welcomed the work as "a crucial first step in the challenge to regain movement" for completely locked-in patients.

He said his team now plans to build on these results to develop the technology further and eventually aim for it to be available to people with paralysis resulting from ALS, stroke, or spinal cord injury.