The product, called "Kokoro-gatari" (Mind-talk), was developed jointly by electronics giant Hitachi, Excel of Mechatronix Corp, and Japan ALS Association for patients in the severest stage of Amyotrophic Lateral Sclerosis.
ALS is a disease of motor neurons, muscle-controlling nerve cells, and ultimately affects all voluntary muscles, making patients incapable of even the slightest movements such as blinking.
But the companies have made use of one function the patients can control, blood flows in their brains.
Although sufferers ultimately lose the power to move and speak, they continue to think normally.
With the new device a patient wears a headband that emits near-infrared rays to measure the flow of blood.
If a patient wants to say "yes" to a question, he or she can activate the brain by doing a simple calculation or singing a song mentally, which causes the blood to gather in the frontal lobe.
The device would then detect the increased blood flow. When patients want to say "no", they would just stay relaxed to keep the blood flow unchanged.
An answer, about 80% correct on average, would come in 36 seconds.
The project started with a telephone call to Hitachi in 1999 by a man who was taking care of his wife in an advanced stage of ALS, according to a director of the ALS Association.
"Care-givers are always wondering if what they have done is okay to patients ... as there is no way to confirm it," said Kensuke Yanagita, whose association groups 6500 patients, families, medical experts and volunteers.
"You may think the accuracy rate is 20% less than perfect but it is a big leap from zero for us. At least care-givers would be able to know the patient is feeling good or not"
"You may think the accuracy rate is 20% less than perfect but it is a big leap from zero for us. At least care-givers would be able to know the patient is feeling good or not," he said.
The project stalled once as Hitachi wanted 100% accuracy, but the association pushed it to market the product, he said.
The association has put up $22,200 for the project, which has also got more than 40 million yen of subsidies from Japan's Health Ministry. Hitachi has not disclosed how much it spent on it.
The developers hope to market the device by the end of the year in Japan, with a price tag of less than $4200.