Facebook has revealed, at its annual developers' conference, that it is studying ways for people to control computers directly by thought.

The social media giant says it is developing "silent speech" software to allow people to type at a rate of up to 100 words per minute.

The project grew from being just an idea six months ago to being the focus of a team of more than 60 scientists, engineers and system integrators.

Brain-computer interface technology currently involves implanting electrodes, but Facebook says it wants to use optical imaging to eliminate the need for invasive surgery.

Such technology could let people fire off text messages or emails by thinking, instead of needing to interrupt what they are doing to use smart-phone touch screens, for example.

Facebook launched a research unit, called Building 8, last year to conduct long-term work that might lead to hardware products.

It is also working on sensors that let people "hear" through their skin, with what they feel being converted into words in a variation on how the ear turns vibrations into comprehensible sounds.

Last December, Facebook signed a deal with 17 universities, including Harvard and Princeton, to allow closer collaboration on projects with Building 8.