There have been significant advances in developing new prostheses with a simple sense of touch, but researchers are looking to go further. Scientists and engineers are working on a way to provide prosthetic users and those suffering from spinal cord injuries with the ability to both feel and control their limbs or robotic replacements by means of directly stimulating the cortex of the brain.

For decades, a major goal of neuroscientists has been to develop new technologies to create more advanced prostheses or ways to help people who have suffered spinal cord injuries to regain the use of their limbs. Part of this has involved creating a means of sending brain signals to disconnected nerves in damaged limbs or to robotic prostheses, so they can be moved by thought, so control is simple and natural.

However, all this had only limited application because as well as being able to tell a robotic or natural limb to move, a sense of touch was also required, so the patient would know if something has been grasped properly or if the hand or arm is in the right position. Without this feedback, it's very difficult to control an artificial limb properly even with constant concentration or computer assistance.

Bioengineers, computer scientists, and medical researchers from the University of Washington's (UW) GRIDLab and the National Science Foundation Center for Sensorimotor Neural Engineering (CSNE) are looking to develop electronics that allow for two-way communication between parts of the nervous system.

The bi-directional brain-computer interface system sends motor signals to the limb or prosthesis and returns sensory feedback through direct stimulation of the cerebral cortex – something that the researchers say they've done for the first time with a conscious patient carrying out a task.

In developing the new system, volunteer patients were recruited, who were being treated for a severe form of epilepsy through brain surgery. As a precursor to this surgery, the patients were fitted with a set of ElectroCorticoGraphic (ECoG) electrodes. These were implanted on the surface of the brain to provide a pre-operational evaluation of the patient's condition and stimulate areas of the brain to speed rehabilitation afterwards.

According to the UW, this allowed for stronger signals to be received than if the electrodes were placed on the scalp, but wasn't as invasive as when the electrodes are put into the brain tissue itself.

While the electrode grid was still installed, the patients were fitted with a glove equipped with sensors that could track the position of their hand, use different electrical current strength to indicate that position and stimulate their brain through the ECoG electrodes.

The patients then used those artificial signals delivered to the brain to "sense" how to move their hand under direction from the researchers. However, this isn't a plug-and-play situation. The sensation is very unnatural and is a bit like artificial vision experiments of the 1970s where blind patients were given "sight" by means of a device that covered their back and formed geometric patterns. It worked in a simple way, but it was like learning another language.

"The question is: Can humans use novel electrical sensations that they've never felt before, perceive them at different levels and use this to do a task?," says UW bioengineering doctoral student James Wu. "And the answer seems to be yes. Whether this type of sensation can be as diverse as the textures and feelings that we can sense tactilely is an open question."

The brain sensor matrix used in the University of Washington neuro-sensation system(Credit: University of Washington/Mark Stone)

For the test, three patients were asked to move their hand into a target position using only the sensory feedback from the glove. If they opened their fingers too far off the mark, no stimulation would occur, but as they closed their hand, the stimulus would begin and increase in intensity. As a control, these feedback sessions would be interspersed with others were random signals were sent.

According to the team, the hope is that one day such artificial feedback devices could lead to improved prostheses, neural implants, and other techniques to provide sensation and movement to artificial or damaged limbs.

"Right now we're using very primitive kinds of codes where we're changing only frequency or intensity of the stimulation, but eventually it might be more like a symphony," says Rajesh Rao, CSNE director. "That's what you'd need to do to have a very natural grip for tasks such as preparing a dish in the kitchen. When you want to pick up the salt shaker and all your ingredients, you need to exert just the right amount of pressure. Any day-to-day task like opening a cupboard or lifting a plate or breaking an egg requires this complex sensory feedback."

The research will be published in IEEE Transactions on Haptics.

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