Pittsburgh - Electrical impulses on four implants on the postcentral gyrus, where the somatosensory cortex responsible for tactile sensations is located, enabled paraplegic man to perceive the signals from pressure sensors of an arm robot as touch signals. The results presented in Science Translational Medicine (2016: doi: 10.1126 / scitranslmed.aaf8083) are further step towards lifelike arm prosthesis, which one day will enable amputees to perform (more than) simple activities via brain-computer interface.
While running makes no great demands on the sensors and wearers of leg prostheses are capable of Olympic-level performances, the situation is different with the hand. The upper extremities are responsible for manual activities. The delicate movements only succeed if the brain receives feedback on whether the hand is holding glass or balloon. The muscles may only be contracted so far that the glass does not break, but the grip must be firm enough so that the balloon does not slip away from the fingers.
Researchers at the Brain Institute at the University of Pittsburgh have taken an important step towards lifelike prosthesis. The experiments were not carried out with an amputee, but with tretraplegic. The 28-year-old patient had been paralyzed and (largely) numb in his arms and legs since an accident ten years ago.
In an operation, neurosurgeons from the Pitt School of Medicine implanted four small plates with 32 microelectrodes each on the Postcentral gyrus of the parietal lobe. This is where the body's sensory signals arrive, from different regions in different areas of homunculus, which the Canadian neurologist Wilder Penfield described in the 1950s and which every medical student gets to know in the anatomy course. The implants were placed where the thumb, fingers and palm are represented in the homunculus.
In the first few days after the operation, as Robert Gaunt's team reports, the patient still felt nothing. Even slightly stronger impulses in the electrodes did not trigger any sensations. Then suddenly there were spontaneous sensations, even without stimulation, which the patient described as tickling in the right arm.
These “phantom” feelings finally disappeared and in the fourth week the patient noticed signals from the implants for the first time which he assigned to different areas of the palm and fingers. 93 percent of the sensations were of normal quality for the patient, and they were clearly different from the pain caused by the electrical irritation of the skin - the patient still had residual sensitivity in the hands.
By stimulating individual electrodes, the researchers were later able to trigger targeted pressure sensations in certain places on the individual fingers and on the thumb. The intensity of the pressure sensations could be varied by changing the electrical impulses (in the range from 20 to 100 micro-amperes). The patient differentiated between four to six different print qualities.
Over the further duration of the study, the number of electrodes to which the patient responded increased even further. So far, there has been no failure of individual electrodes, assures Gaunt, who viewed this as an important prerequisite for permanent implants.
In the next step, the implants were connected to computer with "Modular Prosthetic Limb" (MPL) connected, robotic arm that Johns Hopkins University developed as prototype for touch-sensitive prosthesis. The MPL is equipped with various sensors, including so-called torque sensors (torque transducers) that detect changes in shape and convert them into electrical impulses.
The sensors on the fingers of the robot were connected to the electrodes of the implants that the patient had previously assigned to the corresponding fingers. In the first experiments, the researchers touched individual fingers of the robot hand and the patient, who did not see the robot, was able to name more than 80 percent of the fingers that were touched. When the investigators touched two fingers on the robot arm, the hit rate was around half.
The next step could be to implant additional chips into patient's precentral gyrus, the primary motor cortex. two implants with 96 electrodes each implanted.
The patient was then able to control robotic arm with just her thoughts. However, the movements were still quite rough. The combination of both electrodes could enable patient to adapt the force of his arm and finger movements to the situation using the sensitive signals from the sensors, which could make handling glass or balloons easier.