Neurally Driven Prosthetics: Pt. II Study: (Schwartz, et. Al: primate control of robotic arm) http://news.nationalgeographic.com/news/2008/05/080530-monkey-video-ap.html
Signal is derived from M1 Primary motor cortex produces stereotyped neuron ensemble firing patterns during a particular action Strong neural activity-behavior coupling in this area, ideal for a decoding algorithm Ensemble firing patterns are vectorial, and encoded by firing rate.
Cortical ensemble activity and behavioral pairing is strong This allows engineers to be confident about designing a patterned output from a neural signal Figure shows the high predictability of a neuron’s activity with behavior—machine canpredict accurately what the monkey’s arm would do.
Other motor association cortices Occur in parallel processing networks, and work in concert to initiate, guide/modify, and extinguish movements. These include: supplementary motor cortex (fine/complex movements), cerebellum (coordination/error suppression), basalganglia (initiation/extinction) and possibly parietal association cortices involved in internal representations that govern hand manipulation/grasping.
The interface Researchers found that microelectrode arrays in the 10’s are sufficient as a signal source Most probably 100’s are needed to accommodate full repertoire of limb movements.
Interface is closed-loop Once the microelectrode array is implanted, it cannot be modified-closed system. Manipulation of robot arm is real-time, sampling neuronal recordings once every 30 msec Time delay for arm movement is ~150 msec, comparable to the delay seen in a biological arm. Visual feedback substitutes for sensory feedback, allowing increased accuracy/agility through learning.
PVA (Population Vector Algorithm) “relies on the directional tuning of each unit, characterized by a single preferred direction in which the unit fires maximally. The real-time population vector is essentially a vector sum of the preferred directions of the units in the recorded population, weighted by the instantaneous firing rates of the units” –(Schwartz, et. Al)
Visualizing PVA Neuronal assembly assignment: Upon movement of the arm in one of 8 directions, excitability of that neuron determines calibration in that direction. Tuning!
Inverse Kinematics An additional algorithm is needed to calculate proper joint positioning. Since there is such a large number of possible limb positionings (shoulder flexion/extension, adduction/abduction, rotation, and elbow flexion/extension), to avoid computational overload, an algorithm computes the most probable joint positioning using inverse kinematics equations.
Inverse Kinematics To produce such an output for an inverse kinematics equation, only starting and ending points are needed. Endpoint in 3D space is calculated before being fed to the algorithm real-time by integrating the endpoint velocity to endpoint position, which is then converted to a joint-angle command to the robot.
Movement Quality Different colors represent 4 different targets. Thin grey lines represent the average over all trials Space-filling shapes represent the standard deviation Grey balls represent areas where assistance was provided.
Misc. Methodology It takes about 1,000 trials over a period of 1-2 weeks before high accuracy (80-100%) is achieved. A training period was necessary, where small automated velocity vectors were added (in the direction of interest) for a handicap Kinematics of arm control mirrors bell-shaped profile of a natural arm, but much slower (3-5 sec robotic vs. 1-2 sec biological)
Hurdles Engineering: long-term, stable electrodes must be developed if this technology is to be used for prostheses. Sizable array of immobile recording, computer and robotic control hardware must be sized down. Not autonomic--trained specialist is needed to supervise. Neuroscientific: feedback is purely visual (clumsiness during training is typical for visual-only guidance); for optimal interaction with a physical environment, sensory ganglion, mainly pressure sensors, are needed. Velliste, et. Al. recorded from the primary motor cortex; but as mentioned before, many areas in the brain with unique properties produce signals which may be useful in guiding a prosthetic device.
Areas of great promise Patients of paralysis or locked-in syndrome could feasibly interact with their environment again. Opens up and makes feasible the field of Brain/Machine Interfaces (BMI’s) moving beyond the field of prostheses. Theoretically, as long as neural activity and behavior are coupled stereotypically, we can understand how the information is encoded. BMI engineers can decode it and feed the signal to a computer algorithm which produces an output.
So maybe one day, we’ll finally say BUH-BYE to this…
And Hello to this… Other applications: -Biomimetics (retinal and cochlear implants) -Neural signal feedback therapy -Gaming control
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Video: http://news.nationalgeographic.com/news/2008/05/080530-monkey-video-ap.html Electrode figure: http://www.crunchgear.com/wp-content/uploads/2008/04/windowslivewriterjapansfirstonbraininterfacebeingresearch-9723microelectrode-thumb.jpg Inverse kinematics figure: http://staff.aist.go.jp/eimei.oyama/IKPE.GIF BMI figure: http://www.sms.mavt.ethz.ch/flow_man_machine Cartoon: http://www.usabilitycorner.com/images/hci.JPG Terminator figure: http://www.igargoyle.com/archives/t800arm.jpg Macaque brain figure: http://www.nature.com/nrn/journal/v6/n3/thumbs/nrn1626-f6.jpg Keyboard figure: www.logitech.com “Predicted vs. Actual “kinematic trajectory figure: Wessberg, et. Al. All other figures are from Velliste, et. Al.