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VANDERBILT HON 182 - Study Notes

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REVIEWTHE EMERGING WORLD OF MOTOR NEUROPROSTHETICS:ANEUROSURGICAL PERSPECTIVEEric C. Leuthardt, M.D.Department of Neurological Surgery,University of WashingtonSchool of Medicine,Harborview Medical Center,Seattle, WashingtonGerwin Schalk, M.S.Wadsworth Center,New York State Department of Health,Albany, New YorkState University of New York,Albany, New YorkDaniel Moran, Ph.D.Department of Biomedical Engineering,Washington University in St. Louis,St. Louis, MissouriJeffrey G. Ojemann, M.D.Department of Neurological Surgery,University of WashingtonSchool of Medicine,Harborview Medical Center,Seattle, WashingtonReprint requests:Eric C. Leuthardt, M.D.,Washington UniversitySchool of Medicine,Department of Neurological Surgery,660 South Euclid Avenue,Campus Box 8057,St. Louis, MO 63110.Email: [email protected], November 19, 2005.Accepted, March 30, 2006.A MOTOR NEUROPROSTHETIC device, or brain computer interface, is a machinethat can take some type of signal from the brain and convert that information into overtdevice control such that it reflects the intentions of the user’s brain. In essence, theseconstructs can decode the electrophysiological signals representing motor intent. Withthe parallel evolution of neuroscience, engineering, and rapid computing, the era ofclinical neuroprosthetics is approaching as a practical reality for people with severemotor impairment. Patients with such diseases as spinal cord injury, stroke, limb loss,and neuromuscular disorders may benefit through the implantation of these braincomputer interfaces that serve to augment their ability to communicate and interactwith their environment. In the upcoming years, it will be important for the neurosur-geon to understand what a brain computer interface is, its fundamental principle ofoperation, and what the salient surgical issues are when considering implantation. Wereview the current state of the field of motor neuroprosthetics research, the earlyclinical applications, and the essential considerations from a neurosurgical perspectivefor the future.KEY WORDS: Brain computer interface, Brain machine interface, Electrocorticography,Electroencephalography, Neuroprosthetics, Single unitsNeurosurgery 59:1-14, 2006 DOI: 10.1227/01.NEU.0000221506.06947.AC www.neurosurgery-online.comDuring the past decade, the idea of ma-chines that could be controlled byone’s thoughts has emerged from therealm of fiction to one of serious scientificinquiry. The most common technical term forthese types of devices is a brain computerinterface (BCI). Other synonymous terms in-clude motor neuroprosthetics, direct brain in-terface, brain machine interface, and neuroro-botics. Most simply put, these are machinesthat create a new output channel from thebrain other than the natural motor and hor-monal commands. BCIs recognize some formof electrophysiological alteration in the brainof a subject and use these changes as signals toeither communicate with or control some ele-ment of the outside world that is consistentwith the intentions of that subject. Concreteexamples of such applications would be sometype of brain signal controlling a cursor on acomputer screen, a prosthetic limb, or one’sown limb. These types of devices hold tremen-dous promise for improving the quality of lifeof individuals who are cognitively intact yetmotor impaired. This includes patients withspinal cord injury, stroke, neuromuscular dis-orders, and amputees. These are patients forwhom, until now, the field of neurosurgeryhas not been able to offer any substantiveintervention. Moreover, these populations areincreasing in size and relevance because of theaging population and improved survival afterstroke and trauma.It is important to distinguish the emergingnature of these output BCIs, or devices thatconvert human intentions to overt device con-trol, from those that translate external stimulisuch as light or sound into internally per-ceived visual or auditory perceptions (i.e., in-put BCIs). There has been a rich and extensiveexperience in the sensory prosthetic field. Todate, the most successful example of a sensoryprosthetic is the cochlear implant. Cochlearimplants are a therapeutic option for patientswho lack the cochlear hair cells that transducesound into neural activity but who have sur-viving auditory nerve fibers. In many cases, acochlear prosthesis and associated speech pro-cessor can restore accurate speech reception toa person who otherwise has little or no audi-NEUROSURGERY VOLUME 59 | NUMBER 1 | JULY 2006 | 1tory sensitivity. Indeed, many implant users routinely con-verse on the telephone (1). Cochlear implants have been incommon clinical use for more than two decades, and morethan 60,000 devices have been implanted (52). Auditory im-plants are also being extended to direct stimulation of thebrainstem for those with dysfunctional cochlear nerves (e.g.,neurofibromatosis-2) (57). To date, approximately 300 to 500patients have been implanted with auditory brainstem im-plants (12, 40). Visual prosthetics are also making significantinroads into clinical viability. Prosthetics have been applied toevery aspect of the visual system ranging from cortical im-plants (both surface and intraparenchymal electrodes) (3, 16–20, 71), to optic nerve stimulators (83), to retinal (both subreti-nal and epiretinal) implants (11, 32, 33, 87). Each of theseplatforms is undergoing various stages of clinical trials rang-ing from transient placement to chronic implantation. Themost efficacious clinical platform, however, still has yet to bedetermined, as discussed by Margalit et al. (48).Now, with the improved understanding of the electrophys-iological underpinnings of motor related cortical function,rapid development of inexpensive and fast computing, and agrowing awareness of the needs of the severely motor im-paired, the notion of a practical and clinically viable BCI nowis beginning to deserve serious consideration. It will be essen-tial for the neurosurgical community to understand whatthese devices are and their implications for patient care. Thiswill require a fundamental framework of how these systemsoperate, what the current BCI platforms and their limitationsare, relevant issues when applied clinically, and what theimportant milestones are for their evolution toward enteringstandard neurosurgical practice.This review will provide a reference to which neurosur-geons can refer to critically evaluate the emerging field ofmotor neuroprosthetics. We


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