REVIEW THE EMERGING WORLD OF MOTOR NEUROPROSTHETICS A NEUROSURGICAL PERSPECTIVE Eric C Leuthardt M D Department of Biomedical Engineering Washington University in St Louis St Louis Missouri A MOTOR NEUROPROSTHETIC device or brain computer interface is a machine that can take some type of signal from the brain and convert that information into overt device control such that it reflects the intentions of the user s brain In essence these constructs can decode the electrophysiological signals representing motor intent With the parallel evolution of neuroscience engineering and rapid computing the era of clinical neuroprosthetics is approaching as a practical reality for people with severe motor impairment Patients with such diseases as spinal cord injury stroke limb loss and neuromuscular disorders may benefit through the implantation of these brain computer interfaces that serve to augment their ability to communicate and interact with their environment In the upcoming years it will be important for the neurosurgeon to understand what a brain computer interface is its fundamental principle of operation and what the salient surgical issues are when considering implantation We review the current state of the field of motor neuroprosthetics research the early clinical applications and the essential considerations from a neurosurgical perspective for the future Jeffrey G Ojemann M D KEY WORDS Brain computer interface Brain machine interface Electrocorticography Electroencephalography Neuroprosthetics Single units Department of Neurological Surgery University of Washington School of Medicine Harborview Medical Center Seattle Washington Gerwin Schalk M S Wadsworth Center New York State Department of Health Albany New York State University of New York Albany New York Daniel Moran Ph D Department of Neurological Surgery University of Washington School of Medicine Harborview Medical Center Seattle Washington Reprint requests Eric C Leuthardt M D Washington University School of Medicine Department of Neurological Surgery 660 South Euclid Avenue Campus Box 8057 St Louis MO 63110 Email ericleuthardt sbcglobal net Received November 19 2005 Accepted March 30 2006 NEUROSURGERY Neurosurgery 59 1 14 2006 D DOI 10 1227 01 NEU 0000221506 06947 AC uring the past decade the idea of machines that could be controlled by one s thoughts has emerged from the realm of fiction to one of serious scientific inquiry The most common technical term for these types of devices is a brain computer interface BCI Other synonymous terms include motor neuroprosthetics direct brain interface brain machine interface and neurorobotics Most simply put these are machines that create a new output channel from the brain other than the natural motor and hormonal commands BCIs recognize some form of electrophysiological alteration in the brain of a subject and use these changes as signals to either communicate with or control some element of the outside world that is consistent with the intentions of that subject Concrete examples of such applications would be some type of brain signal controlling a cursor on a computer screen a prosthetic limb or one s own limb These types of devices hold tremendous promise for improving the quality of life of individuals who are cognitively intact yet motor impaired This includes patients with www neurosurgery online com spinal cord injury stroke neuromuscular disorders and amputees These are patients for whom until now the field of neurosurgery has not been able to offer any substantive intervention Moreover these populations are increasing in size and relevance because of the aging population and improved survival after stroke and trauma It is important to distinguish the emerging nature of these output BCIs or devices that convert human intentions to overt device control from those that translate external stimuli such as light or sound into internally perceived visual or auditory perceptions i e input BCIs There has been a rich and extensive experience in the sensory prosthetic field To date the most successful example of a sensory prosthetic is the cochlear implant Cochlear implants are a therapeutic option for patients who lack the cochlear hair cells that transduce sound into neural activity but who have surviving auditory nerve fibers In many cases a cochlear prosthesis and associated speech processor can restore accurate speech reception to a person who otherwise has little or no audi VOLUME 59 NUMBER 1 JULY 2006 1 LEUTHARDT ET AL tory sensitivity Indeed many implant users routinely converse on the telephone 1 Cochlear implants have been in common clinical use for more than two decades and more than 60 000 devices have been implanted 52 Auditory implants are also being extended to direct stimulation of the brainstem for those with dysfunctional cochlear nerves e g neurofibromatosis 2 57 To date approximately 300 to 500 patients have been implanted with auditory brainstem implants 12 40 Visual prosthetics are also making significant inroads into clinical viability Prosthetics have been applied to every aspect of the visual system ranging from cortical implants both surface and intraparenchymal electrodes 3 16 20 71 to optic nerve stimulators 83 to retinal both subretinal and epiretinal implants 11 32 33 87 Each of these platforms is undergoing various stages of clinical trials ranging from transient placement to chronic implantation The most efficacious clinical platform however still has yet to be determined as discussed by Margalit et al 48 Now with the improved understanding of the electrophysiological underpinnings of motor related cortical function rapid development of inexpensive and fast computing and a growing awareness of the needs of the severely motor impaired the notion of a practical and clinically viable BCI now is beginning to deserve serious consideration It will be essential for the neurosurgical community to understand what these devices are and their implications for patient care This will require a fundamental framework of how these systems operate what the current BCI platforms and their limitations are relevant issues when applied clinically and what the important milestones are for their evolution toward entering standard neurosurgical practice This review will provide a reference to which neurosurgeons can refer to critically evaluate the emerging field of motor neuroprosthetics We will discuss the critical features function and platforms of output BCIs in