During the past decade, the idea of ma- chines 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 in- clude motor neuroprosthetics, direct brain in- terface, brain machine interface, and neuroro- botics. Most simply put, these are machines that create a new output channel from the brain other than the natural motor and hor- monal 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 ele- ment 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 tremen- dous promise for improving the quality of life of individuals who are cognitively intact yet motor impaired. This includes patients with spinal cord injury, stroke, neuromuscular dis- orders, 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 con- trol, from those that translate external stimuli such as light or sound into internally per- ceived visual or auditory perceptions (i.e., in- put 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 sur- viving auditory nerve fibers. In many cases, a cochlear prosthesis and associated speech pro- cessor can restore accurate speech reception to a person who otherwise has little or no auditory sensitivity. Indeed, many implant users routinely con- verse 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 im- plants 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 im- plants (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 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 these platforms is undergoing various stages of clinical trials rang- ing 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 electrophys- iological underpinnings of motor related cortical function, rapid development of inexpensive and fast computing, and a growing awareness of the needs of the severely motor im- paired, the notion of a practical and clinically viable BCI now is beginning to deserve serious consideration. It will be essen- tial 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 neurosur- geons can refer to critically evaluate the emerging field of motor neuroprosthetics. We will discuss the critical features, function, and platforms of output BCIs; in addition, we will define the key surgical elements to be considered for an im- plantable BCI and then critically review the literature of the various platforms relative to these considerations.