Device interfaces with the brain is 1 of the most promising areas of research in the diagnosis and treatment of disorders of the nervous system. The ability to monitor brain electrical and chemical activity in real time and with noninvasive or minimally invasive techniques is crucial for both the understanding of nervous system functioning in health and disease and the development of effective treatment options for those disorders. Moreover, the ability to restore the diseased nervous system to an intact and normal-functioning state or substitute lost function with brain-actuated assistive devices is crucially dependent on techniques to translate that monitoring information into effective treatment modalities, ie, to stimulate brain tissue and modulate brain activity. One example of neuroprosthesis/brain-machine interface neuromodulation, deep brain stimulation (DBS), has proven to be the greatest advance in the treatment of Parkinson disease since the demonstration of the effectiveness of L-dopa nearly 50 years ago.
To clarify what techniques this overview addresses, it is important to note the various types of device-tissue interfaces. A first critical aspect of a neural interface concerns its primary function as a stimulation or brain signal monitoring (and translation) device. The term brain-machine interface (BMI)--or, equivalently, brain-computer interface (BCI)--is largely reserved for the latter approaches. In other words, a brain-machine interface is a neuroprosthetic system able to directly convey commands to the external world circumventing the conventional neuromuscular pathways. On the other hand, brain recording and/or stimulating neuroprosthetics are most often employed either for treatment of neurologic conditions and their symptoms (eg, deep brain stimulation) or for the replacement of impaired sensory modalities (eg, retinal implants) and involve neuromodulation through the stimulation of brain tissue. This overview intends to equally address both categories of neuroprostheses.
Another major distinction regards the type of tissue targeted by the neural interface hardware. According to the prevalent definition (77), a brain-machine interface in the strict sense should only rely on the activity of the central nervous system. Therefore, recording and stimulation techniques of the peripheral nervous system and muscles will be excluded from this overview.
Furthermore, one can separate techniques into “noninvasive” and “invasive” devices, implying that noninvasive techniques are preferable due to avoidance of an implantation procedure. However, noninvasive techniques, such as electroconvulsive shock therapy for refractory epilepsy and transcranial magnetic stimulation (TMS) for various nervous disorders, are not without the risk of producing seizures or transient (possibly even permanent) neurologic complications as an unwanted side-effect. Moreover, such noninvasive techniques usually require repeated treatment sessions (sometimes on a daily basis), which is not practical in a patient with a neurologic disorder such as severe depression whose life expectancy may be 50 years or more. On the other hand, an invasive technique such as the vagus nerve stimulator (VNS) for refractory epilepsy is an outpatient procedure with very low risk--potentially requiring only a 20-minute procedure under local anesthesia every 10 years to replace the pulse generator or battery. There is even the option of implanting a rechargeable VNS or DBS device (transcutaneous recharging with a charger that is placed on the skin over the VNS or DBS pulse generator). Most patients, however, prefer a brief procedure every 10 years to the need for daily or weekly recharging episodes, with its significant risk if recharging is overlooked.
Thus, only brief attention is given in this article to brain-machine interface brain stimulation techniques such as transcranial magnetic stimulation and techniques that stimulate the spinal cord (eg, for chronic pain or bladder dysfunction), whereas those targeting the peripheral nervous system (eg, for pain), or muscles (eg, for restoration of function) are entirely excluded. This article focuses primarily on what might be considered “pure” brain-machine interfaces prosthetics, ie, wherein either the recording or stimulating (or both) aspects of the interface are in actual contact with, or in close proximity to, brain tissue. Additionally, techniques that may be of great value in animal models but are unlikely to be used in humans in the near future, such as optogenetics, are not considered here.
Review articles on the field of brain-machine interfaces, brain stimulation, or neuroprosthetics in general are appearing with increasing rapidity as the field develops and its potential for restoring brain function is realized (77; Wolpaw et al 2011; 10; 74; 16; 78). Because it is estimated there are more than 100,000 quadriplegic patients in the United States alone, the need for an effective brain-machine interface for these patients, not to mention the larger number of patients with nervous system disorders ranging from depression to epilepsy to Parkinson disease, is quite large.
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• The brain-machine interface is the communication link between biology and technology, ie, the translation of brain electrical and chemical activity into information that can then be “computed” in order to feed information back to the brain in order to correct a brain disorder or replace lost function.
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• The brain-machine interface involves computationally demanding algorithms to process the vast amounts of brain electrical or chemical activity data acquired.
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• The brain-machine interfaces to date have primarily involved stimulation (eg, in deep-brain stimulation an electrode stimulates a specific region of the brain electrically), but increasingly the brain-machine interface involves recording brain electrical or chemical activity in order to guide brain stimulation or to restore lost functions through coupling with robotic and other assistive devices.
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• The brain-machine interface can be divided into invasive or noninvasive techniques depending on whether a surgical procedure is involved to implant the device.
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• Noninvasive brain-machine interface techniques are not necessarily preferable to invasive techniques as usually they are less precise, require an external device and repeated treatment sessions, and may have undesirable side effects (eg, the risk of seizures with transcranial magnetic stimulation).