Brain–machine interfaces: past, present and future

Since the original demonstration that electrical activity generated by ensembles of cortical neurons can be employed directly to control a robotic manipulator, research on brain–machine interfaces (BMIs) has experienced an impressive growth. Today BMIs designed for both experimental and clinical studies can translate raw neuronal signals into motor commands that reproduce arm reaching and hand grasping movements in artificial actuators. Clearly, these developments hold promise for the restoration of limb mobility in paralyzed subjects. However, as we review here, before this goal can be reached several bottlenecks have to be passed. These include designing a fully implantable biocompatible recording device, further developing real-time computational algorithms, introducing a method for providing the brain with sensory feedback from the actuators, and designing and building artificial prostheses that can be controlled directly by brain-derived signals. By reaching these milestones, future BMIs will be able to drive and control revolutionary prostheses that feel and act like the human arm.

Introduction

Less than a decade ago, hardly anyone could have predicted that attempts to build direct functional interfaces between brains and artificial devices, such as computers and robotic limbs, would have succeeded so readily, and in the process would have led to the establishment of a new area at the frontier of systems neuroscience. Born as a highly multidisciplinary field, basic research on brain–machine interfaces (BMIs) has moved at a stunning pace since the first experimental demonstration in1999 that ensembles of cortical neurons could directly control a robotic manipulator [1]. Since then, a continuous stream of research papers has kindled an enormous interest in BMIs among the scientific community and the lay public. This interest stems from the considerable potential of this technology for restoration of motor behaviors in severely handicapped patients.

Indeed, BMIs have been primarily conceived as a potential new therapy to restore motor control in severely disabled patients, particularly those suffering from devastating conditions such as amyotrophic lateral sclerosis (ALS), spinal cord injury, stroke and cerebral palsy. As this technology advances and the risks of invasive brain recordings decrease, BMIs might also hold promise for amputees. In addition to the systems controlling upper-limb prostheses, BMIs dedicated to the restoration of locomotion and speech are likely to emerge.

However, such stellar progress also breeds unrealistic expectations that such a future is just around the corner. Thus, the understandable eagerness in attaining the lofty goal of helping severely disabled patients has to be carefully calibrated by an objective analysis of the current state and future directions of the field. Such analysis indicates that, despite the optimism raised by a barrage of new accomplishments, there are still many issues that preclude straightforward translation of experimental BMIs into clinical applications. Indeed, most of the invasive BMIs have been tested only in experimental animals. Thus, despite recent enthusiasm to move emergent, and in some cases not thoroughly tested, BMI-related technology into clinical trials, much experimentation remains to be done before BMIs can become a safe and efficient rehabilitation tool.

Here, we highlight some of the fundamental obstacles faced by BMI research and propose a series of milestones that can transform recent experimental advances into viable clinical applications in the next 10–20 years. The roadmap detailed here takes into account the recent history of the field, the factors that influenced its growth, and a critical analysis of the published work.