ABSTRACT NEUROMOTOR PROSTHESES
23 June 2007 - University of Chicago

To translate preclinical results from intact animals to a clinically useful NMP, movement signals must persist in cortex after spinal cord injury and be engaged by movement intent when sensory inputs and limb movement are long absent.

Furthermore, NMPs would require that intention-driven neuronal activity be converted into a control signal that enables useful tasks. Here we show initial results for a tetraplegic human using a pilot NMP. Neuronal ensemble activity recorded through a 96-microelectrode array implanted in primary motor cortex demonstrated that intended hand motion modulates cortical spiking patterns three years after spinal cord injury. Decoders were created, providing a 'neural cursor' with which MN opened simulated e-mail and operated devices such as a television, even while conversing. Furthermore, MN used neural control to open and close a prosthetic hand, and perform rudimentary actions with a multi-jointed robotic arm. These early results suggest that NMPs based upon intracortical neuronal ensemble spiking activity could provide a valuable new neurotechnology to restore independence for humans with paralysis.

Hundreds of thousands of people suffer from forms of motor impairment in which intact movement-related areas of the brain cannot generate movements because of damage to the spinal cord, nerves, or muscles1. Paralysing disorders profoundly limit independence, mobility and communication. Current assistive technologies rely on devices for which an extant function provides a signal that substitutes for missing actions. For example, cameras can monitor eye movements that can be used to point a computer cursor2. Although these surrogate devices have been available for some time, they are typically limited in utility, cumbersome to maintain, and disruptive of natural actions. For instance, gaze towards objects of interest disrupts eye-based control. By contrast, an NMP is a type of brain–computer interface that can guide movement by harnessing the existing neural substrate for that action, that is, neuronal activity patterns in motor areas. An ideal NMP would provide a safe, unobtrusive and reliable signal from the disconnected motor area that could restore lost function. Neurons in the primary motor cortex arm area of monkeys, for example, provide information about intended arm reaching trajectories3,4,5, but this command signal would work for an NMP only if neural signals persist and could be engaged by intention in paralysed humans.

Since this report was written, we added a second trial participant to the study, a 55-yr-old man who has had complete spinal cord injury at C4 since 1999. Recordings were collected starting in the seventh month after implant, after making an electrical contacts repair in the pedestal connector. We recorded an average of 53.2 6.3 units per session during trial months 7–10, again demonstrating the presence of neural activity lasting many months. Another technical issue causing abrupt signal loss at most electrodes, which may be related to the original repair, occurred at month 11 in participant 2; the reason for this change is being evaluated.

Top of pageModulation by intent
Imagined limb motions modulated neural firing on multiple electrodes, upon request, beginning at the first experimental session. Modulation was evaluated during four consecutive sessions when MN was asked to imagine a series of movements. This series revealed a rich variety of firing modulations largely consistent with patterns observed in monkey MI16. Importantly, this activity was evoked by imagined actions in this participant with cervical spinal cord injury. Figure 3a illustrates how certain neurons are selective for one imagined action (hands together/apart), whereas others recorded simultaneously are engaged by different imagined actions (elbow or wrist). This diversity includes neurons that fired with imagined hand or distal arm actions and those that fired during shoulder movements that were actually performed. Non-selective neurons, active with the onset of any imagined upper extremity action, were also observed. This diversity is useful in creating a flexible control signal.

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