Controlled unilateral isometric force generated by epidural spinal cord stimulation in the rat hindlimb

Prediction of isometric forces from combined epidural spinal cord and neuromuscular electrical stimulation in the rat lower limb

Although epidural spinal cord and muscle stimulation have each been separately used for restoration of movement after spinal cord injury, their combined use has not been widely explored. Using both approaches in combination could provide more flexible control compared to using either approach alone, but whether responses evoked from such combined stimulation can be easily predicted is unknown. We evaluate whether responses evoked by combined spinal and muscle stimulation can be predicted simply, as the linear summation of responses produced by each type of stimulation individually. Should this be true, it would simplify the prediction of co-stimulation responses and the development of control schemes for spinal cord injury rehabilitation. In healthy anesthetized rats, we measured hindlimb isometric forces in response to spinal and muscle stimulation. Force prediction errors were calculated as the difference between predicted and observed co-stimulation forces. We found that spinal and muscle co-stimulation could be closely predicted as the linear summation of the individual spinal and muscle responses and that the errors were relatively low. We discuss the implications of these results to the use of combined muscle and spinal stimulation for the restoration of movement following spinal cord injury.

Prediction of isometric forces from combined epidural spinal cord and neuromuscular electrical stimulation in the rat lower limb

Both epidural spinal cord and muscle stimulation have been widely used for restoration of movement after spinal cord injury. However, using both approaches simultaneously could provide more flexible control compared to using either approach alone. We evaluate whether responses evoked by combined spinal and muscle stimulation can be predicted by the linear summation of responses produced by each individually. Should this be true, it would simplify the prediction of co-stimulation responses and the development of control schemes for spinal cord injury rehabilitation. In anesthetized rats, we measured hindlimb isometric forces in response to spinal and muscle stimulation across a range of amplitudes. Force prediction errors were calculated as the difference between predicted co-stimulation vectors and observed co-stimulation vectors whereby small errors signified evidence for linear summation. We found that the errors for spinal and muscle co-stimulation were significantly larger than expected. Using a bootstrapping analysis, we find that these larger errors do not reflect a nonlinear interaction between spinal and muscle responses. Instead, they can be attributed to the variability of spinal stimulation responses. We discuss the implications of these results to the use of combined muscle and spinal stimulation for the restoration of movement following spinal cord injury.

Restoration of Hindlimb Movements after Complete Spinal Cord Injury Using Brain-Controlled Functional Electrical Stimulation

Single neuron and local field potential signals recorded in the primary motor cortex have been repeatedly demonstrated as viable control signals for multi-degree-of-freedom actuators. Although the primary source of these signals has been fore/upper limb motor regions, recent evidence suggests that neural adaptation underlying neuroprosthetic control is generalizable across cortex, including hindlimb sensorimotor cortex. Here, adult rats underwent a longitudinal study that included a hindlimb pedal press task in response to cues for specific durations, followed by brain machine interface (BMI) tasks in healthy rats, after rats received a complete spinal transection and after the BMI signal controls epidural stimulation (BMI-FES). Over the course of the transition from learned behavior to BMI task, fewer neurons were responsive after the cue, the proportion of neurons selective for press duration increased and these neurons carried more information. After a complete, mid-thoracic spinal lesion that completely severed both ascending and descending connections to the lower limbs, there was a reduction in task-responsive neurons followed by a reacquisition of task selectivity in recorded populations. This occurred due to a change in pattern of neuronal responses not simple changes in firing rate. Finally, during BMI-FES, additional information about the intended press duration was produced. This information was not dependent on the stimulation, which was the same for short and long duration presses during the early phase of stimulation, but instead was likely due to sensory feedback to sensorimotor cortex in response to movement along the trunk during the restored pedal press. This post-cue signal could be used as an error signal in a continuous decoder providing information about the position of the limb to optimally control a neuroprosthetic device.