Advisor: Karen Moxon, Ph.D.
The disability inflicted by a Spinal Cord Injury (SCI) is unquestionably severe, especially in the case of complete transection injuries. It has been estimated that nearly 250,000 individuals in the United States alone have suffered SCI, with 11,000 new cases added each year. Even with intensive therapy, gains toward meaningful restoration of voluntary control over movement continue to be disappointing, largely due to the limited capability of the central nervous system (CNS) to regenerate damaged axons.
The isolation imposed by SCI renders ineffective the chief adaptive mechanism possessed by the adult CNS: plasticity, or the ability to alter the functional structure of existing networks to suit new goals. Despite this isolation, some evidence has been presented that the cortex retains its ability to generate output signals. This perseverance of cortical output signals appropriate to movement following SCI highlights the potential for the use of the Neurorobotic Interface following spinal cord injury. 11 adult male Long-Evans rats were trained to press a lever, with either hindlimb, in response to an audible cue in exchange for a reward.
Following behavioral training, animals were implanted stereotaxically with chronic indwelling arrays of 50 μm stainless steel microwires into the hindlimb representation of the sensorimotor cortex. Following implantation, spiking activity was recorded from ensembles of single neurons while the animals resumed their skilled hindlimb lever press behavior. Lever position data was also recorded, and offline analysis quantified the degree of correlation between neuronal firing and te kinematic parameters of the movement, as well as the temporal tuning properties of the cells. In all, 68.3% of individual neurons significantly modulated their activity in response to some aspect of the movement.
Later, control of reward delivery was changed from lever-press activity to the value of the Neural Population Function, a weighted-sum average of the activity of the ensemble that was calculated in real time during the experiments. Once neural control of the reward had been established, the lever was removed, and finally, a complete mid-thoracic spinal transection surgery was performed. Offline analysis of the population activity showed that cortical firing patterns could encode for the intention to move, with or without actual limb movement. Decoding of movement intent could be performed in 87.6% of trials prior to injury, and did not significantly change when the lever was removed. Decoding accuracy did change as a function of the frequency with which the decoding algorithm was updated, possibly indicating that functional reorganization took place in the cortex in response to the neurorobotic paradigm.
Following transection, there were decreases in both the proportion of cells modulating their activity in response to the audible cue, and in the firing rates of those cells. Decoding accuracy did not significantly decrease, however, as a result of spinal transection, and the significant effect on overall accuracy by algorithm update frequency was not apparent following transection.
These results indicates that the presence or absence of an injury condition does not irrevocably prevent the use of a Neurorobotic Interface, as movement intent is preserved following injury. The ability to observe the response properties of the cells both before and after an injury makes the Neurorobotic Interface a powerful experimental paradigm for research into spinal cord injury.
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