Abstract
Cervical spinal cord injury (SCI) paralyzes muscles of the hand and arm, making it difficult to perform activities of daily living. Restoring the ability to reach can dramatically improve quality of life for people with cervical SCI. Any reaching system requires a user interface to decode parameters of an intended reach, such as trajectory and target. A challenge in developing such decoders is that often few physiological signals related to the intended reach remain under voluntary control, especially in patients with high cervical injuries. Furthermore, the decoding problem changes when the user is controlling the motion of their limb, as opposed to an external device. The purpose of this study was to investigate the benefits of combining disparate signal sources to control reach in people with a range of impairments, and to consider the effect of two feedback approaches. Subjects with cervical SCI performed robot-assisted reaching, controlling trajectories with either shoulder electromyograms (EMGs) or EMGs combined with gaze. We then evaluated how reaching performance was influenced by task-related sensory feedback, testing the EMG-only decoder in two conditions. The first involved moving the arm with the robot, providing congruent sensory feedback through their remaining sense of proprioception. In the second, the subjects moved the robot without the arm attached, as in applications that control external devices. We found that the multimodal-decoding algorithm worked well for all subjects, enabling them to perform straight, accurate reaches. The inclusion of gaze information, used to estimate target location, was especially important for the most impaired subjects. In the absence of gaze information, congruent sensory feedback improved performance. These results highlight the importance of proprioceptive feedback, and suggest that multi-modal decoders are likely to be most beneficial for highly impaired subjects and in tasks where such feedback is unavailable.
Highlights
Injuries to the cervical spinal cord can be devastating, resulting in lost function in both the upper and lower limbs
We evaluated the utility of combining disparate signal sources to enhance trajectory control, and compared two different feedback approaches
We examined the effect of the proprioceptive feedback that subjects experienced when interacting with the reaching interface
Summary
Injuries to the cervical spinal cord can be devastating, resulting in lost function in both the upper and lower limbs Many people with such injuries consider the restoration of hand and arm function to be of highest importance for improving their quality of life (Anderson, 2004; Collinger et al, 2013). People with high tetraplegia—injuries at the fourth cervical level (C4) or above—may have no movement in the arm except for possibly shoulder shrug through upper trapezius activity. For these individuals, simple every-day tasks such as feeding and grooming cannot be achieved without assistance. As more complex systems are being developed that can provide control of continuous reach trajectories to people with high tetraplegia (Crema et al, 2011; Hart et al, 2011; Cooman and Kirsch, 2012; Schearer et al, 2013), finding appropriate signal sources and developing intuitive user interfaces is an even greater challenge
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