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Brain–Computer Interfaces - Index of

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Non Invasive BCIs for Neuroprostheses Control <strong>of</strong> the Paralysed Hand 173<br />

a static posture. Due to the weight <strong>of</strong> the upper limb and the non-physiologic synchronous<br />

activation <strong>of</strong> the paralyzed muscles, these systems are difficult to use for<br />

most activities throughout the day.<br />

One <strong>of</strong> the main problems in functional restoration <strong>of</strong> the grasping and reaching<br />

function in tetraplegic patients is the occurrence <strong>of</strong> a combined lesion <strong>of</strong> the central<br />

and peripheral nervous structures. In almost one third <strong>of</strong> the tetraplegic patients, an<br />

ulnar denervation occurs due to damage <strong>of</strong> the motor neurons in the cervical spinal<br />

cord [2]. Flaccid and denervated muscles can not be used for functional restoration<br />

by electrical stimulation where action potentials on the nerve are elicited. Much<br />

higher currents are necessary to directly stimulate the muscle, which can damage<br />

the skin. Here, further research is necessary to evaluate the possibility <strong>of</strong> transferring<br />

results obtained by direct muscle stimulation at the lower extremities [8].<br />

In principle, all types <strong>of</strong> grasp neuroprostheses are controlled with external<br />

control units, such as a shoulder position sensor (see Fig. 1).<br />

Today, residual movements not directly related to the grasping process are usually<br />

used to control the neuroprostheses. In the highest spinal cord injuries, not<br />

enough functions are preserved for control, which so far hampered the development<br />

<strong>of</strong> neuroprostheses for patients with a loss <strong>of</strong> not only hand and finger but also elbow<br />

and shoulder function.<br />

Kirsch [10] presents a new concept <strong>of</strong> using implanted facial and neck muscles<br />

sensors to control an implanted neuroprosthesis for the upper extremity. The system<br />

itself is designed to consist <strong>of</strong> two stimulation units providing 12 stimulation<br />

Fig. 1 Grasp neuroprostheses. Left: Neuroprosthesis using surface electrodes. Right:<br />

Neuroprosthesis with implanted electrodes (e.g., Freehand(R)). Both systems are controlled with a<br />

shoulder position sensor fixed externally on the contralateral shoulder

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