Brain–Computer Interfaces - Index of

172 G.R. Müller-Putz et al.
Electrical Stimulation (FES). The stimulation devices and systems that are used for
this purpose are called neuroprostheses [27].
A restoration of motor functions (e.g., grasping) by using neuroprostheses is possible
if the peripheral nerves connecting the neurons of the central nervous system
to the muscles are still intact [26]. By placing surface electrodes near the motor
point of the muscle and applying short (< 1 ms) constant-current pulses, the potential
of the nerve membrane is depolarized and the elicited action potential leads to
a contraction of the innervated muscle fibers, somewhat similar to natural muscle
contractions. There are some differences between the physiological and the artificial
activation of nerves: physiologically, small and thin motor fibers of a nerve,
which innervate fatigue resistant muscle fibers, are activated first. As people grasp
more strongly, more and more fibers with larger diameters are recruited, ultimately
including muscle fibers that are strong, but fatigue rapidly. Applying artificial stimulation
pulses, this activation pattern is reversed. The current pulses lead to action
potentials in the fibers with large diameter first, and by further increasing the current,
the medium sized and thin fibers also get stimulated [29].
This “inverse recruitment” leads to a premature fatiguing of electrically stimulated
muscles. Additionally, the fact that every stimulation pulse activates the same
nerve fibers all at once further increases muscle fatigue. Muscle fatigue is especially
problematic with higher stimulation frequencies for tetanic contractions. A
frequency around 35 Hz leads to a strong tonus, however the muscle gets tired earlier
than in the case of e.g., 20 Hz. Therefore, the pulse repetition rate has to be carefully
chosen in relation to the desired muscle strength and the necessary contraction
duration.
Some neuroprostheses for the upper extremity are based on surface electrodes
for external stimulation of muscles of the forearm and hand. Examples are the commercially
available (N200, Riddenderk, Netherlands) System [5] and other more
sophisticated research prototypes [12, 31]. To overcome the limitations of surface
stimulation electrodes concerning selectivity, reproducibility and practicability, an
implantable neuroprostheses (the Freehand R○ system, Neurocontrol, Valley View,
OH, USA) was developed, where electrodes, cables and the stimulator reside permanently
under the skin [7]. This neuroprosthesis was proven effective in functional
restoration and user acceptance [21]. All FES systems for the upper extremity can
only be used by patients with preserved voluntary shoulder and elbow function,
which is the case in patients with an injury of the spinal cord below C5. The reason
for this limitation is that the current systems are only able to restore grasp function
of the hand, and hence patients must have enough active control movements for
independent use of the system.
Until now, only two groups have dealt with the problem of restitution of elbow
and shoulder movement. Memberg and colleagues [13] used an extended Freehand
system to achieve elbow extension, which is typically not apparent in patients with
a lesion at cervical level C5. In the mid 80 s, Handa’s group [6] developed a system
based on intramuscular electrodes for restoration of shoulder function in hemiparesis
and SCI. Both systems represent exclusive FES systems, which stimulate the
appropriate muscle groups not only for dynamic movements but also for maintaining