78 S. Norman and R. Bellamkonda
brain to distal portions of the body. Electrical stimulation of the spinal cord distal
to the injury may restore function or sensation to patients with SCI.
When the spinal cord, nerves, or muscles are stimulated directly, they can elicit
a desired response. In one study, cats were implanted with either microwire arrays
in the lumbar spinal cord or microwires in the hindlimb muscles, and stimulation
was sufficient for standing and balance.
80
A review on intraspinal microstimu-
lation states that the technology looks promising for a variety of applications,
including the reestablishment of egestive and sexual function to SCI patients.
81
Intraspinal stimulation has been shown to elicit force from and contraction of
the hindlimbs of anesthetized and decerebrate cats;
82
the ability to control such
muscular activation is of primary interest for stimulation devices designed to help
SCI patients maintain balance and possibly walk.
Microelectrodes for spinal stimulation are often made from silicon or mi-
crowires. Smaller electrodes presumably result in less damage to neural tissue,
so micromachined devices are important for use in the spinal cord. Investigators
using silicon probes manufactured at the University of Michigan and iridium
microwire arrays implanted into the sacral spinal cord of cats have shown them
to be adequate for modulating bladder pressure.
83
In addition to reestablishing lost function, spinal cord stimulation has also
been used to reduce discomfort experienced by chronic pain patients. In an im-
planted spinal cord stimulation study, more than half the patients reported at least
a 50% reduction in pain and some were even able to return to work.
84
Continual
refinement of microfabricated spinal cord devices will likely improve the number
of chronic pain patients that achieve relief using spinal cord stimulators.
4.2.9 Brain Computer Interfaces
While a visual or auditory prosthesis translates signals from the external envi-
ronment into electrical stimulation that patients can interpret, brain computer
interfaces (BCIs) are the reciprocal devices; a BCI uses a patient’s brain activity
to interact with the external environment in some way. Patients with extreme
physical trauma or debilitating disease can undergo a major loss of function, such
that they are rendered unable to move and sometimes even unable even to blink;
such patients may have completely intact thought and mental functioning, but
they are unable to interact with the world around them, and these patients are
said to be “locked-in” to their internal environment.
85
BCIs offer locked-in patients
clinical hope, as they may one day be able to control mobility devices, computers,
and household appliances simply thinking by about them.
At the highest level, a brain computer interface records signals from the patient
while they perform a task and then gives them feedback (Fig. 4.6). Neuronal
signals for BCIs can be acquired from the patient in a number of ways: classical
electroencephalogram (EEG) has been shown in the literature, as well as the more
invasive electrocorticogram (ECoG), local field potentials (LFPs) and single neuron
action potentials (single unit recordings).
86
Generally, more invasive recording
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