AAWSAP DIRD, Technological Approaches to Controlling External Devices, March 2010

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currently 22 electrodes directly implanted into the cochlea of the inner ear. The
microphone and sound processor replaces the outer and middle ear, while the
electrodes replace the frequency selective hair fibers of the inner ear that normally
would transmit electrical signals to the cochlea. The quality of the sound is much less
than natural hearing, but the only option for patients who have lost hearing or were
born deaf. It has been estimated that 100,000 cochlear implants provide sound to users
worldwide (Reference 37). The cochlear implant is an open loop stimulation system that
provides complex signals to the nervous system through a peripheral connection.
BMI technologies have evolved from assistive technology devices solely targeted for the
healthcare industry, into apparatuses designed for intracortical microstimulation to
deliver sensory feedback. Such systems allow the simultaneous recording and
microstimulation of neuronal and behavioral events. The primary device that has
emerged leading this technological transformation is the direct cortical array containing
either surface or neocortex penetrating electrodes. The direct cortical array is implanted
on the surface of the brain near the functional area of interest, and then the connection
is reverse-engineered: individual electrodes are tested to determine if they should be
assigned as input, output, or neither. 12 These devices are designed to fuse neural
signaling pathways with external machinery into a 3D control system, such as those
apparatuses designed to modulate the movement of appendages (e.g. robotic arms),
though the signals could be used for control of any complex machine. The control
circuits on these systems are intricate and often bulky, and they can be designed with
either open- or closed-loop sensory feedback.
An additional device is the penetrating cortical electrode. These electrodes contain a
ladder of input/output probes and can be selected to transmit or receive signals in any
layer of the neocortex. Research is ongoing regarding the optimal layer with which to
place an interface. When using penetrating electrodes, tissue response is an additional
challenge as these devices frequently cause scarring. Animal studies are in progress to
quantify this effect.
Technological challenges in development of all types of invasive BMI devices include
signal decoding/stimulation algorithms, and localization of brain activation near the
implant. Functional MRI (fMRI) is utilized in conjunction with the invasive implants to
confirm additional neural network activity during interface tasks. Modifications in the
design of electrodes is necessary for experiments involving fMRI (Reference 38). 13
The majority of invasive experiments to date use animal or even tissue models, though
a few human trials have been conducted (with the exception of the cochlear implant
described above) .
OPEN-LOOP DIRECT CORTICAL ARRAY ALGORITHM MODELING
In traditional open loop experiments, eye movement systems are utilized to develop a
linear model for a physiological system. 14 In such experiments, invasive signals are
recorded from the cortex and the motion of the eye is recorded in tandem. The location
t i This procedure eliminates the need to directly locate the neuron(s) of interest during implantation.
13 Field potentials at the points of metal electrodes cause concentration of RF energy from the excitation pulses of
the MRI. This effect is used in a similar setup for tissue ablation therapy; however, such energy concentration is
undesirable in the studies at hand .
1• These techniques may be generally applied to any motor control systems.
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