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A Technique for Implantation of a 3-Dimensional Penetrating Electrode Array in the Modiolar Nerve of Cats and Humans
Arunkumar N. Badi, MD;
Todd Hillman, MD;
Clough Shelton, MD;
Richard A. Normann, PhD
Arch Otolaryngol Head Neck Surg. 2002;128:1019-1025.
ABSTRACT
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Background We believe that direct intraneural stimulation of the modiolar nerve
using an array of electrodes will have lower thresholds, offer greater frequency
selectivity and more stimulation sites, and have a greater frequency representation
than conventional cochlear implants.
Objectives To describe a potential auditory prosthesis based on electrical stimulation
of the modiolar cochlear nerve and to report the development of a surgical
approach in human and animal models.
Design Cadaveric human and animal studies conducted in temporal bones indicated
that an array of penetrating microelectrodes could be implanted in the modiolar
nerve. Cat studies using anesthesia were performed to develop the surgical
procedure in an animal model. Nerve viability was assessed by measurement
of electrically evoked auditory brainstem responses at different stages of
the surgery.
Subjects Two fresh cadaveric human temporal bones, 3 cat cadavers, 1 pig cadaver,
and 6 anesthetized cats were used in the experiments.
Results We were able to implant arrays containing 20 microelectrodes in the
human modiolar nerve after exposure by a modified extended facial recess approach.
In animals, the modiolar nerve was accessed by the transbulla and the middle
fossa approach. The cat was chosen as the appropriate animal model, and the
transbulla approach was selected. The round window was exposed by ventral
access to the bulla and after cochleostomy; drilling the modiolar bone exposed
the modiolar nerve. The mean ± SD diameter of the exposed nerve in
cats was 1.64 ± 0.07 mm (n = 9), and the mean ± SD exposed length
was 2.50 ± 0.11 mm (n = 9); this is adequate to accommodate 20 microelectrodes.
The electrically evoked auditory brainstem responses indicated nerve survival
during and after the surgery.
Conclusions The surgical technique allows implantation of up to 20 microelectrodes
in the cat and human modiolar nerve. The nerve survives the surgical procedure.
This work enables studies in the electrophysiological properties and consequences
of long-term implantation.
INTRODUCTION
CONVENTIONAL cochlear implant (CI) prostheses are routinely used to
restore a sense of hearing for the profoundly deaf.1
The spiral ganglion cells are the target of stimulation by the CI electrodes,
and it is postulated that electrodes close to the modiolus may allow more
focused and discrete electrical stimulation, reducing the stimulation threshold
and channel interaction.2-3 We
suggest that direct intraneural stimulation of the modiolar nerve using an
array of penetrating microelectrodes will be better than such attempts at
"modiolus hugging" and, hence, we propose that direct cochlear nerve stimulation
using such an array can be better than conventional intrascalar cochlear stimulation.
A significant feature of intraneural stimulation with an array of penetrating
electrodes is that the active tips of the electrodes are directly apposed
to the fibers of the nerve.4 This apposition,
and the small diameters of the tips of the penetrating electrodes, result
in highly selective stimulation of nerve fibers. Such stimulation can have
the following advantages over conventional intrascalar cochlear stimulation:
- Lower threshold stimulation current: Because the
electrodes are intraneural, they are much closer to the fibers than in a conventional
CI, in which the modiolar bone is intervening. This will result in thresholds
that will be lower than in current CIs.
- Greater selectivity: Because the array is intraneural,
the stimulation is expected to be focal, leading to selective stimulation
of specific frequencies.
- Greater number of stimulation sites: An array of
3-dimensional microelectrodes is likely to provide more than 25 intraneural
electrodes that may evoke different frequency percepts in the patient, potentially
offering better frequency representation than a conventional CI.
The Utah electrode array (UEA) is a candidate for such intraneural stimulation.
The UEA is a novel silicon-based microelectrode array, having up to a 4.2
x 4.2-mm 200-µm-thick p-doped silicon
substrate, from which project up to 100 microelectrodes with 400-µm
spacing.5 These electrodes are in the form
of a rectangular or square grid in a 10 x 10 pattern. Each needle can
be as long as 1.5 mm or as short as 0.5 mm, with a cylindrical diameter of
80 µm at the base tapering to a tip. Figure 1 illustrates a scanning electron micrograph of a 5 x
5 UEA with 25 microelectrodes projecting out of the substrate. The light-colored
sharpened tips are coated with platinum to facilitate charge transfer into
the neural tissue. On implantation, the tips of the electrodes are embedded
in the nervous tissue and provide a direct interface with the nervous system.
On the back of the substrate are metallic pads making electrical contact with
1 electrode each. The bases of the electrodes are isolated from each other
by a moat of glass. The entire electrode array, except for the exposed tip,
is coated with an insulating material. Lead wires connect the metallic pads
to a percutaneous connector that is mounted on a titanium pedestal. This connector
system can be screwed onto bone, thereby providing a stable platform to record
and stimulate from the implanted microelectrode tips. This array has application
in basic and applied neuroprosthetic experiments.4, 6-8
Higher-density arrays are likely to be available in the future, with spacing
between electrodes as small as 200 µm.
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Figure 1. A scanning electron micrograph
of a Utah electrode array shows an unwired array with 25 electrodes projecting
out of the silicon substrate. Each of the electrodes is electrically isolated
from its neighbor by a moat of glass. The light-colored platinum tips of the
electrodes are the active portions. Bar indicates 1 mm.
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While the UEA is composed of electrodes of the same height, the Utah
slanted electrode array has electrode lengths that gradually vary from 1.5
to 0.5 mm. This provides access to fibers at varying depths in a nerve. The
Utah slanted electrode array has application in short- and long-term intraneural
recording and stimulation.4
To realize the potential advantages of the UEA as an intraneural auditory
prosthesis, we first had to create reasonable surgical access to the auditory
nerve. If such surgical access were considerably more complex than the surgery
required to implant a conventional cochlear electrode array, then the potential
benefits of a cochlear nerve auditory neuroprosthesis might not justify the
costs and risks of a more complex surgical procedure. After the development
of surgical access in humans, we had to develop an appropriate animal model
for subsequent electrophysiological experiments. The nerve exposed by the
surgical procedure in the animal model should have the demonstrated capability
to survive the procedure. Intraoperative auditory-evoked and electrically
evoked potential assessment of nerve functioning performed periodically provided
proof that the nerve was not damaged during the surgical procedure. Given
the difficulty of implanting the fragile UEA in a cavity with complex contours,
implantation of dummy UEAs was performed to demonstrate the feasibility of
implanting the UEA in the modiolar nerve.
SUBJECTS AND METHODS
HUMAN TEMPORAL BONE STUDIES
Human anatomical studies were performed at the Temporal Bone Laboratory,
Division of Otolaryngology–Head and Neck Surgery, School of Medicine,
The University of Utah. Two human temporal bone surgical procedures were performed
using an operating microscope and a high-speed drill. The extended facial
recess approach was modified to expose the modiolus.
Dimensions of the exposed nerve were measured with a tungsten microelectrode
(World Precision Instruments, Inc, Sarasota, Fla) with a shaft diameter of
0.25 mm and a tip diameter of 1 to 2 µm, positioned with a calibrated
micromanipulator.
ANIMAL MODEL SELECTION
Animal model selection was performed by trying transbulla and middle
fossa approaches to the modiolar nerve in 1 cadaver pig (Sus scrofa) and 3 cadaver cats (Felis catus).
In each case, the ease of surgery and the origin and insertion of cervical
musculature were identified. Exploratory drilling for the cochleostomy was
performed in all animal cadaver specimens, and dimensions of the exposed nerve
were measured as in the human cadaver experiments. The anatomical features
of the area were reviewed by comparing the cervical musculature and the temporal
bone with data in standard textbooks.9-10
This helped us become familiar with the anatomical features while helping
us select the best animal model.
ANESTHETIZED ANIMAL STUDIES
This part of the study was conducted in 6 cats. Animals were treated
in accordance with guidelines of the Institutional Animal Care and Use Committee,
The University of Utah. Anesthesia was induced using a 1:1 combination of
tiletamine hydrochloride and zolazepam hydrochloride, 9 to 12 mg/kg intramuscularly.
The animal was intubated, and general anesthesia was maintained by 0.5% to
1.5% halothane inhalation. Maintenance of the depth of anesthesia and vital
signs were periodically assessed. Lactated Ringer solution was administered
intravenously, 8 to 12 mL/kg per hour, through an intravenous cannula in the
arm to compensate for blood loss. A warmed water blanket was used to prevent
hypothermia. The surgical site was prepared by shaving the ventral and lateral
part of the neck.
Surgical exploration was performed using the transbulla approach in
the anesthetized cats. An initial auditory-evoked brainstem response (ABR)
was performed to rule out deafness in the cats (the method of performing ABR
is described later). The cat was placed in a specially designed head holder
in a left lateral position, with the head facing anteriorly and extended;
this facilitated surgical access to the tympanic bulla.
Dimensions of the exposed nerve were measured as described in the "Human
Temporal Bone Studies" subsection of this section. We also attempted to measure
the spontaneous electrical activity in the modiolar nerve using a tungsten
microelectrode (0.5-M impedance at 1 kHz) in 21 different sites and
at different depths of the modiolar nerve in one preparation. We were able
to obtain spontaneous action potentials in the nerve at all sites and depths.
The neural activity was recorded differentially between the tungsten microelectrode
and a silver/silver chloride electrode placed in the middle ear as a reference.
The differential signal was amplified 5000 times using a commercially available
system (Neural Signal Acquisition System; Bionic Technologies, LLC, Salt Lake
City). Each channel was high- and low-pass filtered with cutoff frequencies
at 250 Hz and 7.5 kHz, respectively. The filtering reduces noise and minimizes
distortion of the action potential spikes. The filtered signal was digitized
at 30 000 samples per second using a commercial data acquisition system
(Bionic Technologies, LLC), and the data were stored for off-line unsupervised
statistical spike classification using mixture-modeling techniques.11-14 The
sorted spikes were plotted, and results were compared with published spike
characteristics.
Serial ABRs and electrically evoked auditory brainstem responses (EABRs)
were obtained in 3 cats to demonstrate nerve survival throughout the surgery.
Serial measurements of ABRs were performed before surgery, after exposure
of the bulla, and after bullostomy. Electrically evoked auditory brainstem
responses were obtained by placing a ball electrode (Standard Prass monopolar
stimulating probe with a flush 0.5-mm tip diameter; Medtronic Xomed Surgical
Products, Inc, Jacksonville, Fla) on the round window, in the scala, and on
the exposed modiolar nerve. Electrically evoked auditory brainstem responses
were also measured by stimulating a single tungsten microelectrode (0.5-M
impedance at 1 kHz) inserted in the modiolar nerve. For all EABRs, the nerve
was electrically stimulated with a charge-balanced biphasic waveform of amplitudes
ranging from 2 to 350 µA, with 75 microseconds per phase. The pulse
was generated using a stimulator (model S88K; Grass Instrument Division, Astro-Med,
Inc, West Warwick, RI), and the stimulus was delivered by using a pair of
photoelectric stimulus isolation units (model PSIU6; Grass Instrument Division,
Astro-Med, Inc); the nerve was stimulated cathodically, with the return placed
in the clavotrapezius muscle. The ABR and the EABR were measured using a commercial
ABR system (Navigator SE; BioLogic Systems Corp, Mundelein, Ill). The ABR/EABR
measuring electrodes were placed intradermally, as described by Achor and
Starr15; the brainstem response was measured
differentially across the vertex and the base of the auricula, with a distant
ground in the neck. The signal was amplified 100 000 times and averaged
across 1024 stimuli. The resultant waveforms were compared with published
literature, and the waveform peaks were identified by 2 independent observers
(A.N.B. and T.H.). The presence of a consistent positive deflection in waves
I and III was defined as threshold; this was done to avoid the variability
reported in the latency of negative deflection of wave II.16
A dummy UEA was implanted in the exposed modiolar nerve to assess the
possibility of implantation. In each surgical exposure, a UEA of a 4 x
5 (20-electrode) configuration was carefully positioned on the modiolar nerve
using a micromanipulator, and implanted using a process of rapid pneumatic
insertion. The method for pneumatic insertion has been described in detail
elsewhere17 and is briefly described herein.
This is a pneumatically actuated system that was originally developed for
the optimal insertion of a 10 x 10 configuration of the UEA in the cerebral
cortex. The insertion of arrays is accomplished via momentum transferred from
a pneumatically driven piston to the inserter head, which is positioned stereotaxically
against the UEA. The UEA has been previously positioned in the implantation
site. The travel of the inserter head, hence, the insertion depth, is precisely
controlled by mechanical stops. It works by delivering an impulse generated
by a pneumatically driven piston traveling a distance predetermined by a "spacer"
stop. The insertion is accomplished in less than 1 millisecond to ensure complete
implantation. Histological and electrophysiological studies performed in the
cat's nervous system4 demonstrate that this
procedure produces only minimal insult to the tissues. We modified the inserter
head by machining the tip down to a diameter of 1.2 mm to facilitate insertion
of the inserter into the cochleostomy site. We also reduced the insertion
pressure to 20 psi, keeping in mind that the cochlear nerve lacks a tough
epineurium like the cat's sciatic nerve.
HISTOLOGICAL CHARACTERISTICS
A determination of the histological characteristics of the implanted
cochlear nerve was performed in 2 animals to reveal the position of the electrodes.
At the end of the study, the animals were deeply anesthetized and perfused
with the array in place, with formaldehyde acting as a fixative. The array
was left in place while the head was immersed in formaldehyde. This allowed
preservation of the electrode implantation site in the nerve. After a week
in formaldehyde, we removed the array from the nerve and dissected the entire
length of the nerve from the temporal bone. This tissue was embedded in plastic
to facilitate sectioning. Osmium tetroxide staining was used to demonstrate
the lipid-rich myelin sheath of the nerve fibers.
RESULTS
HUMAN TEMPORAL BONE STUDIES
Surgical procedures were performed in 2 human temporal bones to establish
access to the human modiolar nerve and to measure the dimensions of the exposed
nerve. A postauricular incision was made and flaps were raised, the ear was
reflected anteriorly, and a self-retaining retractor was used to expose the
mastoid; a standard mastoidectomy was performed using a high-speed drill;
and the promontory and round window were exposed by the extended facial recess
approach. Next, a cochleostomy was performed by removing the bone of the round
window, exposing the basal turn of the cochlea. The modiolar bone was thinned
with a diamond burr, and the thinned bone was picked with an otologic pick
to expose the nerve. Figure 2 shows
a labeled photograph of the surgical access to the modiolar nerve in the left
temporal bone. The exposure was not significantly more difficult than the
procedure for implanting the CI, as both involve the extended facial recess
approach; while the CI surgery exposes the scala, our technique is modified
to expose the modiolar nerve.
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Figure 2. A completed, modified, extended
facial recess approach in a cadaveric human temporal bone preparation. The
extended facial recess is bound by the curved facial canal.
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Exposure by such a modified extended facial recess approach yielded
a diameter of 2 mm and a length of 3 mm in 2 studies. This was adequate to
implant the 4 x 5 arrays used in this study; however, given the dimensions
of the exposed nerve, it would have been possible to implant 6 x 8 arrays
(48 electrodes).
ANIMAL MODEL SELECTION
The pig was rejected as an animal model because the temporal bone drilling
for a transbulla approach was difficult (the bulla is located more medially
than in cats). The depth of the bulla on exposure of the modiolar nerve created
problems of visualizing the insertion of the array.
A middle fossa approach was rejected in cats and pigs, as this approach
to the modiolar nerve was impractical. In the case of pigs, this surgery was
difficult because of fleshy dorsal cervical muscles and a thick sloping crest
at the back of the skull, formed by the supraoccipital and parietal bones.
There are several significant advantages to the use of cats for these experiments.
The anatomical features of the cat temporal bone are well described,
and the topology of the cat cochlear nerve is well-known18;
this affords a unique opportunity to selectively stimulate fibers for specific
frequencies. This is the most significant advantage of the model.
The present generation of human cochlear prostheses was partly developed
using cat models. Hence, there is much literature relating to the development
and evaluation of an auditory prosthesis in cat models. We may be able to
comparatively evaluate the utility of the UEA as an auditory prosthesis by
using the cat model.
The size of the cat cochlear nerve is comparable to that of the human
cochlear nerve; this is an important factor in choosing the cat model, as
smaller nerves will complicate implantation of the UEA because of size and
access constraints. Logistical and surgical constraints rule out bovine and
simian models.
The cat is a common model used in auditory neurophysiology. There is
a potential that our data and methods can be used to further elucidate the
role of cochlear efferents in auditory neurophysiology.
An incision was made just medial to and parallel with the digastric
muscle in the ventral aspect of the neck. The incision was approximately 7
cm long and located directly over the hollow bulla, the tympanic part of the
temporal bone. The skin, subcutaneous tissue, platysma, and mylohyoid muscle
were incised; the bulla was exposed by blunt dissection between the digastric,
styloglossus, and hyoglossus muscles. Care was taken not to damage the hypoglossal
nerve, the anterior lobules of the sublingual salivary gland, and the internal
maxillary vessels. The lingual artery, which is lateral to the hypoglossal
nerve, was spared. The base of the skull was thus exposed. A 2-cm-diameter
access site in the bulla was made using a rotary burr tool mounted on a dental
drill. Isotonic sodium chloride solution was used for regular suction-irrigation
to prevent dehydration of tissues and heat from the cutting and diamond burr.
The mucosa internal to the bulla was freed with alligator forceps, which allowed
visualization of the round window in the posterolateral wall of the bulla.
This provided access to the cavity of the middle ear without damage to the
tympanic membrane. The location of the round window was verified by identification
of the promontory and the attachment of the stapes to the oval window. The
round window was drilled with a diamond burr under a surgical microscope to
reach the basal turn of the cochlea. The access site was expanded by further
drilling to expose the modiolar bone. The modiolar bone was thinned out, and
the bone fragments were picked with a Rosen needle to expose the modiolar
portion of the VIII nerve. Figure 3
shows a magnified photograph of a typical exposure of the cat modiolar nerve
using the procedure described. The modiolar nerve is seen emerging out of
the basal turn of the cochlea; we expect this portion of the nerve to have
purely auditory fibers.
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Figure 3. The modiolar nerve in cats after
drilling the modiolus (original magnification x40). The previous location
of the round window is indicated (dashed blue line); we drilled through the
round window to expose the basal turn of the cochlea and the modiolar nerve
(dashed yellow line).
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Given these results, the cat was chosen as a good animal model and the
transbulla approach was chosen over the middle fossa approach.
NERVE SURVIVAL
Serial ABRs and EABRs at different stages of the surgery demonstrate
nerve survival during the surgery. Figure
4 shows sample ABRs/EABRs from a cat to suprathreshold stimuli at
different stages of the surgical exposure. The presurgery ABR was obtained
to rule out any hearing loss in the cat. Although the presence of EABRs after
the opening of the modiolus would have been sufficient proof regarding nerve
survival, we recorded EABRs to stimulation at surgical stages before that
to pinpoint the stage of failure and to validate the stimulation and recording
system before modiolar stimulation was attempted. The waveforms corresponded
to the latencies suggested by Achor and Starr15
and to the waveforms published by Black et al19
and Simmons et al.20 The presence of independently
identifiable and consistent peaks in the EABR at each stage of the surgery
suggests that the nerve was intact at each stage. Table 1 shows that the thresholds for stimulation were within limits
of published results for the electrodes used.20-21
The variation in thresholds compared with published data can be attributed
to the different biphasic pulse durations used in the experiments and to intraspecies
differences.
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Figure 4. Sample auditory-evoked brainstem
responses (ABRs) and electrically evoked auditory brainstem responses (EABRs)
to auditory and electrical stimulation in cats. The ABR stimulus was suprathreshold
(150% of the threshold), and the response was averaged over 1024 trials. The
EABRs shown were recorded by presenting suprathreshold biphasic stimuli with
75 milliseconds per phase at each site. Again, responses were averaged over
1024 trials. RW indicates round window.
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Comparison of Stimulation Site and EABR Thresholds*
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In our experiments, the thresholds decrease as we get closer to the
modiolar nerve, with the lowest threshold being for intraneural stimulation
using the single tungsten microelectrode. This indicates that the thresholds
for intraneural stimulation using the UEA are likely to be close to 8 µA,
with a 75-microsecond biphasic stimulation pulse. In all 3 experiments, the
EABR waveforms were never lost. We were able to obtain stable EABR waveforms
from the time of implantation to the time the animal was euthanatized. This
period varied from 4 to 8 hours in the 3 experiments.
We were also able to record spontaneous action potentials in one preparation
when we attempted to record the modiolar nerve activity. Figure 5 shows the spontaneous action potentials recorded from the
nerve. The characteristics of the action potentials were compared with the
typical action potentials in published data,22-23
and we conclude that the spike characteristics are similar to them. Although
this is an anecdotal observation, it also indicates nerve survival after surgery.
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Figure 5. The spontaneous sorted and time-aligned
action potentials recorded from the modiolar nerve of a cat using a single
tungsten electrode. Given the sampling frequency of 30 kHz, the time window
is 1 millisecond.
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DUMMY UEA IN CAT NERVE
The measurement of nerve dimensions was crucial, because we wanted to
show that the size of the cat's modiolar nerve is comparable to that of the
human's modiolar nerve. The dimensions of the exposed nerve would also dictate
the dimensions of the UEA that could be implanted and, hence, the number of
electrodes in the implanted array. The mean ± SD diameter of the exposed
nerve was 1.64 ± 0.07 mm (n = 9), and the mean ± SD length was
2.50 ± 0.11 mm (n = 9), as measured with a tungsten microelectrode
mounted over a micromanipulator. We implanted nonfunctional but otherwise
geometrically accurate UEAs in all exposed modiolar nerves to examine the
feasibility of such implantation. This exposure was adequate to accommodate
a 4 x 5 UEA (20 electrodes) of 400-µm spacing used in the study,
without any insertion difficulty. There was an adequate area in which to work,
and we could gain perpendicular access to the nerve. The inserter spacing
at 1 mm allowed us to insert the array to a depth of 1 mm from the nerve surface.
However, given the exposed nerve dimensions, it should be possible to implant
6 x 10 UEAs (60 electrodes) with 200-µm electrode spacings.
HISTOLOGICAL CHARACTERISTICS
When the animals were humanely killed, a visual inspection was performed
on the site of implantation and of the explanted UEA. In all cases, there
was no visual hemorrhage in the implanted site as evidenced by an unaided
eye examination of the implanted site and by a microscopic examination (with
x40 magnification). None of the explanted UEAs had any broken electrodes,
and the microelectrode morphological features were indistinguishable from
an unimplanted UEA under x40 magnification. This indicates that the
array did not shatter against the medial side of the modiolus on implantation.
Our preliminary histological evaluation of the implanted site did not
reveal any formation of connective tissue. This was expected, as the array
was implanted for the short-term. Electrodes were located inside the nerve
as evidenced by the track. Figure 6 shows a cross section of the nerve taken at the implantation site. It demonstrates
minimal damage to the nerve, as evidenced by the absence of hemorrhage in
the implanted site and by preservation of normal tissue architecture compared
with the proximal control (not shown). The histological features distal to
the implantation site seem normal (not shown), indicating no nerve damage
to the axons going from the implanted site to the brainstem. However, we believe
that the issue of nerve survival can be better investigated using the histological
features of an animal with a long-term implant, and are working toward this
goal.
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Figure 6. Osmium tetroxide–stained
section of a cochlear nerve that was implanted with a short-term 4 x
3 Utah electrode array. The photomicrograph reveals normal fibers, with absence
of hemorrhage into the nerve. Bar indicates 50 µm.
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COMMENT
The feasibility of implanting UEAs in humans and in animal models was
demonstrated in this work. Human temporal bone studies indicated a surgical
access site in which a 20-electrode UEA with 400-µm spacing was implanted.
But the dimensions would theoretically permit up to 48 electrodes to be implanted.
If we use a UEA with 200-µm spacing, we may be able to implant up to
192 electrodes in the human modiolar nerve.
Two surgical techniques were investigated in the candidate animal cadavers
of cat and pig: the suboccipital and transbulla approaches. The cat was chosen
as the animal model, and the transbulla approach was selected. The exposed
modiolar nerve was in the basal turn of the cochlea and, hence, intraneural
stimulation at this site is expected to produce purely auditory sensations.
Serial EABRs obtained during surgical exploration of animals with short-term
implants determined that the nerve survives the procedure. Also, a 20-electrode
UEA with 400-µm spacing can be implanted in live animals. But the dimensions
of the exposed nerve could permit implantation of up to 60 electrodes with
200-µm spacings in the cat's modiolar nerve.
Electrophysiological work should be performed in cats and humans on
nerve VIII using the access site delineated in this article. Such electrophysiological
work will demonstrate the functioning of the implanted UEA. Because EABRs
are an easy and effective assay of whole nerve VIII function, electrophysiological
experiments should involve stimulation of nerve VIII through a UEA implanted
in the modiolar portion of the nerve and recording of EABRs. Single-unit electrically
evoked responses could also be recorded from the primary auditory cortex,
A1. This will allow exploration of the selectivity and sensitivity of such
modiolar nerve stimulation. Future work also has to address the long-term
biocompatibility issues of the implant (ie, tissue reaction, threshold changes,
and the mechanical stability of the implant). Human psychophysical experiments
will allow us to optimize the stimulation variables and refine the system
to transform it into an auditory prosthesis. The experimental setup can also
be used to study cochlear efferents because the implanted UEA can also record
neural activity in the modiolar nerve.
It is anticipated that the present work will lead to the development
of an intraneural auditory prosthesis that is superior to current CIs, by
having lower stimulation thresholds and, thus, lower power requirements and
less cross talk. Also, if the electrodes are more selective than current intracochlear
electrodes, then improved frequency selectivity could be anticipated.
AUTHOR INFORMATION
Accepted for publication March 4, 2002.
This study was supported by the Utah State Center of Excellence Program,
and contract N01-DC-1-2108 from the National Institutes of Health, Bethesda,
Md.
Corresponding author and reprints: Richard A. Normann, PhD, Department
of Bioengineering, The University of Utah, 20 S 2030E, Room 506, Salt Lake
City, UT 84112 (e-mail: Normann{at}m.cc.utah.edu).
From the Department of Bioengineering, The University of Utah (Drs
Badi and Normann), and the Division of Otolaryngology–Head and Neck
Surgery, School of Medicine, The University of Utah (Drs Hillman and Shelton),
Salt Lake City.
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