THE ‘MIVIP’ VISUAL PROSTHESIS FOR OPTIC NERVE STIMULATION
J. Delbeke*,
M.C. Wanet-Defalque*, B. Gérard*, M. Troosters**, G. Michaux*, and C. Veraart*
Université catholique de
Louvain, Neural Rehabilitation Engineering Lab, Brussels *
& NeuroTech S.A.,
Louvain-la-Neuve, Belgium**
SUMMARY
The ‘MIVIP’
visual prosthesis generates visual perceptions well below safety and stimulator
saturation limits. These phosphenes are of reasonably small size and broadly
distributed in the visual field. They can thus be used to convey useful visual
information. Psychophysical evaluations are being performed in order to assess
the implantee’s benefits in the use of the «MiViP» optic nerve visual prosthesis. In a pattern
recognition task, the performance improved regularly with practice with an
increasing score and a decreasing delay to recognition. These observations open
the way towards an evaluation of general mobility improvement with the portable
system. In conclusion, the results obtained so far still support the potential
usefulness of the optic nerve visual prosthesis. A low-resolution artificial
vision can be expected from the prosthesis after extensive training.
STATE OF THE ART
The visual cortex has been the stimulation target for the very first
visual prosthesis implanted in a blind person /1/. More recent attempts /2/ rely on high-tech electrodes and improved stimulators but have not yet
reached the level of chronic human implantation. In industrial countries, a
significant number of the late blind patients who might benefit from a visual
prosthesis are suffering from retinitis pigmentosa. It has been shown /3/ that in this disease, a large number of ganglion cells do survive even
when the photosensitive layer of the retina has dyed out completely, thus
leading to complete blindness. This fact has led several teams to explore the
possibility of a direct retinal stimulation, either with subretinal
electrodes, between the retina and the choroids, or epiretinal
devices placed between vitreous and retina /4/. The main advantage of leaving the brain intact is counterbalanced
however by huge technological hurdles that have not been passed yet. As a
simpler and more immediately available technique, a self sizing cuff electrode /5/ was implanted around the optic nerve of a blind volunteer /6/. In this implementation, control of the potential field generated by
four contacts is used to selectively stimulate a fraction of the nerve /7/. The purpose of this study is to provide an update account of the
potentialities of this approach as can be deduced from results obtained in the
first implanted human volunteer.
MATERIALS AND METHODS
A 59 years old lady with retinitis pigmentosa has been implanted with a
self-sizing spiral cuff electrode around her right optic nerve on February
1998. Later, on August 20 2000, the percutaneous lead
was replaced by an implanted stimulator and antenna for telemetry. The silicone
rubber cuff electrode includes 4 platinum contacts of 0.2 mm˛ area. These are
driven by the independent current sources of the implanted stimulator. Biphasic
pulses with charge recuperation have a time resolution of 21.3 µs, and a
current intensity ranging from 10 µA to 3 mA with a non-linear amplitude
resolution. Each stimulator has an output span of ±
8.5 Volts, which thus provides a range of 17 Volts when one of the contacts is
used as an anode in a bipolar montage. Only half that voltage is obtained with
a monopolar montage whereby the stimulator case is
used as reference. Based on data from the literature /8/, /9/, the stimulation strength was kept below 340 nC
for single pulses and 100 nC for 300 Hz pulse trains,
which corresponds to 170 and 50 µC/cm˛.phase respectively.
The optic nerve activation is achieved under control of external
equipment using radio-frequency transmission with a 3 Mbit/s
data rate. This external equipment includes a dedicated head-worn artificial
retina and a digital signal processor. This system extracts significant pixels
from camera images and defines appropriate stimulating conditions of the nerve
susceptible to elicit corresponding phosphenes in real time. Psychophysical
experiments included a pattern recognition task /10/. Fifty simple patterns were used during a ten-session program with
feedback from the instructor. Four evaluation sessions were embedded in the
training program to assess learning improvement.
This project fully complies with the Declaration of Helsinki, and was
approved by the Ethics committee of the School of medicine and University
Hospital of the University of Louvain, Brussels.
RESULTS
Measured electrode impedance values /11/ have been used to calculate the stimulator saturation levels as plotted
for different pulse durations in figure 1. Another trace of the figure
describes the safety limits complied with in this study. As can be seen from
the left part of the figure, the safety level corresponds to the 200 µs monopolar single pulse saturation point, but falls clearly
below that level for longer pulses. The right part of figure 1 gives the
corresponding values for a burst stimulation: 300 Hz safety limit.
Figure 1.
Left panel: Current ranges for
single pulses of various durations. Open squares represent the saturation level
of the stimulator output in a bipolar montage (17 Volts range), open circles
represent the same limit when the stimulator case is used as a reference (8.5
Volts limit). Asterisks stand for the safety limit as gathered from the
literature. Filled triangles represent the typical phosphene perception
thresholds.
Right panel: Current ranges for
pulse train stimulations. All symbols are similar to those of figure 1. The
safety limit and typical threshold traces are obtained here for high frequency
pulse trains.
The perception threshold bounds the lower side of the operational range.
Again, the single pulse results are plotted on the left while the right panel
of the figure holds the values for 17 pulse trains at 160 Hz. Although the
single pulse thresholds are close to the corresponding monopolar
saturation level, a useful range is still available for pulse durations below
200 µs. A much broader stimulation parameter choice is given with pulse trains
but in this case, safety considerations will have to limit the current values
used.
The phosphenes described have areas from 1 to 50 square degrees. Their
central position in the visual field can reach from 35° upwards to 50° downward
and from 30° to the right to 30° left of the vertical meridian. For repeated
stimuli, flicker fusion is observed between 8 and 10 Hz but perception fades
out between 1 and 3 s. A few phosphenes can be produced simultaneously, be it
through the interlacing of the pulses of individual trains. The phosphene
position is stable enough for these summated stimuli to be perceived as
reproducible shapes.
Phosphenes generated by stimuli selected according to
the processed signal from the head-worn camera have allowed successful pattern
recognition. In this task, the volunteer scans a projection screen with head
movements. Between 4 and 24phosphenes have been used in this test with somewhat
better results for the larger number of phosphenes. There is an obvious
learning curve, which tends towards stabilisation over the 12 sessions. When
the system exploits 24 phosphenes, recognition times including the scanning
fall below 1 minute with a score of 60%.
DISCUSSION
The thresholds to single pulses are markedly higher than for burst
stimulations. This is perfectly in line with the already described link between
perception threshold currents and other stimulation parameters /12/. Safety limits abound in the literature /8/, /9/, but they have been obtained in different nervous structures, using
other electrodes and incompatible stimulation regimen. These must thus be
interpreted with caution because all those parameters could make a significant
difference. However, the stability of our long-term results over more than two
years /12/ do suggest that the maximal values selected here are unlikely to have
damaged the optic nerve in any way. In addition to the given limits, single
pulses durations should be kept at 200 µs or less. Saturation can occur with monopolar single pulse stimulation using an 8.5 volts
stimulator output range. Pulse trains are not likely to lead to such a
limitation.
The phosphenes can be assembled in simple patterns /10/. Even very few phosphenes generated according to the output from a
forehead worn camera can lead to satisfactory recognition of simple shapes,
using scanning head movements. Accepting that a visual prosthesis can by far
not be compared with normal vision, the results obtained still hold much promise
when considering the level required to reach usefulness for the totally blind /13/. Evaluation of general mobility improvement with the portable system is
in preparation.
The results thus obtained demonstrate the potentials of the optic nerve
visual prosthesis. A low-resolution artificial vision can be expected but will
require extensive training. The final objective of the project to allow the
volunteer to better cope with her visual environment during mobility and
grasping remains realistic.
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ACKNOWLEDGEMENTS
CEU grants # 22 527 (MiViP) and IST-2000-25145
(Optivip); FMSR grant # 3.4584.98.
AUTHOR’S ADDRESS
Dr. J. Delbeke
Neural Rehabilitation Engineering Laboratory e-mail: delbeke@gren.ucl.ac.be
Avenue Hippocrate, 54, B-1200 Brussels,
Belgium home page: www.gren.ucl.ac.be