Method for the electrical stimulation of the auditory nerve and multichannel hearing prosthesis for carrying out the method

In the exemplary embodiments, the sound signals are converted into electrical signals, which are wirelessly transmitted to an implanted receiver, and from the latter, electric stimuli in electrode channels are brought to act on the auditory nerve. The transmission proceeds in the time division multiplex technique in such a manner that the electrical signals are subjected to a pulse modulation and are transmitted in an HF-channel. Thus, the receiver can be reduced with regard to the volume of its construction as well as with regard to its energy requirement. The disclosed method and hearing prostheses for its realization are particularly suited for attending to the needs of the extremely hard of hearing.

BACKGROUND OF THE INVENTION 
The invention relates to methods for the electrical stimulation of the 
auditory nerve, and to multichannel hearing prosthesis for carrying out 
the method. Such methods and prostheses are e.g. known from IEEE Journal 
of Solid-State Circuits, Vol. SC-10, No. 6, December 1975, pages 472 
through 479. 
The advancing miniaturization of electronic circuits has led to the 
manufacture of small stimulation current transmitters which can be 
implanted in the body for the electric stimulation of nerves and muscles. 
In addition to serving the function of the stimulation of the heart 
muscles (heart pacemakers), etc., circuits have also become known which 
are suitable for stimulation of the auditory nerves. The single channel 
and multichannel electrode systems are so constructed that they can 
deliver small stimulation currents. However, they can only be employed for 
their function as a hearing prosthesis for deaf individuals if the inner 
ear is, indeed, non-functional, but if the auditory nerve, including 
higher processing locations for information transmission and information 
processing, is still intact. In the case of deaf individuals with such 
impairments, a small receiver can be implanted in the mastoid. Stimulation 
currents can then be transmitted from the implant via an electrode bundle. 
The signals are generated, in a portion of the apparatus worn outside the 
body, from the sound events which are to be conveyed to the wearer of the 
apparatus, and are transmitted wirelessly via a small transmitter to the 
implanted receiver in order to avoid an electrically conductive connection 
through the skin and the related risks of an infection. 
The invention proceeds from the assumption that there are per se no 
significant restrictions regarding construction and size for the portion 
of the apparatus worn outside the body, i.e., the converter of the sound 
events into transmittable signals, and for the transmitter, whereas, for 
the implanted portion, one must proceed from specific constraints such as, 
for example: 
1. The receiver is to possess a small volume (maximally 2 cm.sup.3), so 
that the electrodes can remain short. Long electrode wires result in 
electric (cross-talk) and mechanical (wire breakage during movement) 
problems. 
2. At least ten to twenty electrodes (according to the application up to 
twenty-four) are to be capable of being provided, which, in the range of 
frequencies of between 100 and 5000 Hz, can deliver stimulation currents 
of at least ten microampers (10 .mu.A), whereby the form of the signals is 
to be freely selectable within wide boundaries, in order that, following 
completed implantation, the optimum stimulation current form can be found 
and adjusted with the patient in experiments. 
The internal resistance of the circuit should be as high as possible in 
order that the current is impressed at the transmission points of the 
electrodes (electrode tips). However, simultaneously, with regard to 
electrolysis, the voltage cannot be permitted to become excessively high 
in order to avoid a damage to the surrounding tissue. If necessary, a 
current source with voltage limitation would have to be provided for this 
purpose. 
3. The separation of the present channels is to amount to at least thirty 
decibels (30 dB); i.e., if a stimulation current J.sub.i is generated at 
an electrode, in the interest of high channel separation, the current 
brought about by J.sub.i at another electrode should be less than J.sub.i 
/32. The fraction with the denominator thirty-two results from the 30 dB. 
A value which is greater would further improve the channel separation; 
i.e., reduce the cross-talk; a value which is smaller would reduce the 
channel separation to low values. 
Moreover, the implanted materials must be compatible with the tissue. The 
materials cannot be permitted to change even after years of implantation. 
SUMMARY OF THE INVENTION 
The object underlying the invention, in the case of a prosthesis such as 
described in the preceding section resides in reducing the outlay 
regarding space and energy requirements and increasing the performance 
reliability. 
If various independent signals are to be transmitted via a communications 
channel (designated in the following as the HF-channel), as a rule, one 
employs the multiplex technique. In the case of frequency-division 
multiplex, the low-frequency information (of the AF-channels) is modulated 
onto various high frequency (HF) carriers. High frequency signals in 
different bands result thereby which must be separated again in the 
receiver, for example, by means of band-pass filters. For a good 
separation of the channels, either filters with a steep drop in response 
outside the pass band or a very broad transmission band are necessary. 
However, both lead to a construction which, given present-day technology, 
is counter to the above-cited constraints. 
According to the invention, therefore, the transmission of the signals is 
carried out in time division multiplex technique, in which a number n of 
AF channels are successively sampled (or scanned) from channel number 1 to 
channel number n. After the n.sup.th channel, again the first channel is 
sampled, etc. The sampling values are successively transmitted via an 
HF-channel in the frequency range of around 100 to 500 kilohertz, in 
particular, 240 kHz. A 5 kHz AF-oscillation, according to the sampling 
theorem, would have to be sampled at least twice; thus, every T=100 .mu.s. 
If a total of twenty-four channels are to be transmitted, there remains, 
for the sampling value of each individual AF-channel, only one time span. 
EQU .DELTA.t=100 .mu.s/24=4.2 .mu.s. 
In the receiver, the individual transmitted AF-channels can then again be 
separated by means of synchronously controlled switches (demultiplexers). 
For good separation of the channels, a short switching time (less than one 
microsecond) is desirable. After the switches, holding capacitors ensure 
"charge storage" for the time between two sampling values. 
In order to connect the AF-signals with the HF-channel, preference is to be 
given to pulse amplitude modulation (PAM) as compared with pulse code 
modulation (PCM). The last-cited PCM method is, indeed, less sensitive to 
attenuations in the transmission path. However, during the coding and 
during the decoding, it requires a greater outlay. Given the present state 
of technology, however, this leads to difficulties with regard to 
requirements of space and data flow. The latter is apparent in that, given 
a specified space in the case of PCM, fewer channels, or a smaller band 
width, or a smaller dynamic range can be transmitted. Moreover, the 
reliability of the apparatus decreases because the number of required 
components is greater. In addition, in the case of PAM-demodulation, a 
voltage-limited current impression is provided without additional 
elements. 
For the construction of the inventive implantable receiver, commercial 
integrated components can be employed. CMOS chips have proven expedient 
which are cemented on 12.times.12 mm.sup.2 ceramic plates. Several of 
these ceramic plates (substrates) can be arranged above one another in 
sandwich construction. A multilayer thick laminar module, known per se, 
connects the highly integrated chips with one another. As assembly 
technique, the ultrasonic wire bond method can be employed. Lines which 
can connect the substrates with one another are designed in the form of 
small metal comb-like conductor arrays. In the case of the selected 
technology, it is possible to manufacture a 24-channel receiver in the 
size of 12.times.12.times.5 mm.sup.3 (without housing). 
A reduction of the receiver can be achieved if monolithic, integrated 
components are employed. However, on the other hand, in particular 
instances, an additional reduction in the volume of the receiver can be 
realized through omission of individual substrates, in case e.g. the 
transmission of only eight or sixteen channels should prove satisfactory 
for speech intelligibility. On the other hand, an increase in the number 
of channels can be achieved through addition of substrates and enlargement 
of the volume. However, the transmission band width is thus reduced, 
because the sampling would have to proceed in larger time intervals. 
On the other hand, the housing exhibits connections for the two receiving 
induction coils, which receive the signals transmitted externally to the 
conversion apparatus via two transmitting coils. Moreover, connections of 
the electrodes proceed out of the housing. The latter are combined into a 
bundle in a manner known per se. Jointly with the housing the bundle of 
electrodes is coated with a tissue compatible material, for example, a 
plastic, such as silastic. 
The dimensions of the electrodes result from a compromise between current 
density and space required for the transmission. A bundle of electrodes 
with approximately twenty individual electrodes should have an overall 
diameter less than the diameter of the auditory nerve. For the individual 
electrodes, thus a diameter of approximately one hundred microns (100 
.mu.m) enters into consideration. This is a value which cannot by any 
means be permitted to be randomly fallen short of, because a stimulation 
current of up to ten microampers (10 .mu.A) must be transmitted which 
already yields a current density of 100 mA/cm.sup.2. If such high current 
densities are generated over a long time, the result can be a destruction 
of the tissue resting against the electrode. In addition to the 
stimulation current density, also the stimulation voltage must be taken 
into consideration. Depending upon the electrode metal and stimulation 
frequency, an electrolysis can occur already at one-half to one volt (0.5 
to 1 V), which likewise leads to the destruction of the tissue. The 
electrodes are so designed that they strike the auditory nerve in the 
inner auditory passage in a manner known per se. 
The band width in each AF-channel amounts to five kilohertz (5 kHz). This 
value is determined by the sampling in the 100 .mu.s-interval. The band 
width of 5 kHz has proven optimum in the case of earlier investigations 
(e.g. German Patent Application 29 08 999.4), although also bandwidths of 
100 Hz to 10 kHz are applicable, depending upon whether a smaller or 
greater outlay is desired. The channel separation is greater than 40 dB 
between adjacent channels (measured values on a prototype). Between 
further removed channels, even values of more than 50 dB were attained. 
The harmonic distortion attenuations were measured, in the case of 
transmissions of a one kilohertz (1 kHz) sine tone, in the range of zero 
to five kilohertz (0 to 5 kHz). Depending upon modulation; i.e., depending 
upon the transmitted voltage amplitude, values resulted of between 30 to 
40 dB. The signal-to-noise ratio, measured in a non-evaluated fashion; 
i.e., linearly, amounts to approximately 60 dB in the frequency range of 2 
Hz to 5.6 kHz. 
Further advantages and details of the invention shall be explained in 
greater detail in the following on the basis of the exemplary embodiments 
illustrated in the Figures of the accompanying drawing sheet; and other 
objects, features and advantages will be apparent from this detailed 
disclosure and from the appended claims.

DETAILED DESCRIPTION 
In FIG. 1, 10 designates a microphone which is connected via a line 11 with 
a signal processing installation 12, which forms a portion of the 
transmitter part 13 of the inventive prosthesis to be worn outside on the 
body. 
In the installation, the signal arriving via 11 is first separated e.g. 
into individual frequency bands by processing component 12 (e.g. same band 
widths and mean frequencies as in German patent application No. 29 08 
999.4), and then e.g. matched by component 12a in its dynamic range to the 
dynamic range of the nerve fibers which lie in the immediate vicinity of 
the respective electrode tip. The allocation of the band-pass filters, 
determining the frequency bands, to the individual AF-channels; i.e., to 
the individual electrodes, can, following completed implantation, be 
carried out individually for each patient in the transmitter in the 
crossbar distributor component 12c (matching to the patient). If 
necessary, it is also possible to connect, between the band-pass filter 
(12a) and crossbar (12c), a pulse shaper (12b), with which the output 
signals of the band-pass filters are variable according to the 
requirements of the patient. Since, only in collaboration with the patient 
can the waveform of the signal, etc., which is optimum for him, be 
ascertained, an implanted receiver, whose data, as a rule, can no longer 
be changed, must be so universally constructed that it can deliver a 
plurality of signal waveforms (i.e., stimulation current waveforms). The 
proposed circuit ideally satisfies this demand on account of its high 
bandwidth of 0 to 5 kHz. The processed signals are, as indicated with the 
lines 14, supplied to a multiplexer 15, which then conducts, in 
chronological sequence, a sampling of the channels supplied via 14, so 
that, via a line 16 and an amplifier 17, the signal to be transmitted is 
supplied to a transmitting coil 18. For controlling the multiplexer 15, 
high frequency generator 19 is provided in the part 13 which effects, via 
a logic circuit 20, the control of the multiplexer 15, as indicated by an 
arrow 21. On the other hand, a forwarding to an amplifier 22 takes place, 
to which an induction coil 23 is connected. As indicated by a broken line 
24, both coils 18 and 23 rest externally against the body of the wearer of 
the prosthesis who is implanted with a receiver referenced with 25. Within 
the body, there is disposed, opposite the coils 18 and 23, a coil 26 and 
27, respectively, so that the electric signals arriving from the coil 18 
are transmitted, on the one hand, and the high frequency signal of 
generator 19 is transmitted, on the other hand. The signals of coil 26 are 
supplied via a line 28 to a demultiplexer 29 in which, synchronously with 
the sampling in 15, a sampling takes place which, in the manner indicated 
by 1 . . . n, delivers signals to electrodes indicated by the arrow 
symbols associated with lines 30. The synchronization proceeds, as 
indicated by a line 31, via the high frequency arriving from coil 27, 
which is processed in a circuit logic component 32, so that the 
synchronization takes place, on the one hand, and--as indicated by the 
outputs designated plus and minus of component 32--the supply of the 
receiver 25 with direct current energy from the transmitted high frequency 
takes place, on the other hand. 
The described PAM circuit comes quite close to the demands for current 
impression with voltage limitation: under the assumption that the holding 
capacitors at 30', which are connected between the electrode-leads 
indicated by arrows 30 (FIG. 1) and ground potential, are charged with 
each sampling value and, in the time between two sampling values, are 
largely discharged. An impression of the mean electrode current results 
##EQU1## 
For low-impedance load resistances (tissue resistances), a higher current 
flows for a short time, and, for high-impedance load resistances, a lower 
current flows for a longer time. However, the current mean value remains 
virtually the same, as long as the load resistance does not exceed a 
specific resistance. However, this is not to be expected in the case of an 
apparatus according to the invention, because metal electrodes of the 
indicated diameter yield low resistances. A limitation of the voltage 
results automatically by the operating voltage of the receiver of 
approximately plus and minus four volts (.+-.4 V), which is rather low 
with respect to a small power consumption. A possibly necessary additional 
limitation of the operating voltage, in order to avoid electrolysis, can 
be readily installed, for example in the form of limiter diodes which 
limit the voltage at the coil 26. 
In FIG. 2, the value K of the sampling of the PAM, which fluctuates between 
K.sub.i and -K.sub.i, is plotted on the abscissa relative to the time t, 
whereby it is shown that, in order to achieve d.c. voltage-free 
transmission in the first third of the time .DELTA.t, which is available 
for the transmission of the channel, the sampling value K.sub.i is 
transmitted. In the second third of the time slot, the negative sampling 
value; i.e., -K.sub.i, is transmitted. Without this measure, the PAM 
signal would not be d.c. voltage-free, and since the transmitter (cf. 18, 
26, FIG. 1) cannot transmit any d.c. voltages, a higher outlay would have 
to be expended during the decoding in order to compensate resulting 
transmission errors. In the last third of the time slot, no voltage is 
then transmitted any longer, so that during this time the multiplexer can 
switch over to the next channel. This measure enlarges the channel 
separation. 
In the receiver 25 the first third of the multiplex time slot for each 
channel is sampled. With this sampling value, via the demultiplexer switch 
29, a holding capacitor 30' is then charged (FIG. 1). The discharge of 
this capacitor via the load resistance R.sub.L, which is formed by the 
tissue bordering on the electrode, then corresponds, by way of 
approximation, to the desired current impression of I.sub.i, if the value 
of the capacitor C is greater than .DELTA.t/R.sub.L. The value of C of 
approximately five hundred picofarads (500 pF) has proven favorable. 
However, the sampling values can also be transmitted with another 
arrangement of the time periods. In comparison with the progression of 
curve 35 of the sampling values according to FIG. 2, curve 36 of FIG. 3 
yields a progression in which first the positive sampling proceeds, then a 
voltage-free section 37, and only following this does the negative portion 
38 occur, which is finally again followed by a voltage-free part 39. 
On the other hand, however, it is also possible, as is indicated in FIG. 4, 
to place the positive portion 40 of the sampling and the negative portion 
41 closely together on the first portion of the sampling in order to 
obtain a longer voltage-free portion 42 separating the channels. 
Another variation can be achieved through differentiation of the sampling 
values, so that the progression illustrated in FIGS. 5 and 6 is obtained. 
As is indicated in broken lines in FIG. 1, the differentiation can proceed 
in a differentiation member 15b. The progression of the sampling values to 
be transmitted then exhibits a steep rise 45 and a peak 46. This is 
followed by a somewhat more gradual drop 47 which, with commencement of 
the negative portion of the sampling value, passes into a steep drop 48, 
which then passes into an increase 49 whose rate of change (slope) largely 
corresponds to the rate of change in the drop 47. 
In the manner illustrated in FIG. 6, the rise 45', indeed, corresponds to 
that referenced in FIG. 5 with 45; likewise, the steep drop referenced 
with 48' corresponds to 48 in FIG. 5. Only the drop 50 and the rise 51 are 
more gradual than those in FIG. 5. 
According to FIG. 5 as well as according to FIG. 6, through the 
differentiation, a signal is attained which is free of d.c. current 
components. For generating differentiated signals in component 15b, a 
circuit 15a can also be so designed that, instead of 1/3 of the time slot 
consisting of +K.sub.i and 1/3 of -K.sub.i, etc., only a switching-on 
and-off of the same signal occurs, respectively; i.e., only +K.sub.i or 
only -K.sub.i occur. A signal corresponding to FIG. 5 can already thus be 
obtained e.g. from the positive portion of the curve 35 (portion of the 
curve 35 of FIG. 2 in the range of +K.sub.i lying above the time axis t). 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel concepts and teachings of 
the present invention.