Abstract:
An electronic hearing device for stimulating the auditory nerve by way of implanted electrodes wherein sound signals are converted to electrical signals which are divided into separate channels according to their frequencies. The signals of each channel are amplified and applied to the electrodes. The relative amplifications of each channel, after taking into account a threshold amplification, have a ratio of 6 to 7 dB/octave from the channel having the lowest center frequency to the channel having the highest center frequency.

Description:
BACKGROUND ART 
     This invention relates to an electronic hearing device for stimulating the auditory nerve of a human patient by way of implanted electrodes. Such devices are necessary for people who have malfunctioning or non-functioning inner ear mechanisms. 
     There are some prior art devices which receive sound signals, convert said sound signals to electrical signals, divide those signals into channels based on frequency separation, and then apply the outputs of those channels to separate electrodes along the basilar membrane within the cochlea of a human patient. Many, if not most of those devices, involve the conversion of sound signals into digital electronic signals which are applied to the electrodes. Some systems, however, such as U.S. Pat. Nos. 3,751,605 and 4,400,590 apply analog signals representative of sounds to the auditory nerve. 
     In the normal ear, auditory nerve fibers are distributed along the cochlear basilar membrane. Because of the mechanical tuning of this membrane and its associates structures, nerve fibers that innervate near the basal end are sensitive to high frequencies while the more apical fibers are sensitive to lower frequencies. A multichannel device for auditory nerve stimulation that uses a plurality of filters to produce a plurality of component signals having different frequency ranges should distribute these component signals to electrodes in a relationship that is consistent with the frequency analysis preformed by the normal cochlea. Assume, for instance, that four channels of filter processing are used to produce four component signals named S1, S2, S3 and S4 with S1 having the lowest range of frequencies, S2 the next lowest, S3 the next lowest and S4 the highest range. These signals should be distributed to corresponding electrodes E1, E2, E3, E4, where E1 is placed in the cochlea more apical than E2, E2 more apical than E3 and E3 more apical than E4. Ideally, the position of E1 in the cochlea would correspond to the position of auditory neurons that are most sensitive to the center frequency of the channel that produces S1. Likewise for the other channels and their corresponding electrode positions. 
     It has been discovered that the amount of amplification necessary for each of the frequency channels is not necessarily uniform. In the Michelson Pat. No. 4,400,590, the gains of the respective channels are empirically adjusted to meet the particular needs of the user. This is an extremely difficult task since the user has no basis of comparison by which to tell the proper adjustment. Also, the adjustment is difficult to make with regard to discriminating speech sounds, which vary widely in frequency content. 
     SUMMARY OF THE INVENTION 
     The above and other problems of prior art hearing devices are overcome by the present invention of a multi-channel hearing device of the type which amplifies electrical signals which are representative of sounds, as, for example, for the transducer, and supplies such signals to the auditory nerve of a user through a plurality of electrodes implanted in the user&#39;s cochlea. The improvement of the present device comprises bandpass filter means for separating the electrical signals into a plurality of component signals having different frequency ranges within the audio spectrum, each of the ranges having a predetermined center frequency, and voltage source to current source converter means connected to the bandpass filter means for separately converting said signals into electrical currents representative of the frequency component signals of the original sounds. These currents are supplied to separate, implanted electrodes. An emphasis circuit, connected in series with the bandpass filter means causes the amplitudes of the frequency component signals, beyond what is necessary to produce a predetermined threshold response in the user, to increase at a predetermined rate of between 3 dB/octave and 9 dB/octave and preferably at about 6 to 7 dB/octave from the lowest center frequency to the highest center frequency of the component signals. 
     The respective center frequencies of the bandpass filter means of the preferred embodiment are approximately 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz. 
     In the preferred embodiment, connected in series with each voltage source to current source converter is an adjustable gain amplifier. Establishing a threshold amplitude for each electrode is accomplished by applying a particular standard stimulus, for example, a 200 Hz biphasic signal to the input of each adjustable gain amplifier and adjusting the gain until some standard threshold, patient response is obtained, for example, the softest audible tone with a 50% accuracy of detection. 
     Accordingly, it is an important object of the present invention to provide a hearing device for stimulating the auditory nerve which provides for high speech discrimination. 
     It is yet another object of the present invention to provide a hearing device for stimulating the auditory nerve which is relatively insensitive to the degree of coupling between the electrode and the auditory nerve. 
     The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of certain preferred embodiments of the invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the hearing device according to the present invention. 
     FIG. 2 is a diagram illustrating the magnitude of frequency response of the respective bandpass filters shown in FIG. 1. 
     FIG. 3 is a graph illustrating the relative channel gain with respect to the logarithms of the channel center frequencies. 
     FIG. 4 is a detailed schematic diagram of the hearing device according to the invention. 
     FIG. 5 is a perspective view of the hearing device of the invention together with a cochlear implantable electrode assembly. 
     FIG. 6 is a diagrammatic view showing the installation of the implanted electrodes in the cochlea of a patient. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now more particularly to FIGS. 5 and 6, the general physical assembly and installation of the hearing aid device according to the present invention is illustrated. A sound processor 10 is connected between a microphone assembly 12 and a percutaneous connector 14. In alternative embodiments, the percutaneous connector 14 could be replaced with a transcutaneous transmitter and a receiver. An electrode assembly 16 is connected to the female portion of the percutaneous connector. As shown in FIG. 6, the electrode assembly 16 is inserted into the cochlea so that the electrodes 20a, 20b, 20c, and 20d are disbursed along the length of the basal membrane. The electrodes may be uniformly spaced or spaced according to the patient&#39;s individual auditory response characteristics to the bandpass center frequencies. One or more electrodes 18 are used as grounds. A particularly satisfactory ground is the temporalis muscle. The remaining electrodes 20a-20d, inclusive, are inserted through the round window, into the scala tympani of the cochlea. The first electrode, namely electrode 20a, is inserted as far as possible, typically 24 mm. 
     Each electrode is formed by flaming a 0.5 mm ball on the end of a platinum wire (99% platinum). The platinum wire is 0.005 inches in diameter and is covered with an insulation of 2.5 mil thick layer of polytetrofluoroethylene (known under the trademark as TEFLON), extruded onto the platinum wire. While a particular electrode assembly has been described, other electrode designs could also be suitable for use with the present invention. 
     Referring now more particularly to FIGS. 1 and 4, the microphone 12 converts the sound signals into electrical signals which are supplied to the input of a first adjustable gain amplifier 22 which controls the level of background sound that is audible. With this gain turned down, only the loudest sounds will be within the capture range of the automatic gain control 24 which is connected to the output of the amplifier 22. When the gain of the amplifier 22 is at maximum, most signals produced by sounds audible to a normal hearing listener will be within the capture range of the AGC 24 and therefore audible to the patient. This control should not be confused with a loudness control which will be discussed hereinafter. The control 22 gives the patient the ability to mute background sounds that are softer than the signal of interest. 
     The automatic gain control 24 automatically adjusts its gain based on the input signal level. If the input signal varies slowly in amplitude within the &#34;capture range&#34; at the AGC, it will appear at the output as a signal of constant level. The characteristics of the circuit are used to perform two functions: 
     1. To reduce the dynamic range of the electrical waveforms used as stimuli to the ear. The dynamic range of the normal ear is more than 100,000. The dynamic range for electrical stimulation is typically less than 10 times threshold. 
     2. In conjunction with the preceding variable-gain stage, the AGC 24 gives the patient the ability to eliminate some background sounds. 
     Because the AGC cannot adjust its gain instantaneously, there is a chance that very sharp, loud sounds may slip through without proper attenuation. A clamp circuit built into the AGC sets a limit on the maximum signal level passed to the output of the AGC circuit 24. 
     The output at the AGC 24 is supplied to the input of a second variable gain amplifier 26 which is essentially the loudness control. When the patient increases this gain, the sounds he perceives become louder. 
     The output of the variable gain amplifier 26 is supplied to the inputs of four separate channels which separate the signals by frequency and individually amplify them. Each of the channels, for example the channel connected to electrode 20a is comprised of a bandpass filter 28a connected in series with a variable gain amplifier 30a. The output of the amplifier 30a is supplied to a voltage source to current source converter 32a which drives the electrode 20a. Similarly, the second channel is comprised of a bandpass filter 28b, a variable gain amplifier 30b, and a voltage source to current source converter 32b which are all connected in series to an electrode 20b. The third channel is comprised of a bandpass filter 28c, a variable gain amplifier 30c, and a voltage source to current source converter 32c which are all connected in series to an electrode 20c. The fourth channel is comprised of a bandpass filter 28d, a variable gain amplifier 30d, and a voltage to current converter 32d which are all connected in series to an electrode 20d. 
     The filters are four-pole bandpass filters with cut-off frequencies as illustrated in FIG. 2. The center frequencies of the bandpass filters 28a, 28b, 28c and 28d are, respectively, approximately 500 Hz, 1K Hz, 2K Hz, and 4K Hz. While the above noted center frequencies are used in the preferred embodiment, they could be modified somewhat in other embodiments. 
     A distinguishing feature of the present invention from the prior art is the relative gains of each of the amplifiers 30a, 30b, 30c and 30d with respect to each other. While in prior art systems, such as that proposed by Michelson, these gains were individually adjusted empirically for each patient, the gains of the present system are adjusted with respect to each other to provide the most efficient discrimination for speech information. The gains are adjusted so that, when measured at the center frequency of the particular channel&#39;s bandpass filter and after being normalized with respect to the electrode&#39;s threshold to a 200 Hz stimulus signal, the gains will increase at approximately 6 to 7 dB/octave. This means that when each channel&#39;s dB gain is plotted versus the logarithm of the channel&#39;s center frequency, the result will be a sloped line 34 as illustrated in FIG. 3. 
     The purpose of converting the electrical signal output from the adjustable gain amplifiers 30a-30d to an electric current is so that the amount of electrical charge delivered to each electrode 20a-20d, respectively, depends only on the stimulation waveform and is not a function of electrode to tissue contact resistance. 
     The normalization procedure is necessary because the contact between the various electrodes and the tissue is not constant and can change depending on the position of the electrode and can also change with deterioration in the surrounding tissue. In order for the relative gain settings described above to provide the most efficient discrimination, each amplifier 30a-30d must first have its gain adjusted to meet a certain minimum threshold level. This threshold level can be determined, for example, by applying a stimulus signal having a predetermined frequency to a given electrode. The amplitude of the stimulus signal is then increased until a predetermined response from the patient is produced. For example, the amplitude is adjusted to produce the lowest audible sound as perceived by the patient. This can be done by the ascending or descending methods, for example. See Eddington, D. K., et al.: Auditory Protheses Research with Multiple Channel Intracochlear Stimulation in Man, The Annals of Otology, Rhinology &amp; Laryngology, Supplement 53, Volume 87, Nov.-Dec., 1978, No. 6, part 2, p. 17. After adjustment for the threshold level, the gains are then further adjusted according to the 6-7 dB/octave ratio. 
     Consider for example the case where all the electrodes 20a-20d have the same threshold characteristics for a 200 Hz stimulus. Such a stimulus signal may be a sinusoidal or, for example, a biphasic, pulsatile, current waveform of 50 micro-amperes (peak to peak) and 0.25 ms pulse duration. 
     Assume that each electrode requires no individual threshold gain adjustment, i.e. that all have the same threshold. For this situation, the proper gain adjustments for each channel would result in the following measurements at the outputs of the four channels: 
     
                       TABLE I______________________________________BAND-PASS                       OUTPUT SIGNALCHAN-  INPUT SIGNAL        AMPLITUDENEL    AMPLITUDE   FREQUENCY   AS ADJUSTED______________________________________28a    1 volt       500 Hz      50 μA28b    1 volt      1000 Hz     100 μA28c    1 volt      2000 Hz     200 μA28d    1 volt      4000 Hz     400 μA______________________________________ 
    
     This reflects the relative gains increasing by 6 dB/octave. 
     Now suppose that the threshold of electrode 20c to a 200 Hz stimulus signal increases from 50 μA to 200 μA (by a factor of 4). If the relative channel gains are left as depicted in TABLE I, the &#34;effective&#34; stimulus at electrode 20c would not be the same. The gain of channel must be adjusted upwards to reflect the high threshold of electrode 20c. It must, in fact, be increased by a factor of 4. This setting results in a relationship between the channel gains of 6 dB/octave, normalized for the relative thresholds of the electrodes: 
     
                       TABLE II______________________________________BAND-PASS                       OUTPUT SIGNALCHAN-  INPUT SIGNAL        AMPLITUDENEL    AMPLITUDE   FREQUENCY   AS ADJUSTED______________________________________28a    1 volt       500 Hz      50 μA28b    1 volt      1000 Hz     100 μA28c    1 volt      2000 Hz     800 μA28d    1 volt      4000 Hz     400 μA______________________________________ 
    
     This process for a 6 to 7 dB/octave, normalized relationship can be stated mathematically in the following manner: 
     Let 
     Gi=gain of the channel associated with electrode i. 
     Ti=the threshold measured at electrode i with a standard stimulus (say 200 Hz, biphasic pulsatile waveform). 
     Fi=the center frequency of the bandpass filter associated with electrode i. 
     Ga=gain of channel a. 
     Fa=bandpass center frequency of channel a. 
     Ta=threshold measured at electrode 20a with the standard stimulus. ##EQU1## 
     While in the above-described embodiment each channel was provided with a bandpass filter whose output was supplied to the input of a variable gain amplifier, it should be apparent that, in other embodiments, the series arrangement of the bandpass filters and the individually adjustable gain controlled amplifiers could be reversed so that the bandpass filters were interposed between the adjustable gain amplifiers and the voltage source to current source converters. 
     In another embodiment, the adjustment of the relative mean amplitudes of the frequency components of the electrical signals representing the sound signals does not take place in the separate channels defined by the series circuits of elements 28a-32a, 28b-32b, 28c-32c, 28d-32d, respectively. Instead, the amplifier 26 is designed to have a pre-emphasis of 6 to 7 dB/octave, i.e. the response of the amplifier is frequency dependent so that more gain is given to the higher frequencies at the rate of 6 to 7 dB/octave from the lowest center frequency of 500 Hz to the highest center frequency of 4000 Hz. Thereafter, the bandpass filters 28a-28d, inclusive, separate the frequency components and apply them to the separate electrodes as described above. In this embodiment, the only adjustment made to the gains of amplifiers 30a-30d, inclusive, is the threshold adjustment described above. In still other embodiments, this desired pre-emphasis can be obtained by passive circuits comprised of known filtering networks. 
     While it was found that the best results were obtained by adjusting the relative amplitudes to follow a 6 to 7 dB/octave ratio, in other, less effective embodiments, this range could fall within 3 dB/octave to 9 dB/octave. As the relative ratio falls towards these extremes, however, the ability to recognize speech signals deteriorates significantly. 
     In the above embodiments, only four electrodes and four bandpass filters are shown and described, with the center frequencies of the bandpass filters 28a-28d being spaced apart by whole octaves. It should be understood that in other embodiments more electrodes and more channels could be used. In such case, the center frequencies of the bandpass filters would be uniformly distributed over the logarithmic frequency range of at least 500 Hz to 4000 Hz and the emphasis given to the amplitudes would be at the above described ratio of 6 to 7 dB/octave, taking into account the less than whole octave separation of the channels. Thus, the same relationship between channel gain and channel center frequency as depicted in FIG. 3 would be maintained. 
     While the invention has been described in terms of a device for detecting and hearing sounds, it should be apparent that the microphone 12 could be replaced by, for example, a connection to a telephone, a radioreceiver, or any other source of electrical signals representative of sounds. 
     The terms and expressions which have been employed here are used as terms of description and not of limitations, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.