# Patent

## Title
Encoding fine time structure in presence of substantial interaction across an electrode array

## Abstract

A method for reducing the effects of decreased resolution in a cochlear implant worn by an individual includes 1) determining the value of the individual's resolution region, 2) analyzing a plurality of dominant components within a sound signal, 3) removing a component that has lesser associated energy of two or more components that are within the resolution region of one another, thereby producing a sound signal with a reduced pattern of components, and 4) transmitting the reduced pattern signal to an array of electrodes associated with the cochlear implant.

## Background
TECHNICAL FIELD
This disclosure relates to, systems and methods for stimulating the cochlea, for example, systems and methods for encoding the fine time structure components of a sound signal in the presence of substantial interaction across an electrode array.
BACKGROUND
Prior to the past several decades, scientists generally believed that it was impossible to restore hearing to the deaf. However, scientists have had increasing success in restoring normal hearing to the deaf through electrical stimulation of the auditory nerve. The initial attempts to restore hearing were not very successful, as patients were unable to understand speech. However, as scientists developed different techniques for delivering electrical stimuli to the auditory nerve, the auditory sensations elicited by electrical stimulation gradually came closer to sounding more like normal speech. The electrical stimulation is implemented through a prosthetic device, called cochlear implant, which is implanted in the inner ear to restore partial hearing to profoundly deaf people.
Such cochlear implants generally employ an electrode array that is inserted in the cochlear duct, usually in the scala tympani. One or more electrodes of the array selectively stimulate different auditory nerves at different places in the cochlea based on the pitch of a received sound signal. Within the cochlea, there are two main cues that convey “pitch” (frequency) information to the patient. There are (1) the place or location of stimulation along the length of a cochlear duct and (2) the temporal structure of the stimulating waveform. In the cochlea, sound frequencies are mapped to a “place” in the cochlea, generally from low to high sound frequencies mapped from the apical to basilar direction. The electrode array is fitted to the patient to arrive at a mapping scheme such that electrodes near the base of the cochlea are stimulated with high frequency signals, while electrodes near the apex are stimulated with low frequency signals
The position of each electrode is not very precise. That is, there are only a limited number of electrodes, e.g., numbering about 16 to 24 electrodes, spread along the length of the electrode array, inserted into one of the spiraling ducts of the cochlea. Hence, accurately mapping to a “place” within the cochlea can be difficult, as the mapping is limited by the resolution of the discretely placed electrodes.
In a conventional cochlear implant, an envelope is extracted in each channel, and the remaining information, i.e., fine structure, is discarded. Given the number of channels in current processors, information in the fine structure can be very important for hearing certain sounds, particularly music. In previous disclosures, methods for encoding the fine time structure have been proposed. In these methods, stimulation is presented on one or more virtual or physical channels that has been optimally selected based on the estimate of the fine structure in each analysis band. However, these methods do not take into account the recent findings that non-simultaneous stimulation of nearby physical or virtual electrodes is perceived as a single pitch. For example, if in one band, the desired stimulation location is that corresponding to 1000 Hz, and in an adjacent band the desired location corresponds to 1300 Hz, then if both are present, the subject might perceive a pitch corresponding to 1150 Hz. This is known as decreased resolution, and it has a negative impact on the performance of current implants that seek to improve hearing by increasing the number of electrodes or using virtual electrodes to increase the number places to stimulate on a cochlea.
SUMMARY
The present inventors recognized a need to improve the functionality of newer generation implants that utilize increased numbers of electrodes or virtual electrodes. Disclosed are methods and systems for reducing the effects of decreased resolution in a cochlear implant, for example, by encoding the fine time structure of sound signals in the presence of substantial interaction across an electrode array.
In one aspect, a method of reducing the effects of decreased resolution in a cochlear implant includes analyzing an incoming sound within a plurality of frequency bands. One or more dominant components in each frequency band is estimated. The dominant components are analyzed to determine if any two or more are within a resolution region of one another. Of any two or more components that are within a resolution region of one another, the component with the lesser associated energy is removed, thereby producing a sound signal with a reduced pattern of components. The reduced pattern signal is transmitted to an array of electrodes associated with a cochlear implant.
In another aspect, a cochlear stimulation system includes a plurality of analysis channels. Each channel has a filter associated with a frequency band. A processor is in communication with the plurality of analysis channels. The processor estimates the dominant components in each frequency band. The system also includes means for analyzing the dominant components and determining if any two or more are within a resolution region of one another. Additionally, the system includes means for removing the component that has lesser associated energy of two or more components that are within a resolution region of one another, thereby producing a sound signal with a reduced pattern of components.
In another aspect, a method of reducing the effects of decreased resolution in a cochlear implant includes determining the value of an individual's resolution region. A plurality of dominant components within a sound signal are analyzed. A component that has lesser associated energy of two or more components that are within the resolution region of one another is removed, thereby producing a sound signal with a reduced pattern of components. The reduced pattern signal is transmitted to an array of electrodes associated with the cochlear implant.
In another aspect, a method for estimating a resolution region associated with a cochlear implant used by an individual includes providing a first sound having a predetermined frequency. A second sound in close succession to the first sound is provided. The second sound has a predetermined frequency different from the first sound. It is determined if the individual hears two sounds. These steps are repeated until it is determined that the individual does not hear two separate sounds. The second sound has a frequency closer to the frequency of the first sound upon each consecutive repetition of the above steps.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.



DESCRIPTION OF DRAWINGS
These and other features and advantages will be apparent from the following more particular description thereof, presented in conjunction with the following drawings, wherein:
FIG. 1 is a current stimulation waveform that defines the stimulation rate (1/T) and biphasic pulse width (PW) associated with electrical stimuli, as those terms are commonly used in the neurostimulation art;
FIGS. 2A and 2B, respectively, show a cochlear implant system and a partial functional block diagram of the cochlear stimulation system, which system is capable of providing high rate pulsatile electrical stimuli and virtual electrodes
FIG. 3A schematically illustrates the locations of applied stimuli within a duct of the cochlea, without the benefit of virtual electrodes;
FIG. 3B schematically illustrates the locations of applied stimuli within a duct of the cochlea or other implanted location, with the benefit of virtual electrodes.
FIG. 4 shows a representation of sound wave amplitudes as a function of time and a slowly moving envelope.
FIG. 5A shows a graph depicting four frequency bands or windows with dominant FTS components a, b, and c.
FIG. 5B shows a graph depicting eight frequency bands or windows with dominant FTS components d, g, h, and j.
FIG. 6A shows a block functional diagram representing conventional sound processing in a cochlear implant using an envelope extractor.
FIG. 6B shows a block functional diagram representing a system and method in accordance with one aspect, which includes an FTS analyzer/estimator and a current navigator.
FIG. 7 depicts a filter output, which is one frequency band, wherein the dominant FTS frequency component is calculated by averaging the intervals between zero crossings of a waveform or counting the number of zero crossings of a waveform and averaging the result.
FIG. 8A presents a graphical depiction of perception of a sound having multiple frequency components that are within a resolution region.
FIG. 8B presents a graphical depiction of perception of a sound in which the middle component of FIG. 4A has been removed.
FIG. 9 presents a flowchart of a method for reducing the effects of decreased resolution in a cochlear implant.
FIG. 10 presents another flowchart of a method for reducing the affects of decreased resolution in a cochlear implant.
FIG. 11 presents a flowchart of a method for estimating a resolution region associated with a cochlear implant.



Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
Disclosed are systems and methods for encoding the fine time structure of sound signals in the presence of substantial interaction across an electrode array associated with a cochlear implant. An overview of the structure and functionality of an exemplary cochlear implant system is provided below in connection with the description of FIGS. 1, 2A and 2B. The following description is exemplary and the device and methods described herein can be used with other types and other configurations of cochlear implant systems.
FIG. 1 shows a waveform diagram of a biphasic pulse train. The figure defines stimulation rate (1/T), pulse width (PW) and pulse amplitude as those terms are commonly used in connection with a neurostimulator device, such as a cochlear implant, a spinal cord stimulator (SCS), a deep brain stimulator (DBS), or other neural stimulator. All such systems commonly generate biphasic pulses 6 of the type shown in FIG. 1 in order to deliver stimulation to tissue.
A “biphasic” pulse 6 consists of two pulses: a first pulse of one polarity having a specified magnitude, followed immediately or after a very short delay by a second pulse of the opposite polarity, although possibly of different duration and amplitude, such that the total charge of the first pulse equals the total charge of the second pulse. It is thought that such charge-balancing can prevent damage to stimulated tissue and prevent electrode corrosion. For multichannel cochlear stimulators, it is common to apply a high rate biphasic stimulation pulse train to each of the pairs of electrodes in the implant (described below) in accordance with a selected strategy and modulate the pulse amplitude of the pulse train as a function of information contained within the sensed acoustic signal.
FIG. 2A shows a cochlear stimulation system 5 that includes a speech processor portion 10 and a cochlear stimulation portion 12. The speech processor portion 10 includes a speech processor (SP) 16 and a microphone 18. The microphone 18 may be connected directly to the SP 16 or coupled to the SP 16 through an appropriate communication link 24. The cochlear stimulation portion 12 includes an implantable cochlear stimulator (ICS) 21 and an electrode array 48. The electrode array 48 is adapted to be inserted within the cochlea of a patient. The array 48 includes a plurality of electrodes 50, e.g., sixteen electrodes, spaced along the array length and which electrodes are selectively connected to the ICS 21. The electrode array 48 may be substantially as shown and described in U.S. Pat. No. 4,819,647 or 6,129,753, both patents incorporated herein by reference. Electronic circuitry within the ICS 21 allows a specified stimulation current to be applied to selected pairs or groups of the individual electrodes included within the electrode array 48 in accordance with a specified stimulation pattern defined by the SP 16.
The ICS 21 and the SP 16 are shown in FIG. 2A as being linked together electronically through a suitable data or communications link 14. In some cochlear implant systems, the SP 16 and microphone 18 comprise the external portion of the cochlear implant system and the ICS 21 and electrode array 48 comprise the implantable portion of the system. Thus, the data link 14 is a transcutaneous (through the skin) data link that allows power and control signals to be sent from the SP 16 to the ICS 21. In some embodiments, data and status signals may also be sent from the ICS 21 to the SP 16.
Certain portions of the cochlear stimulation system 5 can be contained in a behind the ear (BTE) unit that is positioned at or near the patient's ear. For example, the BTE unit can include the SP 16 and a battery module, which are coupled to a corresponding ICS 21 and an electrode array 48. A pair of BTE units and corresponding implants can be communicatively linked via a Bionet System and synchronized to enable bilateral speech information conveyed to the brain via both the right and left auditory nerve pathways. The Bionet system uses an adapter module that allows two BTE units to be synchronized both temporally and tonotopically in order to maximize a patient's listening experience.
FIG. 2B shows a partial block diagram of one embodiment of a cochlear implant system capable of providing a high pulsatile stimulation pattern and virtual electrodes, which are described below. At least certain portions of the SP 16 can be included within the implantable portion of the overall cochlear implant system, while other portions of the SP 16 can remain in the external portion of the system. In general, at least the microphone 18 and associated analog front end (AFE) circuitry 22 can be part of the external portion of the system and at least the ICS 21 and electrode array 48 can be part of the implantable portion of the system. As used herein, the term “external” means not implanted under the skin or residing within the inner ear. However, the term “external” can also mean residing within the outer ear, residing within the ear canal or being located within the middle ear.
Typically, where a transcutaneous data link must be established between the external portion and implantable portions of the system, such link is implemented by using an internal antenna coil within the implantable portion, and an external antenna coil within the external portion. In operation, the external antenna coil is aligned over the location where the internal antenna coil is implanted, allowing such coils to be inductively coupled to each other, thereby allowing data (e.g., the magnitude and polarity of a sensed acoustic signals) and power to be transmitted from the external portion to the implantable portion. Note, in other embodiments, both the SP 16 and the ICS 21 may be implanted within the patient, either in the same housing or in separate housings. If in the same housing, the link 14 may be implemented with a direct wire connection within such housing. If in separate housings, as described, e.g., in U.S. Pat. No. 6,067,474, incorporated herein by reference, the link 14 may be an inductive link using a coil or a wire loop coupled to the respective parts.
The microphone 18 senses sound waves and converts such sound waves to corresponding electrical signals and thus functions as an acoustic transducer. The electrical signals are sent to the SP 16 over a suitable electrical or other link 24. The SP 16 processes these converted acoustic signals in accordance with a selected speech processing strategy to generate appropriate control signals for controlling the ICS 21. Such control signals specify or define the polarity, magnitude, location (which electrode pair or electrode group receive the stimulation current), and timing (when the stimulation current is applied to the electrode pair) of the stimulation current that is generated by the ICS. Such control signals thus combine to produce a desired spatio-temporal pattern of electrical stimuli in accordance with a desired speech processing strategy.
A speech processing strategy is used, among other reasons, to condition the magnitude and polarity of the stimulation current applied to the implanted electrodes of the electrode array 48. Such speech processing strategy involves defining a pattern of stimulation waveforms that are to be applied to the electrodes as controlled electrical currents.
FIG. 2B depicts the functions that are carried out by the SP 16 and the ICS 21. It should be appreciated that the functions shown in FIG. 2B (dividing the incoming signal into frequency bands and independently processing each band) are representative of just one type of signal processing strategy that may be employed. Other signal processing strategies could just as easily be used to process the incoming acoustical signal. A description of the functional block diagram of the cochlear implant shown in FIG. 2B is found in U.S. Pat. No. 6,219,580, incorporated herein by reference. The system and method described herein may be used with other cochlear systems other than the system shown in FIG. 2B, which system is not intended to be limiting.
The cochlear implant functionally shown in FIG. 2B provides n analysis channels that may be mapped to one or more stimulus channels. That is, after the incoming sound signal is received through the microphone 18 and the analog front end circuitry (AFE) 22, the signal can be digitized in an analog to digital (A/D) converter 28 and then subjected to appropriate gain control (which may include compression) in an automatic gain control (AGC) unit 29. After appropriate gain control, the signal can be divided into n analysis channels 30, each of which includes at least one bandpass filter, BPFn, centered at a selected frequency. The signal present in each analysis channel 30 is processed as described more fully in the U.S. Pat. No. 6,219,580 patent, or as is appropriate, using other signal processing techniques and the signals from each analysis channel may then be mapped, using mapping function 41, so that an appropriate stimulus current of a desired amplitude and timing may be applied through a selected stimulus channel to stimulate the auditory nerve.
The exemplary system of FIG. 2B provides a plurality of analysis channels, n, wherein the incoming signal is analyzed. The information contained in these n analysis channels is then appropriately processed, compressed and mapped in order to control the actual stimulus patterns that are applied to the user by the ICS 21 and its associated electrode array 48.
The electrode array 48 includes a plurality of electrode contacts 50, 50′, 50′′ and labeled as, E1, E2, . . . Em, respectively, which are connected through appropriate conductors to respective current generators or pulse generators within the ICS. Through these plurality of electrode contacts, a plurality of stimulus channels 127, e.g., m stimulus channels, may exist through which individual electrical stimuli can be applied at m different stimulation sites within the patient's cochlea or other tissue stimulation site.
It can be common to use a one-to-one mapping scheme between the n analysis channels and the m stimulus channels 127 that are directly linked to m electrodes 50, 50′, 50′′, such that n analysis channels=m electrodes. In such a case, the signal resulting from analysis in the first analysis channel may be mapped, using appropriate mapping circuitry 41 or equivalent, to the first stimulation channel via a first map link, resulting in a first cochlear stimulation place (or first electrode). Similarly, the signal resulting from analysis in the second analysis channel of the SP may be mapped to a second stimulation channel via a second map link, resulting in a second cochlear stimulation place, and so on.
In some instances, a different mapping scheme may prove to be beneficial to the patient. For example, assume that n is not equal to m (n, for example, could be at least 20 or as high as 32, while m may be no greater than sixteen, e.g., 8 to 16). The signal resulting from analysis in the first analysis channel may be mapped, using appropriate mapping circuitry 41 or equivalent, to the first stimulation channel via a first map link, resulting in a first stimulation site (or first area of neural excitation). Similarly, the signal resulting from analysis in the second analysis channel of the SP may be mapped to the second stimulation channel via a second map link, resulting in a second stimulation site. Also, the signal resulting from analysis in the second analysis channel may be jointly mapped to the first and second stimulation channels via a joint map link. This joint link results in a stimulation site that is somewhere in between the first and second stimulation sites.
The “in-between” site at which a stimulus is applied may be referred to as a “stimulation site” produced by a virtual electrode. Advantageously, this capability of using different mapping schemes between n SP analysis channels and m ICS stimulation channels to thereby produce a large number of virtual and other stimulation sites provides a great deal of flexibility with respect to positioning the neural excitation areas precisely in the cochlear place that best conveys the frequencies of the incoming sound.
As explained in more detail below in connection with FIGS. 3A and 3B, through appropriate weighting and sharing of currents between two or more physical electrodes, it is possible to provide a large number of virtual electrodes between physical electrodes, thereby effectively steering the location at which a stimulus is applied to almost any location along the length of the electrode array.
The output stage of the ICS 21 which connects with each electrode E1, E2, E3 . . . Em of the electrode array may be as described in U.S. Pat. No. 6,181,969, incorporated herein by reference. Such output stage advantageously provides a programmable N-DAC or P-DAC (where DAC stands for digital-to-analog converter) connected to each electrode so that a programmed current may be sourced to the electrode or sunk from the electrode. Such configuration allows any electrode to be paired with any other electrode and the amplitudes of the currents can be programmed and controlled to gradually shift the stimulating current that flows from one electrode through the tissue to another adjacent electrode or electrodes, thereby providing the effect of “shifting” the current from one or more electrodes to another electrode(s). Through such current shifting, the stimulus current may be shifted or directed so that it appears to the tissue that the current is coming from or going to an almost infinite number of locations.
Additional features of the cochlear implant system shown in FIG. 2B involve the rate of electrical stimuli. The system shown in FIG. 2B can allow the current stimuli to be applied to the target tissue at fast rates and in a way that more naturally elicits a stochastic firing of the target tissue, as taught, e.g., in U.S. patent application Ser. Nos. 10/218,645 and 10/218,616, both of which were filed on Aug. 13, 2002, 60/425,215, filed on Nov. 8, 2002, and International Patent Application Serial No. PCT/US01/25861, filed on Aug. 17, 2002. All of these applications are assigned to the same assignee as the present application, and all are incorporated herein by reference.
Next, with reference to FIG. 3A, a diagram is presented to illustrate the location where a stimulus is applied when virtual electrodes are employed. In FIG. 3A, three electrodes E1, E2 and E3 of an electrode array are illustrated. A reference electrode, not shown, is also presumed to be present some distance from the electrodes E1, E2 and E3, thereby allowing monopolar stimulation to occur between a selected one of the electrodes and the reference electrode. Bipolar stimulation could likewise occur, e.g., between electrodes E1 and E2, between E2 and E3, or between any other pair of electrodes.
The electrodes E1, E2 and E3 are located “in line” on a carrier 150, and are spaced apart from each other by a distance “D”. Each electrode is electrically connected to a wire conductor (not shown) that is embedded within the carrier 150 and which connects the electrode to the ICS 21 (see FIG. 2A or 2B). The carrier 150 is shown inserted into a duct 52 within tissue 54 that is to be stimulated. For a cochlear implant system, the duct 52 typically comprises the scala tympani of a human cochlea.
When a stimulus current is applied to electrode E1, the stimulus location in the tissue 54 is essentially the location 56, adjacent the physical location of the electrode E1. Similarly, when a stimulus current is applied to electrode E2, the stimulus location in the tissue 54 is essentially the location 58, adjacent the physical location of the electrode E2. Likewise, when a stimulus current is applied to electrode E3, the stimulus location in the tissue 54 is essentially the location 60, adjacent the physical location of the electrode E3. It is thus seen that the resolution or precision, with which a stimulus may be applied to the tissue is only as good as is the spacing of the electrodes on the electrode array. That is, each stimulus location in the tissue 54 is separated by approximately the same distance “D” as separates the electrodes.
With reference to FIG. 3B, a diagram is presented to illustrate the location where a stimulus is applied when virtual electrodes are employed, specifically by using current steering. The structure of the electrode array and spacing between electrodes E1, E2 and E3 is the same as in FIG. 3A. Thus, when a stimulus current is applied only to electrode E1, the stimulus location in the tissue 54 is the location 56, the same as was the case in FIG. 3A. Similarly, when a stimulus current is applied only to electrode E2, the stimulus location in the tissue 54 is the location 58. Likewise, when a stimulus current is applied only to electrode E3, a stimulus location in the tissue 54 is the location 60. However, through application of current steering, a stimulus current may be shared, e.g., between electrodes E1 and E2 (and some other paired or reference electrode), and the effective tissue location where the stimulus is directed may be anywhere along the line 62 between points 56 and 58. Alternatively, if the current is shared between electrodes E2 and E3, the location in the tissue where the stimulus is directed may be anywhere along the line 64 between points 58 and 60.
To illustrate further, suppose a stimulus current having an amplitude I1 is applied to the tissue through electrode E1 (and some reference electrode). The location within the tissue 54 where the stimulus would be felt would be the point 56. However, if a stimulus current of only 0.9×I1 were applied through electrode E1 at the same time that a stimulus current of 0.1×I1 where applied through electrode E2, then the location within the tissue 54 where the stimulus would be felt would be a little to the right of the point 56, more or less somewhere on the line 62. If the stimulus current applied through electrode E1 continued to be deceased while, at the same time, the current applied through electrode E2 were increased, then the location in the tissue where the stimulus would be directed would move along the line 62 from left to right, i.e., from point 56 to point 58.
Similarly, by concurrently delivering current stimuli at electrodes E2 and E3, the location in the tissue where the effective stimulus would be felt would lie somewhere along the line 64, depending on the weighting of stimulus currents delivered at the two electrodes. This concept of current steering is described more fully in U.S. Pat. No. 6,393,325, incorporated herein by reference.
It is noted that the concept of virtual electrodes which directs a stimulus to a location on the cochlear location or place is broad concept. One method of implementing virtual electrodes is by concurrently delivering stimuli at two or more electrodes. Another way of implementing virtual electrodes is to present alternating stimuli at two electrodes in a time-multiplexed manner. For example, a first stimulus current is presented at the first electrode, then a second stimulus current is presented at the second electrode then, the first stimulus current is presented at the first electrode, then second stimulus current is presented at the second electrode, and so on, in a time multiplexed sequence. The first and second stimulus signals are usually different, e.g., they have different amplitudes and pulsewidths. Such delivery of stimulation will be perceived as if a virtual electrode were delivering a stimulus, which virtual electrode appears to be located between the two physical electrodes.
FIG. 4 shows a representation of incoming sound waves as a function of time that could be picked up by microphone 18 as shown in FIG. 2B. The envelope 110 of the sound provides the slow moving or lower frequency components of the sound. The faster varying components 120, within this envelope 110 of the incoming sound are FTS (fine time structure) components of the sound. The incoming sounds, as represented in FIG. 4, may be deconstructed into frequency bands as shown in FIGS. 5A and 5B. These frequency bands may correspond to bandpass filters BPF1 . . . BPFN shown in FIG. 2B.
FIG. 5A provides an example in which incoming sounds are processed into four separate frequency bands, shown as 40-100 Hz, 100-200 Hz, 200-500 Hz, and 500-1000 Hz. The four frequency bands may represent four distinct stimulation channels, in the instant where n analysis channels=m stimulation channels. To implement the four channel processing (without the use of virtual electrodes0, at least four stimulating electrodes are required to convey the necessary stimuli to the cochlea. The dominant FTS components, a, b, and c occur in three of four frequency bands shown. Minor peaks may be present but are not illustrated here, as they are ignored in processing. The third frequency band does not show any activity.
To convey the identified, dominant FTS in each band, the stimulation system delivers stimulation pulses to three of four electrodes that are placed on the cochlea. The specific amplitudes of each dominant FTS component that is translated to stimulus pulses through the appropriate three of the electrodes can be translated as a perceived sound intensity. This perceived sound intensity can be increased by increasing the stimulation amplitude of a pulse such that more nerve fibers are recruited (captured) at one time. In addition, the frequency of the train of pulses delivered at the electrode can also be increased to recruit more nerve fibers within a fixed time interval. Such an increase in pulse frequency and amplitude can translate to an increase in perceived sound intensity.
FIG. 5B provides another example of an eight-channel system in which the sound frequency spectrum, from 0 Hz to 1 kHz, is divided into eight frequency bands 125, 125′. Dominant FTS components within each frequency analysis band 125, 125′ shown are d, g, h, and j. These dominant FTS components are identified, selected and captured by the cochlear stimulation system. Minor peaks e, f and i are discarded or are not identified. In such an eight-channel system (where n analysis channels=m stimulation channels), there should be at least eight electrodes in the electrode array, each electrode 50, 50′, 50′′ representing one analysis channel 30. As illustrated in FIG. 4B, when the sound spectrum is broken into more frequency bands to capture more FTS, the resolution of the stimulation system is enhanced because it will be able to capture smaller components such as e and f. Thus, it can be appreciated that higher resolution can be obtained by having more analysis channels (BPF1 . . . BPFN), for example, using sixteen different channels employing a sixteen electrode array 48. On the other hand, there is a practical limit to increasing the number of frequency analysis bands that can be used in a cochlear stimulator because of space and power limitations of the device.
It is important to understand however, that even in a system having many analysis channels, e.g., sixteen, there is a loss of FTS information because, while the dominant FTS component, for instance, g, shown in FIG. 4B, is identified within a single frequency band (40-80 Hz), the conventional stimulation system does not further attempt to convey the exact frequency of the FTS component within that band. Rather, it is conveyed generally by the fixed position of an electrode that represents that particular frequency band. It is assumed that the dominant frequency is centered and fixed somewhere in the middle of an analysis frequency band (or band-pass frequency). This limitation is inherent in any conventional cochlear stimulation systems because each electrode is placed in a fixed location with respect to the electrode array and, when the array is implanted, each electrode is fixed with respect to the cochlea. As a result, FTS frequency information is lost.
FIG. 6A shows a partial block diagram of the processing system and method that are presently practiced in conventional cochlear stimulation systems and can be used to illustrate how FTS is lost. The concept of virtual electrodes is not employed in a conventional system. After filtering out the dominant frequency components in each frequency band using an envelope extractor, as shown by the rectangular box 130, the dominant frequency component is conveyed to the respective electrode 50, 50′, 50′′ that corresponds to that particular frequency band. It can be seen that the position of a dominant frequency, for example, component c in FIG. 6A will be positioned on the cochlea, spectrally in the middle of the frequency band between 500 Hz to 1000 Hz, instead of closer to the 500 Hz as shown.
The block diagrams of FIGS. 6A and 6B show that both the conventional cochlear stimulator system and the system of the present invention includes an envelope extractor 130 and 131 for providing the carrier signal, and a linear array of m electrodes, E1, E2 . . . Em, which are represented as 50, 50′, and 50′′, respectively.
FIG. 6B shows, in accordance with the present invention, a block diagram of a cochlear stimulation system, which captures the FTS information in the auditory stimulation signals and conveys this information spatially (spectrally) to the nerves in the cochlea. The system shown in FIG. 6B includes: (a) an envelope extractor 131; (b) a fine time structure (FTS) analyzer/estimator 135; and (c) a current navigator 140 for creating virtual electrodes in order to precisely direct stimuli to various spatial locations (the place) on the cochlea that correspond to the FTS captured and analyzed by the FTS analyzer 135. The task of the FTS analyzer 135 is to precisely estimate the dominant FTS components in each frequency band as, for example, shown previously in FIG. 6B.
Assuming a stimulation system, which has eight analysis frequency bands, the FTS analyzer 135 determines the dominant FTS component within each analysis frequency band. The dominant FTS component within each band is identified, including the dominant component's precise frequency within the band. The other minor FTS components within a frequency band (such as inferior components e and f, in FIG. 5B) can be discarded.
The obtained FTS is then linearly added to the carrier signal obtained from the envelope extractor 131 by the current navigator 140 which processes and spatially directs the presentation of stimuli on the various cochlear places via one or more electrodes 50, 50′, 50′′ such that the peak of the stimulation will be presented more precisely at the cochlear locations (places) that correspond to the FTS dominant frequency components in each frequency band.
FIG. 7 shows a graph, in accordance with the present invention, one method for determining the dominant FTS component (the precise frequency) within a frequency band. The graph shows a sampled time window, TW, 121 of 20 milliseconds sampling sounds within a single frequency band of 125 to 250 Hz. The dominant FTS component within this frequency band can be calculated by averaging the time intervals between successive zero crossings, 122, 122′, 122′′ also labeled in FIG. 6 as Z1, Z2, Z3, Z4, Z5 and Z6 within a time window, TW, illustrated here as 20 milliseconds. The peak FTS estimate is simply 1 over the averaged intervals of time between successive zero crossings. For example, the intervals of time are represented by the time between Z2 and Z1; the time between Z3 and Z2, the time between Z4 and Z3, the time between Z5 and Z4 and the time between Z6 and Z5. Although the example shows a time window, Tw of 20 milliseconds, a time window of between about 10 to 100 milliseconds may be used to smooth the estimate so that stimulation is not perceived as noisy. Another, alternative embodiment of the present method for determining the FTS of the dominant component is to count zero crossings of the sound waves in a predetermined time window. The number of zero crossings is divided by the total duration of the time window to achieve the frequency estimate. For instance, both positive and negative going crossings are counted (in this case seven), then the number of zero crossings is further divided by 2 and then divided again by the total duration of time, TW, 121, which is 20 milliseconds. Sufficiently accurate estimates can be achieved with a time window, TW, that is at least 10 milliseconds long and preferably between about 10 to 100 milliseconds.
Still another alternative method of the present invention for determining the dominant FTS component is somewhat more sophisticated and employs a Fast Fourier Transform to precisely de-construct the fundamental frequency components in a particular incoming sound. The dominant FTS component in a frequency band should be sustained over a time duration of about between 10 to 100 milliseconds. A formant tracker, which can identify the dominant FTS components in a frequency band can be employed. A hardware and software implementation that uses a Fast Fourier Transform may, however, may require greater processing power and, therefore, use more energy and use up more device space compared to the simple method described which needs only detection and counting of zero crossings.
Although fine time structures have been found to be essential for representing musical melodies, the above methods do not take into account the recent findings that non-simultaneous stimulation of nearby physical or virtual electrodes may be perceived as a single pitch. For example, if in one band, the desired stimulation location is that corresponding to 1000 Hz, and in the adjacent band the desired location corresponds to 1300 Hz, then if both are presented, the subject might perceive a pitch corresponding to 1150 Hz. This phenomenon is sometimes referred to as decreased resolution and occurs when one energy/frequency pair is within a “resolution region” of another energy/frequency pair. It has been found that when two or more energy/frequency pairs are within a resolution region of one another, they are harmonically unrelated. It has further been determined that when an interfering energy/frequency pair or sound signal component is removed, the remaining components or energy/frequency pairs are harmonically related and the individual perceives the distinct components rather than an unintended sound or pitch.
Therefore, once the dominant FTS in each of the frequency bands are identified by one of the methods identified above, a determination must be made as to whether or not any two dominant FTS are within a resolution region, and how to minimize interference in the event that they are. FIG. 8A presents a graphical depiction of perception of a sound having multiple frequency components that are within a resolution region. In the example, the stimulus applied could be a FTS current-steered strategy as described above, or any other time domain strategy, frequency domain strategy, or other strategy that analyzes spectral peaks. The sound signal applied has three components 100, 200, and 300, each corresponding to a different frequency. Component 100 has a frequency of 1000 Hz. Component 200 has a frequency of 1300 Hz, and component 300 has a frequency of 1600 Hz. The perception of these separate pitches by a patient having a resolution region of approximately 1⁄3 octave or 300 Hz is depicted by the region 400 of the graph. As shown, the patient does not perceive three separate pitches associated with each of the three frequencies. Instead, the patient perceives a single pitch characterizing a frequency of somewhere between 1000 Hz and 1600 Hz, which is harmonically unrelated to the original frequencies or pitches.
FIG. 8B presents a graphical depiction of perception of a sound in which the middle component of FIG. 8A has been removed. This graph depicts a masked strategy in which the middle frequency has been masked or removed. The difference between the two frequencies (600 Hz) is greater than the resolution region (300 Hz). Therefore, the patient perceives two separate pitches; one associated with a 1000 Hz sound signal and another associated with a 1600 Hz sound signal. The two frequencies are harmonically related.
FIG. 9 presents a flowchart of one method for reducing the effects of decreased resolution in a cochlear implant. Sound is generated and applied at 200. The sound signal is converted to an electrical signal at 205. This step can be performed by a microphone that senses sound waves and converts the waves to corresponding electrical signals, thus functioning as an acoustic transducer. For example, microphone 18 depicted in FIGS. 2A and 2B can perform step 200. An analog-to-digital converter, such as the converter 28 shown in FIG. 2B, associated with a cochlear implant implanted into a patient converts the electrical sound signal to a digital signal at 210. The signal is divided into frequency bands and analyzed at 220. An analyzer estimates the dominant components within each frequency band at 230. The dominant components are further analyzed at 240 to determine if any two components are within a resolution region of one another. This is accomplished by applying a patient-specific algorithm, which is derived by one or more psychophysical techniques, some of which are described further below. If two or more components are within a resolution region of one another, the component with less energy is masked (i.e., removed) at 250 with a masker. This step applies to any two of a potential multitude of components, thus removing one, two, three, four, or more components from the sound signal. An alternative to removing the component is to reduce the amplitude of the component using a masking function. The reduced pattern signal is then transmitted at 260 to the array of electrodes associated with the cochlear implant. Steps 220-260 are performed by a speech processor, such as the speech processor 16, depicted in FIGS. 2A and 2B.
FIG. 10 presents another flowchart of a method for reducing the effects of decreased resolution in a cochlear implant. Sound is generated at 300 and processed at 310 using any one of a multitude of methods known to those of skill in the art, such as using a cochlear model, Fast Fourier Transformation (FFT) based phase and frequency determination, interval-based analysis of filter outputs, and Hilbert-based analysis of filter outputs. A pattern is generated at 320, which can include values for place, timing information, phase, amplitude, and width, which can represent descriptions of peaks of the spectra or the outputs of the analysis bands. The sound can also be resynthesized at this step to detect how much information is available and to determine the consequence of an implant. The pattern generated at 320 may be too detailed to present to the cochlear implant patient. This can be due both to constraints of the implant and/or, constraints of the physiology of the patient. Therefore, features of the pattern that would produce a sound most similar in pitch, loudness, and timbre to the original sound are selected at 330. For example, certain frequencies are removed. In particular, frequencies that are so close spatially to one another that they would merge if presented to the subject and produce the wrong pitch are removed at 330. A reduced pattern is subsequently generated at 340, again including values representative of, inter alia, cochlear place, timing information, phase, and amplitude. Again, the sound can be resynthesized as described with respect to step 320. The pattern is then ready to be converted into an electric signal at 350. In one embodiment, current-steering can be used to generate the desired pattern in the cochlea. Phase delays may be introduced here, and loudness balance would also be performed at 350. Finally, electric stimuli corresponding with the pattern generated at step 350 is transmitted via electrodes to the cochlea at 360.
FIG. 11 presents a flowchart of a method for estimating the resolution region of an individual using the cochlear implants described herein. A first sound or tone is provided at 400. The first sound or tone has a predetermined frequency and is provided at time T1. A second sound or tone is provided at 410. The second sound or tone is provided at a time T2, which is close in time to T1. Next, it is determined whether or not the individual hears or perceives two separate sounds at 420 by having the individual indicate if two separate sounds or tones are heard. If the individual has heard two separate sounds or tones, then the above steps are repeated until it is determined that the individual does not hear two separate sounds, wherein the second sound at 410 has a frequency closer to the frequency of the first sound at 400 upon each consecutive repetition of the above steps. When it is determined that the individual does not hear two separate sounds, the difference in frequency between the first and second sounds is recorded at 430. This is the estimate of the resolution region for the individual.
In another embodiment, sound 1 and sound 2 are provided by interleaving short pulse trains between two electrodes to provide a dual-sensation stimulus. The alternation rate can be larger than a predetermined value, such as 100 Hz. The resulting stimulus is compared to a sensation evoked by a single electrode near the middle of the two previously-stimulated electrodes and it is determined if the dual-sensation stimulus sounds like a multi-component stimulus as compared to the single electrode stimulus. In yet another embodiment, the method can be performed by heuristically adjusting the resolution region until optimal sound quality is achieved while listening to speech or simple music.
The estimated resolution region can be used to calibrate each cochlear implant to eliminate those sound components that interfere with one another. In this way, spectral resolution achievable by each individual or patient is explicitly taken into account.
While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.

## Claims

1. A method for reducing effects of decreased resolution in a cochlear implant implanted in an individual, the method comprising:
analyzing an incoming sound within a plurality of frequency bands;
estimating one or more dominant components in each frequency band;
analyzing the dominant components to determine if any two or more are within a same resolution region, wherein the resolution region of an individual is estimated using a psychophysical method;
producing a sound signal with a reduced pattern of components by removing at least one of the dominant components within the same resolution region having lesser associated energy than another of the dominant components within the same resolution region; and
transmitting the reduced pattern signal to an array of electrodes associated with a cochlear implant.

2. The method of claim 1, wherein two or more dominant components that are within a same resolution region are not harmonically related.

3. The method of claim 1, wherein the reduced pattern signal comprises two or more components that are harmonically related.

4. The method of claim 1, wherein the psychophysical method comprises:
a. providing a first sound having a predetermined frequency;
b. providing a second sound in close succession to the first sound, the second sound having a predetermined frequency different from the first sound;
c. determining if the individual hears two different sounds;
d. repeating steps a-c until it is determined that the individual does not hear two separate sounds, wherein the second sound has a frequency closer to the frequency of the first sound upon each consecutive repetition of steps a-c; and
e. recording the difference in frequency between the first and second sounds when it is determined that the individual does not hear two separate sounds.

5. A cochlear stimulation system comprising:
a plurality of analysis channels, wherein each channel comprises a filter associated with a frequency band;
a processor in communication with the plurality of analysis channels, wherein the processor estimates one or more dominant components in each frequency band;
means for analyzing the dominant components and determining if any two or more are within a same resolution region, wherein the resolution region of an individual is estimated using a psychophysical method; and
means for removing the component that has lesser associated energy of two or more components that are within a same resolution region, thereby producing a sound signal with a reduced pattern of components.

6. The cochlear stimulation system of claim 5, wherein the filter of each channel is centered at a selected frequency of a frequency band.

7. The cochlear stimulation system of claim 5, wherein the means for analyzing the dominant components and determining if any two or more are within a same resolution region comprises a patient-specific algorithm.

8. A method for reducing the effects of decreased resolution in a cochlear implant worn by an individual, comprising:
determining the value of the individual's resolution region;
analyzing a plurality of dominant components within a sound signal;
producing a sound signal with a reduced pattern of components by removing a component that has lesser associated energy of two or more components that are within a same resolution region; and
transmitting the reduced pattern signal to an array of electrodes associated with the cochlear implant.

9. The method of claim 8, wherein two or more dominant components that are within a same resolution region are not harmonically related.

10. The method of claim 8, wherein the reduced pattern signal comprises two or more components that are harmonically related.

11. The method of claim 8, wherein the resolution region of an individual is estimated using a psychophysical method.

12. The method of claim 11, wherein the psychophysical method comprises:
a. providing a first sound having a predetermined frequency;
b. providing a second sound in close succession to the first sound, the second sound having a predetermined frequency different from the first sound;
c. determining if the individual hears two different sounds;
d. repeating steps a-c until it is determined that the individual does not hear two separate sounds, wherein the second sound has a frequency closer to the frequency of the first sound upon each consecutive repetition of steps a-c; and
e. recording the difference in frequency between the first and second sounds when it is determined that the individual does not hear two separate sounds.

13. A method for estimating a resolution region associated with a cochlear implant used by an individual, comprising:
a. providing a first sound having a predetermined frequency;
b. providing a second sound in close succession to the first sound, the second sound having a predetermined frequency different from the first sound;
c. determining if the individual hears two different sounds;
d. repeating steps a-c until it is determined that the individual does not hear two separate sounds, wherein the second sound has a frequency closer to the frequency of the first sound upon each consecutive repetition of steps a-c; and
e. recording the difference in frequency between the first and second sounds when it is determined that the individual does not hear two separate sounds.

14. A cochlear stimulation system comprising:
a plurality of analysis channels, wherein each channel comprises a filter associated with a frequency band;
a processor in communication with the plurality of analysis channels, wherein the processor estimates one or more dominant components in each frequency band, analyzes the dominant components and determines if any two or more are within a same resolution region, and removes the component that has lesser associated energy of two or more components that are within a same resolution region, thereby producing a sound signal with a reduced pattern of components, wherein the processor comprises an associated masker that removes the component that has lesser associated energy of two or more components that are within a same resolution region.

15. The cochlear stimulation system of claim 14, wherein the filter of each channel is centered at a selected frequency of a frequency band.

16. The cochlear stimulation system of claim 14, wherein a patient-specific algorithm associated with the processor analyzes the dominant components and determines if any two or more are within a same resolution region.

17. The cochlear stimulation system of claim 16, wherein the patient-specific algorithm is derived using a psychophysical method.

18. The cochlear stimulation system of claim 17, wherein the psychophysical method comprises:
a. providing a first sound having a predetermined frequency;
b. providing a second sound in close succession to the first sound, the second sound having a predetermined frequency different from the first sound;
c. determining if the individual hears two different sounds;
d. repeating steps a-c until it is determined that the individual does not hear two separate sounds, wherein the second sound has a frequency closer to the frequency of the first sound upon each consecutive repetition of steps a-c; and
e. recording the difference in frequency between the first and second sounds when it is determined that the individual does not hear two separate sounds.