US 20090024184 A1
A programmable cochlear implant system utilizes multiple-resolution current sources and flexible data-encoding scheme for transcutaneous transmission. In certain embodiments, the number of current sources may be equal to or greater than 2, but equal or less than N−1, where N is the number of electrodes. The multi-resolution current source may introduce offset currents to achieve perceptually-based multiple resolutions with high resolution at low amplitudes and low resolution at high amplitudes. The flexible data-encoding scheme may allow arbitrary waveforms in terms of phase polarity, phase duration, pseudo-analog-waveform, while producing high-rate and high-temporal-precision stimulation. In one embodiment, a 2-current-source system may support simultaneous and non-simultaneous stimulation as well as monopolar, bipolar, pseudo-tripolar, and tripolar electrode configurations.
1. A method for stimulating a plurality of electrodes implanted in a human inner ear comprising:
multiplexing a plurality of current sources in accordance with a stimulation mode set using a command frame;
encoding stimulation data for the stimulation of the plurality of electrodes in a data frame following the command frame, wherein the stimulation data includes electrode address information, phase polarity information and amplitude information;
generating a stimulation pulse based on said stimulation data and using one or more of the plurality of current sources in accordance with the stimulation mode; and
delivering the stimulation pulse to one or more of the plurality of electrodes in accordance with the stimulation data.
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15. A programmable cochlear implant system comprising:
a plurality of electrodes implanted in a human inner ear;
a plurality of current sources multiplexed in accordance with a stimulation mode set using a command frame; and
a processing circuit including a plurality of current sources and electrically connected to the plurality of electrodes, the processing circuit configured to:
encode stimulation data for the stimulation of the plurality of electrodes in a data frame following the command frame, wherein the stimulation data includes electrode address information, phase polarity information and amplitude information,
generate a stimulation pulse based on said stimulation data and using one or more of the plurality of current sources in accordance with the stimulation mode, and
deliver the stimulation pulse to one or more of the plurality of electrodes in accordance with the stimulation data.
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The present invention relates to an implanted auditory prosthesis that utilizes multiple-resolution current sources and a data encoding scheme.
Cochlear implants are electronic medical device to help deaf or severely-hearing-impaired people. They typically consist of an external signal processor, a transmission coil, an implantable package with a receiver coil, a hermetically sealed circuit, and an electrode array. More particularly, these systems include a microphone to receive sounds and convert them into corresponding electrical signals. These electrical signals may then be processed to generate a series of stimulation pulses that are delivered to the inner ear using a series of implanted electrodes. The stimulation of these implanted electrodes allows the implantee to perceive the corresponding ambient sounds.
A typical cochlear implant includes both an external component and an internal component. The external component will typically include a microphone, a speech processor, and a radio-frequency transmitter, while the internal component includes an implanted receiver, a hermetically-sealed decoding circuit, and a series of implanted electrodes. However, there are also numerous other designs currently available. Regardless of the specific configuration, a basic premise of all cochlear implant devices is that ambient sounds are detected by the microphone and a transduced signal representative of this signal is then generated. The transduced signal is then processed by a speech processor in accordance with one of several possible strategies.
One of the primary design considerations for cochlear implants is the current source design. To that end, there are two primary types of current source designs currently in use. The first is to use one current source for all N electrodes, while the second approach is to use N current sources for N electrodes. Some products even use 2N current sources for N electrodes for more flexibility. Each solution has its own advantages and disadvantages. For example, with one current source for all electrodes, the size, complexity, and power consumption of current stimulator are low. But the stimulation mode is also restricted by the ability of one current source. No simultaneous stimulation, current steering, or multi-polar stimulation strategies are supported. For N (or 2N) current sources for N electrodes, more flexibility and functionality of stimulation are achieved at the expense of size, complexity, and power consumption.
Current resolution is an important factor for current source design, especially for low stimulation level. For cochlear implant users, the ratio of current variation versus current Δl/I is more important than the current variation ΔI itself. Traditional linear step-size current source uses a constant ΔI. Therefore, at low stimulation level where I is small, ΔI/I is large. To lower ΔI/I, one solution is to increase the number of current amplitude bit. However, this results in an increase in the number of current sources in the internal circuit and lowers the stimulation rate (8-bit requires 256 unit current sources and 10-bit requires 1024 unit current sources). When the stimulation level is close to the most comfortable loudness (MCL) and the value I is large, ΔI/I is usually too small and the resolution space is wasted. As such, there is a need for an improved cochlear implant which provides a more balanced solution for both complexity and functionality.
Disclosed and claimed herein is a programmable cochlear implant that utilizes multiple current sources and a flexible data-encoding scheme. In one embodiment of the invention, a method for stimulating electrodes implanted in a human inner ear includes the acts of multiplexing current sources in accordance with a stimulation mode set using a command frame, and encoding stimulation data for the stimulation of the plurality of electrodes in a data frame following the command frame, where the stimulation data include electrode address information, phase polarity information and amplitude information. The method further includes generating a stimulation pulse based on the stimulation data and using one or more of the current sources in accordance with the stimulation mode, and delivering the stimulation pulse to the electrodes in accordance with the stimulation data.
Other aspects, features, and techniques of the invention will be apparent to one skilled in the relevant art in view of the following detailed description of the invention.
Described and claimed herein is a programmable cochlear implant system utilizing multiple-resolution current sources and a flexible data-encoding scheme. In one embodiment, the system can support simultaneous and non-simultaneous stimulation as well as monopolar, bipolar, pseudo-tripolar, and tripolar electrode configurations.
One aspect of the invention is a cochlear implant having 2 to N−1 current sources for N electrodes. In one embodiment, the number of amplitude bits are balanced with the current resolution at a low stimulation level. By introducing an offset current circuit consisting of three current sources whose states are controlled by 2-bit range information, a four- range current source may be implemented with four different current resolutions of I, 2I, 4I, and 8I, where I is the minimum reference current. Compared with traditional single range current source, this embodiment improves current resolution by 4 times at low stimulation level, according to one embodiment. In another embodiment, the invention provides high resolution by not overlapping resolution space among different ranges.
Another aspect of the invention is a highly-flexible data encoding scheme to support the aforementioned design of 2 to N−1 current sources. The information for each current source may be modularized and conveniently added or removed from a data frame. With each module, the phase polarity, amplitude, stimulating electrode, and phase duration of electrical stimulation can be individually set for each current source. Therefore, flexible stimulation modes, stimulation strategies, arbitrary pulse polarity, arbitrary stimulation waveforms, flexible pulse duration and inter-phase gap may all be supported. In one embodiment, the range of pulse widths may be from 1 μs to 1024 μs. Similarly, the range of inter-phase gap may be from 0 μs to 31 μs. In one embodiment, the resolution for both may be essentially 1 μs. Finally, a high-rate stimulation mode is supported by a special command specifying only one pulse duration, producing a 31-kHz overall stimulation rate with 1 current source and a 62-kHz rate with 2 current sources.
The timing control for implanted circuit may be an important factor for the normal operation of a current stimulator. While one solution has been to add a local timer in the implanted circuit, this tends to increase the power consumption and lower the reliability of the circuit. Moreover, any minor defect of the timer will cause unpredictable error in the internal circuit and current stimulator. Also, it is very difficult to synchronize the timing between external timer and internal timer. To achieve a reliable synchronization between the two timers, usually a phase lock loop (PLL) circuit is required in the implanted circuit, which itself is complicated and power hungry. Thus, another aspect of the invention is to provide timing from outside such that no timer or PLL circuit is required inside. As will be described in more detail below, with a careful design of bit coding which enables signal level changes at the beginning of each data bit, this data encoding scheme can provide timing information to the implanted circuit, in addition to providing power and data.
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Processing circuit 115 is further depicted as including a data decoder 135 for decoding the incoming data, and mode detector 140 for detecting the mode of the incoming data. Once the data is decoded and the mode detected, the data distributor 145 may use this information to control the current sources 120 1 and 120 2, timing control 150 and electrode selector 155, as shown in
With the 2-current-source configuration of
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The bit coding of the proposed data encoding scheme may be used to provide timing control for internal pulse generation. In this way, the implanted circuit may not require a local timer. After electrode and amplitude information are provided in the beginning of a data frame, the remaining bits in a data frame (e.g., data frame 200) provide pulse width and inter-phase gap information and may also act as clock signal for the timing control of current pulse. Phase extending bits can be added after a data frame to generate long phase duration pulses. The start and end of each phase of a pulse may be synchronized by the onset of bits in a data frame. In one embodiment, each bit may include of 10 to 15 RF cycles to provide cycle error tolerance. It should be appreciated that the proposed timing control may make the implanted circuit more reliable and easier to implement.
As previously mentioned, one embodiment of the coding scheme may support flexible 2 to N−1 current sources for N electrodes. For each current source, one embodiment of the coding scheme has 16 bits in date frame 200 corresponding to it, including 5-bit electrode address information, 2-bit phase polarity information and 8-bit pulse amplitude information, and 1 parity check bit. In this fashion, multiplexing of 2 or more current sources to achieve monopolar, bipolar, and pseudo-tripolar stimulation modes is enabled. The pseudo-tripolar refers to apically or basally applied negative current to sharpen the electric field. A true tripolar will sharpen the field from both sides. In certain embodiments, alternating mono-phasic stimulus may result in the power consumption being at least as good as regular bipolar stimulation. In addition, with 3 or more current sources true tripolar stimulation may be provided.
It should further be appreciated that multiplexing 2 or more current sources may achieve both simultaneous and non-simultaneous virtual channels. To that end, in one embodiment the total number of virtual channel may be at least N+(N−1), where N is the number of electrodes.
In certain embodiments, a pulse (e.g., Pulse1, Pulse2) always starts and finishes within one data frame. After electrode, amplitude and phase polarity information are provided, the width and phase gap bits in a data frame may act as a clock signal for the timing control of present pulse. The start and end of each phase of a pulse may be synchronized by the onset of bits in a data frame.
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In one embodiment, each bit must start with an on-cycle and end with an off-cycle. In this way, the start of a bit may be associated with a rising edge, which can be used as a clock signal to trigger other events in the implanted circuit.
In one embodiment, the flexible coding scheme of the invention may provide a high temporal resolution, with the shortest pulse duration being set to 8 μs and a temporal resolution set to one period of data bit (0.5 μs). This high temporal resolution may also allow accurate encoding of fundamental frequency (F0) and frequency modulated (FM) information.
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In certain embodiments, the stimulation mode may be set in the command frame not in the data frame. As previously mentioned, four of the possible stimulation modes include bipolar, monopolar, pseudo-tripolar and tripolar.
Bipolar stimulation can be implemented by either using one current source multiplexing between different electrodes, or by using two current sources in a monopolar mode. By way of example,
By way of providing another example of bipolar stimulation,
With respect to a monopolar stimulation mode, one embodiment of a command frame 540 and data frame 550 using one current source are depicted in
In the case of a Pseudo-tripolar stimulation mode, one embodiment of a command frame 560 and data frame 570 using one current source are depicted in
An exemplary command frame 580 and data frame 590 for a tripolar stimulation mode are shown in
Finally, a special high-rate stimulation mode can be achieved using a high-rate mode command using a high rate data frame 595, as shown in the embodiment of
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In the practice of programmable current source design, there has been two general approaches. The first approach, which is now largely obsolete, is to control the gate-source voltage to get variable drain-source current. This method, used by first generation cochlear implant products, required that each current source be calibrated due to the fact that the nonlinear VGS-IDS relationship has a large variation among transistors.
The second approach for current source design is to use a linear combination of fixed value current sources to get a desired current value. Usually, a group of high precision fixed value currents is used as reference currents to generate output current values by current mirroring. The majority of current cochlear implant products use this type of current source.
For this type of current sources, to achieve a higher stimulating accuracy, a smaller step size ΔI of current increment is required, under a given current range [Imin, Imax]. Thus, more current amplitude bits B are needed to get a smaller step size, as shown below:
However, more amplitude bits B usually means more unit current sources. For example, B=8 requires 255 unit current sources, and B=10 requires 1023 unit current sources. For an integrated circuit implementation, it is undesirable to have so many current sources, since more chip space is required causing a parasitic effect.
Consider the example of B=8, Imin=0, Imax=2 mA, where there are 256 current levels with a step size of 8 μA. For small stimulations near threshold level T, this step size may be too large such that the actual T level might fall between two current levels. For large stimulations near the MCL, this step size may be too small such that different current levels make no difference to patients, thus wasting limited current levels. Thus, it may be desirable to use a small step size for small currents to get accurate T levels, and a large step size for large currents for adequate sensational variation.
One embodiment of a current source control scheme 700 in accordance with the principles of the invention is depicted in
Referring now to
Table 2 above illustrates that for relatively small currents (e.g., 0-126 μA) the step size is 2 μA. For larger currents (896 μA-1904 μA), the step size is shown as being 16 μA. In the depicted embodiment, the current source uses 8 amplitude bits and a 6-bit DAC to achieve a minimal step size of a 10-bit DAC. Compared with 8-bit DAC, the 6-bit DAC uses 53% less unit current sources, and when compared with a 10-bit DAC, the 6-bit DAC uses 88% less unit current sources.
In certain embodiments, 8-bit 256 level nonlinear step size amplitude control may provide one or more of the following features:
Continuing to refer to
Similarly, the Chn2 data frame is decoded to produce the corresponding pulse for Chn2, as shown in
While the invention has been described in connection with various embodiments, it should be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptation of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within the known and customary practice within the art to which the invention pertains.