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Sound-processing strategy for cochlear implantsRelated Patent Categories: Surgery: Light, Thermal, And Electrical Application, Light, Thermal, And Electrical Application, Electrical Therapeutic Systems, Promoting Auditory FunctionSound-processing strategy for cochlear implants description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070043403, Sound-processing strategy for cochlear implants. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a sound processing strategy for use in hearing prosthesis systems, with particular application to cochlear implant systems. BACKGROUND ART [0002] In cases where individuals have experienced sensorineural deafness, the restoration of hearing sensations to such individuals has been achieved through the use of hearing aids and cochlear implants. Cochlear implants in particular have been in clinical use for many years. A wide variety of different speech processing strategies have been employed in order to process a sound signal into a basis for electrical stimulation via implanted electrode arrays. Some systems have focused upon extracting particular acoustic components of the detected sound signal, which are important to the user's understanding of speech, for example the amplitudes and frequencies of formants, and using these as a basis for generating stimuli. Other approaches have also attempted to utilise the generally tonotopic arrangement of the cochlea, so that each electrode corresponds generally to a particular frequency band. [0003] One such approach, commercially used in speech processors sold by Cochlear Limited, is known as SPEAK. In the SPEAK system, the incoming sound signal is processed to provide an indication of the amplitude of the ambient sound signal in each of a predetermined set of frequency channels, and the channels with the largest amplitudes are selected as the basis for stimulation. In other approaches, the outputs of all channels are used to specify the stimulation patterns, rather than just the channels having the highest short-term amplitudes. The channels are defined by the partially overlapping frequency responses of a bank of band-pass filters. The filters may be implemented using a variety of analog or digital techniques, including the Fast Fourier Transform (FFT). The electrodes corresponding to those channels, determined by a clinical mapping procedure, are selected for activation in each stimulation period and are allocated to the channels according to the tonotopic organization of the cochlear. The rate of stimulation is preferably as high as possible subject to limitations imposed by the processing and power capacity of the external processor and implanted receiver/stimulator unit. [0004] The range of electrical stimulus levels is usually determined by psychophysical measurement of threshold and comfortably loud levels on individual electrodes, using fixed-current pulse trains at the same rate as the stimulus cycle rate of the speech processor output. This may be described as per electrode loudness mapping. The problem with this method of loudness-mapping is that is does not take into consideration the effects of loudness summation when multiple electrodes are activated in quick succession, as they generally are in the output of speech processors. [0005] Although most processing strategies activate a nominal fixed number of electrodes per stimulus cycle, it is important to realise that the actual number of electrodes stimulated in individual cycles is a variable subset of this number, depending on the level and bandwidth of the acoustic stimulus at each point in time. To illustrate this point, a low-level acoustic pure tone will lead to activation of a single electrode, and the electrical level on this electrode, must be at least equal to the psychophysical threshold measured individually for that electrode to be audible. In contrast, a low level broad-band noise may activate (for example) eight electrodes in a stimulus cycle. If each of these eight electrodes are activated close to their individual psychophysical thresholds, as may occur with existing systems, then the resultant loudness will not be close to threshold loudness as intended, but will be closer to the maximum comfortable loudness. [0006] This loudness summation leads to the situation that the output of the processor is too loud, even though the individual levels on each electrode do not exceed a comfortable loudness. Various practical methods have been employed to attempt to overcome this problem, including a global reduction of the upper level limit on each electrode, or the use of complex input signals to set the range of individual levels across electrodes. These methods, although alleviating the discomfort of implant users for loud sounds, do not address a second important issue, and that is the impact of loudness summation on speech perception. [0007] Amplitude envelope fluctuations of a speech signal provide vital cues for speech perception, especially for those people who are less able to make use of spectral cues in the signal (for example, those with few active electrodes or poor electrode discrimination ability). Therefore it is important that the changes in acoustic intensity from moment to moment in a speech signal are accurately conveyed as the appropriate perceptual loudness changes to the implantee. The present loudness coding methods, whereby the acoustic output of a filter is mapped to a fixed range of electrical levels (however determined) on its corresponding electrode, lead inevitably to a perceptual distortion of the amplitude envelope shape because these methods do not take into account the variations from moment to moment of important aspects such as the number of electrodes activated in each stimulus cycle, and the relative loudness contributions from these other electrodes. In summary, the relative loudness of electrically stimulated hearing using present approaches does not accurately convey the relative loudness that a normally-hearing person would hear for the same acoustic input. As well as distorting the perception of the amplitude envelope of the acoustic signal, this effect will lead to narrow-band signals being masked by lower-level broad-band noise, thus disrupting the ability of implantees to understand speech in background noise. [0008] Whilst the SPEAK approach has proven successful clinically, it is an object of the present invention to improve sound processing strategies so as to enhance intelligibility of speech and other sounds, for users of cochlear implants. It is a further object of the present invention to improve the perception of loudness provided to users of cochlear implants. SUMMARY OF THE INVENTION [0009] Broadly, the present invention relates to applying models of sound perception for normal hearing in the sound processing scheme to improve the control of loudness and to provide additional information about sound signals, while ensuring the implant users perceive signals of appropriate loudness. In one aspect the present invention relates to applying shaping algorithms to the amplitudes of the channels after initial analysis to allow further processing of the amplitudes in each channel. The shaping algorithm takes into account the relative importance of each channel for speech perception by normal hearing listeners. The purpose is to not merely emphasise the channels with the largest amplitudes, but to also apply a selective weighting towards those most important for speech perception. [0010] According to another aspect, the input sound signal is processed to determine an overall loudness estimate for a hypothetical listener with normal hearing. After the parameters of the electrical stimuli are determined according to the stimulation scheme employed, the loudness perceived by the implant user with the proposed stimuli is estimated, based on parameters including the previous stimuli applied, and relevant characteristics of the patient's auditory perception with electric stimulation which have been previously determined clinically. If the loudness of the proposed stimuli is not the same as that for a normally hearing person within a predetermined range, then the stimuli are adjusted and the loudness estimated again, until the range is met. This may be termed normalising the overall loudness as perceived by a listener using electric hearing. Preferably, the input values for the electrical stimuli are initially determined using an established sound processing scheme for cochlear implants, such as the SPEAK scheme described above. However other schemes may also be employed, including schemes which generate simultaneous or analog patterns of stimulation, rather than stimulation using sequences of rectangular pulses which do not overlap in time. The intention of this approach is to adjust the electric stimulation so that the overall loudness of the user's percept is comparable to that of a normal hearing listener for the same input sound signal, including taking account of the specific user's characteristics. This approach is particularly applicable when it is desired to produce overall loudness which is not identical to that perceived by a hypothetical normal hearing listener, but has a predetermined relationship to normal loudness. For example, it may be desirable to compress the range of loudness levels perceived using the implant compared with the normal range so as to reduce the effects of background noise, or to enhance speech intelligibility. This approach also provides signals which better emphasise the signals known to be most important to speech perception in normally hearing people. [0011] The present invention also attempts to provide a scheme that can improve the control of not just the overall loudness of signals perceived by cochlear implant users but also to improve the control of the relative loudness of signals presented to implant users, particularly the relative loudness of different components of speech signals, such as phonemes. This is done by using a version of the present scheme which controls the distribution of loudness contributions across frequency or across cochlear position, rather than controlling the overall loudness. This aspect will be described in more detail below. [0012] The above approaches allow for improvement when used with existing implants and processors. However, with an increasing trend to provide more electrodes for possible stimulation, to allow for a higher rate of stimulation, to provide for multiple simultaneous or near simultaneous stimuli, and to provide for stimulation using waveforms that are continuous in time (ie analog stimuli), the above approaches become more important. As the stimulation environment becomes more complex, it is increasingly important to control the overall perceptual effects of stimulation, such as the loudness perceived by the user, to ensure that the full benefits of stimulation with a cochlear implant can be obtained. The present invention will have increased application as more complex schemes are implemented. DETAILED DESCRIPTION [0013] Illustrative embodiments of the present invention will now be described with reference to the accompanying figures, in which [0014] FIG. 1 is a block diagram of prior art arrangement; [0015] FIG. 2 is a block diagram of a first implementation; and [0016] FIG. 3 is a block diagram of a second implementation. [0017] The present invention will be described with reference to particular approaches to speech processing. However, it will be appreciated that the present invention can be applied to many different speech processing strategies, as an addition to assist in providing an outcome where the percepts provided to the user are optimal, particularly when many stimuli are applied within a short time interval and/or to multiple electrode positions. Further to this, each aspect of the present invention can be applied to existing speech processing schemes either separately or in combination to enhance the operational characteristics of such schemes. [0018] The scheme described below incorporates the various aspects of the present invention discussed above. The scheme is similar in many respects to the SPEAK system used commercially by Cochlear Limited, and an understanding of the operation of SPEAK will assist in understanding the present invention. The principles of this system are also described in U.S. Pat. No. 5,597,380, the disclosure of which is incorporated herein by reference. [0019] FIG. 1 is a functional block diagram showing the main components of a typical existing sound processor for cochlear implants. For convenience, a digital implementation is discussed, however, it will be appreciated that analog (or combined analog/digital) implementations of sound processors are also practical, and are not excluded from the field of the invention. [0020] In FIG. 1, the input is from a microphone 11, via a preamplifier stage 12 to an analog-to-digital converter (ADC) 13. The spectral analysis block 14 is generally implemented by a Fast Fourier Transform (preceded by appropriate windowing of the sampled time-domain input signals), or by a bank of band-pass filters. The output of the spectral analysis block is a set of short-term estimates of the level in a number of discrete or partially overlapping frequency bands. In some existing processing algorithms such as SPEAK or ACE (Advanced Combination Encoder--a SPEAK derivative that uses a generally higher stimulation rate), amplitude information from only a subset of the analysis bands is passed on to the following processing stage. This subset includes the bands with the highest amplitudes. In other schemes, such as CIS (Continuous Interleaved Sampling), amplitudes from all bands are used. However, CIS processors typically have fewer analysis bands than SPEAK or ACE processors. The subsequent processing block 15 converts the amplitude data representing the input spectrum into levels of electric pulses appropriate for the cochlear implant user. A number of user-specific parameters 16 are required. These include data to `map` the analysis-band frequencies onto the available intracochlear electrode positions; the current level and/or pulse width for stimulation on each electrode that is just detectable by the user (the `T-level`); and the current level and/or pulse width for stimulation on each electrode that is loud but not uncomfortable for the user (the `C-level`). Continue reading about Sound-processing strategy for cochlear implants... 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