1. Field of the Invention
The present invention relates to electrical stimulation of the auditory nerve by an implanted prosthesis in response to detected sound, in a manner which produces a percept which conveys an increased amount of information regarding that sound to the recipient of the prosthesis.
2. Related Art
Cochlear implants have been developed to assist people who are profoundly deaf or severely hearing impaired, by enabling them to experience hearing sensation representative of the natural hearing sensation. For most such individuals the hair cells in the cochlea, which normally function to transduce acoustic signals into nerve impulses which are interpreted by the brain as sound, are absent or have been destroyed. The cochlear implant therefore bypasses the hair cells to directly deliver electrical stimulation to the auditory nerves with this electrical stimulation being representative of the sound.
Cochlear implants have traditionally consisted of two parts, an external speech processor unit and an implanted receiver/stimulator unit. The external speech processor unit has been worn on the body of the user and its main purpose has been to detect the external sound using a microphone and convert the detected sound into a coded signal through an appropriate speech processing strategy.
This coded signal is then sent to the receiver/stimulator unit which is implanted in the mastoid bone of the user, via a transcutaneous link. The receiver/stimulator unit processes the coded signal into a series of stimulation sequences which are then applied directly to the auditory nerve via a series or an array of electrodes positioned within the cochlea. One such cochlear implant is set out in U.S. Pat. No. 4,532,930, the contents of which are incorporated herein by reference.
Several modes or techniques of stimulation have been proposed. One such mode, referred to generally as bipolar stimulation, typically comprises passing current between a pair of electrodes, which in general may be any two electrodes of the electrode array of the implant. All other electrodes in the array are kept ‘floating’ or inactive during such a stimulation. Bipolar (BP) stimulation is also used in a specific sense to refer to stimulation by passing current between adjacent electrodes of the array. A bipolar+1 (BP+1) stimulation mode involves passing a current between two electrodes which are separated by one inactive electrode. Similarly, a bipolar+n (BP+n) stimulation mode involves passing current between two electrodes which are separated by n inactive electrodes.
Another mode of stimulation is referred to as monopolar (MP) stimulation, which involves passing current between an electrode of the implanted array and an electrode outside the cochlea (known as an extra-cochlear electrode or ECE) which for example may be mounted on the receiver/stimulator unit.
Yet another mode of stimulation, known as variable bipolar stimulation, involves use of a single electrode of the implanted array as the return electrode for all stimulations, regardless of the other active electrode.
Still another mode of stimulation, known as common ground (CG) stimulation, involves applying current via one electrode of the electrode array, with all other electrodes of the array being grounded and thus providing a return path for the stimulation.
With improvements in technology it is possible that the external speech processor and implanted stimulator unit may be combined to produce a totally implantable cochlear implant unit that is capable of operating, at least for a period of time, without the need for any external device. In such a device, a microphone would be implanted within the body of the user, for example in the ear canal or within the stimulator unit, and sounds would be detected and directly processed by a speech processor within the stimulator unit, with the subsequent stimulation signals delivered without the need for any transcutaneous transmission of signals. Such a device would, however, still have the capability to communicate with an external device when necessary, particularly for program upgrades and/or implant interrogation, and if the operating parameters of the device required alteration.
Typically, following the surgical implantation of a cochlear implant, the recipient must have the implant fitted or customised to conform with the specific demands of that recipient. This procedure is usually referred to as “mapping” and is the process of measuring and controlling the amount of electrical current delivered to the cochlea, and determining the manner in which the electrical current should be delivered. The mapping process leads to the creation of a program or map that ensures stimulation from the implant provides a patient with comfortable and useful auditory perception, and is essential in ensuring that the recipient receives maximum benefit from the cochlear implant. As the implant system is designed to present acoustic information, in particular speech, to a patient in a useable form, the initial aim of the mapping process is to optimise the information provided for a particular patient.
The mapping process includes identifying a stimulation mode which produces the best speech percept for the recipient. As noted previously herein, there are several possible modes or techniques of stimulation, and at the time of mapping a selection is made as to the most appropriate stimulation mode based on the percept produced. That stimulation mode is then fixed unless and until clinical remapping should become necessary.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.