1. Field of the Invention
This invention relates to generally to cochlear implants, and more particularly to applying stimulation.
2. Related Art
Sound vibrations that impinge on the outer and middle ear of a person with normal hearing, are conveyed to a tiny shell-like organ called the cochlea where they are spatially dispersed according to their frequency across a vast number of available excitable auditory neurons of the spiral ganglion. Profound deafness often arises when outer, middle or inner ear vibration pathways are severed or when the neural and/or sensory hair cells of the cochlea are damaged.
Implanted hearing prostheses that stimulate the inner ear can be used to assist people with total or partial hearing loss. In general, such devices employ electronic analogue and digital techniques to process, and transform received sound or signals representing sound, into one or more discrete channels of mechanical and/or electrical stimulus information. The resulting stimulus is then conveyed to auditory neurons within the recipient's cochlea associated with the sense of hearing through the central auditory pathways. In this way, persons with severe to profound hearing loss are able to perceive a potentially beneficial approximation of sound. The use of an array of electrodes which are surgically implanted in the recipient's cochlea results in an implant that is unique in every recipient since the final resting place of each electrode is unique to the recipient and the particular spatial distribution of auditory neurons within the recipient's cochlea. The fitting of a coding strategy in a recipient with an implanted hearing prostheses aims to give the individual patient the maximal benefit of the available coding options.
The fidelity of sound perceived by the recipient of an implanted hearing prosthesis is greatly affected by their ability to distinguish pitch and loudness which is severely limited by the relatively small number of discrete electrodes (say 20 or so) which can be fitted within the narrow confines of their cochlea and which are intended to stimulate the available auditory neurons (some 20,000-40,000). The dispersal along the cochlea of applied electrical stimulation to a single electrode further conspires to degrade the recipient's ability to discriminate sounds that are close in frequency. The above processes are further complicated by the need to stimulate multiple electrodes at the same time or at close intervals to better mimic the received audio.
As a consequence, most recipients have difficulty with recognition, perception and appreciation of environmental sounds, speech in noise and music.
Electrical stimulation of the cochlea complicates perception of sound particularly speech in noise and music for the recipient and thereby limits their ability to learn and adapt to their newly acquired prosthetic hearing. This is especially relevant when the recipient is an infant or person with little or no prior experience of sound and its relationship to everyday events. Currently the process of learning to use an implanted hearing prosthesis requires extensive and long-term habilitation provided by health care professionals after the particular physical characteristics of the recipient's cochlear implant are tested and incorporated into the way in which simulation signals are applied to one or more electrodes.
There exist a number of coding schemes and stimulation methods all of which use a combination of current pulses of varying rate and intensity to provide a sensation to the cochlear implant recipient that matches respectively the received pitch (frequency) and loudness of the often quite complex received sound, all with limitations and characteristics that the recipient then lives with or copes with to the limit of their ability and the coding and signaling capability of the implant and its associated audio and signal processing devices.