A cochlear implant is a prosthetic device designed to provide hearing to patients with severe to profound sensorineural hearing loss. A portion of the device, referred to as the ''implant package,'' is surgically placed in the mastoid portion of the temporal bone. An electrode array extending from the implant package is inserted into the cochlea. The remaining portions of the device are worn externally and include a microphone, speech processor, cable, and transmitting coil. Figure 1 illustrates the MED-EL Cochlear Implant System and TEMPO+ Speech Processor.
Sound is collected by the microphone (1) and sent to electronic components within the speech processor (2). The speech processor analyzes the input signal (sound) and converts it into an electronic code. This code travels along a cable (3) to the transmitting coil (4) and is sent across the skin via frequency modulated (FM) radio waves to the implant package (5). Based on characteristics of the code transmitted to the internal device, electrode contacts within the cochlea (6) provide electrical stimulation to the spiral ganglion cells and dendrites extending into the modiolus. Electrical impulses then travel along the auditory nerve (7) and ascending auditory pathways to the brain.
The characteristics and code transmitted by a cochlear implant are dependent, not only on the components of the device itself, but also on the way in which the device is fit. Fitting of a cochlear implant, also referred to as ''programming'' or ''mapping,'' creates a set of instructions (code) that defines the specific characteristics used to stimulate the electrodes of the implanted array. This set of instructions is referred to as the patient's ''program'' or ''map.'' Implant programming is performed by an audiologist with specialized training in the field of cochlear implants. The audiologist uses interactive software and computer hardware to create individualized patient programs that are stored on the hard drive of the computer and downloaded to the memory of the patient's speech processor.
With advances in cochlear implant technology, the fitting process has become more complex. Today's implants offer a number of sophisticated parameters that can be manipulated to improve sound quality and speech understanding. All cochlear implants require at least two levels to be set for each stimulating electrode. These measurements are referred to as the Threshold level (abbreviated as THR or T) and the Most Comfortable Loudness level (abbreviated as MCL, M, or C). Exact terminology and abbreviations are device specific, but the general purpose of these measures is common across all cochlear implants.