Classical T/C level fitting for adults
The classical way of setting map level is the following procedure, which is based on single-electrode psychophysics:
The clinician stimulates a specific electrode with a burst of pulses that occurs at the desired rate. This stimulation starts at a very low level (<100 CL (= 100 mA) in the Nucleus devices) and the stimulation is slowly increased in small steps (e.g. 1 dB or 6 Nucleus CL units). The recipient is asked to report any auditory sensation. The stimulation is increased until the recipient hears the sound, then, using finer steps, an up-down procedure (very similar to audiometry) is used to find the threshold of sensation for this burst. The level at the threshold is used as T-level. Unlike classical audiometry, which aims to find the 50% detection point on the psychometric curve, we usually aim for the 100% detection point on the psychometric curve. Once the T level is established, the clinician increases the stimulation in small steps (1 dB) until the recipient indicates that the sound is loud but comfortable. This is used as C-level.
The above procedure is repeated for all electrodes on the array.
When all electrodes are tested, generally a balancing procedure is performed to make sure that all C-levels generate approximately the same loudness, which, in turn, ensures that equal-levelled audio input generates equal-levelled electrical stimulation. For instance, in the 22 channel Nucleus device, balancing is usually performed by stimulating 5 consecutive electrodes in turn at C level or a specific (fixed) percentage of the dynamic range and asking the recipient if one is softer or louder than the others.
When this is done, the implant is put into speech processing mode, and the map is tested with speech input. If needed the clinician can globally lower or raise the T- and or C-levels to adjust the loudness perception of live speech.
Advantages: Precise, each electrode tested, very low chance of over-stimulation.
Disadvantages: Time-consuming, often overly precise (especially for initial fitting sessions as levels may shift quickly in the first weeks). Also, due to the effect of temporal-spatial summation of stimuli, it may be a bit arbitrary to test on single electrodes at a time.
Remarks: Classically, audiologists tend to spend considerable time in setting T-levels, however, research and experience indicate now that C-level, or balance in the upper regions of the dynamic range may be more important for the map quality, both in terms of subjective judgement and objective speech scores.