2Anatomy of the MAP: Thresholds, Comfort and Dynamic Range

TThreshold (T) levels

The threshold of stimulation, the T level, is the least amount of electrical stimulation a recipient can reliably detect on a given electrode, measured with an ascending and descending procedure. Definitions differ by manufacturer: Advanced Bionics and Cochlear use a T level (Advanced Bionics at 50% detection, Nucleus at 100% detection), while MED-EL uses THR, the highest level that is just sub-audible. The programming signal is usually a train of biphasic pulses with a 300 to 500 ms on/off duty cycle, perceived by most users as two to three beeps, and recipients are asked to raise a hand or count the beeps. Setting T too high can produce a constant buzzing, humming, or bacon-frying percept, especially in CIS strategies where one channel is always stimulating, while setting it too low leaves soft sounds inaudible.[2020][1994][2002]

TComfort (C / M) levels

The upper stimulation level is the maximum current the implant will deliver, set so high-level sounds are loud but not uncomfortable; Nucleus calls this the C level (loud but comfortable), Advanced Bionics the M level (most comfortable), and MED-EL the MCL. Comfort levels critically shape speech recognition, sound quality, and a user's ability to monitor their own voice, so an inappropriate upper level reliably degrades outcomes. After loudness balancing two channels at a time, the C-level profile across the array is usually flatter, with less variation, than the T-level profile. The electrically evoked stapedial reflex threshold correlates strongly with C level and is routinely used as an objective guide to where comfort levels should sit.[2020][2002]

CThe electrical dynamic range

The electrical dynamic range is the difference between T and C levels; in experienced Nucleus recipients it is typically 20 to 60 clinical units, with most around 40 to 50. Long-duration high-frequency loss can narrow the range to 15 to 30 units on basal electrodes, and ranges under 20 units or wider than 60 to 80 units should make the audiologist cautious. Converted to decibels the electrical dynamic range is only about 10 to 25 dB, so manufacturers use audio compression to squeeze the wider input dynamic range, default about 40 to 60 dB, into it. A very narrow dynamic range can sometimes be widened gradually, for example by 2 to 3 units every four to six weeks across a six-month period, provided the recipient tolerates it.[2020][1994]

TCharge, channels and multiple programs

Stimulus intensity is set by charge, the product of current amplitude and pulse width; both can be raised to increase loudness, with each phase of a biphasic pulse typically lasting about 10 to 100 microseconds. Voltage compliance limits the available current, so on high-impedance electrodes the audiologist may have to widen the pulse width to deliver enough charge. Electrode arrays carry roughly 12 to 22 intracochlear contacts; each active contact maps to a frequency channel, and contacts with abnormal impedance (open or short) are flagged and disabled. A single MAP can be stored in multiple processor program slots, for example three programs (P1/P2/P3), letting the user switch between settings such as a quiet-room map and an accessory or telecoil program.[2020][2014]