4Setting Threshold (T) Levels

FWhat the T level is and why each maker defines it differently

The T level fixes the floor of the electrical dynamic range so that a low-level input such as 25 dB SPL is just audible and is perceived as soft, mirroring how a quiet sound is soft to a normal-hearing listener. The definition is not uniform across systems: Advanced Bionics treats threshold like an audiometric threshold (detected on about 50% of presentations), Nucleus defines T as the minimum stimulation detected 100% of the time, and MED-EL defines threshold as the highest level that does not yet produce an audible percept. The threshold parameter is labelled T level in Advanced Bionics and Nucleus software and THR or threshold in MED-EL software. The whole 100 dB acoustic intensity range of everyday listening must be compressed into a narrow electrical dynamic range, which is typically only about 10 to 25 dB when converted from stimulation units to decibels. Threshold is measured with a train of biphasic electrical pulses delivered at the user's everyday stimulation rate, usually gated with an on/off duty cycle of about 300 to 500 ms.[2020][2009]

CMeasuring T behaviourally: ascending, descending, and hysteresis

T-level testing usually starts on a low-frequency apical channel because recipients tend to recognise a familiar low-pitched percept, most having had some low-frequency hearing before surgery. After audibility is established, the audiologist brackets threshold with a modified Hughson-Westlake adaptive procedure, the same approach used for audiometric thresholds. Recipients respond at lower levels on descending runs than on ascending runs, a hysteresis effect, so T should be set from ascending responses to guarantee that soft inputs remain audible. Adults and older children are asked to say yes or raise a hand; at least two responses per channel in ascending mode are recommended before accepting a T value. At initial activation, responses are often suprathreshold because the percept is unfamiliar, the recipient has been deprived at that frequency, and the beep can be confused with tinnitus, so larger step sizes early and smaller steps once the patient is practised are reasonable. The count-the-beeps method, presenting one to five pulses and asking the recipient to count them, helps separate the stimulus from tinnitus and from false positives, and gave better access to low-level sound than the audiometric approach.[2020][1994][1995]

TWhy T is less critical in high-rate strategies

Many adults with contemporary Advanced Bionics and MED-EL systems hear soft sounds well even when T is set to zero or estimated from the upper-stimulation level, because the strategy's mapping function delivers audibility for low inputs based on the typical user's dynamic range. As a rule of thumb, T is set near 50% of detection or just below for CIS-type strategies and to the minimum stimulation giving 100% detection for n-of-m strategies. Estimating T from upper levels is a streamlined approach designed to save time, not to optimise soft-sound audibility, so it can leave some recipients with insufficient access to low-level sound. Set too low, T deprives the recipient of soft sounds; set too high, ambient noise becomes intrusive, soft sounds are too loud, and the usable dynamic range narrows. When T is set too high in CIS-type strategies, users commonly report a continuous bacon-frying, hissing, buzzing, or static-like noise, a useful diagnostic complaint. Behavioural T measurement is still worth doing when soundfield warbled-tone thresholds are poorer than about 25 to 30 dB HL, indicating inadequate audibility for soft sounds.[2020][2005][2011]

CThe audibility sweep and paediatric methods

Sweeping (sequentially beeping each channel, for example from electrode 22 down to 1) at threshold or low levels confirms that every channel is audible and reasonably balanced; the same sweep at higher levels checks tonotopic pitch order and screens for non-auditory side effects. Children are tested with age-appropriate behavioural methods: behavioural observation audiometry in infants, visual reinforcement audiometry, conditioned play audiometry, and standard audiometry as they mature. Because infants respond suprathreshold and fatigue quickly, the audiologist may globally lower T below the minimum response level observed during behavioural observation rather than treating those responses as true threshold. A staggered measurement order (an apical channel, then a basal channel, then a mid-array channel, alternating) captures coverage across the whole frequency range before a short-attention-span child disengages. If a child fatigues, unmeasured channels can be interpolated from the channels that were obtained, and an animated test assistant is essential to elicit and condition reliable responses.[2020]