3The Adult Learning Curve

FThe shape of the curve: rapid gains then plateau

Postlingual adults typically improve fastest in the first weeks to months after activation, with the curve flattening toward an asymptote by roughly 3 to 12 months of implant use. In a 55-subject series, mean sentence and word scores rose steadily out to 12 months and then plateaued, with sentence scores levelling near 90% and word scores near 55% correct. Across the largest series of 2251 adults, performance kept improving through the first 3.5 years, but the majority of that improvement occurred within the first 12 months. The plateau is not a hard stop: the early steep climb reflects fast adaptation, while the long shallow tail reflects continued, slower acclimatisation in some recipients.[2009][2013][2013]

CWhere scores sit at one year

In 114 adults followed closely, mean CNC word recognition reached 61.5%, with the best listener above 90% and the poorest near 10%, and most reached their asymptote by about 6 months. Representative one-year sentence scores in quiet for experienced adults cluster high, on the order of 70 to 90% on AzBio depending on the cohort and material. The contrast with the pre-implant baseline is stark: the same 114-subject cohort averaged only 16.4% on the easier HINT sentence test in quiet before surgery. Even older recipients gain substantially; in one cohort, listeners over 65 rose from a mean pre-implant CNC of 8.4% to 72% after implantation.[2013][2016][2012]

TThe acclimatisation effect

The continued rise in scores over months of use, even with an unchanged map, is an acclimatisation effect attributed largely to central reorganisation rather than peripheral change. The brain learns to interpret a sparse input: a typical device drives only a handful of broadly overlapping nerve sectors, far fewer than the roughly 30,000 fibres of the normal auditory nerve, yet open-set understanding still emerges. Because of acclimatisation, outcome should be judged against a stable post-plateau measure, not a single early post-activation score that has not yet matured. Adaptation explains why a recipient who seems unimpressive at first switch-on can become a strong performer months later, and why early scores under-predict the final result.[2009][2008][2013]

CWhy some keep climbing and others plateau early

A shorter duration of deafness, younger age at implantation, consistent pre-implant hearing-aid use, and earlier onset of severe-to-profound loss are all associated with better and sometimes faster-rising trajectories. A duration of deafness beyond about 20 years tends to predict poorer plateaus, though modern technology has softened this penalty compared with earlier eras. Such pre-implant factors together explain only around 10% of the variance in adult outcome, so the timing and height of an individual's plateau remain hard to forecast. Device placement matters for where the curve settles: perimodiolar arrays sitting wholly within the scala tympani are associated with better word recognition than mislocated or over-inserted arrays.[2013][2013][2016]