10The Electrode Array as a Frequency Ruler

TA cochlea of variable length

Human cochlear duct length averages ~33.5 mm (SD ~2.28 mm) with a ~13.78 mm difference between the shortest and longest cochleae; normal cochleae vary in size by as much as 40%. A fixed frequency table cannot match every patient because the characteristic frequency at a given distance from the base differs greatly between a short and a long cochlea (cross-ref Ch.12 Preoperative Imaging).[2014]

CLength, design and angular depth

Array length and design dictate angular insertion depth; full insertion is typically ~450–630°. MED-EL Standard/FLEXSOFT (31.5 mm) reaches ~630°; Cochlear CI522 Slim Straight (~25 mm) targets 450–540°; lateral-wall arrays need ~33 mm to cover the whole organ of Corti, whereas a perimodiolar array on the inside track needs only ~17 mm to cover the spiral ganglion.[2020]

TFrequency-to-place mapping

Frequency-to-place follows the Greenwood function with the Stakhovskaya spiral-ganglion correction: high frequencies basal, low frequencies apical. Because real insertions rarely reach the tonotopically correct place and the table is one-size-fits-all, stimulation usually produces a basalward mismatch — the electrode sits more basal than its assigned band, so the pitch heard is higher than intended.[2007]

CHow dramatic the mismatch is

The mismatch is quantitatively dramatic: at 24 mm from the base, predicted CF is ~76 Hz in a 28 mm cochlea but ~1020 Hz in a 42 mm cochlea. Skinner et al. found a significant correlation between insertion depth and speech recognition, with shallower insertions producing larger, harder-to-overcome mismatch.[1990]

TBoth extremes degrade outcome

Both extremes degrade outcome: shallow insertions cause basalward mismatch, while very deep insertions (605–720°) cause apicalward mismatch — subjects reported 'very low pitched' sound even after years, and disabling the two most apical electrodes improved speech ~7 percentage points. Patients adapt partially over months to a year, but adaptation is incomplete for large shifts.

CPitch resolution and realignment

Poor pitch resolution motivates place-map realignment and current steering: CI users score only 49% correct at one-semitone pitch ranking (vs 81% normal hearing), with mean limens ~3 semitones. Realigning the place map to measured pitch percepts (Di Nardo 2011) improved music perception (cross-ref Ch.16 Programming).