10Out of Tune: Frequency-to-Place Mismatch

FTwo maps that should agree but often don't

The cochlea is tonotopic: each place along the basilar membrane has a characteristic frequency, described mathematically by the Greenwood place-frequency function. The processor independently divides the input (roughly 100-8000 Hz) into channels and assigns each to an electrode, optimised for speech intelligibility, not for matching the cochlear place frequency. Frequency-to-place mismatch is the gap between the frequency a channel delivers and the natural characteristic frequency of the cochlear location that channel stimulates. Because arrays usually sit at places whose natural frequencies are no lower than ~500-1500 Hz, low-input bands are delivered to a cochlear region that is naturally tuned much higher - a basalward (upward) shift.[1990][2008][2009]

TWhy shallow insertion shifts everything sharp

Most arrays reach only the basal and middle turns; the apical, low-frequency region of the cochlea is rarely stimulated, so the lowest input bands land on places naturally tuned to mid-high frequencies. The net effect is that a note is delivered to a place that 'expects' a higher note, so unprocessed percepts can sound shifted upward (sharp) and compressed. Deeper insertion reduces the mismatch by reaching lower-frequency cochlear places, but trades off against insertion trauma and loss of residual hearing. Insertion depth is rarely uniform across the array, so the size of the mismatch can differ from the basal to the apical electrodes within the same ear.[1990][2008][2020]

CHow mismatch corrupts melody and harmony

Melody is a pattern of pitch intervals; a frequency map that is shifted and compressed distorts those intervals, so the contour can be recognised by direction but the exact notes sound wrong. Harmony depends on precise frequency ratios between simultaneous notes; mismatch and channel interaction smear these ratios, so chords lose their consonant/dissonant identity. Across-ear mismatch in bilateral users (the same input band on differently-placed electrodes in each ear) and in bimodal users (electric pitch versus acoustic pitch) can make a single note sound like two different pitches. A familiar song may be identifiable from rhythm and lyrics yet still sound out of tune, which is why patients often say speech 'works' but music 'sounds wrong'.[2014][2005][2008]

CPutting the notes back: adaptation and place-based maps

The brain partially adapts: over months to years, pitch percepts can drift toward the processor's assigned frequencies, so the map increasingly determines pitch more than cochlear place does. Adaptation is often incomplete; some users never fully resolve the mismatch and across-ear differences can persist. Anatomy-based / place-based frequency allocation uses post-operative imaging to estimate each electrode's Greenwood place frequency and assigns input bands to match, reducing mismatch. Place-based fitting shows the clearest promise for music and for binaural consistency, though speech benefits are more variable; it is a key rationale for image-guided programming.[2008][2014][1990][2020]