The cochlea is a spiral-shaped organ of hearing within the inner ear where acoustic
vibrations are decomposed into different frequencies to create electrical signals that
transmit audio information to the brain. The basilar membrane (BM), which is an internal
soft tissue component of the cochlea, mechanically filters different frequencies at
different distances along the helical shape. This separation is what allows us to discern
different pitches in sound. Due to individual anatomical differences, each person
naturally has their own unique pitch-map, or tonotopic map, that maps nerves at specific
locations along the basilar membrane to perceived frequencies in the brain.
When the cochlea is not functioning properly, cochlear implantation is a successful
treatment to restore the sense of sound. A cochlear implant (CI) is a neural-prosthetic
device that consists of an external portion that sits behind the ear and a surgically
implanted array of electrodes inserted along the cochlea. After surgery, implants are
programmed using a process called pitch mapping, whereby each implanted electrode is
assigned a specific stimulation frequency. A CI must span the entire length of the
cochlea and stimulate with the correct pitch-map (meaning the correct nerves and
locations are stimulated with the correct frequencies) to produce full and accurate
hearing. When a generalized pitch-mapping approach is used, each electrode within a CI
array will stimulate with a pre-specified frequency, independent of a patient's
individual tonotopy or postoperative electrode location. Generalized pitch-mapping can
result in a place-pitch mismatch of over one octave. This mismatch inhibits the pitch
perception required for complex hearing tasks, such as music appreciation or speech
recognition. Neural plasticity can allow auditory perception to adapt over time to reduce
the effect of cochlear implant pitch-map errors, however this requires long periods of
acclimation, is dependent on recipient age and environment, and can only overcome certain
sized pitch-map errors. Customization of CI pitch-maps can reduce rehabilitation time and
the need for implant acclimation.
Patient-specific pitch maps are produced by accurately determining each patient's
cochlear duct length (CDL), or more specifically BM length, from diagnostic images.
Previous methods to determine CDL have traditionally contained uncertainties at the
start- and end-point of the BM, largely due to visualization limitations in the imaging
modality used. Measuring an inaccurate BM length may cause an erroneous shift in all
tonotopic frequencies. Using an enhanced imaging technique, our team has recently
developed an algorithm to automatically and accurately estimate CDL, segment the BM, and
determine CI electrode locations from individual patient computed-tomography (CT) scans
to produce customized CI pitch-maps, called placed-based mapping (Helpard et al., 2021).
The primary objective of this study is to evaluate whether a place-based map improves
hearing outcomes for cochlear implant recipients. We will compare the auditory abilities,
speech recognition and spatial hearing (speech recognition in spatially separated noise,
and sound source localization) for subjects randomized to listen exclusively with a
default map versus our novel place-based map. We hypothesize that the majority of CI
recipients will experience a faster rate of speech recognition and spatial hearing growth
when their cochlear implant is mapped to match the electric stimulation with the
tonotopic place frequency (i.e., using the place-based map).