DOI: 10.5061/dryad.dfn2z359d
title: Musical pitch interval comparisons in cochlear implants

description: Music perception remains challenging for many cochlear implant (CI)
 recipients, due perhaps in part to the frequency mismatch that occurs
 between the electrode-neural interface and the frequencies allocated by
 the programming. Individual differences in ear anatomy, electrode array
 length, and surgical insertion can lead to great variability in the
 positions of electrodes within the cochlea, but these differences are not
 typically accounted for by current CI programming techniques. Flat panel
 computed tomography (FPCT) can be used to visualize the location of the
 electrodes and calculate the corresponding spiral ganglion characteristic
 frequencies. Such FPCT-based CI frequency mapping may improve pitch
 perception accuracy, and thus music appreciation, as well as speech
 perception. The present study seeks to develop a behavioral
 assessment metric for how well place-based pitch is represented across the
 frequency spectrum. Listeners were asked to match the pitch interval
 created by two tones, played sequentially, across different frequency
 ranges to estimate the extent to which pitch is evenly distributed across
 the CI array. This test was piloted with pure tones in normal hearing
 listeners, using both unprocessed and vocoder-processed sounds to simulate
 both matched and mismatched frequency-to-place maps. We hypothesized that
 the vocoded stimuli would be more difficult to match in terms of pitch
 intervals than unprocessed stimuli and that a warped map (as may occur
 with current clinical maps) would produce poorer matches than a veridical
 and even map (as may be achieved using FPCT-based frequency
 allocation). Preliminary results suggest that the task can reveal
 differences between veridical and warped maps in normal-hearing listeners
 under vocoded conditions. A small cohort of CI recipients performed
 similarly to a vocoded condition employing the same pitch map. The next
 steps will be to test this procedure in CI users and compare results with
 traditional clinical maps and FPCT-based frequency allocation to determine
 whether the FPCT-based maps result in improved pitch-interval perception.
description: Subjects Two primary groups were enlisted
 for this study: normal hearing (NH) individuals and cochlear implant (CI)
 recipients. NH listeners were used to establish baseline data that could
 be used to compare against CI recipients. CI recipients are included here
 as a pilot to determine whether this approach is feasible for these
 listeners.   Normal Hearing (NH)
 Participants Recruited through the University of
 Minnesota, 31 NH individuals participated. The group that assessed
 unprocessed stimuli comprised 15 participants (average age: 22.6 years,
 SD: ±1.5; gender distribution: 5 males, 10 females) with an average of 8.1
 years (SD: ±3.9) of musical experience. The vocoded stimuli group included
 16 participants (average age: 28.6 years, SD: ±13.8; gender distribution:
 7 males, 9 females), reporting an average of 11.1 years (SD: ±11.1) of
 musical experience. Testing for both NH groups was completed remotely via
 an online MATLAB platform, requiring the use of headphones.
 Cochlear Implant (CI) Recipients Nine CI
 recipients (Table 1, average age: 57.4 years, SD: ±13.2; gender
 distribution: 6 males, 3 females) were recruited through UCSF. This group
 consisted of one bilateral and eight unilateral CI users, all equipped
 with MED-EL CIs and using their clinical everyday listening programs.
 Their reported musical experience averaged 11.3 years (SD: ±12.3). Similar
 to the NH group, the CI cohort completed the task via an online MATLAB
 platform. CI recipients were instructed to choose the transducer that they
 regularly use with success at home; this could have included sound field
 speakers, headphones, or streaming, with care taken to isolate the test
 ear. Pitch Interval Assessment Procedure
 For these experiments, we focused on pitch interval comparisons
 across a frequency range utilized by contemporary CI processors. This
 frequency range was divided into three regions, assuming a logarithmic
 distribution of frequencies, resulting in low (root note 150 Hz, interval
 range 126-505 Hz), mid (root note 572 Hz, interval range 480-1924 Hz), and
 high (root note 2181 Hz, interval range 1833-7314 Hz)
 categories. NH participants completed the pitch
 interval assessment with pure tones across both frequency test ranges (Low
 vs Mid, Mid vs High), whereas CI recipients and NH subjects with vocoded
 stimuli were tested only in the Mid vs High ranges due to limitations of
 the vocoded conditions in low frequencies and time constraints. 
 The task involved comparing two pitch intervals between two
 frequency regions presented in succession. Participants identified the
 larger of two presented pitch intervals in a forced choice paradigm
 (Zarate et al., 2012; McDermott et al., 2010).  A
 single pitch interval consisted of a 3-tone melody in a low-high-low
 sequence, where the first and last notes were the same (e.g., C4 - G4 -
 C4). The melody's root note was roved within a half-octave range.
 Each note was a pure tone of 300 ms, including 30-ms onset and 50-ms
 offset raised-cosine ramps. The notes within each 3-note sequence were
 separated by 150-ms gaps.  For the NH listeners
 presented with pure tones, the fixed interval was either 4 ST (a major 3rd
 in music notation) or 7 ST (a perfect 5th); for the CI users and NH
 listeners presented with vocoded stimuli, the fixed interval was always 7
 ST. Intervals were defined using equal temperament tuning, where 1 ST
 always represents a change in frequency of 21/12. 
 Adaptive Tracking Procedure The assessment
 employed an adaptive testing approach (e.g., Jesteadt, 1980) to determine
 each participant's point of subjective equality (PSE) for pitch
 intervals across different frequency regions. One of the two intervals was
 fixed, and the other interval was adaptively varied, based on the
 listener’s previous responses. A value of 0 semitones (ST) in this
 procedure indicates that the adaptively varying interval was the same size
 as the fixed interval (either 4 or 7 ST).  Each run
 consisted of four randomly interleaved adaptive tracks, two of which used
 a 2-down 1-up procedure and two of which used a 1-up 2-down procedure,
 tracking the 71% and 29% points of the psychometric function, respectively
 (Levitt, 1971). For each of these pairs of tracks, one pair varied the
 first (lower) interval and the other pair varied the second (higher)
 interval. For each track, the starting size of the varying interval was ±3
 STs, and the starting step size was 4 STs and decreased after the first
 two reversals to 2 STs. Four reversals were required during the initial
 phase and two reversals were required during the measurement
 phase.  Once all the tracks had terminated, the PSE was
 defined as the average of the four tracks (as the mean of the 71% and 29%
 points approximates the 50% point). A total of 5 runs were completed per
 participant in each condition, with a prompt for participants to rest
 between runs. The adaptive tracking procedure was
 limited to values of between -7 and +10 ST. If the adaptive procedure
 called for a value exceeding the maximum or minimum more than 6 times in
 one track, the track was terminated and a value of -8 or +11 ST was
 assigned to that track. Prior to data collection,
 participants completed a short training module in the initial phase of the
 experiment that utilized 7 ST intervals and provided feedback to ensure
 understanding of the task. No feedback was given during
 testing.  Stimuli Normal Hearing
 Cohort Normal hearing (NH) participants were assessed
 using either pure-tone (unprocessed) stimuli or vocoded stimuli (to
 simulate aspects of cochlear implant sound perception). 
 To create the vocoded stimuli, a frequency warp was first applied
 to simulate either a full-length (28 mm) or a shorter (24 mm) electrode
 array placement. The basis for these two frequency warps was generated by
 calculation of the spiral ganglion characteristic frequencies of
 electrodes measured using lateral wall cochlear duct length measurements
 of our previously FPCT-imaged cohort (Jiam et al., 2021; Helpard et al.,
 2020; Li et al., 2021). The most apical electrode of the 28 mm array
 corresponds to 350 Hz (Figures 1A and 1B, black dots), and of the 24 mm
 array corresponds to 500 Hz (Figure 1C, black dots), consistent with
 larger cohorts reported elsewhere (Canfarotta et al., 2020,
 ***).  The second step to create the vocoded stimuli
 was to apply a frequency allocation table to simulate either default or
 custom CI filterbank settings, which yielded the following three
 conditions: (1) Vocoded 28 mm Array with Default Frequencies (“Voc
 Default”, black bars in Figure 1A), (2) Vocoded 28 mm Array with Middle
 Frequencies Matched (“Voc MidFreq Match”, gray bars, Figure 1B), and (3)
 Vocoded 24 mm Array with All Frequencies Matched (“Voc AllFreq Match”,
 gray bars, Figure 1C).  The Voc Default setting (Figure
 1A, black bars) used a frequency range of 70-8500 Hz, logarithmically
 divided into 12 channels, and was modeled after the manufacturer’s default
 frequency allocation table (i.e., “LogFS”). The Voc
 MidFreq Match applied a strict match of center frequencies to the
 mid-frequency range (950-4000 Hz) and the remaining frequency ranges were
 then redistributed across the most apical and basal electrodes (70-950 Hz,
 4000-8500 Hz, as seen in Figure 1B, gray bars). This approach attempted to
 maintain audibility across the entire frequency range while also
 maintaining pitch interval integrity where feasible.
 The Voc AllFreq Match utilized a strictly CT-based approach
 (Figure 1C, gray bars), which matched all channel center frequencies to
 the electrode contact locations as much as is feasible (&lt;4000 Hz).
 To avoid deactivating electrodes that were located above the bandwidth
 limit of the software (8500 Hz), a logarithmic redistribution was applied
 in the highest frequencies (&gt;4000 Hz) to make the best use of
 available electrodes.  Cochlear Implant
 Cohort The CI cohort was presented with only pure-tone
 (unprocessed) stimuli during the assessment. All CI recipients utilized
 the manufacturer's default logarithmic frequency allocation table
 (i.e., LogFS, indicated by the black bars in Figure 1A).
 Additionally, the CI and the vocoded conditions were only
 completed the mid to high-frequency range comparisons (i.e., 480-7314 Hz)
 since the low-frequency range was not accessible to the vocoded condition
 modeling the shorter electrode array (most apical electrode corresponding
 to a characteristic frequency of 500 Hz).
 Standardization of Sound Levels for Participant
 Assessment Given the diversity of hardware used by
 participants conducting the assessment on their computers, inherent
 variability in presentation levels and frequency response was a
 significant challenge. To mitigate this variability and standardize sound
 levels across varying setups, we implemented a personalized
 standardization process for each participant. This
 standardization involved a subjective loudness equivalency task, where
 participants were presented with five tones that spanned the critical
 frequency range of the assessment (125 Hz, 250 Hz, 500 Hz, 2000 Hz, and
 7352 Hz). Accompanied by a graphical user interface, each tone was
 adjustable via a slider allowing for a ±30 dB range of loudness
 modification. Participants adjusted the loudness of each tone to achieve a
 uniformly "most comfortable" listening level across the
 frequency spectrum. The standardization data from these
 five frequencies were then utilized to construct a participant-specific
 frequency response profile. This was achieved through linear interpolation
 of the logged frequency values against the decibels of standardization
 adjustment, using MATLAB's interp1 function. The interpolated values
 provided a tailored correction for each tone's amplitude, allowing
 for more consistent volume across the test's frequency
 range. Consequently, this standardization process
 generated a custom amplitude scaling curve for each participant. This
 curve adjusted the nominal level of each tone to 60 dB, considering the
 individual standardization offsets. These adjustments were applied to all
 stimuli, including those processed through the vocoder, to maintain a more
 consistent stimulus volume throughout the assessment.
 This standardization approach was intended to reduce the
 variability introduced by participants' diverse hardware
 configurations, ensuring that all participants experienced the stimuli at
 a consistent and optimal listening level throughout the
 experiment.
description: # Musical pitch interval comparisons in cochlear implants
 [https://doi.org/10.5061/dryad.dfn2z359d](https://doi.org/10.5061/dryad.dfn2z359d) ## Description of the data and file structure Excel file with summary data: * Participant ID - each subject is given a number * Test ID - the number the subject used to access the test, which matches their raw data file * Age - the age in years when the subject was tested * Music Training - the number of years of formal music training each subject reports having (e.g., 1:1 lessons, school band, etc.) * Cohort - which of the 3 cohorts each subject belonged to: NH (natural hearing), Voc (Vocoder), or CI (cochlear implant) * Condition - the listening condition * Freq Range - either Low vs Mid or Mid vs High * Fixed Semitone Interval - either 4 or 7 * Test Parameters - a field combining the Freq Range and Fixed Semitone Interval fields, used to indicate which subtest was completed * PSE - point of subjective equality, measured in semitones, a continuous dependent variable * Filename - the corresponding raw data file name Raw data files; there is a mismatch between column header names and the data contained in each column. Below is the data contained in each column. * Track number: There were four tracks in each trial. Each set of four rows represents one trial. * LoHiFix: Representing whether the low interval was fixed (0) or the high interval was fixed (1) * ST: Number of semitones of the fixed interval (either 4 or 7, depending on the subtest) * Hz: A value representing which frequency range was being tested, either Low vs Mid (150) or Mid vs High (572) * Oct: A constant indicating the distance, in octaves, between the pitch intervals being tested (always 1.93) * ST: The dependent variable, in semitones, representing the point of subjective equality * A value, in semitones, of the variability of the subject's responses. * Elapsed time: The duration of each trial in seconds. ## Sharing/Access information Data was collected at the University of California, San Francisco, and the University of Minnesota. All subjects gave informed consent under IRB-approved study protocols.