A Model of Incomplete Adaptation to a Severely Shifted Frequency-to-Electrode Mapping by Cochlear Implant Users

Abstract

In the present study, a computational model of phoneme identification was applied to data from a previous study, wherein cochlear implant (CI) users' adaption to a severely shifted frequency allocation map was assessed regularly over 3 months of continual use. This map provided more input filters below 1 kHz, but at the expense of introducing a downwards frequency shift of up to one octave in relation to the CI subjects' clinical maps. At the end of the 3-month study period, it was unclear whether subjects' asymptotic speech recognition performance represented a complete or partial adaptation. To clarify the matter, the computational model was applied to the CI subjects' vowel identification data in order to estimate the degree of adaptation, and to predict performance levels with complete adaptation to the frequency shift. Two model parameters were used to quantify this adaptation; one representing the listener's ability to shift their internal representation of how vowels should sound, and the other representing the listener's uncertainty in consistently recalling these representations. Two of the three CI users could shift their internal representations towards the new stimulation pattern within 1 week, whereas one could not do so completely even after 3 months. Subjects' uncertainty for recalling these representations increased substantially with the frequency-shifted map. Although this uncertainty decreased after 3 months, it remained much larger than subjects' uncertainty with their clinically assigned maps. This result suggests that subjects could not completely remap their phoneme labels, stored in long-term memory, towards the frequency-shifted vowels. The model also predicted that even with complete adaptation, the frequency-shifted map would not have resulted in improved speech understanding. Hence, the model presented here can be used to assess adaptation, and the anticipated gains in speech perception expected from changing a given CI device parameter.

A mathematical model of vowel identification by users of cochlear implants

“…The contribution of a nonzero response bias to lowering vowel percent correct scores for the type of frequency mismatch imposed by FAT No. 1 is addressed in Sagi et al, ͑2010͒ wherein the MPI model was applied to the vowel data of Fu et al ͑2002͒.…”

A mathematical model of medial consonant identification by cochlear implant users

“…The model employed in the present study is an application of the multidimensional phoneme identification (MPI) model (Svirsky, 2000(Svirsky, , 2002Svirsky et al, 2001;Sagi et al, 2010aSagi et al, , 2010b which is a general framework that predicts phoneme identification based on listeners' ability to discriminate speech cues. These speech cues are combined to form a multidimensional perceptual space so that a phoneme's location within the space is defined by that phoneme's set of speech cue values.…”

Optimizing frequency-to-electrode allocation for individual cochlear implant users

“…Başkent and Shannon (2005) conducted a frequency mapping study 17 with MED-EL C40+ cochlear implant users and simulated different insertion depths. 18 They found that, for simulated insertions between 20 and 25 mm, a reduced frequency 19 range map with less spectral distortion resulted in better speech recognition.…”

“…Svirsky et al 2 (2004) found that for three post-lingually deafened adults, acclimatization for vowels 3 had occurred after one day, one month and three months post activation respectively, 4 but for a pre-lingually deafened adult, up to 24 months was needed for acclimatization 5 to occur. Sagi et al (2010) reported that some acclimatization occurred following a 6 severe basal spectral shift, for three CI users who were exposed to a shifted map for 7 three months; two could shift their internal representations to the new sound within 8 one week but one had not completely shifted their representation after three months. 9 10 In summary, it is possible that a frequency allocation matched to the CI recipient's 11 internal frequency map, or one adjusted to make best use of remaining spiral ganglion 12 cells, may offer better speech perception than a default map, which maps the speech 13 frequency range (100 -8500 Hz) to the available electrodes.…”

“…Their data were excluded 16 from the data analysis. 17 18 Figure 2 Post-operative X-ray for P3: all electrodes were visualized 19 20…”