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Sound localization is one of the major challenges for bilateral cochlear implant (CI) users, especially if they suffer from early hearing loss. Their ability to utilize binaural cues, particularly interaural time differences (ITDs), falls below that of their normal hearing peers. Our recent work on neonatally deafened (ND), CI-supplied rats demonstrated excellent ITD sensitivity with remarkably low thresholds (~50 μs) even in the absence of early sensory input, if the CIs present microsecond-accurate ITDs [1]. We could also show that NDCI rats develop excellent sensitivity to interaural level differences (ILDs) [2]. However, to date, it is not known how acoustic hearing experience prior to CI implantation affects the precedence effect, as well as the ITD and ILD sensitivity. To get further insights into sound localization with bilateral CIs and the mechanisms behind it, we investigated binaural cue sensitivity in adult deafened (AD), CI-supplied rats. Additionally, we studied the precedence effect by measuring temporal weighting functions (TWFs) and compared them to TWFs of NDCI and normal hearing rats.

ADCI rats (n=4) were deafened during CI implantation in young adulthood. They were trained to lateralize pulse trains with accurate ITDs in the range of ±\'7b40, 60, 80, 100, 120\'7d µs and ILDs in the range of ±\'7b1, 2, 3, 4, 5\'7d dB at a clinical pulse rate of 900 pps. ITDs were presented congruently in both pulse timing and envelope. After three to five weeks of training, rats were tested on their ITD and ILD sensitivity, as well as their temporal weighting. To determine the TWFs, all rats performed a test similar to that developed by Brown and Stecker (2010) [3]. During testing, each pulse in a stimulus burst of eight pulses had a different, randomly selected ITD between ±120 µs, and the rat had to choose freely where the sound came from. TWFs were calculated using multiple regression analysis to determine the perceptual weight of each pulse. ITD and ILD sensitivity and TWFs were compared with those of normally hearing rats and NDCI rats, tested under similar conditions.

All ADCI rats developed robust lateralization abilities and developed excellent ITD and ILD sensitivity with behavioral thresholds comparable to NDCI and normally hearing rats. Both CI cohorts showed a reduced precedence effect relative to normally hearing rats.

The results indicate that the deaf auditory system can develop good ITD and ILD sensitivity, independent of its prior acoustic hearing experience, if microsecond-accurate ITD information is provided from the outset of biCI stimulation. However, the TWFs were much flatter in both electrically hearing cohorts, suggesting that the precedence effect may require context or domain-specific experience rather than early acoustic experience to develop.

References

[1] Rosskothen-Kuhl N, Buck AN, Li K, Schnupp JW. Microsecond interaural time difference discrimination restored by cochlear implants after neonatal deafness. Elife. 2021 Jan 11;10:e59300. DOI: 10.7554/eLife.59300
[2] Buchholz S, Schnupp JWH, Arndt S, Rosskothen-Kuhl N. Interaural level difference sensitivity in neonatally deafened rats fitted with bilateral cochlear implants. Sci Rep. 2024 Dec 16;14(1):30515. DOI: 10.1038/s41598-024-82978-4
[3] Brown and Stecker. Temporal weighting of interaural time and level differences in high-rate click trains J. Acoust. Soc. Am. 2010;128:332-341. DOI: 10.1121/1.3436540