Cochlear implants (CIs) can restore a high degree of functional hearing in deaf patients but enable only poor spatial hearing or hearing in noise. Early deaf CI users are essentially completely insensitive to interaural time differences (ITDs). A dearth of binaural experience during an early critical period is often blamed for these shortcomings. However, here we show that neonatally deafened rats which are fitted with binaural CIs in early adulthood are highly sensitive to ITDs immediately after implantation. Under binaural synchronized stimulation they can be trained to localize ITDs with essentially normal behavioral thresholds near 50 μs. This suggests that the deficits seen in human patients are unlikely to be caused by lack of experience during their period of deafness. It may instead be due to months or years of CI stimulation with inappropriate binaural parameters provided by CI processors which do not provide sub-millisecond temporal fine structure of sounds . 3 33 34 35 36 37 38 39 40 41 42 43 44 45 The World Health Organization reports that about 466 million people suffer from disabling hearing loss, making it the most common sensory impairment of our age. For people with severe to profound sensorineural hearing loss, cochlear implants (CIs) can be enormously beneficial, quite routinely allowing near normal spoken language acquisition, particularly when CI implantation takes place early in life [1]. Never the less the performance of CI users remains variable, and even in the best cases falls short of natural hearing.Good speech understanding in multi-sound environment requires the ability to separate speech from background, which relies in part on a phenomenon known as "spatial release from masking". This relies on the brain's ability to process binaural spatial cues, including interaural level and interaural time differences (ILDs/ITDs) [2]. To benefit from binaural cues in everyday life, bilateral cochlear implantation is becoming increasingly common for deaf patients [3][4][5] . However, even binaural CI patients perform much below the level of normal listeners in sound localization or auditory scene analysis tasks, particularly when multiple sound sources are present [6,7]. The parameters that would allow CI patients to derive maximum benefits from binaural spatial cues are still only partially understood. A number of technical problems (see [8], chapter 6) limit the fidelity with which CIs can encode binaural cues, particularly ITDs. The fact that contemporary CI speech processors were originally designed for monaural, rather than binaural, hearing likely contributes the observed deficits in ITD performance of bilateral CI users [3]. Standard CI processors provide pulsatile stimulation which is not locked to the temporal fine structure of the incoming sounds, and the timing of the electrical pulses is not synchronized between both ears, which makes these devices fundamentally incapable of encoding sub-millisecond binaural time structure. To be useful, ITDs as small as a few tens ...

Spatial hearing in cochlear implant (CI) patients remains a major challenge, with many early deaf users reported to have no measurable sensitivity to interaural time differences (ITDs). Deprivation of binaural experience during an early critical period is often hypothesized to be the cause of this shortcoming. However, we show that neonatally deafened (ND) rats provided with precisely synchronized CI stimulation in adulthood can be trained to lateralize ITDs with essentially normal behavioral thresholds near 50 μs. Furthermore, comparable ND rats show high physiological sensitivity to ITDs immediately after binaural implantation in adulthood. Our result that ND-CI rats achieved very good behavioral ITD thresholds, while prelingually deaf human CI patients often fail to develop a useful sensitivity to ITD raises urgent questions concerning the possibility that shortcomings in technology or treatment, rather than missing input during early development, may be behind the usually poor binaural outcomes for current CI patients.

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