Discrimination of inconsistent interaural time differences across frequency in simulated bilateral cochlear-implant users

Abstract

When using highly controlled low-rate stimulation, interaural-time-difference (ITD) just-noticeable differences (JNDs) vary greatly across frequency and individual bilateral cochlear-implant (CI) users. It is unclear how across-frequency variability would affect JNDs in multi-electrode stimulation where there are channel interactions from current spread. Therefore, we simulated variable across-frequency ITD sensitivity and channel interactions in 10 normal-hearing listeners presented with a CI simulation that consisted of 100-pulse/s, 1.5-mm bandlimited pulse trains (PTs) in three frequency regions (middle frequency = 7.6 kHz). The simulated electrode spacing was varied by placing the low- and high-frequency PTs Δ = 0.75, 1.5, 3, or 4.5 mm from the middle frequency. Inconsistent ITD sensitivity across frequency was simulated by applying non-zero ITDs in one, two, or three PTs, while the remaining PTs had zero ITD. The pulses in the PTs were simultaneous, nearly simultaneous, or maximally separated. In a left-right discrimination task, JNDs increased for a decreasing number of dichotic PTs. JNDs increased for Δ<3 mm, suggesting an important role of channel interactions. JNDs increased for non-simultaneous stimulation likely due to a rate limitation effect. The relatively poorer listener performance from the factors in our simulations suggest a means to predict multi-electrode ITD performance in CI users.