When the First Pulse Fades: 75% Reduction in Precedence Effect in Cochlear Implanted Rats

It is widely known that users of cochlear implants (CIs) often show deficits in binaural hearing. In particular, interaural time difference (ITD) discrimination tends to be substantially worse in CI users. However, only very few studies have investigated how CI stimulation affects the "precedence effect" (PE), and how this might be affected by abnormal hearing experience during development. We performed behavioral experiments to measure temporal weighting functions (TWFs) to quantify the PE in three different cohorts of rats: normally hearing (NH) acoustically stimulated, neonatally deafened (ND) CI-stimulated, and adult deafened (AD) CI-stimulated animals. Lateralization responses were recorded to bursts of eight stimulus pulses in which the ITD of each pulse varied independently, and probit analysis revealed the perceptual weight of each pulse. The NH and ND-CI cohorts were tested at 50, 300, and 900 pps; the AD-CI animals were tested only at 300 pps. The NH animals show a pronounced "onset dominance" at higher pulse rates, that is, the weighting of the first pulse was much larger than for subsequent pulses, exactly as one would expect given previous studies on human listeners. In contrast, the TWFs for the ND-CI and AD-CI animals were much flatter, with a reduction of onset dominance by ∼75%, especially for higher pulse rates. Since neither of our CI cohorts had any experience of reverberation in their electric hearing, we interpret these results to indicate that the PE may require domain-specific experience to develop.