(a) Close-up of the training setup for CI rats. The central ‘start’ and lateral ‘response’ spouts deliver the water reward and are indicated by arrows. (b) CI rat during a testing session, making a response to the left by making contact with the left reward spout. (c) Calibration measurements of stimulus pulses recorded by connecting the stimulator cable to 10 kΩ resistors instead of the in vivo electrodes and recording voltages using a Tektronix MSO 4034B oscilloscope. Recordings of stimulus pulses are shown with ±50, 100, and 150 µs ITDs as indicated. Pulses delivered to the right ear channel are shown in red, and those delivered in the left ear in blue. (d) To measure the size of artifactual, unintended interaural level differences (ILDs) that our system generates, the root mean square (RMS) amplitudes of the stimulus traces shown in (c) were compared. The resulting ILDs for five repeat presentations are shown. One observes very small, artifactual ILDs that are attributable to a tiny amount of capacitive/inductive channel cross-talk in the wires leading from the programmable stimulator to the implants. A current pushed through one wire will induce a tiny current in the wire running parallel to it by magnetic induction. On careful inspection of the traces in Figure 2c, one can see tiny little red bumps coinciding with big blue rising or falling phases and vice versa, which correspond to these induced currents. (Magnetic induction of currents is proportional to rate of change in field strength and hence occurs during rising and falling phases of the current pulses.) The currents measured by the oscilloscope and used here for stimulus calibration are thus a superposition of the direct stimulus current injected into a given channel by the stimulator, plus the very much smaller induced current from the cross-talk from the neighboring channel. The direct current pulses and the cross-talk current pulses can be either in phase or out of phase with each other depending on the ITD, which will lead to either constructive or destructive interference. This creates the small ILDs and accounts for their dependence on ITD. Note that these very small artifactual ILDs cannot account for our behavioral results because they are an order of magnitude below the animals’ typical ILD thresholds (e) and they lack the required systematic relationship with ITD that would be needed if one tried to account for the ITD psychometric function in terms of sensitivity to the tiny artifactual ILDs. The largest ITD-induced ILD is 0.18 dB, or equivalently 2.17%. At 100 μs ITD, where our rats routinely achieve 80% correct or better (compare Figure 2) the ILD is as low as 0.018 dB and does not change sign with the ITD, and would therefore have to be completely uninformative. (e) Behavioral ILD psychometric curves obtained from two additional ND-CI rats (not part of the cohorts introduced in Figure 1). Two rats were neonatally deafened, fitted with CIs as young adults and trained in sound lateralization tasks exactly as described in the methods, except that for these tests, the ITD of the pulses was kept constant at 0 and the relative amplitude of the left and right ear pulses was varied from trial to trial to introduce ILDs. The pyschometrics are plotted using the same conventions as in Figure 2, with the blue error bars showing Wilson confidence intervals for the proportion of right responses at each ILD and the red lines showing bounded linear psychometric functions fit to the data. Note that, to reach levels of performance >75% correct, both animals need ILDs of >2 dB, at least an order of magnitude larger than the largest 0.18 dB artifactual ILD observed.