The EOD of A. leptorhynchus is neurogenic and therefore is not affected by injection of curare. All stimuli consisting of AMs of the animal’s own EOD were produced by triggering a function generator to emit one cycle of a sine wave for each zero crossing of the EOD, as done previously (36). The frequency of the emitted sine wave was set slightly higher (~40 Hz) than that of the animal’s own EOD, which allowed the output of the function generator to be synchronized to the animal’s discharge. The emitted sine wave was subsequently multiplied with the desired AM waveform (MT3 Multiplier; Tucker Davis Technologies), and the resulting signal was isolated from the ground (A395 Linear Stimulus Isolator; World Precision Instruments). The isolated signal was then delivered through a pair of chloridized silver wire electrodes placed 15 cm away from the animal on either side of the recording tank perpendicular to the fish’s rostrocaudal axis. The resulting signal measured at the fish’s skin was approximated using a dipole (1-mm distance between the two poles) positioned next to the fish 2 mm away.

We used stimuli consisting of a 5- to 15-Hz noise (fourth-order Butterworth) carrier waveform (i.e., AM) whose amplitude (i.e., envelope) was further modulated sinusoidally at frequencies ranging from 0.05 to 1 Hz. This constitutes a behaviorally relevant range of frequencies, which mimicked the envelope signals due to relative movement between two fish (10). Adaptation stimuli that consisted of a 100-s-long 5- to 15-Hz noise (fourth-order Butterworth) carrier signal whose amplitude was also noisy were played repetitively. The noisy amplitude was characterized by power spectra that decayed with exponents as a function of temporal frequency of either −2 (neurons: n = 16 PCells; average recording time, 106.3 ± 4.5 min; behavior: n = 20 fish; average recording time, 312 ± 26.7 min) or 0 (neurons: n = 15 PCells; average recording time, 108.7 ± 5.4 min; behavior: n = 15 fish; average recording time, 232 ± 29.8 min). We note that sensitivity and whiteness index values were already significant when only eight randomly selected neurons or fish were taken into account (αstim = 0: αneuron control = 0.22 ± 0.08, αneuron adapted = 0.5 ± 0.07, P = 3.1 × 10−2, whiteness index (WI): P = 7.9 × 10−3, αbehavior control = −0.83 ± 0.05, αbehavior adapted = −0.46 ± 0.11, P = 7.8 × 10−3; αstim = −2: αneuron control = 0.54 ± 0.12, αneuron adapted = 0.3 ± 0.1, P = 7.8 × 10−3, WI: P = 1.6 × 10−2, αbehavior control = −0.85 ± 0.05, αbehavior adapted = −1.25 ± 0.15, P = 1.6 × 10−2; Wilcoxon signed rank tests). The depth of modulation for both stimulus classes used during experimental stimulation was approximately 20% of the baseline EOD amplitude as in previous studies (13, 24, 37) as measured using a small dipole placed close to the animal’s skin in the middle of the animal’s rostrocaudal and dorsoventral axes (typically 0.2 mV cm−1).