Analysis of experimental recordings.

<p>(A) Experimental setup: the fish in the middle of the tank is restrained in a hammock; another one or two fish swim freely. The compound electric organ discharge (EOD) signals due to all fish were recorded through two electrodes (red dots) 1 cm apart and very close to the skin of the head of the restrained “receiving” fish. The line joining the electrodes was perpendicular to the skin in order to measure the normal component of the electric field. (B) Raw electric field (black) recorded from two static fish (both were held in hammocks), and its corresponding first envelope E1 (red) and second envelope E2 (blue). Their EOD frequencies (EODf) are 827 and 763 Hz, respectively. (C) A stretch of data of 1.2 sec long (black, only the positive part is shown) from one restrained fish (EODf at 827 Hz) with one other fish (763 Hz) freely swimming, and the corresponding E1 (red) and E2 (blue). (D) E2 extracted from the recording in (C) over a 10 second duration. The dip around the middle of this trial (marked by “*”, same event as in (E)) indicates a chirp. (E) 3D spectrogram of the data in (D). The amplitude of the spectral density (ASD) of the restrained fish is almost constant at 827 Hz, but the ASD of the freely swimming fish at 763 Hz varies with a very similar pattern as E2 shown in (D).