Acquisition and stimulation setup.

<p><b>A – hardware:</b> EODs from real (f) and artificial stimulus (S) fish are measured using a 3D array of 8 electrodes [(1)-(7), and (R)]. Signals are differentially amplified (x 100, (R) is the common reference) and digitized at 50 kHz by an ADC board. The artificial fish is a dipole electrode inside a 15 cm PVC pipe driven by a DAC output from a computer running real-time home made stimulation software that mimics a real fish pulse shape with IPIs defined by the experimenter. <b>B – acquisition software:</b> the square of the digitized artificial signal are delayed of 1–3 samples due to the DAC-ADC circuitry and subtracted from the squared and summed electrode signals, because this signal contains both stimulus (S) and real fish (f) and after subtraction, only the real fish pulses will remain. Real fish pulses are then detected with a simple threshold level and the timestamps at each electrode are stored for posterior analyzes. The positive and negative amplitude values at each timestamp are detected in all electrodes. The electrode corresponding to the maximum positive amplitude indicates the position of the fish’s head and the electrode with the most negative amplitude indicates the position of the fish’s tail. If the fish’s head(tail) is in the middle of the tank, the positive (negative) amplitudes will be more or less the same in all electrodes and not as high as if the fish were near the corners of the tank or the reference electrode. The locally induced amplitude (LIA) at each electrode is defined as the difference between the maximum and minimum values of voltage induced during the EOD.