Precision Measurement of Electric Organ Discharge Timing 4 from Freely Moving Weakly Electric Fish

20 Physiological measurements from an unrestrained, untethered, and freely moving animal 21 permit analyses of neural states correlated to naturalistic behaviors of interest. Precise and 22 reliable remote measurements remain technically challenging due to animal movement, which 23 perturbs the relative geometries between the animal and sensors. Pulse-type electric fish generate 24 a train of discrete and stereotyped electric organ discharges (EOD) in order to actively sense 25 their surroundings, and rapid modulation of the discharge rate occurs while free-swimming in 26 Gymnotus sp.. The modulation of EOD rates is a useful indicator of the fish’s central state such 27 as resting, alertness, and learning associated with exploration. However, the EOD pulse 28 waveforms remotely observed at a pair of dipole electrodes continuously vary as the fish swims 29 relative to the electrodes, which biases the judgment of the actual pulse timing. In order to 30 measure the EOD pulse timing more accurately, reliably, and non-invasively from a free31 swimming fish, we propose a novel method based on the principles of waveform reshaping and 32 spatial averaging. Our method is implemented using envelope extraction and multi-channel 33 summation, which is more precise and reliable in comparison to other widely used thresholdor 34 peak-based methods, according to the tests performed under various source-detector geometries. 35 Using the same method, we constructed a real-time electronic pulse detector performing an 36 additional online pulse discrimination routine to further enhance the detection reliability. Our 37 stand-alone pulse detector performed with high temporal precision (<10 μsec) and reliability 38 (error <1 per 10 pulses), and permits longer recording duration by storing only event timestamps 39 (4 bytes/pulse). 40