Pluta SR, Kawasaki M. Temporal selectivity in midbrain electrosensory neurons identified by modal variation in active sensing. weakly electric fish actively sense their surroundings by continuously emitting discrete pulses of electricity separated by varying intervals of silence. The temporal pattern of this pulsing behavior is related to context. While resting in the absence of an overt stimulus, baseline interpulse intervals (IPIs) mostly range 200 -450 ms, and sequential variation is

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... ly high. Spontaneously, or following the presentation of a novel stimulus, IPIs transiently shorten during the performance of an electromotor "burst" display. We made intracellular whole cell recordings in vivo from neurons in the lateral nucleus of the torus semicircularis while the fish's dynamic pulsing behavior modified the temporal pattern of stimulation. Stimulation was designed to simulate the spatial patterns of AM that occur during the electrolocation of a resistive object. We discovered that toral neurons selectively respond to stimulation during a particular mode of electromotor activity. Two types of temporally selective neurons were discovered: baseline-selective neurons that displayed significantly higher postsynaptic potential (PSP) amplitude and spike count per electric organ discharge (EOD) during baseline electromotor activity and burst-selective neurons that displayed significantly higher PSP amplitude and spike count per EOD during electromotor burst displays. Interval-dependent changes in the strength of excitation and inhibition contributed to their selectivity. Action potentials were counted by locating positive derivatives in membrane potential Ͼ12 mV/ms after applying a Savitzky-Golay FIR smoothing filter (The Mathworks, Natick, MA). The filter caused a 1-3 mV reduction in action potential height while practically abolishing the electrosensory stimulus artifact. The minimum time between two action potentials was set to 1.2 ms to avoid counting the 499 TEMPORAL SELECTIVITY IN ELECTROSENSORY PROCESSING