Directional characteristics of tuberous electroreceptors in the weakly electric fish, Hypopomus (Gymnotiformes)

This paper is an electrophysiological study of the directionality of the tuberous electroreceptors of weakly electric fish. We recorded from two classes of tuberous electroreceptors known for pulse gymnotiforms: Burst Duration Coders (BDCs), and Pulse Markers (PMs). Both code for stimulus amplitude, although the dynamic range for BDCs is greater, and both exhibit strong directional preferences. Polar plots of spike number (for BDCs) or spike threshold (for PMs) versus electric field azimuth, are figure-8 shaped with two asymmetrical, elliptical lobes separated by 180 degrees. The best azimuth of these two types of receptors from a given body region correlate with each other and with measures of best azimuth for transepidermal current flow. The shape and asymmetry of the directionality profiles appear to be caused by filter dynamics of the receptors. Pulse Markers are located on the anterior part of the body surface while Burst Duration Coders are located all over. The best directions of receptors in the anterior third of the body vary systematically with location from 0 degrees to 180 degrees. This region is probably critical for determining the direction of local electric fields. Together these receptors provide the CNS with sufficient information to construct a map of horizontal plane electric field directions.