Directional sensitivity of tuberous electroreceptors: polarity preferences and frequency tuning

This paper examines the directionality of tuberous electroreceptor responses and relates them to a polarity bias seen for passive electrolocation by electric fish (Hypopomus). We recorded from Burst Duration Coders (BDCs) while stimulating with 1 kHz single period sine waves with electric fields oriented horizontally in different directions. Electroreceptors have figure-8 directional sensitivity profiles with two, usually unequal lobes of sensitivity separated by 180 degrees. For most units the larger lobe points inward, while for a few, the lobes are symmetrical or the larger lobe points outward. The differences correlate with differences in frequency tuning of the receptors. We can alter, and even reverse, the directional asymmetry of a single unit by changing the frequency of the stimulus. Two general response profiles results, with two corresponding classes of tuning curves. The degree of asymmetries and the effects of stimulus frequency and of tuning can be modeled with a linear/non-linear/linear cascade filter. The behavioral preference for approaching the head end (+) of an electrode is difficult to understand in light of the asymmetry of responses we report for amplitude-coding BDCs but can be understood by reference to the time-coding Pulse Marker (PM) receptors.