DIRECTIONAL SENSITIVITY OF TUBEROUS ELECTRORECEPTORS - POLARITY PREFERENCES AND FREQUENCY TUNING

This paper examines the directionality of tuberous electroreceptor res ponses and relates them to a polarity bias seen for passive electroloc ation by electric fish (Hypopomus). We recorded from Burst Duration Co ders (BDCs) while stimulating with 1 kHz single period sine waves with electric fields oriented horizontally in different directions. Electr oreceptors have figure-8 directional sensitivity profiles with two, us ually unequal lobes of sensitivity separated by 180-degrees. For most units the larger lobe points inward, while for a few, the lobes are sy mmetrical 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 changi ng the frequency of the stimulus. Two general response profiles result , with two corresponding classes of tuning curves. The degree of asymm etry varies with position on the, body surface. The asymmetries and th e effects of stimulus frequency and of tuning can be modeled with a li near/non-linear/linear cascade filter. The behavioral preference for a pproaching 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 Mark er (PM) receptors.