HOFMANN, MH; WILKENS, LA; Univ. of Bonn, Germany; Univ. of Missouri, St. Louis: Physiology of the electrosensory system of the paddlefish

Many aquatic animals use the passive electrosense to detect the weak electric fields from other animals or from geoelectric sources. Paddlefish have greatly enlarged their electrosensory system compared to other animals, with up to 75,000 receptors distributed in the skin of the head and elongated rostrum. Electroreception, however, is different from other skin senses because sources can be localized at some distance to the skin. Distant sources, however, stimulate a large number of receptors at the same time and central circuits have to compute the exact location of the source from this distributed information. In order to gain insights into the algorithms involved, we compared the response properties of neurons in the dorsal octavolateral nucleus (DON) with primary afferent fibers in the paddlefish to a variety of stimuli. Although there are slight differences regarding receptive field size, there are no differences in sensitivity, frequency tuning, or responses to moving objects. Although all DON units show well-defined receptive fields, there is no sign of topographic order within the DON that could serve as the basis for spatial information processing. However, we have found that DON units analyze the signal in the time domain by computing the first derivative. Since there is always a relative movement between source and receiver under natural conditions, we simulated the signal perceived by an animal traversing a dipole field and found that a derivative function would transform distance information into a well defined peak in the frequency domain that is proportional to the distance. These simulations show that there is rich information in the time domain of such a signal and that the first derivative is an important step in extracting this information. We still have to find out whether the paddlefish actually make use of this information.