1. Electrolocation by weakly electric fish Ruben Budelli, Angel Caputi, Leonel Gómez and Adriana Migliaro Facultad de Ciencias and Instituto de Investigaciones Biológicas Clemente Estable, Montevideo, Uruguay.

2. An image Vermeer: The Little Street. In vision, an image is a distribution of light on the retina. In this case the image coincides with a real image, generated by an optical device (cornea, lens, etc.) In general an image is a distribution of some kind of energy on a sensory surface (i.e.: a surface where sensory receptors are distributed.)

3. Electric images An electric image is a distribution of currents or electric fields on the electrosensory surface, from the skin of some animals. This distribution can be generated by external sources in electrocommunication or pasive electrolocation, or by internal sources (the Electric Organ, EO) in active electrolocation. In pulse fishes, the EO fire a very brief stereotyped signal: the EO Discharge (EOD). Currents generated by this pulse are modulated by objects close to the fish skin. The changes produced by the presence of an object is called the electric image of the object.

4. Electric images The electric image is sensed by electro- receptors, that send information about the amplitude and waveform to the Central Nervous System (CNS). With this information the CNS has to construct a representation of the environment. Rules used by the CNS to do this are been studied.

5. Scheme showing how the program, developed by Diego Rother, determine the image of an object in a scene.using the Boundary Element Method (BEM) Skin resistance Fish shape Sources Internal conductivityScene Solid Current flow Absolute values Images

6. Electric Images of Resistive Objects sphere cube pyramid Resistive objects of different shapes, produce images with different profiles. Plots show the images at an horizontal plane passing through the center of the object (see the inset).

7. If the fish internal resistivity were similar to that of water, the basal field generated by The EO would be similar to that of a dipole (figure on top). A cube in the position marked by a white square in the figure at bottom, should produce a field similar to that of a dipole oriented in the direction of the basal field and generating a byphasic image: the current enters through caudal regions and exits through rostral regions (figure at bottom).

8. In a fish with the internal resistivity determined experimentally, the basal field is almost perpendicular to the fish skin (figure at top) and the perturbation produce an increase of the trans- epidermal current just in front of the object and a dicrease in a sourronding region (figure at bottom).

9. The electric image as a function of the internal conductivity The size of the image (measure as current flow) increases with the conductivity (Fig. A). The central part of the image becames wider and more symetric as the internal conductivity increases (Fig. B)

10. Where: x and y are the coordinates at the interface  1 y  2 are the conductivities,  1 y   2 are the electric permittivity, a is the distance between the dipole and the interface and p is the dipole moment. Jn(x y) =  1  2 p(x 2 /a 2 + y 2 /a 2 − 2) 2  1 (  1 +  2 ) a 3 (x 2 /a 2 + y 2 /a 2 + 1) 5/2 The image generated by a dipole perpendicular to the sensory surface has a center sourround (“Mexican hat”) shape.

11. The effect of the skin conductivity The image change when the skin conductivity is changed. The shape becames smoother and a litle wider (profile at right). Measured as current densities (red trace, left) it increase with conductivity. Measured as transepidermal voltage (blue trace, left) it present a maximum for conductivities in the range of those of fish skin (marked by a rectangle). Skin conductivity Peak current (red trace) Peak current (blue trace)

12. The effect of the skin resistance The image measure as current densities (at left) and as transepidermal voltage (at right). The brocken line, corresponds to a fishwith the experimentally determined skin resistance distribution (close to an homogeneour intermediate value, in blue).