Electric images of two low resistance objects in weakly electric fish

Abstract

Electroreceptive fish detect nearby objects by processing the information contained in the pattern of electric currents through their skin. In weakly electric fish, these currents arise from a self-generated field (the electric organ discharge), depending on the electrical properties of the surrounding medium. The electric image can be defined as the pattern of transepidermal voltage distributed over the receptive surface. To understand electrolocation it is necessary to know how electric image of objects are generated. In pulse mormyrids, the electric organ is localized at the tail, far from the receptors and fires a short biphasic pulse. Consequently, if all the elements in the environment are resistive, the stimulus at every point on the skin has the same waveform. Then, any measure of the amplitude (for example, the peak to peak amplitude) could be the unique parameter of the stimulus at any point of the skin. We have developed a model to calculate the image, corroborating that images are spread over the whole sensory surface and have an opposite center-surround, “Mexican-hat” shape. As a consequence, the images of different objects superimpose. We show theoretically and by simulation that the image of a pair of objects is not the simple addition of the individual images of these objects.

Introduction

Electroreceptive fish detect nearby objects by processing the information contained in the pattern of electric currents through their skin. These currents, in weakly electric fish, result from a self-generated field (the electric organ discharge), depending on the relative positions of different parts of the animal (as for example body bending, tail position, etc.) and the electrical properties of the surrounding medium. The electric image can be defined as the pattern of transepidermal voltage (Bastian, 1986, Bell, 1989). From this image the brain constructs a representation of the external world. To understand electrolocation it is necessary to know how electric images of objects are generated in a complex environment. Electric images of isolated objects have been measured in certain specific cases (Aguilera et al., 2001, Hoshimiya et al., 1980, Rasnow, 1996; Rasnow et al., 1993; Rasnow and Bower, 1996, von der Emde and Bleckmann, 1992, von der Emde et al., 1998), but this approach, having the strength of empirical data, lacks the flexibility for describing different scenes and circumstances. Complementing these experimental studies, theoretical analysis of image generation has yielded realistic models that predict with acceptable accuracy the electrosensory stimulus (Assad, 1997, Budelli and Caputi, 2000; Caputi and Budelli, 1995; Caputi et al., 1998, Heiligenberg, 1973, Lissmann and Machin, 1958, Rasnow, 1996). When the image is calculated by these computational models, an unique object is placed either in an infinite environment or in a tank. Experimentally, the image of an object is calculated by the difference in the transcutaneous voltage between two scenes (i.e. the fish and all the objects in the environment) in which the only difference is the presence or absence of that object. But usually the fish is moving in a complex medium with several objects of interest.

We argue in this paper that when including several objects, the resulting image of the scene is not the addition of the images of the individual objects: by the contrary, the presence of an object distorts the image of others, if close enough. As a consequence, when we determine the image of an object as the difference between the current densities in the presence and absence of the object, the result depends on the context; i.e. the presence and characteristics of other objects and the active changes of the field direction related to the orientation of the skin.