Fig. 1. Characterization of reafferent electrosensory image and its changes. (A) The diagram illustrates the methodology employed. Local electric organ discharge (LEOD) of Gymnotus carapo was recorded between an electrode adjacent to the skin, and the closest base of a cylindrical object placed 2 mm away from the skin. The electrode was a 100 µm bare-tip insulated wire; the object consisted of a 2 mm diameter, 10 mm long plastic tube with a carbon plug electrode in each opening. An external variable resistor r₀ was connected to the carbon plugs to set the baseline amplitude (bPP) of the local EOD. A second variable resistor r₁ was periodically connected in parallel, using a timed switch setting the comparison LEOD amplitude (cPP). Changes in object longitudinal resistance resulted in marked changes in image contrast. (B) LEOD recorded at the center of the image of a cylindrical object facing the electrosensory fovea. Left: baseline LEOD obtained without load (r=) and right: comparison LEOD obtained when the same object was loaded with a short circuit (r=0). Wave components are labeled as V₁, V₃ and V₄ (according to the nomenclature introduced by Trujillo-Cenóz et al., 1984; V₂ is not present at the foveal region). (C) The object resistance change mainly effects the contrast of the image. The amplitudes of each of these LEOD peaks are `one-to-one' functions of the peak-to-peak LEOD (PP), indicating that changes in waveform are small and predictable from the change in PP. (D) The electric image of a metal cylinder consists of a Mexican-hat spatial profile. This is illustrated by the plot of the change in the peak of V₃ caused by the presence of the object as a function of distance from the projection of the center of the object. The dotted line indicates the amplitude of V₃ in the absence of the object (modified from Caputi et al., 2003).