### abstract ###
Electroreceptive fish detect nearby objects by processing the information contained in the pattern of electric currents through the skin.
The distribution of local transepidermal voltage or current density on the sensory surface of the fish's skin is the electric image of the surrounding environment.
This article reports a model study of the quantitative effect of the conductance of the internal tissues and the skin on electric image generation in Gnathonemus petersii.
Using realistic modelling, we calculated the electric image of a metal object on a simulated fish having different combinations of internal tissues and skin conductances.
An object perturbs an electric field as if it were a distribution of electric sources.
The equivalent distribution of electric sources is referred to as an object's imprimence.
The high conductivity of the fish body lowers the load resistance of a given object's imprimence, increasing the electric image.
It also funnels the current generated by the electric organ in such a way that the field and the imprimence of objects in the vicinity of the rostral electric fovea are enhanced.
Regarding skin conductance, our results show that the actual value is in the optimal range for transcutaneous voltage modulation by nearby objects.
This result suggests that voltage is the answer to the long-standing question as to whether current or voltage is the effective stimulus for electroreceptors.
Our analysis shows that the fish body should be conceived as an object that interacts with nearby objects, conditioning the electric image.
The concept of imprimence can be extended to other sensory systems, facilitating the identification of features common to different perceptual systems.
### introduction ###
Electroreceptive fish detect nearby objects by processing the information contained in the pattern of electric currents through the skin.
In weakly electric fish, these currents result from a self-generated field, produced by the electric organ discharge.
Local transepidermal voltage or current density is the effective stimulus for electroreceptors.
The distribution of voltage or current on the sensory surface of the fish's skin is the electric image of the surrounding environment CITATION CITATION.
From this image, the brain constructs a representation of the external world.
Therefore, to understand electrolocation it is necessary to know the image-generation strategy used by electrolocating animals.
Theoretical analysis of image generation has yielded realistic models that predict with acceptable accuracy the electrosensory stimulus CITATION CITATION.
One general conclusion of previous reports is that the skin conductance and the conductivity difference between the internal tissues of the fish and the water are the main factors shaping the electric image: the seminal paper by Lissmann and Machin CITATION started a long-lasting controversy about the roles of these factors.
Lissmann and Machin argued that if the fish has approximately the same conductivity as the water and that it does not appreciably distort the perturbing field, the potential distribution around the fish due to the perturbing field can be calculated.
However, several reports CITATION, CITATION, CITATION have indicated that the internal conductivity of freshwater fish is high with respect to the surrounding water, and that the high conductance of internal tissues is critical for enhancing the local EOD field as well as for generating the centre-surround opposition pattern that characterizes electric images and that is coded by primary afferents CITATION .
Experimental studies in pulse gymnotids have confirmed theoretical predictions, showing that the high conductivity of the fish body funnels the self-generated current to the perioral region, where an electrosensory fovea has been described on the basis of electroreceptor density, variety, and central representation CITATION.
This funnelling effect enhances the stimulus at the foveal region.
In addition, two different types of skin have been described in some electric fish of the family Mormyridae: the low-conductance mormyromast epithelium where electroreceptors are present, and the high-conductance non-mormyromast epithelium where electroreceptors are absent CITATION, CITATION.
The mormyromast epithelium is found on the head in front of the gills, as well as along the dorsum of the back and along the ventral surface of the trunk.
The non-mormyromast epithelium is found along the sides of the trunk.
This heterogeneity of skin conductance introduces another factor shaping physical electric images.
This article describes a realistic modelling study of the effect of the internal and skin conductance on electric image generation in G. petersii.
We have calculated the electric image of a metal object on a simulated fish having different magnitudes of conductances for internal tissues and skin.
While the high conductivity of the fish body enhances the electric image by a combination of mechanisms, the skin conductance appears to optimize the transcutaneous voltage modulation by nearby objects.
In contrast, transcutaneous current increases monotonically with skin conductivity.
These results suggest that transcutaneous voltage is the critical proximal stimulus for electroreceptors.
We generalize two concepts: object perturbing field and imprimence, introduced early in electroreception research CITATION, to other sensory systems.
An object perturbs an electric field as if it were adding a new field to the basal one.
This perturbing field can be considered as equivalent to a certain distribution of electric sources.
This distribution is referred to as an object's imprimence.
