Proc. Natl Acad. Sci. USA (2014)


Close your eyes and try to guess the shapes of bodies moving around you. Having trouble? It might help to generate an electric field, like certain fish species. Weakly electric fish (pictured) locate both predators and prey by sensing changes in their transdermal potential in response to objects whose electromagnetic properties differ from those of water. Habib Ammari and colleagues have now devised a scheme that these fish may employ to determine the shape of the objects they sense.

The way that a target perturbs the electric field produced by a weakly electric fish is a nonlinear function of its shape and proximity. And it differs depending on whether the target is living or dead: the electromagnetic properties of biological matter are frequency dependent, whereas those of their inanimate counterparts are not.

Ammari et al. simplified the problem of shape determination by proposing a method based on a multipole expansion of the field perturbations. Within their scheme, sensing a non-biological target involves drawing on a dictionary of shapes — a kind of model memory for the fish. But living targets can be classified using the spectral content of field-modulation data they amass.