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Electric fish measure distance in the dark

Abstract

Distance determination in animals can be achieved by visual or non-visual cues1. Weakly electric fish use active electrolocation for orientation in the dark2. By perceiving self-produced electric signals with epidermal electroreceptors, fish can detect, locate and analyse nearby objects. Distance discrimination, however, was thought to be hardly possible because it was assumed that confusing ambiguity could arise with objects of unknown sizes and materials3,4,5. Here we show that during electrolocation electric fish can measure the distance of most objects accurately, independently of size, shape and material. Measurements of the ‘electric image’ projected onto the skin surface during electrolocation6,7,8 revealed only one parameter combination that was unambiguously related to object distance: the ratio between maximal image slope and maximal image amplitude. However, slope-to-amplitude ratios for spheres were always smaller than those for other objects. As predicted, these objects were erroneously judged by the fish to be further away than all other objects at an identical distance. Our results suggest a novel mechanism for depth perception that can be achieved with a single, stationary two-dimensional array of detectors.

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Figure 1: a, Electric image size and amplitude change with object distance.
Figure 2: Psychometric functions for distance discrimination of (a) two identical metal cubes (64 cm3), (b) two identical metal spheres (33.5 cm3) and (c) two metal cubes of different sizes (125 and 27 cm3).
Figure 3: Electric image measurements.
Figure 4: Psychometric functions for distance discrimination of a metal sphere and a metal cube, and for two identical metal cubes.

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Acknowledgements

The behavioural experiments and the analysis of the electric images were performed by S.S. during work for his diploma thesis. We thank H. Bleckmann for providing laboratory space and for his continuous support throughout this study; C. C. Bell, H. Bleckmann, J. Mogdans, S. F. Perry, F. Schaeffel and H. Wagner for critically reading the manuscript and helpful discussions; C. Gutzen for help with the figures; and W. Alt for statistical advice. G.v.d.E. is a recipient of a Heisenberg stipend of the DFG. This work was financed partly by a research grant from the EC to K.G. by the Franco-German international exchange programme PROCOPE, by the Franco-Uruguayan exchange programme ECOS and by a doctoral fellowship to L.G. from the French Ministry of Foreign Affairs.

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Correspondence to Gerhard von der Emde.

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von der Emde, G., Schwarz, S., Gomez, L. et al. Electric fish measure distance in the dark. Nature 395, 890–894 (1998). https://doi.org/10.1038/27655

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