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Unformatted text preview: ORIGINAL PAPER Stephan Schwarz Gerhard von der Emde Distance discrimination during active electrolocation in the weakly electric sh Gnathonemus petersii Accepted: 19 May 2000 / Published online: 10 January 2001 Springer-Verlag 2001 Abstract Weakly electric sh use active electrolocation for orientation at night. They emit electric signals electric organ discharges) which generate an electrical eld around their body. By sensing eld distortions, sh can detect objects and analyze their properties. It is unclear, however, how accurately they can determine the distance of unknown objects. Four Gnathonemus petersii were trained in two-alternative forced-choice procedures to discriminate between two objects diering in their distances to a gate. The sh learned to pass through the gate behind which the corresponding object was farther away. Distance discrimination thresholds for dierent types of objects were determined. Locomotor and elec- tromotor activity during distance measurement were monitored. Our results revealed that all individuals quickly learned to measure object distance irrespective of size, shape or electrical conductivity of the object material. However, the distances of hollow, water-lled cubes and spheres were consistently misjudged in com- parison with solid or more angular objects, being per- ceived as farther away than they really were. As training continued, sh learned to compensate for these `elec- trosensory illusions' and erroneous choices disappeared with time. Distance discrimination thresholds depended on object size and overall object distance. During dis- tance measurement, the sh produced a fast regular rhythm of EOD discharges. A mechanisms for distance determination during active electrolocation is proposed. Key words Depth perception Environmental imaging Spatial orientation Sensory illusion Behavioral strategies Abbreviations EOD electric organ discharge GD gate distance OD inter-object distance Introduction A feature of particular importance to animals for orientation within their environment is to know how far away are other animals or objects. Depending on the sensory system, several parameters of the physical input to the sense organs can be used for this necessary dis- tance determination. In echolocating bats, for example, the time delay between emitting a sound and hearing the echo reected by an object codes object distance Neu- weiler 1990). Visually oriented animals can use the dynamic properties of visual images; for instance, the change of image size or angle of an object projected on the retina during moving Wagner 1982; Lehrer et al. 1988). In binocular animals with overlapping visual elds of the two eyes, the disparity of the two retinal images depends on object distance and can be used for visual depth perception Collett and Harkness 1982; Howard and Rogers 1995; Timney and Keil 1999)....
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