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Unformatted text preview: REVIEW G. von der Emde Non-visual environmental imaging and object detection through active electrolocation in weakly electric fish Received: 3 November 2004/ Revised: 5 October 2005/ Accepted: 26 December 2005/Published online: 28 January 2006 Springer-Verlag 2006 Abstract Weakly electric fish orient at night by employing active electrolocation. South American and African species emit electric signals and perceive the consequences of these emissions with epidermal elec- troreceptors. Objects are detected by analyzing the electric images which they project onto the animals electroreceptive skin surface. Electric images depend on size, distance, shape, and material of objects and on the morphology of the electric organ and the fishs body. It is proposed that the mormyrid Gnathonemus petersii possesses two electroreceptive foveae at its Schnau- zenorgan and its nasal region, both of which resemble the visual fovea in the retina of many animals in design, function, and behavioral use. Behavioral experiments have shown that G. petersii can determine the resistive and capacitive components of an objects complex impedance in order to identify prey items during for- aging. In addition, fish can measure the distance and three-dimensional shape of objects. In order to deter- mine object properties during active electrolocation, the fish have to determine at least four parameters of the local signal within an objects electric image: peak amplitude, maximal slope, image width, and waveform distortions. A crucial parameter is the object distance, which is essential for unambiguous evaluation of object properties. Keywords Electroreception Object recognition Distance 3-D shape Capacitance detection Electric fovea Abbreviations ELL: Electrosensory lateral line lobe EO: Electric organ EOCD: Electric organ corollary discharge EOD: Electric organ discharge S+: Positive stimulus S : Negative stimulus Introduction Animals that use active location systems actively emit signals into the environment and perceive these signals after they have been modified by the external world. Objects are detected because they change the emitted signal in a way perceivable by the animal. Well-known examples of active location systems are echolocation in bats [for review, see Moss and Sinha ( 2003 )], whales [for review, see Harley et al. ( 2003 )], and some other animals (acoustic signals emitted by the vocal apparatus and perceived by the ears), and electrolocation in weakly electric fish. However, also haptic inspection of objects, e.g., by humans, can be considered to be an active location system [for review, see Newell et al. ( 2001 )]....
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