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 Ó 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 animal’s electroreceptive skin surface. Electric images depend on size, distance, shape, and material of objects and on the morphology of the electric organ and the fish’s 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 object’s 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 object’s 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 Æ Object recognition Æ Distance Æ 3-D shape Æ Capacitance detection Æ Electric fovea Abbreviations ELL: Electrosensory lateral line lobe Æ EO: Electric organ Æ EOCD: Electric organ corollary discharge Æ EOD: Electric organ discharge Æ S+: Positive stimulus Æ SÀ: 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)]. All active location systems have in common that they are used in situations where visual perception is not possible or hindered, such as in nocturnal animals like bats or weakly electric fish. However, echolocation and electrolocation are not mere substitutes for vision, because these systems can provide additional informa- tion about environmental objects, which cannot be gathered through vision. For example, echolocating dolphins have been shown to detect the thickness of the walls of hollow metal cylinders and to find out about the material composition of underwater objects (Pack et al. 2004). Mormyrid weakly electric fish can determine the electrical properties of objects, i.e., they measure the complex electrical impedance of an electrolocation tar- get (von der Emde and Ronacher 1994) and thereby discriminate categorically between animated and dead items (von der Emde 1990). Electrolocation and echolocation thus provide advantages over vision, a fact that is exploited in artifi- cial, biomimetic systems, which were developed after biological models. Well-known examples designed G. von der Emde (&) Neuroethologie/Sensorische O ¨ kologie, Institut fu¨r Zoologie, Universita¨t Bonn, Endenicher Allee 11-13, 53115 Bonn, Germany E-mail: vonderemde@uni-bonn.de Tel.: +49-228-735555 Fax: +49-228-735556 J Comp Physiol A (2006) 192: 601–612 DOI 10.1007/s00359-006-0096-7