REVIEWSNATURE REVIEWS | NEUROSCIENCE VOLUME 5 | DECEMBER 2004 | 943Behaviour,although multifaceted and diverse, also seemsto be convergent across taxa. Even distantly relatedorganisms can show similar behaviours,involvingsensory pattern recognition,locomotion and experience-dependent changes in sensory processing and motoroutput. In neuroscience, the prevalent use of particularsystems as models for understanding the function of thehuman nervous system rests on this functional overlapand structural homology.However,we are only begin-ning to understand whether similarities in behaviour areparalleled by similarities in control mechanisms,neuralcircuitry and processing. This gap in knowledge is notsurprising; the identification of the neural control of anyparticular behaviour or function can be a formidablechallenge.As we learn more about how neural circuitscontrol behaviour,we hope to gain a greater under-standing ofwhy particular solutions have developed1.Integration of information at these two levels will beessential for revealing the uniqueness of particular neuralcircuits2and mechanisms,as well as for understandingthe roles of historical forces in determining the finalarchitecture of neural circuits and processing.Electrosensory systems (BOX 1) are well suited toaddressing these questions.In addition to their estab-lished utility for investigating receptor3and ion channel4function,electric fish have increasingly been used forstudying the neural circuits that control behaviour5.Some fish are purely electroreceptive, whereas others canboth sense and produce electric fields.Most species ofthe latter type continue to produce discharges of theirelectric organs (EODs, electric organ discharges) whenprepared for in vivo neurophysiological recording.Furthermore,changes in these EODs produce a varietyof electrosensory behaviours,permitting investigators tostudy the entire neural circuit for the control of thesebehaviours5,6.The fish’s intended signals can be moni-tored and replaced by substitute signals,thereby openingthe loop between behaviour and its sensory conse-quences6,7.The jamming avoidance responses (JARs) of‘wave-type’gymnotiform fishes have been particularlywell studied.As I will discuss, comparative studies ofJARs and the adaptive cancellation of expected sensoryinformation8have increased our understanding of theneural control of behaviour and its evolution.Jamming avoidance responsesBehaviour. Wav e-type electric fish generate electric fields(FIG.1a) by periodically discharging their electric organs.They can ‘electrolocate’objects9in their environment by sensing perturbations of these fields — that is, bysensing changes in the timing and amplitude of the signals caused by the presence of nearby objects — withINSIGHTS INTO NEURALMECHANISMS AND EVOLUTION OFBEHAVIOUR FROM ELECTRIC FISHGary J.RoseAbstract | Both behaviour and its neural control can be studied at two levels. At the proximatelevel, we aim to identify the neural circuits that control behaviour and to understand howinformation is represented and processed in these circuits. Ultimately, however, we are facedwith questions of why particular neural solutions have arisen, and what factors govern the waysin which neural circuits are modified during the evolution of new behaviours. Only by integratingthese levels of analysis can we fully understand the neural control of behaviour. Recent studiesof electrosensory systems show how this synthesis can benefit from the use of tractablesystems and comparative studies.Department of Biology,University of Utah,Salt Lake City,Utah 84112-0840,USA.e-mail:[email protected]:10.1038/nrn1558944 | DECEMBER 2004 | VOLUME 5 www.nature.com/reviews/neuroREVIEWSBox 1 | ElectroreceptionElectroreception,which is the detection of weak electric fields, is widespread among vertebrates, with cases in all classes offishes, two orders of amphibians and even mammals (the duck-billed platypus). This ‘exotic’sense seems to be an ancestralvertebrate trait, as it is present in lampreys and cartilaginous fishes. Its spotty presence in particular vertebrate groupsindicates that electroreception has evolved (been ‘reinvented’) a number of times during vertebrate evolution.Particularlycompelling evidence for the independent evolution of this sense is its presence in mormyriforms (African) andgymnotiforms (South American), two distantly related (Osteoglossomorpha versus Ostariophysi) orders of teleost(modern, bony) electric fishes,and in the duck-billed platypus, a monotreme mammal.In all cases, electroreception doesnot seem to be the ancestral condition.Modern holostean (the lineage that gave rise to teleosts) fishes are not electro-receptive. Similarly, electroreception in the duck-billed platypus is probably a derived trait because it is not characteristic ofreptilians (from which mammals evolved).Electroreceptors vary in sensitivity (from 0.005 µV cm–1to >0.1 mV cm–1) andfrequency sensitivity (near DC (direct current) to >15 kHz).All electroreceptive animals have ampullary receptors,whichare highly sensitive and best excited by very low frequencies (less than 30 Hz). Other electroreceptor types are found in mostelectrogenic species. Electrogenic fish produce electric signals by discharging their electric organs, which consist of columnsof modified muscle cells (electrocytes)57.Some organs generate strong discharges (hundreds of volts) that are useful forstunning prey, whereas others produce weak discharges (millivolts) that are used for social communication andelectrolocation. Species that have electric organs of the latter type produce either intermittent (pulse species) or periodic(wave species) discharges. Both types of weakly-electric fish also have electroreceptors that are tuned to the species-specifichigher frequencies found in their discharges.b reproduced,with permission,from REF.19  (1999) Company of Biologists.abAgnatha(jawless fishes)Chondrichthys(cartilaginous fishes)Osteichthys(bony fishes)Amphibia ReptiliaMammaliaLampreysRatfishSkates andRaysSharksMonotremesAvesAnuraUrodelaApodaPlatypusSalamandersCaeciliansTele ostsCrossopterygiiSturgeonPaddlefishPolypterusCoelacanthOstracodermsChordateancestorPlacodermsMormyriformesAfrican notopteridsNotopteriformesOsteoglossiformesHiodontiformesGymnotiformesSiluriformesCharaciformesCypriniformesConorynchiformes'Modern' teleostsElopomorpha(157 genera)Clupeomorpha(68 genera)Osteoglossomorpha