Search

Swarm behaviors in nature have inspired the emergence of many heuristic optimization algorithms. They have attracted much attention, particularly for complex problems, owing to their characteristics of high dimensionality, nondifferentiability, and the like. A new heuristic algorithm is proposed in this study inspired by the prey location and communication behaviors of electric fish. Nocturnal electric fish have very poor eyesight and live in muddy, murky water, where visual senses are very limited. Therefore, they rely on their species-specific ability called electrolocation to perceive their environment. The active and passive electrolocation capability of such fish is believed to be a good candidate for balancing local and global search, and hence it is modeled in this study. A new heuristic called electric fish optimization (EFO) is introduced and compared with six well-known heuristics (simulated annealing, SA; vortex search, VS; genetic algorithm, GA; differential evolution, DE; particle swarm optimization, PSO; and artificial bee colony, ABC). In the experiments, 50 basic and 30 complex mathematical functions, 13 clustering problems, and five real-world design problems are used as the benchmark sets. The simulation results indicate that EFO is better than or very competitive with its competitors.

The primary goal of this volume is to provide an updated perspective on the topics of electrogenesis and electroreception in fishes. Throughout, there is an emphasis on how comparative perspectives can inform general issues regarding the neural mechanisms of behavior, from detailed comparisons among related species having divergent phenotypes to broad comparisons across distantly related clades having similar phenotypes. The underlying theme throughout is that evolution provides a natural experiment that can be exploited to relate variation in behavior to variation in its neural substrates. This allows for the development and testing of hypotheses regarding the neural control of behavior and for distinguishing generally applicable principles from clade-specific differences. The chapters cover a range of topics including the evolution and development of electric organs and electroreceptors, electrosensory transduction, evolutionary drivers and biophysical bases of electric signal diversity, influences of hormones and motor systems on electrosensory processing, envelope and temporal coding, use of control theory to characterize active sensing, and the role of active electrolocation and spatial learning in behavior. In this introductory chapter, a brief history of research on electrogenesis and electroreception in fishes is presented, with a summary of some of the most important neuroethological studies in electric fish that have contributed greatly to our understanding of brain function and the neural basis of behavior. The field of electroreception research continues to provide fertile ground for using comparative frameworks to understand the neurobiology of animal communication, social behavior, orientation and navigation, and the evolution of information processing.

Elephant nose fish searches its food such as larvae by active electrolocation. It discharges electric pulse through its electric organ in tail and detects the object by analyzing the geometrical property of projected electrical image on it. The capacitance value found out from that electric image helps the fish to reach near the food source. Shark also uses passive electrolocation for the same purpose. It can target its prey by sensing the electrical wave generated due to the muscle twitching of small living beings in water. Both the above physiological phenomena, concerning the active and passive electrolocation of fish, has been mathematically developed as nature-inspired meta-heuristic technique named fish electrolocation optimization (FEO). A comparative study based on benchmark functions has been done amongst real coded genetic algorithm, accelerated particle swarm optimization, particle swarm optimization, harmony search and the proposed algorithm. Furthermore, comparative study has been done with simulated annealing and differential evolution on eggcrate function. The proposed technique has also been implemented on real-world optimization problem related to cost-based reliability enhancement in radial distribution system. It can be said by comparing percentage of success, mean number of function evaluation and standard deviation that FEO algorithm works better than other mentioned meta-heuristic techniques.

The ability to localize communication signals plays a fundamental role in social interactions. For signal localization to take place, the sensory system of the receiver must extract information about distance and direction to the sender from physical characteristics of the signal. In many sensory systems, information from multiple peripheral receptors must be integrated by central sensory pathways to determine the sender location. Here, we asked whether evolutionary divergence in the electrosensory and visual systems of mormyrid fish is associated with signal localization behavior. In mormyrids, differences in the distribution of electroreceptors on the surface of the skin are associated with differences in the midbrain exterolateral nucleus (EL). Species with electroreceptors clustered in three rosettes on both sides of the head have a small and undifferentiated EL. In contrast, EL is enlarged and subdivided into anterior (ELa) and posterior (ELp) regions in species that have electroreceptors broadly ­distributed throughout the body. Interestingly, species with EL and clustered electroreceptors also have larger visual systems and higher visual acuity than species with ELa/ELp and broadly distributed electroreceptors. Species with broadly distributed electroreceptors and ELa/ELp approached a simulated conspecific by following the curved electric field lines generated by the electrosensory stimulus. In contrast, a species with small EL and clustered electroreceptors, but an enlarged visual system, followed shorter and straighter paths to the stimulus source. In the central electrosensory system, evoked field potentials in response to stimuli delivered from the left versus the right differed more in EL than in ELa/ELp. Our results suggest that signal localization behavior is associated with differences in sensory specializations. We propose that the distribution of electroreceptors on the body affects the ability of individuals to align parallel to electric field lines and maintain such alignment while approaching the signal source. The spatial resolution of sensory information relayed from the periphery to the midbrain in species with clustered electroreceptors may allow for gross, but not fine, processing of sender location. Furthermore, visual information may play an important role in localizing signaling individuals in species with small EL and clustered electroreceptors. In line with previous studies, we suggest that the physiological and behavioral differences associated with signal localization reflect adaptations to different habitats and social environments.

Weakly electric fish use an electric organ discharge (EOD) to generate an electric field around their body. Two different families of teleost fish have independently evolved an active electrosense: the South American gymnotiform fish and the African mormyrid fish. Fish from both families have numerous cutaneous electroreceptors that can sense their own electric field as well as that of neighboring conspecifics. The electroreceptors then project to the hindbrain and provide the input required for these fish to navigate and catch prey in total darkness, as well as to locate and communicate with conspecifics. Over the last six decades, remarkable progress has been made in working out the neural mechanisms that enable electrolocation as well as electrocommunication. Remarkably, many of the same computations found in the more familiar auditory, somatosensory and visual systems also operate in the electrosense. In this Reference Module, I summarize our current understanding of the neural mechanisms of electrolocation for navigation, prey capture, communication and for electrolocation of moving conspecifics.

Weakly electric fish can sense electric signals produced by other animals whether they are conspecifics, preys or predators. These signals, sensed by passive electroreception, sustain electrocommunication, mating and agonistic behavior. Weakly electric fish can also generate a weak electrical discharge with which they can actively sense the animate and inanimate objects in their surroundings. Understanding both sensory modalities depends on our knowledge of how pre-receptorial electric images are formed and how movements modify them during behavior. The inability of effectively measuring pre-receptorial fields at the level of the skin contrasts with the amount of knowledge on electric fields and the availability of computational methods for estimating them. In this work we review past work on modeling of electric organ discharge and electric images, showing the usefulness of these methods to calculate the field and providing a brief explanation of their principles. In addition, we focus on recent work demonstrating the potential of electric image modeling and what the method has to offer for experimentalists studying sensory physiology, behavior and evolution.

International audience; This paper shows how a sensor inspired by an electric fish could be used to help navigate in confined environments. Exploiting the morphology of the sensor, the physics of electric interactions, as well as taking inspiration from passive electrolocation in real fish, a set of reactive control laws encoding simple behaviors, such as avoiding any electrically contrasted object, or seeking a set of objects while avoiding others according to their electric properties, is proposed. These reflex behaviors are illustrated on simulations and experiments carried out on a setup dedicated to the study of electric sense. The approach does not require a model of the environment and is quite cheap to implement.

Weakly electric fish use self-generated electric fields to identify prey, explore their surroundings, and communicate with conspecifics. They have electroreceptors over their entire skin surfaces, and readings from these electroreceptors form electric images. The characteristics of electric images include important information such as the locations, shapes, and electrical properties of target objects. The lateral distance to a target object can be estimated using the rate between maximal slope and maximal amplitude, or relative slope, and provides a direct cue to localize target objects, irrespective of size and electrical properties. Sensor readings acquired from distributions of electroreceptors are interpreted as spatial electric images. Temporal electric images can be assessed using a single electroreceptor by back-and-forth swimming. Some species of weakly electric fish engage in back-and-forth swimming behavior during foraging and can use spatiotemporal information to localize objects. With active body movement and forward swimming, a single electroreceptor can scan the time course of potential perturbation caused by a target object. If there is more than one target object, electric images may be distorted by interactions between individual images. In this paper, we suggest that temporal relative slope is an effective measure for the electrolocation of multiple objects by robotic systems. We investigate the characteristics of temporal relative slopes based on back-and-forth sweeping motions to localize multiple spherical objects via modeling experiments. This method can be applied to biomimetic robotic systems in the underwater environment.

Weakly electric fishes emit electric organ discharges (EODs) from their tail electric organs and sense feedback signals from their EODs by electroreceptors in the skin. The electric sense is utilized for various behaviors, including electrolocation, electrocommunication, and the Jamming avoidance response (JAR). For each behavior, various types of sensory Information are embedded in the transient electrical signals produced by the fish. These temporal signals are sampled, encoded, and further processed by peripheral and central neurons specialized for time coding. There are time codes for the sex or species Identities of other fish or the resistance and capacitance of objects. In the central nervous system, specialized neural elements exist for decoding time codes for different behavioral functions. Comparative studies allow phylogenetic comparison of time-coding neural systems among weakly electric fishes.

Instead of vision, many nocturnal animals use alternative senses for navigation and object detection in their dark environment. For this purpose, weakly electric mormyrid fish employ active electrolocation, during which they discharge a specialized electric organ in their tail which discharges electrical pulses. Each discharge builds up an electrical field around the fish, which is sensed by cutaneous electroreceptor organs that are distributed over most of the body surface of the fish. Nearby objects distort this electrical field and cause a local alteration in current flow in those electroreceptors that are closest to the object. By constantly monitoring responses of its electroreceptor organs, a fish can detect, localize, and identify environmental objects. Inspired by the remarkable capabilities of weakly electric fish in detecting and recognizing objects, we designed technical sensor systems that can solve similar problems of remote object sensing. We applied the principles of active electrolocation to technical systems by building devices that produce electrical current pulses in a conducting medium (water or ionized gases) and simultaneously sense local current density. Depending on the specific task a sensor was designed for devices could (i) detect an object, (ii) localize it in space, (iii) determine its distance, and (iv) measure properties such as material properties, thickness, or material faults. Our systems proved to be relatively insensitive to environmental disturbances such as heat, pressure, or turbidity. They have a wide range of applications including material identification, quality control, non-contact distance measurements, medical applications and many more. Despite their astonishing capacities, our sensors still lag far behind what electric fish are able to achieve during active electrolocation. The understanding of the neural principles governing electric fish sensory physiology and the corresponding optimization of our sensors to solve certain technical tasks therefore remain ongoing goals of our research.

Weakly electric fish are able to detect and localize prey based on microvolt-level perturbations in the fish's self-generated electric field. In natural environments, weak prey-related signals are embedded in much stronger electrosensory background noise. To better characterize the signal and background components associated with natural electrolocation tasks, we recorded transdermal voltage modulations in restrained Apteronotus albifrons in response to moving spheres, tail bends, and large nonconducting boundaries. Spherical objects give rise to ipsilateral images with center-surround structure and contralateral images that are weak and diffuse. Tail bends and laterally placed nonconducting boundaries induce relatively strong ipsilateral and contralateral modulations of opposite polarity. We present a computational model of electric field generation and electrosensory image formation that is able to reproduce the key features of these empirically measured signal and background components in a unified framework. The model comprises an array of point sources and sinks distributed along the midline of the fish, which can conform to arbitrary body bends. The model is computationally fast and can be used to estimate the spatiotemporal pattern of activation across the entire electroreceptor array of the fish during natural behaviors.

The weaklyelectric fish Gnathonemus petersii detects, localizes, and analyzes objects during active electrolocation even in complete darkness. This enables these fish to lead a nocturnal life and find and identifytheir prey(small insect larvae) on the ground of their freshwater habitat. During active electrolocation, fish produce a series of brief electric signals, electric organ discharges (EOD), with an electric organ in their tail. Each EOD builds up a stable electric field around the fish, which is distorted onlyby nearbyobjects. Field distortions lead to changes of the transepidermal electric current flow at a region of the fish’s electroreceptive skin surface called the ‘electric image’. Within the electric image, locallyperceived EODs can be either altered in amplitude or waveform byan object. Fish measure both parameters to assess object properties, such as the capacitive and resistive components of the object’s complex impedance, the object’s size and shape, and its distance from the fish. None of these object properties can be evaluated in isolation, but have to be inferred during parallel processing of electric image spatial and qualitative parameters. Two anterior skin regions of G. petersii appear to possess particular properties for special electrolocation tasks and we therefore refer to them as ‘foveal’ regions. Because of its high electroreceptor density, the electric field geometry around it, and its behavioral use, the ‘nasal region’ between the nares and the mouth at the head of the fish is suggested to be a fovea for long-range guidance and object detection. We propose that the ‘Schnauzenorgan’, a long and flexible chin appendix covered denselywith electroreceptor organs, is a second electroreceptive fovea associated with a short-range (food) identification system. Together, these two electric foveae constitute an effective preydetection and identification sy stem. # 2003 Elsevier Ltd. All rights reserved.

Weakly electric fish use active electrolocation – the generation and detection of electric currents – to explore their surroundings. Although electrosensory systems include some of the most extensively understood circuits in the vertebrate central nervous system, relatively little is known quantitatively about how fish electrolocate objects. We believe a prerequisite to understanding electrolocation and its underlying neural substrates is to quantify and visualize the peripheral electrosensory information measured by the electroreceptors. We have therefore focused on reconstructing both the electric organ discharges (EODs) and the electric images resulting from nearby objects and the fish’s exploratory behaviors. Here, we review results from a combination of techniques, including field measurements, numerical and semi-analytical simulations, and video imaging of behaviors. EOD maps are presented and interpreted for six gymnotiform species. They reveal diverse electric field patterns that have significant implications for both the electrosensory and electromotor systems. Our simulations generated predictions of the electric images from nearby objects as well as sequences of electric images during exploratory behaviors. These methods are leading to the identification of image features and computational algorithms that could reliably encode electrosensory information and may help guide electrophysiological experiments exploring the neural basis of electrolocation.

Weakly electric fish produce electric signals (electric organ discharges, EODs) with a specialised electric organ creating an electric field around their body. Objects within this field alter the EOD-induced current at epidermal electroreceptor organs, which are distributed over almost the entire body surface. The detection, localisation and analysis of objects performed by monitoring self-produced electric signals is called active electrolocation. Electric fish employ active electrolocation to detect objects that are less than 12 cm away and have electric properties that are different from those of the surrounding water. Within this range, the mormyrid Gnathonemus petersii can also perceive the distance of objects. Depth perception is independent of object parameters such as size, shape and material. The mechanism for distance determination through electrolocation involves calculating the ratio between two parameters of the electric image that the object projects onto the fish’s skin. Electric fish can not only locate objects but can also analyse their electrical properties. Fish are informed about object impedance by measuring local amplitude changes at their receptor organs evoked by an object. In addition, all electric fish studied so far can independently determine the capacitative and resistive components of objects that possess complex impedances. This ability allows the fish to discriminate between living and non-living matter, because capacitance is a property of living organisms. African mormyrids and South American gymnotiforms use different mechanisms for capacitance detection. Mormyrids detect capacitance-evoked EOD waveform distortions, whereas gymnotiforms perform time measurements. Gymnotiforms measure the temporal phase shift of their EODs induced at body parts close to the object relative to unaffected body parts further away.

Weakly electric fish of the families Gymnotidae and Hypopomidae (Gymnotiformes) are able to locate the electric discharges from conspecifics or from dipole electrodes, and they demonstrate this by mak

Weakly electric fish of the families Gymnotidae and Hypopomidae (Gymnotiformes) are able to locate the electric discharges from conspecifics or from dipole electrodes, and they demonstrate this by making rapid, well-directed approaches toward these electrical sources. A video tracking system was used to follow the movements of electric fish in a large tank and an analytic method was used for computing the direction and magnitude of the electric field anywhere within the cylindrical test tank. Using a static analysis method, we describe the posture of test fish relative to the electric fields during their approaches to stationary or moving electrical stimuli. Using a dynamic analysis, we examine the movements of the fish including the sign and magnitude of velocity and bending in response to electric fields. Electric fish seek to maintain a zero error angle between their body orientation and the local electric field. They do so by bending their body in the direction of the local electric field. The response has a delay of approximately 0.5 s. Swimming in reverse inverts the direction of the bend. These fish also use 'V-turns' to redirect their swim directions when encountering rapidly-changing electric fields.

SUMMARYWeakly electric fish generate an electric field with their electric organ to navigate in space, detect objects and communicate with conspecifics. Several studies have examined how electric fish identify objects with their electroreceptors and use electric images for electrolocation. It has been argued that sensor readings from electroreceptors along the rostrocaudal line allow fish to determine the location of a target object. It is well known that the ratio between the maximal slope and the maximal amplitude of the electric image can allow the discrimination of object distances, regardless of object size and conductivity. In order to understand the temporal pattern of electric images, we used a model of electric field perturbation. Using the model, we suggest that the temporal pattern generated at an electrosensor during tail bending is another cue that can be used by the fish to discriminate object distances. The time course of electric sensor signals from a specific electroreceptor when tail-bending movements are applied can provide information about the lateral distance of a target object.

Many aquatic vertebrates detect weak, low-frequency electric fields produced by other animals to avoid predators and to find prey or mates. These low-frequency fields are sensed with special electroreceptor organs called the ampullae of Lorenzini. Electroreception is an important sensory system present in all basal fish groups and evolved in early vertebrates. Although the structure of receptor organs was described some time ago, their function was discovered only 50 years ago. Today, we know some details of how the electrosense is used, but many aspects of central information processing remain to be discovered.

Strongly electric fish that use electricity as a weapon have been recognized since antiquity, and they played a key role in the discovery of the electrical nature of neuromuscular signaling. More recent discoveries include low-frequency ampullary electroreceptors used for passive electrolocation, as well as weak electric organs and high-frequency tuberous electroreceptors used for active electrolocation and electrocommunication. Modern research on electrosensory organisms is diverse and continues to play an important role in the development of fundamental principles in areas such as sensory processing, motor control, sensorimotor integration, neuronal plasticity, animal communication, and diversification.

Weakly electric fish serve as model systems for active sensing because they actively emit electric signals into the environment, which they also perceive with an array of electroreceptor organs distributed over their skin surface. In this process called ‘active electrolocation’, animals can detect and analyze objects in their environment, which allows them to perceive a detailed electrical picture of their surroundings even in complete darkness. Electroreceptor organs are specialized to respond to amplitude and phase modulations of the local electrical signal caused by nearby objects. Complex information processing by the brain leads to environmental perception resembling visual sensitivity.

SUMMARYWeakly electric fish generate electric fields with an electric organ and perceive them with cutaneous electroreceptors. During active electrolocation,nearby objects are detected by the distortions they cause in the electric field. The electrical properties of objects, their form and their distance,can be analysed and distinguished. Here we focus on Gnathonemus petersii (Günther 1862), an African fish of the family Mormyridae with a characteristic chin appendix, the Schnauzenorgan. Behavioural and anatomical results suggest that the mobile Schnauzenorgan and the nasal region serve special functions in electroreception, and can therefore be considered as electric foveae. We investigated passive pre-receptor mechanisms that shape and enhance the signal carrier. These mechanisms allow the fish to focus the electric field at the tip of its Schnauzenorgan where the density of electroreceptors is highest (tip-effect). Currents are funnelled by the open mouth (funnelling-effect), which leads to a homogenous voltage distribution in the nasal region. Field vectors at the trunk, the nasal region and the Schnauzenorgan are collimated but differ in the angle at which they are directed onto the sensory surface. To investigate the role of those pre-receptor effects on electrolocation, we recorded electric images of objects at the foveal regions. Furthermore, we used a behavioural response(novelty response) to assess the sensitivity of different skin areas to electrolocation stimuli and determined the receptor densities of these regions. Our results imply that both regions – the Schnauzenorgan and the nasal region – can be termed electric fovea but they serve separate functions during active electrolocation.

Three weakly electric fish (Gnathonemus petersii) were force-choice trained in a two-alternative procedure to discriminate between objects differing in their electrical characteristics. The objects were carbon dipoles in plexiglass tubing (length 2.5 cm, diameter 0.6 cm). Their electrical characteristics could be changed by varying the impedance of an external circuit to which they were connected (Fig. 1). In one (the ‘capacitance dipole’) the resistance was very low(< 3 Ω) and the capcitance variable. In the other (the ‘resistance dipole’) the resistance was variable and the capacitance low (

Publisher Summary Many groups of fishes perceive naturally occurring electric stimuli. They possess ampullary electroreceptor organs that respond to low‐frequency electric fields. Two nocturnally active teleost groups produce weak electric signals (electric organ discharges, EOD), which they perceive with tuberous electroreceptor organs. In addition to “passive electrolocation,” these weakly electric fish perform “active electrolocation,” during which they detect alterations of their EOD caused by nearby objects. This enables them to perceive the complex impedance of objects and thus to identify animated objects. In addition, they can measure distance, size, shape, and other 3D object properties. When inspecting an object, the weakly electric fish Gnathonemus petersii employs two “electrical foveae" that are located on their moveable chin appendage and in the nasal region. The former is used for object inspection, while the latter is used for object detection during foraging. The brain of weakly electric fishes extracts information about objects by analyzing the input from the electroreceptor organs. The study of electroreceptive brain areas has revealed many general principals of neural processing of sensory and motor information.

The electrosensory system of fishes is well matched to the conditions of the aquatic habitat, where electrical fields of biotic or abiotic sources are abundant. During passive electrolocation environmental low-frequency electrical fields are detected for orientation, navigation and prey detection. Weakly electric fish have perfected electro-orientation by developing active electrolocation. They actively emit high-frequency electrical signals with specialized electric organs. Objects are located by sensing the specific distortions they evoke on the electrical field which builds up during each electric organ discharge. The production of their own electric signals makes weakly electric fish independent of existing electrical fields and enables them to actively probe their environment. Several unique adaptations of the electrosensory system provide the means to detect and localize objects three-dimensionally in space and to analyze their electrical properties. The advantage of perceiving an electrically “colorful” picture of their surroundings led South American and African electric fishes to evolve capacitance detection, i.e., the ability to perceive capacitive object properties. Even though the fishes’ sensory capabilities, which are the results of a convergent evolutionary process, look almost identical, South American and African fishes employ different receptor types and different neural algorithms to achieve them.

Weakly electric fishes are nocturnal and orientate in the absence of vision by using their electrical sense. This enables them not only to navigate but also to perceive and recognize objects in complete darkness. They create an electric field around their bodies by producing electric signals with specialized electric organs. Objects within this field alter the electric current at electroreceptor organs, which are distributed over almost the entire body surface. During active electrolocation, fishes detect, localize and analyse objects by monitoring their self–produced electric signals. We investigated the ability of the mormyridGnathonemus petersiito perceive objects three–dimensionally in space. Within a range of about 12cm,G.petersiican perceive the distance of objects. Depth perception is independent of object size, shape and material. The mechanism for distance determination through electrolocation involves calculating the ratio between two parameters (maximal slope and maximal amplitude) of the electrical image which each object projects onto the fish's skin. During active electrolocation, electric fishes cannot only locate objects in space but in addition can determine the three–dimensional shape of an object. Up to certain limits, objects are spontaneously categorized according to their shapes, but not according to their sizes or the materials of which they are made.

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.

The present study describes a measurement-based model of electric image generation in the weakly electric mormyrid fish Gnathonemus petersii. Measurements of skin impedance, internal resistivity and fish body dimensions have been used to generate an electrical-equivalent model of the fish and to calculate electrical images and equivalent dipole sources for elementary resistive objects. These calculations allow us to understand how exafferent and reafferent signals are sensed by electroreceptors. An object's electric image consists of the modulation of the transcutaneous voltage profile generated by the fish's own discharge. The results suggest a set of rules for electrolocation: (1) the side of the fish where modulation is larger indicates the side on which the object is situated; (2) the object's position in the electroreceptive field is indicated by the point of maximum modulation of the transcutaneous voltage; (3) the degree of focus of the image indicates the distance to the object. In addition, center-surround opposition originating at pre-receptor level is proposed. Both experimental measurements and modeling indicate that fish skin impedance is relatively low (400-11 000IMG src="/images/symbols/capomega.gif" WIDTH="13" HEIGHT="13" ALIGN="BOTTOM" NATURALSIZEFLAG="3"cmSUP2/SUP) and mainly resistive. This low skin impedance appears to enhance the local electric organ discharge modulation, the center-surround effect, the signal-to-noise ratio for electrolocation and the active space for electrocommunication.

The weakly electric fish Gnathonemus petersii searches at night for insect larvae in tropical African streams. The aim of this study was to determine the contributions of different sensory modalities to foraging. The time that fish needed to find two randomly placed chironomid larvae was measured. The influence of various senses on search time was investigated by blocking the use of one or more senses. Active electrolocation was used by most fish for prey detection in the dark. In addition, passive electrolocation played a role in some individuals. If light was available, vision could become the dominant sense in some individuals, replacing active electrolocation. The presence of chemical cues decreased prey detection time in most fish. Prey movements also shortened search times when active electrolocation and vision were not possible, indicating that the mechanosensory lateral line also plays a role in the detection of moving prey. The results show that G. petersii uses several senses simultaneously during foraging. Each individual favours a specific combination of the available sensory inputs. If one sensory modality is eliminated, fish can switch to other modalities, indicating that the food detection system is flexible and plastic.

This paper examines the directionality of tuberous electroreceptor responses and relates them to a polarity bias seen for passive electrolocation by electric fish (Hypopomus). We recorded from Burst Duration Coders (BDCs) while stimulating with 1 kHz single period sine waves with electric fields oriented horizontally in different directions. Electroreceptors have figure-8 directional sensitivity profiles with two, usually unequal lobes of sensitivity separated by 180°. For most units the larger lobe points inward, while for a few, the lobes are symmetrical or the larger lobe points outward. The differences correlate with differences in frequency tuning of the receptors. We can alter, and even reverse, the directional asymmetry of a single unit by changing the frequency of the stimulus. Two general response profiles result, with two corresponding classes of tuning curves. The degree of asymmetry varies with position on the body surface. The asymmetries and the effects of stimulus frequency and of tuning can be modeled with a linear/non-linear/linear cascade filter. The behavioral preference for approaching the head end ( + ) of an electrode is difficult to understand in light of the asymmetry of responses we report for amplitude-coding BDCs but can be understood by reference to the time-coding Pulse Marker (PM) receptors.

The African electric fish, Mormyriformes, evolved electric organs and electroreceptors independently from the South American electric fish, Gymnotoidei. Like other electric fish,Gymnarchus is able to electrolocate objects which differ in conductivity from the surrounding water. Its performance in electrolocation can be measured by different behavioral methods (Figs. 1, 2 and 3). Similarly to the gymnotoid fishEigenmannia, Gymnarchus fires its electric organ in a nearly sinusoidal manner at a fairly stable frequency near 300 to 400 Hz, with considerable interindividual variation. Electrolocation deteriorates when sinusoidal signals of sufficient intensity approach the frequency of the animal's electric organ discharges (EODs) (Figs. 4 and 5).Gymnarchus improves its performance by shifting its EOD frequency away from stimulus frequencies and thereby maintaining differences of at least 4 Hz. Thus, in another example of convergent evolution,Gymnarchus andEigenmannia have developed the same behavioral strategy to protect their electrolocation ability from jamming signals.

African freshwater mormyrid fish produce weak electric organ discharges and possess electroreceptors which permit them to see their world via an electric sense. The discharges also form part of an important inter- and intea-specific communication system. In this review Peter Moller looks at some of the physiological characteristics of electrolocation and electrocommunication and examines their function from an ecological and ethological viewpoint.

Weakly electric fish use their electrosensory systems for electrocommunication, active electrolocation and low-frequency passive electrolocation. In electric fish of the family Mormyridae, these three purposes are mediated by separate classes of electro receptors: electrocommunication by Knollenorgan electroreceptors, active electrolocation by Mormyromast electroreceptors and low-frequency passive electrolocation by ampullary electroreceptors. The primary afferent fibres from each class of electroreceptors terminate in a separate central region. Thus, the mormyrid electrosensory system has three anatomically and functionally distinct subsystems. This review describes the sensory coding and initial processing in each of the three subsystems, with an emphasis on the Knollenorgan and Mormyromast subsystems. The Knollenorgan subsystem is specialized for the measurement of temporal information but appears to ignore both intensity and spatial information. In contrast, the Mormyromast subsystem is specialized for the measurement of both intensity and spatial information. The morphological and physiological characteristics of the primary afferents and their central projection regions are quite different for the two subsystems and reflect the type of information which the subsystems preserve. This review also describes the electric organ corollary discharge (EOCD) effects which are present in the central projection regions of each of the three electrosensory subsystems. These EOCD effects are driven by the motor command that drives the electric organ to discharge. The EOCD effects are different in each of the three subsystems and these differences reflect differences in both the pattern and significance of the sensory information that is evoked by the fish’s own electric organ discharge. Some of the EOCD effects are invariant, whereas others are plastic and depend on previous afferent input. The mormyrid work is placed within two general contexts: (a) the measurement of time and intensity in sensory systems, and (b) the various roles of motor command (efferent) signals and self-induced sensory (reafferent) signals in sensorimotor systems.

The electric fish,Eigenmannia, is able to sense objects which differ in electrical conductivity from the surrounding water by detecting distortions of the current field associated with its electric organ discharges. While resting,Eigenmannia hovers near electrically-detectable objects and follows their motions. When such objects are swung in a sinusoidal manner, the fish follows with a certain gain and phase lag. This allows one to quantify performance in “electrolocation” in terms of gain and phase values as functions of frequency of object motion, object size and distance. As these parameters reach threshold values, the gain of the animal's Following Response approaches zero while its phase lag tends toward-π. Electrolocation deteriorates under “jamming” conditions, i.e. in the presence of electric signals with frequencies near the animal's discharge frequency.Eigenmannia prevents the deterioration of electrolocation by shifting its frequency away from noise frequencies.