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PubMed1996

Coding of time-varying electric field amplitude modulations in a wave-type electric fish.

1. The coding of time-varying electric fields in the weakly electric fish, Eigenmannia, was investigated in a quantitative manner. The activity of single P-type electroreceptor afferents was recorded while the amplitude of an externally applied sinusoidal electric field was stochastically modulated. The amplitude modulation waveform (i.e., the stimulus) was reconstructed from the spike trains by mean square estimation. 2. From the stimulus and the reconstructions we calculated the following: 1) the signal-to-noise ratio and thus an effective temporal bandwidth of the units; 2) the coding fraction, i.e., a measure of the fraction of the time-varying stimulus encoded in single spike trains; and 3) the mutual information provided by the reconstructions about the stimulus. 3. Signal-to-noise ratios as high as 7:1 were observed and the bandwidth ranged from 0 up to 200 Hz, consistent with the limit imposed by the sampling theorem. Reducing the cutoff frequency of the stimulus increased the signal-to-noise ratio at low frequencies, indicating a nonlinearity in the receptors' response. 4. The coding fraction and the rate of mutual information transmission increased in parallel with the standard deviation (i.e., the contrast) of the stimulus as well as the mean firing rate of the units. Significant encoding occurred 20-40 Hz above the spontaneous discharge of a unit. 5. When the temporal cutoff frequency of the stimulus was increased between 80 and 400 Hz, 1) the coding fraction decreased, 2) the rate of mutual information transmission remained constant over the same frequency range, and 3) the reconstructed filter changed. This is in agreement with predictions obtained in a simplified neuronal model. 6. Our results suggest that 1) the information transmitted by single spike trains of primary electrosensory afferents to higherorder neurons in the fish brain depends on the contrast and the cutoff frequency of the stimulus as well as on the mean firing rate of the units; and 2) under optimal conditions, more than half of the information about a Gaussian stimulus that can in principle be encoded is carried in single spike trains of P-type afferents at rates up to 200 bits per second.

Action Potentials↗
PubMed1996

Logarithmic time course of sensory adaptation in electrosensory afferent nerve fibers in a weakly electric fish.

1. We recorded single unit activity from individual primary electrosensory afferent axons in the posterior branch of the anterior lateral line nerve of gymnotid weakly electric fish, Apteronotus leptorhynchus. We analyzed the responses of P-type (probability-coding) afferent fibers to externally applied amplitude step changes in the quasi-sinusoidal transdermal potential established by the fish's own electric organ discharge (EOD). 2. In response to AM step increases in transdermal potential, the firing rate of P-type afferents exhibited an abrupt increase followed by an initially rapid and subsequently more gradual decay back toward the baseline level. Afferent responses continued to adapt slowly throughout the duration of prolonged step stimuli lasting > 100 s. The time course of sensory adaptation was similar for all units tested. 3. We introduce a new functional form for describing the time course of sensory adaptation in which the change in firing rate delta r decays logarithmically with time: delta r(t) = A/[B In (t) + 1]. This logarithmic form accurately describes the adaptation time course of P-type afferents over five decades in time, from milliseconds to hundreds of seconds, with only two free parameters. Using a nonlinear least-squares fitting technique, we obtained a mean value of the parameter B, which characterizes the adaptation time course, of 0.149 +/- 0.028 (mean +/- SD, n = 49). 4. We compare logarithmic fits with traditional multiexponential and power law forms and demonstrate that the logarithmic form yields a better characterization of P-type afferent responses. This analysis helps explain the variability in previously reported adaptation time constants, which have ranged from 0.2 to 3.4 s, in gymnotid P-type afferents. 5. We tested the linearity of P-type afferent responses using positive and negative AM steps of varying amplitudes. Aside from nonlinearities associated with rectification (firing rates cannot be negative) and saturation (firing rates cannot exceed the EOD frequency), we found that P-type afferent responses scaled linearly with stimulus amplitude. 6. Based on the observed linearity, we predict the frequency domain response characteristics of P-type afferents and find that the predicted gain and phase are in good agreement with experimental measurements using sinusoidal AM stimuli over a range of AM frequencies from 1 to 100 Hz. Thus the logarithmic parameterization of the step appears to accurately capture the response dynamics of P-type afferents over a wide range of behaviorally relevant AM frequencies. 7. We conclude that the temporal filtering properties of pyramidal cells in the medullary electrosensory nucleus, the electrosensory lateral line lobe (ELL), need to be reevaluated in light of the logarithmic adaptation time course in the periphery, and we discuss implications for the role of P-type afferents in driving a feedback gain control mechanism that regulated ELL pyramidal cell responsiveness.

Action Potentials↗
PubMed1988

Individual prepacemaker neurons can modulate the pacemaker cycle of the gymnotiform electric fish, Eigenmannia.

The prepacemaker nucleus (PPN) in the midbrain of the gymnotiform electric fish Eigenmannia provides the only known neuronal input to the medullary pacemaker nucleus, which triggers each electric organ discharge (EOD) cycle by a single command pulse. Electrical stimulation of the PPN elicited two distinct forms of modulations in the pacemaker activity, brief accelerations, hence referred to as 'chirps', and gradual frequency shifts with a time constant of approximately one second. The associated EOD modulations were indistinguishable from natural communication signals. Depending upon the site of stimulation, the two forms of modulation could be elicited alone or superimposed (Fig. 1). Stimulation sites eliciting only chirps could be separated from sites eliciting only gradual shifts by as little as 60 micron. The magnitude of the elicited chirps depended upon the timing of the pulse stimulus with reference to the phase of the pacemaker cycle (Figs. 2, 3). Extracellular and intracellular recordings of single PPN neurons revealed that an action potential from a single neuron generates a chirp, and that the magnitude of the chirp depends upon the timing of the action potential with reference to the phase of the pacemaker cycle (Figs. 4, 5). The spike activity of these neurons had no relation to the jamming avoidance response (JAR), suggesting independent neuronal mechanisms for chirps and the JAR. Depolarization of such neurons by current injection produced bursts of chirps (Fig. 6), and intracellular injection of Lucifer Yellow identified these cells as a large type of PPN neuron which could also be retrogradely labeled from the pacemaker with horseradish peroxidase (HRP) (Fig. 7).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1983

A new method for the simulation of electric fields, generated by electric fish, and their distorsions by objects.

Among the many species of fishes endowed with electric organs Mormyriformes and Gymnotoidei are known to emit and receive electric signals for the purposes of intraspecific communication and recognition of objects. Models which have been proposed for this electro-sensory system generally assume steady-state conditions. On the other hand, the very character of the signals itself and the idea that the cerebellum might be working as a clock point to the importance of the signal dynamics. Therefore a new approach to the simulation of electric fields is described in the paper. The basic idea is to superpose the fields of point charges in a way that the sum is in accordance with the fish's electric field. The same technique could be used to simulate the influence of objects on the electric field. Following a suggestion of Dr. E. Kasper I used a simpler but equal effective approach for object simulation consisting in the use of a dipole instead of point charges. The model described is easily applied to diverse situations and allows one to estimate the influence of various parameters (size, shape, and position) on the "electric image" of an object. Furthermore, the well-known behaviour of tailbending and its consequences in object recognition can be simulated. The results underline the importance of signal dynamics for species with pulse-type discharges.

Animals↗
PubMed2006

Distribution of Kv1-like potassium channels in the electromotor and electrosensory systems of the weakly electric fish Apteronotus leptorhynchus.

The electromotor and electrosensory systems of the weakly electric fish Apteronotus leptorhynchus are model systems for studying mechanisms of high-frequency motor pattern generation and sensory processing. Voltage-dependent ionic currents, including low-threshold potassium currents, influence excitability of neurons in these circuits and thereby regulate motor output and sensory filtering. Although Kv1-like potassium channels are likely to carry low-threshold potassium currents in electromotor and electrosensory neurons, the distribution of Kv1 alpha subunits in A. leptorhynchus is unknown. In this study, we used immunohistochemistry with six different antibodies raised against specific mammalian Kv1 alpha subunits (Kv1.1-Kv1.6) to characterize the distribution of Kv1-like channels in electromotor and electrosensory structures. Each Kv1 antibody labeled a distinct subset of neurons, fibers, and/or dendrites in electromotor and electrosensory nuclei. Kv1-like immunoreactivity in the electrosensory lateral line lobe (ELL) and pacemaker nucleus are particularly relevant in light of previous studies suggesting that potassium currents carried by Kv1 channels regulate neuronal excitability in these regions. Immunoreactivity of pyramidal cells in the ELL with several Kv1 antibodies is consistent with Kv1 channels carrying low-threshold outward currents that regulate spike waveform in these cells (Fernandez et al., J Neurosci 2005;25:363-371). Similarly, Kv1-like immunoreactivity in the pacemaker nucleus is consistent with a role of Kv1 channels in spontaneous high-frequency firing in pacemaker neurons. Robust Kv1-like immunoreactivity in several other structures, including the dorsal torus semicircularis, tuberous electroreceptors, and the electric organ, indicates that Kv1 channels are broadly expressed and are likely to contribute significantly to generating the electric organ discharge and processing electrosensory inputs.

Animals↗
PubMed1989

Corollary discharge inhibition and preservation of temporal information in a sensory nucleus of mormyrid electric fish.

Knollenorgan electroreceptors in mormyrid fish are concerned with electrocommunication, i.e., with detecting electric organ discharges (EODs) of other electric fish. But knollenorgan electroreceptors are also activated by the fish's own EOD. Potential interference by such self-stimulation is blocked by an inhibitory corollary discharge in the nucleus of the electrosensory lateral line lobe (NELL), the first central relay of the knollenorgan pathway. This study used intracellular recording and staining to examine the mechanism of the corollary inhibition and the specializations in anatomy and physiology that permit the accurate relaying of temporal information about the EODs of other fish. Several events are recorded inside primary knollenorgan afferents in addition to a large orthodromic action potential. The additional events include small orthodromic electronic epsps, postsynaptic action potentials, and a corollary discharge inhibitory postsynaptic potential (ipsp) associated with the EOD motor command. These additional events are also recorded in NELL cells and almost certainly originate there. Electrical coupling between afferents and cells makes it possible to observe the events inside primary afferents. The corollary discharge ipsp in the cell is associated with a conductance increase and inverts rapidly when recorded with chloride-containing electrodes, supporting a hypothesis of GABA mediation. The ipsp lasts longer in cells than in afferents. Each electrotonic excitatory postsynaptic potential (epsp) is probably caused by a single primary afferent, and any one of several epsps in a given cell seems capable of eliciting a postsynaptic spike in that cell. The epsps follow stimulation rates as high as 500/sec with minimal variability. No lateral inhibition is observed in NELL. These and other properties indicate that the knollenorgan pathway is specialized for temporal information rather than spatial or intensity information.

Action Potentials↗
PubMed1999

Systematic biology of gymnotiform and mormyriform electric fishes: phylogenetic relationships, molecular clocks and rates of evolution in the mitochondrial rRNA genes

The phylogenetic relationships of both African and South American electric fish orders are reviewed at their intra-ordinal level taking into consideration recent studies in which cladistic principles have been employed. Several concordant topologies emerge from the different data sets, but some unsettled issues still remain. From the studies available, a consensus topology has been suggested for the Mormyriformes and for the Gymnotiformes. Subsequently, the evolutionary relationships of these two electric fish clades are considered within each respective superorder, i.e. in relation to the other osteoglossomorph and ostariophysan orders. The inter-ordinal phylogenies are used as a framework to test the molecular clock hypothesis with two gene fragments of the mitochondrial genome. Gymnotiformes, Siluriformes and Characiformes are accumulating mutations at the same pace in relation to their respective outgroups, but for all the other combinations of sister clades tested the molecular clock can be statistically rejected. Fossil records are then surveyed and used to calibrate absolute rates of genetic differentiation for each main lineage (orders) of both osteoglossomorphs and ostariophysans. The most conserved regions (stems) of the 12S and 16S gene fragments used are evolving at an average rate of 0.123 % 10(6 )years-1 for the osteoglossomorphs and 0.137 % 10(6 )years-1 for the ostariophysans, with no significant difference between these two values. The rate of mutation in the loops, the faster-evolving segments, estimated for closely related electric fish taxa is 0.82 % 10(6 )years-1 for four Brienomyrus species and 1.01 % 10(6 )years-1 for the four eigenmanniid genera. When the entire molecule (loops + stems) is considered, the rate of mutation in both mormyriforms and gymnotiforms converges to a rounded value of 0.23 % 10(6 )years-1.

Journal Article↗
PubMed1999

Electroreceptor model of weakly electric fish Gnathonemus petersii: II. Cellular origin of inverse waveform tuning.

In part I (. Biophys. J. 75:1712-1726), we presented a cellular model of the A- and B-electroreceptors of the weakly electric fish Gnathonemus petersii. The model made clear the cellular origin of the differences in the response functions of A- and B-receptors, which sensitively code the intensity of the fish's own electric organ discharge (EOD) and the variations in the EOD waveform, respectively. The main purpose of the present paper is to clarify the cellular origin of the inverse waveform tuning of the B-receptors by using the receptor model. Inverse waveform tuning means that B-receptors respond more sensitively to the 180 degrees inverted EOD than to undistorted or less distorted EODs. We investigated how the A- and B-receptor models respond to EODs with various waveforms, which are the phase-shifted EODs, whose shift angle is varied from -1 degrees to -180 degrees, and single-period sine wave stimuli of various frequencies. We show that the tuning properties of the B-receptors arise mainly from the combination of two attributes: 1) The waveform of the stimuli (Bstim) effectively sensed by the B-receptor cells. This consists of a first smaller and a second larger positive peak, even though in the original phase-shifted EOD stimuli, the amplitudes of the two positive peaks are reversed. 2) The effective time constant of dynamical response of the receptor cells. It is on the order of the duration of a single EOD pulse. We also calculated the response properties of the A- and B-receptor models when stimulated with natural EODs distorted by various capacitive and resistive objects. Furthermore, we investigated the effect of EOD amplitude on the receptor responses to capacitive and resistive objects. The models presented can systematically reproduce the experimentally observed response properties of natural A- and B-receptor cells. The mechanism producing these properties can be reasonably explained by the variation in the stimulus waveforms effectively sensed by the A- and B-receptor cells and by time constants.

Action Potentials↗
PubMed1996

Tectal input to the central posterior/prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: an in vitro tract-tracing study.

The weakly electric fish Apteronotus leptorhynchus produces electric organ discharges which are highly stable in waveform and frequency. Short-term modulations of these discharges, typically displayed during social interactions, are controlled by the prepacemaker nucleus (PPn). Neurons of this thalamic cell group intermingle with cells of the central posterior nucleus (CP) to form a complex called 'CP/PPn'. By employing in vitro tract-tracing techniques, we have, in the present investigation, demonstrated that this complex receives input from the tectum opticum. The tectal input is mediated by varicose fibers forming an elongated stripe at the ventral rim of the CP/PPn. As suggested by retrograde tracing from the CP/PPn, this projection is likely to arise from 'multipolar cells with an ascending axon' previously characterized in a Golgi study [14]. As this tectal cell type has been shown to be predominantly driven by electrosensory stimuli [6], information arising from these cells may be used in controlling modulations of the electric organ discharges.

Afferent Pathways↗
PubMed2000

Robustness and variability of neuronal coding by amplitude-sensitive afferents in the weakly electric fish eigenmannia.

We investigated the variability of P-receptor afferent spike trains in the weakly electric fish, Eigenmannia, to repeated presentations of random electric field AMs (RAMs) and quantified its impact on the encoding of time-varying stimuli. A new measure of spike timing jitter was developed using the notion of spike train distances recently introduced by Victor and Purpura. This measure of variability is widely applicable to neuronal responses, irrespective of the type of stimuli used (deterministic vs. random) or the reliability of the recorded spike trains. In our data, the mean spike count and its variance measured in short time windows were poorly correlated with the reliability of P-receptor afferent spike trains, implying that such measures provide unreliable indices of trial-to-trial variability. P-receptor afferent spike trains were considerably less variable than those of Poisson model neurons. The average timing jitter of spikes lay within 1-2 cycles of the electric organ discharge (EOD). At low, but not at high firing rates, the timing jitter was dependent on the cutoff frequency of the stimulus and, to a lesser extent, on its contrast. When spikes were artificially manipulated to increase jitter, information conveyed by P-receptor afferents was degraded only for average jitters considerably larger than those observed experimentally. This suggests that the intrinsic variability of single spike trains lies outside of the range where it might degrade the information conveyed, yet still allows for improvement in coding by averaging across multiple afferent fibers. Our results were summarized in a phenomenological model of P-receptor afferents, incorporating both their linear transfer properties and the variability of their spike trains. This model complements an earlier one proposed by Nelson et al. for P-receptor afferents of Apteronotus. Because of their relatively high precision with respect to the EOD cycle frequency, P-receptor afferent spike trains possess the temporal resolution necessary to support coincidence detection operations at the next stage in the amplitude-coding pathway.

Action Potentials↗
PubMed1991

Electrophysiological characteristics of the Mauthner cell of the weakly electric fish Gymnotus carapo.

The Mauthner cell (M-cell) of the 'weakly electric fish' Gymnotus carapo was investigated with electrophysiological techniques. The antidromic action potential, the recurrent inhibitory input and the posterior VIIIth nerve excitatory input in this cell exhibited characteristics similar to those described in the goldfish. In addition, we found an excitatory input evoked by spinal stimulation at intensities subthreshold for M-cell axons.

Animals↗
PubMed1993

Phase and amplitude maps of the electric organ discharge of the weakly electric fish, Apteronotus leptorhynchus.

The electric organ discharge (EOD) potential was mapped on the skin and midplane of several Apteronotus leptorhynchus. The frequency components of the EOD on the surface of the fish have extremely stable amplitude and phase. However, the waveform varies considerably with different positions on the body surface. Peaks and zero crossings of the potential propagate along the fish's body, and there is no point where the potential is always zero. The EOD differs significantly from a sinusoid over at least one third of the body and tail. A qualitative comparison between fish showed that each individual had a unique spatiotemporal pattern of the EOD potential on its body. The potential waveforms have been assembled into high temporal and spatial resolution maps which show the dynamics of the EOD. Animation sequences and Macintosh software are available by anonymous ftp (mordor.cns.caltech.edu; cd/pub/ElectricFish). We interpret the EOD maps in terms of ramifications on electric organ control and electroreception. The electrocytes comprising the electric organ do not all fire in unison, indicating that the command pathway is not synchronized overall. The maps suggest that electroreceptors in different regions fulfill different computational roles in electroreception. Receptor mechanisms may exist to make use of the phase information or harmonic content of the EOD, so that both spatial and temporal patterns could contribute information useful for electrolocation and communication.

Animals↗
PubMed2004

Unitary giant synapses embracing a single neuron at the convergent site of time-coding pathways of an electric fish, Gymnarchus niloticus.

Phase-locking neurons in the electrosensory lateral line lobe (ELL) of a weakly electric fish, Gymnarchus niloticus, fire an action potential in response to each cycle of the sinusoidal electrosensory signal (350-500 Hz) created by the fish's own electric organ. The exact firing times of the phase-locking neurons are altered (time-shifted) by capacitance of electrolocation objects or by electric organ discharges of other individuals. The magnitude of the time shifts depends on the location of the neurons' receptive field on the skin; thus, time disparities arise between the firing of phase-locking neurons. To compute these disparities, there should be a site where these phase-locking neurons converge. In this study we morphologically identified a novel cell type, which we named the "ovoidal cell", that receives the convergent projections of phase-locking neurons in the inner cell layer (ICL) of the ELL. We labeled these neurons with biocytin and examined them by light and electron microscopy. The giant cells and the S-type primary afferents, two types of phase-locking neurons, respectively terminate on the soma via chemical synapses and on the dendrite of the ovoidal cells via mixed synapses. Each terminal of the giant cells embraces the soma of an ovoidal cell, covering as much as 84% of the somatic membrane. The giant cell terminals and ovoidal cell somata were immunoreactive to SV2, a synaptic vesicle protein, but the S-afferent terminals were not, even though they contain numerous synaptic vesicles. The dendrite of the ovoidal cells also contacts the pyramidal cells of the ICL, which are known to be sensitive to time disparities. The anatomical connections of the phase-locking neurons to the ovoidal cells strongly suggest that they are involved in computing time disparity.

Animals↗
PubMed1996

Hormonal modulation of communication signals in electric fish.

Steroid hormones have profound effects on vertebrate behaviors. Yet, in most cases we can neither identify the neurons responsible for particular behaviors nor how steroids alter their excitability. Electric fish emit sexually dimorphic electric organ discharges (EODs) the waveform of which may be altered by steroid treatment. These communication signals are generated by a medullary pacemaker nucleus (PMN) composed of two cell types. The output neurons of the PMN synapse on spinal cord electromotoneurons which then innervate the electric organ. The PMN receives input from only two sources, and these are responsible for brief modulations that occur during social interactions. The small numbers and stereotyped electrical behaviors of the cells in the EOD-generating circuitry and their access for biophysical recording makes them a convenient model system to study the actions of steroids on identified cells.

Animal Communication↗
PubMed1997

Characterization and modeling of P-type electrosensory afferent responses to amplitude modulations in a wave-type electric fish.

The first stage of information processing in the electrosensory system involves the encoding of local changes in transdermal potential into trains of action potentials in primary electrosensory afferent nerve fibers. To develop a quantitative model of this encoding process for P-type (probability-coding) afferent fibers in the weakly electric fish Apteronotus leptorhynchus, we recorded single unit activity from electrosensory afferent axons in the posterior branch of the anterior lateral line nerve and analyzed responses to electronically generated sinusoidal amplitude modulations of the local transdermal potential. Over a range of AM frequencies from 0.1 to 200 Hz, the modulation transfer function of P-type afferents is high-pass in character, with a gain that increases monotonically up to AM frequencies of 100 Hz where it begins to roll off, and a phase advance with a range of 15-60 degrees. Based on quantitative analysis of the observed gain and phase characteristics, we present a computationally efficient model of P-type afferent response dynamics which accurately characterizes changes in afferent firing rate in response to amplitude modulations of the fish's own electric organ discharge over a wide range of AM frequencies relevant to active electrolocation.

Action Potentials↗
PubMed1989

Distinct mechanisms of modulation in a neuronal oscillator generate different social signals in the electric fish Hypopomus.

The medullary pacemaker nucleus of the gymnotiform electric fish, Hypopomus, is a relatively simple neuronal oscillator which contains pacemaker cells and relay cells. The pacemaker cells generate a regular discharge cycle and drive the relay cells which trigger pulse-like electric organ discharges (EODs). The diencephalic prepacemaker nucleus (PPN) projects to the pacemaker nucleus and modulates its activity to generate a variety of specific discharge patterns which serve as communicatory signals (Figs. 2 and 3). While inducing such signals by microiontophoresis of L-glutamate to the region of the PPN (Fig. 4) of curarized animals, we monitored the activity of neurons in the pacemaker nucleus intracellularly. We found that pacemaker cells and relay cells were affected differently in a manner specific to the type of EOD modulation (Figs. 5-10). The normal sequence of pacemaker cell and relay cell firing was maintained during gradual rises and falls in discharge rate. Both types of cells ceased to fire during interruptions following a decline in discharge rate. During sudden interruptions, however, relay cells were steadily depolarized, while pacemaker cells continued to fire regularly. Short and rapid barrages of EODs, called "chirps", were generated through direct and synchronous activation of the relay cells whose action potentials invaded pacemaker cells antidromically and interfered with their otherwise regular firing pattern.

Action Potentials↗
PubMed2000

Urbanization, place of experiment and how the electric fish was caught by Emil du Bois-Reymond.

Laboratories, along with the researchers, organisms, instruments, and experiments associated with these places of investigation, are not isolated from the world beyond their physical and institutional boundaries. Both laboratories and the cities in which they are embedded are subject to change, as was most dramatically apparent in their dynamic and far-reaching transformation during the industrial revolution of the nineteenth century. Using the example of Berlin and the institutionalization of experimental physiology by Emil du Bois-Reymond, in this paper I study experimental work on frogs and fish in order to demonstrate how a city and a laboratory cooperate in the production of knowledge. Emil du Bois-Reymond's research on electric fish illustrates how an exotic organism and a laboratory came together in a city and how the research was driven by innovation and development in urban and industrial technology. Ongoing changes in the urban landscape entered du Bois-Reymond's workplace and became part of the material culture of his experimental physiology and his attempts to demonstrate that the electric fish discharge is fundamentally similar to the excitation of nerve and muscle.

Academies and Institutes↗
PubMed1986

A time-comparison circuit in the electric fish midbrain. I. Behavior and physiology.

Behavioral experiments show that the weakly electric fish, Eigenmannia, detects differences in timing as small as 400 nsec between electric signals from different parts of its body surface. The neural basis of this remarkable temporal resolution was investigated by recording from elements of the phase-coding system, a chain of electrotonically connected neurons devoted to the processing of temporal information. Each element of this system fires a single action potential for every cycle of the electric signal (either the fish's own electric organ discharge or a sinusoidal signal of similar frequency). For phase-coding primary afferents and midbrain neurons, the temporal resolution was determined by measuring each unit's capacity to lock its spike to a particular phase of the stimulus cycle. The jitter of a neuron's response (measured as the standard deviation of the timing of the spikes with respect to the stimulus) decreases from the level of the primary afferent (mean = 30 microsec) to the midbrain torus (mean = 11 microsec); these results can be correlated with morphological measures of convergence. The temporal resolution of single neurons is still inferior to that displayed at the behavioral level.

Action Potentials↗