Effects of global electrosensory signals on motion processing in the midbrain of <Emphasis Type="Italic">Eigenmannia</Emphasis>

Wave-type weakly electric fish such as Eigenmannia produce continuous sinusoidal electric fields. When conspecifics are in close proximity, interaction of these electric fields can produce deficits in electrosensory function. We examined a neural correlate of such jamming at the level of the midbrain. Previous results indicate that neurons in the dorsal layers of the torus semicircularis can (1) respond to jamming signals, (2) respond to moving electrosensory stimuli, and (3) receive convergent information from the four sensory maps of the electrosensory lateral line lobe (ELL). In this study we recorded the intracellular responses of both tuberous and ampullary neurons to moving objects. Robust Gaussian-shaped or sinusoidal responses with half-height durations between 55 ms and 581 ms were seen in both modalities. The addition of ongoing global signals with temporal-frequencies of 5 Hz attenuated the responses to the moving object by 5 dB or more. In contrast, the responses to the moving object were not attenuated by the addition of signals with temporal frequencies of 20 Hz or greater. This occurred in both the ampullary and tuberous systems, despite the fact that the ampullary afferents to the torus originate in a single ELL map whereas the tuberous afferents emerge from three maps.     

An internal current source yields immunity of electrosensory information processing to unusually strong jamming in electric fish

Summary The electric organ of a fish represents an internal current source, and the largely isopotential nature of the body interior warrants that the current associated with the fish's electric organ discharges (EODs) recruits all electroreceptors on the fish's body surface evenly. Currents associated with the EODs of a neighbor, however, will not penetrate all portions of the fish's body surface equally and will barely affect regions where the neighbor's current flows tangentially to the skin surface. The computational mechanisms of the jamming avoidance response (JAR) in Eigenmannia exploit the uneven effects of a neighbor's EOD current to calculate the correct frequency difference between the two interfering EOD signals even if the amplitude of a neighbor's signal surpasses that of the fish's own signal by orders of magnitude. The particular geometry of the fish's own EOD current thus yields some immunity against the potentially confusing effects of unusually strong interfering EOD currents of neighbors.Abbreviations DF frequency difference - ELL electrosensory lateral line lobe - EOD electric organ discharge - JAR jamming avoidance response     

A neural mechanism of hyperaccurate detection of phase advance and delay in the jamming avoidance response of weakly electric fish

 The weakly electric fish Eigenmannia can detect the phase difference between a jamming signal and its own signal down to 1 s. To clarify the neuronal mechanism of this hyperaccurate detection of phase difference, we present a neural network model of the torus of the midbrain which plays an essential role in the detection of phase advances and delays. The small-cell model functions as a coincidence detector and can discriminate a time difference of more than 100 s. The torus model consists of laminae 6 and 8. The model of lamina 6 is made with multiple encoding units, each of which consists of a single linear array of small cells and a single giant cell. The encoding unit encodes the phase difference into its spatio-temporal firing pattern. The spatially random distribution of small cells in each encoding unit improves the encoding ability of phase modulation. The neurons in lamina 8 can discriminate the phase advance and delay of jamming electric organ discharges (EODs) compared with the phase of the fish's own EOD by integrating simultaneously the outputs from multiple encoding units in lamina 6. The discrimination accuracy of the feature-detection neurons is of the order of 1 s. The neuronal mechanism generating this hyperacuity arises from the spatial feature of the system that the innervation sites of small cells in different encoding units are distributed randomly and differently on the dendrites of single feature-detection neurons. The mechanism is similar to that of noise-enhanced information transmission. Received: 10 July 2000 / Accepted in revised form: 19 January 2001     

The coding of signals in the electric communication of the gymnotiform fish Eigenmannia: From electroreceptors to neurons in the torus semicircularis of the midbrain

Summary In the context of aggression and courtship, Eigenmannia repeatedly interrupts its electric organ discharges (EODs) These interruptions (Fig. 1) contain low-frequency components as well as high-frequency transients and, therefore, stimulate ampullary and tuberous electroreceptors, respectively (Figs. 2, 3). Information provided by these two classes of receptors is relayed along separate pathways, via the electrosensory lateral line lobe (ELL) of the hindbrain, to the dorsal torus semicircularis (TSd) of the midbrain. Some neurons of the torus receive inputs from both types of receptors (Figs. 14, 15), and some respond predominantly to EOD interruptions while being rather insensitive to other forms of signal modulations (Figs. 12, 13). This high selectivity appears to result from convergence and gating of inputs from individually less selective neurons.Abbreviations CP central posterior thalamic nucleus - Df frequency difference between neighbor's EOD and fish's own - DPn dorsal posterior nucleus (thalamus) - EOD electric organ discharge - ELL electrosensory lateral line lobe - JAR jamming avoidance response - LMR lateral mesencephalic reticular formation - nE nucleus electrosensorius - nEb nucleus electrosensorius, beat-related area - nE nucleus electrosensorius, area causing rise of EOD frequency - nE nucleus electrosensorius, area causing fall of EOD frequency - nEar nucleus electrosensorius-acusticolateralis area - NPd nucleus praeeminentialis, pars dorsalis - PPn prepacemaker nucleus - PT pretectal nucleus - SE nucleus subelectrosensorius - TeO optic tectum - TSd dorsal (electrosensory) torus semicircularis - TSv ventral (mechano-sensory and auditory) torus semicircularis     

Sensory cues for the gradual frequency fall responses of the gymnotiform electric fish, Rhamphichthys rostratus

The sensory cues for a less known form of frequency shifting behavior, gradual frequency falls, of electric organ discharges (EODs) in a pulse-type gymnotiform electric fish, Rhamphichthys rostratus, were identified. We found that the gradual frequency fall occurs independently of more commonly observed momentary phase shifting behavior, and is due to perturbation of sensory feedback of the fish's own EODs by EODs of neighboring fish. The following components were identified as essential features in the signal mixture of the fish's own and the neighbor's EOD pulses: (1) the neighbor's pulses must be placed within a few millisecond of the fish's own pulses, (2) the neighbor's pulses, presented singly at low frequencies (0.2–4 Hz), were sufficient, (3) the frequency of individual pulse presentation must be below 4 Hz, (4) amplitude modulation of the sensory feedback of the fish's own pulses induced by such insertions of the neighbor's pulses must contain a high frequency component: sinusoidal amplitude modulation of the fish's own EOD feedback at these low frequencies does not induce gradual frequency falls. Differential stimulation across body surfaces, which is required for the jamming avoidance response (JAR) of wave-type gymnotiform electric fish, was not necessary for this behavior. We propose a cascade of high-pass and low-pass frequency filters within the amplitude processing pathway in the central nervous system as the mechanism of the gradual frequency fall response.Abbreviations EOD electric organ discharge - f frequency of EOD or pacemaker command signal - JAR jamming avoidance response - S ₁ stimulus mimicking fish's own EOD - f ₁ frequency of S₁ - S ₂ stimulus mimicking neighbor's EOD - f ₂ frequency of S₂     

Structure and function of neurons in the complex of the nucleus electrosensorius of the gymnotiform fish Eigenmannia: Detection and processing of electric signals in social communication

Summary The complex of the diencephalic nucleus electrosensorius (nE) provides an interface between the electrosensory processing performed by the torus semicircularis and the control of specific behavioral responses. The rostral portion of the nE comprises two subdivisions that differ in the response properties and projection patterns of their neurons. First, the nEb (Fig. 1 B), which contains neurons that are driven almost exclusively by beat patterns generated by the interference of electric organ discharges (EODs) of similar frequencies. Second, the area medial to the nEb, comprising the lateral pretectum (PT) and the nE-acusticolateralis region (nEar, Fig. 1 B-D), which contains neurons excited predominantly by EOD interruptions, signals associated with aggression and courtship. Neurons in the second area commonly receive convergent inputs originating from ampullary and tuberous electroreceptors, which respond to the low-frequency and high-frequency components of EOD interruptions, respectively. Projections of these neurons to hypothalamic areas linked to the pituitary may mediate modulations of a fish's endocrine state that are caused by exposure to EOD interruptions of its mate.Abbreviations a axon - ATh anterior thalamic nucleus - CCb corpus cerebelli - CE central nucleus of the inferior lobe - CP central posterior thalamic nucleus - Df frequency difference between neighbor's EOD and fish's own - DFl nucleus diffusus lateralis of the inferior lobe - DFm nucleus diffusus medialis of the inferior lobe - DTn dorsal tegmental nucleus - EOD electric organ discharge - G glomerular nucleus - Hc caudal hypothalamus - Hd dorsal hypothalamus - Hl lateral hypothalamus - Hv ventral hypothalamus - JAR jamming avoidance response - LL lateral lemniscus - MGT magnocellular tegmental nucleus - MLF medial longitudinal fasciculus - nB nucleus at the base of the optic tract - nE nucleus electrosensorius - nEar nucleus electrosensorius-acusticolateral region - nEb nucleus electrosensorius-beat related area - nE nucleus electrosensorius, area causing rise of EOD frequency - nE nucleus electrosensorius, area causing fall of EOD frequency - nLT nucleus tuberis lateralis - nLV nucleus lateralis valvulae - PC posterior commissure - Pd nucleus praeeminentialis, pars dorsalis - PeG periglomerular complex - PG preglomerular nucleus - PLm medial division of the perilemniscal nucleus - Pn pacemaker nucleus - PPn prepacemaker nucleus - PT pretectal nucleus - PTh prethalamic nucleus - R red nucleus - Sc suprachiasmatic nucleus - SE nucleus subelectrosensorius - TAd nucleus tuberis anterior-dorsal subdivision - TAv nucleus tuberis anterior-ventral subdivision - TeO optic tectum - TL torus longitudinalis - TSd dorsal (electrosensory) torus semicircularis - TSv ventral (mechanosensory and auditory) torus semicircularis - tTB tecto-bulbar tract - VCb cerebellar valvula - VP valvular peduncle - VPn nucleus of the valvular peduncle     

Discrimination of the sign of frequency differences bySternopygus,an electric fish without a jamming avoidance response

Several species of weakly electric fish reflexively change their frequency of electric organ discharge (EOD) in response to sensing signals of similar frequency from conspecifics; that is, they exhibit jamming avoidance responses (JAR).Eigenmannia increases its EOD frequency if jammed by a signal of lower frequency and decreases its EOD frequency if jammed by a signal of higher frequency. This discrimination is based on an analysis of the patterns of amplitude modulations and phase differences resulting from signal interference. Fish of the closely related genus,Sternopygus, however, do not exhibit a JAR. Here we show that despite lacking this behavior,Sternopygus shares many sensory processing capacities withEigenmannia:

‘Ancestral’ neural mechanisms of electrolocation suggest a substrate for the evolution of the jamming avoidance response

Summary The genusSternopygus, believed to reflect ancestral traits of gymnotiform electric fish, is closely related to the more modern genusEigenmannia (Mago-Leccia 1978; Fink and Fink 1981).Sternopygus is the only known genus of electric fish that does not perform a jamming avoidance response (JAR) to minimize the potentially detrimental effects of signal interference between discharging neighbors (Bullock et al. 1972, 1975), and its ability to electrolocate objects is rather immune to jamming (Matsubara and Heiligenberg 1978).By studying the responses of midbrain neurons to stimulus regimes effective in eliciting the JAR inEigenmannia, we found thatSternopygus has neurons capable of discriminating the sign of the difference frequency between interfering electric organ discharges (EODs). These sign-selective neurons, which are believed to be important elements in the control of the JAR inEigenmannia, may, therefore, fulfill a more general function in the detection of moving objects and conspecifics but could potentially be assembled for the evolution of a JAR inSternopygus. The relative immunity to jamming in this genus may result, in part, from a stronger reliance upon the ampullary electrosensory system which operates in the DC and low-frequency range, outside the EOD spectrum of these fish.Abbreviations AM amplitude modulation - Df frequency difference - EOD electric organ discharge - JAR jamming avoidance response     

Communication in the weakly electric fish Sternopygus macrurus

There is a sexual dimorphism in the frequency of the quasi-sinusoidal electric organ discharge (EOD) of Sternopygus macrurus, with males, on average, an octave lower. EODs are detected by tuberous electroreceptor organs, which exhibit V-shaped frequency tuning with maximal sensitivity near the fish's own EOD frequency. This would seem to limit the ability of a fish to detect the EODs of opposite-sex conspecifics. However, electroreceptor tuning has always been based on single-frequency stimulation, while actual EOD detection involves the addition of a conspecific EOD to the fish's own. In the present study, recordings were made from single electroreceptive units while the fish were stimulated with pairs of sine waves: one (S1) representing the fish's own EOD added to a second (S2) representing a conspecific EOD. T unit response was easily predicted by assuming that the electroreceptor acts as a linear filter in series with a threshold-sensitive spike initiator. P unit response was more complex, and unexpectedly high sensitivity was found for frequencies of S2 well displaced from the fish's EOD frequency. For both P and T units, detection thresholds for S2 were much lower when added to S1, than when presented alone.     

From distributed sensory processing to discrete motor representations in the diencephalon of the electric fish,Eigenmannia

Summary During their jamming avoidance response (JAR), weakly electric fish of the genusEigenmannia shift their electric organ discharge (EOD) frequency away from a similar EOD frequency of a neighboring fish. The behavioral rules and neural substrates for stimulus recognition and motor control of the JAR have been extensively studied (see review by Heiligenberg 1986). The diencephalic nucleus electrosensorius (nE) links sensory processing within the torus semicircularis and optic tectum with the mesencephalic prepacemaker nucleus which, in turn, modulates the medullary pacemaker nucleus and hence the EOD frequency. Two separate areas within the nE responsible for JAR-related EOD frequency rises and frequency falls, respectively, were identified by iontophoresis of the excitatory amino acid L-glutamate. Bilateral lesion of the areas causing EOD frequency rises resulted in elimination of JAR-related frequency rises above a baseline frequency obtained in the absence of a jamming stimulus. Similarly, bilateral lesion of the areas causing frequency falls resulted in a loss of JAR-related frequency falls below the baseline frequency. Whether these areas are also responsible for non-JAR-related frequency shifts is not known. The strength of response and spatial extent of the areas causing frequency shifts varied among fish and also varied in individual fish, reflecting the strength of JAR-related frequency shifts and the balance of activities in frequency-rise and frequency-fall areas. Local application of bicuculline-methiodide or GABA demonstrated a tonic inhibitory input to each area and suggests a reciprocal inhibitory interaction between the two ipsilateral areas, possibly accounting for much of the individual plasticity.The nE thus is a site for neuronal transformation from distributed, topographically organized processing within the laminated structures of the torus and tectum to discrete cell clusters which control antagonistic motor responses.Abbreviations EOD electric organ discharge - JAR jamming avoidance response - Df difference frequency between jamming signal and the fish's own EOD - nE nucleus electrosensorius - PPn prepacemaker nucleus     

Stimulus discrimination in the diencephalon ofEigenmannia: the emergence and sharpening of a sensory filter

Summary Neuronal reliability and sensitivity to behaviorally relevant stimulus patterns were investigated in a higher-order nucleus of the diencephalon believed to participate in the jamming avoidance response (JAR) of the weakly electric fish,Eigenmannia. The fish raises or lowers its frequency of electric organ discharge (EOD) to minimize interference from a neighboring fish's EOD. Proper JARs require determination of the sign of the difference frequency (Df) between the neighboring fish's EOD and the fish's own EOD. Bastian and Yuthas (1984) recently described diencephalic neurons within the nucleus electrosensorius that are able to make this determination. In the present study, response properties of such neurons were compared with those of lower-level sign-selective cells found in the torus semicircularis and the optic tectum (Heiligenberg and Rose 1985) as well as with properties of the intact behavior.Most sign-selective cells within the nucleus electrosensorius show a high degree of selectivity for one sign of the difference frequency; cells with either sign preference were found in approximately equal numbers. The sign preference and the degree of sign selectivity is most often independent of the spatial orientation of the jamming stimulus. In contrast, the responses of toral and tectal cells are less robust and consistent and are often highly dependent on the geometry of the jamming stimulus.Determination of the sign of the difference frequency requires the analysis of amplitude modulations coupled with modulations in phase (timing) differences between pairs of areas of the body surface. The most sensitive cells recorded in the nucleus electrosensorius can determine the sign of the difference frequency with timing differences of 1 s or less, roughly comparable to the behavioral threshold of 400 ns (Carr et al. 1986). The best toral/tectal response required at least a 16 s modulation.Cells within the nucleus electrosensorius thus code the sign of Df with a high degree of reliability and sensitivity. Ambiguities persist, however, which suggest that single cells at this level cannot completely account for the behavioral discrimination. Additional processing may be necessary to transform a still primarily sensory code into a motor program for control of the JAR (Rose et al. 1988).Abbreviations EOD electric organ discharge - JAR jammning avoidance response - Df difference frequency between jamming signal and fish's own EOD - S ₁ sinusoidal EOD mimic of subject fish - S ₂ sinusoidal EOD mimic of neighbor     

Immunohistochemical identification of substance P in cutaneous sensory organs (electroreceptors) of gymnotid teleost fish

Summary The distribution of the neuropeptide substance P, which is considered to be a neurotransmitter or neuromodulator of the central nervous system, has been studied in the cutaneous electroreceptor organs (tuberous and ampullary organs) of 3 species of gymnotid fish: Apteronotus leptorhynchus, Eigenmannia virescens and Sternopygus sp. Immunohistochemical data have revealed that substance P is never present in the afferent fibers but is specifically localized in the electroreceptors of the three species examined. Substane P immunoreactivity is strictly localized in the sensory cells of the ampullary organs of all three species and in those of the tuberous organs of Apteronotus leptorhynchus and Sternopygus sp. In contrast, weak substance P immunoreactivity was observed only in certain tuberous sensory cells of Eigenmannia. Substance P immunoreactivity was also found in the accessory cells of certain organs: it was detected in the two types of accessory cells of the tuberous organs of Eigemmannia virescens, in the accessory cells type 2 of the tuberous organs of Sternopygus sp., and in all accessory cells of ampullary organs of Sternopygus sp. and Apteronotus leptorhynchus. In Sternopygus sp., positive staining was only evident if the substance P antibody was used at low concentration. Immunoreactivity for substance P in the sensory cells suggests that it has a transmitter or modulator function in these electroreceptors; the presence of substance P in the accessory cells remains to be explained.     

The responses of peripheral and central mechanosensory lateral line units of weakly electric fish to moving objects

Mechanosensory lateral line afferents of weakly electric fish (Eigenmannia) responded to an object which moved parallel to the long axis of the fish with phases of increased spike activity separated by phases of below spontaneous activity. Responses increased with object speed but finally may show saturation. At increasingly greater distances the responses decayed as a power function of distance. For different object velocities the exponents (mean±SD) describing this response falloff were -0.71±0.4 (20 cm/s object velocity) and-1.9±1.25 (10 cm/s). Opposite directions of object movement may cause an inversion of the main features of the response histograms. In terms of peak spike rate or total number of spikes elicited, however, primary lateral line afferents were not directionally sensitive.Central (midbrain) lateral line units of weakly electric fish (Apteronotus) showed a jittery response if an object moved by. In midbrain mechanosensory lateral line, ampullary, and tuberous units the response to a rostral-tocaudal object movement may be different from that elicited by a caudal-to-rostral object motion. Central units of Apteronotus may receive input from two or more sensory modalities. Units may be lateral line-tuberous or lateral line-ampullary. Multimodal lateral line units were OR units, i.e., the units were reliably driven by a unimodal stimulus of either modality. The receptive fields of central units demonstrate a weak somatotopic organization of lateral line input: anterior body areas project to rostral midbrain, posterior body areas project to caudal midbrain.Abbreviation EOD electric organ discharge     

Responses of electrosensory neurons in the torus semicircularis of Eigenmannia to complex beat stimuli: testing hypotheses of temporal filtering

The weakly electric fish, Eigenmannia, changes its frequency of electric organ discharges (EODs) to increase the frequency difference between its EODs and those of a jamming neighbor. This jamming avoidance response is greatest for frequency differences (i.e., beat rates) of approximately 4 Hz and barely detectable at beat rates of 20 Hz. A neural correlate of this behavior is found in the torus semicircularis, where most neurons act as low-pass or band-pass filters over this range of beat rates.This study examines two mechanisms that could possibly underlie low-pass temporal filtering: 1) Inhibition by a high-pass interneuron. 2) Voltage and time-dependent conductances associated with ligand-gated channels. These mechanisms were tested by recording intracellularly while employing stimuli consisting of simultaneous low and high beat rates. A neuron's response to the low beat rate was not diminished by the addition of the higher frequency jamming signal (thereby superimposing a high rate of amplitude and phase modulation onto the lower rate), and the inhibitory interneuron hypothesis is, therefore, not supported. Also, the responses to the high beat rate were not facilitated during maintained depolarization in response to the low beat rate.In some cases, particularly band-pass neurons, accommodation processes appeared to contribute to the decline in the amplitude of psps at high beat rates.     

Electrosensory phase sensitivity in the weakly electric fish Eigenmannia in the detection of signals similar to its own

The electric organ discharge (EOD) of the South American knifefish Eigenmannia sp. is a permanently present wave signal of usually constant amplitude and frequency (similar to a sine wave). A fish perceives discharges of other fish as a modulation of its own. At frequency identity (F = 0 Hz) the phase difference between a fish's own electric discharge and that of another fish affects the superimposed waveform. It was unclear whether or not the electrosensory stimulus-intensity threshold as behaviourally determined depends on the phase difference between a fish's own EOD and a sine-wave stimulus (at F = 0 Hz). Also the strength of the jamming avoidance response (JAR), a discharge frequency shift away from a stimulus that is sufficiently close to the EOD frequency, as a function of phase difference was studied. Sine-wave stimuli were both frequency-clamped and phase-locked to a fish's discharge frequency (F = 0 Hz). In food-rewarded fish, the electrosensory stimulus-intensity threshold depended significantly on the phase difference between a fish's discharge and the stimulus. Stimulus-intensity thresholds were low (down to 3 V/cm, peak-to-peak) when the superimposed complex wave changed such that the shift in zero-crossings times relative to the original EOD was large but amplitude change minimal; stimulus-intensity thresholds were high (up to 16.9 V/cm, peak-to-peak) when the shift in zero-crossings times was small but amplitude change maximal. Similar results were obtained for the non-conditioned JAR: at constant supra-threshold stimulus intensities and F = 0 Hz, the phase difference significantly affected the strength of the JAR, although variability between individuals was higher than that observed in the conditioned experiments.Abbreviations ACP active phase coupling - EOD electric organ discharge - JAR jamming avoidance response - F frequency (fish) — frequency (stimulus) [Hz] - p-p peak-to-peak     

The development of the Jamming Avoidance Response (JAR) in Eigenmannia: An innate behavior indeed

Summary In its Jamming Avoidance Response (JAR), the gymnotiform electric fish Eigenmannia shifts its electric organ discharge (EOD) frequency away from similar interfering frequencies. Continual behavioral measurements were carried out in 164 juvenile fish until a correct JAR emerged. Sixty-four of these fish were raised in complete isolation, the remainder in a community of their siblings. A correct JAR emerged in fish of 1.2–1.6 cm in body length, corresponding to a developmental age of 24–32 days. In 6 of 164 fish, the emergence of a correct JAR followed an interim appearance of an incorrect JAR, which involved frequency shifts in the direction opposite to those of a correct JAR. The fish raised in isolation developed the same forms of behavior and showed the same sequence in their appearance as did socially raised fish. This indicates that the JAR and its developmental schedule are innate. The appearance of an incorrect JAR suggests initial errors or incompleteness in the wiring of central nervous connections. A correct JAR ultimately emerged even if a stimulus regimen was offered that rewarded frequency shifts in the direction opposite to those of a correct JAR. This indicates that the development of the JAR is immune to experimental alterations of sensory experience.Abbreviations Df frequency difference between a jamming stimulus and fish's EOD - ELL electrosensory lateral line lobe - EO electric organ - EOD electric organ discharge - JAR Jamming Avoidance Response - nE nucleus electrosensorius - nE subnucleus of nE, causing drop of EOD frequency - nE subnucleus of nE, causing rise of EOD frequency - Pn pacemaker nucleus - PPn prepacemaker nucleus     

Distinct mechanisms of modulation in a neuronal oscillator generate different social signals in the electric fishHypopomus

Summary 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.Abbreviations EOD electric organ discharge - HRP horseradish peroxidase - NMDA N-Methyl-D-Aspartate - PPN prepacemaker nucleus     

Electrolocation in the presence of jamming signals: electroreceptor physiology

Responses of ampullary and tuberous electroreceptor afferents were studied using moving electrolocation targets and electrical modulations of the animal's electric organ discharge as stimuli. The ability of the electroreceptors to encode these stimuli was measured with and without various forms of electrical jamming signals. The goal of this study was to measure the deterioration in electroreceptor responses due to the jamming signals, and to compare these results with the behavioral measures of electrolocation under the same conditions of jamming as described in the preceding report (Bastian 1987). 1. Three types of jamming stimuli were used to interfere with the tuberous electroreceptor afferents' ability to respond to the test stimuli mentioned above: Broad-band noise, high frequency stimuli consisting of a sinusoidal waveform having a frequency maintained at a chosen difference frequency (DF) from the EOD frequency of the fish being studied, and 5 or 50 Hz sinusoidal stimuli. 2. The tuberous receptor afferents' spontaneous frequency was sensitive to continuous presentation of all but the 5 Hz jamming signals. The 4 Hz DF signal caused the largest increase in spontaneous activity, the 50 Hz stimulus was intermediate in effectiveness, and the noise stimulus caused the smallest increase. Estimates of the variability of the ongoing receptor activity were also made, and both the 4 Hz DF and the 50 Hz stimuli reduced the coefficient of variation of the receptor activity, but noise had no significant effect on this parameter. Noise, 4 Hz DF, and 50 Hz jamming signals also reduced the tuberous receptors' responses to a 100 ms EOD amplitude modulation, and the 5 Hz stimulus was again ineffective. 3. Noise and 4 Hz DF jamming were also effective in reducing tuberous receptor afferents' responses to a moving metal electrolocation target. The 4 Hz DF stimulus was most effective in reducing the receptor's ability to encode information about the target. Receptor responses showed about a three-fold larger decrease per 10 dB increase in DF jamming amplitude as compared to similar sized increases in noise amplitude. Threshold target distances were also determined with and without noise and DF jamming, and again, the noise stimulus was less effective in reducing the distance at which electrolocation targets were just detectable. 4. Recordings from ampullary receptor afferents confirmed that the galvanic potentials produced by metal electrolocation targets stimulate these receptors while EOD distortions caused by such objects probably do not.(ABSTRACT TRUNCATED AT 400 WORDS)     

Directional sensitivity of lateral line units in the clawed toadXenopus laevis Daudin

Summary Following lateral line stimulation with surface waves single unit activity was recorded from the periphery, torus semicircularis, and tecturn opticum ofXenopus laevis. The reaction of units to varying stimulus directions was examined.The directional specificity (DS) was calculated on the basis of spike counts per stimulus using circular statistics. It was expressed as the length of the mean vector.Discharges of primary afferents of the ramus supraorbitalis and ramus infraorbitalis were phase locked to the stimulus to a varying degree depending on the location of the corresponding groups of neuromasts. Their DS was not better than 0.26.Lemniscal fibers, representing the ascending output of the medulla and units of the torus semicircularis reached a DS of 0.10–0.24 and 0.11–0.36 respectively. Neurons in the tectum opticum were the most sharply tuned with DS ranging between 0.81 and 0.96.The surroundings were represented by the best directions of two arrays of tectal units forming a map which is in register with the representation of the corresponding visual field of the animal.Abbreviations DS directional specificity - r.i.o. ramus infraorbitalis - r.s.o. ramus supraorbitalis     

Continuous detection of weak sensory signals in afferent spike trains: the role of anti-correlated interspike intervals in detection performance

An important problem in sensory processing is deciding whether fluctuating neural activity encodes a stimulus or is due to variability in baseline activity. Neurons that subserve detection must examine incoming spike trains continuously, and quickly and reliably differentiate signals from baseline activity. Here we demonstrate that a neural integrator can perform continuous signal detection, with performance exceeding that of trial-based procedures, where spike counts in signal- and baseline windows are compared. The procedure was applied to data from electrosensory afferents of weakly electric fish (Apteronotus leptorhynchus), where weak perturbations generated by small prey add ~1 spike to a baseline of ~300 spikes s^–1. The hypothetical postsynaptic neuron, modeling an electrosensory lateral line lobe cell, could detect an added spike within 10–15 ms, achieving near ideal detection performance (80–95%) at false alarm rates of 1–2 Hz, while trial-based testing resulted in only 30–35% correct detections at that false alarm rate. The performance improvement was due to anti-correlations in the afferent spike train, which reduced both the amplitude and duration of fluctuations in postsynaptic membrane activity, and so decreased the number of false alarms. Anti-correlations can be exploited to improve detection performance only if there is memory of prior decisions.Abbreviations B binomial - CV coefficient of variation - EOD electric organ discharge - ELL electrosensory lateral line lobe - EPSP excitatory postsynaptic potential - ISI interspike interval - M₀ Markov order zero - M₁ Markov order one - N noise - OC operating characteristic - PDF probability density function - ROC receiver operating characteristic - S signal - SNR signal-to-noise ratio - S+N signal in noise