Behavioral evidence of a latency code for stimulus intensity in mormyrid electric fish

Mormryid electric fish (Gnathonemus petersii) respond to novel stimuli with an increase in the rate of the electric organ discharge (EOD). These novelty responses were used to measure the fish's ability to detect small changes in the amplitude and latency of an electrosensory stimulus. Responses were evoked in curarized fish in which the EOD was blocked but in which the EOD motor command continued to be emitted. An artificial EOD was provided to the fish at latencies of 2.4 to 14.4 ms following the EOD motor command.Novelty responses were evoked in response to transient changes in artificial EOD amplitude as small as 1% of baseline amplitude, and in latency as small as 0.1 ms. Changes in latency were effective only at baseline delays of less than 12.4 ms.The sensitivity to small changes in latency supports the hypothesis that latency is used as a code for stimulus intensity in the active electrolocation system of mormyrid fish. The results also indicate that a corollary discharge signal associated with the EOD motor command is used to measure latency.Abbreviations EOD electric organ discharge - ELL electrosensory lateral line lobe - epsp excitatory post synaptic potential     

Interval-specific event related potentials to omitted stimuli in the electrosensory pathway in elasmobranchs: an elementary form of expectation

Multiunit activity and slow local field potentials show Omitted Stimulus Potentials (OSP) in the electrosensory system in rays (Platyrhinoidis triseriata, Urolophus halleri) after a missing stimulus in a 3 to >20 Hz train of V pulses in the bath, at levels from the primary medullary nucleus to the telencephalon. A precursor can be seen in the afferent nerve. The OSP follows the due-time of the first omitted stimulus with a, usually, constant main peak latency, 30–50 ms in medullary dorsal nucleus, 60–100 ms in midbrain, 120–190 ms in telencephalon — as though the brain has an expectation specific to the interstimulus interval (ISI). The latency, form and components vary between nerve, medulla, mid-brain and forebrain. They include early fast waves, later slow waves and labile induced rhythms. Responsive loci are quite local. Besides ISI, which exerts a strong influence, many factors affect the OSP slightly, including train parameters and intensity, duration and polarity of the single stimulus pulses. Jitter of ISI does not reduce the OSP substantially, if the last interval equals the mean; the mean and the last interval have the main effect on both amplitude and latency.Taken together with our recent findings on visually evoked OSPs, we conclude that OSPs do not require higher brain levels or even the complexities of the retina. They appear in primary sensory nuclei and are then modified at midbrain and telencephalic levels. We propose that the initial processes are partly in the receptors and partly in the first central relay including a rapid increase of some depressing influence contributed by each stimulus. This influence comes to an ISI-specific equilibrium with the excitatory influence; withholding a stimulus and hence its depressing influence causes a rebound excitation with a specific latency.Abbreviations DN dorsal nucleus of medullary lateral line lobe - EEG electroencephalogram - EP evoked potential - ERP event related potential - IR induced rhythm - ISI interstimulus interval - OSP omitted stimulus potential - MLN mesencephalic lateral nucleus - P75 positive peak at 75 ms     

A new electroreceptive teleost:Xenomystus nigri (Osteoglossiformes: Notopteridae)

Summary The African knife fish,Xenomystus nigri, is found to be sensitive to weak electric fields by the method of averaged evoked potentials from the brain. Slow waves and spikes were recorded in or near the lateral line area of the medulla and the torus semicircularis of the mesencephalon in response to long pulses (best > 50 ms) and low frequency sine waves (best ca. 10 Hz) of voltage gradients down to < 10 V/cm. Evoked waves in the lateral line area are a sequence of negative and positive deflections beginning with a first peak at ca. 24 ms; in the torus semicircularis the first peak is at ca. 37 ms. Spikes are most likely in the torus between 50 and 80 ms after ON. At each recording locus there is a best axis of the homogeneous electric field and a better polarity. Effects of stimulus intensity, duration and repetition are described. The physiological properties are similar to those of ampullary receptor systems in mormyriforms, gymnotiforms and siluriforms.Confirming Braford (1982),Xenomystus has a large medullary nucleus resembling the nucleus otherwise peculiar to mormyriforms, gymnotiforms and siluriforms and now called the electrosensory lateral line lobe (ELLL; formerly the posterior lateral line lobe). We describe the projections of anterior and posterior lateral line nerves by HRP applied to the proximal stump of a cut nerve. A descending central ramus of the anterior lateral line nerve and a lateral component of the ascending ramus of the posterior lateral line nerve end in part in the ELLL.Electroreception, including the system of discrete central structures mediating it, is for the first time found to be less than an ordinal or even a family character, but apparently a characteristic of the subfamily Xenomystinae. Species of the other subfamily, Notopterinae as well as of the other families of osteoglossiforms (Osteoglossidae, Hiodontidae and Pantodontidae), lack the ELLL.Notopterus andPantodon are found to lack the evoked potential.The positive finding of evoked activity to feeble electric field is found to be the most practical method for searching widely among fishes for the presence of the electrosense modality and its central pathways. The anatomical criterion of an ELLL can now be taken to be a good criterion for the presence of this sensory system. The absence of evoked response correlates well with the absence of an ELLL.Abbreviations ELLL electrosensory lateral line lobe - HRP horseradish peroxidase - TS torus semicircularis     

Active electric imaging: body-object interplay and object's "electric texture"

This article deals with the role of fish's body and object's geometry on determining the image spatial shape in pulse Gymnotiforms. This problem was explored by measuring local electric fields along a line on the skin in the presence and absence of objects. We depicted object's electric images at different regions of the electrosensory mosaic, paying particular attention to the perioral region where a fovea has been described. When sensory surface curvature increases relative to the object's curvature, the image details depending on object's shape are blurred and finally disappear. The remaining effect of the object on the stimulus profile depends on the strength of its global polarization. This depends on the length of the object's axis aligned with the field, in turn depending on fish body geometry. Thus, fish's body and self-generated electric field geometries are embodied in this "global effect" of the object. The presence of edges or local changes in impedance at the nearest surface of closely located objects adds peaks to the image profiles ("local effect" or "object's electric texture"). It is concluded that two cues for object recognition may be used by active electroreceptive animals: global effects (informing on object's dimension along the field lines, conductance, and position) and local effects (informing on object's surface). Since the field has fish's centered coordinates, and electrosensory fovea is used for exploration of surfaces, fish fine movements are essential to perform electric perception. We conclude that fish may explore adjacent objects combining active movements and electrogenesis to represent them using electrosensory information.     

Representation of lateral line and electrosensory systems in the midbrain of the axolotl,Ambystoma mexicanum

Summary The study focussed on the representation of the electrosensory and lateral line units in the midbrain of the axolotl Ambystoma mexicanum. In addition, the responses to photic and acoustic/vibrational stimuli were determined. Unit properties were characterized with respect to baseline activity, sensitivity, latency, directional specificity and number of input modalities. The anatomical arrangement of the units was determined using stereotactic and histological measurements of the electrode positions.Of 106 units recorded, 29 units were unimodal, 77 units responded to more than one modality. Most units discharged only in response to stimuli. Thresholds of electrosensory units were about 100 V/cm field strength; lateral line units had thresholds below 5 m pp amplitude. The shortest latencies (8–17 ms) were found for responses to visual stimuli. Lateral line and vestibular units responded after 35–58 ms, electroreceptive units after 79–150 ms. All electrosensory and about 50% of the lateral line units were sharply tuned to definite stimulus directions.Electrosensory and lateral line units formed topographical maps in the tectum. The map in each tectal hemisphere contained information about the contralateral surroundings. The electrosensory, lateral line and visual representations were only partly in register; especially in the caudal areas of the midbrain the alignment was poor.     

Spatiotemporal burst coding for extracting features of spatiotemporally varying stimuli

Encoding features of spatiotemporally varying stimuli is quite important for understanding the neural mechanisms of various sensory coding. Temporal coding can encode features of time-varying stimulus, and population coding with temporal coding is adequate for encoding spatiotemporal correlation of stimulus features into spatiotemporal activity of neurons. However, little is known about how spatiotemporal features of stimulus are encoded by spatiotemporal property of neural activity. To address this issue, we propose here a population coding with burst spikes, called here spatiotemporal burst (STB) coding. In STB coding, the temporal variation of stimuli is encoded by the precise onset timing of burst spike, and the spatiotemporal correlation of stimuli is emphasized by one specific aspect of burst firing, or spike packet followed by silent interval. To show concretely the role of STB coding, we study the electrosensory system of a weakly electric fish. Weakly electric fish must perceive the information about an object nearby by analyzing spatiotemporal modulations of electric field around it. On the basis of well-characterized circuitry, we constructed a neural network model of the electrosensory system. Here we show that STB coding encodes well the information of object distance and size by extracting the spatiotemporal correlation of the distorted electric field. The burst activity of electrosensory neurons is also affected by feedback signals through synaptic plasticity. We show that the control of burst activity caused by the synaptic plasticity leads to extracting the stimulus features depending on the stimulus context. Our results suggest that sensory systems use burst spikes as a unit of sensory coding in order to extract spatiotemporal features of stimuli from spatially distributed stimuli.     

Steroid influences upon the electrosensory system of weakly electric fish: direct effects upon discharge frequencies with indirect effects upon electroreceptor tuning

Summary Previous studies indicated that gonadal steroids can induce changes in both motor and sensory aspects of the electrosensory system of weakly electric fish: androgens decrease the electric organ discharge frequencies and electroreceptor best frequencies of the South American gymnotoidSternopygus. The relationship between these two effects, however, was not known. In the present study, electric organ discharges (EODs) ofSternopygus dariensis were eliminated by means of spinal cord transections. This was done in order to allow an independent assessment of the influences of gonadal steroids upon electroreceptor tuning and those structures in the CNS responsible for establishing the discharge frequency. Transection alone affected neither the rhythmic discharges of the pacemaker nucleus that normally controls the discharge frequency, nor the best frequencies of electroreceptors. Similarly, administration of the androgen 5-dihydrotestosterone (DHT) to transected animals also had no significant effect upon electroreceptor tuning. DHT did, however, cause significant decreases in the discharge rates of the pacemaker nucleus. Thus, the effects of gonadal steroids upon discharge frequencies in intact animals are a direct consequence of CNS influences, while effects upon electroreceptor tuning likely arise as a secondary consequence of the changed discharges of hormone-treated animals.Abbreviations ALLN anterior lateral line nerve - BF best frequency - DHT 5-dihydrotesterone - EOD electric organ discharge     

Detection of multiple stimulus features forces a trade-off in the pyramidal cell network of a gymnotiform electric fish's electrosensory lateral line lobe

Modification of an existing neural structure to support a second function will produce a trade-off between the two functions if they are in some way incompatible. The trade-off between two such sensory functions is modeled here in pyramidal neurons of the gymnotiform electric fish's medullar electrosensory lateral line lobe (ELL). These neurons detect two electric stimulus features produced when a nearby object interferes with the fish's autogenous electric field: (1) amplitude modulation across a cell's entire receptive field and (2) amplitude variation within a cell's receptive field produced by an object's edge. A model of sensory integration shows that detection of amplitude modulation and enhancement of spatial contrast involve an inherent mechanistic trade-off and that the severity of the trade-off depends on the particular algorithm of sensory integration. Electrophysiology data indicate that of the two algorithms for sensory integration modeled here for the gymnotiform fish Brachyhypopomus pinnicaudatus, the algorithm with the better trade-off function is used. Further, the intrinsic trade-off within single cells has been surmounted by the replication of ELL into multiple electrosensory map segments, each specialized to emphasize different sensory features. Accepted: 14 June 1997     

Habituation and prepulse inhibition of acoustic startle in rodents

The acoustic startle response is a protective response, elicited by a sudden and intense acoustic stimulus. Facial and skeletal muscles are activated within a few milliseconds, leading to a whole body flinch in rodents(1). Although startle responses are reflexive responses that can be reliably elicited, they are not stereotypic. They can be modulated by emotions such as fear (fear potentiated startle) and joy (joy attenuated startle), by non-associative learning processes such as habituation and sensitization, and by other sensory stimuli through sensory gating processes (prepulse inhibition), turning startle responses into an excellent tool for assessing emotions, learning, and sensory gating, for review see( 2, 3). The primary pathway mediating startle responses is very short and well described, qualifying startle also as an excellent model for studying the underlying mechanisms for behavioural plasticity on a cellular/molecular level(3). We here describe a method for assessing short-term habituation, long-term habituation and prepulse inhibition of acoustic startle responses in rodents. Habituation describes the decrease of the startle response magnitude upon repeated presentation of the same stimulus. Habituation within a testing session is called short-term habituation (STH) and is reversible upon a period of several minutes without stimulation. Habituation between testing sessions is called long-term habituation (LTH)(4). Habituation is stimulus specific(5). Prepulse inhibition is the attenuation of a startle response by a preceding non-startling sensory stimulus(6). The interval between prepulse and startle stimulus can vary from 6 to up to 2000 ms. The prepulse can be any modality, however, acoustic prepulses are the most commonly used. Habituation is a form of non-associative learning. It can also be viewed as a form of sensory filtering, since it reduces the organisms' response to a non-threatening stimulus. Prepulse inhibition (PPI) was originally developed in human neuropsychiatric research as an operational measure for sensory gating(7). PPI deficits may represent the interface of "psychosis and cognition" as they seem to predict cognitive impairment(8-10). Both habituation and PPI are disrupted in patients suffering from schizophrenia(11), and PPI disruptions have shown to be, at least in some cases, amenable to treatment with mostly atypical antipsychotics(12, 13). However, other mental and neurodegenerative diseases are also accompanied by disruption in habituation and/or PPI, such as autism spectrum disorders (slower habituation), obsessive compulsive disorder, Tourette's syndrome, Huntington's disease, Parkinson's disease, and Alzheimer's Disease (PPI)(11, 14, 15) Dopamine induced PPI deficits are a commonly used animal model for the screening of antipsychotic drugs(16), but PPI deficits can also be induced by many other psychomimetic drugs, environmental modifications and surgical procedures.     

Decrease in occurrence of fast startle responses after selective Mauthner cell ablation in goldfish (Carassius auratus )

A single action potential in one of a pair of reticulospinal neurons, the Mauthner cells, precedes a short-latency electromyographic response of the trunk and tail musculature on the opposite side of the body and a fast startle response in goldfish. It has been postulated that not only the Mauthner cell, but also an array of neurons can trigger or participate in fast startle responses (Eaton et al. 1991). We have selectively ablated the Mauthner cells in goldfish to study how neurons of the brainstem fast startle response network interact. The probability of eliciting a fast startle response was significantly less in fish with double Mauthner cell ablations, as compared to the responsiveness of control fish. The finding that there is a significant decrease in the occurrence of fast startle responses in animals with no Mauthner cells, implies that the Mauthner cell may play a role in triggering the involvement of the other network elements in fast startle responses. We hypothesize that Mauthner cell activation may be important in bringing those reticulospinal neurons that are “primed” by the behavioral context to threshold and provides the basis for studies focused on the interactive nature of the brainstem startle response network. Accepted: 4 November 1998     

Electrosensory systems in the mormyrid fish, Gnathonemus petersii : special emphasis on the fast conducting pathway

1. Rhombencephalic and mesencephalic structures involved in electroreception were investigated by electrophysiological methods in the weakly electric fish Gnathonemus petersii. 2. The existence of a synchronous response to electric field stimulation of the fish in the mesencephalic exterolateral nucleus (n.ext.-lat.mes) with 2.5-3 ms latency was confirmed. The lateral line lobe nucleus (nLLL) is identified as the rhombencephalic relay for the mesencephalic responses because of the short latency synchronous response in the nLLL obtained by threshold stimulation of the posterior lateral line nerve. Responses in both the nLLL and the n.ext.-lat.mes. appear and their amplitudes increase simultaneously with increasing stimulus intensity. 3. Comparison of latencies supports a three-neuron pathway hypothesis which also agrees well with the various functional properties described. 4. The nLLL-n.ext.-lat.mes. pathway is blocked sharply for a period of 1 ms occurring 3 ms after the electric organ discharge (EOD). This inhibitory period is phase-related to the Mesencephalic Command Associated Signal (MCAS) of Aljure (1946) ; The phase relation is such that no response is observed to the fish's own EOD. 5. Long-lasting responses of 10-12 ms duration to higher stimulation intensities were obtained in the ganglionic layer of the lateral line lobe (LLL). Intensities evoking maximal responses in the nLLL and n.ext.lat.mes. are still threshold stimulation for lateral line lobe responses. 6. Long-lasting responses (of the same order as in the LLL) to the fish' own EOD were observed in the mesencephalic lateral nucleus. Responses to artificial electric pulses were obtained only if delivered in a certain phase realtion to the MCAS. The MCAS displays a facilitating effect on the slow conducting electrosensory system. 7. Results indicate the existence in mormyrids of a double, fast and slow conducting, electrosensory system similar to that of gymnotid fish. The mormyrids can control both of these electrosensory systems by means of the MCAS, the effect of which is opposite for the same time period on the two systems.     

Mauthner cell-initiated abrupt increase of the electric organ discharge in the weakly electric fishGymnotus carapo

Stimulation of the spinal cord of the electric fish Gymnotus carapo, evoked an abrupt increase in the discharge rate of the electric organ. At the maximum of this response, the rate increased an average of 26 ± 11.8%. The duration of the response was 4.9 ± 2.12 s; its latency was 10.4 ± 1.1 ms. Activation of the Mauthner axon played a decisive role in this phenomenon as indicated by the following: (1) recordings from the axon cap of the Mauthner cell demonstrated that the response was evoked if the Mauthner axon was antidromically activated and (2) a response that was similar to that produced by spinal cord stimulation, was elicited by intracellular stimulation of either Mauthner cell. Stimulation of the eighth nerve could also increase the discharge rate of the electric organ. The effect was greater if a Mauthner cell action potential was elicited. The findings described in the present report, indicate the existence of a functional connection between the Mauthner cell and the electromotor system in Gymnotus carapo. This connection may function to enhance the electrolocative sampling of the environment during Mauthner-cell mediated behaviors. This is a novel function for the Mauthner cell.Abbreviations EHP extrinsic hyperpolarizing potential - EOD electric organ discharge - M-AIR Mauthner initiated abrupt increase in rate - M-cell Mauthner cell - M-axon Mauthner axon - PM pacemaker nucleus - PM-cell pacemaker cell - PPn prepacemaker nucleus - SPPn sublemniscal prepacemaker nucleus     

Characterization of the Aerial Escape Response of the African Butterfly Fish, Pantodon buchholzi Peters

We studied the startle response of the African butterfly fish, Pantodon buchholzi (Osteoglossomorpha, Osteoglossoidea). It is an upward movement, mediated by abduction of the pectoral fins, and is elicited by mechanical and visual stimuli. Because this fish inhabits the first few centimeters beneath the water surface, its startle response results in an aerial excursion that may be described as ballistic-like, following a motion as defined by linear acceleration. We show that the aerial excursion is well-modeled by a parabola. On average, a fish jumps no more than twice its height and travels horizontally about five times its standard length. The fish may exhibit variable in-flight trunk and fin movements, but neither increases the travel distance in air following the initial in-water propulsive event. Similar vertical jumps also occur entirely within the water column suggesting that this motor behavior of Pantodon is a general escape behavior analogous to a Mauthner neuron-induced escape response. The variability in its posture in air and its direction of motion after reentering the water enhances this act of vertical flight as a step in this fish's escape behavior. The aerial aspect of its escape behavior is only a consequence of its position in the water column.     

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     

Gap detection in the starling (Sturnus vulgaris)

Summary Gap-detection thresholds of single units were determined from auditory forebrain neurons of the awake starling. Nine different response types were statistically defined from the discharge pattern to a 400 ms broadband noise stimulus. The gap stimuli consisted of two broadband noise bursts which were separated by a gap ranging from 0.4 to 204.8 ms duration. The median minimumdetectable gap for 121 out of 145 units that had a significant threshold 204.8ms was 12.8 ms; 20% of the neurons showed thresholds between 0.4 and 3.2 ms. The neurons of the nine response types differed significantly in their minimum-detectable gaps; neurons with phasic-tonic and phasic excitation exhibited the best (i.e. shortest) minimum-detectable gaps. The neurons of the three different recording areas (field L, NCM and HV) were significantly different in their minimumdetectable gaps; field L neurons showed the best temporal resolution for gaps in broadband noise. Gap-detection thresholds are compared with psychophysical thresholds determined with the same stimuli and the relevance of forebrain units for temporal resolution is discussed.Abbreviations CS control stimulus - HV hyperstriatum ventrale - HVc hyperstriatum ventrale pars caudalis - NB noise burst - NCM neostriatum caudale pars medialis - NS noise stimulus - SGS standard gap series - TW time window     

Omnidirectional sensory and motor volumes in electric fish

Active sensing organisms, such as bats, dolphins, and weakly electric fish, generate a 3-D space for active sensation by emitting self-generated energy into the environment. For a weakly electric fish, we demonstrate that the electrosensory space for prey detection has an unusual, omnidirectional shape. We compare this sensory volume with the animal's motor volume—the volume swept out by the body over selected time intervals and over the time it takes to come to a stop from typical hunting velocities. We find that the motor volume has a similar omnidirectional shape, which can be attributed to the fish's backward-swimming capabilities and body dynamics. We assessed the electrosensory space for prey detection by analyzing simulated changes in spiking activity of primary electrosensory afferents during empirically measured and synthetic prey capture trials. The animal's motor volume was reconstructed from video recordings of body motion during prey capture behavior. Our results suggest that in weakly electric fish, there is a close connection between the shape of the sensory and motor volumes. We consider three general spatial relationships between 3-D sensory and motor volumes in active and passive-sensing animals, and we examine hypotheses about these relationships in the context of the volumes we quantify for weakly electric fish. We propose that the ratio of the sensory volume to the motor volume provides insight into behavioral control strategies across all animals.     

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     

Responses of neurons in the electrosensory lateral line lobe of the weakly electric fish <Emphasis Type="Italic">Gnathonemus petersii</Emphasis> to simple and complex electrosensory stimuli

Mormyrid fish use active electrolocation to detect and analyze objects. The electrosensory lateral line lobe in the brain receives input from electroreceptors and an efference copy of the command to discharge the electric organ. In curarized fish, we recorded extracellularly from neurons of the electrosensory lateral line lobe while stimulating in the periphery with either a local point stimulus or with a more natural whole-body stimulus. Two classes of neurons were found: (1) three types of E-cells, which were excited by a point stimulus; and (2) two types of I-cells, which were inhibited by point stimulus and responded with excitation to the electric organ corollary discharge. While all neurons responded to a point stimulus, only one out of two types of I-units and two of the three types of E-units changed their firing behavior to a whole-body stimulus or when an object was present. In most units, the responses to whole-body stimuli and to point stimuli differed substantially. Many electrosensory lateral line lobe units showed neural plasticity after prolonged sensory stimulation. However, plastic effects during whole body stimulation were often unlike those occurring during point stimuli, suggesting that under natural conditions electrosensory lateral line lobe network effects play an important role in shaping neural plasticity.     

Electric organ corollary discharge pathways in mormyrid fish

Corollary discharge signals associated with the motor command that elicits the electric organ discharge are prominent in the electrosensory lobe of mormyrid fish (Gnathonemus petersii). Central pathways and structures that convey these signals from the motor command nucleus to the electrosensory lobe are known anatomically, but these structures and their contributions to the various corollary discharge phenomena have not been examined physiologically. This study examines one such structure, the mesencephalic command associated nucleus (MCA).Recordings from MCA cells show a highly stereotyped two spike response. The first spike of the response has a latency of about 2.5 ms following the initiation of the electric organ discharge (EOD) motor command which is about 5.5 ms before the occurrence of the EOD.Results from stimulation and lesion experiments indicate that MCA is responsible for: 1) the gate-like corollary discharge-driven inhibition of the knollenorgan pathway; 2) the gate-like corollary discharge-driven excitation of granule cells in the mormyromast regions of the electrosensory lobe; and 3) various excitatory effects on other cells in the mormyromast regions.Some corollary discharge phenomena are still present after MCA lesions, including the earliest corollary discharge effects and the plasticity that follows pairing with electrosensory stimuli. These phenomena must be mediated by structures other than MCA.Abbreviations BCA bulbar command associated nucleus - C EOD motor command - C3 central cerebellar lobule 3 - COM EOD motor command nucleus - DLZ dorsolateral zone of ELL cortex - EGa eminentia granularis anterior - EGp eminentia granularis posterior - ELa nucleus exterolateralis anterior - ELL electrosensory lobe - ELLml molecular layer of ELL cortex - EOD electric organ discharge - gang ganglion layer - gran granule layer - jlem juxtalemniscal region - JLl lateral juxtalobar nucleus - JLm medial juxtalobar nucleus - lat nucleus lateralis - ll lateral lemniscus - MCA mesencephalic command associated nucleus - mol molecular layer - MOml molecular layer of the medial octavolateral nucleus - MRN medullary relay nucleus - MZ medial zone of ELL cortex - nALL anterior lateral line nerve - NELL nucleus of the electrosensory lobe - nX cranial nerve X (vagus) - OT optic tectum - PCA paratrigeminal command associated nucleus - pee praeeminentialis electrosensory tract - plex plexiform layer - prae nucleus praeeminentialis - sublem sublemniscal nucleus - TEL telencephalon - VLZ ventrolateral zone of ELL cortex - vped valvular peduncle     

Redox-controlled,in vivo and in vitro phosphorylation of the α subunit of the light-harvesting complex I in Rhodobacter capsulatus

Labelling of Rhodobacter capsulatus cells with (³²P)Pi in a phototrophic culture results in phosphorylation of a membrane-bound polypeptide identified as the subunit of the LHI antenna complex of the photosynthetic apparatus. Phosphorylation of the same polypeptide was also observed by incubation of chromatophores with (³²P)ATP or under conditions of photophosphorylation with ADP and (³²P)Pi. The identity of the phosphorylated LHI- subunit was demonstrated by N-terminal protein sequencing of the phosphorylated polypeptide and by failure of labelling in LHI-defective mutants. Pre-aeration of the samples or addition of the oxidant potassium ferrcyanide stimulated the kinase activity whereas the presence of soluble cytoplasmic proteins impaired phosphorylation in an in vitro assay. No effect resulted from addition of reductants to the assay medium. The results indicate the presence of a membrane-bound protein kinase in R. capsulatus that phosphorylates the subunit of the LHI antenna complex under redox control.Abbreviations Pi inorganic phosphate - SDS-PAGE sodium dodecyl-sulfate polyacrylamide gel electrophoresis