Motor Output Patterns during Random and Rhythmic Stimulation of Locust Thoracic Ganglia

This paper employs new statistical techniques to further analyze the flight control system of grasshoppers. The quantitative results confirm some hypotheses which arise from previous studies of this system. After decapitation and ablation of wing proprioceptors, stimulation of the nerve cord at random intervals can elicit a coordinated response closely resembling the normal flight motor output pattern. The coordinated response begins only after many stimuli and there are usually many cycles of after-discharge. The frequency of the cyclic output is rather low and may be increased only slightly by large increases in stimulus frequency. Input from the stretch receptors is necessary to attain normal wingstroke frequency. Frequency of wingbeat rises with a time constant of about 2 seconds (or about 25 wingbeats) when stretch receptor stimulation is initiated. Frequency decay after cessation of stimulation has about the same time constant. No special phase relationship between stimulation and output is necessary for the increase in frequency or maintenance of normal pattern. When input frequency is adjusted as closely as possible to output frequency it is still not possible to force the output to maintain a particular phase with respect to the stimulation, all phase relationships still occur. In some animals all phases occurred with equal probability; in others a particular phase was preferred. When there was a strong phase preference the normal output pattern was disrupted.     

Effects of octopamine, dopamine, and serotonin on production of flight motor output by thoracic ganglia of Manduca sexta

Effects of biogenic amines on a centrally generated motor pattern in Manduca sexta were examined by pressure injecting nanomole to micromole amounts of octopamine, dopamine or serotonin into thoracic ganglia. Motor output was recorded extracellularly from a pair of antagonistic flight muscles and their motor neurons. The monoamines were found to alter production of a motor pattern that produces rhythmic wing flapping (10 Hz) and exhibits phase relationships similar to those in the flight pattern of intact moths. In mesothoracic ganglia with sensory nerves intact, octopamine (4 X 10(-9) mol) injected into lateral regions evoked regular firing of a single motor neuron, whereas a higher dose (4 X 10(-8) mol) often elicited the flight motor pattern. In the absence of sensory input, these doses of octopamine had little effect. Low doses (10(-10) mol) greatly enhanced motor responses to electrical stimulation of a wing sensory nerve. Dopamine (2 X 10(-10) mol) injected into the medial region of the mesothoracic ganglion elicited the flight motor pattern in the presence or absence of sensory input. Rhythmic output induced by dopamine (5 X 10(-10) mol) was suppressed by injecting serotonin (5 X 10(-10) mol) into the same region. These findings demonstrate that dopamine, octopamine, and serotonin have different effects on motor output in Manduca and suggest that these amines are involved in initiating, maintaining and terminating flight behavior, respectively. Octopamine may elicit flight production by enhancing the efficacy of sensory transmission thereby increasing excitability or arousal. Dopamine may act on interneurons involved in generating the flight motor pattern.     

Reflex activation of locust flight motoneurones by proprioceptors responsive to muscle contractions

 This report investigates the reflex activation of locust flight motoneurones following their spiking activity. As shown elsewhere, an electrical stimulus applied to a flight muscle produces multiple waves of delayed excitation in wing elevator and depressor motoneurones. Nerve ablation experiments show that this response is initiated by the mechanical movement of the stimulated muscle, and not the antidromic spike evoked in the motoneurone. The delayed excitation still occurs in the absence of inputs from the wing receptor systems, and also when all other sources of afferent feedback are abolished, excepting thoracic nerve 2. Following complete deafferentation, spikes in flight motoneurones had no influence on other flight motoneurones. Numerous afferents in the purely sensory nerve 2 are excited by flight muscle contractions. The responses are consistent for repeated contractions of the same muscle, but differ when other muscles are stimulated. During tethered flight, changes in the activation of single flight muscles are reflected in changes of the nerve 2 discharge pattern. Electrical stimulation of this nerve causes delayed excitation of flight motoneurones, and can initiate flight activity. It is suggested that internal proprioceptors, such as those associated with nerve 2, will contribute to shaping the final motor output for flight behaviour. Accepted: 24 April 1996     

The role of delayed excitation in the co-ordination of some metathoracic flight motoneurons of a locust

Summary Intracellular recordings have been made from the somata of two metathoracic flight motoneurons, one innervating an elevator muscle of the hindwing, the tergosternal muscle 113 and the other a depressor, the first basalar muscle 127. The locust,Ghortoicetes terminifera was mounted ventral side uppermost with the thorax restrained and opened for access to the thoracic ganglia. Patterns of electrical activity recorded from the thoracic muscles were similar to those shown by a locust during flight when tethered in a more normal posture. In flight the left and right 113 motoneurons each receive a single impulse together at every stroke of the wing, with the 127 muscles active in approximate antiphase. A spike in a 113 motoneuron causes a delayed wave of excitation simultaneously upon itself and its contralateral partner (Fig. 2). The epsp's which form these waves summate and may cause a spike which follows the original one with a delay equal to the wingbeat period. The delayed excitation of the contralateral motoneuron is of larger amplitude than the ipsilateral one so that spikes in either motoneuron must activate separate but symmetrical pathways. A single spike may cause multiple waves in either motoneuron, each separated by intervals equal to the wingbeat period (Fig. 3). In the pathway must be neurons capable of reverberation.A spike in a 113 motoneuron causes a delayed excitation of the ipsilateral 127 motoneuron so that its membrane potential is lowered antiphasically to that of 113 (Fig. 17). A spike in a 127 motoneuron has no effect on the 113 motoneurons. In flight these pathways causing delayed excitation may co-ordinate the motoneurons.The left and right 113 motoneurons receive common synaptic inputs from at least two sources (Fig. 8). These occur as bursts of epsp's at intervals approximately equal to or multiples of the wingbeat period and in the absence of flight. Epsp's of sufficient amplitude cause a spike in the motoneuron which is in the correct phase in the flight pattern relative to any other active motoneurons (Fig. 9). During sustained flight epsp's contribute to the wave of depolarization that the motoneuron undergoes at each wingbeat (Fig. 11). In the absence of the epsp's the motoneuron does not oscillate on its own. At the end of flight bursts of epsp's may continue at the flight frequency long after all activity in the muscles has ceased.Beit Memorial Research Fellow.     

Auditory input to motor neurons of the dorsal longitudinal flight muscles in a noctuid moth (Barathra brassicae L.)

Summary Motor neurons innervating the dorsal longitudinal muscles of a noctuid moth receive synaptic input activated by auditory stimuli. Each ear of a noctuid moth contains two auditory neurons that are sensitive to ultrasound (Fig. 1). The ears function as bat detectors. Five pairs of large motor neurons and three pairs of small motor neurons found in the pterothoracic ganglia innervate the dorsal longitudinal (depressor) muscles of the mesothorax (Figs. 2 to 5). In non-flying preparations the motor neurons receive no oscillatory synaptic input. Synaptic input to a cell resulting from ultrasonic stimulation is consistent and can be either depolarizing or hyperpolarizing (Figs. 6 to 9). Quiescent neurons only rarely fire a spike in response to auditory inputs. Motor neurons in flying preparations receive oscillatory synaptic drive from the flight pattern generator and usually fire a spike for each wingbeat cycle (Figs. 10 to 12). Ultrasonic stimulation can provide augmented synaptic drive causing a neuron to fire two spikes per wingbeat cycle thus increasing flight vigor (Fig. 11). The same stimulus presented on another occasion can also inhibit spiking in the same motor neuron, but the rhythmic drive remains (Fig. 12). Thus, when the flight oscillator is running auditory stimuli can modulate neuronal responses in different ways depending on some unknown state of the nervous system. Sound intensity is the only stimulus parameter essential for activating the auditory pathway to these motor neurons. The intensity must be sufficient to excite two or three auditory neurons. The significance of these responses in relation to avoidance behavior to bats is discussed.     

Discharge patterns of tonically firing human motoneurones

We have attempted to reconcile the different patterns of distribution of interspike intervals that are found in motoneurones made to discharge by intracellular injection of constant current in reduced animal preparations and by voluntary control in human subjects. We recorded long spike trains from single motor units in three human muscles made to discharge at constant mean frequencies with the help of auditory and visual feedback. The distribution of interspike intervals in each spike train was analysed quantitatively. We found that the different pattern of discharge of the human motor units could be accounted for when due allowance was made for the variability of the drive to the human motoneurone which arose because of the feedback process used to maintain the target frequency. A model testing this hypothesis gave results that were qualitatively consistent with the human data.     

Brainstem lateral line responses to sinusoidal wave stimuli in the goldfish, Carassius auratus

Extracellular recordings were made from single lateral line units in the medial octavolateralis nucleus in the brainstem of goldfish, Carassius auratus. Units were defined as receiving lateral line input if they responded to the water motions generated by a stationary, sinusoidally oscillating sphere and/or a moving sphere but not to airborne sound and vibrations. Units which responded to airborne sound or vibrations were assumed to receive input from the inner ear and were not further investigated. Responses of lateral line units were quantified in terms of the number of evoked spikes and the degree of phase-locking to a 50 Hz vibrating sphere presented at various stationary locations along the side of the fish. Receptive fields were characterized based on spike rate, degree of phase-locking and average phase angle as a function of sphere location. Four groups of units were distinguished: 1, units with receptive fields comparable to those of primary afferents; 2, units with receptive fields which consisted of one excitatory and one inhibitory area; 3, units with receptive fields which consisted of more than two excitatory and/or inhibitory areas; 4, units with receptive fields which consisted of a single excitatory or a single inhibitory area. The receptive fields of most units were characterized by adjacent excitatory and inhibitory areas. This organization is reminiscent of excitatory-inhibitory receptive field organizations in other vertebrate sensory systems.     

Intracellular recordings from spinal neurons during 'swimming' in paralysed amphibian embryos

Intracellular microelectrode recordings have been made from probable motoneurons in the spinal cord of Xenopus laevis embryos during fictive 'swimming' in preparations paralysed with the neuromuscular blocking agent tubocurarine. These cells had resting potentials of -50 mV or more. During spontaneous or stimulus-evoked 'swimming' episodes: (a) the cells were tonically excited; the level of tonic synaptic excitation and the conductance increase underlying it were both inversely related to the 'swimming' cycle period; (b) the cells usually fired one spike per cycle in phase with the motor root burst on the same side; spikes did not overshoot zero and were evoked by phasic excitatory synaptic input on each cycle, superimposed on the tonic excitation; (c) in phase with motor root discharge on the opposite side of the body, the cells were hyperpolarized by a chloride-dependent inhibitory postsynaptic potential. The nature of synaptic potentials during 'swimming' was evaluated by means of intracellular current injections. The 'swimming' activity could be controlled by natural stimuli. The results provide clear evidence on the relation of tonic excitation to rhythmic locomotory pattern generation, and indirect evidence for reciprocal inhibitory coupling between antagonistic motor systems.     

Modulation of auditory responsiveness in the locust

The auditory responsiveness of a number of neurones in the meso- and metathoracic ganglia of the locust, Locusta migratoria, was found to change systematically during concomitant wind stimulation. Changes in responsiveness were of three kinds: a suppression of the response to low frequency sound (5 kHz), but an unchanged or increased response to high frequency (12 kHz) sound; an increased response to all sound; a decrease in the excitatory, and an increase in the inhibitory, components of a response to sound. Suppression of the response to low frequency sound was mediated by wind, rather than by the flight motor. Wind stimulation caused an increase in membrane conductance and concomitant depolarization in recorded neurones. Wind stimulation potentiated the spike response to a given depolarizing current, and the spike response to a high frequency sound, by about the same amount. The strongest wind-related input to interneuron 714 was via the metathoracic N6, which carries the axons of auditory receptors from the ear. The EPSP evoked in central neurones by electrical stimulation of metathoracic N6 was suppressed by wind stimulation, and by low frequency (5 kHz), but not high frequency (10 kHz), sound. This suppression disappeared when N6 was cut distally to the stimulating electrodes. Responses to low frequency (5 kHz), rather than high frequency (12 kHz), sounds could be suppressed by a second low frequency tone with an intensity above 50-55 dB SPL for a 5 kHz suppressing tone. Suppression of the electrically-evoked EPSP in neurone 714 was greatest at those sound frequencies represented maximally in the spectrum of the locust's wingbeat. It is concluded that the acoustic components of a wind stimulus are able to mediate both inhibition and excitation in the auditory pathway. By suppressing the responses to low frequency sounds, wind stimulation would effectively shift the frequency-response characteristics of central auditory neurones during flight.     

Spike latency and jitter of neuronal membrane patches with stochastic Hodgkin-Huxley channels

Ion channel stochasticity can influence the voltage dynamics of neuronal membrane, with stronger effects for smaller patches of membrane because of the correspondingly smaller number of channels. We examine this question with respect to first spike statistics in response to a periodic input of membrane patches including stochastic Hodgkin-Huxley channels, comparing these responses to spontaneous firing. Without noise, firing threshold of the model depends on frequency—a sinusoidal stimulus is subthreshold for low and high frequencies and suprathreshold for intermediate frequencies. When channel noise is added, a stimulus in the lower range of subthreshold frequencies can influence spike output, while high subthreshold frequencies remain subthreshold. Both input frequency and channel noise strength influence spike timing. Specifically, spike latency and jitter have distinct minima as a function of input frequency, showing a resonance like behavior. With either no input, or low frequency subthreshold input, or input in the low or high suprathreshold frequency range, channel noise reduces latency and jitter, with the strongest impact for the lowest input frequencies. In contrast, for an intermediate range of suprathreshold frequencies, where an optimal input gives a minimum latency, the noise effect reverses, and spike latency and jitter increase with channel noise. Thus, a resonant minimum of the spike response as a function of frequency becomes more pronounced with less noise. Spike latency and jitter also depend on the initial phase of the input, resulting in minimal latencies at an optimal phase, and depend on the membrane time constant, with a longer time constant broadening frequency tuning for minimal latency and jitter. Taken together, these results suggest how stochasticity of ion channels may influence spike timing and thus coding for neurons with functionally localized concentrations of channels, such as in “hot spots” of dendrites, spines or axons.     

The responses of central octavolateralis cells to moving sources

Mechanosensory lateral line units recorded from the medulla (medial octavolateralis nucleus) and midbrain (torus semicircularis) of the bottom dwelling catfish Ancistrus sp. responded to water movements caused by an object that passed the fish laterally. In terms of peak spike rate or total number of spikes elicited responses increased with object speed and sometimes showed saturation (Figs. 7, 14). At sequentially greater distances the responses of most medullary lateral line units decayed with object distance (Fig. 11). Units tuned to a certain object speed or distance were not found. The signed directionality index of most lateral line units was between –50 and +50, i.e. these units were not or only slightly sensitive to the direction of object motion (Figs. 10, 17). However, some units were highly directionally sensitive in that the main features of the response histograms and/or peak spike rates clearly depended on the direction of object movement (e.g. Fig. 9C, D and Fig. 16). Midbrain lateral line units of Ancistrus may receive input from more than one sensory modality. All bimodal lateral line units were OR units, i.e., the units were reliably driven by a unimodal stimulus of either modality. Units which receive bimodal input may show an extended speed range (e.g. Fig. 18).Abbreviations MON medial octavolateralis nucleus - MSR mean spike rate - PSR peak spike rate - p-p peak-to-peak - SDI signed directionality index     

Coping with variability in small neuronal networks

Experimental and corresponding modeling studies indicate that there is a 2- to 5-fold variation of intrinsic and synaptic parameters across animals while functional output is maintained. Here, we review experiments, using the heartbeat central pattern generator (CPG) in medicinal leeches, which explore the consequences of animal-to-animal variation in synaptic strength for coordinated motor output. We focus on a set of segmental heart motor neurons that all receive inhibitory synaptic input from the same four premotor interneurons. These four premotor inputs fire in a phase progression and the motor neurons also fire in a phase progression because of differences in synaptic strength profiles of the four inputs among segments. Our work tested the hypothesis that functional output is maintained in the face of animal-to-animal variation in the absolute strength of connections because relative strengths of the four inputs onto particular motor neurons is maintained across animals. Our experiments showed that relative strength is not strictly maintained across animals even as functional output is maintained, and animal-to-animal variations in strength of particular inputs do not correlate strongly with output phase. Further experiments measured the precise temporal pattern of the premotor inputs, the segmental synaptic strength profiles of their connections onto motor neurons, and the temporal pattern (phase progression) of those motor neurons all in the same animal for a series of 12 animals. The analysis of input and output in this sample of 12 individuals suggests that the number (four) of inputs to each motor neuron and the variability of the temporal pattern of input from the CPG across individuals weaken the influence of the strength of individual inputs. Moreover, the temporal pattern of the output varies as much across individuals as that of the input. Essentially, each animal arrives at a unique solution for how the network produces functional output.     

Neural correlates to flight‐related density‐dependent phase characteristics in locusts

Locust phase polymorphism is an extreme example of behavioral plasticity; in response to changes in population density, locusts dramatically alter their behavior. These changes in behavior facilitate the appearance of various morphological and physiological phase characteristics. One of the principal behavioral changes is the more intense flight behavior and improved flight performance of gregarious locusts compared to solitary ones. Surprisingly, the neurophysiological basis of the behavioral phase characteristics has received little attention. Here we present density‐dependent differences in flight‐related sensory and central neural elements in the desert locust. Using techniques already established for gregarious locusts, we compared the response of locusts of both phases to controlled wind stimuli. Gregarious locusts demonstrated a lower threshold for wind‐induced flight initiation. Wind‐induced spiking activity in the locust tritocerebral commissure giants (TCG, a pair of identified interneurons that relay input from head hair receptors to thoracic motor centers) was found to be weaker in solitary locusts compared to gregarious ones. The solitary locusts' TCG also demonstrated much stronger spike frequency adaptation in response to wind stimuli. Although the number of forehead wind sensitive hairs was found to be larger in solitary locusts, the stimuli conveyed to their flight motor centers were weaker. The tritocerebral commissure dwarf (TCD) is an inhibitory flight‐related interneuron in the locust that responds to light stimuli. An increase in TCD spontaneous activity in dark conditions was significantly stronger in gregarious locusts than in solitary ones. Thus, phase‐dependent differences in the activity of flight‐related interneurons reflect behavioral phase characteristics. © 2003 Wiley Periodicals, Inc. J Neurobiol 57: 152–162, 2003     

A reconsideration of the central pattern generator concept for locust flight

Summary Although it is generally agreed that locusts can generate flight similar rhythmic motor activity in the absence of sensory feedback from the wings, recent studies indicate that functional deafferentation produces significant changes in the flight motor pattern (Hedwig and Pearson 1984). These findings have raised doubts on the adequacy of the central pattern generator concept for the locust flight system (Pearson 1985). In this paper, we re-investigate the effects of deafferentation on the capacity of adult migratory locusts to generate the flight motor pattern. For this purpose, the experimental animals were dissected to various degrees, ranging from head-ventral nerve cord, to isolated pterothoracic nerve cord, and finally single isolated ganglion preparations. Flight motor activity was released by either wind stimulation, the more traditional method, or by applying octopamine (Sombati and Hoyle 1984; Stevenson and Kutsch 1986). In all cases the released motor activity was analysed, giving details of latency, and phase relationships between specific synergistic and antagonistic motor units, and then compared with the flight motor pattern generated by intact tethered locusts.This analysis shows that deafferentation, although reducing the frequency, does not necessarily disrupt the basic flight motor pattern. By using octopamine we could show that even isolated thoracic nerve cord preparations can generate activity, which in all major aspects corresponds to this motor program. This could also be shown for the fully isolated metathoracic ganglion and we provide some evidence that the mesothoracic ganglion may be capable of a similar performance. In addition to releasing flight activity, octopamine was also found to enhance the responsiveness of deafferentated locusts to wind stimulation. This resulted in a considerable elevation of the frequency and prolongation of the flight motor activity to values comparable to the performance of intact tethered locusts.     

EGF converts transit-amplifying neurogenic precursors in the adult brain into multipotent stem cells

We studied the transformation of sensory input as it progresses from vibrissa primary sensor (S1) to motor (M1) cortex. Single-unit activity was obtained from alert adult rats that did not to whisk upon application of punctate, rhythmic stimulation of individual vibrissae. The spike response of units in S1 cortex largely reproduced the shape of the stimulus. In contrast, the spiking output of units in M1 cortex were modulated solely as a sinusoid at the repetition rate of the stimulus for frequencies between 5 and 15 Hz; this range corresponds to that of natural whisking. Thus, the S1 to M1 transformation extracts the fundamental frequency from a spectrally rich stimulus. We discuss our results in terms of a band-pass filter with a center frequency that adapts to the change in stimulation rate.     

Computer simulations of synchrony and oscillations evoked by two coherent inputs

Coherent oscillations have been reported in multiple cortical areas. This study examines the characteristics of output spikes through computer simulations when the neural network model receives periodic/aperiodic spatiotemporal spikes with modulated/constant populational activity from two pathways. Synchronous oscillations which have the same period as the input are observed in response to periodic input patterns regardless of populational activity. The results confirm that the output frequency of synchrony is essentially determined by the period of the repeated input patterns. On the other hand, weak periodic outputs are observed when aperiodic spikes are input with modulated populational activity. In this case, higher firing rates are necessary to input for higher frequency oscillations. The spike-timing-dependent plasticity suppresses the spikes which do not contribute to the synchrony for periodic inputs. This effect corresponds to the experimental reports that learning sharpens the synchrony in the motor cortex. These results suggest that spatiotemporal spike patterns should be entrained on modulated populational activity to transmit oscillatory information effectively in the convergent pathway.     

Transfer characteristics between EMG activity and muscle tension under isometric conditions in man.

The relationship between motor unit activity and a voluntarily produced, sinusoidally modulated isometric tension was evaluated as a function of the modulation frequency. These date are reported in terms of the gain and phase difference of the motor activity (input) and tension (output) relationship, the gain being the logarithmic ratio of the amplitudes of the output and input sinusoids. It was found that an increase in the modulation amplitude of the motor unit activity was required to produce the same amount of modulation of the output tension as the modulation frequency was increased. For example, the modulation amplitude of the activity is about twice as much at 2 Hz as at 0.25 Hz and about 4 times as much at 5 Hz. It was also found that the maximum tension which could be produced voluntarily during brief jerks at 5 Hz was the same as the maximum sustained tension which could be attained. This latter finding emphasizes the importance of recruitment and especially synchronization of motor unit activity to the gradation of output tension.     

Neural correlates to flight-related density-dependent phase characteristics in locusts

Locust phase polymorphism is an extreme example of behavioral plasticity; in response to changes in population density, locusts dramatically alter their behavior. These changes in behavior facilitate the appearance of various morphological and physiological phase characteristics. One of the principal behavioral changes is the more intense flight behavior and improved flight performance of gregarious locusts compared to solitary ones. Surprisingly, the neurophysiological basis of the behavioral phase characteristics has received little attention. Here we present density-dependent differences in flight-related sensory and central neural elements in the desert locust. Using techniques already established for gregarious locusts, we compared the response of locusts of both phases to controlled wind stimuli. Gregarious locusts demonstrated a lower threshold for wind-induced flight initiation. Wind-induced spiking activity in the locust tritocerebral commissure giants (TCG, a pair of identified interneurons that relay input from head hair receptors to thoracic motor centers) was found to be weaker in solitary locusts compared to gregarious ones. The solitary locusts' TCG also demonstrated much stronger spike frequency adaptation in response to wind stimuli. Although the number of forehead wind sensitive hairs was found to be larger in solitary locusts, the stimuli conveyed to their flight motor centers were weaker. The tritocerebral commissure dwarf (TCD) is an inhibitory flight-related interneuron in the locust that responds to light stimuli. An increase in TCD spontaneous activity in dark conditions was significantly stronger in gregarious locusts than in solitary ones. Thus, phase-dependent differences in the activity of flight-related interneurons reflect behavioral phase characteristics.     

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