Multiunit Recordings Research Articles

Can motor volition be extracted from the spinal cord?

BackgroundSpinal cord injury (SCI) results in the partial or complete loss of movement and sensation below the level of injury. In individuals with cervical level SCI, there is a great need for voluntary command generation for environmental control, self-mobility, or computer access to improve their independence and quality of life. Brain-computer interfacing is one way of generating these voluntary command signals. As an alternative, this study investigates the feasibility of utilizing descending signals in the dorsolateral spinal cord tracts above the point of injury as a means of generating volitional motor control signals.MethodsIn this work, adult male rats were implanted with a 15-channel microelectrode array (MEA) in the dorsolateral funiculus of the cervical spinal cord to record multi-unit activity from the descending pathways while the animals performed a reach-to-grasp task. Mean signal amplitudes and signal-to-noise ratios during the behavior was monitored and quantified for recording periods up to 3 months post-implant. One-way analysis of variance (ANOVA) and Tukey’s post-hoc analysis was used to investigate signal amplitude stability during the study period. Multiple linear regression was employed to reconstruct the forelimb kinematics, i.e. the hand position, elbow angle, and hand velocity from the spinal cord signals.ResultsThe percentage of electrodes with stable signal amplitudes (p-value < 0.05) were 50% in R1, 100% in R2, 72% in R3, and 85% in R4. Forelimb kinematics was reconstructed with correlations of R2 > 0.7 using tap-delayed principal components of the spinal cord signals.ConclusionsThis study demonstrated that chronic recordings up to 3-months can be made from the descending tracts of the rat spinal cord with relatively small changes in signal characteristics over time and that the forelimb kinematics can be reconstructed with the recorded signals. Multi-unit recording technique may prove to be a viable alternative to single neuron recording methods for reading the information encoded by neuronal populations in the spinal cord.

Advances in Sympathetic Nerve Recording in Humans

In humans, sympathetic activity is commonly assessed by measuring the efferent traffic in the peroneal nerve. The firing activity is the sum of several active neurons, which have the tendency to fire together in a bursting manner. While the estimation of overall sympathetic nervous activity using this multiunit recording approach has advanced our understanding of sympathetic regulation in health and disease no information is gained regarding the underling mechanisms generating the bursts of sympathetic activity. The introduction of single-unit recording has been a major step forward, enabling the examination of specific sympathetic firing patterns in diverse clinical conditions. Disturbances in sympathetic nerve firing, including high firing probabilities, high firing rates or high incidence of multiple firing, or a combination of both may impact on noradrenaline release and effector response, and therefore have clinical implications with regards to the development and progression of target organ damage. Understanding the mechanisms and consequences of specific firing patterns would permit the development of therapeutic strategies targeting these nuances of sympathetic overdrive.

Optimal Sequential Detection of Stimuli from Multiunit Recordings Taken in Densely Populated Brain Regions

We address the problem of detecting the presence of a recurring stimulus by monitoring the voltage on a multiunit electrode located in a brain region densely populated by stimulus reactive neurons. Published experimental results suggest that under these conditions, when a stimulus is present, the measurements are gaussian with typical second-order statistics. In this letter we systematically derive a generic, optimal detector for the presence of a stimulus in these conditions and describe its implementation. The optimality of the proposed detector is in the sense that it maximizes the life span (or time to injury) of the subject. In addition, we construct a model for the acquired multiunit signal drawing on basic assumptions regarding the nature of a single neuron, which explains the second-order statistics of the raw electrode voltage measurements that are high-pass-filtered above 300 Hz. The operation of the optimal detector and that of a simpler suboptimal detection scheme is demonstrated by simulations and on real electrophysiological data.

Tetrode recordings in the cerebellar cortex

Multi-unit recordings with tetrodes have been used in brain studies for many years, but surprisingly, scarcely in the cerebellum. The cerebellum is subdivided in multiple small functional zones. Understanding the proper features of the cerebellar computations requires a characterization of neuronal activity within each area. By allowing simultaneous recordings of neighboring cells, tetrodes provide a helpful technique to study the dynamics of the cerebellar local networks. Here, we discuss experimental configurations to optimize such recordings and demonstrate their use in the different layers of the cerebellar cortex. We show that tetrodes can also be used to perform simultaneous recordings from neighboring units in freely moving rats using a custom-made drive, thus permitting studies of cerebellar network dynamics in a large variety of behavioral conditions.

Topographic Maps within Brodmann's Area 5 of Macaque Monkeys

Brodmann's area 5 has traditionally included the rostral bank of the intraparietal sulcus (IPS) as well as posterior portions of the postcentral gyrus and medial wall. However, different portions of this large architectonic zone may serve different functions related to reaching and grasping behaviors. The current study used multiunit recording techniques in anesthetized macaque monkeys to survey a large extent of the rostral bank of the IPS so that hundreds of recording sites could be used to determine the functional subdivisions and topographic organization of cortical areas in this region. We identified a lateral area on the rostral IPS that we term area 5L. Area 5L contains neurons with receptive fields on mostly the shoulder, forelimb, and digits, with no apparent representation of other body parts. Thus, there is a large magnification of the forelimb. Receptive fields for neurons in this region often contain multiple joints of the forelimb or multiple digits, which results in imprecise topography or fractures in map organization. Our results provide the first overall topographic map of area 5L obtained in individual macaque monkeys and suggest that this region is distinct from more medial portions of the IPS.

Multisensory information facilitates reaction speed by enlarging activity difference between superior colliculus hemispheres in rats.

Animals can make faster behavioral responses to multisensory stimuli than to unisensory stimuli. The superior colliculus (SC), which receives multiple inputs from different sensory modalities, is considered to be involved in the initiation of motor responses. However, the mechanism by which multisensory information facilitates motor responses is not yet understood. Here, we demonstrate that multisensory information modulates competition among SC neurons to elicit faster responses. We conducted multiunit recordings from the SC of rats performing a two-alternative spatial discrimination task using auditory and/or visual stimuli. We found that a large population of SC neurons showed direction-selective activity before the onset of movement in response to the stimuli irrespective of stimulation modality. Trial-by-trial correlation analysis showed that the premovement activity of many SC neurons increased with faster reaction speed for the contraversive movement, whereas the premovement activity of another population of neurons decreased with faster reaction speed for the ipsiversive movement. When visual and auditory stimuli were presented simultaneously, the premovement activity of a population of neurons for the contraversive movement was enhanced, whereas the premovement activity of another population of neurons for the ipsiversive movement was depressed. Unilateral inactivation of SC using muscimol prolonged reaction times of contraversive movements, but it shortened those of ipsiversive movements. These findings suggest that the difference in activity between the SC hemispheres regulates the reaction speed of motor responses, and multisensory information enlarges the activity difference resulting in faster responses.

Brain–Gut Interactions Increase Peripheral Nociceptive Signaling in Mice With Postinfectious Irritable Bowel Syndrome

To investigate the peripheral sensory effects of repeated stress in patients with postinfectious irritable bowel syndrome (IBS), we tested whether stress following self-limiting bacterial colitis increases colonic dorsal root ganglia (DRG) nociceptive signaling. C57BL/6 mice were infected with Citrobacter rodentium. Stress was induced using a 9-day water avoidance paradigm (days 21-30 after infection). Colonic DRG neuronal excitability was measured using perforated patch clamp techniques, in vitro multi-unit afferent recordings, and measurements of visceromotor reflexes. Combined stress and prior infection increased corticosterone and epinephrine levels, compared with infected animals, but did not alter the resolution of colonic inflammation. These changes were associated with increased neuronal excitability and parallel changes in multi-unit afferent recordings and visceromotor reflex thresholds. Protease activity was increased at day 30 following infection with C rodentium. Protease inhibitors markedly reduced the effects of colonic supernatants on neuronal excitability from C rodentium but not stressed animals. Colonic DRG neurons expressed messenger RNAs for the β(2) adrenergic and glucocorticoid receptors; incubation with stress mediators recapitulated the effects on neuronal excitability observed with chronic stress alone. PAR2 activation with concentrations of the activating peptide SLIGRL that had no effect on neuronal excitability in controls caused marked increases in excitability when applied to neurons from chronically stressed animals. Stress, combined with prior acute colitis, results in exaggerated peripheral nociceptive signaling. Proteases and stress mediators can signal directly to colonic DRG neurons; further analysis of these pathways could provide new targets for treatment of patients with postinfectious IBS.

Selectivity for the rate of frequency-modulated sweeps in the mouse auditory cortex

Frequency-modulated (FM) sweeps are common components of vocalizations, including human speech. Both sweep direction and rate influence discrimination of vocalizations. Across species, relatively less is known about FM rate selectivity compared with direction selectivity. In this study, FM rate selectivity was studied in the auditory cortex of anesthetized 1- to 3-mo-old C57bl/6 mice. Neurons were classified as fast pass, band pass, slow pass, or all pass depending on their selectivity for rates between 0.08 and 20 kHz/ms. Multiunit recordings were used to map FM rate selectivity at depths between 250 and 450 μm across both primary auditory cortex (A1) and the anterior auditory field (AAF). In terms of functional organization of rate selectivity, three patterns were found. First, in both A1 and AAF, neurons clustered according to rate selectivity. Second, most (∼60%) AAF neurons were either fast-pass or band-pass selective. Most A1 neurons (∼72%) were slow-pass selective. This distribution supports the hypothesis that AAF is specialized for faster temporal processing than A1. Single-unit recordings (n = 223) from A1 and AAF show that the mouse auditory cortex is best poised to detect and discriminate a narrow range of sweep rates between 0.5 and 3 kHz/ms. Third, based on recordings obtained at different depths, neurons in the infragranular layers were less rate selective than neurons in the granular layers, suggesting FM processing undergoes changes within the cortical column. On average, there was very little direction selectivity in the mouse auditory cortex. There was also no correlation between characteristic frequency and direction selectivity. The narrow range of rate selectivity in the mouse cortex indicates that FM rate processing is a useful physiological marker for studying contributions of genetic and environmental factors in auditory system development, aging, and disease.

Spatial organization of receptive fields in the auditory midbrain of awake mouse

Efficient encoding of sensory information can be implemented by heterogeneous response properties of neurons within sensory pathways. In the auditory system, neurons in the main auditory midbrain nucleus, the inferior colliculus (IC), show heterogeneous response properties to various types of acoustic stimuli including behaviorally relevant sounds. The receptive fields of these neurons, and their spatial organization, may reveal mechanisms that underlie response heterogeneity in the IC. The mouse is becoming an increasingly popular system for auditory studies and although some studies have examined spectral characteristics in the IC, most of these have been conducted in anesthetized animals. There were two goals of this study. The first goal was to examine the frequency representation of awake mouse IC in fine spatial resolution. The second goal was to determine whether there is a spatial organization of excitatory frequency tuning curves in the IC of awake mice. We achieved these goals by histologically reconstructing locations of single and multiunit recordings throughout the IC in a mouse strain with normal hearing (CBA/CaJ). We found that the tonotopic progression is discontinuous in mouse IC, and we found that there is no clear spatial organization of frequency tuning curve types. Rather, there is heterogeneity of receptive fields in the bulk of the IC such that frequency tuning characteristics and hence the structure of excitatory and inhibitory inputs does not depend on location in the IC. This heterogeneity likely provides a mechanism for efficient encoding of auditory stimuli throughout the extent of the mouse IC.

A preliminary study on the arrangement of sympathetic fibres within the human common fibular nerve

Recently, interest has grown in the firing patterns of individual or multi-unit action potentials (AP) in human muscle sympathetic nerve recordings using microneurography. Little is known, however, about sympathetic fibre distribution in human lower limb nerves that will affect the multi-unit recordings. Therefore, the purpose of this study was to examine the sympathetic fibre distribution within the human common fibular nerve using immunohistochemical techniques (tyrosine hydroxylase, avidin–biotin complex technique).

Synchronization hubs may arise from strong rhythmic inhibition during gamma oscillations in primary visual cortex

Parallel multiunit recordings from V1 in anesthetized cat were collected during the presentation of random sequences of drifting sinusoidal gratings at 12 fixed orientations while gamma oscillations were present. In agreement with the seminal work [1], most units were orientation selective to varying degrees and synchronization was evident in spike train crosscorrelograms computed between units with similar preferred orientations, particularly during the presentation of optimal stimuli. Interestingly, a subset of units, which we refer to as synchronization hubs, were additionally found to synchronize with units having differing preferred orientations which was consistent with a previous study [2]. Moreover, oscillatory patterning in spike train autocorrelograms was also found to be strongest in units denoted as synchronization hubs, and synchronization hubs also tended to have narrower tuning curves relative to other units. We used simplified computational models of small networks of V1 neurons to demonstrate that neurons subject to a sufficiently strong level of inhibitory input can function as synchronization hubs. Neurons were endowed either with integrate-and-fire or conductance-based dynamics and each neuron received a combination of excitatory (AMPA) synaptic inputs that were Poisson-distributed and inhibitory (GABA) inputs that were coherent at a gamma-frequency range. If the strength of rhythmic inhibition was increased for a subset of neurons in the network, and excitation was increased simultaneously to maintain a fixed firing rate, then these neurons produced stronger oscillatory patterning in their discharge probabilities. The oscillations in turn synchronized these neurons with other neurons in the network. Importantly, the strength of synchronization increased with neurons of differing orientation preferences even though no direct synaptic coupling existed between the hubs and the other neurons. Enhanced levels of inhibition account for the emergence of synchronization hubs in the following way: Inhibitory inputs exhibiting a gamma rhythm determine a time window within which a cell is likely to discharge. Increased levels of inhibition narrow down this window further simultaneously leading to (i) even stronger oscillatory patterning of the neuron's activity and (ii) enhanced synchronization with other neurons. This enables synchronization even between cells with differing orientation preferences. Additionally, the same increased levels of inhibition may be responsible for the narrow tuning curves of hub neurons. In conclusion, synchronization hubs may be the cells that interact most strongly with the network of inhibitory interneurons during gamma oscillations in primary visual cortex.

Construction of a brain–machine hybrid system to evaluate adaptability of an insect

Insects perform adaptive behavior according to changing environmental conditions using comparatively small brains. Because adaptability is generated through the relationship among brain, body and environment, it is necessary to examine how a brain works under these conditions. In this study, to understand neural processing involved in adaptive behavior, we constructed a brain–machine hybrid system using motor signals related to the steering behavior of the male silkworm moth for controlling a two-wheeled mobile robot. We developed this hybrid system according to the following steps. (1) We selected steering signals corresponding to walking direction that were activated during neck swinging induced by optic flow and pheromone stimuli. (2) To control a robot by neural activity, we implemented a spike-behavior conversion rule such that frequency of the left and right neck motor neurons’ spikes was linearly converted into rotation of the wheels. (3) For electrophysiological multi-unit recordings on a robot, we developed small amplifiers. Using this hybrid system, we could observe the programmed behavioral pattern and orientation toward a pheromone source. Moreover, we compared the orientation behavior of moths and that of the hybrid system at different pheromone stimulus frequencies. From these experiments, we concluded that we could reconstruct silkworm moth behavior on the hybrid system.

The sympathetic nervous system through the ages: from Thomas Willis to resistant hypertension

The 17th century London neuroanatomical school headed by Thomas Willis provided us with the first identifiable images of the sympathetic nervous system. Nineteenth century giants of European physiology (Bernard, Waller and Brown-Sequard) identified these as the ‘pressor nerves’. Von Euler's demonstration that the sympathetic transmitter was noradrenaline brought the field into the modern era. The development of ganglion-blocking drugs by Paton, whose name this review commemorates, allowed comprehensive pharmacological antagonism of this system in patients. With the development of contemporary techniques for recording from human sympathetic nerves and quantifying rates of noradrenaline release, the sympathetic nervous system became accessible to clinical scientists investigating possible contributions to cardiovascular and other diseases. Sympathetic nervous system responses typically are regionally differentiated, with activation in one outflow sometimes accompanying no change or sympathetic inhibition in another. Regional sympathetic activity is best studied in humans by recording from postganglionic sympathetic efferents (multi-unit or single-fibre recording) and by isotope dilution-derived measurement of organ-specific noradrenaline release to plasma from sympathetic nerves (regional ‘noradrenaline spillover’). With the application of these techniques, evidence has been assembled in the past three decades which indicates that sympathetic nervous system activation is crucial in the development of cardiovascular disorders, most notably heart failure and essential hypertension. An important goal for clinical scientists is translation of knowledge of pathophysiology, such as this, into better treatment for patients. The achievement of this ‘mechanisms to management’ transition is mature in cardiac failure, with knowledge of cardiac neural pathophysiology having led to introduction of β-adrenergic blockers, an effective therapy. Perhaps we are now on the cusp of effective translation in patients with essential hypertension, with recent successful testing of selective catheter-based renal sympathetic nerve ablation in patients with resistant hypertension, an intervention firmly based on prior demonstration in them of activation of the renal sympathetic outflow.

Learning alters theta amplitude, theta-gamma coupling and neuronal synchronization in inferotemporal cortex

BackgroundHow oscillatory brain rhythms alone, or in combination, influence cortical information processing to support learning has yet to be fully established. Local field potential and multi-unit neuronal activity recordings were made from 64-electrode arrays in the inferotemporal cortex of conscious sheep during and after visual discrimination learning of face or object pairs. A neural network model has been developed to simulate and aid functional interpretation of learning-evoked changes.ResultsFollowing learning the amplitude of theta (4-8 Hz), but not gamma (30-70 Hz) oscillations was increased, as was the ratio of theta to gamma. Over 75% of electrodes showed significant coupling between theta phase and gamma amplitude (theta-nested gamma). The strength of this coupling was also increased following learning and this was not simply a consequence of increased theta amplitude. Actual discrimination performance was significantly correlated with theta and theta-gamma coupling changes. Neuronal activity was phase-locked with theta but learning had no effect on firing rates or the magnitude or latencies of visual evoked potentials during stimuli. The neural network model developed showed that a combination of fast and slow inhibitory interneurons could generate theta-nested gamma. By increasing N-methyl-D-aspartate receptor sensitivity in the model similar changes were produced as in inferotemporal cortex after learning. The model showed that these changes could potentiate the firing of downstream neurons by a temporal desynchronization of excitatory neuron output without increasing the firing frequencies of the latter. This desynchronization effect was confirmed in IT neuronal activity following learning and its magnitude was correlated with discrimination performance.ConclusionsFace discrimination learning produces significant increases in both theta amplitude and the strength of theta-gamma coupling in the inferotemporal cortex which are correlated with behavioral performance. A network model which can reproduce these changes suggests that a key function of such learning-evoked alterations in theta and theta-nested gamma activity may be increased temporal desynchronization in neuronal firing leading to optimal timing of inputs to downstream neural networks potentiating their responses. In this way learning can produce potentiation in neural networks simply through altering the temporal pattern of their inputs.

P9.9 Towards noninvasive multi-unit spike recordings: mapping 1 kHz EEG signals over human somatosensory cortex

P9.9 Towards noninvasive multi-unit spike recordings: mapping 1 kHz EEG signals over human somatosensory cortex

A mutual information analysis of neural coding of speech by low-frequency MEG phase information

Recent work has implicated low-frequency (<20 Hz) neuronal phase information as important for both auditory (<10 Hz) and speech [theta (∼4-8 Hz)] perception. Activity on the timescale of theta corresponds linguistically to the average length of a syllable, suggesting that information within this range has consequences for segmentation of meaningful units of speech. Longer timescales that correspond to lower frequencies [delta (1-3 Hz)] also reflect important linguistic features-prosodic/suprasegmental-but it is unknown whether the patterns of activity in this range are similar to theta. We investigate low-frequency activity with magnetoencephalography (MEG) and mutual information (MI), an analysis that has not yet been applied to noninvasive electrophysiological recordings. We find that during speech perception each frequency subband examined [delta (1-3 Hz), theta(low) (3-5 Hz), theta(high) (5-7 Hz)] processes independent information from the speech stream. This contrasts with hypotheses that either delta and theta reflect their corresponding linguistic levels of analysis or each band is part of a single holistic onset response that tracks global acoustic transitions in the speech stream. Single-trial template-based classifier results further validate this finding: information from each subband can be used to classify individual sentences, and classifier results that utilize the combination of frequency bands provide better results than single bands alone. Our results suggest that during speech perception low-frequency phase of the MEG signal corresponds to neither abstract linguistic units nor holistic evoked potentials but rather tracks different aspects of the input signal. This study also validates a new method of analysis for noninvasive electrophysiological recordings that can be used to formally characterize information content of neural responses and interactions between these responses. Furthermore, it bridges results from different levels of neurophysiological study: small-scale multiunit recordings and local field potentials and macroscopic magneto/electrophysiological noninvasive recordings.

The sympathetic nervous system through the ages: from Thomas Willis to resistant hypertension

The 17th century London neuroanatomical school headed by Thomas Willis provided us with the first identifiable images of the sympathetic nervous system. Nineteenth century giants of European physiology (Bernard, Waller and Brown-Sequard) identified these as the 'pressor nerves'. Von Euler's demonstration that the sympathetic transmitter was noradrenaline brought the field into the modern era. The development of ganglion-blocking drugs by Paton, whose name this review commemorates, allowed comprehensive pharmacological antagonism of this system in patients. With the development of contemporary techniques for recording from human sympathetic nerves and quantifying rates of noradrenaline release, the sympathetic nervous system became accessible to clinical scientists investigating possible contributions to cardiovascular and other diseases. Sympathetic nervous system responses typically are regionally differentiated, with activation in one outflow sometimes accompanying no change or sympathetic inhibition in another. Regional sympathetic activity is best studied in humans by recording from postganglionic sympathetic efferents (multi-unit or single-fibre recording) and by isotope dilution-derived measurement of organ-specific noradrenaline release to plasma from sympathetic nerves (regional 'noradrenaline spillover'). With the application of these techniques, evidence has been assembled in the past three decades which indicates that sympathetic nervous system activation is crucial in the development of cardiovascular disorders, most notably heart failure and essential hypertension. An important goal for clinical scientists is translation of knowledge of pathophysiology, such as this, into better treatment for patients. The achievement of this 'mechanisms to management' transition is mature in cardiac failure, with knowledge of cardiac neural pathophysiology having led to introduction of β-adrenergic blockers, an effective therapy. Perhaps we are now on the cusp of effective translation in patients with essential hypertension, with recent successful testing of selective catheter-based renal sympathetic nerve ablation in patients with resistant hypertension, an intervention firmly based on prior demonstration in them of activation of the renal sympathetic outflow.

Olfactory sensitivity to amino acids in the blackspot sea bream (Pagellus bogaraveo): a comparison between olfactory receptor recording techniques in seawater

The current study investigated the olfactory sensitivity of the blackspot sea bream to amino acids, odorants associated with food detection in fish, and compared the efficacy of two different experimental methods: multi-unit recording from the olfactory nerve and the electro-olfactogram (EOG). Twenty essential amino acids plus L-DOPA evoked clear, concentration-dependent olfactory responses using both methods, with estimated thresholds of 10(-8.5)-10(-6.2) M (nerve recording) and 10(-7.5)-10(-4.8) M (EOG). The most potent amino acids were L-cysteine, L-methionine (both sulphur-containing), L-alanine, L-leucine (both neutral), L-glutamine (amide-containing) and L-serine (hydroxyl-containing). The least potent were L-proline (secondary α-amino group), the aromatic amino acids and glycine (simplest). Although the rank order of olfactory potency was similar for the two methods used, and the calculated thresholds given by the two methods were positively correlated, the sensitivity of the EOG was consistently lower than multi-unit recording by approximately one order of magnitude, presumably due to the electrical shunting effect of seawater. As in freshwater, the EOG could be a valid method for comparing olfactory potency of different odorants in stenohaline marine fish; however, for absolute 'biological' thresholds, a more invasive recording technique, such as multi-unit recording from the olfactory nerve, should be used.

Hypoxic fetal programming of the sympathetic nervous system

The link between intra uterine growth restriction (IUGR) and increased risk of adult cardiovascular disease in the offspring is well established. Chronic hypoxia in utero (CHU) is a well established cause of IUGR, and using in vitro methods, has been shown to cause endothelial dysfunction and sympathetic hyperinnervation in the offspring. Our aim is to assess the impact of CHU upon sympathetic nerve activity. We have made direct recordings of MSNA from arteries supplying the Spinotrapezius muscle under Alfaxan anaesthesia in Normal (N, n=7) and CHU rats (n=6), discriminating single unit activity from multiunit recordings. Arterial blood pressure, measured from the femoral artery, was similar in N and CHU rats (N:123±3, CHU: 119±5, mmHg). All single units (N=17, CHU=26) had cardiac and respiratory rhythmicity. Mean firing rates were similar (N: 0.48±0.09, CHU: 0.52±0.1, Hz). Further analysis showed that high frequency doublets and triplets of activity (>20hz) were more common in CHU. This may suggest increased release of Neuropeptide Y from sympathetic varicosities, and hence, impact upon cardiovascular health. Recordings during baro- and chemoreceptor stimulation are under analysis.

Cardiac‐related and other rhythmic activity of neurons in the rostral ventrolateral medulla (RVLM) of conscious cats: effects of vestibular lesions

Recordings were made from neurons in the RVLM in conscious cats before and after removal of vestibular input through a bilateral vestibular neurectomy. Blood flow through the carotid artery was also measured so we could determine whether a unit had cardiac-related activity (CRA). Only a small fraction of RVLM neurons exhibited CRA, but the fraction increased from 4% to 9% following vestibular lesions. CRA neurons were often clustered: multiunit recordings revealed that when a CRA unit was present, another was in the same field approximately 50% of the time. In 32% of the instances where pairs of RVLM neurons with CRA were examined at the same location, short time scale interactions between their discharges were demonstrated. Frequency-domain analysis revealed that a small fraction of RVLM neurons with CRA displayed higher-frequency components (10–30 Hz) as well, most often after vestibular lesion. These data suggest that rhythmic firing of RVLM neurons including CRA may be ‘state dependent,’ and is masked until the animal is alerted or agitated.