Neural substrates of two different rhythmical vibrissal movements in the rat

Vibrissa movement elicited by rhythmic electrical microstimulation to motor cortex in the aroused rat mimics exploratory whisking

The rhythmic motor activity of the vibrissae that rodents use for the tactile localization of objects provides a model system for understanding patterned motor activity in mammals. Evidence suggests that neural circuitry in the brain stem provides rhythmic drive to the vibrissae. Yet multiple brain structures at higher levels of organization, including vibrissa primary motor cortex (M1), have direct projections to brain stem nuclei that are implicated in whisking. We thus asked whether output from M1 can control vibrissa movement on the approximately 10-Hz scale of the natural rhythmic movement of the vibrissae. Our assay of cortical control made use of periodic intracortical microstimulation (ICMS) to excite a region of vibrissa M1 cortex in awake, behaving animals and measurements of the stimulus-locked electromyogram (EMG) in both the intrinsic and extrinsic muscles that drive the vibrissae. We observed that ICMS evoked a prompt activation of the extrinsic muscles and a delayed and prolonged response in the intrinsic muscles. The relative timing and shape of these waveforms approximates the EMG waveforms seen during natural exploratory whisking. We further observed prompt activation of the intrinsic muscles, an occurrence not seen during exploratory whisking. Despite the latter difference in muscular activation, the motion of the vibrissae evoked by periodic ICMS strongly resembled the motion during natural, exploratory whisking. Interestingly, the extent of the movement was proportional to the level of arousal, as quantified by the amplitude of hippocampal activity in the theta frequency band. We interpret these data as demonstrating that M1 cortex can, in principle, initiate the full pattern of whisking on a cycle-by-cycle basis in aroused animals. Beyond issues of natural motor control, our result may bear on the design of algorithms for neuroprosthetic control of motor output.

Single-cell study of motor cortex projections to the barrel field in rats

In freely moving rats, whisking is associated with a slow modulation of neuronal excitability in the primary somatosensory cortex. Because it persists after the blockade of vibrissa input, it was suggested that the slow modulation might be mediated by motor-sensory corticocortical connections and perhaps result from the corollary discharges of corticofugal cells. In the present study, we identified motor cortical cells that project to the barrel field and reconstructed their axonal projections after juxtacellularly staining single cells with a biotinylated tracer. On the basis of the final destination of main axons, two groups of neurons contribute to motor-sensory projections: callosal cells (87.5%) and corticofugal cells (12.5%). Axon collaterals of callosal cells arborize in layers five to six of the granular and dysgranular zones and give off several branches that ascend between the barrels to ramify in the molecular layer. In contrast, the axon collaterals of corticofugal cells do not ramify in the infragranular layers but in layer 1. The origin of the majority of motor sensory projections from callosally projecting cells does not support the notion that the slow modulation results from the corollary discharges of corticofugal axons. It would rather originate from a separate population of cells, which could output the slow signal to the barrel field in parallel with the corticofugal commands to a brainstem pattern generator. As free whisking is characterized by bilateral concerted movements of the vibrissae, the transcallosal contribution of motor-sensory axons represents a substrate for synchronizing the slow modulation across both hemispheres.

Serotonin regulates rhythmic whisking

Many rodents explore their environment by rhythmically palpating objects with their mystacial whiskers. These rhythmic whisker movements ("whisking"; 5-9 Hz) are thought to be regulated by an unknown brainstem central pattern generator (CPG). We tested the hypothesis that serotonin (5-HT) inputs to whisking facial motoneurons (wFMNs) are part of this CPG. In response to exogenous serotonin, wFMNs recorded in vitro fire rhythmically at whisking frequencies, and selective 5-HT2 or 5-HT3 receptor antagonists suppress this rhythmic firing. In vivo, stimulation of brainstem serotonergic raphe nuclei evokes whisker movements. Unilateral infusion of selective 5-HT2 or 5-HT3 receptor antagonists suppresses ipsilateral whisking and substantially alters the frequencies and symmetry of whisker movements. These findings suggest that serotonin is both necessary and sufficient to generate rhythmic whisker movements and that serotonergic premotoneurons are part of a whisking CPG.

Rhythmic whisking by rat: retraction as well as protraction of the vibrissae is under active muscular control

The rhythmic motor activity of the vibrissae that rodents use for the tactile localization of objects provides a model system for understanding patterned motor activity in mammals. The muscles that drive this whisking are only partially fixed relative to bony attachments and thus shift their position along with the movement. As a means to characterize the pattern of muscular dynamics during different patterns of whisking, we recorded electromyogram (EMG) activity from the muscles that propel individual follicles, as well as EMG activity from a muscle group that moves the mystacial pad. The dominant pattern of whisking in our behavioral paradigm, referred to as exploratory whisking, consisted of large amplitude sweeps in the frequency range of 5-15 Hz. The frequency remained remarkably constant within a bout of whisking but changed values between bouts. The extrinsic musculature, which shifts the surface of the pad backwards, was found to be activated in approximate antiphase to that of the intrinsic muscles, which rotate individual vibrissae forward. Thus retraction of the vibrissae was driven by a backward shift in the attachment point of the follicles to the mystacial pad. In a less frequent pattern of whisking, referred to as foveal whisking, the vibrissae are thrust forward and palpate objects with low-amplitude movements that are in the higher frequency range of 15-25 Hz. Protraction of the vibrissae remains driven by the intrinsic muscles, while retraction in this pattern is largely passive. Interestingly, a mechanical argument suggests that activation of the extrinsic muscles during foveal whisking is not expected to affect the angle of the vibrissae. As a means to establish if the phasic control of the intrinsic versus extrinsic muscles depended on sensory feedback, we characterized whisking before and after bilateral transections of the infraorbital branch of the trigeminal sensory nerve. The loss of sensory feedback had no net effect on the antiphase relation between activation of the intrinsic versus extrinsic muscles over the full frequency range for exploratory whisking. These data point to the existence of a dual-phase central pattern generator that drives the vibrissae.

Dynamic shifting in thalamocortical processing during different behavioural states

Recent experiments in our laboratory have indicated that as rats shift the behavioural strategy employed to explore their surrounding environment, there is a parallel change in the physiological properties of the neuronal ensembles that define the main thalamocortical loop of the trigeminal somatosensory system. Based on experimental evidence from several laboratories, we propose that this concurrent shift in behavioural strategy and thalamocortical physiological properties provides rats with an efficient way to optimize either the detection or analysis of complex tactile stimuli.

Oscillatory local field potentials recorded from the subthalamic nucleus of the alert rat

Hitherto, high-frequency local field potential oscillations in the upper gamma frequency band (40-80 Hz) have been recorded only from the region of subthalamic nucleus (STN) in parkinsonian patients treated with levodopa. Here we show that local field potentials recorded from the STN in the healthy alert rat also have a spectral peak in the upper gamma band (mean 53 Hz, range 46-70 Hz). The power of this high-frequency oscillatory activity was increased by 30 +/- 4% (+/-SEM) during motor activity compared to periods of alert immobility. It was also increased by 86 +/- 36% by systemic injection of the D2 dopamine receptor agonist quinpirole. The similarities between the high-frequency activities in the STN of the healthy rat and in the levodopa-treated parkinsonian human argue that this oscillatory activity may be physiological in nature and not a consequence of the parkinsonian state.

Thalamocortical [correction of Thalamcortical] optimization of tactile processing according to behavioral state

We propose a conceptual model that describes the operation of the main thalamocortical loop of the rat somatosensory system. According to this model, the asynchronous convergence of ascending and descending projections dynamically alters the physiological properties of thalamic neurons in the ventral posterior medial (VPM) nucleus as rats shift between three behavioral states. Two of these states are characterized by distinct modes of rhythmic whisker movements. We posit that these simultaneous shifts in exploratory behavioral strategy and in the physiological properties of VPM neurons allow rats to either (i) optimize the detection of stimuli that are novel or difficult to sense or (ii) process complex patterns of multi-whisker stimulation.

A proposed function for hippocampal theta rhythm: separate phases of encoding and retrieval enhance reversal of prior learning

The theta rhythm appears in the rat hippocampal electroencephalogram during exploration and shows phase locking to stimulus acquisition. Lesions that block theta rhythm impair performance in tasks requiring reversal of prior learning, including reversal in a T-maze, where associations between one arm location and food reward need to be extinguished in favor of associations between the opposite arm location and food reward. Here, a hippocampal model shows how theta rhythm could be important for reversal in this task by providing separate functional phases during each 100-300 msec cycle, consistent with physiological data. In the model, effective encoding of new associations occurs in the phase when synaptic input from entorhinal cortex is strong and long-term potentiation (LTP) of excitatory connections arising from hippocampal region CA3 is strong, but synaptic currents arising from region CA3 input are weak (to prevent interference from prior learned associations). Retrieval of old associations occurs in the phase when entorhinal input is weak and synaptic input from region CA3 is strong, but when depotentiation occurs at synapses from CA3 (to allow extinction of prior learned associations that do not match current input). These phasic changes require that LTP at synapses arising from region CA3 should be strongest at the phase when synaptic transmission at these synapses is weakest. Consistent with these requirements, our recent data show that synaptic transmission in stratum radiatum is weakest at the positive peak of local theta, which is when previous data show that induction of LTP is strongest in this layer.

Coherent electrical activity between vibrissa sensory areas of cerebellum and neocortex is enhanced during free whisking

We tested if coherent signaling between the sensory vibrissa areas of cerebellum and neocortex in rats was enhanced as they whisked in air. Whisking was accompanied by 5- to 15-Hz oscillations in the mystatial electromyogram, a measure of vibrissa position, and by 5- to 20-Hz oscillations in the differentially recorded local field potential (nablaLFP) within the vibrissa area of cerebellum and within the nablaLFP of primary sensory cortex. We observed that only 10% of the activity in either cerebellum or sensory neocortex was significantly phase-locked to rhythmic motion of the vibrissae; the extent of this modulation is in agreement with the results from previous single-unit measurements in sensory neocortex. In addition, we found that 40% of the activity in the vibrissa areas of cerebellum and neocortex was significantly coherent during periods of whisking. The relatively high level of coherence between these two brain areas, in comparison with their relatively low coherence with whisking per se, implies that the vibrissa areas of cerebellum and neocortex communicate in a manner that is incommensurate with whisking. To the extent that the vibrissa areas of cerebellum and neocortex communicate over the same frequency band as that used by whisking, these areas must multiplex electrical activity that is internal to the brain with activity that is that phase-locked to vibrissa sensory input.

Divergent movement of adjacent whiskers

The current view of whisker movement is that approximately 25 whiskers on each side of the face move in synchrony. To determine whether whiskers are constrained to move together, we trained rats to use two whiskers on the same side of the face in simple behavioral tasks and videotaped the whiskers during the task. Here we report that the movement of adjacent whiskers is usually synchronous but can diverge: 1) the distance between whiskers can vary dramatically during movement; 2) one whisker can move while the second one remains stationary; 3) two whiskers can simultaneously move in opposite directions; and 4) one whisker can be maintained in contact with an object while the other is retracted and protracted. The frequency of whisker movement during the task falls within the previously reported range for rats whisking freely into air or performing roughness discrimination with their whiskers. Our data also suggest that whisker movement can be divided into three distinct phases: protraction, retraction, and a measurable delay between these movements. We conclude that, although whiskers often move in concert, adjacent caudal whiskers can be moved independently of each other.

Functional circuitry involved in the regulation of whisker movements

Neuroanatomical tract-tracing methods were used to identify the oligosynaptic circuitry by which the whisker representation of the motor cortex (wMCx) influences the facial motoneurons that control whisking activity (wFMNs). Injections of the retrograde tracer cholera toxin subunit B into physiologically identified wFMNs in the lateral facial nucleus resulted in dense, bilateral labeling throughout the brainstem reticular formation and in the ambiguus nucleus as well as predominantly ipsilateral labeling in the paralemniscal, pedunculopontine tegmental, Kölliker-Fuse, and parabrachial nuclei. In addition, neurons in the following midbrain regions projected to the wFMNs: superior colliculus, red nucleus, periaqueductal gray, mesencephalon, pons, and several nuclei involved in oculomotor behaviors. Injections of the anterograde tracer biotinylated dextran amine into the wMCx revealed direct projections to the brainstem reticular formation as well as multiple brainstem and midbrain structures shown to project to the wFMNs. Regions in which retrograde labeling and anterograde labeling overlap most extensively include the brainstem parvocellular, gigantocellular, intermediate, and medullary (dorsal and ventral) reticular formations; ambiguus nucleus; and midbrain superior colliculus and deep mesencephalic nucleus. Other regions that contain less dense regions of combined anterograde and retrograde labeling include the following nuclei: the interstitial nucleus of medial longitudinal fasciculus, the pontine reticular formation, and the lateral periaqueductal gray. Premotoneurons that receive dense inputs from the wMCx are likely to be important mediators of cortical regulation of whisker movements and may be a key component in a central pattern generator involved in the generation of rhythmic whisking activity.

Thalamic bursting in rats during different awake behavioral states

Thalamic neurons have two firing modes: tonic and bursting. It was originally suggested that bursting occurs only during states such as slow-wave sleep, when little or no information is relayed by the thalamus. However, bursting occurs during wakefulness in the visual and somatosensory thalamus, and could theoretically influence sensory processing. Here we used chronically implanted electrodes to record from the ventroposterior medial thalamic nucleus (VPM) and primary somatosensory cortex (SI) of awake, freely moving rats during different behaviors. These behaviors included quiet immobility, exploratory whisking (large-amplitude whisker movements), and whisker twitching (small-amplitude, 7- to 12-Hz whisker movements). We demonstrated that thalamic bursting appeared during the oscillatory activity occurring before whisker twitching movements, and continued throughout the whisker twitching. Further, thalamic bursting occurred during whisker twitching substantially more often than during the other behaviors, and a neuron was most likely to respond to a stimulus if a burst occurred approximately 120 ms before the stimulation. In addition, the amount of cortical area activated was similar to that during whisking. However, when SI was inactivated by muscimol infusion, whisker twitching was never observed. Finally, we used a statistical technique called partial directed coherence to identify the direction of influence of neural activity between VPM and SI, and observed that there was more directional coherence from SI to VPM during whisker twitching than during the other behaviors. Based on these findings, we propose that during whisker twitching, a descending signal from SI triggers thalamic bursting that primes the thalamocortical loop for enhanced signal detection during the whisker twitching behavior.

An outline of brain function

An outline of how the brain may compute is proposed. In the cerebral cortex memories are stored through long-term potentiation at synapses from layer 1 cortical inputs (representing contexts) on layer 2/3 pyramidal cells linked with the thalamus in a cortico-thalamic (CT) unit. The signals which are memorized are the layer 3 inputs from the thalamus or other cortical areas. Signals are memorized (and later recalled) at the gamma frequency. A conscious thought comprises the outputs of layer 5 cells in CT units in different cortical regions firing in synchrony through the contribution of oscillatory thalamic and cortical inputs. This cortical output influences sub-cortical areas to cause or participate in a movement. Cerebral cortical outputs may be stored in the cerebellum and generated later in a particular context by the basal ganglia and cerebellum. Thus the brain may either generate 'conscious' outputs using the cerebral cortex or 'automatic' outputs using the basal ganglia and cerebellum. When contexts are recognized by the basal ganglia it permits outputs stored in the cerebellum to commence and in this way the basal ganglia can control complex sequences of outputs or movements. Working memory involves the prefrontal cortex using similarly the basal ganglia and cerebellum. The hippocampus has a role in the storage and recall of cortical outputs by providing unique layer 1 contexts to all the CT loops in different cortical areas in a conscious thought. With further recall of the thought new layer 1 contexts may become associated with the CT loops enabling recall without the hippocampal input.

Whisker deafferentation and rodent whisking patterns: behavioral evidence for a central pattern generator

Even in the absence of explicit stimulation, rats emit patterns of rhythmic whisking movements. Because of their stereotyped nature and their persistence after sensory denervation and cortical ablation, whisking movements have been assumed to reflect the output of a central pattern generator (CPG). However, identification of a movement pattern as the product of a CPG requires evidence that its generation, patterning, and coordination are independent of sensory input. To provide such evidence, we used optoelectronic instrumentation to obtain high-resolution records of the movement trajectories of individual whiskers in rats whose heads were fixed to isolate their exploratory whisking from exafferent inputs. Unconditioned whisking patterns were quantitatively characterized by a biometric analysis of the kinematics, rhythmicity, and coordination of bilaterally homologous vibrissa movements. Unilateral and bilateral sectioning of the infraorbital nerve, which innervates the whiskers, was then performed to block reafferent inputs generated by the animal's own whisking movements. Unilateral sectioning of the nerve has no effect on whisking kinematics but is followed by a significant but relatively transient bilateral increase in whisking frequency. However, bilateral deafferentation, when performed in a single-stage procedure, does not disrupt the generation, patterning, or bilateral coordination of whisking patterns in the rat. These findings provide strong behavioral evidence for a whisking CPG and are discussed in relation to its possible location and properties.

Corticothalamic projections from layer 5 of the vibrissal barrel cortex in the rat

This study bears on the projections of layer 5 cells of the vibrissal sensory cortex to the somatosensory thalamus in rats. Small groups of cells were labeled with biotinylated dextran amine (BDA), and their axonal arborizations were individually reconstructed from horizontal sections counterstained for cytochrome oxidase. Results show that the vast majority ( approximately 95%) of layer 5 axons that innervate the somatosensory thalamus are collaterals of corticofugal fibers that project to the brainstem. The anterior pretectal nucleus, the deep layers of the superior colliculus, and the pontine nuclei are among the structures most often coinnervated. In the thalamus, layer 5 axons terminate exclusively in the dorsal part of the posterior group (Po), where they form clusters of large terminations. Because dorsal Po projects to multiple cortical areas, we sought to determine whether all recipient areas return a layer 5 projection to this part of the thalamus. Additional experiments using fluoro-gold and BDA injections provided evidence that the primary somatosensory area is the sole source of layer 5 projections to dorsal Po but that this thalamic region receives convergent layer 6 projections from the primary and second somatosensory areas and from the motor and insular cortices. These results show that layer 5 projections do not overlap in associative thalamic nuclei, thus defining area-related subdivisions. Furthermore, the coinnervation of brainstem nuclei by layer 5 CT axons suggests that this pathway conveys to the thalamus a copy of the cortical output aimed at brainstem structures.

Behavioral modulation of tactile responses in the rat somatosensory system

We investigated the influence of four different behavioral states on tactile responses recorded simultaneously via arrays of microwires chronically implanted in the vibrissal representations of the rat ventral posterior medial nucleus (VPM) of the thalamus and the primary somatosensory cortex (SI). Brief (100 microsecond) electrical stimuli delivered via a cuff electrode to the infraorbital nerve yielded robust sensory responses in VPM and SI during states of quiet immobility. However, significant reductions in tactile response magnitude and latency were observed in VPM and SI during large-amplitude, exploratory movements of the whiskers (at approximately 4-6 Hz). During small-amplitude, 7-12 Hz whisker-twitching movements, a significant reduction in SI response magnitude and an increase in VPM and SI response latencies were observed as well. When pairs of stimuli with interstimulus intervals <100 msec were delivered during quiet immobility, the response to the second stimulus in the pair was reduced and occurred at a longer latency compared with the response to the first stimulus. In contrast, during large-amplitude whisker movements and general motor activity, paired stimuli yielded similar sensory responses at interstimulus intervals >25 msec. These response patterns were correlated with the amount and duration of postexcitatory firing suppression observed in VPM and SI during each of these behaviors. On the basis of these results, we propose that sensory responses are dynamically modulated during active tactile exploration to optimize detection of different types of stimuli. During quiet immobility, the somatosensory system seems to be optimally tuned to detect the presence of single stimuli. In contrast, during whisker movements and other exploratory behaviors, the system is primed to detect the occurrence of rapid sequences of tactile stimuli, which are likely to be generated by multiple whisker contacts with objects during exploratory activity.

Oscillatory activity in the cerebellar hemispheres of unrestrained rats

We recorded multiunit neural activity in the granule cell layer of cerebellar folium Crus IIa in unrestrained rats. Seven- to 8-Hz oscillatory activity was seen during behavioral states in which the animal was immobile; any movement the animal made coincided with termination of the oscillations. However, nearly one-third of oscillatory episodes appeared to cease spontaneously, in the absence of any observable sensory input or movement. Oscillations were synchronized both within and between cerebellar hemispheres, demonstrating precise temporal coordination among multiple, bilateral levels of the somatosensory system. We interpret these data in the context of similar oscillations observed in other brain structures and suggest that the oscillations are an underlying dynamic property of the entire somatosensory network.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

Central versus peripheral determinants of patterned spike activity in rat vibrissa cortex during whisking

We report on the relationship between single-unit activity in primary somatosensory vibrissa cortex of rat and the rhythmic movement of vibrissae. Animals were trained to whisk freely in air in search of food. Electromyographic (EMG) recordings from the mystatial pads served as a reference for the position of the vibrissae. A fast, oscillatory component in single-unit spike trains is correlated with vibrissa position within the whisk cycle. The phase of the correlation for different units is broadly distributed. A second, slowly varying component of spike activity correlates with the amplitude of the whisk cycle. For some units, the phase and amplitude correlations were of sufficient strength to allow the position of the whiskers to be accurately predicted from a single spike train. To determine whether the observed patterned spike activity was driven by motion of the vibrissae, as opposed to central pathways, we reversibly blocked the contralateral facial motor nerve during the behavioral task so that the rat whisked only on the ipsilateral side. The ipsilateral EMG served as a reliable reference signal. The fast, oscillatory component of the spike-EMG correlation disappears when the facial motor nerve is blocked. This implies that the position of vibrissae within a cycle is encoded through direct sensory activation. The slowly varying component of the spike-EMG correlation is unaffected by the block. This implies that the amplitude of whisking is likely to be mediated by corollary discharge. Our results suggest that motor cortex does not relay a reference signal to sensory cortex for positional information of the vibrissae during whisking.

Short-term plasticity in thalamocortical pathways: cellular mechanisms and functional roles

Information reaches the neocortex through different types of thalamocortical pathways. These differ in many morphological and physiological properties. One interesting aspect in which thalamocortical pathways differ is in their temporal dynamics, such as their short-term plasticity. Primary pathways display frequency-dependent depression, while secondary pathways display frequency-dependent enhancement. The cellular mechanisms underlying these dynamic responses involve pre- and post-synaptic and circuit properties. They may serve to synchronize, amplify and/or filter neural activity in neocortex depending on behavioral demands, and thus to adapt each pathway to its specific function.

Short-term plasticity of a thalamocortical pathway dynamically modulated by behavioral state

The neocortex receives information about the environment and the rest of the brain through pathways from the thalamus. These pathways have frequency-dependent properties that can strongly influence their effect on the neocortex. In 1943 Morison and Dempsey described "augmenting responses," a form of short-term plasticity in some thalamocortical pathways that is triggered by 8- to 15-hertz activation. Results from anesthetized rats showed that the augmenting response is initiated by pyramidal cells in layer V. The augmenting response was also observed in awake, unrestrained animals and was found to be dynamically modulated by their behavioral state.

The relationship of vibrissal motor cortex unit activity to whisking in the awake rat

Rats actively sweep their whiskers back and forth to locate and palpate objects within their immediate environment. Microstimulation studies in anesthetized rats have demonstrated the presence of a large vibrissal motor representation in agranular cortex. However, the manner in which motor cortex neurons contribute to whisking behavior in the awake animal is unknown. This study represents an initial investigation of the relationship between the activity of task-related neurons in vibrissal motor cortex and the electromyographic (EMG) activity of the deep vibrissal pad muscles in the awake, freely whisking rat. Each animal was gently held in an experimenter's hands while the animal whisked the air. A spring-loaded, metal microelectrode mounted in a removable, miniature microdrive provided stabile recordings of motor cortex unit activity. Fine-wire electrodes implanted in the mystacial pad simultaneously recorded facial muscle activity. Results showed that the discharge of task-related neurons was correlated with changing levels of muscle output. Unit discharge was related in a tonic or phasic-tonic fashion to EMG activity. No units were found to discharge rhythmically in a 1:1 fashion with the periodicity of the whisking pattern. These findings support a role for vibrissal motor cortex in the initiation and modulation of the overall level of mystacial pad muscular output, but not in the generation of bursts of EMG activity responsible for individual whisking sweeps.

The superior colliculus relays signals descending from the vibrissal motor cortex to the facial nerve nucleus in the rat

Electrical stimulation of the vibrissal motor cortex (VMCx), the superior colliculus and the facial nerve nucleus elicits vibrissal movements. A possibility of the superior colliculus as one of the relay nuclei between the VMCx and the facial nerve nucleus was investigated by injecting an anterograde neural tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), into the physiologically identified VMCx and a retrograde tracer, cholera toxin B subunit (CTb), into the facial nerve nucleus in Wistar rats. In the lateral half of the superior colliculus, the termination field of axons from the VMCx overlapped with the distribution field of projection neurons to the facial nerve nucleus. In some cases, PHA-L-labeled terminal swellings were closely apposed to the somas of CTb-labeled neurons. We conclude that signals descending from the VMCx are relayed through the lateral half of the superior colliculus to the facial nerve nucleus.

Sensorimotor encoding by synchronous neural ensemble activity at multiple levels of the somatosensory system

Neural ensemble processing of sensorimotor information during behavior was investigated by simultaneously recording up to 48 single neurons at multiple relays of the rat trigeminal somatosensory system. Cortical, thalamic, and brainstem neurons exhibited widespread 7- to 12-hertz synchronous oscillations, which began during attentive immobility and reliably predicted the imminent onset of rhythmic whisker twitching. Each oscillatory cycle began as a traveling wave of neural activity in the cortex that then spread to the thalamus. Just before the onset of rhythmic whisker twitching, the oscillations spread to the spinal trigeminal brainstem complex. Thereafter, the oscillations at all levels were synchronous with whisker protraction. Neural structures manifesting these rhythms also exhibited distributed spatiotemporal patterns of neuronal ensemble activity in response to tactile stimulation. Thus, multilevel synchronous activity in this system may encode not only sensory information but also the onset and temporal domain of tactile exploratory movements.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

A neurosynaptic model of state-dependent EEG wave generation in the subcortico-cortical system

A neurosynaptic model of the subcortico-cortical system is presented in order to analyze the mechanism for the generation of EEG rhythms with specific state-dependent spectral patterns. The model is based on the interaction among the infraslow, as well as basic, rhythms of the PSP's (postsynaptic potentials) trains from which CSD's (current source densities) or cortical surface potentials emerge. The model system was simulated by two trains of positive and negative cortical surface potentials within the same period, according to the thalamic clock as modulated by the infraslow rhythms of the midbrain reticular system. The simulated EEG's showed rhythmic waxing and waning sawtooth-like waves with no frequency fluctuation, but with some spectral broadband peaks at the basic repetitive frequency, as well as its harmonics.

Genetic models of absence epilepsy, with emphasis on the WAG/Rij strain of rats

In this review, the main characteristics of genetic models of absence epilepsy, in particular with respect to WAG/Rij rats, are presented. Genetic models are important and relevant, since evidence exists that these models mimic spontaneously occurring human epilepsy more than models in which epilepsy is artificially induced. Genetic models can be divided into models in which seizures are elicited and into those in which epilepsy appears without any sensory stimulation. The majority of genetic models show that absence type of epilepsy; during the last few years, we and others have noticed that rats of various strains exhibit spontaneously occurring spike-wave discharges in the EEG. Among the strains highly affected is the WAG/Rij strain, which is a fully inbred strain. Individuals are homozygous and because of this property, genetic studies are meaningful. Electrophysiological studies have indicated that abnormal discharges in the cortical EEG are generalized and that the hippocampus is not involved. Parts of the thalamus, together with the thalamic reticular nucleus, apparently act as a pacemaker for the abnormal discharges. There is a circadian modulation in the number of spike-wave discharges. Discharges mainly occur during intermediate levels of vigilance such as passive wakefulness and light slow-wave sleep and at transitions of sleep states. Pharmacological studies with clinically effective antiepileptic drugs have shown a close agreement in seizure response between man and rat. Studies with new compounds have emphasized the role of the GABAergic and glutamatergic system in this type of epilepsy. Particularly striking is the role of the GABAergic system. GABA agonists enhance and GABA antagonists reduce the occurrence of spike-wave discharges, which deviates from the effects of GABAergic drugs in non-convulsive epilepsy. Even more striking is the role of the benzodiazepines, generally seen as GABA agonists; these drugs do not act as such in absence epilepsy since they reduce spike-wave discharges. Also good evidence for an involvement of other neurotransmitters such as noradrenaline, dopamine and opioid peptides exists in absence epilepsy. Genetic data obtained from the WAG/Rij model for absence epilepsy show a relatively simple pattern of inheritance with one gene determining whether an individual is epileptic or not, and with other genes regulating the number and duration of seizures. This is in good agreement with the more restricted human data. Cognitive studies have shown two important features of epilepsy in the WAG/Rij strain: modulation of the number of spike-wave discharges by mental or physical activity and on the other hand, the disruption of cognitive activity by spike-wave discharges.(ABSTRACT TRUNCATED AT 400 WORDS)

Genetic absence epilepsy in rats from Strasbourg--a review

We have selected a strain of rats and designated it the Genetic Absence Epilepsy Rat from Strasbourg (GAERS). In this strain, 100% of the animals present recurrent generalized non-convulsive seizures characterized by bilateral and synchronous spike-and-wave discharges accompanied with behavioural arrest, staring and sometimes twitching of the vibrissae. Spontaneous SWD (7-11 cps, 300-1,000 microV, 0.5-75 sec) start and end abruptly on a normal background EEG. They usually occur at a mean frequency of 1.5 per min when the animals are in a state of quiet wakefulness. Drugs effective against absence seizures in humans (ethosuccimide, trimethadione, valproate, benzodiazepines) suppress the SWD dose-dependently, whereas drugs specific for convulsive or focal seizures (carbamazepine, phenytoin) are ineffective. SWD are increased by epileptogenic drugs inducing petit mal-like seizures, such as pentylenetetrazol, gamma-hydroxybutyrate, THIP and penicillin. Depth EEG recordings and lesion experiments show that SWD in GAERs depend on cortical and thalamic structures with a possible rhythmic triggering by the lateral thalamus. Most neurotransmitters are involved in the control of SWD (dopamine, noradrenaline, NMDA, acetylcholine), but GABA and gamma-hydroxybutyrate (GHB) seem to play a critical role. SWD are genetically determined with an autosomal dominant inheritance. The variable expression of SWD in offsprings from GAERS x control reciprocal crosses may be due to the existence of multiple genes. Neurophysiological, behavioural, pharmacological and genetic studies demonstrate that spontaneous SWD in GAERS fulfill all the requirements for an experimental model of absence epilepsy. As the mechanisms underlying absence epilepsy in humans are still unknown, the analysis of the genetic thalamocortical dysfunction in GAERS may be fruitful in investigations of the pathogenesis of generalized non-convulsive seizures.

Representation of the body by single neurons in the dorsolateral striatum of the awake, unrestrained rat

Single cell recordings in awake monkeys and cats have demonstrated that individual body parts are represented within striatal subregions receiving projections from somatic sensorimotor cortex. Literature indicating that the lateral striatum of the rat receives similar cortical inputs and subserves sensorimotor functions prompted a study of whether this subregion contains similar representations of the body. Single cell recordings were obtained from 923 neurons of 24 awake, unrestrained rats. Of 788 neurons categorized according to body part, 264 (34%) discharged in relation to active movement, passive manipulation, and/or cutaneous stimulation of a particular part of the body; the remainder were related to global, whole body movement (38%) or were unresponsive (28%). Neurons related to individual body parts were recorded throughout the entire anterior-posterior extent of the dorsolateral striatum (+1.60 to -2.12 mm A-P, from bregma), intermingled among each other in all 3 dimensions. Two topographic arrangements were observed. First, neurons that fired rhythmically, in phase with low frequency (5-6 Hz) whisking of the vibrissae were segregated in the caudal striatum (-0.2 to -2.12 mm A-P) from neurons related to other body parts, which were distributed from +1.6 to -0.8 mm A-P. Second, representations of the head and face were located ventral to those of the limbs, despite substantial overlap in their overall distributions. A prominent feature of individual electrode tracks was the clustering together of cells related to the same body part. Neurons related to body parts exhibited substantial diversity, which took several distinct forms. Some neurons fired during movement or sensory stimulation in any direction, whereas others showed selectivity for a particular direction. Certain neurons responded to sensory stimulation of a large unilateral region of the body (e.g., all vibrissae or the entire forelimb), whereas others responded to stimulation of highly restricted regions (e.g., a single vibrissa or a single forepaw digit). Finally, neurons differed in the extent to which they exhibited active and passive properties. Among vibrissae-related neurons, one group fired rhythmically during whisking but did not respond to sensory stimulation of the vibrissae; a second group responded to sensory stimulation of the vibrissae but did not fire rhythmically during whisking; a third group showed both properties. Among limb-related neurons, firing during active movement was a property of every cell; none showed sensory responsiveness without showing a relation to active movement of one limb. Of the limb-related neurons, 89% tested responded to passive manipulation of the limb to which the neuron was actively related, and 71% also responded to cutaneous stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Mapping of spontaneous spike and wave discharges in Wistar rats with genetic generalized non-convulsive epilepsy

Electrical activity was recorded in different parts of the brain in Wistar rats from a strain with genetic generalized non-convulsive epilepsy (GNCE or absence epilepsy). Movable bipolar electrodes were lowered stereotaxically by 1 mm steps into the brain in immobilized animals. Spontaneous spike and wave discharges (SWD) of the largest amplitude were recorded in the cortex and in lateral nuclei of the thalamus where they appeared occasionally to precede. Smaller amplitude SWD were recorded in the striatum, hypothalamus, tegmentum and substantia nigra. No SWD were recorded in limbic structures. Partial limbic seizures induced by the introduction of the electrode did not interfere with occurrence of cortical SWD. These results confirm the primacy of thalamocortical involvement in SWD of GNCE. The absence of spread to limbic structures and the implication of a precisely limited substrate in GNCE accounts for the clinical and pharmacological specificity of this particular kind of epilepsy.

Petit mal epilepsy and parkinsonian tremor: hypothesis of a common pacemaker

Rhythmic oscillation in neuronal systems may serve physiological purposes or may interfere with normal functions of the brain. In disorders of petit mal epilepsy and parkinsonian tremor, centrally and peripherally observable rhythmic patterns are due to network oscillations of thalamocortical cells. This article reviews the afferent mechanisms that might be critically involved in controlling the ionic conductances of thalamic neurons in the behaving organism. We propose that during active behavior the subcortical aminergic and cholinergic inputs to the thalamus act as anti-burst and anti-oscillation mechanisms. We suggest further that the thalamopetal GABAergic inputs (pars reticulata of substantia nigra, entopeduncular nucleus, pallidum) are burst- and oscillation-promoting systems, whose output is controlled by the striatum. Experimental or disease-related decrease of the striatal dopamine levels is hypothesized to increase the efficacy of the GABAergic burst-promoting systems resulting in rhythmic network oscillation of thalamocortical neurons during rest. The recognition of the overlapping neuronal mechanisms in petit mal epilepsy and parkinsonian tremor, and the multistage control of thalamic oscillation suggests that drugs effectively used in petit mal attacks may be effective in levodopa-refractory parkinsonian tremor, and conversely, epileptic patients may benefit from drugs acting on the extrapyramidal system.

Common fur and mystacial vibrissae parallel sensory pathways: 14 C 2-deoxyglucose and WGA-HRP studies in the rat

Stimulation of mystacial vibrissae in rows A,B, and C increased (14C) 2-deoxyglucose (2DG) uptake in spinal trigeminal nucleus pars caudalis (Sp5c) mostly in ventral portions of laminae III-IV with less activation of II and V. Stimulation of common fur above the whiskers mainly activated lamina II, with less activation in deeper layers. The patterns of activation were compatible with an inverted head, onion skin Sp5c somatotopy. Wheatgerm Agglutinin-Horseradish Peroxidase (WGA-HRP) injections into common fur between mystacial vibrissae rows A-B and B-C led to anterograde transganglionic labeling only of Sp5c, mainly of lamina II with less label in layer V, and very sparse label in III and IV. WGA-HRP skin injections appear to primarily label small fibers, which along with larger fibers, were metabolically activated during common fur stimulation. Mystacial vibrissae stimulation increased 2DG uptake in ventral ipsilateral spinal trigeminal nuclei pars interpolaris (Sp5i) and oralis (Sp5o) and principal trigeminal sensory nucleus (Pr5). Common fur stimulation above the whiskers slightly increased 2DG uptake in ventral Sp5i, Sp5o, and possibly Pr5. The most dorsal aspect of the ventroposteromedial (VPM) nucleus of thalamus was activated contralateral to whisker stimulation. Stimulation of the common fur dorsal to the whiskers activated a region of dorsal VPM caudal to the VPM region activated during whisker stimulation. This is consistent with previous data showing that ventral whiskers and portions of the face are represented rostrally in VPM, and more dorsal whiskers and dorsal portions of the face are represented progressively more caudally in VPM. Mystacial vibrissae stimulation activated the contralateral primary sensory SI barrelfield cortex and a separate region in the second somatosensory SII cortex. Common fur stimulation above the whiskers activated a cortical region between the SI and SII whisker activated regions of cortex. It is proposed that this region represented the combined SI and SII common fur regions of somatosensory neocortex. Both whisker and common fur stimulation activated all layers of cortex, with layer IV being most activated followed by II-III, V, and VI. These data indicate that sensory input from the mystacial vibrissae in the adult rat is processed in brainstem, thalamic, and cortical pathways which are predominantly parallel to those which process information from the neighboring common fur sensory receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Computation, rhythm, and cerebral slow waves

The trajectories of data and modelling from formerly divergent research efforts now seem to be converging to an unexpected region of the phase space of neuroscience. Computational network theory and simulation assume that temporal rhythm may be a significant parameter for the successful organization of nonlinear analog computation effected by hierarchical sets of biological neurons and of nonbiological circuitry alike. For neurobiology, the apparently chaotic rhythms of cerebral compound field potentials--the electroencephalogram (EEG) and slow waves of event related brain potentials (ERBP)--have long been a phenomenological embarrassment, of only marginal clinical utility. But recent data from molecular biophysics, nonlinear dynamics, artificial intelligence, and scalp-conducted human electrocorticography suggest a possible functional role in the serial gating of neural network computations for the familiar theta-alpha-beta rhythms of the EEG clinic.

Cerebral slow waves and time parsing

Cerebral compound field potentials, observed as either the electroencephalogram (EEG) or stimulus-synchronized event related brain potentials (ERBP), have received thirty years of experimental study as possible indicators of general brain state or of sensorimotor information processing respectively. They have received relatively little attention in the context of subjective awareness of a undirectional dimension of time, and then mainly in relation to nonhuman sensorimotor rhythms and hippocampal theta waves. This report is a pilot study of human EEG and ERBP during subjective familiarization with simple motor-sensory temporal patterns, using both small-averaged and unaveraged time series of the traditional theta-alpha-beta band and computer-unaided, human recognition of temporal patterns in the cerebral slow waves. The data support experimental use of more sophisticated methods to analyze possible time-parsing functions of primate extracellular field oscillations.

How to use EEG/ERBP phenomena

Hitherto the study of cerebral slow waves has been mainly empirical phenomenology, in spite of 50 years of effort to discover their origins and possible functions. Since 1970 the increasing use of both analog and digital filters, combined with more sensitive averaging techniques, has led to better understanding of some possible functional significance for the electroencephalogram (EEG) and the endogenous slow waves of event-related brain potentials (ERBP). The developing concepts and mathematics of non-linear dynamicsand coupled oscillators, when considered in the light of cerebral circuit geometry and topology, of local neuronal circuitry, and of both observed and experienced biological behavior, now offer hope for future, more complete, understanding of the development of 'conscious and goal-directed' activity from robotic and reflexive stimulus-response paradigms. Immediate practical results of a biophysical approach to brain waves should be: Models of vertebrate brains as distributed systems of non-linearly coupled oscillators; and revision of traditional methodologies for recording and interpreting EEG/ERBP data.

Difference in projections to the lateral and medial facial nucleus: anatomically separate pathways for rhythmical vibrissa movement in rats

The present paper demonstrates that the lateral and medial subdivisions of the rat facial motor nucleus (NVII) receive differing mesencephalic and metencephalic projections. In order to study brain projections to facial nucleus, horseradish peroxidase (HRP) was injected iontophoretically into the entire facial nucleus or the following subdivisions: lateral, dorsolateral, medial, intermediate, and ventral. In the mesencephalic region, the retrorubral nucleus was found to project to the contralateral medial subdivision of NVII, while the red nucleus was found to project to the contralateral lateral subdivision of NVII. Other mesencephalic projections to the facial nucleus arose from the deep mesencephalic nucleus, oculomotor nucleus, central gray including interstitial nucleus of Cajal and nucleus Darkschewitsch, superior colliculus and substantia nigra (reticular). In the mesencephalic region, the Kölliker-Fuse nucleus, parabrachial nucleus, and the ventral nucleus of the lateral lemniscus projected mainly to the ipsilateral lateral subdivision of NVII. In addition, the trapezoid, pontine reticular, vestibular, and motor trigeminal nuclei were observed to have predominantly ipsilateral connections to the facial nucleus. In contrast, projections from the myelencephalic region were to both the lateral and medial subdivision of NVII. The medullary reticular nucleus, ambiguus nucleus, spinal trigeminal nucleus and parvocellular reticular nucleus projected to both lateral and medial subdivisions of NVII with an ipsilateral predominance. The gigantocellular and paragigantocellular reticular nuclei, raphe magnus, external cuneate nucleus and the nucleus of the solitary tract also projected to the facial motor nucleus. Surprisingly, no direct projections to the NVII were observed from diencephalic and telencephalic regions. Our findings that the lateral subdivision of NVII which innervates vibrissa-pad-muscles (Dom et al. 1973; Martin and Lodge 1977; Watson et al. 1982) receives different metencephalic and mesencephalic projections than medial subdivision which controls pinna movement (Henkel and Edwards 1978), suggest that the functional difference between these subdivisions is mediated by the anatomically separate pathways. We confirmed our anatomical findings by eliciting exclusively vibrissa responses by electrical stimulation of the nuclei which project to the lateral subdivision of NVII.

The facial "motor" nerve of the rat: control of vibrissal movement and examination of motor and sensory components

Rhythmical whisking of the mystacial vibrissae at about 7 Hz during exploration is one of the most conspicuous behavioral patterns in the rat. To identify the final common pathway for vibrissal movement, individual motor branches of the facial nerve, including the posterior auricular, temporal, zygomatic, buccal, marginal mandibular, cervical, stylohyoid, and posterior digastric branches, were cut, either singly or in various combinations. We found that vibrissal movement could be abolished only by transection involving the buccal branch and the upper division of the marginal mandibular branch. To trace back the central origins of the buccal and marginal mandibular, as well as the other branches of the facial nerve, all distal to the stylomastoid foramen, horseradish peroxidase (HRP) was applied to the cut proximal ends of these individual branches. The retrograde HRP labelling in the facial motor nucleus revealed topographical representation of these branches in which the buccal and marginal mandibular branches were represented laterally. The stylohyoid and posterior digastric branches originated from cells in the suprafacial nucleus. Consistent with earlier observations with intramuscular HRP injections, the motoneuronal population devoted to vibrissal movement did not seem to be substantially larger than that for other facial movements. An additional examination was made of the labelled afferent component of the facial motor nerve. We confirmed and extended previous findings that none of the above facial motor nerve branches, except the posterior auricular branch, contained a significant number of afferent fibers originating from the geniculate ganglion, the sensory ganglion of the seventh nerve. In addition, no labelling was seen in the mesencephalic trigeminal nucleus or trigeminal ganglion. These findings, in combination, suggest that, with the exception of the posterior auricular branch, all the facial motor nerve branches, including those involved in vibrissal movement, are almost entirely efferent.