Dynamic and distributed properties of many-neuron ensembles in the ventral posterior medial thalamus of awake rats

Intracortical brain-computer interfaces for improved motor function: a systematic review

In this systematic review, we address the status of intracortical brain-computer interfaces (iBCIs) applied to the motor cortex to improve function in patients with impaired motor ability. This study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 Guidelines for Systematic Reviews. Risk Of Bias In Non-randomized Studies - of Interventions (ROBINS-I) and the Effective Public Health Practice Project (EPHPP) were used to assess bias and quality. Advances in iBCIs in the last two decades demonstrated the use of iBCI to activate limbs for functional tasks, achieve neural typing for communication, and other applications. However, the inconsistency of performance metrics employed by these studies suggests the need for standardization. Each study was a pilot clinical trial consisting of 1-4, majority male (64.28 %) participants, with most trials featuring participants treated for more than 12 months (55.55 %). The systems treated patients with various conditions: amyotrophic lateral sclerosis, stroke, spinocerebellar degeneration without cerebellar involvement, and spinal cord injury. All participants presented with tetraplegia at implantation and were implanted with microelectrode arrays via pneumatic insertion, with nearly all electrode locations solely at the precentral gyrus of the motor cortex (88.88 %). The development of iBCI devices using neural signals from the motor cortex to improve motor-impaired patients has enhanced the ability of these systems to return ability to their users. However, many milestones remain before these devices can prove their feasibility for recovery. This review summarizes the achievements and shortfalls of these systems and their respective trials.

"I Felt the Ball"-The Future of Spine Injury Recovery

Spinal cord injury (SCI) has no cure and individuals with SCI become dependent on others for life. After injury, the signals below the lesion are disrupted, but the brain still produces motor commands. Researchers have bypassed this obstacle, which has given rise to the brain-machine interface (BMI). BMI devices are implanted in the brain or spinal cord, where they decode and send signals beyond the injured segment. Experiments were initially conducted on animals, with favorable results. BMIs are classified according to their type, function, signal generation, and so on. Because of invasiveness, their long-term use is questionable, because of infections and complications. The use of an implantable epidural array in patients with chronic SCI showed that participants were able to walk with the help of a stimulator, and after months of training, they were able to walk with the stimulator turned off. Another innovation is a robotic suit for paraplegics and tetraplegics that supports the movement of limbs. The research on stem cells has not shown favorable results. In future, one of these cutting-edge technologies will prevail over the other, but BMI seems to have the upper hand. The future of BMI with fusion of robotics and artificial intelligence is promising for patients with chronic SCI. These modern devices need to be less invasive, biocompatible, easily programmable, portable, and cost-effective. After these hurdles are overcome, the devices may become the mainstay of potential rehabilitation therapy for partial recovery. The time may come when all patients with severe SCI are told "You will walk again."

Incisional Nociceptive Input Impairs Attention-related Behavior and Is Associated with Reduced Neuronal Activity in the Prefrontal Cortex in Rats

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Cognitive capacity may be reduced from inflammation, surgery, anesthesia, and pain. In this study, we hypothesized that incision-induced nociceptive input impairs attentional performance and alters neuronal activity in the prefrontal cortex.

METHODS

Attentional performance was measured in rats by using the titration variant of the 5-choice serial reaction time to determine the effect of surgical incision and anesthesia in a visual attention task. Neuronal activity (single spike and local field potentials) was measured in the medial prefrontal cortex in animals during the task.

RESULTS

Incision significantly impaired attention postoperatively (area under curve of median cue duration-time 97.2 ± 56.8 [n = 9] vs. anesthesia control 25.5 ± 14.5 s-days [n = 9], P = 0.002; effect size, η = 0.456). Morphine (1 mg/kg) reduced impairment after incision (area under curve of median cue duration-time 31.6 ± 36.7 [n = 11] vs. saline 110 ± 64.7 s-days [n = 10], P < 0.001; η = 0.378). Incision also decreased cell activity (n = 24; 1.48 ± 0.58 vs. control, 2.93 ± 2.02 bursts/min; P = 0.002; η = 0.098) and local field potentials (n = 28; η = 0.111) in the medial prefrontal cortex.

CONCLUSIONS

These results show that acute postoperative nociceptive input from incision reduces attention-related task performance and decreases neuronal activity in the medial prefrontal cortex. Decreased neuronal activity suggests nociceptive input is more than just a distraction because neuronal activity increases during audiovisual distraction with similar behavioral impairment. This suggests that nociceptive input and the medial prefrontal cortex may contribute to attentional impairment and mild cognitive dysfunction postoperatively. In this regard, pain may affect postoperative recovery and return to normal activities through attentional impairment by contributing to lapses in concentration for routine and complex tasks.

Brain-Machine Interfaces: From Basic Science to Neuroprostheses and Neurorehabilitation

Brain-machine interfaces (BMIs) combine methods, approaches, and concepts derived from neurophysiology, computer science, and engineering in an effort to establish real-time bidirectional links between living brains and artificial actuators. Although theoretical propositions and some proof of concept experiments on directly linking the brains with machines date back to the early 1960s, BMI research only took off in earnest at the end of the 1990s, when this approach became intimately linked to new neurophysiological methods for sampling large-scale brain activity. The classic goals of BMIs are 1) to unveil and utilize principles of operation and plastic properties of the distributed and dynamic circuits of the brain and 2) to create new therapies to restore mobility and sensations to severely disabled patients. Over the past decade, a wide range of BMI applications have emerged, which considerably expanded these original goals. BMI studies have shown neural control over the movements of robotic and virtual actuators that enact both upper and lower limb functions. Furthermore, BMIs have also incorporated ways to deliver sensory feedback, generated from external actuators, back to the brain. BMI research has been at the forefront of many neurophysiological discoveries, including the demonstration that, through continuous use, artificial tools can be assimilated by the primate brain's body schema. Work on BMIs has also led to the introduction of novel neurorehabilitation strategies. As a result of these efforts, long-term continuous BMI use has been recently implicated with the induction of partial neurological recovery in spinal cord injury patients.

Audiovisual Distraction Increases Prefrontal Cortical Neuronal Activity and Impairs Attentional Performance in the Rat

Involvement of attentional processes is generally evidenced by disruption of behavior in the presence of distracting stimuli. The medial prefrontal cortex (mPFC) seems to play a role in fine-tuning activity during attentional tasks. A novel titration variant of the 5-choice serial reaction time task (5-choice serial reaction time titration variant [5CTV]) that adjusts task difficulty based on subject performance was used to evaluate the effects of audiovisual distraction (DSTR) on performance and mPFC single spike activity and local field potential (LFP). Attention was impaired in the 5CTV from DSTR, and mPFC spike activity was increased, whereas LFP was reduced. The increased spike activity in the mPFC in conjunction with DSTR suggests that conflicting attentional demands may contribute to the reduced task performance. As both hypo- and hyperactivation of the mPFC may contribute to attentional disruption, further studies using the 5CTV are needed to understand mPFC activity changes in real time during disruption of performance by other types of behavioral or neurobiological manipulations.

Temporary Nerve Block at Selected Digits Revealed Hand Motor Deficits in Grasping Tasks

Peripheral sensory feedback plays a crucial role in ensuring correct motor execution throughout hand grasp control. Previous studies utilized local anesthesia to deprive somatosensory feedback in the digits or hand, observations included sensorimotor deficits at both corticospinal and peripheral levels. However, the questions of how the disturbed and intact sensory input integrate and interact with each other to assist the motor program execution, and whether the motor coordination based on motor output variability between affected and non-affected elements (e.g., digits) becomes interfered by the local sensory deficiency, have not been answered. The current study aims to investigate the effect of peripheral deafferentation through digital nerve blocks at selective digits on motor performance and motor coordination in grasp control. Our results suggested that the absence of somatosensory information induced motor deficits in hand grasp control, as evidenced by reduced maximal force production ability in both local and non-local digits, impairment of force and moment control during object lift and hold, and attenuated motor synergies in stabilizing task performance variables, namely the tangential force and moment of force. These findings implied that individual sensory input is shared across all the digits and the disturbed signal from local sensory channel(s) has a more comprehensive impact on the process of the motor output execution in the sensorimotor integration process. Additionally, a feedback control mechanism with a sensation-based component resides in the formation process for the motor covariation structure.

Characterisation of sensitivity and orientation tuning for visually responsive ensembles in the zebrafish tectum

Sensory coding relies on ensembles of co-active neurons, but these ensembles change from trial to trial of the same stimulus. This is due in part to wide variability in the responsiveness of neurons within these ensembles, with some neurons responding regularly to a stimulus while others respond inconsistently. The specific functional properties that cause neurons to respond more or less consistently have not been thoroughly explored. Here, we have examined neuronal ensembles in the zebrafish tectum responsive to repeated presentations of a visual stimulus, and have explored how these populations change when the orientation or brightness of the stimulus is altered. We found a continuum of response probabilities across the neurons in the visual ensembles, with the most responsive neurons focused toward the spatial centre of the ensemble. As the visual stimulus was made dimmer, these neurons remained active, suggesting higher overall responsiveness. However, these cells appeared to represent the most consistent end of a continuum, rather than a functionally distinct “core” of highly responsive neurons. Reliably responsive cells were broadly tuned to a range of stimulus orientations suggesting that, at least for this stimulus property, tight stimulus tuning was not responsible for their consistent responses.

Cortex dynamically modulates responses of thalamic relay neurons through prolonged circuit-level disinhibition in rat thalamus in vivo

Cortex actively modulates the responses of thalamic relay neurons through corticothalamic (CT) projections. Here we investigated the temporal precision of CT modulation on sensory responses of relay neurons in rat ventral posterior medial thalamus (VPM) to direction-specific whisker stimuli. CT feedback levels were either augmented by cortical electrical microstimulation or depressed by cortical application of muscimol, a potent agonist of γ-aminobutyric acid A-type (GABA_A) receptors. To evaluate the temporal specificity of CT influence, we compared the early (3-10 ms after stimulus onset) and late (10-100 ms) response components of VPM single units to whisker deflections in preferred or nonpreferred directions before and after altering CT feedback levels under urethane anesthesia. The data showed that cortical feedback most strongly affected the late responses of single VPM units to whisker stimulation. That is, cortical stimulation consistently increased the late responses of VPM units in the corresponding (homologous) barreloids to the stimulus direction preferred by neurons in the cortical locus stimulated. However, cortical stimulation could either increase or decrease the early response, depending on whether or not cortical and thalamic loci were tuned to the same direction. Such bidirectional regulation of the early and late VPM responses is consistent with a mechanism of circuit-level disinhibition in vivo. The results support the theory that CT feedback on thalamic sensory responses is mediated by a time-dependent shift of the excitation-inhibition balance in the thalamo-cortico-thalamic loop, such as would occur during sensory feature integration, plasticity, and learning in the awake state.

Distribution and Morphology of Calcium-Binding Proteins Immunoreactive Neurons following Chronic Tungsten Multielectrode Implants

The development of therapeutic approaches to improve the life quality of people suffering from different types of body paralysis is a current major medical challenge. Brain-machine interface (BMI) can potentially help reestablishing lost sensory and motor functions, allowing patients to use their own brain activity to restore sensorimotor control of paralyzed body parts. Chronic implants of multielectrodes, employed to record neural activity directly from the brain parenchyma, constitute the fundamental component of a BMI. However, before this technique may be effectively available to human clinical trials, it is essential to characterize its long-term impact on the nervous tissue in animal models. In the present study we evaluated how chronic implanted tungsten microelectrode arrays impact the distribution and morphology of interneurons reactive to calcium-binding proteins calbindin (CB), calretinin (CR) and parvalbumin (PV) across the rat’s motor cortex. Our results revealed that chronic microelectrode arrays were well tolerated by the nervous tissue, with recordings remaining viable for up to 6 months after implantation. Furthermore, neither the morphology nor the distribution of inhibitory neurons were broadly impacted. Moreover, restricted microglial activation was observed on the implanted sites. On the whole, our results confirm and expand the notion that tungsten multielectrodes can be deemed as a feasible candidate to future human BMI studies.

Seeing the whole picture: A comprehensive imaging approach to functional mapping of circuits in behaving zebrafish

In recent years, the zebrafish has emerged as an appealing model system to tackle questions relating to the neural circuit basis of behavior. This can be attributed not just to the growing use of genetically tractable model organisms, but also in large part to the rapid advances in optical techniques for neuroscience, which are ideally suited for application to the small, transparent brain of the larval fish. Many characteristic features of vertebrate brains, from gross anatomy down to particular circuit motifs and cell-types, as well as conserved behaviors, can be found in zebrafish even just a few days post fertilization, and, at this early stage, the physical size of the brain makes it possible to analyze neural activity in a comprehensive fashion. In a recent study, we used a systematic and unbiased imaging method to record the pattern of activity dynamics throughout the whole brain of larval zebrafish during a simple visual behavior, the optokinetic response (OKR). This approach revealed the broadly distributed network of neurons that were active during the behavior and provided insights into the fine-scale functional architecture in the brain, inter-individual variability, and the spatial distribution of behaviorally relevant signals. Combined with mapping anatomical and functional connectivity, targeted electrophysiological recordings, and genetic labeling of specific populations, this comprehensive approach in zebrafish provides an unparalleled opportunity to study complete circuits in a behaving vertebrate animal.

Spike count, spike timing and temporal information in the cortex of awake, freely moving rats

OBJECTIVE

Sensory processing of peripheral information is not stationary but is, in general, a dynamic process related to the behavioral state of the animal. Yet the link between the state of the behavior and the encoding properties of neurons is unclear. This report investigates the impact of the behavioral state on the encoding mechanisms used by cortical neurons for both detection and discrimination of somatosensory stimuli in awake, freely moving, rats.

APPROACH

Neuronal activity was recorded from the primary somatosensory cortex of five rats under two different behavioral states (quiet versus whisking) while electrical stimulation of increasing stimulus strength was delivered to the mystacial pad. Information theoretical measures were then used to measure the contribution of different encoding mechanisms to the information carried by neurons in response to the whisker stimulation.

MAIN RESULTS

We found that the behavioral state of the animal modulated the total amount of information conveyed by neurons and that the timing of individual spikes increased the information compared to the total count of spikes alone. However, the temporal information, i.e. information exclusively related to when the spikes occur, was not modulated by behavioral state.

SIGNIFICANCE

We conclude that information about somatosensory stimuli is modulated by the behavior of the animal and this modulation is mainly expressed in the spike count while the temporal information is more robust to changes in behavioral state.

Heterogeneity in the spatial receptive field architecture of multisensory neurons of the superior colliculus and its effects on multisensory integration

Multisensory integration has been widely studied in neurons of the mammalian superior colliculus (SC). This has led to the description of various determinants of multisensory integration, including those based on stimulus- and neuron-specific factors. The most widely characterized of these illustrate the importance of the spatial and temporal relationships of the paired stimuli as well as their relative effectiveness in eliciting a response in determining the final integrated output. Although these stimulus-specific factors have generally been considered in isolation (i.e., manipulating stimulus location while holding all other factors constant), they have an intrinsic interdependency that has yet to be fully elucidated. For example, changes in stimulus location will likely also impact both the temporal profile of response and the effectiveness of the stimulus. The importance of better describing this interdependency is further reinforced by the fact that SC neurons have large receptive fields, and that responses at different locations within these receptive fields are far from equivalent. To address these issues, the current study was designed to examine the interdependency between the stimulus factors of space and effectiveness in dictating the multisensory responses of SC neurons. The results show that neuronal responsiveness changes dramatically with changes in stimulus location - highlighting a marked heterogeneity in the spatial receptive fields of SC neurons. More importantly, this receptive field heterogeneity played a major role in the integrative product exhibited by stimulus pairings, such that pairings at weakly responsive locations of the receptive fields resulted in the largest multisensory interactions. Together these results provide greater insight into the interrelationship of the factors underlying multisensory integration in SC neurons, and may have important mechanistic implications for multisensory integration and the role it plays in shaping SC-mediated behaviors.

A review of cell assemblies

Since the cell assembly (CA) was hypothesised, it has gained substantial support and is believed to be the neural basis of psychological concepts. A CA is a relatively small set of connected neurons, that through neural firing can sustain activation without stimulus from outside the CA, and is formed by learning. Extensive evidence from multiple single unit recording and other techniques provides support for the existence of CAs that have these properties, and that their neurons also spike with some degree of synchrony. Since the evidence is so broad and deep, the review concludes that CAs are all but certain. A model of CAs is introduced that is informal, but is broad enough to include, e.g. synfire chains, without including, e.g. holographic reduced representation. CAs are found in most cortical areas and in some sub-cortical areas, they are involved in psychological tasks including categorisation, short-term memory and long-term memory, and are central to other tasks including working memory. There is currently insufficient evidence to conclude that CAs are the neural basis of all concepts. A range of models have been used to simulate CA behaviour including associative memory and more process- oriented tasks such as natural language parsing. Questions involving CAs, e.g. memory persistence, CAs' complex interactions with brain waves and learning, remain unanswered. CA research involves a wide range of disciplines including biology and psychology, and this paper reviews literature directly related to the CA, providing a basis of discussion for this interdisciplinary community on this important topic. Hopefully, this discussion will lead to more formal and accurate models of CAs that are better linked to neuropsychological data.

The flexDrive: an ultra-light implant for optical control and highly parallel chronic recording of neuronal ensembles in freely moving mice

Electrophysiological recordings from ensembles of neurons in behaving mice are a central tool in the study of neural circuits. Despite the widespread use of chronic electrophysiology, the precise positioning of recording electrodes required for high-quality recordings remains a challenge, especially in behaving mice. The complexity of available drive mechanisms, combined with restrictions on implant weight tolerated by mice, limits current methods to recordings from no more than 4-8 electrodes in a single target area. We developed a highly miniaturized yet simple drive design that can be used to independently position 16 electrodes with up to 64 channels in a package that weighs ~2 g. This advance over current designs is achieved by a novel spring-based drive mechanism that reduces implant weight and complexity. The device is easy to build and accommodates arbitrary spatial arrangements of electrodes. Multiple optical fibers can be integrated into the recording array and independently manipulated in depth. Thus, our novel design enables precise optogenetic control and highly parallel chronic recordings of identified single neurons throughout neural circuits in mice.

Comprehensive mapping of whisker-evoked responses reveals broad, sharply tuned thalamocortical input to layer 4 of barrel cortex

Cortical neurons are organized in columns, distinguishable by their physiological properties and input-output organization. Columns are thought to be the fundamental information-processing modules of the cortex. The barrel cortex of rats and mice is an attractive model system for the study of cortical columns, because each column is defined by a layer 4 (L4) structure called a barrel, which can be clearly visualized. A great deal of information has been collected regarding the connectivity of neurons in barrel cortex, but the nature of the input to a given L4 barrel remains unclear. We measured this input by making comprehensive maps of whisker-evoked activity in L4 of rat barrel cortex using recordings of multiunit activity and current source density analysis of local field potential recordings of animals under light isoflurane anesthesia. We found that a large number of whiskers evoked a detectable response in each barrel (mean of 13 suprathreshold, 18 subthreshold) even after cortical activity was abolished by application of muscimol, a GABA(A) agonist. We confirmed these findings with intracellular recordings and single-unit extracellular recordings in vivo. This constitutes the first direct confirmation of the hypothesis that subcortical mechanisms mediate a substantial multiwhisker input to a given cortical barrel.

Spike timing, spike count, and temporal information for the discrimination of tactile stimuli in the rat ventrobasal complex

The aim of this work was to investigate the role of spike timing for the discrimination of tactile stimuli in the thalamic ventrobasal complex of the rat. We applied information-theoretic measures and computational experiments on neurophysiological data to test the ability of single-neuron responses to discriminate stimulus location and stimulus dynamics using either spike count (40 ms bin size) or spike timing (1 ms bin size). Our main finding is not only that spike timing provides additional information over spike count alone, but specifically that the temporal aspects of the code can be more informative than spike count in the rat ventrobasal complex. Virtually all temporal information--i.e., information exclusively related to when the spikes occur--is conveyed by first spikes, arising mostly from latency differences between the responses to different stimuli. Although the imprecision of first spikes (i.e., the jitter) is highly detrimental for the information conveyed by latency differences, jitter differences can contribute to temporal information, but only if latency differences are close to zero. We conclude that temporal information conveyed by spike timing can be higher than spike count information for the discrimination of somatosensory stimuli in the rat ventrobasal complex.

Tactile responses of hindpaw, forepaw and whisker neurons in the thalamic ventrobasal complex of anesthetized rats

The majority of studies investigating responses of thalamocortical neurons to tactile stimuli have focused on the whisker representation of the rat thalamus: the ventral–posterior–medial nucleus (VPM). To test whether the basic properties of thalamocortical responses to tactile stimuli could be extended to the entire ventrobasal complex, we recorded single neurons from the whisker, forepaw and hindpaw thalamic representations. We performed a systematic analysis of responses to stereotyped tactile stimuli − 500 ms pulses (i.e. ON–OFF stimuli) or 1 ms pulses (i.e. impulsive stimuli) − under two different anesthetics (pentobarbital or urethane). We obtained the following main results: (i) the tuning of cells to ON vs. OFF stimuli displayed a gradient across neurons, so that two-thirds of cells responded more to ON stimuli and one-third responded more to OFF stimuli; (ii) on average, response magnitudes did not differ between ON and OFF stimuli, whereas latencies of response to OFF stimuli were a few milliseconds longer; (iii) latencies of response to ON and OFF stimuli were highly correlated; (iv) responses to impulsive stimuli and ON stimuli showed a strong correlation, whereas the relationship between the responses to impulsive stimuli and OFF stimuli was subtler; (v) unlike ON responses, OFF responses did not decrease when stimuli were moved from the receptive field center to a close location in the excitatory surround. We obtained the same results for hindpaw, forepaw and whisker neurons. Our results support the view of a neurophysiologically homogeneous ventrobasal complex, in which OFF responses participate in the structure of the spatiotemporal receptive field of thalamocortical neurons for tactile stimuli.

Properties of tactile responses of neurons in rat thalamic ventroposterolateral nucleus

OBJECTIVE

To determine whether the convergences of tactile information also occur at thalamic ventroposterolateral nucleus in rats, we investigated the properties of tactile responses of the thalamic ventroposterolateral nucleus in rats.

METHODS

Unit responses were recorded extracellularly from thalamic ventroposterolateral nucleus in anesthetized rats.

RESULTS

Among 156 neurons examined, 140 neurons (89.7%) had the single, continual and small receptive fields, and 16 neurons (10.3%) had two discrete receptive fields. Some neurons?exhibited different responses to the same intensity stimulation which delivered to different points in their receptive fields. In addition, 4.5% neurons (n = 7) responded only to locomotive stimulation but?not to a punctiform tactile stimulation.

CONCLUSION

The majority of neurons in ventroposterolateral nucleus of rats have the spatial, temporal and submodal characteristics of cutaneous receptors, while the minority of neurons exhibit the responses of interaction of different peripheral receptors. Therefore, it is concluded that there are convergences of tactile information at the ventroposterolateral nucleus of rats.

Enhanced short-latency responses in the ventral posterior medial (VPM) thalamic nucleus following whisker trimming in the adult rat

This study examined the effects of whisker trimming on the functional organization of the adult somatosensory thalamus. In vivo extracellular unit recordings were made on ventral posterior medial (VPM) thalamic neurons in urethane anaesthetised adult rats. Neuronal response properties to controlled whisker deflection were recorded in untrimmed control animals and in animals where one row of whiskers had been trimmed for a median of 18 days. Trimming significantly increased short-latency responses to stimulation of the centre receptive field whisker (mean increase of 36%, p<.001). Longer latency responses to surround receptive field whiskers were unaffected. Spontaneous firing was significantly decreased in trimmed animals. A condition-test paradigm indicated that thalamic inhibition was reduced following whisker trimming, although this effect failed to reach statistical significance. These results demonstrate a capacity of the adult somatosensory thalamus to undergo functional reorganization in response to non-traumatic and innocuous whisker trimming.

I.v. cocaine induces rapid, transient excitation of striatal neurons via its action on peripheral neural elements: single-cell, iontophoretic study in awake and anesthetized rats

Cocaine's (COC) direct interaction with the dopamine (DA) transporter is usually considered the most important action underlying the psychomotor stimulant and reinforcing effects of this drug. However, some physiological, behavioral and psycho-emotional effects of COC are very rapid and brief and they remain intact during DA receptor blockade, suggesting possible involvement of peripheral non-DA neural mechanisms. To assess this issue, single-unit recording with microiontophoresis was used to examine changes in impulse activity of dorsal and ventral striatal neurons to i.v. COC (0.25-0.5 mg/kg) in the same rats under two conditions: awake with DA receptor blockade and anesthetized with urethane. In the awake preparation approximately 70% striatal neurons showed rapid and transient (latency approximately 6 s, duration approximately 15 s) COC-induced excitations. These effects were stronger in ventral than dorsal striatum. During anesthesia, these phasic effects were fully blocked and COC slowly decreased neuronal discharge rate. Cocaine-methiodide (COC-M), a derivative that cannot cross the blood-brain barrier, also caused phasic excitations in the awake, but not anesthetized condition. In contrast to regular COC, COC-M had no tonic effect on discharge rate in either preparation. Most striatal neurons that were phasically excited by both COC forms also showed short-latency excitations during tail-touch and tail-pinch in the awake preparation, an effect strongly attenuated during anesthesia. Finally, most striatal neurons that in awake conditions were phasically excited by somato-sensory stimuli and COC salts were also excited by iontophoretic glutamate (GLU). Although striatal neurons were sensitive to GLU in both preparations, the response magnitude at the same GLU current was higher in awake than anesthetized conditions. These data suggest that in awake animals i.v. COC, like somato-sensory stimuli, transiently excites striatal neurons via its action on peripheral neural elements and rapid neural transmission. While the nature of these neuronal elements needs to be clarified using other analytical techniques, they might involve voltage-gated K(+) and Na(+) channels, which have a high affinity for COC and are located on terminals of visceral sensory nerves that densely innervate peripheral vessels. Therefore, along with direct action on specific brain substrates, central excitatory effects of COC may occur via indirect action, involving afferents of visceral sensory nerves and rapid neural transmission. By providing a rapid sensory signal and triggering transient neural activation, such a peripherally triggered action might play a crucial role in the sensory effects of COC, thus contributing to learning and development of drug-taking behavior.

Differential taste coding of salt and acid by correlative activities between taste-sensitive neuron types in rat gustatory cortex

Using a multi-electrode recording technique, the present study aimed to elucidate the role of broadly-tuned taste-sensitive neurons in the rat gustatory cortex in discriminating between salt and acid. A majority of taste-sensitive neurons (94/119 neurons; 78%) were classified as NaCl (N)-, HCl (H)- or NaCl and HCl (NH)-best neurons. Of 63 neuron pairs (94 neurons), 31 showed significant peaks and/or troughs in their cross-correlograms (CCs) during taste stimulation periods. During NaCl stimulation, the incidence of significant correlation and the mean frequency of correlated discharges (FC) in the N/N and NH/NH pairs were higher than those in the other best-taste pairs. In contrast, during HCl stimulation both indices in the N/N or H/H pairs were very low, while those in the NH/NH pairs were high. These results suggest that (1) correlated activities between N-best neurons and those between NH-best neurons play a significant role in taste quality coding of salt, and that (2) correlated activities between NH-best neurons may be important for sour taste coding as well. Peak formation in CCs tended to be more frequent in the homo-types (N/N, H/H and NH/NH pairs) than in the hetero-types (N/NH, N/H and H/NH pairs). In contrast, troughs were observed mostly in the hetero-types. Inhibitory interaction in hetero-type pairs together with coactivation in homo-type pairs may enhance taste discrimination by taste-sensitive neuron populations.

State-dependent modulation of time-varying gustatory responses

Sensory processing is modulated by attention, which is a function of network states. Here we show that changes in such states do more than a simple gating of stimuli: they actually re-arrange cortical coding space to emphasize emotional valences. We delivered taste stimuli to rats before and after a spontaneous state change ("disengagement") that is associated with a reduction in attention and a concurrent emergence of cortical mu rhythms. The percentage of cortical neurons that responded to tastes, and the average response across neurons, remained stable with disengagement, but the particulars of the responses changed drastically. The distinctiveness of sucrose and quinine-which represent the high and low ends of the palatability spectrum-increased, the distinctiveness of the two aversive tastes (quinine and citric acid) decreased, and the distinctiveness of sucrose and NaCl, which were almost identically palatable to start with, did not change. Overall, then, the changes appeared to be palatability-specific. Two additional findings were consistent with this conclusion: rats' palatability-related behavioral responses to the tastes changed in similar ways with disengagement and disengagement-related neural changes specifically appeared late in the response, when palatability-specific information emerges in cortical responses. These data suggest that neural state changes can change the content of neural codes.

Dynamic-clamp analysis of the effects of convergence on spike timing. II. Few synaptic inputs

Sensory pathways in the nervous system possess mechanisms for decreasing spike-timing variability ("jitter"), probably to increase acuity. Most studies of jitter reduction have focused on convergence of many subthreshold inputs. However, many neurons receive only a few active inputs at any given time, and jitter reduction under these conditions is not well understood. We examined this issue using dynamic-clamp recordings in slices from mouse auditory brain stem. Significant jitter reduction was possible with as few as two inputs, provided the inputs had several features. First, jitter reduction was greatest and most reliable for supra-threshold inputs. Second, significant jitter reduction occurred when the distribution of input times had a rapid onset, i.e., for alpha- but not for Gaussian-distributed inputs. Third, jitter reduction was compromised unless late inputs were suppressed by the refractory period of the cell. These results contrast with the finding in the previous paper in which many subthreshold inputs contribute to jitter reduction, whether alpha- or Gaussian-distributed. In addition, convergence of many subthreshold inputs could fail to elicit any postsynaptic response when the input distribution outlasted the refractory period of the cell. These significant differences indicate that each means of reducing jitter has advantages and disadvantages and may be more effective for different neurons depending on the properties of their inputs.

The barrel cortex--integrating molecular, cellular and systems physiology

A challenge for neurobiology is to integrate information across many levels of research, ranging from behaviour and neuronal networks to cells and molecules. The rodent whisker signalling pathway to the primary somatosensory neocortex with its remarkable somatotopic barrel map is emerging as a key system for such integrative studies.

Chronic, multisite, multielectrode recordings in macaque monkeys

A paradigm is described for recording the activity of single cortical neurons from awake, behaving macaque monkeys. Its unique features include high-density microwire arrays and multichannel instrumentation. Three adult rhesus monkeys received microwire array implants, totaling 96-704 microwires per subject, in up to five cortical areas, sometimes bilaterally. Recordings 3-4 weeks after implantation yielded 421 single neurons with a mean peak-to-peak voltage of 115 +/- 3 microV and a signal-to-noise ratio of better than 5:1. As many as 247 cortical neurons were recorded in one session, and at least 58 neurons were isolated from one subject 18 months after implantation. This method should benefit neurophysiological investigation of learning, perception, and sensorimotor integration in primates and the development of neuroprosthetic devices.

Hand posture modulates neuronal interaction in the primary somatosensory cortex of humans

OBJECTIVE

To investigate the effects of hand posture on the modulation of neuronal interactions in the cortical finger regions of the human somatosensory cortex.

METHODS

Neuronal magnetic fields, evoked by electrical stimuli to the thumb and/or to the index finger of the right hand, were recorded in different hand postures ('OPEN': opened hand and 'CLOSE': both fingers in opposite position to pick up something) by using a whole head type magnetoencephalography. The equivalent current dipole (ECD) for components in the primary (SI) and secondary somatosensory cortices (SII) was calculated. The interaction ratio (IR) was calculated as a ratio of the vector sum of ECD moments evoked by respective stimulation of each finger to the ECD moment evoked by simultaneous stimulation of both fingers.

RESULTS

The mean IR of N20m was significantly larger in CLOSE than in OPEN (p=0.033, ANOVA). On the contrary, the IR of P40m was larger in OPEN than in CLOSE (p=0.042). The IR of SII components was not significantly different between the different hand postures (p=0.35).

CONCLUSIONS

Neuronal interaction between the thumb and index finger in the human SI is modulated by hand posture. Provided that forming hand posture is related to receiving sensory input, the interaction modulation may play a role in the facilitation of somatosensory processing.

SIGNIFICANCE

Our results suggest experimental evidence for the immediate modulation of neuronal activity in the somatosensory area.

Inhibitory control of sensory gating in a computer model of the CA3 region of the hippocampus

A model of the CA3 region of the hippocampus was used to simulate the P50 auditory-evoked potential response to repeated stimuli in order to study the neuronal circuits involved in a sensory-processing deficit associated with schizophrenia. Normal subjects have a reduced P50 auditory-evoked potential amplitude in response to the second of two paired auditory click stimuli spaced 0.5 s apart. However, schizophrenic patients do not gate or reduce their response to the second click. They have equal auditory-evoked response amplitudes to both clicks. When schizophrenic patients were medicated with traditional neuroleptics, the evoked potential amplitude to both clicks increased, but gating of the second response was not restored or improved. Animal studies suggest a role for septohippocampal cholinergic activity in sensory gating. We used a computational model of this system in order to study the relative contributions of local processing and afferent activity in sensory gating. We first compared the effect of information representation as average firing rate to information representation as cell assemblies in order to evaluate the best method to represent the response of hippocampal neurons to the auditory click. We then studied the effects of nicotinic cholinergic input on the response of the network and the effect of GABA(B) receptor activation on the ability of the local network to suppress the test response. The results of our model showed that nicotinic cholinergic input from the septum to the hippocampus can control the flow of sensory information from the cortex into the hippocampus. In addition, postsynaptic GABA(B) receptor activation was not sufficient to suppress the test response when the interstimulus interval was 500 ms. However, presynaptic GABA(B) receptor activity may be responsible for the suppression of the test response at this interstimulus interval.

A system for MEA-based multisite stimulation

The capability for multisite stimulation is one of the biggest potential advantages of microelectrode arrays (MEAs). There remain, however, several technical problems which have hindered the development of a practical stimulation system. An important design goal is to allow programmable multisite stimulation, which produces minimal interference with simultaneous extracellular and patch or whole cell clamp recording. Here, we describe a multisite stimulation and recording system with novel interface circuit modules, in which preamplifiers and transistor transistor logic-driven solid-state switching devices are integrated. This integration permits PC-controlled remote switching of each substrate electrode. This allows not only flexible selection of stimulation sites, but also rapid switching of the selected sites between stimulation and recording, within 1.2 ms. This allowed almost continuous monitoring of extracellular signals at all the substrate-embedded electrodes, including those used for stimulation. In addition, the vibration-free solid-state switching made it possible to record whole-cell synaptic currents in one neuron, evoked from multiple sites in the network. We have used this system to visualize spatial propagation patterns of evoked responses in cultured networks of cortical neurons. This MEA-based stimulation system is a useful tool for studying neuronal signal processing in biological neuronal networks, as well as the process of synaptic integration within single neurons.

Adaptive responses of monkey somatosensory cortex to peripheral and central deafferentation

This study deals with two kinds of activity-dependent phenomena in the somatosensory cortex of adult monkeys, both of which may be related: (1) mutability of representational maps, as defined electrophysiologically; (2) alterations in expression of genes important in the inhibitory and excitatory neurotransmitter systems. Area 3b of the cerebral cortex was mapped physiologically and mRNA levels or numbers of immunocytochemically stained neurons quantified after disrupting afferent input peripherally by section of peripheral nerves, or centrally by making lesions of increasing size in the somatosensory thalamus. Survival times ranged from a few weeks to many months. Mapping studies after peripheral nerve lesions replicated results of previous studies in showing the contraction of representations deprived of sensory input and expansion of adjacent representations. However, these changes in representational maps were in most cases unaccompanied by significant alterations in gene expression for calcium calmodulin-dependent protein kinase isoforms, for glutamic acid decarboxylase, GABA(A) receptor subunits, GABA(B) receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) or N-methyl-D-aspartate (NMDA) receptor subunits. Mapping studies after lesions in the ventral posterior lateral nucleus (VPL) of the thalamus revealed no changes in cortical representations of the hand or fingers until >15% of the thalamic representation was destroyed, and only slight changes until approximately 45% of the representation was destroyed, at which point the cortical representation of the finger at the center of a lesion began to shrink. Lesions destroying >60% of VPL resulted in silencing of the hand representation. Although all lesions were associated with a loss of parvalbumin-immunoreactive thalamocortical fiber terminations, and of cytochrome oxidase staining in a focal zone of area 3b, no changes in gene expression could be detected in the affected zone until >40-50% of VPL was destroyed, and even after that changes in mRNA levels or in numbers of GABA-immunoreactive neurons in the affected zone were remarkably small. The results of these studies differ markedly from the robust changes in gene expression detectable in the visual cortex of monkeys deprived of vision in one eye. The results confirm the view that divergence of the afferent somatosensory pathways from periphery to cerebral cortex is sufficiently great that many fibers can be lost before neuronal activity is totally silenced in area 3b. This divergence is capable of maintaining a high degree of cortical function in the face of diminishing inputs from the periphery and is probably an important element in promoting representational plasticity in response to altered patterns of afferent input.

Cortical sensory suppression during arousal is due to the activity-dependent depression of thalamocortical synapses

The thalamus serves as a gate that regulates the flow of sensory inputs to the neocortex, and this gate is controlled by neuromodulators from the brainstem reticular formation that are released during arousal. Here we show in rats that sensory-evoked responses were suppressed in the neocortex by activating the brainstem reticular formation and during natural arousal. Sensory suppression occurred at the thalamocortical connection and was a consequence of the activity-dependent depression of thalamocortical synapses caused by increased thalamocortical tonic firing during arousal. Thalamocortical suppression may serve as a mechanism to focus sensory inputs to their appropriate representations in neocortex, which is helpful for the spatial processing of sensory information.

Different temporal processing of sensory inputs in the rat thalamus during quiescent and information processing states in vivo

Sensory inputs from the whiskers reach the primary somatosensory thalamus through the medial lemniscus tract. The main role of the thalamus is to relay these sensory inputs to the neocortex according to the regulations dictated by behavioural state. Intracellular recordings in urethane-anaesthetized rats show that whisker stimulation evokes EPSP-IPSP sequences in thalamic neurons. Both EPSPs and IPSPs depress with repetitive whisker stimulation at frequencies above 2 Hz. Single-unit recordings reveal that during quiescent states thalamic responses to repetitive whisker stimulation are suppressed at frequencies above 2 Hz, so that only low-frequency sensory stimulation is relayed to the neocortex. In contrast, during activated states, induced by stimulation of the brainstem reticular formation or application of acetylcholine in the thalamus, high-frequency whisker stimulation at up to 40 Hz is relayed to the neocortex. Sensory suppression is caused by the depression of lemniscal EPSPs in relatively hyperpolarized thalamocortical neurons. Sensory suppression is abolished during activated states because thalamocortical neurons depolarize and the depressed lemniscal EPSPs are able to reach firing threshold. Strong IPSPs may also contribute to sensory suppression by hyperpolarizing thalamocortical neurons, but during activated states IPSPs are strongly reduced altogether. The results indicate that the synaptic depression of lemniscal EPSPs and the level of depolarization of thalamocortical neurons work together in thalamic primary sensory pathways to suppress high-frequency sensory inputs during non-activated (quiescent) states while permitting the faithful relay of high-frequency sensory information during activated (processing) states.

Whisker maps of neuronal subclasses of the rat ventral posterior medial thalamus, identified by whole-cell voltage recording and morphological reconstruction

Whole-cell voltage recordings were made in vivo in the ventral posterior medial nucleus (VPM) of the thalamus in urethane-anaesthetised young (postnatal day 16-24) rats. Receptive fields (RFs) on the whisker pad were mapped for 31 neurones, and 10 cells were recovered for morphological reconstruction of their dendritic arbors. Most VPM neurones had antagonistic subthreshold RFs that could be divided into excitatory and inhibitory whiskers. VPM cells comprised different classes, the most frequently occurring being single-whisker excitation (SWE) and multi-whisker excitation (MWE) cells. In SWE cells (36 % of VPM neurones), only principal whisker (PW) deflection evoked an EPSP and was followed by a single action potential (AP) or remained subthreshold. The depolarisation was terminated by a large, delayed IPSP. A stimulus evoked on average 0.74 +/- 0.46 APs (mean +/- S.D.) with short latency (8.1 +/- 1.0 ms) and small temporal scatter (0.31 +/- 0.23 ms dispersion of 50 % of the first APs). In MWE cells (29 % of VPM neurones), deflection of several whiskers evoked EPSPs. PW responses were either subthreshold EPSPs or consisted of an EPSP followed by one or several APs (0.96 +/- 0.99 APs per stimulus). AP responses were often associated with putative low-threshold calcium-dependent regenerative potentials and were followed by a small delayed IPSP. AP responses had a longer latency (12.3 +/- 2.6 ms) and larger temporal scatter (2.5 +/- 1.6 ms) than responses of SWE cells. MWE cells had a lower input resistance than SWE cells. The elongation of dendritic arbors along the representation fields of rows and arcs in VPM barreloids was weakly correlated with the subthreshold RF elongation along whisker rows and arcs, respectively. Evoked EPSP-AP responses exhibited a sharper directional tuning than subthreshold EPSPs, which in turn exhibited a sharper directional tuning than IPSPs. In conclusion, we document two main classes of VPM neurones. SWE cells responded with a precisely timed single AP to the deflection of the PW. In contrast, MWE cell RFs were more broadly tuned and the temporally dispersed multiple AP responses of these cells represented the degree of collective deflection of the PW and several adjacent whiskers.

A new approach to neural cell culture for long-term studies

We have developed a new method for culturing cells that maintains their health and sterility for many months. Using conventional techniques, primary neuron cultures seldom survive more than 2 months. Increases in the osmotic strength of media due to evaporation are a large and underappreciated contributor to the gradual decline in the health of these cultures. Because of this and the ever-present likelihood of contamination by airborne pathogens, repeated or extended experiments on any given culture have until now been difficult, if not impossible. We surmounted survival problems by using culture dish lids that form a gas-tight seal, and incorporate a transparent hydrophobic membrane (fluorinated ethylene-propylene) that is selectively permeable to oxygen (O(2)) and carbon dioxide (CO(2)), and relatively impermeable to water vapor. This prevents contamination and greatly reduces evaporation, allowing the use of a non-humidified incubator. We have employed this technique to grow dissociated cortical cultures from rat embryos on multi-electrode arrays. After more than a year in culture, the neurons still exhibit robust spontaneous electrical activity. The combination of sealed culture dishes with extracellular multi-electrode recording and stimulation enables study of development, adaptation, and very long-term plasticity, across months, in cultured neuronal networks. Membrane-sealed dishes will also be useful for the culture of many other cell types susceptible to evaporation and contamination.

Temporal frequency of whisker movement. I. Representations in brain stem and thalamus

How does processing of information change the internal representations used in subsequent stages of sensory pathways? To approach this question, we studied the representations of whisker movements in the lemniscal and paralemniscal pathways of the rat vibrissal system. We recently suggested that these two pathways encode movement frequency in different ways. We proposed that paralemniscal thalamocortical circuits, functioning as phase-locked loops (PLLs), translate temporally coded information into a rate code. Here we focus on the two major trigeminal nuclei of the brain stem, nucleus principalis and subnucleus interpolaris, and on their thalamic targets, the ventral posteromedial nucleus (VPM) and the medial division of the posterior nucleus (POm). This is the first study in which these brain stem and thalamic nuclei were explored together in the same animals and using the same stimuli. We studied both single- and multi-unit activity. We moved the whiskers both mechanically and by air puffs; here we present air-puff-induced movements because they are more similar to natural movements than movements induced by mechanical stimulations. We describe the basic properties of the responses in these brain stem and thalamic nuclei. The responses in both brain stem nuclei were similar; responses to air puffs were mostly tonic and followed the trajectory of whisker movement. The responses in the two thalamic nuclei were similar during low-frequency stimulations or during the first pulses of high-frequency stimulations, exhibiting more phasic responses than those of brain stem neurons. However, with frequencies >2 Hz, VPM and POm responses differed, generating different representations of the stimulus frequency. In the VPM, response amplitudes (instantaneous firing rates) and spike counts (total number of spikes per stimulus cycle) decreased as a function of the frequency. In the POm, latencies increased and spike count decreased as a function of the frequency. Having described the basic response properties in the four nuclei, we then focus on a specific test of our PLL hypothesis for coding in the paralemniscal pathway. We used short-duration air puffs, much shorter than whisker movements during natural whisking. The activity in this situation was consistent with the prediction we made on the basis of the PLL hypothesis.

High-resolution spatio-temporal mapping of visual pathways using multi-electrode arrays

The parallel processing of visual information was studied with penetrating microelectrode arrays. We studied the high-resolution visuotopic organization of cat primary visual cortex, and the encoding of simple visual stimuli by ensembles of ganglion cells in the isolated turtle retina. The high-resolution visuotopic organization of visual cortex is non-conformal. Regions of visual cortex separated by 400 mu may have receptive field centers that are separated by as much as 3 degrees, or they may superimpose. Ganglion cells are 'generalists', and are poor specifiers of the color of full field visual stimuli. Groups of 'luminosity' type ganglion cells can assist in the specification of stimulus color, but even individual 'chromatic' ganglion cells are not capable of quality color specification. These basic studies have relevance to the development of visual neuroprostheses based upon electrical stimulation of the retina and cortex.

Three-dimensional estimation of the distribution and size of putative functional units in rat gustatory cortex as assessed from the inter-neuronal distance between two neurons with correlative activity

Two neurons recorded simultaneously in rat gustatory cortex often show, when the oral mucosa is stimulated with effective taste solutions, correlative activities (CAs), which can be quantified with a cross-correlation technique. It has been suggested that the CAs are involved in taste quality discrimination and the neuron pairs with CA (CA pairs) may function as a unit (putative functional unit). The present experiment was undertaken (1) to determine the three-dimensional size of putative functional units in rat gustatory cortex on the basis of the interneuronal distance (IND) in individual CA pairs, and (2) to examine to what extent the CA is related to the intrapair similarity in taste responses. The findings were as follows: (1) The CAs occurred in 30 out of 55 pairs with INDs < or =270 microm and often at INDs <100 microm. (2) The numbers of putative functional units were calculated tentatively by using the longest or the mean INDs; at least 200 or over 4,000 units, respectively. (3) In the CA pairs, the "best taste" was identical (homo-type) in many pairs, the "taste profiles" were often similar, and the temporal courses of taste responses resembled each other in more than one half of the pairs. A possible role of hetero-type pairs was discussed. (4) In a small number of cases, troughs were observed in the CCs, suggesting the involvement of local inhibitory neurons. In conclusion we point out that nearby neurons are prone to CA, if (1) their best tastes are identical, (2) their taste profiles are similar, and (3) the temporal patterns of taste response resemble each other.

An imaging system for monitoring receptive field dynamics

The paper describes a computerized method, termed receptive field imaging (RFI), for the rapid mapping of multiple receptive fields and their respective sensitivity distributions. RFI uses random stimulation of multiple sites, in combination with an averaging procedure, to extract the relative contribution from each of the stimulated sites. Automated multi-electrode stimulation and recording, with spike detection and counting, are performed on-line by the RFI programme. Direct user interpretation of receptive field changes is made possible by a user-friendly graphic interface. A series of imaging experiments was carried out to evaluate the functional capacity of the system. RFI was tested on the receptive fields in the nociceptive withdrawal reflex (NWR) system in the rat. RFI replicates the results obtained with conventional methods and allows the display of receptive field dynamics induced by topical spinal cord application of morphine and naloxone on a minute-to-minute time scale. Data variance was estimated, and proved to be small enough to yield a stable representation of the receptive field, thereby achieving a high sensitivity in dynamic imaging experiments. The large number of stimulation and registration sites that can be monitored in parallel permits detailed network analysis of synaptic sets, corresponding to 'connection weights' between individual neurones.

Cortical and subcortical contributions to activity-dependent plasticity in primate somatosensory cortex

After manipulations of the periphery that reduce or enhance input to the somatosensory cortex, affected parts of the body representation will contract or expand, often over many millimeters. Various mechanisms, including divergence of preexisting connections, expression of latent synapses, and sprouting of new synapses, have been proposed to explain such phenomena, which probably underlie altered sensory experiences associated with limb amputation and peripheral nerve injury in humans. Putative cortical mechanisms have received the greatest emphasis but there is increasing evidence for substantial reorganization in subcortical structures, including the brainstem and thalamus, that may be of sufficient extent to account for or play a large part in representational plasticity in somatosensory cortex. Recent studies show that divergence of ascending connections is considerable and sufficient to ensure that small alterations in map topography at brainstem and thalamic levels will be amplified in the projection to the cortex. In the long term, slow, deafferentation-dependent transneuronal atrophy at brainstem, thalamic, and even cortical levels are operational in promoting reorganizational changes, and the extent to which surviving connections can maintain a map is a key to understanding differences between central and peripheral deafferentation.

Segregation of nociceptive and non-nociceptive networks in the squirrel monkey somatosensory thalamus

The somatosensory thalamus (here we examine neurons in the caudal cutaneous portion of ventral posterior lateral nucleus, VPL) is composed of a somatotopic arrangement of anteroposteriorly oriented rods. Each rod is a collection of neurons with homogeneous properties that relay sensory information to specific cortical columns. We developed a multi-electrode recording technique, using fixed-geometry four-tip electrodes that allow simultaneous recordings from small populations of neurons (4-11), in a approximately 150 x 150 x 150 micrometer(3) volume of brain tissue (i.e., the approximate diameter of rods) and study of their spatiotemporal interactions. Due to the fixed geometry of the four-tip electrodes, the relative locations of these neurons can be determined, and due to the simultaneity of the recordings, their spike-timing coordination can be calculated. With this method, we demonstrate the existence of two distinct functional networks: nociceptive and non-nociceptive networks. The population dynamics of these two types of networks are different: cross-correlations in each type of network were different in direction and strength, were a function of the distance between neurons, had an opponent organization for nociceptive networks and a non-opponent organization for non-nociceptive networks, and rapidly changed under different stimulus conditions independent of changes in firing rates. A simple neural network model mimicked these physiological findings, demonstrating the necessity of inhibitory interneurons and different amounts of afferent input synchronization. Based on these results, we conclude that the somatosensory thalamus is composed of two modules, nociceptive and non-nociceptive rods, and that the response dynamics differences between these modules are due to spatiotemporal differences of their afferent inputs.

Patterns in the brain. Neuronal population coding in the somatosensory system

The aim of this article is to review some basic principles of neural coding, with an emphasis on mechanisms of stimulus representation in ensembles of neurons. The theory of "across-neuron response patterns" (ANRPs), first suggested by Thomas Young (1802) and fully developed by Robert Erickson (1963-2000), is summarized and applied to the problem of coding in primary afferent fibers and cortical neurons of the somatosensory system. The basic premise of the theory is that precise information about stimulus features cannot be encoded by single neurons, but is encoded by patterns of activity across populations of neurons. Different stimuli produce uniquely different patterns of ensemble activity (ANRPs)-discrimination between two stimuli is based on the absolute difference in total amount of activity (neural mass difference) of the ANRPs for those stimuli. Review of the literature shows that ANRPs and related population codes can accurately represent and differentiate among various stimulus parameters that cannot be distinguished by single neurons alone. Finally, the behavior of neuronal ensembles can be used to account for the sensory-perceptual changes associated with plasticity of thalamocortical circuits following selective sensorimotor deprivation or experience.

Principal component analysis of neuronal ensemble activity reveals multidimensional somatosensory representations

Principal components analysis (PCA) was used to define the linearly dependent factors underlying sensory information processing in the vibrissal sensory area of the ventral posterior medial (VPM) thalamus in eight awake rats. Ensembles of up to 23 single neurons were simultaneously recorded in this area, either during long periods of spontaneous behavior (including exploratory whisking) or controlled deflection of single whiskers. PCA rotated the matrices of correlation between these n neurons into a series of n uncorrelated principal components (PCs), each successive PC oriented to explain a maximum of the remaining variance. The fact that this transformation is mathematically equivalent to the general Hebb algorithm in linear neural networks provided a major rationale for performing it here on data from real neuronal ensembles. Typically, most information correlated across neurons in the ensemble was concentrated within the first 3-8 PCs. Each of these was found to encode distinct, and highly significant informational factors. These factor encodings were assessed in two ways, each making use of fact that each PC consisted of a matrix of weightings, one for each neuron. First, the neurons were rank ordered according to the locations of the central whiskers in their receptive fields, allowing their weightings within different PCs to be viewed as a function of their position within the whisker representation in the VPM. Each PC was found to define a distinctly different topographic mapping of the cutaneous surface. Next, the PCs were used to weight-sum the neurons' simultaneous activities to create population vectors (PVs). Each PV consisted of a single continuous time series which represented the expression of each PC's 'magnitude' in response to stimulation of different whiskers, or during behavioral events such as active tactile whisking. These showed that each PC functioned as a feature detector capable of selectively predicting significant sensory or behavioral events with far greater statistical reliability than could any single neuron. The encoding characteristics of the first few PCs were remarkably consistent across all animals and experimental conditions, including both spontaneous exploration and direct sensory stimulation: PC1 positively weighted all neurons, mainly according to their covariance. Thus it encoded global magnitude of ensemble activity, caused either by combined sensory inputs or intrinsic network activity, such as spontaneous oscillations. PC2 encoded spatial position contrast, generally in the rostrocaudal dimension, across the whole cutaneous surface represented by the ensemble. PC3 more selectively encoded contrast in an orthogonal (usually dorsoventral) dimension. A variable number of higher numbered PCs encoded local position contrast within one or more smaller regions of the cutaneous surface. The remaining PCs typically explained residual 'noise', i.e. the uncorrelated variance that constituted a major part of each neuron's activity. Differences in behavioral or sensory experience produced relatively little in the PC weighting patterns but often changed the variance they explained (eigenvalues) enough to alter their ordering. These results argue that PCA provides a powerful set of tools for selectively measuring neural ensemble activity within multiple functionally significant 'dimensions' of information processing. As such, it redefines the 'neuron' as an entity which contributes portions of its variance to processing not one, but several tasks.

Sensory loss by selected whisker removal produces immediate disinhibition in the somatosensory cortex of behaving rats

This study used extracellular unit recordings in behaving animals to evaluate thalamocortical response transformations in the rat whisker-barrel system. Based on previous acute studies using controlled whisker stimulation, we hypothesized that in a cortical barrel adjacent (non-principal) whiskers exert a net inhibitory effect. In contrast, in thalamic barreloid neurons, the effects of neighboring whiskers should be net facilitatory. We evaluated these hypotheses by recording unit activity at 21 sites in 17 animals trained to explore a wire mesh screen with their whiskers. In the middle of the recording session, selected vibrissae were clipped close to the skin surface. The absence of whiskers surrounding the principal whisker was associated with a mean 20% increase in cortical activity and, conversely, a 37% decrease in the thalamic activity. Removal of the principal whisker resulted in a 50% decrease in cortical unit firing. Findings are consistent with the idea that, in the behaving animal, each barrel uses multi-whisker thalamic inputs and local inhibitory circuitry to sharpen the receptive field properties of its constituent neurons. Cortical disinhibition as a consequence of selective whisker removal is likely to be an important factor underlying altered receptive field properties in sensory-deprived animals.

Sensory loss by selected whisker removal produces immediate disinhibition in the somatosensory cortex of behaving rats

This study used extracellular unit recordings in behaving animals to evaluate thalamocortical response transformations in the rat whisker-barrel system. Based on previous acute studies using controlled whisker stimulation, we hypothesized that in a cortical barrel adjacent (non-principal) whiskers exert a net inhibitory effect. In contrast, in thalamic barreloid neurons, the effects of neighboring whiskers should be net facilitatory. We evaluated these hypotheses by recording unit activity at 21 sites in 17 animals trained to explore a wire mesh screen with their whiskers. In the middle of the recording session, selected vibrissae were clipped close to the skin surface. The absence of whiskers surrounding the principal whisker was associated with a mean 20% increase in cortical activity and, conversely, a 37% decrease in the thalamic activity. Removal of the principal whisker resulted in a 50% decrease in cortical unit firing. Findings are consistent with the idea that, in the behaving animal, each barrel uses multi-whisker thalamic inputs and local inhibitory circuitry to sharpen the receptive field properties of its constituent neurons. Cortical disinhibition as a consequence of selective whisker removal is likely to be an important factor underlying altered receptive field properties in sensory-deprived animals.

Functional independence of layer IV barrels in rodent somatosensory cortex

Layer IV of rodent primary somatosensory cortex is characterized by an array of whisker-related groups of neurons, known as "barrels." Neurons within each barrel respond best to a particular whisker on the contralateral face, and, on deflection of adjacent whiskers, display relatively weak excitation followed by strong inhibition. A prominent hypothesis for the processing of vibrissal information within layer IV is that the multiwhisker receptive fields of barrel neurons reflect interconnections among neighboring barrels. An alternative view is that the receptive field properties of barrel neurons are derived from operations performed on multiwhisker, thalamic inputs by local circuitry within each barrel, independently of neighboring barrels. Here we report that adjacent whisker-evoked excitation and inhibition within a barrel are unaffected by ablation of the corresponding adjacent barrel. In supragranular neurons, on the other hand, excitatory responses to the ablated barrel's associated whisker are substantially reduced. We conclude that the layer IV barrels function as an array of independent parallel processors, each of which individually transforms thalamic afferent input for subsequent processing by horizontally interconnected circuits in other layers.

Simultaneous reorganization in thalamocortical ensembles evolves over several hours after perioral capsaicin injections

Reorganization of the somatosensory system was quantified by simultaneously recording from single-unit neural ensembles in the whisker regions of the ventral posterior medial (VPM) nucleus of the thalamus and the primary somatosensory (SI) cortex in anesthetized rats before, during, and after injecting capsaicin under the skin of the lip. Capsaicin, a compound that excites and then inactivates a subset of peripheral C and Adelta fibers, triggered increases in spontaneous firing of thalamocortical neurons (10-15 min after injection), as well as rapid reorganization of the whisker representations in both the VPM and SI. During the first hour after capsaicin injection, 57% of the 139 recorded neurons either gained or lost at least one whisker response in their receptive fields (RFs). Capsaicin-related changes continued to emerge for >/=6 h after the injection: Fifty percent of the single-neuron RFs changed between 1-2 and 5-6 h after capsaicin injection. Most (79%) of these late changes represented neural responses that had remained unchanged in the first postcapsaicin mapping; just under 20% of these late changes appeared in neurons that had previously shown no plasticity of response. The majority of the changes (55% immediately after injection, 66% 6 h later) involved "unmasking" of new tactile responses. RF change rates were comparable in SI and VPM (57-49%). Population analysis indicated that the reorganization was associated with a lessening of the "spatial coupling" between cortical neurons-a significant reduction in firing covariance that could be related to distances between neurons. This general loss of spatial coupling, in conjunction with increases in spontaneous firing, may create a situation that is favorable for the induction of synaptic plasticity. Our results indicate that the selective inactivation of a peripheral nociceptor subpopulation can induce rapid and long-evolving (>/=6 h) shifts in the balance of inhibition and excitation in the somatosensory system. The time course of these processes suggest that thalamic and cortical plasticity is not a linear reflection of spinal and brainstem changes that occur following the application of capsaicin.

Real-time control of a robot arm using simultaneously recorded neurons in the motor cortex

To determine whether simultaneously recorded motor cortex neurons can be used for real-time device control, rats were trained to position a robot arm to obtain water by pressing a lever. Mathematical transformations, including neural networks, converted multineuron signals into 'neuronal population functions' that accurately predicted lever trajectory. Next, these functions were electronically converted into real-time signals for robot arm control. After switching to this 'neurorobotic' mode, 4 of 6 animals (those with > 25 task-related neurons) routinely used these brain-derived signals to position the robot arm and obtain water. With continued training in neurorobotic mode, the animals' lever movement diminished or stopped. These results suggest a possible means for movement restoration in paralysis patients.

Discrete domains mediate the light-responsive nuclear and cytoplasmic localization of Arabidopsis COP1

The Arabidopsis CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) protein plays a critical role in the repression of photomorphogenesis during Arabidopsis seedling development. We investigated the control of COP1 partitioning between nucleus and cytoplasm, which has been implicated in the regulation of COP1 activity, by using fusion proteins between COP1 and beta-glucuronidase or the green fluorescent protein. Transient expression assays using onion epidermal cells and data from hypocotyl cells of stably transformed Arabidopsis demonstrated that COP1 carries a single, bipartite nuclear localization signal that functions independently of light. Nuclear exclusion was mediated by a novel and distinct signal, bordering the zinc-finger and coiled-coil motifs, that was able to redirect a heterologous nuclear protein to the cytoplasm. The cytoplasmic localization signal functioned in a light-independent manner. Light regulation of nuclear localization was reconstituted by combining the individual domains containing the nuclear localization signal and the cytoplasmic localization signal; the WD-40 repeat domain of COP1 was not required. However, phenotypic analysis of transgenic seedlings suggested that the constitutively nuclear-localized WD-40 repeat domain was able to mimic aspects of COP1 function, as indicated by exaggerated hypocotyl elongation under light conditions.

Intrinsic firing patterns and whisker-evoked synaptic responses of neurons in the rat barrel cortex

We have used whole cell recording in the anesthetized rat to study whisker-evoked synaptic and spiking responses of single neurons in the barrel cortex. On the basis of their intrinsic firing patterns, neurons could be classified as either regular-spiking (RS) cells, intrinsically burst-spiking (IB) cells, or fast-spiking (FS) cells. Some recordings responded to current injection with a complex spike pattern characteristic of apical dendrites. All cell types had high rates of spontaneous postsynaptic potentials, both excitatory (EPSPs) and inhibitory (IPSPs). Some spontaneous EPSPs reached threshold, and these typically elicited only single action potentials in RS cells, bursts of action potentials in FS cells and IB cells, and a small, fast spike or a complex spike in dendrites. Deflection of single whiskers evoked a fast initial EPSP, a prolonged IPSP, and delayed EPSPs in all cell types. The intrinsic firing pattern of cells predicted their short-latency whisker-evoked spiking patterns. All cell types responded best to one or, occasionally, two primary whiskers, but typically 6-15 surrounding whiskers also generated significant synaptic responses. The initial EPSP had a relatively fixed amplitude and latency, and its amplitude in response to first-order surrounding whiskers was approximately 55% of that induced by the primary whisker. Second- and third-order surrounding whiskers evoked responses of approximately 27 and 12%, respectively. The latency of the initial EPSP was shortest for the primary whiskers, longer for surrounding whiskers, and varied with the neurons' depth below the pia. EPSP latency was shortest in the granular layer, longer in supragranular layers, and longest in infragranular layers. The receptive field size, defined as the total number of fast EPSP-inducing whiskers, was independent of each cell's intrinsic firing type, its subpial depth, or the whisker stimulus parameters. On average, receptive fields included >10 whiskers. Our results show that single neurons integrate rapid synaptic responses from a large proportion of the mystacial vibrissae, and suggest that the whisker-evoked responses of barrel neurons are a function of both synaptic inputs and intrinsic membrane properties.