Neuronal Ensemble Activity Research Articles

Neural basis of localized and delocalized fMRI patterns

What connections can we draw between fMRI and neural firing patterns? The basic requirement of differencing in neuroimaging has limitations because subtraction of baseline activity removes an important part of the total activity 1. Our prior studies have quantitatively revealed that most neurons in the ensemble (in a voxel) contribute differently before, during, and after the stimulation 2. Since the ensemble activity of pyramidal neurons in layer 4 depends on sensory (or localized) input by the thalamus and global (or delocalized) input from other cortical regions, delocalized signals from a global workspace may affect localized stimulus-induced responses. To test the hypothesis that delocalized signals, which have been proposed to include subjective contributions 3, can modulate localized signals we conducted fMRI and electrophysiological studies in rats under varied conditions of anesthesia and forepaw stimulation. The fMRI experiments of -chloralose (40 mg/kg/hr) and halothane (0.7%) anesthetized rats were conducted on 7.0 T or 9.4 T systems using calibrated fMRI 1, 2. The electrical activity was measured by Tungsten microelectodes. The extracellular signals were filtered to separate field and action potentials (FP, AP), where the AP data were binned for spiking frequency. While the fMRI data from both baselines (i.e., halothane and -chloralose) showed sensory-induced changes in the contralateral primary somatosensory and motor areas, there were increased delocalized activities observed with (light) halothane anesthesia. These regions were ipsilateral primary somatosensory, contralateral secondary somatosensory, as well as perirhinal and retrosplenial agranular areas. In contrast signals from delocalized regions were notably absent with (deep) -chloralose anesthesia. These results were supported by correlated changes in AP and FP time courses. The spiking frequency distributions measured from the most activated foci in the contralateral (and ipsilateral) primary somatosensory area(s). The contralateral distributions with (deep) -chloralose anesthesia showed a significant shift from low to high frequency values upon stimulation. The contralateral distribution shift was less noteworthy with (light) halothane anesthesia. The distributions for the baselines with halothane and -chloralose were significantly different, in which the degrees of global inputs assigned to higher frequencies dominated under (light) halothane anesthesia. The contralateral distributions upon stimulation with halothane and -chloralose were also significantly different, suggesting that under the (light) halothane anesthesia level the global inputs dominated whereas local changes were dominated under the (deep) -chloralose anesthesia. Since neuronal populations in different regions do not function as modules, the resultant fMRI or electrophysiological activity patterns arise from interactions of afferent and efferent connections – the latter of which was modulated by anesthesia in this study. These results provide the initial steps to explore the interactions of localized and delocalized underpinnings of the concept of the global workspace model 3 (See Figure 1).

Role of Gap Junctions in Synchronized Neuronal Oscillations in the Inferior Olive

Inferior olivary (IO) neurons are electrically coupled through gap junctions and generate synchronous subthreshold oscillations of their membrane potential at a frequency of 1-10 Hz. Whereas the ionic mechanisms of these oscillatory responses are well understood, their origin and ensemble properties remain controversial. Here, the role of gap junctions in generating and synchronizing IO oscillations was examined by combining intracellular recordings with high-speed voltage-sensitive dye imaging in rat brain stem slices. Single cell responses and ensemble synchronized responses of IO neurons were compared in control conditions and in the presence of 18beta-glycyrrhetinic acid (18beta-GA), a pharmacological gap junction blocker. Under our experimental conditions, 18beta-GA had no adverse effects on intrinsic electroresponsive properties of IO neurons, other than the block of gap junction-dependent dye coupling and the resulting change in cells' passive properties. Application of 18beta-GA did not abolish single cell oscillations. Pharmacologically uncoupled IO neurons continued to oscillate with a frequency and amplitude that were similar to those recorded in control conditions. However, these oscillations were no longer synchronized across a population of IO neurons. Our optical recordings did not detect any clusters of synchronous oscillatory activity in the presence of the blocker. These results indicate that gap junctions are not necessary for generating subthreshold oscillations, rather, they are required for clustering of coherent oscillatory activity in the IO. The findings support the view that oscillatory properties of single IO neurons endow the system with important reset dynamics, while gap junctions are mainly required for synchronized neuronal ensemble activity.

Inhibitory avoidance learning altered ensemble activity of amygdaloid neurons in rats

In this study, we examined single-unit activity in the amygdala before and after a rat had acquired an inhibitory avoidance task. Long-Evans rats with microwires chronically implanted into the central nucleus (CeA) or basolateral complex (BLC) of the amygdala were acclimatized to the apparatus of a step-through inhibitory avoidance task for three sessions. On the fourth session, rats in the experimental group received an inescapable footshock (3 mA, 1 s) as they stepped from the lit side into the dark side of the task apparatus, whereas rats in the control group received the same amount of shock on a different apparatus. All rats were tested for retention in the task apparatus 1 day after shock training. The experimental rats showed better retention than the controls as they stayed longer in the lit side. Ensemble unit activities were recorded in the amygdala nuclei from the indwelling wire bundles during the acclimation and test sessions. The data collected from well-isolated amygdala units showed that neuronal discharge habituated from the first to the third acclimation session. In the test session, the experimental group, but not the control group, showed elevated firing rates in the CeA or BLC neurons located on either side of the brain. These findings provide the first piece of evidence showing that learning of an inhibitory avoidance task leads to an increase in amygdala neuronal discharges during a retention test.

Theta rhythm of navigation: Link between path integration and landmark navigation, episodic and semantic memory

Five key topics have been reverberating in hippocampal-entorhinal cortex (EC) research over the past five decades: episodic and semantic memory, path integration ("dead reckoning") and landmark ("map") navigation, and theta oscillation. We suggest that the systematic relations between single cell discharge and the activity of neuronal ensembles reflected in local field theta oscillations provide a useful insight into the relationship among these terms. In rats trained to run in direction-guided (1-dimensional) tasks, hippocampal cell assemblies discharge sequentially, with different assemblies active on opposite runs, i.e., place cells are unidirectional. Such tasks do not require map representation and are formally identical with learning sequentially occurring items in an episode. Hebbian plasticity, acting within the temporal window of the theta cycle, converts the travel distances into synaptic strengths between the sequentially activated and unidirectionally connected assemblies. In contrast, place representations by hippocampal neurons in 2-dimensional environments are typically omnidirectional, characteristic of a map. Generation of a map requires exploration, essentially a dead reckoning behavior. We suggest that omnidirectional navigation through the same places (junctions) during exploration gives rise to omnidirectional place cells and, consequently, maps free of temporal context. Analogously, multiple crossings of common junction(s) of episodes convert the common junction(s) into context-free or semantic memory. Theta oscillation can hence be conceived as the navigation rhythm through both physical and mnemonic space, facilitating the formation of maps and episodic/semantic memories.

Alteration of neuronal firing properties after in vivo experience in a FosGFP transgenic mouse.

Identifying the cells and circuits that underlie perception, behavior, and learning is a central goal of contemporary neuroscience. Although techniques such as lesion analysis, functional magnetic resonance imaging, 2-deoxyglucose studies, and induction of gene expression have been helpful in determining the brain areas responsible for particular functions, these methods are technically limited. Currently, there is no method that allows for the identification and electrophysiological characterization of individual neurons that are associated with a particular function in living tissue. We developed a strain of transgenic mice in which the expression of the green fluorescent protein (GFP) is controlled by the promoter of the activity-dependent gene c-fos. These mice enable an in vivo or ex vivo characterization of the cells and synapses that are activated by particular pharmacological and behavioral manipulations. Cortical and subcortical fosGFP expression could be induced in a regionally restricted manner after specific activation of neuronal ensembles. Using the fosGFP mice to identify discrete cortical areas, we found that neurons in sensory-spared areas rapidly regulate action potential threshold and spike frequency to decrease excitability. This method will enhance our ability to study the way neuronal networks are activated and changed by both experience and pharmacological manipulations. In addition, because activated neurons can be functionally characterized, this tool may enable the development of better pharmaceuticals that directly affect the neurons involved in disease states.

Differential morphine effects on short- and long-latency laser-evoked cortical responses in the rat

Evoked potential and ensemble neuronal activities were used to study the responses of the primary sensorimotor cortex (SmI) to noxious CO 2 laser irradiation of the middle part of the tail in conscious behaving rats. The hypothesis that systemic morphine treatment preferentially attenuates the longer-latency laser-evoked cortical responses was also tested. Laser-evoked potentials (LEPs) and multiple single-unit (SU) activities were, respectively, recorded from chronically implanted stainless-steel screws and microwire electrodes. When examined individually, many SmI neurons showed either short-latency (<100 ms) or long-latency (300–500 ms) responses to laser irradiation. These neurons are widely dispersed in the tail region and hind limb region of the SmI, and also in the forelimb and head regions of the primary motor cortex (MI). Quantitatively, a higher percentage of neurons in the SmI tail region responded with shorter latencies compared to those in the SmI hind limb region or in the MI. When responses of many simultaneously recorded SU were examined together, short-latency and long-latency SmI ensemble activities matched the LEP1 and LEP2, respectively. Systemic morphine significantly attenuated the long-latency but not the short-latency component in both LEPs as well as ensemble neuronal activity in the tail region of the SmI. These effects were blocked by naloxone pretreatment.

Differential Corticostriatal Plasticity during Fast and Slow Motor Skill Learning in Mice

Background: Motor skill learning usually comprises “fast” improvement in performance within the initial training session and “slow” improvement that develops across sessions. Previous studies have revealed changes in activity and connectivity in motor cortex and striatum during motor skill learning. However, the nature and dynamics of the plastic changes in each of these brain structures during the different phases of motor learning remain unclear.Results: By using multielectrode arrays, we recorded the simultaneous activity of neuronal ensembles in motor cortex and dorsal striatum of mice during the different phases of skill learning on an accelerating rotarod. Mice exhibited fast improvement in the task during the initial session and also slow improvement across days. Throughout training, a high percentage of striatal (57%) and motor cortex (55%) neurons were task related; i.e., changed their firing rate while mice were running on the rotarod. Improvement in performance was accompanied by substantial plastic changes in both striatum and motor cortex. We observed parallel recruitment of task-related neurons in both structures specifically during the first session. Conversely, during slow learning across sessions we observed differential refinement of the firing patterns in each structure. At the neuronal ensemble level, we observed considerable changes in activity within the first session that became less evident during subsequent sessions.Conclusions: These data indicate that cortical and striatal circuits exhibit remarkable but dissociable plasticity during fast and slow motor skill learning and suggest that distinct neural processes mediate the different phases of motor skill learning.

The Olivo-Cerebellar Circuit as a Universal Motor Control System

The olivo-cerebellar system is one of the central networks organizing movement coordination in vertebrates. This system consists of three main anatomical structures: the inferior olive (IO), the cerebellar nuclei, and the cerebellar cortex. Over the last four decades studies in many laboratories have contributed significantly to our understanding of the electrophysiology of IO and cerebellar neurons. However, addressing the dynamic properties of olivo-cerebellar network requires information beyond the limits attainable using single cell recordings. Research at the neuronal network level is presently being implemented in order to determine the spatiotemporal activity profiles of ensemble neuronal activity using optical imaging of voltage-sensitive dye signals. We summarize here results of such type of study using the in vitro IO slices. The dynamic characteristic of the system is addressed using the imaging results as well as mathematical modeling of the network, as a heuristic tool. A computer-based control system based on such biological findings is outlined.

Layer-specific somatosensory cortical activation during active tactile discrimination.

Ensemble neuronal activity was recorded in each layer of the whisker area of the primary somatosensory cortex (SI) while rats performed a whisker-dependent tactile discrimination task. Comparison of this activity with SI activity evoked by similar passive whisker stimulation revealed fundamental differences in tactile signal processing during active and passive stimulation. Moreover, significant layer-specific functional differences in SI activity were observed during active discrimination. These differences could not be explained solely by variations in ascending thalamocortical input to SI. Instead, these results suggest that top-down influences during active discrimination may alter the overall functional nature of SI as well as layer-specific mechanisms of tactile processing.

Long-lasting novelty-induced neuronal reverberation during slow-wave sleep in multiple forebrain areas.

The discovery of experience-dependent brain reactivation during both slow-wave (SW) and rapid eye-movement (REM) sleep led to the notion that the consolidation of recently acquired memory traces requires neural replay during sleep. To date, however, several observations continue to undermine this hypothesis. To address some of these objections, we investigated the effects of a transient novel experience on the long-term evolution of ongoing neuronal activity in the rat forebrain. We observed that spatiotemporal patterns of neuronal ensemble activity originally produced by the tactile exploration of novel objects recurred for up to 48 h in the cerebral cortex, hippocampus, putamen, and thalamus. This novelty-induced recurrence was characterized by low but significant correlations values. Nearly identical results were found for neuronal activity sampled when animals were moving between objects without touching them. In contrast, negligible recurrence was observed for neuronal patterns obtained when animals explored a familiar environment. While the reverberation of past patterns of neuronal activity was strongest during SW sleep, waking was correlated with a decrease of neuronal reverberation. REM sleep showed more variable results across animals. In contrast with data from hippocampal place cells, we found no evidence of time compression or expansion of neuronal reverberation in any of the sampled forebrain areas. Our results indicate that persistent experience-dependent neuronal reverberation is a general property of multiple forebrain structures. It does not consist of an exact replay of previous activity, but instead it defines a mild and consistent bias towards salient neural ensemble firing patterns. These results are compatible with a slow and progressive process of memory consolidation, reflecting novelty-related neuronal ensemble relationships that seem to be context- rather than stimulus-specific. Based on our current and previous results, we propose that the two major phases of sleep play distinct and complementary roles in memory consolidation: pretranscriptional recall during SW sleep and transcriptional storage during REM sleep.

Heterogeneity of rhythmic suprachiasmatic nucleus neurons: Implications for circadian waveform and photoperiodic encoding.

Circadian rhythms in neuronal ensemble, subpopulations, and single unit activity were recorded in the suprachiasmatic nuclei (SCN) of rat hypothalamic slices. Decomposition of the ensemble pattern revealed that neuronal subpopulations and single units within the SCN show surprisingly short periods of enhanced electrical activity of approximately 5 h and show maximal activity at different phases of the circadian cycle. The summed activity accounts for the neuronal ensemble pattern of the SCN, indicating that circadian waveform of electrical activity is a composed tissue property. The recorded single unit activity pattern was used to simulate the responsiveness of SCN neurons to different photoperiods. We inferred predictions on changes in peak width, amplitude, and peak time in the multiunit activity pattern and confirmed these predictions with hypothalamic slices from animals that had been kept in a short or long photoperiod. We propose that the animals' ability to code for day length derives from plasticity in the neuronal network of oscillating SCN neurons.

Learning to control a brain-machine interface for reaching and grasping by primates.

Reaching and grasping in primates depend on the coordination of neural activity in large frontoparietal ensembles. Here we demonstrate that primates can learn to reach and grasp virtual objects by controlling a robot arm through a closed-loop brain–machine interface (BMIc) that uses multiple mathematical models to extract several motor parameters (i.e., hand position, velocity, gripping force, and the EMGs of multiple arm muscles) from the electrical activity of frontoparietal neuronal ensembles. As single neurons typically contribute to the encoding of several motor parameters, we observed that high BMIc accuracy required recording from large neuronal ensembles. Continuous BMIc operation by monkeys led to significant improvements in both model predictions and behavioral performance. Using visual feedback, monkeys succeeded in producing robot reach-and-grasp movements even when their arms did not move. Learning to operate the BMIc was paralleled by functional reorganization in multiple cortical areas, suggesting that the dynamic properties of the BMIc were incorporated into motor and sensory cortical representations.

Representation of the voice onset time (VOT) speech parameter in population responses within primary auditory cortex of the awake monkey.

Voice onset time (VOT) signifies the interval between consonant onset and the start of rhythmic vocal-cord vibrations. Differential perception of consonants such as /d/ and /t/ is categorical in American English, with the boundary generally lying at a VOT of 20-40 ms. This study tests whether previously identified response patterns that differentially reflect VOT are maintained in large-scale population activity within primary auditory cortex (A1) of the awake monkey. Multiunit activity and current source density patterns evoked by the syllables /da/ and /ta/ with variable VOTs are examined. Neural representation is determined by the tonotopic organization. Differential response patterns are restricted to lower best-frequency regions. Response peaks time-locked to both consonant and voicing onsets are observed for syllables with a 40- and 60-ms VOT, whereas syllables with a 0- and 20-ms VOT evoke a single response time-locked only to consonant onset. Duration of aspiration noise is represented in higher best-frequency regions. Representation of VOT and aspiration noise in discrete tonotopic areas of A1 suggest that integration of these phonetic cues occurs in secondary areas of auditory cortex. Findings are consistent with the evolving concept that complex stimuli are encoded by synchronized activity in large-scale neuronal ensembles.

Independent neuronal oscillators of the rat globus pallidus.

In vivo, neurons of the globus pallidus (GP) and subthalamic nucleus (STN) resonate independently around 70 Hz. However, on the loss of dopamine as in Parkinson's disease, there is a switch to a lower frequency of firing with increased bursting and synchronization of activity. In vitro, type A neurons of the GP, identified by the presence of I(h) and rebound depolarizations, fire at frequencies (<or=80 Hz) in response to glutamate pressure ejection, designed to mimic STN input. The profile of this frequency response was unaltered by bath application of the GABA(A) antagonist bicuculline (10 microM), indicating the lack of involvement of a local GABA neuronal network, while cross-correlations of neuronal pairs revealed uncorrelated activity or phase-locked activity with a variable phase delay, consistent with each GP neuron acting as an independent oscillator. This autonomy of firing appears to arise due to the presence of intrinsic voltage- and sodium-dependent subthreshold membrane oscillations. GABA(A) inhibitory postsynaptic potentials are able to disrupt this tonic activity while promoting a rebound depolarization and action potential firing. This rebound is able to reset the phase of the intrinsic oscillation and provides a mechanism for promoting coherent firing activity in ensembles of GP neurons that may ultimately lead to abnormal and pathological disorders of movement.

Chapter 27 Modulation of cortical excitability by weak direct current stimulation – technical, safety and functional aspects

Publisher Summary Achieving short- or even long-term neuroplastic functional modifications of cortical networks through the modulation of activity and excitability of neuronal ensembles has been the focus of many research activities in the last decades. The application of weak direct currents has been shown to elicit cortical excitability and activity shifts during, and after the end of stimulation in animals and humans, and thus could evolve as a promising technique in this field of research. In animals, intracortical or epidural electrodes have been used for direct current (DC) stimulation. Weak direct currents can be applied to humans non-invasively, transcranially and painlessly to induce focal, prolonged, but yet reversible shifts of cortical excitability, the duration and direction of which depend on stimulation duration and polarity. This chapter provides an overview of the basic and functional effects of weak direct current stimulation in animals and in humans. The chapter discusses the technical considerations and summarizes the available safety criteria that are expected to prevent harmful or unwanted effects of the stimulation.

On the amazing olivocerebellar system.

Over the last four decades elegant sets of a single-cell studies, originating from various research groups, have contributed significantly to our understanding of olivary and cerebellar physiology. Nevertheless questions relating to the dynamic properties of olivocerebellar network, as a system, remain unsolved. We may be reaching the limits of what can be learned using the single-cell recordings. Further research on this subject may require study of the spatiotemporal activity profiles of ensemble neuronal activity. This paper summarizes results obtained using voltage-sensitive dye imaging in inferior olive slices, and the use of mathematical modeling to address such activity profiles.

Parallel processing of serial movements in prefrontal cortex.

A key idea in Lashley's formulation of the problem of serial order in behavior is the postulated neural representation of all serial elements before the action begins. We studied this question by recording the activity of individual neurons simultaneously in small ensembles in prefrontal cortex while monkeys copied geometrical shapes shown on a screen. Monkeys drew the shapes as sequences of movement segments, and these segments were associated with distinct patterns of neuronal ensemble activity. Here we show that these patterns were present during the time preceding the actual drawing. The rank of the strength of representation of a segment in the neuronal population during this time, as assessed by discriminant analysis, predicted the serial position of the segment in the motor sequence. An analysis of errors in copying and their neural correlates supplied additional evidence for this code and provided a neural basis for Lashley's hypothesis that errors in motor sequences would be most likely to occur when executing elements that had prior representations of nearly equal strength.

A simulator for the analysis of neuronal ensemble activity: application to reaching tasks

A biologically based, multi-cortical computational model was developed to investigate how ensembles of neurons learn to execute a three-dimensional reaching task. The model produces outputs of spike trains that can be analyzed using a variety of multivariate analysis tools. Simulations show that after learning, the model neurons exhibit broad directional tuning that depend on the defined muscle directions of the simulated arm, and that these neurons form functional clusters within cortical areas. The utility of the model is demonstrated by testing arm movement prediction strategies using ensemble activity.

Neuronal responses in the frontal cortico-basal ganglia system during delayed matching-to-sample task: ensemble recording in freely moving rats.

Electrophysiological recording of single neuron activity has been conducted in rats to investigate the patterns of distributed neuronal responses in the frontal cortico-basal ganglia system that code information during a spatial-delayed matching-to-sample task (DMTSt). Rats were trained to press one of the two retractable levers presented randomly as a sample response. The first valid nose-poke after a delay resulted in the presentation of both levers. Pressing the same lever as the sample lever led to a water reward (match to sample), whereas pressing the lever opposite the sample lever resulted in a time-out (house light turned off). One hundred seventy-one neurons in the medial prefrontal cortex (mPFC), 51 in the dorsal striatum (STR), and 93 in the nucleus accumbens (NAc) were recorded during DMTSt. Different patterns of neuronal responses were observed during different behavioral episodes (sample, delay, and match phases) in all three recording areas. Space-related neuronal responses specific to the side of the lever pressed were more often found in the sample phase than in the match phase in all three areas studied. Neuronal responses specific to either correct or error trials were observed with similar percentages in the mPFC and the NAc, while the incidence of correct/error-coded activity in the STR was lower. Ensemble neuronal activity that coded sample versus match lever presses was observed in three out of five rats in sets of trials with similar speed and trajectory of lever press. The results reveal specific patterns of neural responses in the frontal cortico-basal ganglia system in rats during the DMTSt and suggest the existence of specific neuronal coding for different behavioral events associated with a learned short-term memory process.

Functional Convergence of Response Properties in the Auditory Thalamocortical System

One of the brain's fundamental tasks is to construct and transform representations of an animal's environment, yet few studies describe how individual neurons accomplish this. Our results from correlated pairs in the auditory thalamocortical system show that cortical excitatory receptive field regions can be directly inherited from thalamus, constructed from smaller inputs, and assembled by the cooperative activity of neuronal ensembles. The prevalence of functional thalamocortical connectivity is strictly governed by tonotopy, but connection strength is not. Finally, spectral and temporal modulation preferences in cortex may differ dramatically from the thalamic input. Our observations reveal a radical reconstruction of response properties from auditory thalamus to cortex, and illustrate how some properties are propagated with great fidelity while others are significantly transformed or generated intracortically.