Estimation of Thalamocortical and Intracortical Network Models from Joint Thalamic Single-Electrode and Cortical Laminar-Electrode Recordings in the Rat Barrel System

A new method is presented for extraction of population firing-rate models for both thalamocortical and intracortical signal transfer based on stimulus-evoked data from simultaneous thalamic single-electrode and cortical recordings using linear (laminar) multielectrodes in the rat barrel system. Time-dependent population firing rates for granular (layer 4), supragranular (layer 2/3), and infragranular (layer 5) populations in a barrel column and the thalamic population in the homologous barreloid are extracted from the high-frequency portion (multi-unit activity; MUA) of the recorded extracellular signals. These extracted firing rates are in turn used to identify population firing-rate models formulated as integral equations with exponentially decaying coupling kernels, allowing for straightforward transformation to the more common firing-rate formulation in terms of differential equations. Optimal model structures and model parameters are identified by minimizing the deviation between model firing rates and the experimentally extracted population firing rates. For the thalamocortical transfer, the experimental data favor a model with fast feedforward excitation from thalamus to the layer-4 laminar population combined with a slower inhibitory process due to feedforward and/or recurrent connections and mixed linear-parabolic activation functions. The extracted firing rates of the various cortical laminar populations are found to exhibit strong temporal correlations for the present experimental paradigm, and simple feedforward population firing-rate models combined with linear or mixed linear-parabolic activation function are found to provide excellent fits to the data. The identified thalamocortical and intracortical network models are thus found to be qualitatively very different. While the thalamocortical circuit is optimally stimulated by rapid changes in the thalamic firing rate, the intracortical circuits are low-pass and respond most strongly to slowly varying inputs from the cortical layer-4 population

Slow-wave activity in the S1HL cortex is contributed by different layer-specific field potential sources during development

Spontaneous correlated activity in cortical columns is criticalfor postnatal circuit refinement.We used spatial discriminationtechniques to explore the late maturation of synaptic pathways through the laminar distribution of the field potential (FP) generators underlying spontaneous and evoked activities ofthe S1HL cortex in juvenile (P14 –P16) and adult anesthetized rats. Juveniles exhibit an intermittent FP pattern resembling Up/Down states in adults, but with much reduced power and different laminar distribution. Whereas FPs in active periods are dominated by a layer VI generator in juveniles, in adults a developing multipart generatortakes over, displaying current sinks in middle layers (III–V). The blockade of excitatory transmission in upper and middle layers of adults recovered the juvenile-like FP profiles. In additiontothe layer VI generator, a gamma-specific generator in supragranular layers wasthe same in both age groups.While searching for dynamical coupling among generators in juveniles we found significant cross-correlation in one-half of the tested pairs, whereas excessive coherence hindered their efficient separation in adults. Also, potentials evoked by tactile and electrical stimuli showed different short-latency dipoles between the two age groups, and the juveniles lacked the characteristic long latency UP state currents in middle layers. In addition, the mean firing rate of neurons was lower in juveniles. Thus, cortical FPs originate from different intracolumnar segments as they become active postnatally. We suggest that although some cortical segments are active early postnatally, a functional sensory-motor control relies on a delayed maturation and network integration of synaptic connections in middle layers

Imaging fast neural activity in the brain with Electrical Impedance Tomography

Electrical impedance tomography (EIT) is an emerging medical imaging technique that can be employed to reconstruct the internal conductivity of an object from measurements made on the boundary. One proposed application for EIT is in head imaging, including imaging of impedance changes that occur with neuronal depolarisation and the imaging of acute stroke. The work of this thesis was aimed at advancing the imaging of brain pathology and function, with particular focus on the imaging of fast neural activity. Chapter 1 is a review of other brain imaging techniques, the principles of bioimpedance and EIT, and of previous impedance recordings of fast neural activity. Chapter 2 was a comparison of reconstruction algorithms for the detection of acute stroke using EIT in a realistic head-shaped tank, which entailed assessing boundary voltage rejection methods and quantitative analysis of image quality to determine the best reconstruction algorithms for the detection of acute stroke. In chapter 3, an EIT imaging dataset of fast neural activity, previously collected in a rat model, was assessed using second-level statistical parametric mapping (SPM) and the spatio-temporal propagation of the activity assessed and compared to the neurophysiological literature, which was reviewed in chapter 1. The analysis undertaken in chapter 3 illustrated some key methodological issues, which were addressed in chapter 4: new high resolution meshes and better optimised matrix inversion were employed, a new algorithm for electrode alignment was developed, also the use of SPM was validated by applying it to control datasets and through the use of statistical non-parametric mapping. Chapters 5 and 6 detail work attempting to cross-validate the use of EIT to image fast neural activity by employing a physiological stimulus, mechanical whisker displacement, and comparing the findings to other neurophysiological techniques recorded in the same model. Chapter 5 details work to validate the model and the impedance findings in this model as compared to previously published neurophysiological results, while chapter 6 details the use of other neurophysiological techniques for cross-validation

Probing for local synaptic connectivity in the adult mouse auditory cortex

Die grundlegenden Funktionsweisen wie unser Gehirn neue Informationen speichert beruht auf zwei Theorien. Erstens, Neuronen verbinden sich zu einem Netzwerk mit unterschiedlichen starken Verbindungen zueinander. Zweitens, äußere Einflüsse können diese Verbindungen verändern. Dadurch können sich neue Neuronen dem Netzwerk anschließen oder sich auch die Stärke der Verbindungen von bereits im Netzwerk vorhandenen Neuronen ändern. Um mehr über die Funktionsweise unseres Gehirns zu erfahren ist es absolut notwendig ein Schaltdiagramm corticaler Netzwerke zu haben der alle Verbindungen der Neuronen zueinander enthält. In dieser Arbeit untersuchten wir die synaptischen Verbindungen im auditorischen Cortex, eine Hirnregion wichtig für die Prozessierung von Tönen in verschiedenen assoziativen Lernparadigmen. Wir verwendete coronale Hirnschnitte von erwachsenen (8-14 Wochen alten) C57Bl6/6J Mäusen. Wir machten gleichzeitig ganz-Zell Ableitungen von vier Pyramidenzellen der Schicht 2/3 und der Schicht 5. Diese Methode erlaubt es die synaptische Verbindungsstärke zwischen diesen vier Neuronen zu messen. Wir fanden eine niedrige Verbindungswahrscheinlichkeit zwischen gleichzeitig gemessen Neuronen und weiters dass die Wahrscheinlichkeit einer bidirektionalen Verbindung zwischen zwei zufällig ausgewählte Neuronen höher war als erwartet. Die Verteilung der Stärken der synaptischen Verbindungen (der höchste Punkt der Amplitude des postsynaptischen Potentials (EPSP)) zeigte wenige starke Verbindungen. Dies deutet darauf hin, dass synaptische Verbindungen in lokalen Netzwerken von seltenen aber dafür starken Verbindungen dominiert werden. Wir fanden diese Verbindungen in beiden untersuchten Hirnschichten was darauf hindeutet, dass diese seltenen aber starken Verbindungen die Grundlage der Informationsverarbeitung in corticalen Netzwerken sein könnte. Wir fanden auch, dass die Variabilität der EPSP Amplitude entweder durch die veränderte Wahrscheinlichkeit der Neurotransmitterfreisetzung oder durch eine veränderte Anzahl der freigesetzten Neurotransmittervesikel auf der praesynaptischen Seite entstehen kann. Dies deutet darauf hin, dass beide Parameter Wahrscheinlichkeit und Anzahl unabhängig voneinander sind. Im zweiten Teil untersuchten wir den relativen Anteil des erregenden und inhibierenden Stroms zu Schicht 2/3 Neuronen mit präziser zeitlicher Auflösung. Die strikte Balance zwischen diesen Strömen ist kritisch für die Funktion corticaler Netzwerke und für die Anpassung der Eigenschaften corticaler Neuronen. Daher ist es notwendig herauszufinden, wie diese Balance aufrechterhalten wird. Wir stimulierten extrazellulär zwei unabhängige von einander zu Schicht 2/3 führende Nervenbahnen und maßen die synaptisch erregende und inhibierende Leitfähigkeit von Schicht 2/3 Neuronen. Wir fanden das die Balance zwischen erregenden und inhibierenden Strömen gleich groß für beide Nervenbahnen war und weiters, dass das Eintreffen der inhibierenden Ströme 2ms schneller war als das der erregenden Ströme. Dies deutet darauf hin, dass diese intercorticalen Nervenbahnen monosynaptisch mit Schicht 2/3 Neuronen verbunden sind. Interessanterweise fanden wir, dass fast 50 Prozent aller erregenden Ströme gleichzeitig mit zwei inhibitorischen Strömen eintrafen. Dies wurde vorher nicht beschrieben und könnte auf ein feedback oder feedforward Netzwerk lokaler Interneuronen zurückzuführen sein. In dritten Teil untersuchten wir ob optogenetische Manipulation während eines Verhaltensexperiments die Eigenschaften lokaler corticaler Netzwerke verändert. Wir expremierten Channelrhodopsin in Pyramidenzellen des auditorischen Cortex und photostimulierten diese Zellen während eines Verhaltensexperiments. Dadurch ist es uns Möglich festzustellen ob diese Neuronen ein bestimmtes Verhalten auslösen können. Wir untersuchten auch, ob sich die spezifischen Verbindungen dieser Nervenzellen während des Lernens einer Verhaltensaufgabe ändern. Wir fanden, dass die Stimulation durch Channelrhodopsin dazu verwendet werden kann, den Einfluss präzise getimter Aktionspotentiale auf das erlernen einer Verhaltensaufgabe zu untersuchen. Weiters führten wir ganz-Zell Ableitungen an Schicht 2/3 Neuronen von Mäusen die die Verhaltensaufgabe gelernt hatten durch. Wir fanden heraus, dass sich die Erregbarkeit von Neuronen in diesen Mäusen nicht von der Erregbarkeit von Neuronen in wildtyp Mäusen unterscheidet. Wir konnten keine Unterschiede in den EPSP Amplituden verbundenen Neuronen feststellen. Dies deutet darauf hin, dass die durch Channelrhodopsin ausgelöste Depolarisation nicht zu einer stärkeren Verbindung zwischen diesen Neuronen führt.The current mechanistic view on how the brain is able to store memories over long periods of time is based on two key concepts. The first is that memories are stored in the configuration of the connectivity of neurons in an assembly and in the set of synaptic weights of those connections; the second being that experience can mold and rewire the network connectivity and its synaptic weights. It becomes clear that the understanding of cortical function will always require the unraveling of synaptic connectivity in cortical circuits, that is, establishing the wiring diagrams between individual neurons. In the present work, a first effort was made in order to investigate the excitatory synaptic local circuitry in the adult mouse auditory cortex, a brain area critically involved in sound encoding required for proper associative motivational leaning. For this purpose, coronal whole-brain slices from adult (8-14 weeks old) C57Bl6/6J mice were used. Several simultaneous quadruple whole-cell recordings from layer 2/3 and layer 5 pyramidal neurons were made, a method that allows for quantitative functional measures of synaptic connectivity at the level of individually indentified neurons. It was observed that local circuitry is characterized by low connection probabilities between pairs of neurons, and that bidirectional connections are more common than expected in a random network. The distribution of synaptic connections strengths (defined as the peak of excitatory postsynaptic potential (EPSP) amplitude), has a heavier tail and implies that synaptic weight is concentrated among few synaptic connections. In both layers it was found the existence of rare but reliable large-amplitude synaptic connections, which are likely to contribute strongly to reliable information processing. Moreover, another central finding is that the EPSP amplitude variability can be ascribed to changes in the number of release presynaptic sites, or due to the probability of neurotransmission release, implying that modulations in synaptic transmission can be described by changes in both parameters independently. In the second part, the relative contribution, with precise temporal resolution, of excitatory and inhibitory drives that impinge onto layer 2/3 pyramidal neurons was investigated. The strict balance of these two synaptic conductances plays a critical role in cortical function and in the shaping of the tuning properties of cortical neurons. It is of utmost importance to describe how this balance is achieved and maintain. By means of intracortical extracellular stimulation of two independent but convergent input pathways into layer 2/3 neurons, synaptic conductances could be recorded and decomposed into their excitatory and inhibitory components. It was observed that excitatory/inhibitory balance is of equal magnitude in both stimulated pathways, and that on average a time difference less than 2 ms between the arrival of inhibition compared with the excitation favors for a monosynaptic nature of the stimulated intracortical projections that synapses onto the recorded layer 2/3 pyramidal neurons. On the other hand, it was observed that on almost half of the recorded neurons, the excitation conductance was flanked by two inhibitory barrages, a phenomenon never described so far. A possible feedback or feedforward inhibitory circuitry made by local interneurons could explain this observation. In the third part, one final question was posed: are the features that describe local synaptic circuitry changed upon optogenetic manipulation in a behavioural task? By means of combining expression of channelrhodopsin in auditory cortex pyramidal neurons, with their direct photostimulation in the context of a behaviour task, it was possible to assess the role of a subset of neurons in driving behaviour. Possible changes in their intrinsic interconnectivity were also studied upon learning. Though extremely labour intense, it was concluded that ChR2-based optical microstimulation can be used to dissect the impact of precisely timed action potentials in a subset of neurons in driving behaviour. Whole-cell recordings from layer 2/3 neurons from the subset of mice that reached correct performance levels were performed as before. It was observed that ChR2-expressing neurons in trained mice had similar intrinsic excitability features when compared with non-trained mice. The recorded EPSP amplitudes from pairs of connected neurons had similar rages among both groups of mice, indicating that periodic depolarizations of ChR2-positive neurons does not induce any synaptic scaling effect in these neurons

The Scientific Case for Brain Simulators

A key element of the European Union’s Human Brain Project (HBP) and other large-scale brain research projects is the simulation of large-scale model networks of neurons. Here, we argue why such simulations will likely be indispensable for bridging the scales between the neuron and system levels in the brain, and why a set of brain simulators based on neuron models at different levels of biological detail should therefore be developed. To allow for systematic refinement of candidate network models by comparison with experiments, the simulations should be multimodal in the sense that they should predict not only action potentials, but also electric, magnetic, and optical signals measured at the population and system levels

A model of delta frequency neuronal network activity and theta-gamma interactions in rat sensorimotor cortex in vitro

In recent decades, advances in electrophysiological techniques have enabled understanding of neuronal network activity, with in vitro brain slices providing insights into the mechanisms underlying oscillations at various frequency ranges. Understanding the electrical and neuro-pharmacological properties of brain networks using selective receptor modulators in native tissue allows to compare such properties with those in disease models (e.g. epilepsy and Parkinson’s). In vivo and in vitro studies have implicated M1 in execution of voluntary movements and, from both local network in vitro and whole brain in vivo perspectives. M1 has been shown to generate oscillatory activity at various frequencies, including beta frequency and nested theta and gamma oscillations similar to those of rat hippocampus. In vivo studies also confirmed slow wave oscillations in somatosensory cortex including delta and theta band activity. However, despite these findings, non-thalamic mechanisms underlying cortical delta oscillations remain almost unexplored. Therefore, we determined to explore these oscillations in vitro in M1 and S1. Using a modified sagittal plane slice preparation with aCSF containing neuroprotectants, we have greatly improved brain slice viability, enabling the generation and study of dual rhythms (theta and gamma oscillations) in deep layers (LV) of the in vitro sensorimotor slice (M1 and S1) in the presence of KA and CCh. We found that theta-gamma activity in M1 is led by S1 and that the amplitude of gamma oscillations was (phase-amplitude) coupled to theta phase in both regions. Oscillations were dependent on GABAAR, AMPAR and NMDAR and were augmented by DAR activation. Experiments using cut/reduced slices showed both M1 and S1 could be intrinsic generators of oscillatory activity. Delta oscillations were induced in M1 and S1 by maintaining a neuromodulatory state mimicking deep sleep, characterised by low dopaminergic and low cholinergic tone, achieved using DAR blockade and low CCh. Delta activity depends on GABAAR, GABABR and AMPAR but not NMDAR, and once induced was not reversible. Unlike theta-gamma activity, delta was led by M1, and activity took >20mins to develop in S1 after establishement of peak power in M1. Unlike M1, S1 alone was unable to support delta activity. Dopamine modulates network activity in M1 and it is known that fast-spiking interneurons are the pacemakers of network rhythmogenesis. Recent studies reported that dopamine (DA) controled Itonic in medium spiny, ventrobasal thalamus and nucleus accumbens neurons by modulation of GABARs or cation channels. In the current study, voltage-clamp whole cell recordings were performed in fast spiking interneurons (FS cells) in Layer V of M1. These recordings revealed tonic and phasic GABAAR inhibition and when DA was bath applied, a slow inward current (IDA) was induced. IDA was mediated by non-specific cationic TRPC channels following D2R-like receptor activation. Overall, my studies show the strong interdependence of theta-gamma rhythmogenesis between M1 and S1, dominanace of M1 at delta frequency and the crucial role of dopamine in controlling FS cell activity. Further exploration of these rhythms in models of pathological conditions such as Parkinson`s disease and Epilepsy may provide insights into network changes underlying these disease conditions

Stimulus-specific adaptation and deviance detection in the auditory cortex

Tesis por compendio de publicaciones[EN] Neurons in primary auditory cortex, thalamus and midbrain show stimulus-specific adaptation (SSA), a reduction in response to repetitive stimuli that does not affect neuronal responses to deviant tones. This has been proposed as a neuronal correlate of the mismatch negativity (MMN), a special evoked potential in response to deviant tones. However, three important requirements remain to be demonstrated in order to support the SSA-MMN link: (1) MMN is generated mainly within higher-order auditory cortical areas, whereas cortical SSA has only been recorded in A1 of different species. (2) MMN is a mid-long latency response, peaking between 100-200 ms in humans, whereas SSA has only been observed in early responses of A1 neurons. And finally, (3) neuronal responses to oddball stimulation have not been tested for deviance detection–enhancement of responses to deviant events—in addition to SSA, which is an essential property of any bona-fide mismatch response. In this study, I set specific objectives to investigate the relation between SSA and MMN, and moreover, I will test the Hierarchical Predictive Coding account for the MMN at the neuronal level, showing that single neuron responses to oddball stimulation represent prediction error, which is hierarchically organized along the auditory system

Sensorimotor integration in dystonia: pathophysiology and possible non-invasive approaches to therapy

Dystonia is a condition characterized by excessive and sustained muscle contractions causing abnormal postures and involuntary movements. The pathophysiology of dystonia includes loss of inhibition and abnormal plasticity in the somatosensory and motor systems; however, their contribution to the phenomenology of dystonia is still uncertain, and the possibility to target these abnormalities in an attempt to devise new treatments has not been thoroughly explored. This thesis describes how abnormal inhibition and plasticity in the somatosensory system of dystonic patients can be manipulated to ameliorate motor symptoms by means of peripheral stimulation. First, we characterized electrophysiological and behavioural markers of inhibition in the primary somatosensory cortex in a group of patients with idiopathic cervical dystonia (CD). Outcome measures included a) somatosensory temporal discrimination threshold (STDT); b) paired-pulse somatosensory evoked potentials (PP-SEP) tested with interstimulus intervals (ISIs) of 5, 20 and 40 ms; c) spatial somatosensory inhibition ratio (SIR) by measuring SEP interaction between simultaneous stimulation of the digital nerves in thumb and index finger; d) high-frequency oscillations (HFO) extracted from SEP obtained with stimulation of digital nerves of the index finger. This first investigation demonstrated that increased STDT in dystonia is related to reduced activity of inhibitory circuits within the primary somatosensory cortex, as reflected by reduced PP-SEP inhibition at ISI of 5 ms and reduced area of the late part of the HFO (l-HFO). In a second set of experiments, we applied high frequency repetitive somatosensory stimulation (HF-RSS), a patterned electric stimulation applied to the skin through surface electrodes, to the index finger in a sample of healthy subjects, with the aim to manipulate excitability and inhibition of the primary somatosensory (S1) and motor (M1) cortices. The former was assessed by the same methods used before (STDT, PP-SEP, HFO), with the addition of two psychophysical tasks designed to assess tactile spatial discrimination (grating orientation and bumps tests). Assessment of physiology of M1 was performed by means of short intracortical inhibition (SICI) assessed with TMS; this was performed with multiple conditioning stimulus (CS) intensities (70%, 80%, 90% of the active motor threshold) and with a insterstimulus interval (ISI) between conditioning and test stimulus of 3 ms. It was found that HF-RSS increased inhibition in S1 tested by PP-SEP and HFO; these changes were correlated with improvement in STDT. HF-RSS also enhanced bumps detection, while there was no change in grating orientation test. Finally, there was an increase in SICI, suggesting widespread changes in cortical sensorimotor interactions. Overall, these findings demonstrated that HF-RSS is able to modify the effectiveness of inhibitory circuitry in S1 and M1. The results obtained so far led us to hypothesize that HF-RSS could restore inhibition in dystonic patients, similar to what observed in healthy subjects. To test this, we applied HF-RSS on the index finger in a sample of patients with CD, and tested its effects with some of the outcome measures used before (STDT, PP-SEP, HFO, SIR, SICI). Unexpectedly, the results were opposite to what was predicted. Patients with CD showed a consistent, paradoxical response: after HF-RSS, they had reduced suppression of PP-SEP, as well as decreased HFO area and SICI, and increased SIR. STDT deteriorated after the stimulation protocol, and correlated with reduced measures of inhibition within S1 (PP-SEP at 5 ms ISI, l-HFO area). It was hypothesized that patients with CD have abnormal homeostatic inhibitory plasticity within the sensorimotor cortex and that this is responsible for their abnormal response to HF-RSS. Interestingly, this alteration in plasticity seems to be specific to idiopathic dystonia: when the same protocol was applied to patients with dystonia caused by lesions in the basal ganglia, the response was similar to healthy controls. This result suggests that reduced somatosensory inhibition and abnormal cortical plasticity are not strictly required for the clinical expression of dystonia, and that the abnormalities reported in idiopathic dystonia are not necessarily linked to basal ganglia damage. We then directed our attention to another form of peripheral electrical stimulation, delivered at low frequency (LF-RSS). Previous literature demonstrated that this pattern of stimulation had effects opposite to HF-RSS on tactile performance in healthy subjects; therefore, given the previous findings of abnormal response to HF-RSS in CD, we hypothesized that an inverse response might occur in these patients following LF-RSS as well. Our hypothesis was confirmed by the observation that LF-RSS, applied to the fingers in patients with CD, induced an increase in inhibition in the primary somatosensory and motor cortices. This was reflected by an improvement of STDT and an increase in PP-SEP suppression, HFO area and SICI. With this in mind, in the final project of the thesis, we tested the effects of HF-RSS and LF-RSS applied directly over two affected muscles in different groups of patients with focal hand dystonia (FHD), in an attempt to modulate involuntary muscle activity and, consequently, to ameliorate motor symptoms. Whereas HF-RSS was delivered synchronously over the two muscles, LF-RSS was given either synchronously or asynchronously. Outcome measures included a) PP-SEP obtained by direct stimulation of affected muscles, with ISIs of 5 and 30 ms; b) quantification of electromyographic (EMG) activity from tested muscles; c) SICI recorded from the affected muscles, with CS intensities ranging from 50% to 100% RMT and with an ISI of 3 ms; d) evaluation of hand function, assessed by the box and blocks test (BBT) and the nine-hole peg test (NHPT); e) SIR by measuring SEP interaction between simultaneous stimulation of the two muscles receiving repetitive stimulation. We confirmed the paradoxical response of dystonic patients to HF-RSS, which was reflected in decreased PP-SEP suppression and SICI and increased SIR. Importantly, this was paralleled by an increase in involuntary EMG activity and worse scores at the BBT and NHPT. This results were opposite when LF-RSS was delivered, either in its synchronous or asynchronous version, the latter being slightly more effective. Thus, LF-RSS was able to increase PP-SEP suppression and SICI, decrease SIR and reduce involuntary EMG activity, with consequent improvement in performance in the BBT and NHPT. Overall, our data provide novel insight into the neural mechanisms underlying loss of inhibition and deranged somatosensory plasticity in idiopathic dystonia and bring preliminary evidence that peripheral electrical stimulation can be used as a treatment in idiopathic focal hand dystonia