Dans le but de mieux comprendre le fonctionnement du cerveau nous devons examiner les domaines structuraux qui le composent, de la simple cellule à des régions entières du cerveau interconnectées. Cependant, bien que le fonctionnement d’une ou plusieurs cellules soit relativement bien connu, il n’y a que peu d’informations concernant les groupements de neurones interagissant fonctionnellement dans une même tâche, les réseaux neuronaux. De plus, l'activité équilibrée et concertée des réseaux excitateurs et inhibiteurs joue un rôle clé pour les intégrations corticales appropriées. Par ailleurs, il existe plusieurs outils afin d’enregistrer l’activité des réseaux excitateurs, ce qui n’est pas le cas pour les réseaux inhibiteurs. L’imagerie du colorant sensible au voltage (VSDI) est une technique permettant l’enregistrement de l’activité neuronale au moyen d’une émission de fluorescence proportionnelle au changement de potentiel de membrane. Par rapport aux autres techniques employant des électrodes, le VSDI permet l’enregistrement non invasif de l’activité de centaines de sites en même temps. Au cours des dernières décennies, le VSDI a été largement utilisé tant in vitro qu’in vivo pour étudier l’activité d’une cellule et des réseaux excitateurs. Néanmoins, en utilisant le VSDI, les recherches quant à l’activité des réseaux excitateurs ont été principalement réalisées par quantification d’émission de fluorescence en définissant des régions d’intérêts à des temps fixes, alors que l’activité inhibitrice n’a été évaluée qu’à l’échelle cellulaire. La première approche ne permet pas l’obtention de toutes les informations de la dynamique de propagation de la transmission glutamatergique du fait qu’elle ne prend en considération ni la vitesse ni la direction de propagation du signal. En revanche, la seconde approche n’offre pas la possibilité d’étudier l’activité du réseau inhibiteur ce qui serait toutefois important de définir du fait de la propagation spatiale extensive des interneurones au sein des aires corticales. Durant mon doctorat, le but de mon travail a été d’étudier en détail les réseaux neuronaux excitateurs et inhibiteurs de l’aire CA1 de l’hippocampe de souris à l’aide du VSDI. Pour les étudier de façon plus compréhensive, en collaboration avec une équipe de mathématicien, nous avons développé un algorithme permettant de mesurer la vitesse et la direction de propagation du signal VSDI, ce qui représente une nouvelle méthode pour analyser le flux optique. Après la validation réussie de l’algorithme avec des données de substitution pour tester sa précision, nous avons analysé deux séries d’expériences dans lesquelles l’activité des réseaux excitateurs a été manipulée soit par augmentation de l’intensité de stimulation passant de 10 à 30 Volts ou en bloquant la transmission GABAergique avec la picrotoxine, un antagoniste du récepteur GABAA. Les résultats de ces manipulations montrent une diminution significative de la vitesse alors que l’application de picrotoxine modifie de façon significative la direction de propagation, ce qui rend le signal de dépolarisation médié par le VSDI moins dispersé par rapport au contrôle. L’utilisation du VSDI a permis l’entière caractérisation des signaux hyperpolarisants médiés par les récepteurs GABAA dans toutes les sous-couches de CA1 (champ IPSP), offrant ainsi une nouvelle façon d’étudier les événements inhibiteurs à l’échelle d’un réseau. De plus, j’ai montré qu’en activant les récepteurs mGluR5, j’étais capable d’augmenter de façon durable le champ IPSP du VSDI, avec la durée et l’ampleur au niveau des sous-couches spécifiques de CA1. Globalement, je présente dans cette thèse de nouvelles méthodes et nouveaux résultats qui peuvent représenter une avancée dans la quête d’une meilleure compréhension des réseaux neuronaux, excitateurs et inhibiteurs, ce qui, espérons-le, pourra contribuer à réduire l’écart de connaissance entre l’activité d’une seule cellule et celle du comportement. / In order to better understand brain functioning we need to investigate all the structural domains present in it, from single cell to interconnected entire brain regions. However, while our knowledge in terms of single/few cells functioning is vast, very little is known about neuronal networks, which are interacting collections of neurons functionally related to the same task. Moreover, the balanced and concerted activity of excitatory and inhibitory networks plays a key role for proper cortical computations. However, while exist several tools to record excitatory networks activity, this is not the case for inhibitory networks. Voltage sensitive dye imaging (VSDI) is a technique that allows the recording of neuronal activity by mean of proportional emission of fluorescence according to changes in membrane potential. The advantage of using VSDI over other recording techniques using electrodes is that VSDI allows not invasive recording of neuronal activity from hundreds of sites at the same time. During my doctoral course I aimed at studying in detail excitatory and inhibitory neuronal networks in the CA1 area of mouse hippocampus with VSDI. To study excitatory networks more comprehensively, in collaboration with a team of mathematicians, we developed a mathematical algorithm that allowed measuring the velocity and the direction of spreading of the VSDI signal and it represents a new method to determine an optical flow. After successful validation of the algorithm with surrogate data to test its accuracy, we analysed two set of experiments in which network excitatory activity has been manipulated either by increasing Schaffer’s collaterals stimulation intensity or by blocking GABAergic transmission with the GABAA receptor antagonist picrotoxin in order to increase the depolarization in the CA1 region of the hippocampus. The results of these manipulations significantly decreased signal velocity whereas picrotoxin application significantly modified the direction of spreading, making the depolarization-mediated VSDI signal less dispersed compared to control. Using VSDI I was able to fully characterize GABAA receptor-mediated hyperpolarizing signals in all the CA1 sublayers (field IPSPs), thus providing a new way of monitoring inhibitory events at network level. Moreover, I found that the activation of mGluR5 receptors induced an increase in a long-lasting manner of the VSDI-recorded field IPSPs, with duration and magnitude that relied on the specific CA1 sublayer considered. Overall, my work shows new methodologies and new findings that may represent a step forward in the quest for a better understanding of neuronal networks, both excitatory as well as inhibitory, which hopefully can contribute to reduce the gap of knowledge between single cell activity and behaviour.

A principal goal of neural engineering is to control the activation of neural circuits across space and time. The ability to control neural circuits with surrogate inputs is needed for the development of clinical neural prostheses and the experimental interrogation of connectivity between brain regions. Electrical stimulation provides a clinically viable method for activating neural tissue and the emergence of optogenetic stimulation has redefined the limitations on stimulating neural tissue experimentally. However, it remains poorly understood how these tools activate complex neural circuits. The goal of this proposed project was to gain a greater understanding of how to control the activity of neural circuits in-vivo using a combination of experimental and computational approaches. Voltage sensitive dye imaging was used to observe the spatiotemporal activity within the rodent somatosensory cortex in response to systematically varied patterns of sensory, electrical, and optogenetic stimulation. First, the cortical response to simple patterns of sensory and artificial stimuli was characterized and modeled, revealing distinct neural response properties due to the differing synchrony with which the neural circuit was engaged. Then, we specifically designed artificial stimuli to improve the functional relevance of the resulting downstream neural responses. Finally, through direct optogenetic modulation of thalamic state, we demonstrate control of the nonlinear propagation of neural activity within the thalamocortical circuit. The combined experimental and computational approach described in this thesis provides a comprehensive description of the nonlinear dynamics of the thalamocortical circuit to surrogate stimuli. Together, the characterization, modeling, and overall control of downstream neural activity stands to inform the development of central nervous system sensory prostheses, and more generally provides the initial tools and framework for the control of neural activity in-vivo.

One of the most common risk factors for stroke is diabetes. Diabetics are 2 to 4 times more likely to have a stroke and are also significantly more likely to show poor functional recovery. In order to determine why diabetes is associated with poor stroke recovery, we tested the hypotheses that diabetes either exacerbates initial stroke damage, or inhibits neuronal circuit plasticity in surviving brain regions that is crucial for successful recovery. Type 1 diabetes was chemically induced in mice four weeks before receiving a targeted photothrombotic stroke in the right forelimb somatosensory cortex to model a chronic diabetic condition. Following stroke, a subset of diabetic mice were treated with insulin to determine if controlling blood glucose levels could improve stroke recovery. Consistent with previous studies, one behavioural test revealed a progressive improvement in sensory function of the forepaw in non-diabetic mice after stroke. By contrast, diabetic mice treated with and without insulin showed persistent deficits in sensori-motor forepaw function. To determine whether these different patterns of stroke recovery correlated with changes in functional brain activation, forepaw evoked responses in the somatosensory cortex were imaged using voltage sensitive dyes at 1 and 14 weeks after stroke. In both diabetic and non-diabetic mice that did not have a stroke, brief mechanical stimulation of the forepaw evoked a robust and near simultaneous depolarization in the primary (FLS1) and secondary somatosensory (FLS2) cortex. One week after stroke, forepaw-evoked responses had not been remapped in the peri-infarct cortex in both diabetic and non-diabetic mice. Fourteen weeks after stroke, forepaw evoked responses in non-diabetic mice re-emerged in the peri-infarct cortex whereas diabetic mice showed very little activation, reminiscent of the 1 week recovery group. Moreover, controlling hyperglycemia using insulin therapy failed to restore sensory evoked responses in the peri-infarct cortex. In addition to these differences in peri-infarct responsiveness, we discovered that stroke was associated with increased responsiveness in FLS2 of non-diabetic, but not diabetic or insulin treated mice. To determine the importance of FLS2 in stroke recovery, we silenced the FLS2 cortex and found that it re-instated behavioural impairments in stroke recovered mice, significantly more so than naïve mice that still had a functioning FLS1. Collectively, these results indicate that both diabetes and the secondary somatosensory cortex play an important role in determining the extent of functional recovery after ischemic cortical stroke. Furthermore, the fact that insulin therapy after stroke did not normalize functional recovery, suggests that prolonged hyperglycemia (before stroke) may induce pathological changes in the brain’s circulation or nervous system that cannot be easily reversed. / Graduate

No description available.

Lorsque les rongeurs explorent leur environnement, ils contactent activement les objets environnants avec leurs vibrisses qui sont ainsi défléchies selon des séquences spatiotemporelles complexes. Le système vibrissal est néanmoins capable d'extraire des informations pertinentes de ces stimulations pour générer un comportement tactile-dépendant. Une question se pose alors: Comment l’information multivibrissale globale est-elle encodée? La représentation corticale des vibrisses au sein du cortex somatosensoriel primaire (S1) du rongeur est dotée de structures anatomiquement remarquables, nommées "tonneaux", au niveau de la couche IV, qui sont organisées de la même manière que les vibrisses sur le museau de l’animal. A chaque "tonneau" correspond une colonne corticale, unité de traitement de l’information, qui reçoit en priorité les informations provenant la vibrisse principale (VP) correspondante. Des enregistrements extracellulaires réalisés dans notre équipe chez le rat ont révélé que les réponses des neurones du cortex S1 et du thalamus sont non seulement sensibles à la direction de déflection locale de leur VP, mais aussi à la direction d'un mouvement global de l’ensemble de leurs vibrisses. Afin de mieux comprendre la manière dont le réseau cortical traite ces scènes tactiles globales, nous avons construit un poste expérimental permettant d’enregistrer en temps réel l’activité du cortex S1 chez la souris par imagerie sensible au potentiel, tout en appliquant des stimuli tactiles complexes à l'aide d'une matrice de 24-stimulateurs vibrissaux. Nous avons de plus développé une méthode permettant d’aligner les données fonctionnelles ainsi obtenues par rapport la carte cytoarchitecturale du réseau cortical sous-jacent. Nous avons ainsi étudié premièrement la distribution spatiale de la sélectivité à la direction de déflection locale d’une vibrisse au niveau d’une colonne corticale. Les réponses aux différentes directions étaient localisées de manière légèrement distincte, autour du centre de la colonne, mais selon une organisation différente de celle précédemment décrite chez le rat. Nous avons montré par la suite que la sélectivité à la direction globale est spatialement organisée dans le cortex "en tonneaux" à l’échelle supra-colonnaire. Les colonnes correspondant aux vibrisses rostrales étant plus sélectives à la direction globale que les colonnes associées aux vibrisses caudales. En outre, les colonnes correspondant aux vibrisses dorsales répondent préférentiellement aux directions globales ventrales, tandis que les colonnes associées aux vibrisses ventrales répondent préférentiellement aux directions globales caudales. Enfin, les réponses induites par des directions globales caudo-ventrales étaient en moyenne les plus fortes pour toutes les colonnes. Nous avons montré que la répartition spatiale de la sélectivité à la direction globale peut être expliquée ni par la saillance prédominante de la position de départ de la séquence de stimulation multivibrissale (effet de bord), ni par la sommation linéaire des réponses aux déflections de quelques vibrisses. Les réponses aux stimulations globales de l'ensemble des vibrisses sont en effet fortement sous-linéaires, indépendamment de la direction de la stimulation. Brièvement, nous montrons ici que sortir de la vision classique du système vibrissal permet une meilleure compréhension de la façon dont les différentes caractéristiques des stimuli complexes sont traitées et de la manière dont les propriétés émergentes du cortex, comme la sélectivité à la direction globale, sont construites. / While rodents explore their environment they actively contact surrounding objects with their array of whiskers, resulting in a complex pattern of multiwhisker deflections. Despite this complexity, the whisker system is able to extract relevant information from the spatiotemporal sequence of deflections to generate touch-dependent behavior. The question that arises is: How is global multiwhisker information encoded? Whiskers are mapped onto layer 4 of the primary somatosensory cortex (S1) as discrete units named “barrels”. Each barrel-related vertical column processes information coming primarily from its corresponding principal whisker (PW). Previous experiments in our lab done with extracellular recordings have revealed that neurons in the rat S1 and thalamus not only show a preferred direction for the local deflection of the PW but also for the direction of a global motion across the whisker pad. To further understand how the cortical network processes global tactile scenes, we built a set-up that enables to perform voltage sensitive dye imaging of the mouse barrel cortex while applying precise tactile stimuli using a 24-multi-whisker stimulator. We further developed a technical method to map the recorded functional data onto the cortical structure. We first studied whether local direction selectivity is spatially distributed within the barrel-related column. Responses to different directions were slightly segregated on space close to the barrel center, but the distribution differed from the one previously described in rat S1, namely a pinwheel-like structure. We then showed that global direction selectivity is spatially organized in the barrel cortex. Columns related to rostral whiskers were more selective to the global direction than columns related to caudal whiskers. Moreover, the columns related to dorsal whiskers preferred ventral global directions, while the columns related to ventral whiskers preferred caudal global directions. Overall the responses to the caudo-ventral global directions were the strongest in average for all the columns. We showed that the spatial distribution of the global direction selectivity can be explained neither by the high salience of the starting position of the deflections on the whiskerpad (a border effect), nor by the linear summation of the responses to deflections of several whiskers. Responses to the global motion of the whisker array are indeed highly sublinear independently of the direction of stimulation. In conclusion, we show here that stepping aside from the classical view of the whisker-to-barrel cortex system allows a better understanding of how different features of complex stimuli are processed and how the emergent properties of the cortex, like the global direction selectivity, are built-up.

We can affirm to apprehend a system in its totality only when we know how it behaves under its natural operating conditions. However, in the face of the complexity of the world, science can only evolve by simplifications, which paradoxically hide a good deal of the very mechanisms we are interested in. On the other hand, scientific enterprise is very tightly related to the advances in technology and the latter inevitably influences the manner in which the scientific experiments are conducted. Due to this factor, experimental conditions which would have been impossible to bring into laboratory not more than 20 years ago, are today within our reach. This thesis investigates neuronal integrative processes by using a variety of theoretical and experimental techniques wherein the approximation of ecologically relevant conditions within the laboratory is the common denominator. The working hypothesis of this thesis is that neurons and neuronal systems, in the sensory and higher cortices, are specifically adapted, as a result of evolutionary processes, to the sensory signals most likely to be received under ecologically relevant conditions. In order to conduct the present study along this line, we first recorded movies with the help of two microcameras carried by cats exploring a natural environment. This resulted in a database of binocular natural movies that was used in our theoretical and experimental studies. In a theoretical study, we aimed to understand the principles of binocular disparity encoding in terms of spatio-temporal statistical properties of natural movies in conjunction with simple mathematical expressions governing the activity levels of simulated neurons. In an unsupervised learning scheme, we used the binocular movies as input to a neuronal network and obtained receptive fields that represent these movies optimally with respect to the temporal stability criterion. Many distinctive aspects of the binocular coding in complex cells, such as the phase and position encoding of disparity and the existence of unbalanced ocular contributions, were seen to emerge as the result of this optimization process. Therefore we conclude that the encoding of binocular disparity by complex cells can be understood in terms of an optimization process that regulates activities of neurons receiving ecologically relevant information. Next we aimed to physiologically characterize the responses of the visual cortex to ecologically relevant stimuli in its full complexity and compare these to the responses evoked by artificial, conventional laboratory stimuli. To achieve this, a state-of-the-art recording method, voltage-sensitive dye imaging was used. This method captures the spatio-temporal activity patterns within the millisecond range across large cortical portions spanning over many pinwheels and orientation columns. It is therefore very well suited to provide a faithful picture of the cortical state in its full complexity. Drifting bar stimuli evoked two major sets of components, one coding for the position and the other for the orientation of the grating. Responses to natural stimuli involved more complex dynamics, which were locked to the motion present in the natural movies. In response to drifting gratings, the cortical state was initially dominated by a strong excitatory wave. This initial spatially widespread hyper-excitatory state had a detrimental effect on feature selectivity. In contrast, natural movies only rarely induced such high activity levels and the onset of inhibition cut short a further increase in activation level. An increase of 30% of the movie contrast was estimated to be necessary in order to produce activity levels comparable to gratings. These results show that the operating regime within which the natural movies are processed differs remarkably. Moreover, it remains to be established to what extent the cortical state under artificial conditions represents a valid state to make inferences concerning operationally more relevant input. The primary visual cortex contains a dense web of neuronal connections linking distant neurons. However the flow of information within this local network is to a large extent unknown under natural stimulation conditions. To functionally characterize these long-range intra-areal interactions, we presented natural movies also locally through either one or two apertures and analyzed the effects of the distant visual stimulation on the local activity levels. The distant patch had a net facilitatory effect on the local activity levels. Furthermore, the degree of the facilitation was dependent on the congruency between the two simultaneously presented movie patches. Taken together, our results indicate that the ecologically relevant stimuli are processed within a distinct operating regime characterized by moderate levels of excitation and/or high levels of inhibition, where facilitatory cooperative interactions form the basis of integrative processes. To gather better insights into the motion locking phenomenon and test the generalizability of the local cooperative processes toward larger scale interactions, we resorted to the unequalized temporal resolution of EEG and conducted a multimodal study. Inspired from the temporal properties of our natural movies, we designed a dynamic multimodal stimulus that was either congruent or incongruent across visual and auditory modalities. In the visual areas, the dynamic stimulation unfolded neuronal oscillations with frequencies well above the frequency spectrum content of the stimuli and the strength of these oscillations was coupled to the stimuli's motion profile. Furthermore, the coupling was found to be stronger in the case where the auditory and visual streams were congruent. These results show that the motion locking, which was so far observed in cats, is a phenomenon that also exists in humans. Moreover, the presence of long-range multimodal interactions indicates that, in addition to local intra-areal mechanisms ensuring the integration of local information, the central nervous system embodies an architecture that enables also the integration of information on much larger scales spread across different modalities. Any characterization of integrative phenomena at the neuronal level needs to be supplemented by its effects at the behavioral level. We therefore tested whether we could find any evidence of integration of different sources of information at the behavioral level using natural stimuli. To this end, we presented to human subjects images of natural scenes and evaluated the effect of simultaneously played localized natural sounds on their eye movements. The behavior during multimodal conditions was well approximated by a linear combination of the behavior under unimodal conditions. This is a strong indication that both streams of information are integrated in a joint multimodal saliency map before the final motor command is produced. The results presented here validate the possibility and the utility of using natural stimuli in experimental settings. It is clear that the ecological relevance of the experimental conditions are crucial in order to elucidate complex neuronal mechanisms resulting from evolutionary processes. In the future, having better insights on the nervous system can only be possible when the complexity of our experiments will match to the complexity of the mechanisms we are interested in.

There is a difference between the raw sensory input to the brain and our stable perception of entities in the environment. A first approach to investigate perception is to study relationships between properties of currently presented stimuli and biological correlates of perceptual processes. However, it is known that such processes are not only dependent on the current stimulus. Sampling of information and the concurrent neuronal processing of stimulus content rely on contextual relationships in the environment, and between the environment and the body. Perceptual processes dynamically adjust to relevant context, such as the current task of the organism and its immediate history. To understand perception, we have to study how processing of current stimulus content is influenced by such contextual factors. This thesis investigates the influence of such factors on visual processing. In particular, it investigates effects of temporal context in early visual processing and the effect of task context in eye movement control. To investigate effects of contextual factors on early visual processing of current stimulus content, we study neuronal processing of visual information in the primary visual cortex. We use real-time optical imaging with voltage sensitive dyes to capture neuronal population activity in the millisecond range across several millimeters of cortical area. To characterize the cortical layout concerning the mapping of orientation, previous to further investigations, we use smoothly moving grating stimuli. Investigating responses to this stimulus type systematically, we find independent encoding of local contrast and orientation, and a direct mapping of current stimulus content onto cortical activity (Study 1). To investigate the influence of the previous stimulus as context on processing of current stimulus content, we use abrupt visual changes in sequences of modified natural images. In earlier studies, investigating relatively fast timescales, it was found that the primary visual cortex continuously represents current input (ongoing encoding), with little interference from past stimuli. We investigate whether this coding scheme generalizes to cases in which stimuli change more slowly, as frequently encountered in natural visual input. We use sequences of natural scene contours, comprised of vertically and horizontally filtered natural images, their superpositions, and a blank stimulus, presented with 10 or 33 Hz. We show that at the low temporal frequency, cortical activity patterns do not encode the present orientations but instead reflect their relative changes in time. For example, when a stimulus with horizontal orientation is followed by the superposition of both orientations, the pattern of cortical activity represents the newly added vertical orientations instead of the full sum of orientations. Correspondingly, contour removal from the superposition leads to the representation of orientations that have disappeared rather than those that remain. This is in sharp contrast to more rapid sequences for which we find an ongoing representation of present input, consistent with earlier studies. In summary, we find that for slow stimulus sequences, populations of neurons in the primary visual cortex are no longer tuned to orientations within individual stimuli but instead represent the difference between consecutive stimuli. Our results emphasize the influence of the temporal context on early visual processing and consequentially on information transmission to higher cortical areas (Study 2). To study effects of contextual factors on the sampling of visual information, we focus on human eye movement control. The eyes are actively moved to sample visual information from the environment. Some traditional approaches predict eye movements solely on simple stimulus properties, such as local contrasts (stimulus-driven factors). Recent arguments, however, emphasize the influence of tasks (task context) and bodily factors (spatial bias). To investigate how contextual factors affect eye movement control, we quantify the relative influences of the task context, spatial biases and stimulus-driven factors. Participants view and classify natural scenery and faces while their eye movements are recorded. The stimuli are composed of small image patches. For each of these patches we derive a measure that quantifies stimulus-driven factors, based on the image content of a patch, and spatial viewing biases, based on the location of the patch. Utilizing the participants’ classification responses, we additionally derive a measure, which reflects the information content of a patch in the context of a given task. We show that the effect of spatial biases is highest, that task context is a close runner-up, and that stimulus-driven factors have, on average, a smaller influence. Remarkably, all three factors make independent and significant contributions to the selection of viewed locations. Hence, in addition to stimulus-driven factors and spatial biases, the task context contributes to visual sampling behavior and has to be considered in a model of human eye movements. Visual processing of current stimulus content, in particular visual sampling behavior and early processing, is inherently dependent on context. We show that already in the first cortical stage, temporal context strongly affects the processing of new visual information and that visual sampling by eye movements is significantly influenced by the task context, independently of spatial factors and stimulus-driven factors. The empirical results presented provide foundations for an improved theoretical understanding of the role of context in perceptual processes.