Two fundamental questions in the investigation of any sensory system are what physical signals drive its primary sensory neurons and how such signals are encoded by the successive neural levels during natural behaviour. Due to the complexity of experiments with awake, actively sensing animals, most previous studies focused on anesthetized animals, where the motor component of sensation is abolished and therefore those questions are so far largely unanswered. The aim of this thesis is to exploit recent advance in electrophysiological, behavioural and computational techniques to address those questions in the sub-cortical whisker system of the mouse. To determine the input to the whisker system, in Chapter 2 I recorded from primary whisker afferents (PWAs) of awake, head-fixed mice as they explored a pole with their whiskers, and simultaneously measured both whisker motion and forces with high-speed videography. To predict PWA firing, I used Generalised Linear Models. I found that PWA responses were poorly predicted by whisker angle, but well predicted by rotational force (moment) acting on the whiskers. This concept of “moment encoding” could account for the activity of PWAs under diverse conditions - whisking in air, active whisker-mediated touch and passive whisker deflection. The discovery that PWAs encode moment raises the question of how mice employ moment to control their tactile behaviours. In Chapter 3 I therefore measured moment at the base of the whiskers of head-fixed mice, performing a novel behavioural task, which involved whisker-based object localisation. I then tested which features of moment during whiskerobject touch could predict mouse choice. By using probabilistic classifiers, I discovered that mouse choices could be accurately predicted from moment magnitude and direction during touch, combined with a non-sensory variable - the mouse choice in the previous trial. Finally, in Chapter 4 I asked how tactile coding generalized to whisker system sub-cortical brains regions during a natural active whisker-based behaviour. I therefore combined a naturalistic whisker-guided navigation task and extracellular recording with a novel generation of high density silicon probes (O3 Neuropixel probes) and studied how touch and locomotion were encoded by the whisker first (ventral posterior nucleus, VPM) and higher order thalamic relay (posterior complex, PO) and hypothalamic regions and (zona incerta, ZI). Using multiple linear regressions, I found that neurons in the relay nucleus VPM encoded not only touch, but also locomotion signals. Similarly, neurons in the higherorder regions PO and ZI were driven by both touch and locomotion. My study showed that in the awake animal, in the central part of the whisker system, peripheral signals were preserved, but were encoded concomitantly with motor variables, such as locomotion. In summary, in this thesis I identified the mechanical variable representing the major sensory input to the whisker system. I showed that mice are able to employ it to guide behaviour and found that correlate of this signal was encoded by central neurons of the whisker system in VPM, PO and ZI, concomitantly with locomotion.

La perception menant à une prise de décision implique de multiples aires corticales. Il a été proposé que l'information sensorielle se propage des aires sensorielles primaires, codant principalement la nature du stimulus, aux aires de haut-niveau - plus frontales - codant d'avantage la valence du stimulus ou la décision. Pour mieux comprendre l'intégration corticale des signaux sensoriels, nous avons enregistré les réponses sensorielles évoquées (RSE) simultanément dans différentes aires corticales, tandis que des souris apprenaient une tâche de détection sensorielle. Chez les souris ayant appris la tâche, une RSE est observée dans toutes les aires enregistrées suivant la stimulation de la vibrisse, avec des latences croissantes des aires somatosensorielles primaire (vS1) et secondaire (vS2), vers le cortex moteur primaire des vibrisses (vM1), le cortex pariétal associatif (PtA), l'hippocampe dorsal (dCA1) et enfin le cortex préfrontal médian (mPFC). Nous avons constaté une réduction des RSEs lors des échecs par rapport aux essais réussis dans toutes les aires, sauf vS1. Toutefois, seule l'inactivation de vS1, vS2 ou mPFC affecte significativement la performance des souris. Pendant l'apprentissage de la tâche, une augmentation sélective de la RSE est observée dans le mPFC en corrélation avec la performance. Des enregistrements unitaires dans le mPFC démontrent la nature excitatrice de la réponse sensorielle chez les souris entrainées. Nos résultats confirment ainsi que la réponse sensorielle dans le mPFC reflète l'importance comportementale du stimulus et corrèle avec la prise de décision, tandis que la réponse des aires sensorielles reflète plutôt la nature du stimulus / Sensory perception leading to goal-directed behavior involves multiple, spatially-distributed cortical areas. It has been hypothesized that sensory information flows from primary sensory areas encoding mainly the nature of the stimulus, to higher-order, more frontal, areas encoding the valence of the stimulus or the decision. To further understand the cortical integration of sensory signals, we recorded sensory evoked potentials (SEPs) simultaneously from different areas while mice learned a whisker-based sensory detection task. In mice that have learned the task, the whisker stimulus evoked SEP in all recorded areas with latencies increasing from the whisker primary (wS1) to the secondary somatosensory area (wS2), the whisker motor area (wM1), the parietal area (PtA), the dorsal hippocampus (dCA1) and the medial prefrontal cortex (mPFC). We found a reduction of SEPs during Miss trials compared with Hit trials in all areas except wS1. However, only the local inactivation of either wS1, wS2 or mPFC significantly impaired the mice performance. During training to the detection task, we observed a selective increase of the SEPs in mPFC that correlated with performance. Finally, using high-density extracellular recordings in mPFC, we found that whisker stimulation in trained mice evoked an early increase in the firing rate of putative excitatory neurons (regular spiking units) that was positively correlated with behavioral outcome. Our results support the idea that mPFC could signal the relevance of a sensory stimulus in the context of a well-defined behavior, whereas sensory areas would be more constrained by the nature of the stimulus