Transduction mécanique et représentation tactile des textures par les vibrisses de rat et le doigt humain. – TRANSTACT

Rodents and Humans possess exquisite tactile sensitivity which allows them to extract, mainly through active exploration, rich information about their immediate environment. In both sensory systems, tactile information is contained in the sequence of deformations of the sensory organ ' the facial whisker hairs in rodents and the digital skin in humans ' induced by its rubbing against the probed surface. These deformations are then converted into neuronal signals by specific mechanoreceptors and processed by the central nervous system. Although we have a comprehensive understanding of how the brain reacts to simple sensory stimuli, we are only starting to explore the way sensory systems respond to natural stimuli in realistic exploratory conditions. For example, in the vibrissal system of the rat, one of the dominant models for investigating the mechanisms of sensory information processing, the situation is further confounded by the lack of knowledge on how the receptor organ itself - the whisker hair - transmits the information that is then passed onto the nervous system. Yet, the central treatment of sensory information depends crucially on the encoding performed by the relevant sensory organ. In order to understand this phenomenon, it is necessary to describe the mechanical process by which the information about the texture is transduced into the motion of the sensory organ through dynamic frictional contact. This topic is far beyond the limits of a "classical" physiological study, and necessitates joint expertise in Neurophysiology and Physics. We propose a two phases project a) to explore the pre-neural component of tactile information transduction for both the rodent and human sensory systems using a common experimental strategy and b) to study the cortical representation of tactile information shaped by the pre-neuronal filtering in the somatosensory cortex of anesthetized rats. The first phase consists in directly recording the deformation of tactile organs (skin and whisker hair) upon sweeping textured substrates manufactured in our laboratory and for which we have a complete knowledge of their topography. We will investigate the filtering characteristics of this process and how they are related to the physical properties (shape, surface and elastic properties) of the organ and to the exploratory conditions (applied normal force, rubbing velocity). Identical white-noise stimuli substrates and signal analysis schemes will be employed in both systems in order to allow direct comparison of the associated pre-neural filtering processes. For fingertip touch, a bio-mimetic approach will be developed in order to estimate the stress signals in the vicinity of the mechanoreceptors associated with the rubbing of the skin on textured substrates. Different models of the fingertip will be developed as well as original techniques of spatially resolved stress field measurements. In the case of tactile perception with vibrissae, experiments will be carried out on anesthetized rats and the whisker deflections will be optically recorded as their extremities are rubbed against the same textured substrates. In the second phase of this project, the neural coding of texture will be studied by recording neuronal activity in the cortical areas receiving sensory information from the whiskers while replaying at the follicle base the deflections generated by the texture and recorded in the first part of the project. This interdisciplinary project will benefit from the association of a physics group at the Laboratoire de Physique Statistique (Ecole Normale Supérieure, Paris) which has expertise in the domain of friction, mechanics and fabrication of controlled texture, with a neurophysiology group at the Unité de Neurosciences Intégratives et Computationnelles (CNRS, Gif sur Yvette) specialized in the analysis of cortical mechanisms of sensory information processing with electrophysiological methods.