Pre-neuronal Encoding of Object Position in the Vibrissal System of the Rat.
Résumé
The capacity of the sensory organs to detect environment features determines all subsequent processing of sensory information. In the somatosensory vibrissal system, the physical event of touch is conveyed by the whisker to the follicle mechanoreceptors, which then provide the input to the brain. We used high speed video and artificial whisking to assess how mechanical variables of the whisker reflect the coordinates of the object in space. During active touch, the radial (from the snout outward) and horizontal (front-back) position of an object was reflected in the whiskers' curvature, whisker angle absorption (the amount of protraction movement blocked by the object), and change of velocity. The phase plane spanned by angle absorption and curvature contained invariant representations of horizontal and radial object location. The slope of whisker trajectory in this plane represented radial positions independent of horizontal ones, and the magnitude represented horizontal position independent of radial ones. We also present preliminary results which suggest that during continuous touch (i.e. when a whisker is sweeping on a surface) the radial distance from the surface is encoded by the frequency of the whisker vibrations and their amplitude. We conclude that the first encoding step is composed of mechanical transformations from external physical variables to whisker-related mechanical variables. This computation is done before the first-order neurons come into play.