FUNCTIONAL ARCHITECTURE OF THE MYSTACIAL VIBRISSAE

We investigated the transduction operation and function of the mystaci al vibrissae, using a comparative morphological analysis and behaviora l experiments in rats. Vibrissal architecture was documented in a seri es of mammals to identify evolutionary conserved features of vibrissal organization. As a result of this analysis, we distinguish between a frontal microvibrissal system and macrovibrissal system of the mystaci al pad. The latter was invariably comprised of whiskers aligned in reg ular rows. In each row, whiskers were oriented perpendicular to the an imal's rostrocaudal axis; all shared a specific dorsoventral orientati on. In all species, progressing from rostral to caudal in any vibrissa l row, there was a precisely exponential increase in whisker length. E ach whisker appeared to act as a lever-like transducer, providing info rmation as to whether or not - but not where - an individual vibrissa had been deflected. The rat's frontal microvibrissae system was found to have a vibrissa tip density that was about 40 times higher than tha t of the mystacial macrovibrissae. In behavioral studies spatial tasks and object recognition tasks were used to investigate (a) search beha viors; (b) single whisker movements; (c) object recognition ability; a nd (d) effects of selective macro- or microvibrissae removal on task p erformances. A clear distinction between the functional roles of macro - and microvibrissae was demonstrated in these studies. Mystacial macr ovibrissae were critically involved in spatial tasks, but were not ess ential for object recognition. Microvibrissae were critically involved in object recognition tasks, but were not essential for spatial tasks . A synthesis of these morphological and behavioral data led to the fo llowing functional concept: The mystacial macrovibrissae row is a dist ance decoder. Its function is to derive head centered obstacle/opening contours at the various dorsoventral angles represented by vibrissal rows. This distance detector model is functionally very different from traditional concepts of whisker function, in which the mystacial whis kers were hypothesized to form a fine grain skin-like object-recognizi ng tactile surface.