Resonance and texture coding in the rat whisker system

The sense of touch provides information about the texture of an object with much better resolution than the other senses. Primates perceive texture by actively moving their fingertips over an object and encoding the resulting spatiotemporal patterns of activation of mechanoreceptors in the skin. Within the primate fingertip many types of mechanoreceptors are densely packed in the skin making it difficult to study the individual components of these complex patterns. Rats' ability to discriminate texture using the whiskers is comparable to humans using the fingertips and due to its discrete array of whiskers on the facial pad the rat whisker system offers considerable advantages for studying texture coding. However, to date, very little is known about how behaving rats actively use their whiskers to extract texture information and how this information is encoded in the nervous system. Currently there are two main hypotheses on how rats perform texture discrimination using their whiskers. In one hypothesis, whisker motion across a texture is underdamped, and texture properties drive whisker resonance as the whisker moves across a surface. In this model, the systematic variation in resonance frequency across whiskers enables texture to be encoded by the differential amplitude of vibration across whiskers. In a second, alternative hypothesis, resonance is diminished due to damping and texture information is encoded in the precise motion of each whisker across the surface. Both of these hypotheses are supported by experiments performed in anesthetized animals, however to resolve this debate it is critical to examine whisker dynamics in awake animals to determine 1) to what extent whisker resonance shapes sensory input during active whisker use and 2) how whisker dynamics reflect texture properties when the whiskers are under active muscular control. Chapter 2 of this dissertation examines whisker vibrations measured in behaving animals trained to whisk in air. We show that, in the absence of any sensory stimulus, high frequency whisker vibrations are presents and that these are filtered by whisker resonance. This suggests that, in the awake behaving animal, resonance can play a role in shaping sensory responses in the whiskers. Chapter 3 shows that active palpation onto textures can induce whisker resonance. However we argue that the degree to which resonance occurs is not dependent on the spatial properties of the texture and therefore it is unlikely to be critical for texture encoding arguing against the resonance hypothesis. We present an alternative model for texture encoding based on the magnitude of stick-slip events that were found to occur during texture palpation. We also examine the role that muscles may play in causing these stick-slip events. Chapter 4 concludes by presenting preliminary results of the neural response to stick-slip events. We show that the spiking probability of a subset of neurons is greatly increased following a texture-induced slip and that the spiking probability increases with slip acceleration, as would be required for the texture encoding hypothesis presented in chapter