Rodent Models of Behavior: Texture Coding in the Rat Vibrissa System

Rodent models are increasingly being used to study circuits and physiological processes underlying behavior. Key questions in systems neuroscience such as sensory coding, learning and memory, decision-making, attention and motor control are being addressed in rodents. In this talk, I will focus on coding of sensory stimuli during active, natural sensation in the rat vibrissa system. Rodent whiskers (vibrissae) are highly sensitive tactile detectors, similar to primate fingertips that are actively moved through the environment to extract information about object position, shape and surface features such as texture. Rats can discriminate texture roughness with their whiskers with an acuity rivaling those of primate fingertips. Whisker movement across objects creates dynamic input streams, but which features of these input streams are encoded by the nervous system and are relevant for perception are largely unknown. To identify the basic tactile features encoded during active whisker sensation, we simultaneously measured whisker micro-motion and neural activity in primary somato-sensory cortex (S1) in rats trained to whisk voluntarily on textured surfaces. We found that whisker movement across surfaces generates complex whisker micro-motion, including discrete, transient slip-stick events driven by frictional interactions, which carry information about surface properties. Slip-stick motion events (referred to as “slips”) were encoded by S1 neurons with sparse, low-probability, precisely timed spikes during continuous whisking on surfaces, resulting in a probabilistically sparse ensemble code. Temporally precise slip-evoked responses drove transient correlation between S1 neurons. These transient spiking correlations on ~20-ms time scale encoded slips very efficiently in small (~100 neuron) populations, provided higher signal-to-noise detection of surfaces than slower modulation of firing rate and robustly encoded surface roughness. We thus identified whisker slips as a fundamental tactile feature that is encoded by sparse, temporally precise, synchronous spiking in cortex (Jadhav et al., Nat. Neurosci., 2009). Based on our results, we propose a slip-stick model for texture coding in the whisker system (Jadhav and Feldman, Curr. Opi. Neurobio., 2010).