Algorithms of whisker-mediated touch perception Miguel Maravall 1 and Mathew E Diamond 2 Comparison of the functional organization of sensory modalities can reveal the specialized mechanisms unique to each modality as well as processing algorithms that are common across modalities. Here we examine the rodent whisker system. The whisker’s mechanical properties shape the forces transmitted to specialized receptors. The sensory and motor systems are intimately interconnected, giving rise to two forms of sensation: generative and receptive. The sensory pathway is a test bed for fundamental concepts in computation and coding: hierarchical feature detection, sparseness, adaptive representations, and population coding. The central processing of signals can be considered a sequence of filters. At the level of cortex, neurons represent object features by a coordinated population code which encompasses cells with heterogeneous properties. Addresses 1 Instituto de Neurociencias de Alicante UMH-CSIC, Campus de San Juan, Apartado 18, 03550 Sant Joan d’Alacant, Spain 2 Tactile Perception and Learning Lab, International School for Advanced Studies-SISSA, Via Bonomea 265, 34136 Trieste, Italy Corresponding author: Diamond, Mathew E (diamond@sissa.it) Current Opinion in Neurobiology 2014, 25:176–186 This review comes from a themed issue on Theoretical and computational neuroscience Edited by Adrienne Fairhall and Haim Sompolinsky 0959-4388/$ – see front matter, # 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conb.2014.01.014 Introduction In the process that culminates in sensing and identifying an object, the starting point is the encoding of physical parameters by sensory receptors. A growing set of inves- tigations focuses on transformations along sensory path- ways as a means to understand the conversion from raw physical signals into sensations and percepts. A long- standing hypothesis is that those transformations are built up from a set of standard ‘canonical’ computations, imple- mented repeatedly [1] and combined to generate responses that are selective, specific and flexible [2]. Taking this hypothesis as a point of departure, this review aims to identify canonical computations, or algorithms, implemented in the rodent whisker system, an ‘expert’ system [3]. We focus on computations along the receptor-to-cortex ascending pathway; nevertheless a complete picture of tactile sensation will only be achieved by understanding how sensory and motor computations are woven together [4,5]. Mechanical forces in the follicle As in any sensory pathway, transduction from physical entities into action potentials constrains all later proces- sing. Input signals — fluctuations in mechanical energy at the whisker base — are shaped by the interaction be- tween the whisker’s motion, its mechanical properties (e.g., compliance) and properties of the contacted object. The whisker-follicle junction is rigid, allowing robust transmission and readout of the forces induced by whisker motion [6  ]. The form of whiskers (Fig. 1a) determines their mech- anical behavior. Bending stiffness decreases from whisker base to tip due to taper [7  ], and the concomitant increase in flexibility enables the slippage of whiskers during object exploration [8,9  ]. Additional flexibility is achieved by their hollow structure [10]. New methods for tracking whisker motion have allowed detailed analysis of how whiskers interact with objects [11,12]. The combination of whisker measurements with models of whisker deflection has begun to specify bend- ing [9  ,13] and changes in forces at the whisker base [6  ,7  ,9  ,14]. Contact-induced whisker deformations can be decomposed into a slow bending component and a transient vibrational component [15]; the relative contributions of different components depend on the specific interaction [6  ,7  ,16]. Algorithms involving comparison of components require those components to be effectively transduced by mechanoreceptors. When whiskers sweep across a textured surface (Fig. 1b), they are trapped and released by surface ridges and grains [17–19,20  ] (reviewed in [21]). These brief (2 ms) ‘stick–slip’ events cause transient, high-frequency vibrations [18]. The consequent sequence of fluctuations in mechanical energy provides a signature of texture [17,18]. Stick–slip events excite primary sensory neurons and their targets [17,18,20  ,22]. However, differences in ‘stick–slip’ events across trials are not well-correlated with trial-to-trial choices in a texture discrimination task [20  ]; other features of whisker motion may also con- tribute. Transduction of touch into neuronal signals Transduction is carried out at the terminals of neurons whose cell body resides in the trigeminal ganglion (TG; Fig. 1c). The many mechanoreceptor types, distributed Available online at www.sciencedirect.com ScienceDirect Current Opinion in Neurobiology 2014, 25:176–186 www.sciencedirect.com