The coding of roughness.

Abstract This review examines the way information about textures is captured, encoded, and processed by the somatosensory system to produce sensations of roughness/smoothness. Textures with spatial periods exceeding about 200 µm are encoded spatially, so roughness is nearly independent of the speed and direction of their movement across the skin. The information consists of spatial variations in activity among slowly adapting (SA1) mechanoreceptors, and appears to be extracted by specialized cortical neurons. Perception of the roughness of finer surfaces is mediated by detection, primarily by Pacinian afferents, of cutaneous vibrations generated when textures move across the skin. Movement is necessary to the perception of these textures, and vibrotactile adaptation interferes with it. The code is an intensitive one (i.e., the amount of activity in Pacinian afferents). Resume Dans cette revue, nous examinons comment le systeme somatosensoriel extrait, encode et traite l'information tactile pour produire des sensations de rugosite. Les textures dont la periode spatiale depasse 200µm sont encodees de maniere spatiale: la rugosite est quasiment independante de la vitesse et de la direction du mouvement des textures sur la peau. L'information est contenue dans la variation spatiale de l'activite d'une population de mecanorecepteurs a adaptation lente (Type 1) et semble etre extraite par des neurones corticaux specialises. La perception de rugosite pour les surfaces plus lisses depend en revanche de la detection, principalement par les afferents Paciniens, des vibrations cutanees engendrees par le mouvement des textures sur la peau. Ce mouvement est necessaire pour percevoir les textures, et l'adaptation vibrotactile interfere avec cette perception. Le code neuronal est un code d'intensite, c'est-a-dire qu'il exprime l'activite totale des afferents Paciniens. Everyone uses the term roughness, but no one knows exactly what it means. For example, if we haptically examine a piece of fine sandpaper and a page of Braille, we cannot say for certain which is rougher until we are told what aspect of the surfaces to attend to, because the sandpaper and the Braille feel rough in different ways. This complexity of the roughness experience suggests that there is more than one physiological mechanism or code for using tactile information to create the perception of roughness. It would not be surprising if this were so. Physical properties of tangible surfaces that correlate with their roughness (especially the size and spacing of texture elements) vary over wide ranges. In other sensory systems, it has been found that encoding mechanisms that work over one part of a stimulus dimension (such as light intensity) may not work over another part of the same dimension. In a number of cases, this problem has been solved by having separate mechanisms process information from different parts of a dimension (e.g., rods for low- and cones for high-light intensities). Other examples are the use of a time code and a place code for auditory pitch, and interaural time and intensity cues for binaural localization, over different parts of the sound frequency spectrum. Recent research has shown that at least two codes exist for roughness as well: A spatial code that works for coarse and medium surfaces, and a vibrotactile code that works for very fine surfaces (with spatial periods - the centre-to-centre distance between texture elements - less than about 200 pm). These codes depend on signals that arise in different classes of mechanoreceptors and are processed using different cortical algorithms. In the following sections, we first review the psychophysical evidence for the existence of these two roughness codes, and then explore the physiological mechanisms underlying each code. Finally, we ask whether the two codes can work together in some situations. Psychophysical Evidence for Two Codes The Role of Movement Rubbing a surface is the best way to examine its texture, and people spontaneously execute this exploratory procedure when asked to make textural judgments (Lederman & Klatzky, 1987). …