Three experiments are reported bearing on Katz's hypothesis that tactile texture perception is mediated by vibrational cues in the case of fine textures and by spatial cues in the case of coarse textures. Psychophysical responses when abrasive surfaces moved across the skin were compared with those obtained during static touch, which does not provide vibrational cues. Experiment 1 used two-interval forced-choice procedures to measure discrimination of surfaces. Fine surfaces that were readily discriminated when moved across the skin became indistinguishable in the absence of movement; coarse surfaces, however, were equally discriminable in movingand stationary conditions. This was shown not to result from any inherently greater difficulty of fine-texture discrimination. Experiments 2 and 3 used free magnitude estimation to obtain a more comprehensive picture of the effect of movement on texture (roughness) perception. Without movement, perception was seriously degraded (the psychophysical magnitude function was flattened) for textures with element sizes below 100 llm; above this point, however, the elimination of movement produced an overall decrease in roughness, but not in the slope of the magnitude function. Thus, two components of stimulation (presumably vibrational and spatial) contribute to texture perception, as Katz maintained; mechanisms for responding to the latter appear to be engaged at texture element sizes down to 100llm, a surprisingly small value.In his classic treatise The World of Touch, David Katz (1925 advanced the view that the tactile perception of the texture of surfaces is a complex process, depending on a "spatial sense" for discernment of coarse textures and a "vibration sense" for an appreciation offiner textures. Katz offered a number of ingenious observations to support his theory: For example, he demonstrated that papers can be discriminated by an observer drawing a wooden rod across them but that wrapping the rod in cloth greatly impairs performance (p. 115). These results suggest the use of vibrational cues. Katz's view, referred to here as the duplex theory oftactile texture perception, is succinctly captured in his statement that surfaces touched very lightly could not be clearly perceived, because "the spatial sense ofthe skin could no longer discern the coarse texture of the materials, and the vibrations necessary for the recognition of fine texture were lost as a result of the minimal friction" (p. 138).

Ratio scaling was used to obtain from 5 subjects estimates of the subjective dissimilarity between the members of all possible pairs of 17 tactile surfaces. The stimuli were a diverse array of everyday surfaces, such as corduroy, sandpaper, and synthetic fur. The results were analyzed using the multidimensional scaling (MDS) program ALSCAL. There was substantial, but not complete, agreement across subjects in the spatial arrangement of perceived textures. Scree plots and multivariate analysis suggested that, for some subjects, a two-dimensional space was the optimal MDS solution, whereas for other subjects, a three-dimensional space was indicated. Subsequent to their dissimilarity scaling, subjects rated each stimulus on each of five adjective scales. Consistent with earlier research, two of these (rough/smooth and soft/hard) were robustly related to the space for all subjects. A third scale, sticky/slippery, was more variably related to the dissimilarity data: regressed into three-dimensional MDS space, it was angled steeply into the third dimension only for subjects whose scree plots favored a nonplanar solution. We conclude that the sticky/slippery dimension is perceptually weighted less than the rough/smooth and soft/hard dimensions, materially contributing to the structure of perceptual space only in some individuals.In touch, as in other sensory modalities, stimuli can differ in a large number of physical properties. For example, the surfaces ofobjects presented to the sense of touch can differ from one another in their frictional resistance to lateral movement of a finger across them, in their compressibility in response to radial force, their thermal conductivity, and in the presence, density, size, composition, and arrangement of structural elements that disturb the flatness of the surface. Considerable research documents the close relationship between these properties and subjective qualities such as roughness, softness, and slipperiness (Connor, Hsiao, Phillips, & Johnson, 1990;Connor & Johnson, 1992;Katz, 1925Katz, /1989Lederman & Taylor, 1972;Srinivasan & LaMotte, 1996;Srinivasan, Whitehouse, & LaMotte, 1990;S. S. Stevens & Harris, 1962;Taylor & Lederman, 1975). When an overall impression of such a stimulus is obtained in a relatively brief exposure, it is reasonable to ask (I) which and how many of its properties enter into this impression, and how fully they are combined, and (2) how the stimuli seem to the subject to be related to one another-for example, whether they are arranged in an orderly dimensional structure (analogous, e.g., to color space) defined by the subjective dimensions corresponding to their component properties.

The Pacinian channel has been implicated in the perception of fine textures (Hollins et al., Somatosens Mot Res 18: 253-262, 2001a). In the present study, we investigate candidate codes for Pacinian-mediated roughness perception. We use a Hall effect transducer to record the vibrations elicited in the skin when a set of textured surfaces is passively presented to the index finger. The peak frequency of the vibrations is found to decrease systematically as spatial period increases. The power of the vibrations--weighted according to the spectral sensitivity of the Pacinian system--increases with spatial period for all but the coarsest surfaces. By varying the scanning velocity, we manipulate the temporal and intensive characteristics of the texture-induced vibrations and assess the effect of the manipulation on perceived roughness. We find that doubling the scanning velocity does not result in the substantial decrease in roughness predicted by a frequency theory of vibrotactile roughness perception. On the other hand, the effects of speed on roughness match those of speed on power. We propose that the roughness of a fine surface (spatial period<200 microm) is a function of the Pacinian-weighted power of the vibrations it elicits.

Subjects were presented with pairs of finely textured stimuli and were instructed to rate their dissimilarity, using free magnitude estimation. The subjects also rated the stimuli along each of four textural continua: roughness, hardness, stickiness, and warmth. In subsequent experimental sessions, we used a Hall effect transducer to measure the vibrations produced in the subjects' fingertip skin as the stimuli were scanned across it. We wished to assess the extent to which the perceptual dissimilarity of the textures could be explained in terms of the perceptual dissimilarity of the vibrations they elicited in the skin. To that end, we invoked a model characterizing the Pacinian representation of a vibratory stimulus. From the model, we computed the difference in the vibratory representations of the two stimuli in each pair. We found that the bulk of the variance in perceived dissimilarity of the textures was accounted for by differences in the Pacinian representations of the vibrations they produced. Our results further suggested that the textural information conveyed by the Pacinian system concerns surface roughness and, possibly, stickiness.

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