RESEARCH ARTICLE Roughness of simulated surfaces examined with a haptic tool: effects of spatial period, friction, and resistance amplitude Allan M. Smith • Georges Basile • Jonathan Theriault-Groom • Pascal Fortier-Poisson • Gianni Campion • Vincent Hayward Received: 16 June 2009 / Accepted: 16 November 2009 / Published online: 11 December 2009 Ó Springer-Verlag 2009 Abstract A specifically designed force-feedback device accurately simulated textures consisting of lateral forces opposing motion, simulating friction. The textures were either periodic trapezoidal forces, or sinusoidal forces spaced at various intervals from 1.5 mm to 8.5 mm. In each of two experiments, 10 subjects interacted with the virtual surfaces using the index finger placed on a mobile plate that produced the forces. The subjects selected their own speed and contact force for exploring the test surface. The apparatus returned force fields as a function of both the finger position and the force normal to the skin allowing full control over the tangential interaction force. In Experiment #1, subjects used an integer, numerical scale of their own choosing to rate the roughness of eight identical, varyingly spaced force ramps superimposed on a back- ground resistance. The results indicated that subjective roughness was significantly, but negatively, correlated (mean r =-0.84) with the spatial period of the resistances for all subjects. In a second experiment, subjects evaluated the roughness of 80 different sinusoidal modulated force fields, which included 4 levels of resistance amplitude, 4 levels of baseline friction, and 5 spatial periods. Multiple regression was used to determine the relationship between friction, tangential force amplitude, and spatial period to roughness. Together, friction and tangential force ampli- tude produced a combined correlation of 0.70 with sub- jective roughness. The addition of spatial period only increased the multiple regression correlation to 0.71. The correlation between roughness estimates and the rate of change in tangential force was 0.72 in Experiment #1 and 0.57 in Experiment #2. The results suggest that the sen- sation of roughness is strongly influenced by friction and tangential force amplitude, whereas the spatial period of simulated texture alone makes a negligible contribution to the sensation of roughness. Keywords Roughness scaling  Simulated textures  Friction  Tangential force modulation Introduction In an effort to establish objectively quantifiable surfaces for the purpose of studying the subjective scaling of roughness, many investigators resorted to using surfaces with precisely controlled gratings or two-dimensional raised dots or truncated cones. These manufactured surfaces with peri- odically spaced elements could be characterized in terms of the ridge or groove width or inter-dot spacing and height. Lederman and Taylor (1972) were one among the first to note that for engraved linear metal gratings, the groove width between the ridges exerted a greater effect on per- ceived roughness than the ridge width. This observation has since been repeatedly confirmed (Cascio and Sathian 2001; Lederman 1974; Sathian et al. 1989), and conse- quently it appeared initially that the sensation of roughness increased monotonically with increasing groove width, whereas an increase in ridge width tended to decrease the roughness. A similar observation was made using A. M. Smith (&)  G. Basile  J. Theriault-Groom  P. Fortier-Poisson Groupe de Recherche sur le Syste `me Nerveux Central, De ´partement de Physiologie, Centre de Recherche en Sciences Neurologiques, Universite ´ de Montre ´al, C.P. 6128 Succursale Centre ville, Montreal, QC H3C 3T8, Canada e-mail: allan.smith@umontreal.ca G. Campion  V. Hayward Department of Electrical Engineering, Center for Intelligent Machines, McGill University, Montreal, QC, Canada 123 Exp Brain Res (2010) 202:33–43 DOI 10.1007/s00221-009-2105-x