Roughness discrimination of textured gratings using multiple contact methods

Many studies on texture perception have focused on the perceived roughness of texture samples. The present study continues this analysis by conducting a set of nearly identical amplitude discrimination experiments using three common texture contact methods: fingertip on real gratings (Finger-real), stylus on real gratings (Stylus-real), and stylus on virtual gratings (Stylus-virtual). Two types of textures were used: sinusoidal gratings and triangular gratings that varied along one dimension. Both grating types had a spatial period of 2.5 mm and had the same macro-scale dimensions. The real gratings were fabricated from stainless steel by an electrical discharge machining process while the virtual gratings were rendered via a programmable force-feedback device called the ministick. On each trial, participants compared test gratings with 55, 60, 65, or 70 mum amplitudes (i.e., heights) against a 50-mum reference using a two-interval two-alternative forced-choice paradigm. The results show discrimination thresholds did not differ significantly between sinusoidal and triangular gratings. With sinusoidal and triangular grating data combined, the average (mean +/- standard error) for the Stylus-real threshold (2.5 +/- 0.2 mum) was significantly smaller (p < 0:01) than that for the Stylus-virtual condition (4.9 +/- 0.2 mum). Differences between the Finger-real threshold (3.8 +/- 0.2 mum) and those from the other two conditions were not statistically significant. These results demonstrate strengths and weaknesses of different contact methods for discriminating the roughness of small-scale surface features that create a useful basis for further investigation of human texture perception.