Abstract
The ability to recognize and discriminate complex surface textures through touch is essential to the survival of living beings, including humans. Most studies of tactile texture perception have emphasized perceptual impacts of lower-order statistical structures of stimulus surfaces that can be described in terms of amplitude spectra or spatial-frequency/orientation subband histograms (e.g., root mean squares of carving depth and inter-ridge distance). However, real-world surfaces we encounter in everyday life differ also in higher-order statistics that appear in phase spectra or joint subband histograms. Though human vision has sensitivity to higher-order statistics, and some studies have revealed similarities between visual and tactile information processing, it remains obscure whether human touch has sensitivity to higher-order statistics. Here we show that patterns different from each other in higher-order statistics, which can be easily distinguished by vision, cannot be distinguished by touch. We 3D-printed textured surfaces transcribed from different ‘photos’ of natural scenes such as stones and leaves. The textures look sufficiently different, and the maximum carving depth (2 mm) was well above the haptic detection threshold. Nevertheless, observers (n=10) could not accurately discriminate some texture pairs. Analysis of these stimuli showed that the more similar the amplitude spectrum was, the more difficult the discrimination became, suggesting a hypothesis that the high-order statistics have minor effects on tactile texture discrimination. We directly tested this hypothesis by matching the subband histogram of each texture using a texture synthesis algorithm. Haptic discrimination of these textures was found to be nearly impossible, although visual discrimination remained feasible due to differences in higher-order statistics. These findings suggest that human tactile texture perception qualitatively differs from visual texture perception with regard to insensitivity to higher-order statistical differences.
Significance Humans sense spatial patterns in the surrounding world through their eyes and hands. Researchers have revealed a detailed hierarchical processing of image features for visual texture perception, while that for haptic texture perception remains obscure. One big bottleneck in tactile research has been difficulty in controlling stimulus patterns, but this issue is being resolved by recent technological progress. Here we move a step ahead by using a high-resolution 3D printer. We invented textured surfaces that were 3D-printed from visual images, and controlled low- and high-order statistics of the surfaces by changing features of the original images. Behavioural experiments showed that human observers have sensitivity to lower-order statistics, which is in line with previous studies, but we could not observe any positive evidence of sensitivity to higher-order statistics. Although recent studies have emphasized the similarity between touch to vision with regard to spatiotemporal processing, the present findings indicate a qualitative difference between the two modalities. That is, touch differs from vision not only in spatio-temporal resolution but also in (in)sensitivity to high-level image statistics. Our findings support the view that the two modalities sense spatial information using different and complementary strategies.