Investigation of human perception of tactile graphics and its dependence on fundamental friction mechanisms

Abstract

Accurate depiction of visual information presents a formidable obstacle to the inclusion of people with visual impairments in standardized educational assessment. This challenge is often addressed by using tactile graphics, such as the use of a collection of tactile patterns representing various zones similar to the use of colors to represent political boundaries on a visual map. This study investigated friction mechanisms involved during fingertip exploration of tactile graphics, as well as the role that friction played in forming perceptions. Perception experiments were run using four texture types published by the Braille Authority of North America (BANA): two parallel-ridge textures of different ridge widths and spacings, and two ordered-dot textures of different dot spacings. To minimize tactile confusion, BANA guidelines prohibit the use of similar textures adjacent to each other. In this study, texture pairs were printed adjacent to each other on thermally activated (‘swell touch’) tactile paper. Sighted human subjects participated in a blinded task of determining if the two textures in the pair were the same or different. Friction measurements of the fingertip exploration of the texture pairs was recorded using a dynamometer in a range of normal loads from 0.4 to 0.8 N. A generalized linear model (GLM) with a logistic link function was used to predict the influence of texture pair and participant on the likelihood of perceiving each texture pair as the same or different. Results showed that the particular texture pair had a strong correlation to the ability of participants to discern between textures. These quantitative results support the BANA guidelines. The friction data showed that there was not a significant amount of difference among the coefficients of friction of various textures, such that COF was not a strong predictor of perceptive ability. However, there was some evidence that penetration of the fingertip between raised texture elements, and interlocking with these elements, allowed for dissimilar dot patterns to be better discerned than dissimilar ridge patterns. The results indicate that there are other aspects, possibly still tribological in nature, that govern tactile perception when exploring tactile graphics. These phenomena may include such things as finger deformation and vibrational response produced during contact with individual texture elements.