Abstract
Images that are easy to recognize visually often prove difficult to understand via touch when embossed. This has been a critical limiting factor in blind individuals’ access to graphical information, such as scientific diagrams. The requirement of spatio-temporally integrating information from a small ‘aperture’ (the fingertip, as it moves across an embossed drawing) is clearly a challenging one. Here we examine whether stimulus scale modulates the magnitude of this challenge. Previous studies suggest that larger images are better recognized than small ones (Kennedy & Bai, 2002; Wijntjes et al., 2008). However, there is lack of systematic investigation of tactile shape perception as a function of scale. To address this issue, we conducted an experiment where blindfolded, sighted subjects explored novel tactile patterns of eight randomly configured dots at five spatial scales (the diameter of the convex hull of the dots ranged from approximately 8-160 mm). We also motion-tracked their hands as they explored the presented patterns. After each exploration, the tactile stimulus was removed, and the subjects (now without their blindfold) were asked to draw the spatial configuration they had experienced. The subjects’ pattern reconstruction accuracy was evaluated using the Procrustes similarity function. Results reveal an intermediate scale of patterns as optimal for conveying spatial configuration; the smallest patterns introduce the complexity of distinguishing the individual elements while the largest scale makes it cognitively taxing to integrate individual element positions into a whole. Subjects state imagining the scattered dots as a holistic shape while exploring. This compels us to discuss parallels with the visual image encoding processes, recruiting working memory and mental imagery. Even as these data provide practical guidelines for tactile graphics production, it also serves as a starting point for modelling spatio-temporal integration processes in the tactile system, and their comparison with analogous mechanisms in the visual modality.