Haptics disambiguates vision in the perception of pictorial relief

In this study we demonstrate that touch decreases the ambiguity in a visual image. It has been previously found that visual perception of three-dimensional shape is subject to certain variations. These variations can be described by the affine transformation. While the visual system thus seems unable to capture the Euclidean structure of a shape, touch could potentially be a useful source to disambiguate the image. Participants performed a so-called 'attitude task' from which the structure of the perceived three-dimensional shape was calculated. One group performed the task with only vision and a second group could touch the stimulus while viewing it. We found that the consistency within the haptics+vision group was higher than in the vision-only group. Thus, haptics decreases the visual ambiguity. Furthermore, we found that the touched shape was consistently perceived as having more relief than the untouched the shape. It was also found that the direction of affine shear differences within the two groups was more consistent when touch was used. We thus show that haptics has a significant influence on the perception of pictorial relief.