Haptic discrimination of paper

Ian R. Summers, Richard J. Irwin and Alan C. Brady


This chapter describes a study on the haptic discrimination of different types of plain paper. The experiment is designed to replicate features of a ‘banknote’ scenario in which it may be possible to identify a counterfeit on the basis of only a few seconds contact. Multidimensional scaling (MDS) techniques are used to investigate the perceptual dimensions involved in the discrimination task. A related study of tactile perceptual space is summarised in Appendix 1.
It is an everyday experience to handle paper – turning the page of a book, opening mail, handling a banknote – and it takes only a short time to assess the paper in terms of its characteristic ‘feel’. (See Lederman and Klatsky’s investigation [1] of manipulation strategies for obtaining information about objects in general.) The present study investigates some of the perceptual processes which are involved in making such assessments, particularly in relation to features which are significant in the handling of banknotes. (In the United Kingdom, the distinctive feel of banknotes is officially recommended as an indicator for the detection of counterfeits, and there is anecdotal evidence that counterfeits may indeed be detected in this way.) Such features might include gross physical parameters such as paper thickness and stiffness, as well as parameters which relate to surface texture.
There have been few previous studies on the perception of thickness or stiffness for material in the form of thin sheets. Thickness discrimination might in principle be based either on direct perception of thickness (for example, when holding a sheet between finger and thumb, in terms of joint angle) or on perception of stiffness (which is determined both by the thickness of sheet and the mechanical properties of the material). Such discrimination has been investigated by John, Goodwin and Darian-Smith [2] and Ho [3]. The author of the latter study proposed an explanation of results from both investigations on the basis that, when sheets are sufficiently thin to deform under finger contact, thickness discrimination is based primarily on perception of the curvature of the deformed sheet.