A.  Bicchi, Scilingo, E. P., Dente, D., and Sgambelluri, N., ?Tactile Flow and Haptic Discrimination of Softness?, in Multi-point Interaction with Real and Virtual Objects, F.  Barbagli, Prattichizzo, D., and Salisbury, K., Eds. Springer Verlag, 2005, pp. 165-176.