Shape-elasticity tactile confound

Recent studies, [1, 2], suggest that the mechanics of fingertip-object interactions can provide insight into the functional link between the hand, the brain, and our haptic perception of the world. When two convex objects, a finger′푓′and a surface′푠′, come into Hertzian contact, the respective elastic moduli퐸&, and 퐸', the radii of curvature, 푅&and 푅', the pressing force,푓, the deflection, 푑, and the size of the contact area, 푎, of these bodies are interrelated. If we assume that the finger and the surface are incompressible, and that the finger is a homogeneous, isotropic sphere, (its behaviour does not deviate much from that theory [3]), then it can be shown that 퐸'and 푅'play interchangeable roles in the relationship that link 푎to 푓. If 푎is an adequate representation of the mechanical state of the finger, then two behavioural hypotheses follow, namely:There exists perceptual metamers of compliance, i.e, physically distinct stimuli give rise to the same perception becauseRsandEsare confounds.An object's surface curvature and compliance are increasingly confused with increasedEs.Our findings suggest that, indeed, the interaction between a fingertip and a convex surface produces a perceptual outcome equivalent to decreasing 퐸', while the interaction between the fingertip and a concave surface creates a perceptual estimate of surface compliance equivalent to an increase in 퐸'. A low or high 퐸'both yield a metameric response, however, as predicted, for low 퐸', the effect diminishes significantly. These results, for the first time, provide evidence of a tactile perceptual metamer that can be predicted basic mechanics. The implications of this finding for perceiving and rendering object properties are discussed with respect to the functional link between the hand, brain and environment.