Haptic curvature contrast in raised lines and solid shapes

It is known that our senses are influenced by contrast effects and aftereffects. For haptic perception, the curvature aftereffect has been studied in depth but little is known about curvature contrast. In this study we let observers explore two shapes simultaneously. The shape felt by the index finger could either be flat or convexly curved. The curvature at the thumb was varied to quantify the curvature of a subjectively flat shape. We found that when the index finger was presented with a convex shape, a flat shape at the thumb was also perceived to be convex. The effect is rather strong, on average 20% of the contrasting curvature. The contrast effect was present for both raised line stimuli and solid shapes. Movement measurements revealed that the curvature of the path taken by the metacarpus (part of the hand that connects the fingers) was approximately the average of the path curvatures taken by the thumb and index finger. A failure to correct for the movement of the hand could explain the contrast effect.