Haptic perception of curved shapes

In order to grasp and manipulate objects with our hands, we need accurate and constantly updated information about the shape and changes in the shape of objects that we employ. The sensitivity of the fingers for shape is indispensable in providing this information. In this thesis, the ability to use the hand in the perception of object shape is investigated. In Chapter 2, we investigated how the ability to discriminate between two curved surfaces depends on the way in which the fingers are employed in order to execute the task. Performance did not depend on the finger that was used but decreased when two fingers were employed. Performance decreased even further when the surfaces of the stimuli were explored simultaneously instead of sequentially. In Chapter 3, we studied the ability to compare the curvature of two opponent shapes. The findings were contrasted against the ability to compare congruent shapes. When only a single finger was used, higher thresholds were found for the discrimination of opponent shapes than for the discrimination of congruent shapes. However, similar thresholds were found when the bimanual discrimination of opponent shapes was compared to that of congruent shapes. Furthermore, similar thresholds were found for the unimanual and bimanual discrimination of opponent shapes. In addition, systematic biases were found: the curvature of concave shapes was overestimated compared to the curvature of convex shapes. In Chapter 4, we used objects with complete surfaces to investigate whether curvature information could be used in the perception of 3D objects. We found that the ability to distinguish elliptical shapes from circular shapes was about four times better than the ability to distinguish rectangular shapes from square shapes. Performance in dynamic touch was only slightly better than performance in static touch. These findings show that humans are able to perceive and employ curvature information within 3D objects. The studies that are described in Chapters 5 and 6 were developed to obtain more insight into how the representation of curvature information depends on the manner of exploration. Therefore we investigated the properties of curvature aftereffects in static and dynamic touch. Chapter 5 shows the existence of a curvature aftereffect when stimuli are touched statically with a single finger. Only a partial transfer of this aftereffect was found, either between fingers of the same hand or of opposite hands. These findings suggest that curvature information is mainly represented at the level that is directly connected to the individual finger but partially represented at a level that the fingers of both hands share in common. In Chapter 6, we studied the occurrence and intermanual transfer of a curvature aftereffect in dynamic touch on surfaces with a low curvature. Surprisingly, we found a complete intermanual transfer of this aftereffect, which suggests that curvature information is represented at a high, bilateral level in the brain