Human Ability to Scale and Discriminate Forces Typical of Those Occurring during Grasp and Manipulation

Abstract

When humans manipulate objects, the sensorimotor system coordinates three-dimensional forces to optimize and maintain grasp stability. To do this, the CNS requires precise information about the magnitude and direction of load force (tangential to skin surface) plus feedback about grip force (normal to skin). Previous studies have shown that there is rapid, precise coordination between grip and load forces that deteriorates with digital nerve block. Obviously, mechanoreceptive afferents innervating fingerpad skin contribute essential information. We quantify human capacity to scale tangential and normal forces using only cutaneous information. Our paradigm simulated natural manipulations (a force tangential to the skin superimposed on an indenting force normal to the skin). Precisely controlled forces were applied by a custom-built stimulator to an immobilized fingerpad. Using magnitude estimation, subjects (n ϭ 8) scaled the magnitude of tangential force (0.25-2.8 N) in two experiments (normal force, 2.5 and 4 N, respectively). Performance was unaffected by normal force magnitude and tangential force direction. Moreover, when both normal (2-4 N) and tangential forces were varied in a randomized-block factorial design, the relationship between applied and perceived tangential force remained near linear, with a minor but statistically significant nonlinearity. Our subjects could also discriminate small differences in tangential force, and this was the case for two different reference stimuli. In both cases, the Weber fraction was 0.16. Finally, scaling functions for magnitude estimates of normal force (1-5 N) were also approximately linear. These data show that the cutaneous afferents provide a wealth of precise information about both normal and tangential force.

A comprehensive mechanotransduction model for tactile feedback based on multi-axial stresses at the fingertip-contact interface

“…1): 2,3) correspond to the components of the multi-axial force F applied over A, whose magnitude ||F|| can be expressed as (Eq. (1a)):…”

“…Human hand tactile dexterity [1,2], such as in object manipulation and slipping detection [1,3], relies on the tactile feedback provided by mechanoreceptors. In the glabrous skin at the fingertips, these mainly comprise slowly adapting type I (SAI)-Merkel cells, rapidly adapting type I (RAI)-Meissner corpuscles, rapidly adapting type II (RAII)-Pacinian corpuscles, and slow adapting type II (SAII)-Ruffini corpuscles [1,2].…”

“…In the glabrous skin at the fingertips, these mainly comprise slowly adapting type I (SAI)-Merkel cells, rapidly adapting type I (RAI)-Meissner corpuscles, rapidly adapting type II (RAII)-Pacinian corpuscles, and slow adapting type II (SAII)-Ruffini corpuscles [1,2]. Mechanoreceptors produce trains of action potentials in response to the multi-axial (pressure and shear) stresses applied on the skin, with this feedback tactile information ultimately delivered to the brain.…”

“…In fact, studies performed on human subjects [1,2] have demonstrated the ability of SA and RA mechanoreceptors at the fingertips to detect and encode multi-axial mechanical stimuli [1,2], with the brain relying on this pressure and shear information for object manipulation and haptic identification [2]. Thus, the development of comprehensive models, with modelled SAI, RAI, RAII and SAII units accounting for the conversion of pressure and shear stresses, is crucial towards a more in-depth covering and understanding of tactile perception, also in order to ensure a more effective and direct tactile feedback.…”

A Multi-Modal Haptic Interface for Virtual Reality and Robotics

“…When we lift an object there is frequently a microscopic slip (between the object and the skin) that is perceived by Meissner corpuscles. This information is then sent to the reflex circuits of the spinal cord that act in order to increase the grip force and therefore to avoid the slip [18]. These fibers are insensitive to static forces and to very low frequency vibrations; if not, during a gripping the big force exerted would mask the small signal due to slippage.…”